Why neutrinos? Hitoshi Murayama (Berkeley) Double Beta Decay and Neutrinos Osaka, June 12, 2007
Introduction • Neutrino physics has been full of surprises • We’ve learned a lot in the last ~8 years • We want to learn more. Why? • Window to short distance, early universe • What exactly can we learn from neutrinos? – Origin of neutrino mass? – Origin of baryon asymmetry? – Origin of universe? • Need data from neutrino oscillations, colliders, 0 νββ , dark matter, cosmology, rare decays Osaka, June 12, 2007 2
Outline • Past • What we now know • The Big Questions • Seesaw • Synergy • Conclusion Osaka, June 12, 2007 3
Past Why Neutrinos?
Interest in Neutrino Mass • So much activity on neutrino mass already. Why am I interested in this? Window to (way) high energy scales beyond the Standard Model! • Two ways: – Go to high energies – Study rare, tiny effects ⇐ Osaka, June 12, 2007 5
Rare Effects from High-Energies • Effects of physics beyond the SM as effective operators • Can be classified systematically (Weinberg) Osaka, June 12, 2007 6
Unique Role of Neutrino Mass • Lowest order effect of physics at short distances • Tiny effect ( m ν / E ν ) 2 ~(0.1eV/GeV) 2 =10 –20 ! • Interferometry ( i.e. , Michaelson-Morley) – Need coherent source – Need interference ( i.e. , large mixing angles) – Need long baseline Nature was kind to provide all of them! • “neutrino interferometry” (a.k.a. neutrino oscillation) a unique tool to study physics at very high scales Osaka, June 12, 2007 7
Ubiquitous Neutrinos They must have played some important role in the universe! Osaka, June 12, 2007 8
What we now know
The Data • Atmospheric – ∆ m 23 2 ~2.5 × 10 -3 eV 2 – sin 2 2 θ 23 ~1 • Solar – ∆ m 12 2 ~3–12 × 10 -5 eV 2 – sin 2 2 θ 12 ~0.9 • Reactor – ∆ m 12 2 ~8 × 10 -5 eV 2 • Accelerator (K2K/MINOS) • LSND vs Mini-BooNE Osaka, June 12, 2007 10
What we learned • Lepton Flavor is not conserved • Neutrinos have tiny mass, not very hierarchical • Neutrinos mix a lot the first evidence for incompleteness of Minimal Standard Model Very different from quarks Osaka, June 12, 2007 11
Typical Theorists’ View ca. 1990 • Solar neutrino solution must be small angle MSW solution because it’s cute Wrong! • Natural scale for ∆ m 2 23 ~ 10–100 eV 2 Wrong! because it is cosmologically interesting • Angle θ 23 must be ~ V cb =0.04 Wrong! • Atmospheric neutrino anomaly must go Wrong! away because it needs a large angle Osaka, June 12, 2007 12
The Big Questions • What is the origin of neutrino mass? • Did neutrinos play a role in our existence? • Did neutrinos play a role in forming galaxies? • Did neutrinos play a role in birth of the universe? • Are neutrinos telling us something about unification of matter and/or forces? • Will neutrinos give us more surprises? Big questions ≡ tough questions to answer Osaka, June 12, 2007 13
Immediate Questions • Dirac or Majorana? • Absolute mass scale? • How small is θ 13 ? • CP Violation? • Mass hierarchy? • Is θ 23 maximal? Osaka, June 12, 2007 14
Immediate Questions • Dirac or Majorana? • Absolute mass scale? • How small is θ 13 ? • CP Violation? • Mass hierarchy? • Is θ 23 maximal? Osaka, June 12, 2007 15
Extended Standard Model • Massive Neutrinos ⇒ Minimal SM incomplete • How exactly do we extend it? • Abandon either – Minimality: introduce new unobserved light degrees of freedom (right-handed neutrinos) – Lepton number: abandon distinction between neutrinos and anti- neutrinos and hence matter and anti-matter • Dirac or Majorana neutrino • Without knowing which, we don’t know how to extend the Standard Model Osaka, June 12, 2007 16
0 νββ • The only known practical approach to discriminate Majorana vs Dirac neutrinos 0 νββ : nn → ppe – e – with no neutrinos • Matrix element ∝ < m ν e >= Σ i m ν i U ei 2 • Current limit |< m ν e >| ≤ about 1eV • m 3 ~( ∆ m 2 23 ) 1/2 ≈ 0.05eV looks a promising goal • Good chance to discover it for degenerate and inverted spectra < m ν e > > 0.01eV • Not clear if we can see it for the normal spectrum, need ~0.001 eV sensitivity • Majorana, CANDLES, Cuore, GERDA, MOON, 17 EXO, XMASS, SuperNEMO, COBRA, …
Immediate Questions • Dirac or Majorana? • Absolute mass scale? • How small is θ 13 ? • CP Violation? • Mass hierarchy? • Is θ 23 maximal? Osaka, June 12, 2007 18
