Neutrino Physics III

Hitoshi Murayama

Taiwan Spring School

March 28, 2002

中性微子物理（三）

村山　斉台湾春期学校

二千二年三月二十八日

3

Outline

• LSND

• Implications of Neutrino Mass

• Why do we exist?

• Models of flavor

• Conclusions

4

5

LSND

6

ν μ

ν e?

ν ep→ e+n

μ+→ e+νeν μ

p→ π +

π+→ μ+νμ

7

3.3 Signal

• Excess positron events over calculated BG

P(ν μ → ν e)

=(0.264±0.067±0.045)%

8

Mini-BooNE

• LSND unconfirmed• Neutrino beam from

Fermilab booster• Settles the issue of

LSND evidence• Start data taking later

this year

9

SN1987A neutrino burstdoesn’t like LSND

HM, Yanagida

• Kamiokande’s 11 events:– 1st event is forward

may well be e from deleptonization burst

(p e- n e to become neutron star)– Later events most likely e

• LSND parameters cause complete MSW conversion ofeif light side (e lighter)

eif dark side (e heavier)

• Either mass spectrum disfavored

_

_ _

10

SN1987A neutrino burstdoesn’t like LSND

HM, Yanagida

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Sterile Neutrino

• LSND, atmospheric and solar neutrino oscillation signalsm2

LSND ~ eV2

m2atm ~ 310–3eV2

m2solar < 10–3eV2

Can’t be accommodated with 3 neutrinos

Need a sterile neutrino

New type of neutrino with no weak interaction

• 3+1 or 2+2 spectrum?

12

Sterile Neutrino getting tight

• 3+1 spectrum: sin22LSND=4|U4e|2|U4|2

– |U4|2 can’t be big because of CDHS, SK U/D

– |U4e|2 can’t be big because of Bugey

– Marginally allowed (90% excl. vs 99% allw’d)

• 2+2 spectrum: past fits preferred– Atmospheric mostly

– Solar mostly es (or vice versa)

– Now solar sterile getting tight(Barger et al, Giunti et al, Gonzalez-Garcia et al, Strumia)

13

Not Quite Excluded Yet…

• Global fit to four-neutrino oscillation– Solar, Atmospheric,

LSND(Gonzalez-Garcia, Maltoni, Peña-

Garay@EPS01)

• One can still find a reasonable fit with 2+2 Disfavored at 90-99%

CL

es

e s

14

CPT Violation?“A desperate remedy…”

• LSND evidence: anti-neutrinos

• Solar evidence: neutrinos

• If neutrinos and anti-neutrinos have different mass spectra, atmos-pheric, solar, LSND accommodated without a sterile neutrino(HM, Yanagida)

15

CPT Theorem

• Based on three assumptions:– Locality– Lorentz invariance– Hermiticity of Hamiltonian

• Violation of any one of them: big impact on fundamental physics

• Neutrino mass: tiny effect from high-scale physics– Non-commutative geometry? (HM, Yanagida)

– Brane world? (Barenboim, Borissov, Lykken, Smirnov)

16

Implications on Experiments

• Mini-BooNE experiment will not see oscillation in neutrino mode, but will in anti-neutrino mode

• SNO, Borexino establish LMA, while KamLAND will not see oscillation

• Katrin may see endpoint distortion

We’ll see!

17

Maybe even more surprisesin neutrinos!

Implications of Neutrino Mass

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Mass Spectrum

What do we do now?

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Two ways to go

(1) Dirac Neutrinos:– There are new

particles, right-handed neutrinos, after all

– Why haven’t we seen them?

– Right-handed neutrino must be very very weakly coupled

– Why?

21

Extra Dimensions

• Right-handed neutrinos SM gauge singlet• Can propagate in the “bulk”• Makes neutrino mass small

(Arkani-Hamed, Dimopoulos, Dvali, March-Russell;Dienes, Dudas, Gherghetta; Grossman, Neubert)

• m ~ 1/R if one extra dim R~10m• An infinite tower of sterile neutrinos• Need also inter-generational mixing now

22

Two ways to go

(2) Majorana Neutrinos:– There are no new light

particles

– Why if I pass a neutrino and look back?

– Must be right-handed anti-neutrinos

– No fundamental distinction between neutrinos and anti-neutrinos!

23

Seesaw Mechanism

• Why is neutrino mass so small?

• Need right-handed neutrinos to generate neutrino mass

νL νR( )mD

mD

⎛

⎝ ⎜

⎞

⎠ ⎟

νL

νR

⎛

⎝ ⎜

⎞

⎠ ⎟ νL νR( )

mD

mD M

⎛

⎝ ⎜

⎞

⎠ ⎟

νL

νR

⎛

⎝ ⎜

⎞

⎠ ⎟ mν =

mD2

M<<mD

To obtain m3~(m2atm)1/2, mD~mt, M3~1015GeV (GUT!)

