Takaaki Kajita ICRR, Univ. of Tokyo

Nufact05, Frascati, June 2005 A Megaton water detector for particle and astroparticle physics Takaaki Kajita ICRR, Univ. of Tokyo Based on reports at NNN05 Next generation of Nucleon decay and Neutrino detectors http://nnn05.in2p3.fr/

Outline • Introduction • Neutrino oscillation physics with super-beams (This topic should be discussed extensively in this workshop.  Skip.) • Neutrino oscillation physics with atmospheric neutrinos • Neutrino physics with atmospheric neutrinos + super-beams • Solar and supernova neutrinos • A Mton water detector with Gd • Proton decay • R&D • Summary Apology: references incomplete…

Introduction: Future oscillation physics Present: Study of dominant oscillation channels Future: Study of sub-dominant oscillations Known: Unknown: q12, Dm122 q23, |Dm232| q13 Sign of Dm2 nenmnt n3 or n2 n1 If q23 ≠p/4, is it >p/4 or <p/4 ? CP ? Solar, KamLAND Atmospheric Long baseline Future Mton class detector

Refs: many talks at NNN05 Many, many talks in this meeting Future long baseline neutrino experiments - example - J-PARC ＢＮＬ Fermilab NuMI b-beam Megawatt class super (or b)-beam + Megaton class water detector Hierarchy, CP, …. MEMPHYS Hyper-K UNO

Neutrino oscillation physics with atmospheric neutrinos • Sign of Dm232 • Is q23 >p/4 or <p/4 ? Atmospheric neutrino beam : Long neutrino flight length in matter Very wide L/E  solar L/E range is relevant TK NNN05

Sign of Dm2 ? cosQ If Dm232 is positive, resonance for neutrinos If Dm232 is negative, resonance for anti-neutrinos Dm2=0.002eV2 s2q23 = 0.5 s2q13 = 0.05 (0.45 Mtonyr) En(GeV) Multi-ring e-like Single-ring e-like Relatively high anti-ne fraction More events if Dm2<0 Relatively high ne fraction More events if Dm2>0 Positive Dm2 Negative Dm2 null oscillation cosQ cosQ

c2 difference (inverted-normal) True= normal mass hierarchy assumed. Dm2: fixed, q23: free, q13: freeExposure: 1.8Mtonyr 3s 3s 3s (A similar but slightly worse sensitivity for inverted mass hierarchy.)

Solar term effect to atmospheric n However, due to the cancellation between nmne and nenm, the change in the ne flux is small. s2q23=0.4 =0.6 =0.5 Oscillation probability is different between s2q23=0.4 and 0.6  discrimination between q23 >p/4 and <p/4 might be possible. Peres & Smirnov NPB 680 (2004) 479 Because of the LMA solution, atmospheric neutrinos should also oscillate by (q12, Dm122). s22q12=0.825 Dm212=8.3×10-5 Dm223=2.5×10-3 sin2q13=0

Discrimination between q23 >p/4 and <p/4 with the (12) and (13) terms s2q23=0.40 ~ 0.60 s2q13=0.00~0.04 dcp=45o 1.8Mtonyr 90%CL 90%CL sin22q23=0.96 sin22q23=0.99 Fit result Test point sin2q13 sin2q23 sin2q23 Discrimination between q23>p/4 and <p/4 is possible for all q13. Discrimination between q23>p/4 and <p/4 is marginally possible only for q13 >0.04.

T.Schwetz NNN05, also in this meeting, Huber, Maltoni, Schwetz hep-ph/0501037 Neutrino physics with atmospheric neutrinos + super beams Atmospheric neutrinos: Sensitive to mass hierarchy and octant of q23 Parameter degeneracies in T2K-II Combine LBL and atm data to resolve the degeneracies

Resolving the degeneracies  4MW beam, 2yr neutrino run, 6yr anti-neutrino run, 1Mton detector at 295km  9Mtonyr atmospheric neutrino data

Identifying the mass hierarchy s2q23=0.5 assumed Atm only 1s 2s 3s LBL only LBL+atm

Solar+KamLAND 99.73% 95% Vacuum osc. dominant KamLAND P(ne ne) Solar global matter osc. (MeV) Solar neutrino physics with Mton detectors M.Nakahata NNN05 Do we want further evidence for matter effect ? Day-night asymmetry

sin2q=0.28, Dm2 =8.3×10-5 eV2 Expected signal 8B spectrum distortion Day-night stat. significance Correlated sys. error of SK 1/2 of SK Data/SSM 5 Mton·years Ee (MeV) Enough statistics to see distortion. Energy scale calibration should be better than ~0.3%. 3s signal can be obtained with 0.5% day-night systematic error. In both cases, systematic errors or background are assumed to be better than SK.

