Review on Sterile Neutrinos Carlo Giunti INFN, Torino, Italy SSP 2018 7th Symposium on Symmetries in Subatomic Physics 11-15 June 2018, Aachen, Germany C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 1/25
Beyond Three-Neutrino Mixing: Sterile Neutrinos L osc = 4 π E σ had [ nb ] m 2 ν . . ∆ m 2 . . . . 3 ν ALEPH 30 DELPHI ν 5 ν s 2 4 ν L3 OPAL 20 average measurements, error bars increased ν 4 ν s 1 by factor 10 � 1 eV 2 ∆ m 2 10 SBL ν 3 ≃ 2.5 × 10 − 3 eV 2 ∆ m 2 0 86 88 90 92 94 ATM E cm [ GeV ] ν 2 ≃ 7.4 × 10 − 5 eV 2 ∆ m 2 SOL N LEP ν active = 2 . 9840 ± 0 . 0082 ν 1 ν e ν µ ν τ Terminology: a eV-scale sterile neutrino means: a eV-scale massive neutrino which is mainly sterile C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 2/25
Sterile Neutrinos from Physics Beyond the SM ◮ Neutrinos are special in the Standard Model: the only neutral fermions ◮ Active left-handed neutrinos can mix with non-SM singlet fermions often called right-handed neutrinos ◮ Light left-handed anti- ν R are light sterile neutrinos ν c R → ν sL (left-handed) ◮ Sterile means no standard model interactions [Pontecorvo, Sov. Phys. JETP 26 (1968) 984] ◮ Active neutrinos ( ν e , ν µ , ν τ ) can oscillate into light sterile neutrinos ( ν s ) ◮ Observables: ◮ Disappearance of active neutrinos (neutral current deficit) ← CE ν NS ◮ Indirect evidence through combined fit of data (current indication) ◮ Short-baseline anomalies + 3 ν -mixing: ∆ m 2 21 ≪ | ∆ m 2 31 | ≪ | ∆ m 2 41 | ≤ . . . ν 1 ν 2 ν 3 ν 4 . . . ν e ν µ ν τ ν s 1 . . . C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 3/25
Effective 3+1 SBL Oscillation Probabilities Appearance ( α � = β ) Disappearance � ∆ m 2 � ∆ m 2 � � 41 L 41 L ≃ sin 2 2 ϑ αβ sin 2 ≃ 1 − sin 2 2 ϑ αα sin 2 P SBL P SBL ( − ) ( − ) ( − ) ( − ) 4 E 4 E ν α → ν β ν α → ν α sin 2 2 ϑ αα = 4 | U α 4 | 2 � sin 2 2 ϑ αβ = 4 | U α 4 | 2 | U β 4 | 2 1 − | U α 4 | 2 � U e 1 U e 2 U e 3 U e 4 ◮ CP violation is not observable in SBL U µ 1 U µ 2 U µ 3 U µ 4 experiments! U = U τ 1 U τ 2 U τ 3 U τ 4 ◮ Observable in LBL accelerator exp. U s 1 U s 2 U s 3 U s 4 sensitive to ∆ m 2 ATM [de Gouvea et al, PRD 91 (2015) SBL 053005, PRD 92 (2015) 073012, arXiv:1605.09376; Palazzo et al, PRD ◮ 6 mixing angles 91 (2015) 073017, PLB 757 (2016) 142; Kayser et al, JHEP 1511 (2015) ◮ 3 Dirac CP phases 039, JHEP 1611 (2016) 122] and solar exp. sensitive to ∆ m 2 ◮ 3 Majorana CP phases SOL [Long, Li, CG, PRD 87, 113004 (2013) 113004] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 4/25
3+1: Appearance vs Disappearance ◮ Amplitude of ν e disappearance: sin 2 2 ϑ ee = 4 | U e 4 | 2 � 1 − | U e 4 | 2 � ≃ 4 | U e 4 | 2 ◮ Amplitude of ν µ disappearance: sin 2 2 ϑ µµ = 4 | U µ 4 | 2 � 1 − | U µ 4 | 2 � ≃ 4 | U µ 4 | 2 ◮ Amplitude of ν µ → ν e transitions: sin 2 2 ϑ e µ = 4 | U e 4 | 2 | U µ 4 | 2 ≃ 1 4 sin 2 2 ϑ ee sin 2 2 ϑ µµ quadratically suppressed for small | U e 4 | 2 and | U µ 4 | 2 ⇓ Appearance-Disappearance Tension [Okada, Yasuda, IJMPA 12 (1997) 3669; Bilenky, CG, Grimus, EPJC 1 (1998) 247] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 5/25