, but R SM neutral

24

Grand Unification

• electromagnetic, weak, and strong forces have very different strengths

• But their strengths become the same at 1016 GeV if supersymmetry

• To obtain

m3~(m2atm)1/2, mD~mt

M3~1015GeV!Neutrino mass may be probing unification:

Einstein’s dream

M3

Dimopoulos, Raby, Wilczek

Matter Anti-matter AsymmetryWhy do we exist?

26

Big-Bang NucleosynthesisCosmic Microwave Background

η =nB

nγ= 4.7−0.8

+1.0( )×10−10

5.0±0.5( )×10−10

(Thuan, Izatov)

(Burles, Nollett, Turner)

27

28

Baryon AsymmetryEarly Universe

q q

They basically have all annihilated away except a tiny difference between them

10,000,000,001 10,000,000,000

29

Baryon AsymmetryCurrent Universe

q q

They basically have all annihilated away except a tiny difference between them

1

us

30

Sakharov’s Conditionsfor Baryogenesis

• Necessary requirements for baryogenesis:– Baryon number violation– CP violation– Non-equilibrium(B>0) > (B<0)

• Possible new consequences in– Proton decay– CP violation

31

Original GUT Baryogenesis

• GUT necessarily breaks B. • A GUT-scale particle X decays out-of-equilibrium

with direct CP violation

• Now direct CP violation observed: ’!

• But keeps B–L0 “anomaly washout”• Also monopole problem

B(X → q) ≠B(X → q)

B(K0 → π+π−) ≠B(K0 → π+π−)

32

Anomaly washout

• Actually, SM violates B (but not B–L).– In Early Universe (T >

200GeV), W/Z are massless and fluctuate in W/Z plasma

– Energy levels for left-handed quarks/leptons fluctuate correspon-dingly

L=Q=Q=Q=B=1 B=L=0

33

Two Main Directions

• BL0 gets washed out at T>TEW~174GeV• Electroweak Baryogenesis (Kuzmin, Rubakov, Shaposhnikov)

– Start with B=L=0– First-order phase transition non-equilibrium– Try to create BL0

• Leptogenesis (Fukugita, Yanagida)

– Create L0 somehow from L-violation– Anomaly partially converts L to B

Leptogenesis

35

Leptogenesis

• You generate Lepton Asymmetry first.• Generate L from the direct CP violation in right-handed

neutrino decay

– Two generations enough for CP violation because of Majorana nature (choose 1 & 3)

• L gets converted to B via EW anomaly More matter than anti-matter We have survived “The Great Annihilation”

ε =Γ(N1 → νiH)−Γ(N1 → ν iH)Γ(N1 → νiH)+Γ(N1 → ν iH)

~1

8πIm(h13h13h33

* h33* )

h132

M1

M3

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Does Leptogenesis Work?

• Much more details worked out(Buchmüller, Plümacher; Pilaftsis)

• ~1010 GeV R OK

• Some tension with supersymmetry because of unwanted gravitino overproduction

• Ways around: coherent oscillation of right-handed sneutrino (HM, Yanagida+Hamaguchi)

37

Does Leptogenesis Work?

• Some tension with supersymmetry:– unwanted gravitino

overproduction– gravitino decay

dissociates light nuclei– destroys the success of

Big-Bang Nucleosynthesis

– Need TRH<109 GeV

(Kawasaki, Kohri, Moroi)

38

Leptogenesis Works!

• Coherent oscillation of right-handed sneutrino (Bose-Einstein condensate) (HM, Yanagida+Hamaguchi)

– Inflation ends with a large sneutrino amplitude

– Starts oscillation – dominates the Universe– Its decay produces asymmetry– Consistent with observed

oscillation pattern– isocurvature fluctuation ~10-7 nB

s~ε

Tdecay

M1~

nB

s⎛ ⎝ ⎜

⎞ ⎠ ⎟

obs

Tdecay

106GeVarg

h132

h332

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Can we prove it experimentally?

• We studied this question at Snowmass2001 (Ellis, Gavela, Kayser, HM, Chang)

– Unfortunately, no: it is difficult to reconstruct relevant CP-violating phases from neutrino data

• But: we will probably believe it if– 0 found

– CP violation found in neutrino oscillation

– EW baryogenesis ruled out

Archeological evidences

40

Conclusions

• Neutrinos are weird

• Strong evidence for neutrino mass

• Small but finite neutrino mass:– Need drastic ideas to understand it

• If Majorana, neutrino mass may be responsible for our existence

• A lot more to learn in the next few years