Spontaneous baryogenesis from axions with generic couplings

Axion-like particles can source the baryon asymmetry of our Universe through spontaneous baryogenesis. Here we clarify that this is a generic outcome for essentially any coupling of an axion-like particle to the Standard Model, requiring only a non-zero velocity of the classical axion field while baryon or lepton number violating interactions are present in thermal bath. In particular, coupling the axions only to gluons is sufficient to generate a baryon asymmetry in the presence of electroweak sphalerons or the Weinberg operator. Deriving the transport equation for an arbitrary set of couplings of the axion-like particle, we provide a general framework in which these results can be obtained immediately. If all the operators involved are efficient, it suffices to solve an algebraic equation to obtain the final asymmetries. Otherwise one needs to solve a simple set of differential equations. This formalism clarifies some theoretical subtleties such as redundancies in the axion coupling to the Standard Model particles associated with a field rotation. We demonstrate how our formalism automatically evades potential pitfalls in the calculation of the final baryon asymmetry.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case

Kyrylo Bondarenko, Alexey Boyarsky, Marco Drewes, Shintaro Eijima, Oleg Ruchayskiy, Lesya Shchutska, Anurag Tripathi

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, tau -> 3 mu and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Institute of Physics2016

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

Exploring beyond the Standard Model: the Heavy and the Dark

Kin Shanho Eliaou Mimouni

Physics beyond the Standard Model can appear as new particles too heavy to be produced in experiments (the energy frontier) or interacting too weakly to be seen in our measurements (the intensity frontier). In the first part of this thesis, we study two dark matter models of electroweak WIMP coming from two limiting cases in the MSSM: the Higgsino and the Wino model. The dark matter candidate is a particle too heavy to be directly seen in colliders but it could be seen indirectly by astrophysical observations. Dark matter particles in the galaxy can annihilate to Standard Model particles and be detected by satellites. We consider the charged neutral mass splitting as a free parameter in the theory and investigate its effect on the Sommerfeld enhancement which gives an important boost to the annihilation cross-section, the main observable of indirect detection. In the second part of this thesis, we present an idea of a fixed target experiment so search for dark sector models, a class of model with very weakly interacting particles. This experiment uses the experimental setup needed for a future muon collider but is independent from the muon collider program. We study the reach of our experiment for three benchmark models, the dark photon, the dark Higgs, the heavy neutral lepton and we compare its expected performances to current and future experiments. In the third part of this thesis, we consider a more model-independent approach to the search for new physics. If new states are heavy, they can be integrated out and their leading effects are encoded in effective operators made of Standard Model fields. Assuming baryon and lepton number conservation, one can classify the effective operators of dimension-6 which give the leading contribution and constrain the coefficient of these operators by looking at precision observables. We compile results from LEP-I and LEP-II experiments as well as neutrino scattering and other low-energy observables. We allow all operators to be present with an arbitrary flavour structure. Our result can then be used to translate these constaints to specific models of new physics.