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

◮ There is a wind of crisis in traditional Particle Physics (mitigated by the fmourishing of Astroparticle Physics and Cosmology). ◮ 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 with high precision all the known interactions of the known elementary particles. ◮ Curse: its perfect working is hiding the way of further understanding of the fundamental properties of nature.

◮ 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

◮ 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 leptogenesis).

◮ 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 neutrino masses?) ◮ Hierarchy problem (why the electroweak scale is so much smaller than the Planck scale?). ◮ The strong CP problem. ◮ Accidental conservation of B − L global symmetry (broken by Majorana neutrino masses?).

• 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

◮ Neutrinos are the lightest known elementary particles with a huge gap in the mass scale of about 6-7 orders of magnitude.

m [eV]

10−4 10−3 10−2 10−1 1 10 102 103 104 105 106 107 108 109 1010 1011 1012

νe e u d νµ µ s c ντ τ b t ν1 ν2 ν3

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 5/27

Quasi-Degenerate → Inverted Ordering→ Normal Ordering→

m [eV]

10−4 10−3 10−2 10−1 1 10 102 103 104 105 106 107 108 109 1010 1011 1012

e u d µ s c τ b t ν1 ν2 ν3 NO IO QD

◮ 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

1st Generation 2nd Generation 3rd Generation Quarks: uL dL uR dR cL sL cR sR tL bL tR bR Leptons: νeL eL

• νeR

eR νµL µL

• νµR

µR ντL τL

• ντR

τR ◮ Standard Model extension: νR ⇒ Dirac mass term LD ∼ mDνLνR ◮ This is Standard Model physics, because mD is generated by the standard Higgs mechanism: yLL ΦνR

Symmetry

− − − − − − →

Breaking

yvνLνR ⇒ mD ∼ yv ◮ Bad: extremely small Yukawa couplings: y 10−11

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 7/27

Beyond the Standard Model

◮ The introduction of νR leads us beyond the Standard Model because they can have the Majorana mass term LM ∼ mMνRνc

R

singlet under SM symmetries! ◮ This is beyond the Standard Model because mM 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 conservation which should anyway be explained by some physics beyond the Standard Model.

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 8/27

Seesaw Mechanism

without lepton number conservation LD+M = −1 2

• νc

L

νR mD mD mM νL νc

R

• + H.c.

mM can be arbitrarily large (not protected by SM symmetries) mM ∼ scale of new physics beyond Standard Model ⇒ mM ≫ mD diagonalization of mD mD mM

• =

⇒ mν ≃ m2

D

mM mN ≃ mM

Φ

N ν

seesaw mechanism natural explanation of smallness

• f light neutrino masses

massive neutrinos are Majorana ⇒ ββ0ν ν ≃ −i

• νL − νc

L

• N ≃ νR + νc

R

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: ◮ A Majorana fjeld is simpler than a Dirac fjeld: it corresponds to the 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 elementary particle as the neutrino is naturally Majorana. ◮ The seesaw mechanism if νR is introduced to generate neutrino masses. ◮ A general Efgective Field Theory argument from high-energy new physics: L = LSM + g5 M O5 + g6 M2 O6 + . . .

◮ O5: Majorana neutrino masses (Lepton number violation and ββ0ν decay). O5 = (L Φ) ( ΦTLc) L =

• νL

ℓL

• Φ =
• φ0

−φ+

• ◮ O6: Baryon number violation (proton decay)

and Neutrino Non-Standard Interactions (NSI).

