The Standard Model at 50 Years, Case Western Reserve University, June 1, 2018 Neutrino Oscillations

Takaaki Kajita Institute for Cosmic Ray Research, The Univ. of Tokyo Outline • Early days - Solar neutrinos - Atmospheric neutrinos • Discovery of neutrino oscillations - Atmospheric neutrino oscillations - Solar neutrino oscillations • The third oscillation channel • Agenda for the future neutrino measurements • Summary This is an experimental talk (no theory, I apologize). Many thanks to Art McDonald for various information related to SNO 2 Early days

3 Solar neutrino problem

http://www.sns.ias.edu/~jnb/

J. N. Bahcall https://www.bnl.gov/bnlweb/raydavis/ R. Davis Jr.

http://www.astronomynotes.com/starsun/s4.htm 600ton C2Cl4

Pioneering Homestake experiment observed solar neutrinos for the first time. However, the observed event rate was only about 1/3 of the prediction (since 1960’s).

4 Results from solar neutrino experiments (before ~2000) Following the initial observation, several experiments observed solar neutrinos. Bahcall et al.

Theory

Kam pp ν 8B ν 7Be, pep ν Solar neutrino experiments in the 80’s and 90’s confirmed the deficit of solar neutrinos.

5 Atmospheric neutrinos

Cosmic ray Incoming cosmic rays Air nucleus

Pion Muon Electron 2 muon- neutrinos 1 electron- neutrino

Up/down ratio well understood (and 1). νµ / νe ratio well understood

© David Fierstein, originally published in Scientific American, August 1999 6 Atmospheric νµ deficit (1980’s to 90’s)  Proton decay experiments in the 1980’s observed many atmospheric neutrino events.  Because atmospheric neutrinos are the most serious background to the proton decay searches, it was necessary to understand atmospheric neutrino interactions.  During these studies, a significant deficit of atmospheric νµ events was observed.

Kamiokande (1988, 92, 94)

IMB (1991, 92)

7 Discovery of neutrino oscillations: - Atmospheric neutrino oscillations

8 Super-Kamiokande detector 50,000 ton water Cherenkov detector (22,500 ton fiducial volume)

~20 times larger mass

~160 collaborators ４２ｍ

３９ｍ １０００ｍ underground

9 Constructing the Super-Kamiokande detector (1995)

Side wall construction (Summer 1995) Bottom PMT installation (Nov, 1995) 10 Filling water in Super-Kamiokande Jan. 1996

11 Event type and neutrino energy

Partially contained

1 ring µ-like

1 ring e-like

Upward going µ All these events are used in the analysis. 12 What will happen if the νµ deficit is due to neutrino oscillations ) Cosmic ray µ Down-going Not long enough ν to oscillate remain remain µ Probability Probability ν Long enough ( Up-going to oscillate

1 10 100 1000 104 L(km) for 1GeV neutrinos

A deficit of upward going νµ’s should be observed! Cosmic ray A large detector needed.  Super-Kamiokande

13 Evidence for neutrino oscillations (Super-Kamiokande @Neutrino ’98)

Super-K, Neutrino 98, Super-K., PRL 81 (1998) 1562

Super-Kamiokande concluded that the observed zenith angle dependent deficit (and the other supporting data) gave evidence for neutrino oscillations.

14 Studies of νµντ oscillations Atmospheric neutrinos Soudan-2 ANTARES MACRO Super-K

IceCube Accelerator based long baseline experiments T2K K2K

OPERA NOvA

MINOS

15 T2K T2K, PRD 91 (2015) 072010 νµ disappearance studies (accelerator experiments) K2K K2K, PRD 74 (2006) 072003 MINOS

MINOS PRL 110 (2013) 251801

NOvA NOvA, Jan. 2018

16 ντ appearance OPERA Super-Kamiokande Super-K,arXiv:1711.09436 5 tau-neutrino candidates observed. Expected BG = 0.25 evens. (5.1σ) τ-appearance OPERA PRL 115 (2015) 121602 signal at 4.6σ

The fifth candidate event

17 Discovery of neutrino oscillations: - Solar neutrino oscillations

18 Initial idea

Herbert Chen, PRL 55, 1534 (1985) “Direct Approach to Resolve the Solar-neutrino Problem”

A direct approach to resolve the solar-neutrino problem would be to observe neutrinos by use of both neutral-current and charged-current reactions. Then, the total neutrino flux and the electron-neutrino flux would be separately determined to provide independent tests of the neutrino-oscillation hypothesis and the standard solar model. A large heavy-water Cherenkov detector, sensitive to neutrinos from 8B decay via the neutral-current reaction ν+d→ν+p+n and the charged-current reaction - νe+d→e +p+p, is suggested for this purpose.

