The Theoretical Perspective on Future Neutrino Experiments Carlo - PowerPoint PPT Presentation

The Theoretical Perspective on Future Neutrino Experiments Carlo Giunti INFN, Torino, Italy Gordon Research Conference on Particle Physics: New Tools for the Next Generation of Particle Physics and Cosmology 30 June - 5 July 2019, Hong Kong, China C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 1/27

fmourishing of Astroparticle Physics and Cosmology). with high precision all the known interactions of the known elementary particles. fundamental properties of nature. ◮ There is a wind of crisis in traditional Particle Physics (mitigated by the ◮ The discovery of the Higgs boson in 2012 at LHC was the triumph of the Standard Model of Glashow, Weinberg and Salam. ◮ After this peak of success now we live in an era in which the Standard Model is both a blessing and a curse: ◮ Blessing: it is a consistent Quantum Field Theory that allows to compute ◮ Curse: its perfect working is hiding the way of further understanding of the ◮ Neutrinos can be powerful messengers of the physics beyond the SM. C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 2/27

Open problems that require New Physics Planck scale?). leptogenesis). neutrino masses?). neutrino masses?) ◮ From experiment: ◮ Neutrino masses. ◮ Dark Matter (keV sterile neutrino is a candidate). ◮ Dark Energy (connection with the neutrino mass scale?). ◮ Matter-antimatter asymmetry in the Universe (neutrino-induced ◮ From theory: ◮ Too many free numerical parameters (19 + 7 neutrino masses and mixing). ◮ Why neutrino masses are so small? (seesaw Majorana neutrino masses?) ◮ Why neutrino mixing is so difgerent from quark mixing? (due to Majorana ◮ Hierarchy problem (why the electroweak scale is so much smaller than the ◮ The strong CP problem. ◮ Accidental conservation of B − L global symmetry (broken by Majorana C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 3/27

The Power of Neutrinos ◮ Neutrinos are neutral and the weakest-interacting known particles. ◮ Fantastic astrophysical messenger in the arising multimessenger era. ◮ Sensitive to very weak new interactions beyond the Standard Model: ◮ New non-standard interactions. ◮ Electromagnetic interactions (magnetic moments and charges). C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 4/27

the mass scale of about 6-7 orders of magnitude. ◮ Neutrinos are the lightest known elementary particles with a huge gap in 10 12 t 10 11 10 10 b c 10 9 τ s 10 8 ν τ µ d 10 7 u 10 6 ν µ e 10 5 m [eV] 10 4 10 3 10 2 10 ν e ν 1 ν 2 ν 3 1 10 − 1 10 − 2 10 − 3 10 − 4 C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 5/27

10 12 t 10 11 b 10 10 c 10 9 τ s 10 8 µ d 10 7 u 10 6 e 10 5 m [eV] 10 4 10 3 10 2 10 ν 3 ν 1 ν 2 Quasi-Degenerate → 1 QD Inverted Ordering → 10 − 1 IO 10 − 2 10 − 3 NO Normal Ordering → 10 − 4 ◮ The neutrino mass ordering is a model selector. ◮ The small neutrino masses can be Majorana masses beyond the Standard Model that break Lepton number conservation ( L and B − L ). C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 6/27

Origin of Neutrino Masses s L Breaking e R e L standard Higgs mechanism: b R b L s R Leptons : Symmetry d L d R Quarks : 1 st Generation 2 nd Generation 3 rd Generation � u L � u R � c L � c R � t L � t R � ν eL � � ν µ L � � ν τ L � ν µ R ν eR ν τ R µ L τ L τ R µ R ◮ Standard Model extension: ν R ⇒ Dirac mass term L D ∼ m D ν L ν R ◮ This is Standard Model physics, because m D is generated by the yL L � Φ ν R − − − − − − → yv ν L ν R ⇒ m D ∼ yv y � 10 − 11 ◮ Bad: extremely small Yukawa couplings: C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 7/27

Beyond the Standard Model R singlet under SM symmetries! conservation which should anyway be explained by some physics beyond the Standard Model. ◮ The introduction of ν R leads us beyond the Standard Model because they can have the Majorana mass term L M ∼ m M ν R ν c ◮ This is beyond the Standard Model because m M is not generated by the Higgs mechanism of the Standard Model ⇒ new physics is required. ◮ The Majorana mass term can be avoided by imposing lepton number C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 8/27

Seesaw Mechanism seesaw mechanism of light neutrino masses m M D massive neutrinos are Majorana L R without lepton number conservation m M m D m D diagonalization of natural explanation of smallness R m M 2 L m D m D � � 0 � � ν L � � L D+M = − 1 ν c ν R + H.c. ν c m M can be arbitrarily large (not protected by SM symmetries) m M ∼ scale of new physics beyond Standard Model ⇒ m M ≫ m D � 0 � = ⇒ m ν ≃ m 2 m N ≃ m M ν ⇒ ββ 0 ν N � Φ � � � ν ≃ − i ν L − ν c N ≃ ν R + ν c 3-GEN ⇒ efgective low-energy 3- ν mixing C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 9/27

Majorana Neutrinos There are compelling arguments in favor of Majorana Neutrinos: fundamental spinor representation of the Lorentz group. A Dirac fjeld is more complicated: it is made of two Majorana fjelds degenerate in mass. If there is no additional constraint (as L conservation), a neutral and Neutrino Non-Standard Interactions (NSI). ◮ A Majorana fjeld is simpler than a Dirac fjeld: it corresponds to the elementary particle as the neutrino is naturally Majorana. ◮ The seesaw mechanism if ν R is introduced to generate neutrino masses. ◮ A general Efgective Field Theory argument from high-energy new physics: L = L SM + g 5 M O 5 + g 6 M 2 O 6 + . . . ◮ O 5 : Majorana neutrino masses (Lepton number violation and ββ 0 ν decay). � � � � ν L φ 0 O 5 = ( L � Φ) ( � � Φ T L c ) L = Φ = ℓ L − φ + ◮ O 6 : Baryon number violation (proton decay) C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 10/27

Leptogenesis R inaccessible measurable [Casas, Ibarra, NPB 618 (2001) 171] plausible CP-violating Y R ◮ Ofg-equilibrium L and CP violating heavy Majorana neutrino decays at T ∼ M N : Φ L I ∼ L � Φ Y ν R N k Y αk � � � Γ( N k → Φ ℓ α ) − Γ( N k → ¯ Φ¯ ℓ α ) k ,α A L ∼ � � � Γ( N k → Φ ℓ α ) + Γ( N k → ¯ Φ¯ ℓ α ) ℓ α k ,α ◮ The lepton asymmetry A L is converted into a baryon asymmetry A B at T ∼ 100 GeV by electroweak sphalerons that conserve B − L and break B + L . ( RR T = 1 ) v M 1 / 2 m 1 / 2 ◮ Seesaw ⇒ Y ∼ 1 U 3 × 3 ν � �� � � �� � ◮ CP-violating U 3 × 3 ⇒ C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 11/27

matter-antimatter asymmetry in the Universe. would be a strong indication in favor of leptogenesis as the origin of the and CP violation in the lepton sector (through neutrino oscillations) [Mofgat, Pascoli, Petcov, Turner, JHEP 1903, 034] 6 4 2 η B × 10 10 M 1 ≃ 5 × 10 10 GeV 0 M 1 ≪ M 1 ≪ M 3 − 2 − 4 − 6 0 50 100 150 200 250 300 350 δ [ ◦ ] ◮ The discovery of L violation ( ββ 0 ν decay due to Majorana neutrinos) C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 12/27

(left-handed) connected with dark matter). [Pontecorvo, Sov. Phys. JETP 26 (1968) 984] ◮ Seesaw with leptogenesis is a very attractive and compelling theory. ◮ However, in general there is no constraint on the number and mass scale of the ν R ’s. ◮ It is possible and interesting that there is low-energy new physics (maybe ◮ Light fermions beyond the Standard Model are neutral and can mix with neutrinos: they are ν R ’s. ◮ Light left-handed anti- ν R are light sterile neutrinos ν c R → ν sL ◮ Sterile means no standard model interactions C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 13/27

Light Sterile Neutrinos Short-Baseline Anomalies Reactor Anomaly: ¯ ν e → ¯ ν x ( ∼ 3 σ ) Gallium Anomaly: ν e → ν x ( ∼ 3 σ ) 1.20 Bugey−3 Daya Bay Krasnoyarsk RENO Bugey−4+Rovno91 Double Chooz Nucifer Rovno88 1.1 GALLEX SAGE 1.10 Chooz Gosgen+ILL Palo Verde SRP Cr1 Cr R = N exp N cal 1.00 1.0 R = N exp N cal GALLEX SAGE 0.90 Cr2 Ar 0.9 0.80 R = 0.934 ± 0.024 0.8 0.70 10 2 10 3 10 R = 0.84 ± 0.05 L [m] 0.7 m L osc = 4 π E . . . . . . LSND Anomaly: ¯ ν µ → ¯ ν e ( ∼ 4 σ ) ∆ m 2 ν 5 ν s 2 ν 4 ν s 1 ∆ m 2 � 1 eV 2 SBL ν 3 ≃ 2.5 × 10 − 3 eV 2 ∆ m 2 ATM ν 2 ≃ 7.4 × 10 − 5 eV 2 ∆ m 2 SOL ν 1 ν e ν µ ν τ C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 14/27