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 10/27

Leptogenesis

◮ Ofg-equilibrium L and CP violating heavy Majorana neutrino decays at T ∼ MN: LI ∼ L Φ Y νR AL ∼

• k,α
• Γ(Nk → Φℓα) − Γ(Nk → ¯

Φ¯ ℓα)

• k,α
• Γ(Nk → Φℓα) + Γ(Nk → ¯

Φ¯ ℓα)

• Nk

ℓα Φ Yαk

◮ The lepton asymmetry AL is converted into a baryon asymmetry AB at T ∼ 100 GeV by electroweak sphalerons that conserve B − L and break B + L. ◮ Seesaw ⇒ Y ∼ 1 v M1/2

R

R

inaccessible

m1/2

ν

U3×3

• measurable

(RRT = 1)

[Casas, Ibarra, NPB 618 (2001) 171]

◮ CP-violating U3×3 ⇒ plausible CP-violating Y

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 11/27

50 100 150 200 250 300 350

δ [◦]

−6 −4 −2 2 4 6

ηB × 1010

M1 ≃ 5 × 1010 GeV M1 ≪ M1 ≪ M3

[Mofgat, Pascoli, Petcov, Turner, JHEP 1903, 034]

◮ The discovery of L violation (ββ0ν decay due to Majorana neutrinos) and CP violation in the lepton sector (through neutrino oscillations) would be a strong indication in favor of leptogenesis as the origin of the matter-antimatter asymmetry in the Universe.

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 12/27

◮ Seesaw with leptogenesis is a very attractive and compelling theory. ◮ However, in general there is no constraint on the number and mass scale

• f the νR’s.

◮ It is possible and interesting that there is low-energy new physics (maybe connected with dark matter). ◮ Light fermions beyond the Standard Model are neutral and can mix with neutrinos: they are νR’s. ◮ Light left-handed anti-νR are light sterile neutrinos νc

R→νsL

(left-handed) ◮ Sterile means no standard model interactions

[Pontecorvo, Sov. Phys. JETP 26 (1968) 984]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 13/27

Light Sterile Neutrinos

Short-Baseline Anomalies Reactor Anomaly: ¯ νe → ¯ νx (∼ 3σ)

L [m] R = N exp N cal

10 102 103 0.70 0.80 0.90 1.00 1.10 1.20

R = 0.934 ± 0.024

Bugey−3 Bugey−4+Rovno91 Chooz Daya Bay Double Chooz Gosgen+ILL Krasnoyarsk Nucifer Palo Verde RENO Rovno88 SRP

Losc = 4πE ∆m2 νe νµ ντ

∆m2

SOL

∆m2

ATM

. . . νs2 νs1 ∆m2

SBL

ν4 ν3 ν2 ν1 . . . ν5 m 1 eV2

≃ 2.5 × 10−3 eV2

≃ 7.4 × 10−5 eV2

Gallium Anomaly: νe → νx (∼ 3σ)

0.7 0.8 0.9 1.0 1.1

R = N exp N cal

Cr1 GALLEX Cr SAGE Cr2 GALLEX Ar SAGE

R = 0.84 ± 0.05

LSND Anomaly: ¯ νµ → ¯ νe (∼ 4σ)

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 14/27

Reactor Spectral Ratios

NEOS [PRL 118 (2017) 121802 (arXiv:1610.05134)]

1 2 3 4 5 6 7 10 Prompt Energy [MeV] 1 2 3 4 5 6 7 10 Data/Prediction 0.9 1.0 1.1 NEOS/Daya Bay Systematic total , 0.050)

2

(1.73 eV , 0.142)

2

(2.32 eV

(c)

⋅ ⋅

|U e4|2 ∆m41

2 [eV2]

10−4 10−3 10−2 10−1 10−1 1 10

2σ NEOS DANSS NEOS+DANSS 1σ 2σ 3σ

DANSS

[PLB 787 (2018) 56, arXiv:1804.04046] Positron Energy [MeV] Ratio Down/Up

1.0 2.0 3.0 4.0 5.0 6.0 7.0 0.64 0.68 0.72 0.76 DANSS No−Oscillations Oscillations Best Fit

MODEL INDEPENDENT! ∼ 3.5σ

[Gariazzo, CG, Laveder, Li, arXiv:1801.06467] [Dentler, Hernandez-Cabezudo, Kopp, Machado, Maltoni, Martinez-Soler, Schwetz, arXiv:1803.10661]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 15/27

Model-Independent νe and ¯ νe Disappearance

|U e4|2 ∆m41

2 [eV2]

10−4 10−3 10−2 10−1 10−1 1 10

MIνeDis 1σ 2σ 3σ STEREO (1yr, 2σ) PROSPECT (3+3yr, 3σ) SoLiD (1+3yr, 3σ) KATRIN (90% CL)

∆m2

41 = 1.29 ± 0.03

|Ue4|2 = 0.012 ± 0.003 Huge potential for epochal New Physics discovery!