Now that LMA is confirmed... � ∆ m 12 2 , s 12 came out as large it could be (LMA) • Dream case for neutrino oscillation physics! � ∆ m 2 solar within reach of long-baseline expts • Even CP violation may be probed – neutrino superbeam – muon-storage ring neutrino factory 2 s 23 c 23 P ( ν µ → ν e ) − P ( ν µ → ν e ) = − 16 s 12 c 12 s 13 c 13 sin δ sin ∆ m 12 2 sin ∆ m 13 2 sin ∆ m 23 2 4 E L 4 E L 4 E L • What it would take to see it depends on θ 13 ! Osaka, June 12, 2007 19
θ 13 • Two approaches • Reactor anti-neutrino experiments – Disappearance of anti- ν e – measures purely sin 2 2 θ 13 – Double-CHOOZ, Daya Bay, RENO, ANGRA, … • Long-baseline accelerator experiments – Appearance of ν e from ν µ – Combination of θ 13 , matter effect, CP phase – MINOS, T2K, NO ν A, T2KK, … Osaka, June 12, 2007 20
The Big Questions • What is the origin of neutrino mass? • Did neutrinos play a role in our existence? • Did neutrinos play a role in forming galaxies? • Did neutrinos play a role in birth of the universe? • Are neutrinos telling us something about unification of matter and/or forces? • Will neutrinos give us more surprises? Big questions ≡ tough questions to answer Osaka, June 12, 2007 21
Seesaw
Seesaw Mechanism • Why is neutrino mass so small? • Need right-handed neutrinos to generate , but ν R SM neutral neutrino mass ν L ν L m D m D 2 ( ( ) ) ν L ν L ν R ν R m ν = m D M << m D ν R ν R m D m D M To obtain m 3 ~( ∆ m 2atm ) 1/2 , m D ~ m t , M 3 ~10 14 GeV Osaka, June 12, 2007 23
Grand Unification M 3 • electromagnetic, weak, and strong forces have very different strengths • But their strengths become the same at ~2 × 10 16 GeV if supersymmetry • To obtain m 3 ~( ∆ m 2 atm ) 1/2 , m D ~ m t ⇒ M 3 ~10 14 GeV! Osaka, June 12, 2007 24
Matter and Anti-Matter Early Universe 1,000,000,001 1,000,000,000 Matter Anti-matter Osaka, June 12, 2007 25
Matter and Anti-Matter Current Universe us 1 Matter Anti-matter The Great Annihilation Osaka, June 12, 2007 26
Baryogenesis • What created this tiny excess matter? • Necessary conditions for baryogenesis (Sakharov): – Baryon number non-conservation – CP violation (subtle difference between matter and anti-matter) – Non-equilibrium ⇒ Γ ( ∆ B >0) > Γ ( ∆ B <0) • It looks like neutrinos have no role in this… Osaka, June 12, 2007 27
Electroweak Anomaly • Actually, SM converts L QuickTime™ and a ( ν ) to B (quarks). decompressor are needed to see this picture. – In Early Universe ( T > 200GeV), W is massless and fluctuate in W plasma – Energy levels for left- QuickTime™ and a QuickTime™ and a decompressor decompressor are needed to see this picture. are needed to see this picture. handed quarks/leptons fluctuate correspon- dingly ∆ L = ∆ Q = ∆ Q = ∆ Q = ∆ B =1 ⇒ ∆( B–L ) = 0 Osaka, June 12, 2007 28
Leptogenesis • You generate Lepton Asymmetry first. (Fukugita, Yanagida) • Generate L from the direct CP violation in right-handed neutrino decay * h lj * ) Γ ( N 1 → ν i H ) − Γ ( N 1 → ν i H ) ∝ Im( h 1 j h 1 k h lk • L gets converted to B via EW anomaly ⇒ More matter than anti-matter ⇒ We have survived “The Great Annihilation” • Despite detailed information on neutrino masses, it still works ( e.g., Bari, Buchmüller, Plümacher) 29
Origin of Universe V • Maybe an even bigger role: inflation • Need a spinless field that – slowly rolls down the potential QuickTime™ and a Cinepak decompressor – oscillates around it minimum are needed to see this picture. – decays to produce a thermal bath • The superpartner of right-handed neutrino fits the bill ~ ν R • When it decays, it produces the size of the universe lepton asymmetry at the same time (HM, Suzuki, Yanagida, Yokoyama) • Decay products: supersymmetry and hence dark matter QuickTime™ and a Cinepak decompressor Neutrino is mother of the Universe? are needed to see this picture. 30 t
⇐ ⇐ ⇐ Osaka, June 12, 2007 31
Synergy
Can we prove it experimentally? • Short answer: no. We can’t access physics at >10 10 GeV with accelerators directly • But: we will probably believe it if the following scenario happens Archeological evidences Osaka, June 12, 2007 33
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