[386] | 1 | |
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| 2 | |
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| 3 | In Aussois, the session on ``Present and Future Neutrino Beams'' |
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| 4 | reviewed the long baseline experiments that will help to understand |
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| 5 | the neutrino mixing parameters phenomenology in the coming years. This is |
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| 6 | a long and step-by-step process. In the first step, the MINOS |
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| 7 | (see M. Bishai's talk), OPERA and ICARUS (see D. Duchesneau's talk) |
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| 8 | will confirm and improve the SuperK atmospheric oscillation result. This |
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| 9 | phase will provide an improvement of the limit on \thetachooz ($\simeq < 0.06$ 90\% C.L.) |
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| 10 | In the second step, T2K (see Kobayashi's talk) and NO$\nu$A |
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| 11 | (see Ray's talk) will focus on measuring \thetachooz ($\simeq < 0.006$ 90\% C.L.) . This |
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| 12 | measurement is a prerequisite before attempting to look for |
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| 13 | CP violation in the leptonic sector: this will be the task of |
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| 14 | the third step, and the VLBL (see M. Bishai's talk), |
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| 15 | the T2K-II (see Kobayashi's talk) and the CERN-Fr\'ejus |
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| 16 | (see M. Mezzetto's talk) proposals. The ultimate tool in neutrino |
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| 17 | physics -- the neutrino factory -- was not discussed in this meeting. |
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| 18 | |
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| 19 | \section{First step} |
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| 20 | There are presently four experiments running or planned to confirm |
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| 21 | the atmospheric neutrino result and improve on the knowledge of the |
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| 22 | oscillation parameters (\deltaatm, \sinatm): K2K, MINOS, OPERA and |
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| 23 | ICARUS. The last 3 will also search for the sub-leading |
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| 24 | \numunue ~oscillations, attempting at a |
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| 25 | first measurement of the \thetachooz angle. |
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| 26 | The four experiments rely on very different experimental options |
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| 27 | (beam and/or detector techniques). |
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| 28 | |
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| 29 | \subsection{K2K} |
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| 30 | (http://neutrino.kek.jp/) |
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| 31 | |
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| 32 | The K2K (KEK to Kamioka) long-baseline neutrino oscillation experiment, |
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| 33 | is the first accelerator-based project to explore neutrino |
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| 34 | oscillations in the same \deltaatm\ region as the atmospheric neutrinos. |
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| 35 | By using a low energy \numu\ beam and a flight distance of 250~km, |
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| 36 | the oscillation process should manifest itself as a reduction of the \numu\ |
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| 37 | flux at Kamioka (a disappearance) since the \nutau\ produced |
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| 38 | in the oscillation are below the CC threshold. |
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| 39 | In addition, the energy spectrum of the observed \numu\ should also |
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| 40 | be affected by the oscillation. |
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| 41 | |
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| 42 | The K2K neutrino beam is produced by 12~GeV protons from the |
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| 43 | KEK proton synchrotron. |
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| 44 | The positively charged secondary particles, mainly pions, are |
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| 45 | then focused by a horn system. |
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| 46 | The resulting neutrino beam is $98$~\% pure $\nu_\mu$ |
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| 47 | with a mean energy of $1.3$~GeV. |
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| 48 | It traverses first the near detector (ND) system, |
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| 49 | located 300~m downstream from the proton target, |
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| 50 | and then the SuperKamioka detector, 250~km away. |
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| 51 | |
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| 52 | To estimate the ratio of neutrino flux and spectra between Kamioka and KEK |
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| 53 | (far to near, F/N), |
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| 54 | a combination of experimental measurements and simulation has to be done. |
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| 55 | Indeed, due to different geometrical acceptances (the neutrino production |
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| 56 | place cannot be approximated to a point as seen from the near detector), |
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| 57 | the neutrino spectra seen by |
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| 58 | the 2 detectors differ, even with no oscillation. |
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| 59 | |
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| 60 | The beam MC simulation is first tuned on the PIMON measurements (pion |
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| 61 | monitoring detectors intermitently installed upstream of the decay pipe) and then |
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| 62 | used to compute the F/N ratio in energy bins. |
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| 63 | This ratio allows then to extrapolate to SK the integrated flux and |
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| 64 | energy spectrum as measured in the ND, before neutrino oscillate. |
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| 65 | %(1KT Water Cherenkov detector , |
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| 66 | %SciFi and SciBar detectors). |