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 16/27

Neutrinoless Double-Beta Decay

mββ =

• |Ue1|2 m1 + |Ue2|2 eiα21 m2 + |Ue3|2 eiα31 m3 + |Ue4|2 eiα41 m4
• Lightest mass: m1 [eV]

|Uek|2mk [eV] 10−4 10−3 10−2 10−1 1 10−4 10−3 10−2 10−1 1

|Ue1|2m1 |Ue2|2m2 |Ue3|2m3 |Ue4|2m4

Normal 3ν Ordering 1σ 2σ 3σ ν4 1σ 2σ 3σ

Lightest mass: m1 [eV] mββ [eV]

90% C.L. UPPER LIMIT

10−4 10−3 10−2 10−1 1 10−4 10−3 10−2 10−1 1

Normal 3ν Ordering 3ν (3σ) 3+1 (3σ)

Lightest mass: m3 [eV] |Uek|2mk [eV] 10−4 10−3 10−2 10−1 1 10−4 10−3 10−2 10−1 1

|Ue1|2m1 |Ue2|2m2 |Ue3|2m3 |Ue4|2m4

Inverted 3ν Ordering 1σ 2σ 3σ ν4 1σ 2σ 3σ

Lightest mass: m3 [eV] mββ [eV]

90% C.L. UPPER LIMIT

10−4 10−3 10−2 10−1 1 10−4 10−3 10−2 10−1 1

Inverted 3ν Ordering 3ν (3σ) 3+1 (3σ)

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 17/27

Non-Standard Interactions

◮ Observable non-renormalizable efgective NSI of left-handed neutrinos: Charged-Current-like NSI: (α, β = e, µ, τ) HCC

NSI = 2

√ 2GFVud

• α,β
• ℓαLγρνβL

εudL

αβ uLγρdL + εudR αβ uRγρdR

• + H.c.

+2 √ 2GF

• α,β

(ναLγρνβL)

• σ=δ
• εσδL

αβ ℓσLγρℓδL + εσδR αβ ℓσRγρℓδR

• Neutral-Current-like or Matter NSI:

(εfP

αβ = εfP∗ βα )

HNC

NSI = 2

√ 2GF

• α,β

(ναLγρνβL)

• f =e,u,d
• εfL

αβfLγρfL + εfR αβfRγρfR

• ◮ Obtained in Efgective Field Theory from operators of dimension 6 and

higher: O6 =

• α,β,σ,δ

Cαβσδ

• LαγρLβ

LσγρLδ

• + . . .

Constraints are required to suppress unobserved large charged lepton transitions as µ → 3e.

[see: Gavela, Hernandez, Ota, Winter, PRD 79 (2009) 013007]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 18/27

NSI Efgects on Oscillations

◮ Standard oscillations with matter efgects:

• s

• ✘

• s

• s

• ❅

◮ NC NSI in neutrino propagation in matter ∼ ε:

• s

• ✘

• s

• s

• ❅

◮ CC NSI in neutrino production and detection ∼ ε2:

• s

• ✘

• s

• s

• ❅

• s

• ✘

• s

• s

• ❅

[Kopp, Lindner, Ota, PRD 76 (2007) 013001]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 19/27

1 4 9 ∆χ2 2.4 2.6 2.8 |∆m2

3l|[10−3eV2]

90 180 270 360 δCP 0.1 0.2 |ε⊕

eµ|

0.5 1 |ε⊕

eτ|

0.05 0.1 |ε⊕

µτ|

90 180 270 360 φ⊕

eµ

90 180 270 360 φ⊕

eτ

90 180 270 360 φ⊕

µτ

• 2

2 ε⊕

ee − ε⊕ µµ

0.4 0.5 0.6 0.7 sin2 θ23

• 0.5

0.5 ε⊕

ττ − ε⊕ µµ

1 4 9 ∆χ2 2.4 2.6 2.8 |∆m2

3l|[10−3eV2]