Supernova events in a Mega-ton detector A.Dighe NNN05 ◆Initial spectra rather poorly known. ◆Only anti-ne observed • Difficult find a “clean” observable, which is (almost) independent of the assumptions on the initial spectra. Number of anti-ne+p interactions = 200,000 - 300,000 for a galactic Supernova (@10kpc)

Reverse shock Forward shock Dm132 resonance Dm122 resonance Supernova shock and neutrino oscillations Assume: nature = inverted hierarchy Anti-ne sin2q13 If sudden change in the average energy is observed  Inverted mass hierarchy and sin2q13>10-5. A.Dighe NNN05 R.Tomas et al., astro-ph/0407132

S.Ando NNN05 Mton is large: The detectors can see extragalactic SNe Nearby SN rate Detection probability SK HK

S.Ando, M.Nakahata NNN05 SRN prediction Supernova relic neutrinos (SRN)  get information on galaxy evolution and cosmic star formation rate SK result SNR limit 90%CL Invisible m e ne+anti-ne 90%CL limit: 1.2 /cm2/sec (En>19MeV) (which is just above the most recent prediction 1.1/cm2/sec) With a Mton detector, it must be possible to see SRN signal

Mton detector with Gd loaded water GADZOOKS! M.Vagins NNN05 M.Vagins, J.Beacom hep-ph/0309300 Simulation: M.Nakahta NNN05 (0.2% GdCl3) No neutron tagging B.G. reduction by neutron tagging Invisible m e Statistically 4.6s excess (Evis > 15 MeV)

Search for proton decay How long is the predicted proton lifetime ? C.K.Jung NNN05 J.Ellis NNN05 Lifetime in benchmark scenarios SK limit (ep0) SK limit (nK+)

Ptot < 250 MeV/c, BG 2.2ev/Mtyr, eff=44% • Ptot < 100 MeV/c, BG 0.15ev/Mtyr, eff=17.4% Search for pge+p0 pep0 Monte Carlo M.Shiozawa NNN05 e+ p0gg Main target is free proton decays for the tight cut. Atm n 20Mtonyr bound proton decay free proton decay

Lifetime sensitivity for pge+p0 pge+p0 sensitivity 5Mtonyrs 5Mtonyrs  ~1035 years@90%CL ~4x1034 years@3sCL Normal cut, 90%CL 3s CL Tight cut, 90%CL 3s CL

12nsec 2.2msec νK+ sensitivity (based on SK criteria) 5Mtonyrs τ/B > 2 × 1034yr (5Mtonyr, 90%CL) Most updated number = 2,3×1033 yrs Question: How much photo cathode coverage is necessary?

Remarks on R&D Photo-detector and excavation are the most important items for the construction of the Mton detector. My impression at NNN05 ◆Excavation of an underground cavity for a Mton class detector seems to be possible, but more site specific R&D are necessary. ◆Photo-detector R&D are going on, but it is still unclear if a much cheaper (and better) photo-detector can be ready by the time of the start of the construction.  More R&D are necessary. ◆Also, a more serious discussion on the physics of Mton detector and the detector design might be necessary. One example: what will be the optimal photo-cathode coverage, 10, 20 or 40% ? (Jung, Nakagawa, Levy, Duffaut, NNN05) (Aihara, Ferenc, Pouthas, Hamamatsu, Photonics, Electron tubes, NNN05) It is very good that it was decided to have the NNN workshop every year.

Summary • A Mton water detector will be an excellent neutrino detector for super-beam experiments. (This was not discussed in this talk.) • A Mton water detector will have a lot of physics opportunities. • A Mton detector can not be cheap. Therefore it is very nice that it can carry out many important physics. • Serious detector R&D are necessary.

End

c2 difference (normal – inverted) True= inverted mass hierarchy assumed. Dm2: fixed, q23: free, q13: free Exposure: 1.8Mtonyr 3s 3s 3s

Effect of the solar term to sub-GeV e-like zenith angle Dm212 = 8.3 x 10-5 eV2 Dm223 = 2.5 x 10-3 eV2 sin2 2q12 = 0.82 sub-GeV e-like (Pe :100 ~ 1330 MeV) (Pe :100 ~ 400 MeV) (Pe :400 ~ 1330 MeV) sin2 q23 = 0.4 sin2 q23 = 0.5 sin2 q23 = 0.6 N_e (3 flavor) / N_e (2 flavor full-mixing) cosqzenith (Much smaller and opposite effect for m-like events.) m/e ratio @low energy is useful to discriminate q23>p/4 and <p/4.

Takaaki Kajita ICRR, Univ. of Tokyo