Performance of a Water Cherenkov Detector for e Appearance Shoei NAKAYAMA (ICRR, University of Tokyo) November 18-19, 2005 International Workshop on a Far Detector in Korea for the J-PARC Neutrino KIAS, Seoul, Korea

Motivation  SK : OA 2.5 o  A far detector in Korea has the option to choose a site for a smaller off-axis angle.  higher energy neutrino beam  Check the signal/BG separation performance for higher energy neutrinos   0 BG is dominant for e search.  efficient  0 BG rejection   

Current T2K Selection Criteria for e Appearance  Standard SK cuts : fiducial volume, fully-contained, Evis > 100 MeV single-ring electron-like PID no decay-e 0.35 < E rec < 0.85 GeV E rec = (m N E e -m e 2 /2)/(m N -E e +P e cos-e )  Further  0 cuts : cose < 0.9 L < 80, M 0 < 100 MeV/c 2 of  0 fitter (POLfit)

POL(Pattern of Light)fit –  0 fitter – 500 MeV/c  0 true P  2 = 55.5MeV/c rec.M  0 =140.4MeV/c 2 11 22 standard fitter POLfit Target: FCFV 1R-elike events L ≡ Likelihood(2 assump.) – Likelihood(electron assump.) Try to reconstruct two  rings Input: vertex, visible energy, and the 1 st  direction by the standard fitter Compare observed & expected (direct+scatter) charge Vary the 2 nd  direction and the energy fraction until the best match found  M 0 etc.

Events vs. Selections Event selections: 1.FCFV, E vis > ring 3. e-like 4. no decay-e < E rec <  0 cuts: - cos < L < 80, M 0 < 100 events/22.5kt/5yrs event selections  BG e signal e BG m 2 = 2.5x10 -3 eV 2 sin 2 2 13 = 0.1 S. Mine NP04

Events vs. Selections (cont ’ d)  CC BG  NC BG beam e BG e (CC) Signal FCFV E vis > ring 1313 (46%)277 (26%)114 (46%)243 (84%) e-like 51 (1.8%)219 (20%)111 (45%)240 (83%) no decay-e 15 (0.5%)195 (18%)92 (37%)222 (77%) 0.35 < E rec < (0.1%)58 (5%)27 (11%)173 (60%) cos < 0.9 L < 80 M < ±0.8 (0.3%) 16±0.4 (6%) 122±3 (42%) (events / 22.5kt / 5yrs) m 2 = 2.5x10 -3 eV 2 sin 2 2 13 = 0.1 S. Mine NP04 ~20% ~70%

This study uses …  the SK-I atmospheric MC sample to study the higher neutrino energy range than that of the OA2.5 beam MC contains both e and  events livetime: 100yrs  the maximum-likelihood analysis to select e CC signals efficiently pre-cut : FCFV, Evis>100MeV single-ring e-like no-decay-e

Dist. for making a e CC-enriched likelihood (1) E vis : 100 ~ 250 MeV 250 ~ 400 MeV  CC + NC e CCQE 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV ring-counting parameter

Dist. for making a e CC-enriched likelihood (2) 250 ~ 400 MeV  CC + NC e CCQE 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV PID parameter E vis : 100 ~ 250 MeV

Dist. for making a e CC-enriched likelihood (3) 250 ~ 400 MeV  CC + NC e CCQE 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV cos -e E vis : 100 ~ 250 MeV

Dist. for making a e CC-enriched likelihood (4) 250 ~ 400 MeV  CC + NC e CCQE 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV  0 mass by POLfit E vis : 100 ~ 250 MeV

Dist. for making a e CC-enriched likelihood (5) 250 ~ 400 MeV  CC + NC e CCQE 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV L by POLfit E vis : 100 ~ 250 MeV

Likelihood distribution  CC + NC e CCQE Likelihood FCFV, Evis>100MeV single-ring e-like no decay-e  good separation

Likelihood distributions for each energy range 250 ~ 400 MeV 400 ~ 600 MeV 600 ~ 900 MeV 900 ~ 1500 MeV 1500 ~ MeV E vis : 100 ~ 250 MeV Likelihood  CC + NC e CCQE

Events vs. Selections (100yr atm- MC) FCFV,evis : ring : e-like : no decay-e : likelihood : 43 (25.9%) 70 (39.5%) 59 (25.2%) 116 (22.5%) FCFV,evis : ring : e-like : no decay-e : likelihood : (90.8%) (83.3%) 9379 (78.2%) 7328 (58.9%) FCFV,evis : ring : e-like : no decay-e : likelihood : 290 (12.2%) 667 (25.5%) 325 (27.5%) 545 (39.5%) e CC  CC NC rec E : 0~ ~ ~ ~

Likelihood distributions for each sample rec E: 0~ ~ ~1.51.5~ e CC  CC NC

Efficiency by the likelihood cut e CC  CC NC rec E : 0~ ~ ~ ~ Efficiency Likelihood>0.0 Likelihood>0.5

Composition of e CC interactions in the final sample True neutrino energy (GeV) Fraction e CC QE e CC single  e CC multi  WC detector cannot reconstruct the neutrino energy of these events.

Summary  The signal/BG separation in a water Cherenkov detector for the e appearance experiment has been developed based on a likelihood analysis.  The performance of the signal/BG separation was checked in the higher neutrino energy for a smaller off-axis site of a far detector in Korea.  In the higher energy region, the performance of the separation is worse. In addition, the fraction of CC non-QE interaction in the final sample is larger, whose parent neutrino energy cannot be reconstructed correctly.

Supplement

Events vs. Selections (100yr atm- MC) FCFV,evis : ring : e-like : no decay-e : likelihood : 1 (25.0%) 61 (31.8%) 79 (32.5%) 147 (22.5%) FCFV,evis : ring : e-like : no decay-e : likelihood : (92.8%) (85.1%) (78.1%) 8818 (60.5%) FCFV,evis : ring : e-like : no decay-e : likelihood : 2 ( 3.9%) 204 (16.5%) 356 (20.7%) 1265 (27.9%) e CC  CC NC true E : 0~ ~ ~ ~

Likelihood distributions for each sample true E: 0~ ~ ~1.51.5~ e CC  CC NC

Efficiency by the likelihood cut e CC  CC NC true E : 0~ ~ ~ ~ Efficiency Likelihood>0.0 Likelihood>0.5