Baryogenesis. David Morrissey. SLAC Summer Institute, July 26, 2011

Transcription

1 Baryogenesis David Morrissey SLAC Summer Institute, July 26, 2011

2 Why is There More Matter than Antimatter? About 5% of the energy density of the Universe consists of ordinary (i.e. non-dark) matter. By mass, this is mostly baryons in the form of H and He. There are almost no antibaryons. If there were, they would produce cosmic rays. But the Standard Model treats matter and antimatter nearly identically. How do we explain this baryon asymmetry?

3 Why is There More Matter than Antimatter? [ files.com]

4 Evidence Baryonic Energy Fraction: Ω B = ± CMB Spectrum: η n B n γ = (7.04) n B s = (6.1±0.3) CMB temperature fluctuations % higher η value accepted η value 15% lower η value WMAP data multipole, l

5 Primordial Nucleosynthesis also takes η as an input. [Cyburt et al.] Consistent with the value of η from the CMB! (Well, 7 Li is a bit problematic, but never mind for now.)

6 What We Need......a concrete mechanism to produce a baryon excess: Baryogenesis! It should operate after inflation. Any initial-state asymmetry would be diluted away. n B nγ make baryons, antibaryons. Annihilation at T 50MeV removes the antibaryons. + = 11 11_ (10 +1) p 10 p 1 p One part in 10 10, easy right?

7 Ingredients for Baryogenesis [Sakharov 67] 1. B Violation: B = 0 B 0 obviously requires B violation 2. C and CP violation Without violating both, B-violating processes would make just as many baryons as antibaryons 3. Departure from thermodynamic equilibrium B = constant in equilibrium

8 Simple Toy Model of Baryogenesis Massive particles X and X decay at T m X. Start with n X = n X T3 n eq X departure from thermodynamic equilibrium Decay modes: X A+B ; B = B 1 C +D ; B = B 2 X Ā+ B ; B = B 1 C + D ; B = B 2 B 1 B 2 requires B violation

9 Decay rates : Γ(X A+B) = Γ AB + Γ AB Γ(X C +D) = Γ CD Γ CD Γ( X Ā+ B) = Γ AB Γ AB Γ( X C + D) = Γ CD + Γ CD C and CP violation needed for Γ ij 0 Γ tot X = Γtot X by CPT conservation. Γ AB = Γ CD Γ * Recall that BR(X ij) = Γ(X ij)/γ tot X

10 Net baryon density produced by decays of X, X: n B = n X [B 1 BR(X AB)+B 2 BR(X CD) B 1 BR( X AB) B 2 BR( X CD) ] = 2 Γ Γ X (B 1 B 2 )n X (B 1 B 2 ) 0 from B violation Γ 0 requires C and CP violation n X n eq X requires departure from equilibrium Decay of X at T m X means that washout is turned off. (e.g. A+B X ( ) C + D scatterings)

11 BG Ingredients and the Standard Model The Standard Model (SM) has all three ingredients: spacetime expansion gives a departure from equilibrium C is violated by the chiral fermion structure of the SM CP is violated by the phase in the CKM matrix [talk by D. MacFarlane] B is violated by non-perturbative processes!!!

12 B Violation in the SM B and L seem like symmetries of the SM Lagrangian. They aren t at the quantum level due to SU(2) L : µ j µ B = µj µ L = n g g2 32π 2Wa αβ W bαβ W j ψ µ L a α W b β But maybe we re still OK since W a µν W aαβ = µ K µ, K µ = ǫ µναβ [W a ναw a β g 3 ǫ abcw a νw b αw c β ]? Nope, life is not always perturbative...

13 Non-Abelian gauge theories have multiple vacua characterized by the integer N CS : [ Belavin et al., Jackiw+Rebbi 76] N CS = d 3 xk 0 ( Z by topology) E[W] instanton N CS Instantons = tunnelling between vacua: N CS 0. For each instanton transition, [ t Hooft 76] B = L = n g N CS

14 Instantons violate (B +L)! Instanton rate at zero temperature (T = 0): [ t Hooft 76] Γ inst e 16π2 /g At finite temperature they can go via thermal fluctuations called sphaleron transitions [Klinkhamer+Manton 84] Γ sp T 4 e 4π H /gt H 0 [Arnold+McLerran 87] κα 4 wt 4 H = 0 [Bodeker,Moore,Rummukainen 99]. Result: (B +L) violation is active in the early Universe for T 100GeV (when EW symmetry is unbroken).

15 Ingredients Are Not Enough Baryogenesis requires a mechanism with these ingredients. + We don t know of any mechanisms that work using the Standard Model alone.

16 Some Popular Mechanisms Leptogenesis BG related to the origin of neutrino masses Electroweak Baryogenesis BG related during the EW phase transition GUT Baryogenesis BG from B-violating decay of heavy GUT stuff Affleck-Dine BG from rolling scalars carrying B charges Hidden Sector Asymmetric Baryogenesis BG in an exotic sector related to dark matter

17 Leptogenesis (and Neutrino Masses) We need new physics beyond the SM for neutrino masses. Minimal Requirement: new gauge singlet RH Neutrino N. Dirac Mass Term: L y ν LH N (y ν v) LN, after H v +h/ 2 H = Higgs breaking EW symmetry, v = H 174GeV Neutrino Mass: m ν = y ν v But m ν 1eV why is y ν so small?

18 Alternative Option: Seesaw Mechanism (Type I) [Mikowski,Gell-Mann,Ramond,Yanagida 79] L y ν LH N M N N c N For M N v integrate out the heavy RH state to give L eff yt νy ν M N ( L c H)(LH) Light neutrino masses: m ν = (y νv) 2 M N Gives m ν ev for y ν 1, M N GeV. Note: N is Majorana so that N N.

