source: Backup NB/Talks/MEMPHYSetal/LAGUNA/EU I3/PhysicsLatex/Laguna-before-xarchiv/nusolar_det.tex @ 416

\section{Solar neutrinos}
\label{sec:Solar}
%\REDBLA{Version 0 from J.E.C 3/3/06}
%\REDBLA{update by A. Bueno 23/3/06}
%\REDBLA{update by T. Marrodan Undagoitia  13/4/06}
%\REDBLA{update by JEC 26/9/06}
%\REDBLA{update by JEC 16/10/06: this is a section now}
%
In the past years Water \v{C}herenkov detectors have measured the high energy tail ($E>5$~MeV)
of the solar $^{8}$B neutrino flux using electron-neutrino 
elastic scattering \cite{Smy:2002rz}. 
Since such detectors could record the time of an interaction and reconstruct 
the energy and direction of the recoiling electron, unique information 
of the spectrum and time variation of the solar neutrino flux was extracted. 
This provided further insights into the ``solar neutrino problem'', 
the deficit of the neutrino flux (measured by several experiments) 
with respect to the flux expected by the standard solar models. 
It also constrained the neutrino flavor oscillation solutions in a fairly 
model-independent way.

With MEMPHYS,  Super-Kamiokande's measurements obtained from 1258 days 
of data could be repeated in about half a year (the seasonal flux variation 
measurement requires of course a full year). In particular, a first 
measurement of the flux of the rare "hep" neutrinos may be possible.
Elastic neutrino-electron scattering is strongly forward peaked. 
To separate the solar neutrino signal from the isotropic background events (mainly due to low radioactivity), this 
directional correlation is exploited. Angular resolution is limited 
by multiple scattering.  The reconstruction algorithm first reconstructs 
the vertex from the PMT times and then the direction assuming a single 
Cherenkov cone originating from the reconstructed vertex. 
%JEC 26/9/06 START
Reconstructing 7~MeV events in MEMPHYS seems not to be a problem but decreasing the threshold would imply serious care of the PMT dark current rate as well as the laboratory and detector radioactivity level.
%JEC 26/9/06 END

%T. Marrodan Undagoitia  13/4/06 START

%T. Marrodan Undagoitia  10/12/06 START Some corrections
%\REDBLA{

With LENA, a large amount of neutrinos from ${}^{7}$Be, around $\sim
5.4\times10^3$/day ($\sim 2.0\times10^6$/y) would be
detected. Depending on the signal-to-background ratio, this would
provide a sensitivity for time variations in the $^{7}$Be neutrino
flux of $\sim 0.5$\% during one month of measuring time. Such a
sensitivity may give information at a unique level on helioseismology
(pressure or temperature fluctuations in the center of the sun) and on
a possible magnetic moment interaction with a timely varying solar
magnetic field.

The {\it pep} neutrinos are expected to be recorded at a
rate of $210$/day ($\sim 7.7\times10^4$/y), these neutrinos would
provide a better understanding of the global solar neutrino
luminosity. Due to the value of their energy, they could probe the
transition region of vacuum to matter-dominated neutrino oscillation.

The neutrino flux from the CNO cycle is theoretically
predicted with the lowest accuracy (30\%) of all solar neutrino
fluxes. Therefore, LENA would provide a new opportunity for a detailed
study of solar physics. However, the observation of such solar
neutrinos in these detectors, i.e. through elastic scattering, is not
a simple task, since neutrino events cannot be separated from the
background, and it can be accomplished only if the detector
contamination will be kept very
low~\cite{Alimonti:1998aa,Alimonti:1998nt}. Moreover, only
mono-energetic sources as such mentioned can be detected, taking
advantage of the Compton-like shoulder edge produced in the event
spectrum.

Recently, the possibility to register ${}^8$B solar neutrinos by means
of the charged current interaction with the
${}^{13}$C~\cite{Ianni:2005ki} nuclei naturally contained in organic
scintillators has been investigated. Even if the event signal does not
keep the directionality of the neutrino, it can be separated from the
background by exploiting the time and space coincidence with the
subsequent decay of the produced ${}^{13}$N nuclei (remaining
background of about $~60$/year corresponding to a reduction factor of
$\sim 3~10^{-4}$) \cite{Ianni:2005ki}. Around 360~events of this type
per year can be estimated for LENA. A deformation due to the
MSW-effect should be observable in the low-energy regime after a
couple of years of measurements.