Gallium Anomaly Gallium Radioactive Source Experiments: GALLEX and SAGE e − + 51 Cr → 51 V + ν e e − + 37 Ar → 37 Cl + ν e ν e Sources: E ≃ 0 . 75 MeV E ≃ 0 . 81 MeV ν e + 71 Ga → 71 Ge + e − Test of Solar ν e Detection: 1.1 GALLEX SAGE Cr1 Cr 1.0 R = N exp N cal GALLEX SAGE Cr2 Ar 0.9 0.8 R = 0.84 ± 0.05 0.7 ≈ 2 . 9 σ deficit � L � GALLEX = 1 . 9 m � L � SAGE = 0 . 6 m [SAGE, PRC 73 (2006) 045805; PRC 80 (2009) 015807; Laveder et al, Nucl.Phys.Proc.Suppl. 168 (2007) 344, SBL � 1 eV 2 ≫ ∆ m 2 MPLA 22 (2007) 2499, PRD 78 (2008) 073009, ∆ m 2 PRC 83 (2011) 065504] ATM ◮ 3 He + 71 Ga → 71 Ge + 3 H cross section measurement [Frekers et al., PLB 706 (2011) 134] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 6/25
Reactor Electron Antineutrino Anomaly [Mention et al, PRD 83 (2011) 073006] New reactor ¯ ν e fluxes: Huber-Mueller (H-M) [Mueller et al, PRC 83 (2011) 054615; Huber, PRC 84 (2011) 024617] 1.20 Bugey−3 Daya Bay Krasnoyarsk RENO Bugey−4+Rovno91 Double Chooz Nucifer Rovno88 Chooz Gosgen+ILL Palo Verde SRP 1.10 R = N exp N cal 1.00 0.90 0.80 R = 0.934 ± 0.024 0.70 10 2 10 3 10 L [m] ≈ 2 . 8 σ deficit C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 7/25
1.20 Bugey−4 Rovno88 Gosgen Krasnoyarsk Nucifer Rovno91 Bugey−3 ILL SRP 1.10 1.00 P ν e →ν e R 0.90 DC DB DB DC E ≈ 4MeV − sin 2 2 ϑ ee = 0.1 R 0.80 2 = 0.1 eV 2 ∆ m 41 2 = 0.5 eV 2 ∆ m 41 2 = 1.0 eV 2 ∆ m 41 0.70 10 2 10 3 1 10 L [m] SBL � 0 . 5 eV 2 ≫ ∆ m 2 ∆ m 2 ATM ◮ SBL oscillations are averaged at the Daya Bay, RENO, and Double Chooz near detectors = ⇒ no spectral distortion C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 8/25
Reactor Antineutrino 5 MeV Bump (Data - MC) / MC 0.2 ◮ Cannot be explained by neutrino 0.1 oscillations (SBL oscillations are 0 averaged in RENO, DC, DB). 0.1 − 1 2 3 4 5 6 7 8 ◮ It is likely due to a theoretical Prompt Energy (MeV) miscalculation of the spectrum. [RENO, arXiv:1511.05849] ◮ Heretic solution: detector energy Data / Predicted Data 1.4 0.25 MeV No oscillation nonlinearity. [Mention et al, PLB 773 (2017) 307] Reactor flux uncertainty Total systematic uncertainty 1.2 2 Best fit: sin 2 θ = 0.090 13 ◮ ∼ 3% effect on total flux, but if it is 1.0 an excess it increases the anomaly! 0.8 ◮ No post-bump complete calculation 0.6 of the neutrino fluxes. 1 2 3 4 5 6 7 8 Visible Energy (MeV) ◮ Nominal Huber-Mueller flux [Double Chooz, arXiv:1406.7763] calculation uncertainty: ∼ 2 . 5%. ◮ Guessed true flux uncertainty: ∼ 5%. [Hayes and Vogel, ARNPS 66 (2016) 219] ◮ Bottom line: the status of the reactor anomaly is controversial! [Daya Bay, arXiv:1508.04233] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 9/25
