Reconstruction of stop quark mass at the LHC

The cascade mass reconstruction approach was applied to simulated production of the lightest stop quark at the LHC in the cascade decay \tilde{g} \to \sTop \, \Top \to \tilde{\chi}_{2}^{0} \, \Top \, \Top \to \tilde{\ell}_{R} \, \ell \, \Top \, \Top \to \tilde{\chi}_{1}^{0} \, \ell \, \ell \, \Top \, \Top with top quarks decaying into hadrons. The stop quark mass was reconstructed assuming that the masses of gluino, slepton and of the two lightest neutralinos were reconstructed in advance. A data sample set for the SU3 model point containing 400k SUSY events was generated which corresponded to an integrated luminosity of about 20 $\rm fb^{-1}$ at 14 TeV. These events were passed through the AcerDET detector simulator, which parametrized the response of a generic LHC detector. The mass of the $\tilde{t}_{1}$ was reconstructed with a precision of about $10\%$.Comment: 6 pages, 6 figure

A New Approach for Reconstructing SUSY Particle Masses with a few fb^(-1) at the LHC

We describe a new cascade mass reconstruction approach to allow reconstruction of SUSY particle masses in long cascades (five or more particles) at the LHC with integrated luminosity as low as a few fb^(-1). This approach is based on a consecutive use of the endpoint method, an event filter and a combinatorial mass reconstruction method. The endpoint method gives a preliminary estimate of light sparticle masses. An event filter combining the maximum likelihood distributions for all events in the data sample allows suppression of backgrounds and gives a preliminary estimate of heavy sparticle masses. Finally, SUSY particle masses are reconstructed by a search for a maximum of a combined likelihood function constructed for each possible combination of five events in the data sample. SUSY data sample sets for the SU3 model point containing 80k events each were generated, corresponding to an integrated luminosity of 4.2 fb^(-1). These events were passed through the AcerDET detector simulator, which parametrized the response of a detector. To demonstrate the stability and precision of the approach five different 80k event data sets were considered. Masses were reconstructed with a precision of about 10% for heavy sparticles and 10-20% for light sparticles.Comment: 23 pages, 9 figures, added 1 referenc

Reconstruction of the bubble nucleating potential

We calculate analytically the bubble nucleation rate in a model of first order inflation which is able to produce large scale structure. The computation includes the first-order departure from the thin-wall limit, the explicit derivation of the pre-exponential factor, and the gravitational correction. The resulting bubble spectrum is then compared with constraints from the large scale structure and the microwave background. We show that there are models which pass all the constraints and produce bubble-like perturbations of interesting size. Furthermore, we show that it is in principle possible to reconstruct completely the inflationary two-field potential from observations.Comment: Accepted for publication in Phys. Rev. D, 19 pages, 2 ps figs include

Robust constraint on Lorentz violation using Fermi-LAT gamma-ray burst data

Models of quantum gravity suggest that the vacuum should be regarded as a medium with quantum structure that may have nontrivial effects on photon propagation, including the violation of Lorentz invariance. Fermi Large Area Telescope (LAT) observations of gamma-ray bursts (GRBs) are sensitive probes of Lorentz invariance, via studies of energy-dependent timing shifts in their rapidly varying photon emissions. We analyze the Fermi-LAT measurements of high-energy gamma rays from GRBs with known redshifts, allowing for the possibility of energy-dependent variations in emission times at the sources as well as a possible nontrivial refractive index in vacuo for photons. We use statistical estimators based on the irregularity, kurtosis, and skewness of bursts that are relatively bright in the 100 MeV to multi-GeV energy band to constrain possible dispersion effects during propagation. We find that the energy scale characterizing a linear energy dependence of the refractive index should exceed a few×1017 GeV, and we estimate the sensitivity attainable with additional future sources to be detected by Fermi-LAT.</p

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements