Baryon Asymmetry of the Universe: a Window to Physics Beyond the Standard Model

Baryon asymmetry of the Universe: a window to physics beyond the standard model

Mikhail Shaposhnikov

2 November 2018

IPA colloquium, ETH, Zurich

ETH, November 2, 2018 – p. 1 Outline

The problem of baryon asymmetry of the universe Matter and antimatter Baryon asymmetry in the present universe Baryon asymmetry in the early universe

Sakharov proposal

How to create baryons in the Universe Electroweak baryogenesis GUT baryogenesis Leptogenesis Baryogenesis via light “heavy neutral lepton” oscillations

Conclusions

ETH, November 2, 2018 – p. 2 Matter and antimatter

In general, all elementary particles can be divided in three groups:

Truly neutral, like photon, intermediate Z-boson, Majorana neutrino, π0, etc. They do not carry any charges.

Particles, like proton, neutron, and electron

Antiparticles, like antiproton, antineutron, and positron

CPT theorem: particles and antiparticles have the same mass, the same lifetime, but opposite charges (electric, baryonic, leptonic, etc). Naturally, a matter is a substance which consists of particles, and antimatter is a substance consisting of antiparticles.

ETH, November 2, 2018 – p. 3 Baryon asymmetry in the present universe

Main questions: Why do the Earth, the Solar system and our galaxy consists of of matter and not of antimatter?

Why we do not see any traces of antimatter in the universe except of those where antiparticles are created in collisions of ordinary particles?

This looks really strange, as the properties of matter and antimatter are very similar.

ETH, November 2, 2018 – p. 4 Some history

ETH, November 2, 2018 – p. 5 Some history

Before 1930 : no antimatter - no problem.

ETH, November 2, 2018 – p. 5 Some history

Before 1930 : no antimatter - no problem. The only known elementary particles were protons, neutrons, electrons and photons

ETH, November 2, 2018 – p. 5 Some history

Before 1930 : no antimatter - no problem. The only known elementary particles were protons, neutrons, electrons and photons

1 Dirac: equations for spin 2 particle which pre- dicted the existence of ”electron" with positive electric charge - positron

ETH, November 2, 2018 – p. 5 Dirac’s Nobel Prize Lecture, 1933:

“If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regards it rather as an accident that the Earth (and presumably the whole solar system), contains a predominance of electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods."

ETH, November 2, 2018 – p. 6 Detection of antimatter

Dirac was wrong: cosmological observations do not support the hypothesis that the distant stars and galaxies may consist of antimatter. There are several methods for detection of antimatter:

The search of antinuclei in cosmic rays: the probability of the process

pp → antinuclei + etc is very small! Result: no antinuclei in cosmic rays have been found. However, antiprotons have been observed

pp → ppp + p¯ + etc

ETH, November 2, 2018 – p. 7 Detection of antimatter

The antiproton/proton ratio from AMS experiment:

ETH, November 2, 2018 – p. 8 Detection of antimatter

AMS mounted in the back of the Space Shuttle Payload Bay (May 1998)

ETH, November 2, 2018 – p. 9 Detection of antimatter

Annihilation: particles and antiparticles annihilate:

pp¯ → π+ π− π0 → γγ + ← ← −

νµµ µ ν¯µ + ← ← − e νeν¯µ e ν¯eνµ

Detection of γ-rays?

hEγ i ∼ (2GeV )/(5 − 6)/2 ∼ 150MeV

However, this has not been observed!

ETH, November 2, 2018 – p. 10 Detection of antimatter

Data and expectations for the diffuse γ-ray spectrum (upper curve d = 20 Mpc, lower curve d = 1000 Mpc)

100

-1 ] 10 -1 sr

-1 10-2 MeV

-1 s -2 10-3

10-4 COMPTEL -5 Schönfelder et al. (1980) Flux [photons cm 10 Trombka et al. (1977) White et al. (1977) 10-6 1 10 Photon Energy [MeV] ETH, November 2, 2018 – p. 11 Global baryon asymmetry

There are two possibilities:

Observed universe is asymmetric and does not contain any antimatter

The universe consists of domains of matter and antimatter separated by voids to prevent annihilation. The size of these zones should be greater than 1000 Mps, in order not to contradict observations of the diffuse γ spectrum.

The second option, however, contradicts to the large scale isotropy of the cosmic microwave background. Thus, we are facing the question: Why the universe is globally asymmetric?

ETH, November 2, 2018 – p. 12 Matter-antimatter asymmetry in particle physics

Till 1956: general believe that the nature is symmetric with respect to change of particle into antiparticle. C - charge conjugation: p ↔ p,¯ n ↔ n,¯ e− ↔ e+ P - parity transformation: ~x → −~x, ~p → −~p, but ~m → + ~m

1956: discovery of P and C breaking in weak interactions (Lee, Yang). Many manifestations, e.g. in π± decays. In particular, C- transformation change left-handed neutrino into left-handed antineutrino, which does not exist.

Conclusion: properties of particles and antiparticles are in fact (somewhat) different.

ETH, November 2, 2018 – p. 13 Matter-antimatter asymmetry in particle physics

However, the combined CP symmetry was believed to be exact: change particles to antiparticles AND simultaneously their momenta. Now, CP works for neutrino

CP : ν(~v) −→ ν¯(−~v)

So, still no solution for the problem of baryon asymmetry of the universe!