1 4 9 ∆χ2 90 180 270 360 δCP 1 4 9 ∆χ2 0.1 0.2 |ε⊕

eµ|

1 4 9 ∆χ2 0.5 1 |ε⊕

eτ|

1 4 9 ∆χ2 0.05 0.1 |ε⊕

µτ|

1 4 9 ∆χ2 90 180 270 360 φ⊕

eµ

1 4 9 ∆χ2 90 180 270 360 φ⊕

eτ

1 4 9 ∆χ2 90 180 270 360 φ⊕

µτ

1 4 9 ∆χ2

• 2

2 ε⊕

ee − ε⊕ µµ

• 0.5

0.5 ε⊕

ττ − ε⊕ µµ

1 4 9 ∆χ2

[Esteban, Gonzalez-Garcia, Maltoni, JHEP 1906 (2019) 055, arXiv:1905.05203]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 20/27

Electromagnetic Interactions

◮ Efgective Hamiltonian: H(ν)

em (x) = j(ν) µ (x)Aµ(x) =

• k,j=1

νk(x)Λkj

µ νj(x)Aµ(x)

◮ Efgective electromagnetic vertex: νi(pi) Λ γ(q) νf (pf ) νf (pf )|j(ν)

µ (0)|νi(pi) = uf (pf )Λfi µ(q)ui(pi)

q = pi − pf ◮ Vertex function: Λµ(q) =

• γµ − qµ/

q/q2 FQ(q2) + FA(q2)q2γ5

• − iσµνqν

FM(q2) + iFE(q2)γ5

• form factors:

Lorentz-invariant charge anapole magnetic electric q2 = 0 = ⇒ q a µ ε

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 21/27

Neutrino Charge Radii

◮ In the Standard Model neutrinos are neutral and there are no electromagnetic interactions at the tree-level. ◮ Radiative corrections generate an efgective electromagnetic interaction vertex Λµ(q) =

• γµ − qµ/

q/q2 F(q2)

W ℓ ℓ γ ν ν ℓ W W γ ν ν

◮ F(q2) = ✟✟

✟ ❍❍ ❍

F(0) + q2 dF(q2) dq2

• q2=0

+ . . . = q2 r2 6 + . . . ◮ In the Standard Model:

[Bernabeu et al, PRD 62 (2000) 113012, NPB 680 (2004) 450]

r2

νℓSM = −

GF 2 √ 2π2

• 3 − 2 log

m2

ℓ

m2

W

• r2

νeSM = −8.2 × 10−33 cm2

r2

νµSM = −4.8 × 10−33 cm2

r2

ντ SM = −3.0 × 10−33 cm2

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 22/27

Experimental Bounds

Method Experiment Limit [cm2] CL Year Reactor ¯ νe e− Krasnoyarsk |r2

νe| < 7.3 × 10−32

90% 1992 TEXONO −4.2 × 10−32 < r2

νe < 6.6 × 10−32

90% 2009 Accelerator νe e− LAMPF −7.12 × 10−32 < r2

νe < 10.88 × 10−32

90% 1992 LSND −5.94 × 10−32 < r2

νe < 8.28 × 10−32

90% 2001 Accelerator νµ e− BNL-E734 −5.7 × 10−32 < r2

νµ < 1.1 × 10−32

90% 1990 CHARM-II |r2

νµ| < 1.2 × 10−32

90% 1994

[see the review Giunti, Studenikin, RMP 87 (2015) 531, arXiv:1403.6344 and the update in Cadeddu, Giunti, Kouzakov, Y.F. Li, Studenikin, Y.Y. Zhang, PRD 98 (2018) 113010, arXiv:1810.05606]

◮ Neutrino charge radii contribute coherently to standard neutral-current weak interactions ⇒ shifts sin2ϑW → sin2ϑW

• 1 + 1

3m2

W r2 νℓ

• ◮ The current limits are not too far from the SM prediction: about 1 order of

magnitude. ◮ Powerful precision test of the SM. ◮ A failure to measure the SM values would imply BSM physics!