19 SNO detector

2km

1000 ton of heavy water

20 Constructing the SNO detector One million pieces transported down in the 3 m x 3 m x 4 m mine cage and re-assembled under ultra-clean conditions.

Filled with pure and heavy water in April 1999.

21 3 neutron detection methods (for ν d  ν pn measurement)

3 Phase I (D2O) Phase II (salt) Phase III ( He) Nov. 99 - May 01 July 01 - Sep. 03 Nov. 04-Dec. 06 n captures on 2 tonnes of NaCl 400 m of proportional 2H(n, γ)3H n captures on counters Eff. ~14.4% 35Cl(n, γ)36Cl 3He(n, p)3H Eff. ~40% Effc. ~ 30% capture 35Cl+n 2H+n 8.6 MeV 6.25 MeV

3H 36Cl

22 Evidence for solar neutrino oscillations

SNO PRL 89 (2002) 011301 (A plot based on the SNO PRC 72, 055502 (2005) salt-phase data)

Three (or four) different measurements intersect at a point.  Evidence for (νµ+ντ) flux SSM 68%CL

SNO NC 68%CL SNO CC SNO ES SK ES 68%CL 68%CL 68%CL

23 SNO results from 3 phases

all νx Theory

νeonly

24 Really neutrino oscillations!

KamLAND, PRD 83, 052002 (2011) KamLAND

Reactors Really neutrino oscillations ! 25 Consistent with MSW ! Borexino, PRL 101, 091302 (2008), PRD 82 (2010) 033006, PRL 108, 051302 (2012), Borexino Nature 512, 383 (2014), PRD 89, 112007 (2014), G. Bellini, JINR, Sep. 2016 Measurement of sub-MeV solar neutrinos MSW prediction

Borexino (Also KamLAND measurement of 7Be.)

The data are consistent with the MSW prediction! 26 The third oscillation channel

27 Experiments for the third neutrino oscillations Accelerator based long baseline neutrino oscillation experiments NOν A MINOS T2K

Reactor based (short baseline) neutrino oscillation experiments Daya Bay RENO Double Chooz

28 Discovery of the third neutrino oscillations (2011-2012)

Accelerator based νe appearance experiments MINOS PRL 107 (2011) 181802 T2K PRL 107 (2011) 041801 Note: these data are those in 2011-2012. The updated data are much better (including those from NOvA).

Reactor based anti-νe disappearance experiments Daya Bay PRL 108 (2012) 171803 RENO PRL 108 (2012) 191802 Double Chooz PRL 108 (2012) 131801

Tthe basic structure for 3 flavor neutrino oscillations has been understood!

29 Oscillation parameters P.F.de Salas et al.,arXiv:1708.01186 (v2) (April 2018)

Review of Particle Physics (2017) K. Nakamura and S.T. Petcov, “14. Neutrino mass, mixing and oscillations” See also, A. Marrone et al., AIP Conf.Proc. 1894 (2017) 020015 and many other references 30 Agenda for the future neutrino measurements

31 Agenda for the future neutrino measurements Neutrino mass hierarchy? CP violation?

P(να →ν β ) ≠ P(να →ν β ) ? Baryon asymmetry of the Universe?

Are neutrinos Majorana particles?  Neutrinoless (taken from R. B. Patterson, Ann Rev Nucl Part Sci 65 (2015) 177-192.) double beta decay Absolute neutrino mass?

Beyond the 3 flavor framework? http://wwwkm.phys.sci.osaka- (Sterile neutrinos?) u.ac.jp/en/research/r01.html 32 Summary

• Proton decay experiments in the 1980’s observed many contained atmospheric neutrino events, and discovered the atmospheric νµ deficit. Subsequently, in 1998, Super-Kamiokande discovered neutrino oscillations. • Solar neutrino experiments began in the 1960’s. Various solar neutrino experiments before 2000 observed the deficit of solar neutrinos. Then the SNO experiment discovered solar neutrino oscillations by the measurements of CC and NC reactions of solar neutrinos. • Since then, various experiments have studied neutrino oscillations. The basic structure for 3 flavor neutrino oscillations has been understood. • The discovery of non-zero neutrino masses opened a window to study physics beyond the Standard Model.

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