For the proposed location of LENA in Pyh\"asalmi ($\sim 4000$~m.w.e.),
the cosmogenic background will be sufficiently low for the mentioned
measurements. Notice that Fréjus site would also be adequate for this
topic ($\sim 4800$~m.w.e.). The radioactivity of the detector would
have to be kept very low ($10^{-17}$~g/g level U-Th) as in the KamLAND
detector.
%} 
 %T. Marrodan Undagoitia 10/12/06 END Some corrections

%T. Marrodan Undagoitia 13/4/06 END

%%With LENA, one would in principle get a large amount of  neutrinos from ${}^{7}$Be ($\sim 5.4~10^3$/day) to test small flux fluctuation in time over the general seasonal variation. The {\it pep} neutrinos as well as the CNO cycle induced neutrinos are expected also to be recorded at a rate of $300$/day, this would constraint the CNO contribution to the solar energy release and to better understand the global solar neutrino luminosity. However, the observation of such solar neutrinos in these detectors, through í.e elastic scattering, is not a simple task, since neutrino
%%events cannot be separated from the background, and it can be accomplished
%%only if the detector contamination will be kept very low \cite{AlmontiSolar}. Moreover, only
%%mono-energetic sources as such mentioned can be detected, taking
%%advantage of the Compton-like shoulder edge produced in the event spectrum.
%%Recently, it has been investigated the possibility to register  $\sim 1000$/year ${}^8$B solar neutrinos by means of the charged current interaction with the ${}^{13}$C nuclei naturally contained in organic scintillators. Even, if the event signal does not keep the directionality of the neutrino, it can be separated from the background by exploiting the time and space coincidence with the subsequent decay of the produced ${}^{13}$N nuclei (remaining background of about $~60$/year corresponding to a redution factor of $\sim 3~10^{-4}$.) \cite{Ianni:2005ki}. The propose LENA location in Pyh\"asalmi ($\sim 4000$~m.w.e.) means that the cosmogenic background will be sufficiently low for the proposed measure. Notice that Fréjus location would be also good in this respect ($\sim 4800$~m.w.e.). The radioactivity of the detector would have to be kept very low ($10^{-17}$~g/g level U-Th) as in the KamLAND detector. \REDBLA{To be completed...}

The solar neutrinos in GLACIER can be registered through the elastic scattering $\nu_x + e^- \rightarrow \nu_x + e^-$ (ES) and the absorption reaction $\nu_e + {}^{40}Ar \rightarrow e^- + {}^{40}K^*$ (ABS) followed by $\gamma$s emission. Even if these reactions have low threshold (e.g $1.5$~MeV for the second one), one expects to operate in practice with a threshold set at 5~MeV on the primary electron kinetic energy to reject background from neutron capture followed by gamma ray emission which constitute the main background in some underground laboratory \cite{Arneodo:2001tx} as for the LNGS (Italy). These neutrons are induced by the spontaneous fission of the cavern rock (note that in case of a salt mine this background may be significantly reduced). 

The expected raw event rate is 330,000/year (66\% from ABS, 25\% from ES and 9\% from neutron background induced events) assuming the above mentioned threshold on the final electron energy. Then, applying further offline cuts to purify separatly the ES sample and the ABS sample, one gets the rates shown on \refTab{tab:GLACIER-Solar}.
\begin{table}
                \caption{\label{tab:GLACIER-Solar} Number of events expected in GLACIER per year, compared with the computed background (no oscillation) in the Gran Sasso Laboratory (Italy) rock radioactivity condition (i.e. $0.32~10^{-6}$~n \flux ($> 2.5$~MeV). The Absorption channel have been split into the contributions of events from Fermi transition and from Gamow-Teller transition of the ${}^{40}$Ar to the different ${}^{40}$K excited levels and that can be separated using the emitted gamma energy and multiplicity} 
                \begin{tabular}{lr}\hline\hline
                                                        & Events/year \\ \hline
Elastic channel ($E\geq5$~MeV)   &   45,300 \\
Neutron bkgd                                                    &         1,400 \\
Absorption events contamination   & 1,100 \\ \hline
Absorption channel (Gamow-Teller transition)    & 101,700 \\
Absorption channel (Fermi transition)   & 59,900 \\
Neutron bkgd                                                    & 5,500 \\                                              
Elastic events contamination      & 1,700 \\            
                        \hline\hline
                \end{tabular}
\end{table}

A possible way to combine the ES and the ABS channels similar to the NC/CC flux ratio measured by SNO collaboration \cite{Aharmim:2005gt}, is to compute the following ratio:
\begin{equation}
        R = \frac{N^{ES}/N^{ES}_0}{\frac{1}{2}\left( N^{Abs-GT}/N^{Abs-GT}_0 + N^{Abs-F}/N^{Abs-F}_0\right)}
\end{equation}
%Antonio Bueno 23/03/06 START
where the numbers of expected events without neutrino oscillations are labeled with a $0$). 
This double ratio has the following advantages: 
first it is independent of the ${}^{8}$B total neutrino flux, predicted by different solar models, 
and second it is free of experimental threshold energy bias and of the adopted cross-sections 
for the different channels. 
With the present fit to solar and KamLAND data, one expects a value of $R = 1.30\pm 0.01$ after one year of data taking with GLACIER. 
The quoted error for R only takes into account statistics. 
%Antonio Bueno 23/03/06 END