NEOS [PRL 118 (2017) 121802 (arXiv:1610.05134)] 10 (a) Prompt Energy [MeV] ε 60 7 ◮ Hanbit Nuclear Power Complex in − 10 1 6 50 Events /day/100 keV 5 Yeong-gwang, Korea. − 2 10 4 40 3 − 3 10 2 ◮ Thermal power of 2.8 GW. 1 2 3 4 5 6 7 8 12 30 Neutrino Energy [MeV] Data signal (ON-OFF) 20 Data background (OFF) ◮ Detector: a ton of Gd-loaded ν MC 3 (H-M-V) 10 ν MC 3 (Daya Bay) liquid scintillator in a gallery approximately 24 m from the 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 10 1.1 reactor core. NEOS/Daya Bay (c) Data/Prediction Systematic total ◮ The measured antineutrino event 1.0 rate is 1976 per day with a signal 2 (1.73 eV , 0.050) 2 to background ratio of about 22. (2.32 eV , 0.142) 0.9 ⋅ ⋅ 1 2 3 4 5 6 7 10 Prompt Energy [MeV] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 10/25
DANSS [Solvay Workshop, 1 December 2017; La Thuile 2018, 3 March 2018; Neutrino 2018, 8 June 2018] ◮ Installed on a movable platform 0.76 under a 3 GW reactor. Ratio Down/Up ◮ Large neutrino flux. 0.72 ◮ Reactor shielding of cosmic rays. 0.68 ◮ Variable source-detector distance DANSS with the same detector! No−Oscillations 0.64 Oscillations Best Fit Down = 12 . 7 m 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Up = 10 . 7 m Positron Energy [MeV] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 11/25
Model-Independent ¯ ν e SBL Oscillations [Gariazzo, CG, Laveder, Li, PLB 782 (2018) 13, arXiv:1801.06467] 10 2 σ DANSS NEOS ∼ 3 . 7 σ ∆ m 2 41 = 1 . 29 ± 0 . 03 2 [eV 2 ] sin 2 2 ϑ ee = 0 . 049 ± 0 . 011 1 ∆ m 41 sin 2 ϑ 14 = | U e 4 | 2 NEOS+DANSS sin 2 ϑ 14 = 0 . 012 ± 0 . 003 1 σ 2 σ 3 σ sin 2 ϑ 13 = 0 . 022 ± 0 . 001 10 − 1 10 − 3 10 − 2 10 − 1 1 sin 2 2 ϑ ee C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 12/25
Comparison with the Reactor and Gallium Anomalies 10 2−3 σ (solid−dashed) Reactor Anomaly Gallium Anomaly ◮ 3 σ agreement. 2 [eV 2 ] ◮ 2 σ tension. 1 ◮ Small overestimate of the ∆ m 41 reactor fluxes. ◮ Small overestimate of the NEOS+DANSS GALLEX and SAGE 1 σ 2 σ efficiencies. 3 σ 10 − 1 10 − 3 10 − 2 10 − 1 1 sin 2 2 ϑ ee C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 13/25
Global Model-Independent ν e and ¯ ν e Disappearance 10 MI ν e Dis ◮ NEOS and DANSS. 1 σ 2 σ ◮ Reactor rates with free 235 U 3 σ and 239 Pu fluxes: r 235 and r 239 . 2 [eV 2 ] ◮ Gallium data with free 1 GALLEX and SAGE ∆ m 41 efficiencies: η G and η S . STEREO (1yr, 2 σ ) PROSPECT ◮ New reactor experiments: (3+3yr, 3 σ ) SoLiD STEREO, Neutrino-4, (1+3yr, 3 σ ) SoLiD, PROSPECT KATRIN (90% CL) 10 − 1 ◮ Kinematic ν 4 mass 10 − 3 10 − 2 10 − 1 1 measurement: KATRIN sin 2 2 ϑ ee [See also Dentler, Hernandez-Cabezudo, Kopp, Machado, Maltoni, Martinez-Soler, Schwetz, arXiv:1803.10661] C. Giunti − Review on Sterile Neutrinos − SSP 2018 − 11 June 2018 − 14/25
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