Matter-Antimatter asymmetry of the universe

Observations tell us that there is no antimatter in our universe. However, if we believe the Standard Model, we expect that there should have been equal amounts of matter and antimatter at the beginning. What happened to the antimatter? How did  it disappear? When did it disappear? In order to answer these questions we need to go beyond the Standard Model.

In order to produce a matter–antimatter asymmetry in the early universe, a particle physics model has to satisfy the following Sakharov Conditions: 1- Baryon number cannot be a conserved quantity. 2- Charge and Charge-Parity symmetries must be violated. 3- There must be processes that fall out of thermal equilibrium.  (The Standard Model does not satisfy conditions 2&3.)

In a 2016 paper I proposed a new mechanism that could generate this asymmetry. This was a supersymmetric model in which the supersymmetric partner of the photon, bino, is a pseudo-Dirac fermion. This particle can turn into its antiparticle and vice versa. (These processes are called particle-antiparticle oscillations.) However, these oscillations are not symmetric. It is easier for a particle to turn into an antiparticle than the opposite. This asymmetry in the oscillations can create a matter-antimatter asymmetry after the binos decay into Standard Model particles. 

My 2019 paper, published in PRL, proposes another novel way to address this question. Here we explore the possibility of changing the strong interactions in the early universe. New particles that interact with the gluon field can change the strength of the strong interactions in the early universe. We propose a scenario where the quarks and gluons confine into protons and neutrons (hadronic phase) at a temperature of ~TeV, much higher than what is expected in the Standard Model (~GeV). In such a universe, this confinement transition is a 1st order phase transition and proceeds via ‘bubble nucleation’. Bubbles of the hadronic phase start forming within the quark-gluon plasma phase. These bubble grow and merge with each other and eventually fill out the whole universe. Bubble nucleation can happen out of thermal equilibrium, satisfying one of the Sakharov conditions. There can also be large CP violation in the strong sector, via the axion. Finally, baryon number is violated via weak interactions at these high temperatures. Hence the observed matter-antimatter asymmetry is produced during the phase transition.

Dark matter

About 25% of our universe is made up of ‘dark matter’. It is called ‘matter’ because it interacts gravitationally with other massive particles and ‘dark’ because it does not emit any radiation we can detect. There are numerous ways we know dark matter exists. The first hints of dark matter came from galactic rotation curves, observed by Vera Rubin. Most of the visible matter in a galaxy is concentrated at the center. From Newtonian mechanics, we expect that as one gets away from the center, the rotational velocity of stars to get smaller. However this is not what is observed! Stars have the same (sometimes higher) velocity regardless of their distance from a galactic center. This means there is some extra gravitation force helping the stars go faster. This extra force can be explained with the existence of dark matter, distributed around a galaxy. Other experimental hints of dark matter include the Cosmic Microwave Background and the bullet cluster.

Although we know dark matter exists, we don’t know what it is or if it interacts with the Standard Model particles in any way other than gravitationally. There are many types of experiments searching for dark matter scattering off of atoms, or dark matter particles annihilating to produce radiation. (LHC is also searching for dark matter production at particle colliders.) One hint of such annihilations come from Fermi LAT, a telescope that sees gamma rays. There is some extra gamma rays coming from our galactic center, more than what is expected in astrophysics. It has been claimed that these photons are produced by dark matter annihilation to Standard Model particles. In 2014, my collaborators and I published a paper describing the minimal ingredients of a model that would produce this signal. We showed that there are many other signals we expect to besides the gamma rays and showed the constraints from non-observation of these signals.

In another 2014 paper we explored the possibility of dark matter interactions with neutrinos. We showed how these interactions can change the structure formation in the universe. There are small density perturbations in our universe, resulting in points of overdensities that can attract other masses due to gravity. However it is hard to compress Standard Model particles too much due to photon pressure. Hence, the Standard Model particles in the early universe go under a pull-push motion, producing “baryon acoustic oscillations”. If dark matter does not have any interactions with the Standard Model, it does not feel the photon pressure, only the gravitational pull. However, if the dark matter interacts with neutrinos, it also goes under these oscillations. This results in washing out small structure in the universe. Meaning, we expect a scale below which there are no galaxies. Our work assumed a cutoff scale of billion solar masses for the smallest galaxies. These interactions with the dark matter are also expected to change the spectrum of neutrinos coming from supernovae as they travel to Earth. A great opportunity to test this model would be when Eta Carinae goes supernova, which is expected to happen in a decade (or 10 million years)!

Neutrino masses

Neutrinos are very interesting particles. They are expected to be massless in the Standard Model because they do not interact directly with the Higgs field in contrast to, for example, electrons. It was quite a shock in the particle physics community when they were shown to have small but non-zero mass. (2015 Nobel Physics Prize was awarded for this discovery.) Explaining why neutrinos are massive requires beyond the Standard Model physics. Often physicists introduce new particles called right-handed neutrinos in order to couple the neutrinos to the Higgs field. However, we haven’t detected these new particles yet.

In a 2016 PRL paper my collaborator Pilar Coloma and I showed that in certain supersymmetric models, the supersymmetric particle bino can be responsible for generating neutrino masses. This model also requires a gravitino particle (supersymmetric partner of graviton) which has a mass about 10 keV and it could be the dark matter! In a recent paper we looked at the constraints coming from the LHC on our model. In minimal supersymmetric models the lightest supersymmetric particle is stable. However, in our model it decays to neutrinos and other Standard Model particles. This predicts very interesting signatures at the LHC. We recast current searches for final states with 4-6 jets and missing energy as well as final states with leptons and jets in order to constrain the parameters of our model. We show that, unlike in minimal supersymmetric models, squarks can be as light as 350 GeV in this model.