source: Backup NB/Talks/MEMPHYSetal/ISS KEK WC/session4.tex @ 386

\section{Monte Carlo Generators}



 Accurate measurements of neutrino oscillation parameters by future experiments could be significantly hampered 

by the large uncertainties in neutrino cross-section in the sub-GeV range. Neutrino interactions with nucleon in

nuclei are not well understood from a theoretical point of view, especially at low energies, and experimental 

data are sparse. Futhermore, most of available data come from Bubble chamber experiments made in the late 70s and have 

large systematic errors induced by the determination of the neutrino flux. Calulations for charged current $\nu_\mu$ are 

shown in Fig \ref{fig:neutrinoxsection}. \\



 New generation of high intensity and well controlled neutrino beams allow to collect much precised data that will 

attend to futher understand interactions and better constrain models.\\



\begin{figure}[hbt]

\begin{center}

\vspace{0.1cm}

\includegraphics[width=85mm]{./figures/neutrinoXsection.epsf}

\vspace{0.5cm}

\caption{ $\nu_\mu$ charged current cross-section calculations compared with experimental data}

\label{fig:neutrinoxsection}

\end{center}

\end{figure}



 Many Monte-carlo generator codes exist but are optimised for a dedicated experiment, ${\it{e.g.}}$ tuned for specific 

target materials. The GENIE collaboration\footnote{http://hepunx.rl.ac.uk/~candreop/generators/GENIE/} \cite{genie} gathers 

experimentalists from major neutrino experiments as well as theorits and proposes a Universal neutrino generator 

that will work for all nuclear targets in all energies. 

The code of the framework is developped in Object-Oriented language to ease the interface with standard libraries like 

the CERNLIB or CLHEP packages, with other existing simulation softwares (Geant4, Pythia7, $\ldots$) and with standard 

analysis tools such as ROOT.\\ 

 An additional feature that is included in the GENIE framework is an interface with a database containing the 

world's neutrino data \cite{xsectiondata} for model validation.\\

 

\section{Background rejection in large water Cerenkov}



 Large underground water Cherenkov detectors can measure $\nu_{\rm{e}}$ appearance as well as $\nu_\mu$

disappearance. Projects have different configurations in neutrino flux and energy spectrum, although with 

a similar overall shape with a the dip from oscillation minimum in the oscillated $\nu_\mu$ distribution.



\par 

 For a $\nu_\mu$ disapearance experiment, the signal is muons from charged current quasi elestic interactions,

$\nu_\mu + n \rightarrow p + \mu^-$.



\par

 For a $\nu_{\rm{e}}$ appearance experiment, the signal comes from oscillated $\nu_{\rm{e}}$ neutrinos, 

$\nu_\mu \rightarrow \nu_{\rm{e}}$, $\nu_{\rm{e}} + n \rightarrow p + e^-$ and is detected as a fully 

contained single electron-ring event.\\



\par

 Realistic monte-carlo studies for background rejection in $\nu_{\rm{e}}$ appearance experiments are 

the essential groundwork for the quest for the last unknown mixing angle of the mixing matrix and 

precise measurement of $\theta_{13}$. Main background sources are the $\nu_{\rm{e}}$ contamination in 

the beam and neutral current events with one pion decaying into two photons, $\nu + N \rightarrow N' + \nu + \pi^0 (\gamma\gamma)$. 

The latter can be reduced by the reconstruction of the second fainter photon-ring. Indeed, it is likely that 

one of the photon will carry away most of the energy, and when the energy fraction of one photon is very small, 

the event closely resembles electron signal. Algorthims for $\pi^0$ identification have thus been developped 

both at T2K \cite{dunmore} and at a megaton class detector on a Very Long Base Line neutrino beam \cite{yanagisawa}. 

Background can be subtracted for values of $\theta_{13}$ at the CHOOZ limit, understanding of systematic uncertainties becomes yet crucial 

as $\theta_{13}$ gets smaller. \\



 Estimated performances can be further improved with a better energy reconstruction for all charged 

current events.