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| 67 | This extrapolation is compared to the SuperKamiokande measurements. |
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| 68 | The latest results from the K2K experiment quotes |
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| 69 | $151^{+12}_{-10}$ fully contained events expected in SK |
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| 70 | and 107 observed. |
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| 71 | |
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| 72 | The main sources of uncertainty in this experiment are the following: |
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| 73 | \begin{itemize} |
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| 74 | \item The Monte Carlo F/N ratio relies on a neutrino interaction model, |
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| 75 | which includes QE, single meson production |
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| 76 | via baryon resonance and coherent pion production. |
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| 77 | The relative importance of these componants as a function of energy |
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| 78 | is poorly known. |
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| 79 | \item The efficiency of the 1KT Cherenkov detector (affecting |
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| 80 | the normalization) is dominated by uncertainties in the fiducial |
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| 81 | volume. |
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| 82 | \item The energy scale in both Cherenkov detectors |
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| 83 | %\item autres?? |
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| 84 | \end{itemize} |
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| 85 | A two flavor neutrino oscillation analysis for $\nu_\mu$ disappearance |
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| 86 | is performed by the maximum-likelihood method, using both the number of |
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| 87 | events and the spectrum shape. |
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| 88 | The best fit point in the physical region |
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| 89 | is found at (\sinatm, \deltaatm)=$(1.0, 2.8\times 10^{-3}~{\rm eV^2})$. |
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| 90 | This result is consistent with the results from atmospheric neutrinos. |
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| 91 | The final result of the experiment is expected by the end of 2005. |
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| 92 | |
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| 93 | \subsection{MINOS} |
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| 94 | (http://www-numi.fnal.gov/Minos/) |
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| 95 | |
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| 96 | MINOS is a long baseline neutrino oscillation experiment utilizing the |
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| 97 | NuMI beam at Fermilab. |
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| 98 | This beam is obtained through an intense (0.25 MW) proton beam |
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| 99 | hitting a graphite target at 120 GeV/c. |
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| 100 | The movable 2 horn focussing system allows for selecting different |
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| 101 | energy spectra: low, medium and high. |
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| 102 | The experimental setup consists of 2 detectors separated by 730 km |
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| 103 | and as identical as possible: same transverse and longitudinal granularity, |
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| 104 | same composition and modularity. The basic components are magnetized |
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| 105 | iron plates interlayed with scintillator |
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| 106 | strips (the good timing resolution allows background rejection from |
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| 107 | atmospheric neutrinos and separation of events piling up in the near detector) |
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| 108 | |
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| 109 | The main aim of the experiment is to probe |
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| 110 | the region of parameter space indicated by the atmospheric neutrinos, |
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| 111 | to demonstrate the oscillation behaviour |
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| 112 | and to make a precise measurement of |
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| 113 | \deltaatm\ in a high statistics beam experiment. |
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| 114 | This will be done through \numu\ disappearance: |
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| 115 | by plotting, as a function of energy, |
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| 116 | the ratio of the yield at the far detector |
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| 117 | to the one expected from near detector measurements. The |
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| 118 | location and depth of the dip will allow to measure \deltaatm\ and \sinatm . |
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| 119 | The low energy beam spectrum is best suited to match the latest SK results |
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| 120 | and has been choosen as the present running condition. |
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| 121 | A measurement at 10\% |
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| 122 | (3 years at upgraded intensity: $25\times 10^{20} pot$) %??? |
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| 123 | can be achieved and could then also rule out exotic oscillation models. |
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| 124 | The limitation in sensitivity comes mainly from statistics and from |
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| 125 | the uncertainty in the extrapolation process of the neutrino |
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| 126 | spectrum from the near to the far detector. |
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| 127 | |
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| 128 | A second goal of the experiment is the search for the sub-dominant |
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| 129 | \numunue\ oscillations aiming at a first \thetachooz\ measurement. |
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| 130 | In this \nue\ appearance search, the |
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| 131 | background is dominated by NC produced \pizero 's |
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| 132 | and some intrinsic \nue\ from the beam. |
