Background Understanding and Suppression in Very Long Baseline Neutrino Oscillation Experiments with Water Cherenkov Detector Chiaki Yanagisawa Stony Brook Talk at NNN05, Aussois, France April 7-9, 2005

VLBNO Setting the stage   e and  e +N   e + invisible N' + (invisible n   s, n  0)  e   e  '    invisible n   s, n  0) Look for single electron events Major background  e  contamination in beam (typically 0.7%)  How do we find the signal for   v e ~ a half megaton F.V. water Cherenkov detector, for example UNO BNL very long baseline neutrino beam VLB neutrino oscillation experiment See, for example, PRD68 (2003) for physics argument ● Very long baseline neutrino oscillation

Neutrino spectra of on- and off-axis BNL Superbeams on-axis beam 1 o off-axis beam Neutrino energy (GeV)  s/GeV/m 2 /POT PRD68 (2003) 12002; private communication w/ M.Diwan VLBNO

How is analysis done ? Use of SK atmospheric neutrino MC  Flatten SK atm. spectra and reweight with BNL beam spectra  Normalize with QE events: 12,000 events for   events for beam  e for 0.5 Mt F.V. with 5 years of running, 2,540 km baseline  Reweight with oscillation probabilities for   and for e  Standard SK analysis package +   m 2 21 =7.3 x eV 2,  m 2 31 =2.5 x eV 2  sin 2 2  ij (12,23,13)=0.86/1.0/0.04,  CP =0,+45,+135,-45,-135 o Probability tables from Brett Viren of BNL Oscillation parameters used: distance from BNL to Homestake special  0 finder

VLBNO Selection criteria used to improve Likelihood analysis using the following eight variables: Initial cuts:  One and only one electron-like ring with energy and reconstructed neutrino energy more than 100 MeV without any decay electron     mass, energy fraction, costh,   -likelihood , e-likelihood    -likelihood, total charge/electron energy, Cherenkov angle To reduce events with invisible charged pions Traditional SK cuts only With   finder

VLBNO QE events only before likelihood cut All CC events before likelihood cut Erec Reconstructed energy E E Erec All CC events that survive the initial cuts are signals What are sources of the signal (QE and nonQE) ? Only single e-like events left after initial cut

 0 finder    finder    reconstruction efficiency with standard SK software measured opening angle vs.    mass with    finder m  (MeV/c 2 ) true opening angle (deg) efficiency opening angle measured(deg) Single e-like events from single    int. All single    interactions SK atm. neutrino spectra Always looks for an extra ring in a single ring event inefficiency due to overlap inefficiency due to weak 2 nd ring ●    Finder

   reconstruction efficiency    reconstruction efficiency with standard SK +    finder efficiency All the single    int. True opening angle (deg)    finder  0 mass cut:1- and 2-ring events  0 mass cut:2-ring events With atmospheric neutrino spectra with  0 finder without  0 finder with  0 finder w/o  0 finder

Variables GeV GeV GeV GeV GeV GeV GeV GeV background signal  0 mass Useful Variables All the distributions of useful variables are obtained with neutrino oscillation “on” with CPV phase angle  0 mass

Variables GeV GeV GeV GeV GeV GeV GeV GeV background signal Energy fraction of 2 nd ring Fake ring has less energy than real one

Variables Primary electron ring An undetected weak ring initially - One algorithm optimized to find an extra ring near the primary ring (forward region) - Another algorithm optimized to find an extra ring in wider space (wide region) - See the difference ln   -likelihood (forward) - ln   -  likelihood (wide) Difference between ln of two   -likelihood (wide vs. forward)

Variables Difference between ln of two   -likelihood (wide vs. forward) GeV GeV GeV GeV GeV GeV GeV GeV background signal

Variables GeV GeV GeV GeV GeV GeV GeV GeV background signal costh = cos  e

Preliminary Likelihood Cut  ln likelihood distributions Trained with e CC events for signal,  CC/NC & e  NC for bkg GeV GeV GeV GeV GeV3.0- GeV  likelihood signal background Difference in ln likelihood between sig and bkg ln likelihood ratio Preliminary

Bkgs 169 (112 from   +others) ( 57 from e )  ln likelihood cut (100%  signal retained) Background from   CP+45 o Signal 700 evBkgs 2005 (1878 from   +others) ( 127 from e ) Signal e background o CP+45  ln likelihood cut (~50%  signal retained) Effect of cut on  ln likelihood E rec Preliminary Signal/Background Signal 321 ev  e CC for signal ; all  e NC, e beam  for background TRADITIONAL ANALYSIS After initial cuts

 ln likelihood cut (40%  signal retained) Background from   CP+45 o Signal 251 evBkgs 118 ( 74 from   +others) ( 44 from e ) Signal e background o CP-45  ln likelihood cut (~40%  signal retained) Effect of cut on  likelihood E rec Preliminary Signal/Background Signal 142 ev  e CC for signal ; all  e NC, e beam  for backgrounds Bkgs 118 ( 75 from   +others) ( 43 from e )

Effect of cut on likelihood  ln likelihood cut (~40%  signal retained) Background from   CP+135 o Signal e background o CP-135  ln likelihood cut (~40%  signal retained) E rec Preliminary Signal/Background Signal 342 evBkgs 126 ( 81 from   +others) ( 45 from e ) Signal 233 evBkgs 122 ( 78 from   +others) ( 44 from e )  e CC for signal ; all  e NC, e beam  for backgrounds

Preliminary Summary of BNL Variable removed SignalBkgSignal Bkg Effic None e CC  all, e,  NC e CC e CC e CC 50%  pi0lh pi0-lh poa e-lh 50% Beam e % e CC Issues ● S/B and variables e CC 50% efrac pi0mass costh Neutrino oscillation was on to define template distributions For analysis CPV=+45 o Some issues  all, e,  NC ange e CC 50%

Conclusion ● Conclusion Realistic MC simulation studies have been performed for the BNL very long baseline scenario with a water Cherenkov detector. It was found that BNL VLB combined with a UNO type detector seems to DO A GREAT JOB – Very exciting news but needs confirmation. It was demonstrated that there is some room to improve S/B ratio beyond the standard water Cherenkov detector software even with currently available software We may need further improvement of algorithm/software, which is quite possible A larger detector such as UNO has an advantage over a smaller detector such as SK (we learned a lesson from 1kt at K2K) Detailed studies on sensitivity on oscillation parameters needed