\section{Photodetection}



The remarkable successes of SuperK, Kamland, and SNO experiments have triggered

future extrapolated projects aiming the improvement on the accuracy of the

actual neutrinos family parameters, the exploration of the other ones as well as

the search for proton lifetime; sensitive volumes should reach the megaton

scale,

which is an extrapolation by a factor 10-20 of the SK size. In the same

inflatory direction, the detection of very high energy cosmic neutrinos in ice

or water Cerenkov-based detectors will also lead to large numbers of

photomultipliers. It exists then a strong motivation for R\&D trying to decrease

the price of photo-sensitive $cm^2$, which is a major component of projects

budgets. Note that for the calculation of these  "surface unit prices", HV,

front-end electronics and cables have of course to be included.



  In another hand, the use of Cerenkov light requires conflicting qualities

  concerning the single photoelectron sensitivity, the fast time response

  needed for a good vertex determination, the best photodetection efficiency for

  setting lower energy thresholds and a robust water pressure resistant

  envelop able to work at 10 atmospheres pressure without fatal implosion. The

  process of fabrication should also take account of the time needed to built

  large quantities ( scale: 100000 u). Clearly common R\&D with industry are

  needed.

  

  Price lowering can follow one or several recepices:

\begin{itemize}

\item

Remove the glass blowing (\cite{ferenc})



This leads to a very elegant development using sealed glass planes (\cite{ferenc})

\item  

Simplify the electron multiplicative element (\cite{ferenc},\cite{sk},\cite{photonis})



 The basic idea is to accelerate photoelectrons from photocathode with a large

 potential (10-20 KV); for shaped field, it exists a small surface of

 convergence where can be placed either scintillator+small pm (\cite{photonis}),

 or an APD ( \cite{sk}). The total gain is then the product of the acceleration gain (

 $\sim$ 4500) followed by the detecting device gain ( $\sim$ 30 or more for an APD).

 Such system disposes of a  fast time response even for large size photocathods 

 and of an impressive single

 p.e performance. The main drawbacks are  the problems brought with the isolation of

 the very high voltage and a

 frontend fast amplification needed for the APD case.

\item 

Optimize the unit size (\cite{photonis})



For classical big pmts, there is a not obvious relation between size, price/$cm^2$

,time performance, total efficiency and investments for production tools.

Photonis (\cite{photonis}) evaluated this and found as the best candidate a 12

inches tube, compared to bigger ones. 

\item

Increase the photocathode efficiencies (\cite{ferenc},\cite{photonis})



The use of $\sim$ 20 KV hv permits an excellent collection efficiency. Improvement

of photocathode QE efficiency can be found in the use of reflective photo-cathod

(30-44 $\%$ instead of $\sim 20 \%$)

\end{itemize}  

 

\begin{thebibliography}{99}



\bibitem{genie} C. Andreopoulos and H. Gallagher, "Tools for Neutrino Interaction Model Validation", Nucl.Phys.Proc.Suppl.139:247-252,2005 

\bibitem{xsectiondata} Mike Whalley, "A New Neutrino Cross Section Data Resource", Nucl.Phys.Proc.Suppl.139:241-246,2005 



\bibitem{costas} Neutrino Interactions and MC Event Generators

Presented by C. Andreopoulos (Rutherford Lab)



\bibitem{dunmore} Analysis and background aspects in large water Cherenkov detectors

Presented by J. Dunmore (Irvine)

 \bibitem{yanagisawa} Background understanding and suppression in Very Long Baseline Neutrino Oscillation experiments with water Cherenkov detectors

 Presented by C. Yanagisawa (Stony Brook)



\bibitem{ferenc}

Development of new large-aera photosensors in the USA



Presented by D. Ferenc (Davis) 



\bibitem{sk}

 R\&D of a large format hybrid photo-detector (HPD) for a next

generation water Cherenkov detector. 



Presented by H. Aihara ( Tokyo)



\bibitem{pouthas}

Large photodetector developments in Europe



Presented by J. Pouthas (Orsay)



\bibitem{photonis}

Revisiting the optimum PMT size for water Cherenkov megaton detectors



Presented by C. Marmonier (Photonis)



\bibitem{hama}

Large formats PMTs from Hamamatsu Photonics



Presented by M.A. Birkel (Hamamatsu)



\bibitem{burle}

Burle Indistries: Recent photomultiplier and device developments



Presented by R. Caracciolo (Burle)



\bibitem{etube}

Electron Tubes: Detector considerations for neutrino physic



Presented by T. Wright (Electron Tubes)

\end{thebibliography}