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| 133 | But the sensitivity could reach $\sinchooz < 0.07 $ |
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| 134 | (twice better as the CHOOZ limit). |
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| 135 | |
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| 136 | The detector has been extensively calibrated, both |
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| 137 | at Cern with a micro-MINOS and with cosmic |
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| 138 | muons (the shadow of the moon has been observed!). |
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| 139 | The NuMI beam line has been successfully commissioned and |
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| 140 | the near and far detectors are presently fully operational with |
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| 141 | more than 90\% live time. They both have observed their first |
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| 142 | beam neutrinos in March this year. |
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| 143 | |
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| 144 | |
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| 145 | \subsection{OPERA} |
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| 146 | (http://operaweb.web.cern.ch/operaweb/) |
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| 147 | |
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| 148 | The main goal of OPERA is to focus on providing an unambiguous |
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| 149 | evidence for \numunutau ~oscillations in the |
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| 150 | region of the oscillation parameters indicated by the atmospheric |
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| 151 | neutrino results by looking for \nutau ~appearance in a \numu ~beam. |
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| 152 | This implies both a high energy beam |
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| 153 | (above the $\tau $ production threshold) and a very precise tracking |
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| 154 | detector to observe the $\tau $ produced in charged current |
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| 155 | \nutau\ interactions (through the characteristic short kink of the |
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| 156 | $\tau $ decay). |
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| 157 | |
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| 158 | The CNGS \numu\ beam will have an average energy of about 17 GeV, |
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| 159 | a \nue\ and \nuebar\ |
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| 160 | contamination of 0.87\% and a neglegible prompt \nutau\ contamination. |
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| 161 | This high energy choice means that, given the 730 km distance between |
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| 162 | the neutrino source (at CERN) and the OPERA detector, |
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| 163 | the experiment will be running off the |
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| 164 | oscillation peak |
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| 165 | (as expected from the atmospheric neutrino results) |
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| 166 | and thus will not be sensitive to the oscillation pattern. |
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| 167 | |
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| 168 | The $\tau$ detection relies on the photographic emulsion technique. |
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| 169 | 200000 bricks (emulsion and lead sanwiches) amounting to 1.8 kt will be |
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| 170 | complemented by electronic trackers and a muon spectrometer.. |
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| 171 | Although the expected signal is very low (115 \nutau\ CC interactions |
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| 172 | in the detector and 13 identified by the selection procedure) |
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| 173 | the expected background will stay around 1 event: the evidence |
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| 174 | for \numunutau\ should then be very clear. |
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| 175 | |
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| 176 | Besides the technical difficulties and complexity of constructing |
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| 177 | the apparatus, the chalenges in this experiment mainly concern |
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| 178 | efficiencies: tracking, matching between tracker and emulsions, |
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| 179 | scanning and selection. |
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| 180 | |
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| 181 | Given the good electron identification capabilities in the |
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| 182 | emulsion bricks, OPERA can look as well for \numunue\ oscillations. |
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| 183 | The main backgrounds to the \numunue ~oscillations search are the \pizero\ |
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| 184 | identified as electrons in \numu\ neutral current events, |
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| 185 | the intrinsic \nue\ beam contamination and |
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| 186 | the electrons coming from $\tau$ decays in \numunutau\ oscillations. |
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| 187 | The signal to background ratio can however be enhanced by performing |
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| 188 | a simultaneous fit to the distribution of the visible energy, electron |
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| 189 | energy and missing transverse momentum. This yields a 5 years sensitivity |
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| 190 | corresponding to an upper limit on $sin^2(2 \theta_{13})$ of 0.06 for the |
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| 191 | nominal beam, similar to the MINOS sensitivity. |
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| 192 | |
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| 193 | \subsection{ICARUS} |
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| 194 | (http://www.aquila.infn.it/icarus/) |
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| 195 | |
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| 196 | Located in the Gran Sasso Laboratory in the same CNGS beam as OPERA, |
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| 197 | the ICARUS experiment is a liquid Argon TPC with imaging capabilities, |
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| 198 | able to produce high granularity 3D reconstruction of recorded events. |