19 Leptogenesis: Step #1 [Fukugita+Yanagida 86] Heavy neutrinos N i (i = 1,2,...) decay in the early Universe. Two decay modes (related to neutrino masses): N i H +l L (L = +1) N i H + l L (L = 1) These decays violate lepton number L. Originates from the heavy neutrino couplings: if [N] L = 1, the Majorana mass term has L = 2.

20 Total decay width of N i (i = 1,2,...): Γ Ni = Γ(N i H +l L )+Γ(N i H + l L ) = (y νy ν) ii M Ni 8π Departure from Equilibrium if n N > n eq N at decay: Decays are slow : Γ Ni H. N i are created non-thermally at T M Ni (e.g. N i are created by (p)reheating with T RH M Ni ) Non-equilibrium is needed to avoid washout processes.

21 Leptogenesis: Step #2 CP violation in N decays gives : ǫ 1 Γ(N 1 Hl L ) Γ(N 1 H l L ) Γ(N 1 Hl L )+Γ(N 1 H l L ) 3 16π i>1 Im(y ν y ν) 1i (y ν y ν) 11 (M 2,... M 1 ) Non-zero ǫ 1 comes from tree-loop interference: H H H l l l l N k N k N j N k N j H H l i l i l i

22 Lepton Asymmetry from N 1 decays: Y L = n L s = ǫ 1κ g (tdec ) g (tdec ) = number of rel. degrees of freedom at decay κ = efficiency factor Efficiency κ < 1 from washout processes e.g. H +l L H l L, l L l L HH ( L = 2 scattering) κ 1 for Γ N1 H(t dec ) Aside: n i /s = constant for any decoupled species n i a 3 due to dilution by expansion s = S/a 3 a 3 for adiabatic expansion (S = constant)

23 Leptogenesis: Step #3 Sphalerons are active at T 100GeV. They violate (B +L) by n g = 3 units. convert L charge to B charge Q 1 L L L τ e µ Q Q 2 3 [Cline 06]

24 After sphaleron reprocessing, [Harvey+Turner 90] Y B (final) = C[Y B (initial) Y L (initial)] = CY L (initial) (Leptogenesis) C 1 depends on the degrees of freedom in the plasma Final Answer: BG Ingredients: η B = (7.04)Y B (final) = (7.04)C ǫ 1κ g (tdec ) C 0 requires B violation ǫ 1 0 requires C and CP violation κ 0 requires departure from equilibrium

25 Electroweak Baryogenesis [Kuzmin,Rubakov,Shaposhnikov 85] Departure from EQ can occur during phase transitions. Really Important PT: electroweak symmetry breaking SU(2) L U(1) Y U(1) em Electroweak Baryogenesis (EWBG) B production during the electroweak PT 1. First-order EWPT produces expanding bubbles 2. CP violation near the bubble walls creates a chiral asymmetry 3. Sphalerons reprocess this asymmetry to B

26 The EW Phase Transition Order parameter Higgs VEV H φ: H = 0 SU(2) L U(1) Y is unbroken. H 0 SU(2) L U(1) Y U(1) em. Effective potential: V eff = ( µ 2 +αt 2 )φ 2 γtφ 3 + λ 4 φ V eff Τ >> µ Τ << µ φ

27 Bubble Nucleation First order phase transition: V eff T > T c tunnel φ T = T T < T Bubbles of broken phase are nucleated at T < T c. c c <ϕ> = 0 <ϕ> = 0 <ϕ> = 0 <ϕ> = 0

28 Producing Baryons CP violation in the bubble wall creates a chiral asymmetry. Sphalerons transform the chiral charge into baryons. These baryons are swept up into the bubbles. Sphaleron Bubble Wall CP χ + L χ R χ L B Sphaleron <φ> = 0 <φ> = 0

29 Chiral Asymmetry = n(lh quarks) n(rh quarks). Sphalerons (SU(2) L ) only act on LH quarks Only the LH quarks get reprocessed by sphalerons CP q L q R q L q R

30 EWBG in the Standard Model and Beyond It doesn t work for two reasons: 1. The electroweak phase transition is not first-order in the Standard Model [Kajantie et al. 98] 2. Not enough CP violation [Gavela et al. 94] It can work with new physics extending the SM. e.g. additional scalar doublets, supersymmetry,... New particles and CPV must appear near the EW scale. can test directly in upcoming collider experiments!

31 Other Popular Baryogenesis Mechanisms GUT Baryogenesis (e.g. SU(3) c SU(2) L U(1) Y G simple ) can have quarks and leptons in the same representation heavy GUT decays can violate B, like leptogenesis Affleck-Dine Baryogenesis excite scalar field directions carrying (B L) charge scalar condensates decay to particles with net B Asymmetric Dark Matter Ω DM 5Ω B could they be related? DM carries a net conserved charge related to B density of DM set by the B charge asymmetry

32 Summary More matter than antimatter in the Universe. No known explanation within the Standard Model. Leptogenesis: baryon production from heavy neutrinos. Electroweak BG: baryon production during the EWPT. Other promising mechanisms... If we re lucky, we ll see evidence of new physics soon!

33 Some Nice Reviews A. Ritto, hep-ph/ M.Quirós, hep-ph/ (electroweak BG) M. Dine, A. Kusenko, hep-ph/ W.Buchmüller, R. Peccei, T. Yanagida, hep-ph/ (leptogenesis) A. Strumia, F. Vissani, hep-ph/ (neutrinos and leptogenesis) J. Cline, hep-ph/ S. Davidson, E. Nardi, Y. Nir, hep-ph/ (leptogenesis)

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