Kimmo Kainulainen

Update on scalar singlet dark matter

One of the simplest models of dark matter is where a scalar singlet field S comprises some or all of the dark matter and interacts with the standard model through an vertical bar H vertical bar S-2(2) coupling to the Higgs boson. We update the present limits on the model from LHC searches for invisible Higgs decays, the thermal relic density of S, and dark matter searches via indirect and direct detection. We point out that the currently allowed parameter space is on the verge of being significantly reduced with the next generation of experiments. We discuss the impact of such constraints on possible applications of scalar singlet dark matter, including a strong electroweak phase transition, and the question of vacuum stability of the Higgs potential at high scales.

Stringent cosmological bounds on inert neutrino mixing

We study neutrino oscillations in the early universe induced by a non-trivial vacuum mixing between an inert SU(2)×U(1) singlet and a weakly interacting doublet neutrino. Excluded regions in the (sin2 2θ0 − δm2) plane are determined carefully for mixing involving νe, νγ and ντ neutrinos with an inert partner. The bounds are much more stringent than those obtained from earthbound experiments. Much of the large-angle MSW-solution parameter range is excluded and severe constraints are placed on the proposed 17 keV netrino.

Supersymmetric electroweak baryogenesis

We re-examine the generation of the baryon asymmetry in the minimal supersymmetric standard model (MSSM) during the electroweak phase transition. We find that the dominant source for baryogenesis arises from the chargino sector. The CP-violation comes from the complex phase in the μ parameter, which provides CP-odd contributions to the particle dispersion relations. This leads to different accelerations for particles and antiparticles in the wall region which, combined with diffusion, leads to the separation of higgsinos and their antiparticles in the front of the wall. These asymmetries get transported to produce perturbations in the left-handed chiral quarks, which then drive sphaleron interactions to create the baryon asymmetry. We present a complete derivation of the semiclassical WKB formalism, including the chargino dispersion relations and a self-consistent derivation of the diffusion equations starting from semiclassical Boltzmann equations for WKB-excitations. We stress the advantages of treating the transport equations in terms of the manifestly gauge invariant physical energy and kinetic momentum, rather than in the gauge variant canonical variables used in previous treatments. We show that a large enough baryon asymmetry can be created for the phase of the complex μ-parameter as small as ~ 10−3, which is consistent with bounds from the neutron electric dipole moment.

Supersymmetric electroweak baryogenesis in the WKB approximation

Abstract We calculate the baryon asymmetry generated at the electroweak phase transition in the minimal supersymmetric standard model, treating the particles in a WKB approximation in the bubble wall background. A set of diffusion equations for the particle species relevant to baryon generation, including source terms arising from the CP violation associated with the complex phase δ of the μ parameter, are derived from Boltzmann equations, and solved. The conclusion is that δ must be ≳0.1 to generate a baryon asymmetry consistent with nucleosynthesis. We compare our results to several other recent computations of the effect, arguing that some are overestimates.

Electroweak baryogenesis and dark matter from a singlet Higgs

If the Higgs boson H couples to a singlet scalar S via ?m|H|2S2, a strong electroweak phase transition can be induced through a large potential barrier that exists already at zero temperature. In this case properties of the phase transition can be computed analytically. We show that electroweak baryogenesis can be achieved using CP violation from a dimension-6 operator that couples S to the top-quark mass, suppressed by a new physics scale that can be well above 1 TeV. Moreover the singlet is a dark matter candidate whose relic density is 3% of the total dark matter density, but which nevertheless interacts strongly enough with nuclei (through Higgs exchange) to be just below the current XENON100 limits. The DM mass is predicted to be in the range 80?160 GeV.