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| 199 | %as well as high precision measurements over large sensitive volumes. |
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| 200 | The operating principle of the LAr TPC is based on the fact that |
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| 201 | in highly purified LAr ionization tracks can indeed be transported |
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| 202 | undistorted by a uniform electric field over distances of the order |
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| 203 | of meters. |
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| 204 | |
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| 205 | The detector is not only a tracking |
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| 206 | device with a precise event topology reconstruction but it can |
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| 207 | also estimate momentum |
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| 208 | via multiple scattering, measure local energy deposition ($dE/dx$, |
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| 209 | providing $e/\pi^0$ separation and particle identification via range |
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| 210 | versus $dE/dx$ measurement) and reconstruct the total energy of |
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| 211 | the event from charge integration providing excellent accuracy for |
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| 212 | contained events. |
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| 213 | |
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| 214 | A 600 ton prototype has been extensively tested at surface during |
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| 215 | the summer 2001, demonstrating that the LAr TPC technique can be operated |
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| 216 | at the {\it kton} scale with a drift length up to 1.5 {\it m}. |
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| 217 | Installation at the Gran Sasso Underground Laboratory is currently on-going. |
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| 218 | Cloning the T600 module will permit to gradually increase the mass and reach a |
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| 219 | sensitive mass of 2.35ktons. In the present sensitivity estimates |
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| 220 | only 3 such modules are used since this is the actual guaranteed funding |
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| 221 | level. |
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| 222 | |
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| 223 | Given the good detector performance and the beam conditions (energy |
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| 224 | and flux), ICARUS will also % as OPERA and MINOS |
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| 225 | have as a first goal to prove the \numunutau\ oscillation. |
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| 226 | The analysis will rely on the golden channel: |
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| 227 | $\tau \rightarrow e\nue\nutau$. |
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| 228 | The suppression of the background, dominated by |
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| 229 | \nue\ CC interactions from the beam contamination, will be |
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| 230 | done through a kinematical analysis, using a 3-dimensional |
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| 231 | likelihood, including visible energy |
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| 232 | and missing transverse momentum. |
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| 233 | With a T1800 detector and 5 years data taking (2.25 $10^{20}$ pot) |
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| 234 | 6.2 signal events at $2.5 \times 10^{-3} eV^2$ ($\epsilon . BR = 6\%$) |
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| 235 | for 0.3 background event are expected. |
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| 236 | |
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| 237 | The excellent capability of identifying electrons by the ICARUS |
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| 238 | detector obviously allows to also search for \nue\ appearance. |
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| 239 | This \numunue\ oscillation component would appear as a distorsion |
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| 240 | in the energy spectrum of the \nue\ CC interaction sample. |
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| 241 | The sensitivity to an expected $E_{vis}$ distorsion at low energy |
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| 242 | has been evaluated to $\sinchooz < 0.07$ |
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| 243 | at 90\% CL (for $\deltaatm = 2.5 \times 10^{-3} eV^2$ and full mixing). |
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| 244 | |
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| 245 | % uncertainties, difficulties |
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| 246 | |
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| 247 | \section{Second step} |
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| 248 | MW-class proton accelerators are being constructed for several physics needs. |
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| 249 | High intensity conventional horn-focused neutrino beams, |
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| 250 | ``super beam'', will then provide a new opportunity to further develop |
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| 251 | neutrino physics: |
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| 252 | several super beam LBL experiments are proposed as |
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| 253 | next generation ($\sim$10 years) high sensitivity, |
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| 254 | high precision experiments before the neutrino-factory era. |
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| 255 | Their most important goal is discovery of the $\nu_\mu\rightarrow \nu_e$ |
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| 256 | oscillation but one should not neglect other interests |
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| 257 | such as detailed study of the neutrino interactions or spin structure of |
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| 258 | nucleons. |
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| 259 | |
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| 260 | In all the future super beam experiments (T2K-I and $\rm{NO\nu A}$), |
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| 261 | ``off-axis (OA)'' beam plays a key role to achieve high sensitivity: |
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| 262 | the proton beam line is shooting |
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| 263 | a few degrees away from the direction to a far detector. |