ETH, November 2, 2018 – p. 14 Matter-antimatter asymmetry in particle physics

1964 (Cronin, Fitch, Christenson, Turlay): decays of K0 mesons.

In a small fraction of cases (∼ 10−3), long- 0 0 lived KL (a mixture of K and K¯ decays into pair of two pions, what is forbidden by CP-conservation.

There are other manifestations of CP breaking. For example, if CP were exact symmetry, an equal number of K0 and K¯ 0 would produce an equal number of electrons and positrons in the reaction

0 − + 0 + − K → π e νe, K¯ → π e ν¯e,

ETH, November 2, 2018 – p. 15 Matter-antimatter asymmetry in particle physics

However, this is not the case: the number of positrons is somewhat larger (∼ 10−3) than the number of electrons.

Conclusion: so, there is indeed a tiny difference between particles and antiparticles, on the level of 10−3

How can this very small distinction be transformed in the 100% asymmetry of the universe we observe today?

ETH, November 2, 2018 – p. 16 Big Bang and baryon asymmetry

Cosmic microwave background

In 1965, Penzias and Wilson observed radio-waves in sub-millimeter range which were coming from all directions of the sky. They have a spectrum of black-body radiation with temperature 2.730.

Wavelength (cm) − 10 1.0 0.1 10 17

− 10 18 ) 1 −

Hz −19 1 10 − 2.726 K blackbody sr 2 − −20 10 FIRAS COBE satellite W m

( DMR COBE satellite

ν UBC sounding rocket I − 10 21 LBL-Italy White Mt. & South Pole Princeton ground & balloon Cyanogen optical − 10 22 110 100 1000 Frequency (GHz)

ETH, November 2, 2018 – p. 17 Big Bang and baryon asymmetry

Big Bang theory: Friedmann, Lemaitre, Gamov

ETH, November 2, 2018 – p. 18 Big Bang and baryon asymmetry

Why do we care? The existence of CMB is a proof of the Big Bang theory: universe expands and it was hot and dense in the past:

M0 1 MPl t = , t[sec] ∼ ,M0 = T 2 T 2[MeV ] 1.66N 1/2

Let us compute the baryon asymmetry of the universe

nB − n¯B ∆(t) = nB +n ¯B as a function of time. We already know that ∆(1010years) = 1.

ETH, November 2, 2018 – p. 19 Big Bang and baryon asymmetry

−6 Another interesting point: t0 ∼ 10 s (or T ∼ 1GeV ∼ mp).

Thermodynamics: nB ∼ n¯B ∼ nγ

nB − n¯B ∆(t0) = nγ

This ratio is in fact almost time-independent! Baryon number is conserved

3 (nB − nB¯ )R = const, where R is the scale factor, and the temperature of the CMB scales as 3 T ∼ 1/R =⇒ nγ R = const.

ETH, November 2, 2018 – p. 20 Big Bang and baryon asymmetry

So, to ﬁnd ∆(t0) just take nB/nγ today. We have

3 0 nγ ≃ 410 photons/cm (corresponds to temperature 2.73 K)

3 nB ≃ 0.25 nucleons/m

−10 =⇒ nB/nγ ≃ 6 × 10

Conclusion: Big Bang theory tells that the baryon asymmetry of the early universe is a very small number

(nB − n¯B) ∼ 10−10 (nB +n ¯B)

ETH, November 2, 2018 – p. 21 Big Bang and baryon asymmetry

At t ∼ 10−6s after the big Bang n − n BB 10 n + n for every 10 quarks we have B B 1 (1010 − 1) antiquarks. Some-

annihilation of what later the symmetric back- symmetric background

−10 ground annihilates into photons 10

−6 and neutrinos while the asym- 10 s t Dependence of baryon asymmetric part survives and gives metry on time rise to galaxies, stars, planets.

ETH, November 2, 2018 – p. 22 Big Bang and baryon asymmetry

Problems to solve:

Why in the early universe the number of baryons is greater than the number of antibaryons

How to compute the primordial baryon asymmetry? (from observations, ∼ 10−10)

Now, the comparison between of the baryon asymmetry in the early universe and the measure of the difference between particles and antiparticles in K0 decays (10−3) is much more comfortable!

ETH, November 2, 2018 – p. 23 Andrei Sakharov proposal, 1967:

“According to our hypothesis, the occurrence of C-asymmetry is the consequence of viola- tion of CP-invariance in the nonstationary ex- pansion of the hot universe during the super- dense stage, as manifest in the difference between the partial probabilities of the charge- conjugate reactions."

In short: the universe is asymmetric because baryon number is not conserved in C- and CP-violating reactions which produce more baryons than antibaryons in expanding universe. Consequence: Proton is not stable!

ETH, November 2, 2018 – p. 24 Sakharov model

Superheavy particles with mass m ∼ 1019 GeV = 10−5 grams, and lifetime 10−43 s Decay modes:

X → p p, p e+, X¯ → p¯ p¯, p¯ e−

If C and CP are broken, X and X¯ will produce different number of protons et antiprotons, as K0 and K¯ 0 produce the different numbers of electrons and positrons. It is sufﬁcient to produce a very small asymmetry 10−10 which will then be converted into 100% asymmetry later on.

ETH, November 2, 2018 – p. 25 Sakharov model