Refraction and oscillations of neutrinos in the early universe

We have investigated the behaviour of the electron neutrinos νc and antineutrinos νc in the heat bath of the early Universe. We have determined the effective energies of νc and νc as functions of the temperature of the Universe. We assume that νc and νc mix with inert states νx and νinx, respectively, and consider the νc ↔ νx and νc ↔ νx oscillations. We have paid special attention to the adiabaticity and coherence of these transitions and have found that for a large region of the parameter space the system will evolve through a Mikheyev-Smirnov resonance. The numerical study of the combined neutrino and antineutrino systems shows that the dynamical lepton asymmetry is driven to zero. Thus no significant νc t νc asymmetry can be created. If a resonance exists, both neutrinos and antineutrinos will go through it simultaneously. A large conversion of the νc and νc populations in the early Universe into the inert states νx and νx is thus possible.

Light singlet neutrinos and the primordial nucleosynthesis

Abstract We study the production of light SU (2)-singlet neutrinos in the early universe. It is shown that a singlet neutrino v x may be brought into thermal equilibrium if it mixes with an ordinary doublet neutrino. The nucleosynthesis limit on the number of the light neutrinos then places more stringent limits on the oscillation parameters than what has been obtained in the earthbound experiments. The v e −v x oscillation still remains a viable solution for the solar neutrino problem.

First principle derivation of semiclassical force for electroweak baryogenesis

We perform a systematic gradient expansion on kinetic equations and derive the CP-violating semiclassical force for fermions propagating in presence of a CP-violating wall at a first order electroweak phase transition. The force appears at order in the flow term of the kinetic equation and agrees with the semiclassical force used for baryogenesis computations. In particular we consider the force for charginos in both the MSSM and NMSSM. We then study the continuity equations for the vector and axial vector currents and stress the role of the latter as the one containing baryogenesis sources. We also show that there is no CP-violating force for bosons to order in gradient expansion.

New source for electroweak baryogenesis in the minimal supersymmetric standard model.

One of the most experimentally testable explanations for the origin of the baryon asymmetry of the Universe is that it was created during the electroweak phase transition, in the minimal supersymmetric standard model. Previous efforts have focused on the current for the difference of the two Higgsino fields, H1-H2, as the source of biasing sphalerons to create the baryon asymmetry. We point out that the current for the orthogonal linear combination, H1+H2, is larger by several orders of magnitude. Although this increases the efficiency of electroweak baryogenesis, we nevertheless find that large CP-violating angles > or = 0.15 are required to get a large enough baryon asymmetry.I review a computation of the baryon asymmetry arising from a first order elec-troweak phase transition in the Minimal Supersymmetric Standard model by classical force mechanism (CFM). I focus on CP violation provided by the charginos and show that it is the usually neglected sum of the two Higgsino fields, H 1 + H 2 , which gives a larger contribution to the baryon asymmetry than does the combination H 1 − H 2. In fact, the latter contribution is exactly zero in CFM, because it is associated with a phase transformation of the fields. Baryogenesis is found to be most effective in MSSM CFM when only˜t R is light, which lends independent support for the light stop scenario , and it remains viable for CP-violating phases as small as δµ ∼ few × 10 −3 .

Electroweak baryogenesis in two Higgs doublet models and B meson anomalies

A bstractMotivated by 3.9σ evidence of a CP-violating phase beyond the standard model in the like-sign dimuon asymmetry reported by D∅, we examine the potential for two Higgs doublet models (2HDMs) to achieve successful electroweak baryogenesis (EWBG) while explaining the dimuon anomaly. Our emphasis is on the minimal flavour violating 2HDM, but our numerical scans of model parameter space include type I and type II models as special cases. We incorporate relevant particle physics constraints, including electroweak precision data, bu2009→u2009sγ, the neutron electric dipole moment, Rb, and perturbative coupling bounds to constrain the model. Surprisingly, we find that a large enough baryon asymmetry is only consistently achieved in a small subset of parameter space in 2HDMs, regardless of trying to simultaneously account for any B physics anomaly. There is some tension between simultaneous explanation of the dimuon anomaly and baryogenesis, but using a Markov chain Monte Carlo we find several models within 1σ of the central values. We point out shortcomings with previous studies that reached different conclusions. The restricted parameter space that allows for EWBG makes this scenario highly predictive for collider searches. We discuss the most promising signatures to pursue at the LHC for EWBG-compatible models.