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| 264 | In this way, a high intensity low energy narrow band neutrino beam can be |
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| 265 | obtained and its energy can be adjusted close to the L/E oscillation |
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| 266 | maximum. Moreover this ``trick'' effectively reduces 2 important |
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| 267 | background sources: the high energy NC production of $\pi^0$'s |
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| 268 | and the intrinsic contamination of the beam by $\nu_e$. |
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| 269 | |
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| 270 | \subsection{T2K-I} |
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| 271 | (http://neutrino.kek.jp/jhfnu/) |
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| 272 | |
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| 273 | The Tokai-to-Kamioka (T2K) experiment is the next generation LBL |
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| 274 | experiment in Japan. |
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| 275 | The $\nu_\mu$ beam |
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| 276 | is produced using a 50-GeV proton synchrotron from the Japan |
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| 277 | Proton Accelerator Research Complex (J-PARC). |
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| 278 | The peak position of its energy spectrum is tunable from 500 MeV to 900 MeV by |
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| 279 | changing the OA angle from 2 to 3 degrees. |
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| 280 | The narrow band is important because it increases the |
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| 281 | fux at the oscillation maximum, maximizing |
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| 282 | the appearance signal. |
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| 283 | The far detector is located |
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| 284 | at Kamioka, 295 km from J-PARC. |
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| 285 | |
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| 286 | In the first phase of T2K (T2K-I), the design beam power of the 50-GeV |
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| 287 | PS is 0.75~MW (more than 100 time as powerful as the K2K beam) |
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| 288 | and the far detector is Super-Kamiokande (SK) of 22.5-kt |
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| 289 | fiducial mass. The main purpose of T2K-I is a measurement of |
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| 290 | $\theta_{13}$ with more than one order of magnitude sensitivity |
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| 291 | better than any existing experiment ($\simeq < 0.006$ 90\% C.L.). The second goal would be |
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| 292 | a determination of the ``atmospheric'' |
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| 293 | parameters, $\theta_{23}$ to an accuracy of 0.01 |
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| 294 | and $\Delta m_{23}^2$ to $10^{-4}$ $eV^2$. |
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| 295 | \sinatm\ is presently known to be at least 0.95. |
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| 296 | Maximal mixing could lead to an underlying new symmetry and thus |
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| 297 | being able to measure \thetaatm\ |
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| 298 | to high enough precision to distinguish maximal and nearly maximal mixing |
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| 299 | is very important. |
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| 300 | |
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| 301 | With an OA beam at $2.5^o$ the expected statistics at SK would be 1600 \numu\ |
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| 302 | CC events per year. The intrinsic beam contamination by \nue\ is 0.4\%. |
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| 303 | The appearance signal events in SK are interactions with a single |
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| 304 | showering Cherenkov ring. |
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| 305 | The neutrino energy is reconstructed assuming quasi-elastic two-body kinematics. |
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| 306 | An excess of events over the expected background is the signal for |
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| 307 | \nue\ appearance. Since the signal and background events have different |
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| 308 | energy spectra, it is essential to control both the flux and the |
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| 309 | shape of the input spectrum. This is why the experiment will rely on |
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| 310 | 2 near detectors: at 280m the first near detector will be magnetized |
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| 311 | (contained in the UA1/NOMAD magnet) and permit detailed studies of |
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| 312 | the flux and spectra of the different beam components (\numu, \nue, |
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| 313 | \numubar\ and \nuebar). This knowledge is important in the extrapolation |
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| 314 | to SK procedure. The second near detector will be situated at 2km, where |
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| 315 | the energy spectra of the neutrinos crossing a 1kT water Cherenkov |
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| 316 | and those crossing SK are identical. |
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| 317 | The uncertainty sources listed above for the K2K experiment should |
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| 318 | then be drastically reduced. |
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| 319 | |
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| 320 | The neutrino beam line construction has started, together with an |
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| 321 | intensive R\&D and design work on each of its components. The tecnical |
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| 322 | design report of the 280m detector is expected this summer, while the |
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| 323 | 2km detector is not yet approved. The first neutrinos in T2K should |
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| 324 | be delivered in spring 2009 for 5 years. |
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| 325 | |
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| 326 | %T2K and NO$\nu$A are complementary experiments. |
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| 327 | %They use different detector technologies (water cherenkov vs low-Z calorimeter) |