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 23/27

Neutrino Magnetic and Electric Moments

◮ Extended Standard Model with right-handed neutrinos and ∆L = 0: µD

kk ≃ 3.2 × 10−19µB

mk eV

• εD

kk = 0

µD

kj

iεD

kj

• ≃ −3.9 × 10−23µB

mk ± mj eV

ℓ=e,µ,τ

U∗

ℓkUℓj

mℓ mτ 2

• fg-diagonal moments are GIM-suppressed

[Fujikawa, Shrock, PRL 45 (1980) 963; Pal, Wolfenstein, PRD 25 (1982) 766; Shrock, NPB 206 (1982) 359; Dvornikov, Studenikin, PRD 69 (2004) 073001, JETP 99 (2004) 254]

◮ Extended Standard Model with Majorana neutrinos (|∆L| = 2): µM

kj ≃ −7.8 × 10−23µBi (mk + mj)

• ℓ=e,µ,τ

Im [U∗

ℓkUℓj] m2 ℓ

m2

W

εM

kj ≃ 7.8 × 10−23µBi (mk − mj)

• ℓ=e,µ,τ

Re [U∗

ℓkUℓj] m2 ℓ

m2

W

[Shrock, NPB 206 (1982) 359]

GIM-suppressed, but additional model-dependent contributions of the scalar sector can enhance the Majorana transition dipole moments

[Pal, Wolfenstein, PRD 25 (1982) 766; Barr, Freire, Zee, PRL 65 (1990) 2626; Pal, PRD 44 (1991) 2261]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 24/27

dσνe− dTe

• mag

= πα2 m2

e

1 Te − 1 Eν µν µB 2

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 25/27

Method Experiment Limit [µB] CL Year Reactor ¯ νe e− Krasnoyarsk µνe < 2.4 × 10−10 90% 1992 Rovno µνe < 1.9 × 10−10 95% 1993 MUNU µνe < 9 × 10−11 90% 2005 TEXONO µνe < 7.4 × 10−11 90% 2006 GEMMA µνe < 2.9 × 10−11 90% 2012 Accelerator νe e− LAMPF µνe < 1.1 × 10−9 90% 1992 Accelerator (νµ, ¯ νµ) e− BNL-E734 µνµ < 8.5 × 10−10 90% 1990 LAMPF µνµ < 7.4 × 10−10 90% 1992 LSND µνµ < 6.8 × 10−10 90% 2001 Accelerator (ντ, ¯ ντ) e− DONUT µντ < 3.9 × 10−7 90% 2001 Solar νe e− Super-Kamiokande µS(Eν 5 MeV) < 1.1 × 10−10 90% 2004 Borexino µS(Eν 1 MeV) < 2.8 × 10−11 90% 2017

[see the review Giunti, Studenikin, RMP 87 (2015) 531, arXiv:1403.6344]

◮ Gap of about 8 orders of magnitude between the experimental limits and the 10−19 µB prediction of the minimal Standard Model extensions. ◮ µν ≫ 10−19 µB discovery ⇒ non-minimal new physics beyond the SM. ◮ Neutrino spin-fmavor precession in a magnetic fjeld

[Lim, Marciano, PRD 37 (1988) 1368; Akhmedov, PLB 213 (1988) 64]

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 26/27

Conclusions

◮ Neutrinos can be powerful messengers of the physics beyond the SM. ◮ The discovery of L violation through ββ0ν decay is of paramount importance. ◮ The additional discovery of CP violation in the lepton sector in LBL neutrino oscillation experiments will represent a strong indication in favor of leptogenesis as the origin of the matter-antimatter asymmetry in the Universe. ◮ The search for sterile neutrinos may open a cornucopia of new phenomena. ◮ Look out for neutrino Non-Standard Interactions and Electromagnetic Interactions.

• C. Giunti − The Theoretical Perspective on Future Neutrino Experiments − Hong Kong − 1 July 2019 − 27/27