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| 328 | %but alos their different baseline could help disentangle the effects |
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| 329 | %of CP violation and matter effects in neutrino oscillations. |
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| 330 | |
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| 331 | \subsection{NO$\nu$A} |
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| 332 | (http://www-nova.fnal.gov/) |
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| 333 | |
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| 334 | With the same physics goal as T2K, the NO$\nu$A proposal is |
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| 335 | in addition raising the neutrino mass ordering question: the mass |
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| 336 | ordering can be resolved only by matter effects in the earth |
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| 337 | over long baselines. At 810 km - the NO$\nu$A chosen basline - |
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| 338 | the matter effect should be about 30\% |
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| 339 | for a NuMI off-axis beam and only 10\% for T2K. |
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| 340 | |
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| 341 | NO$\nu$A is proposed to be a 30 kT totally active low-Z calorimeter |
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| 342 | (15m x 15m x 130m) placed 15 mrad off the NuMI beam axis. |
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| 343 | The far detector will consist of 1984 planes of liquid |
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| 344 | scintillator strips contained in |
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| 345 | extruded rigid PVC and readout by APD's through wls fibers. |
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| 346 | Water cherenkov has been discarded because |
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| 347 | it does not provide sufficient NC rejection at NuMI energy |
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| 348 | (2 GeV) while a 0.15 $X_0$ sampling calorimeter provides |
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| 349 | good \pizero-electron discrimination. The fast timing of the |
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| 350 | scintillator allows to install the detector at ground level: |
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| 351 | this will mean less than 10 cosmic rays in the 10 $\mu s$ |
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| 352 | beam spill. |
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| 353 | The near detector, very similar to the far detector but complemented |
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| 354 | with a veto and a muon catcher, can fit in several existing locations |
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| 355 | in the NuMI access tunnel. No single location optimizes all parameters, |
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| 356 | and the collaboration is considering making it movable or building 2 |
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| 357 | detectors. |
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| 358 | |
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| 359 | NO$\nu$A has been granted stage-I approval by Fermilab in april 2005 |
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| 360 | and benefits a strong support as being the only approved US experiment |
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| 361 | in the post 2010 era. The expected beam intensity could reach |
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| 362 | $\rm 6.5 x 10^{20}$ pot/yr, i.e. 0.65 MW after the collider stops operating |
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| 363 | in 2009. The construction could start in FY2006, have the first kT operational |
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| 364 | in 2009 and the full detector operational in 2011. |
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| 365 | With this optimistic schedule, the experiment is expected to reach an |
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| 366 | order of magnitude better sensitivity in \thetachooz\ event faster than |
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| 367 | T2K. |
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| 368 | %Pending NuSAG/P5 and OMB approval. |
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| 369 | |
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| 370 | \section{Third step} |
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| 371 | T2K and NO$\nu$A |
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| 372 | will have very limited sensitivity to the CP phase $\delCP$ |
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| 373 | even if complemented by high sensitivity reactor experiments. |
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| 374 | A third generation of LBL neutrino experiments will then be required |
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| 375 | to start a sensitive search for leptonic CP violation. |
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| 376 | These future experiments will push conventional neutrino beams to their |
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| 377 | ultimate performances (neutrino SuperBeams), or will require new concepts |
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| 378 | in the production of neutrino beams. |
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| 379 | |
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| 380 | \subsection{VLBL-Brookhaven} |
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| 381 | The BNL proposal of a Very Long BaseLine conceptual design is |
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| 382 | advocating the use of a broad band low energy (1-6 GeV) on axis beam |
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| 383 | heading on a megaton class detector to be sensitive both to \delCP\ |
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| 384 | and to the sign of \deltaatm. |
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| 385 | |
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| 386 | The CP contribution is dependent on both atmospheric and solar $\Delta m^2$ |
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| 387 | and is affecting the \nue\ appearance spectrum (and \numu\ disappearance) |
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| 388 | in the 1-3 GeV range. |
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| 389 | On the other hand, the matter effect causes the \numunue\ conversion probability |
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| 390 | to rise with energy and is mostly confined to energies $>$ 3 GeV. |
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| 391 | this energy dependence can |
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| 392 | be used to measure the value of $\delta_{CP}$ and $\sin^2 2\theta_{13}$. |
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| 393 | The detector requirements for such an experiment -- both in size and |
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| 394 | performance -- are well-matched to other important goals in particle |
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| 395 | physics, such as detection of proton decay and astrophysical neutrinos. |
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| 396 | |
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| 397 | In the present design the neutrino beam is produced |
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| 398 | by the 28~GeV proton beam from AGS (Brookhaven) and is detected by a |
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| 399 | Mton UNO type water Cherenkov detector in Homestake mine at 2540~km |
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| 400 | from BNL or in Henderson mine (2700 km). |
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| 401 | The AGS beam power is supposed to be upgraded to 1~MW from present |
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| 402 | 0.1~MW by introducing a 1.2~GeV superconducting LINAC for direct injection |
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| 403 | and increasing repetition rate. |
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| 404 | The beam is a horn-focused on-axis wide band beam with the spectrum |
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| 405 | ranging up to about 6~GeV and the peak at around 2~GeV. |
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| 406 | The expected number of $\nu_\mu$ CC interactions without oscillation is |
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| 407 | $\sim$13,000/500kt/year. Running with anti-neutrinos could improve further |
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| 408 | a \delCP\ measurement. |
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| 409 | |
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| 410 | \subsection{T2K-II} |
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| 411 | |
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| 412 | A future extension of T2K is already envisaged with an upgrading |
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| 413 | of the proton synchrotron to 4~MW and the construction of a 1-Mt |
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| 414 | ``Hyper-Kamiokande''. |
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| 415 | With $\sim$5 times higher intensity and about 25 times larger fiducial |
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| 416 | mass, statistics at HK will be 2 orders of magnitude higher |
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| 417 | than at T2K-I. The expected number of $\nu_\mu$ CC interactions is |
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| 418 | $\sim360,000$/year with a 2$^\circ$ off-axis beam. |
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| 419 | The goals of T2K-II are the discovery |
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| 420 | of CP violation and the precise measurement of $\nu_e$ appearance. |
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| 421 | |
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| 422 | Preliminary studies on a possible upgrade of the 50-GeV PS to 4~MW have |
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| 423 | been made by the J-PARC accelerator group. |
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| 424 | A first gain (by a factor $\sim$2.5) can be obtained by |
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| 425 | increasing the repetition rate (doubling the number of RF |
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| 426 | cavities) and by eliminating some idle time in the acceleration cycle. |
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| 427 | Second, another factor $\sim$2 could be gained by doubling the |
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| 428 | number of circulating protons when adopting the |
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| 429 | ``barrier bucket'' method. |
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| 430 | |
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| 431 | The design of the neutrino beam line presently under construction |
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| 432 | at JPARC includes the property of being off-axis (tunable between |
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| 433 | $2^\circ$ and $3^\circ$) both for T2K-I and T2K-II: the HK site would |
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| 434 | be in the Tochibora mine, $\sim$8 kilometers away from the SK location at 500 m depth (1400 mwe). |
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| 435 | Two 250m long parallel tunnels would host huge water Cherenkov detectors |
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| 436 | similar in principle to SK, amounting to 0.54 Mt fiducial mass. |
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| 437 | |
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| 438 | The expected sensitivity on CP violation in T2K-II, based on a full |
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| 439 | detector simulation (SK scaled to HK), very much depends on |
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| 440 | the size of the systematic errors. |
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| 441 | If 2\% error is achieved, then the CP violating phase \delCP\ can be |
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| 442 | explored down to $\sim 20^\circ$ for $\sin^22\theta_{13}$ greater than 0.01. |
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| 443 | |
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| 444 | \subsection{CERN-Fr\'ejus SPL} |
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| 445 | |
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| 446 | This new proposal has been stimulated by 2 converging ``opportunities''. |
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| 447 | First CERN is considering the construction of a new proton driver, |
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| 448 | a Superconducting Proton Linac of low energy (2-3 GeV/c kinetic energy) |
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| 449 | but very high intensity (4 MW, i.e. $10^{23}$ protons/yr!). Second the |
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| 450 | drilling machines of the new safety tunnel in Fr\'ejus should meet at the |
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| 451 | center around 2009, giving the opportunity to dig a new cavity that could |
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| 452 | be ready by 2012 and host a Megaton class detector at about 1750m depth i.e 4800 mwe . |
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| 453 | |
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| 454 | Although no definite decision on the SPL construction will be taken |
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| 455 | by CERN before 2009, intense R\&D is already going on for a liquid |
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| 456 | mercury target station able to cope with the 4 MW beam and |
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| 457 | for the neutrino beam optics, capable to stand heat, radiation and |
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| 458 | mercury. Recently an optimization of the SPL neutrino superbeam has been |
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| 459 | made and found that, given the 130 km baseline (from CERN to the Fr\'ejus |
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| 460 | tunnel), a 3.5 GeV proton beam plus a 40 m long and 2 m diameter decay tunnel |
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| 461 | would greatly improve the performances over the 2.2 GeV initial option: |
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| 462 | the \numu\ CC interaction rate at 130 km would rise from 42 to 122 |
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| 463 | events/kton/year in case of no oscillation. |
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| 464 | |
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| 465 | In these running conditions, both the NC \pizero\ background and |
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| 466 | the intrinsic \nue\ beam contamination are expected to be low and |
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| 467 | the sensitivity to \thetachooz\ an order of magnitude better than |
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| 468 | in T2K-I for a 5 years run of with \numu\ beam |
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| 469 | (using a 2\% systematic error both on the background |
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| 470 | normalization and on the signal efficiency). The discovery potential |
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| 471 | (at 3$\sigma$) to \delCP\ could reach 45$^\circ$ if \sinatm = 0.001 |
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| 472 | by running 2 years with \numu\ and 8 years with \numubar. |
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| 473 | |
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| 474 | Conventional neutrino beams are going to hit their ultimate limitations, |
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| 475 | specifically in the search for CP violation. But when |
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| 476 | combined with BetaBeams they can improve the CP sensitivity and allow for |
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| 477 | T and CPT searches in the appearance mode. |
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| 478 | |
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| 479 | \subsection{CERN-Fréjus Beta-beam} |
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| 480 | |
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| 481 | The recently proposed beta-beam idea is taking advantage of the |
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| 482 | possibility of accelerating and storing radioactive ions within |
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| 483 | their lifetime, thus producing just one flavor neutrino beam |
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| 484 | (\nue\ or \nuebar). Its energy spectrum is precisely defined |
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| 485 | by the end point energy of the beta decay and by the $\gamma$ |
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| 486 | of the parent ion. The flux normalization is given by the |
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| 487 | number of ions circulating in the storage ring and the beam |
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| 488 | divergence is determined by the $\gamma$: the beam control is |
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| 489 | then virtually systematics free. |
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| 490 | |
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| 491 | Beta-beam studies are essentially done in Europe presently |
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| 492 | and synergies with nuclear physics are emphasized. |
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| 493 | A EURISOL-like complex fed by the SPL could produce |
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| 494 | $6 \times 10^{18}$ $\rm ^6He$ ions (\nuebar) and |
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| 495 | $2.5 \times 10^{18}$ $\rm ^{18}Ne$ (\nue) ions |
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| 496 | per year boosted with a $\rm \gamma = 100$. |
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| 497 | |
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| 498 | The superbeam and beta-beam have the advantage of having similar energies |
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| 499 | which allows usage of the same far detector and explore CP violation |
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| 500 | in two different channels with different backgrounds and systematics. |
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| 501 | The disadvantages however are the low cross section at these energies, |
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| 502 | wich implies very massive detectors, and the limitation in the energy |
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| 503 | resolution due to Fermi motion. A 10 year experiment, combining |
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| 504 | a superbeam (running 2 years with \numu\ and 8 years with \numubar) |
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| 505 | and a beta-beam (running 5 years with \nue\ and 5 years with \nuebar) |
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| 506 | would give a discovery potential (at 3$\sigma$) to \delCP\ of |
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| 507 | 30$^\circ$ if \sinatm = 0.001. |
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| 508 | |
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| 509 | Ideas about storing radioactive ions that can only decay by electron |
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| 510 | capture have been recently proposed: this could lead to monochromatic |
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| 511 | \nue\ beams and should be studied further. |
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| 512 | |
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| 513 | \subsection{Neutrino factory} |
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| 514 | |
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| 515 | This subject was not discussed in the meeting but could be viewed |
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| 516 | as the ultimate step for a full understanding of the neutrino mixing |
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| 517 | and neutrino phenomenology. |
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| 518 | |
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| 519 | |
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