Decay Of Massive Particles Research Articles

Triggering On Hadronic Signatures In The Atlas Experiment-Developments For 2017 And 2018

Hadronic signatures are critical to the ATLAS physics program, and are used extensively for both the StandardModel measurements and searches for new physics. These signatures include generic quark and gluon jets, as well as jets originating from b-quarks or the decay of massive particles (such as electroweak bosons or top quarks). Additionally, missing transverse momentum from non-interacting particles provides an interesting probe in the search for new physics beyond the Standard Model. Developing trigger selections that target these events is a hugechallenge at the LHC due to the enormous rates associated with hadronic signatures. This challenge is exacerbated by the amount of pile-up activity, which continues to grow. In order to address these challenges, several new techniques were developed to significantly improve the potential of the 2017 dataset. An overview of how we triggeron hadronic signatures at the ATLAS experiment is presented, outlining the challenges of hadronic object triggering and describing the improvements performed over the course of the Run 2 LHC data-taking program. The performance in Run 2 data is shown, including demonstrations of the new techniques being used in 2017. We also discuss further critical developments implemented for the rest of Run 2 and their performance in early 2018 data.

Hybrid quasi-single field inflation

The decay of massive particles during inflation generates characteristic signals in the squeezed limit of the primordial bispectrum. These signals are in particular distinctive in the regime of the quasi-single field inflation, where particles are oscillating with masses comparable to the Hubble scale. We apply the investigation to a class of scalar particles that experience a so-called waterfall phase transition in the isocurvature direction driven by the symmetry-breaking mechanism based on the hybrid inflation scenario. With a time-varying mass, a novel shape of oscillatory bispectrum is presented as the signature of a waterfall phase transition during inflation.

Cosmological implications of high-energy neutrino emission from the decay of long-lived particle

We study cosmological scenarios in which high-energy neutrinos are emitted from the decay of long-lived massive particles at the cosmic time later than a redshift of 10^6. The high-energy neutrino events recently observed by the IceCube experiment suggest a new source of high-energy cosmic-ray neutrinos; decay of a heavy particle can be one of the possibilities. We calculate the spectrum of the high-energy neutrinos emitted from the decay of long-lived particles, taking account of the neutrino scattering processes with background neutrinos. Then, we derive bounds on the scenario using the observation of high-energy cosmic-ray neutrino flux. We also study constraints from the spectral distortions of the cosmic microwave background and the big-bang nucleosynthesis. In addition, we show that the PeV neutrinos observed by the IceCube experiment can originate from the decay of a massive particle with its mass as large as O(10^10 GeV).

Weak scale baryogenesis in a supersymmetric scenario with R-parity violation

We investigate the connection between R-parity violation (RPV) in supersymmetric models and Baryogenesis. First we discuss in detail the assumptions of a theorem by Nanopoulos and Weinberg on the CP asymmetry generated from the decay of massive particles. In light of this statement, we analyse some interesting models of Baryogenesis through RPV. We then explore, in the context of RPV SUSY, the possibility to generate the baryon asymmetry through the out-of-equilibrium decay of a metastable Weakly Interacting Massive Particle (WIMP), as proposed in \cite{Cui:2012jh}. This setting is also motivated by the observed coincidence between the abundances of dark and baryonic matter. In this framework, we propose two models of out-of-equilibrium decay of a would-be cold relic, and compute the associated CP asymmetry. With a TeV-scale parent, the observed baryon abundance can be reproduced in these models when the stop is in the multi-TeV region. Furthermore, annihilation of the metastable particle into SM states must be characterised by a very weak coupling, $g\sim 10^{-2}$ and by a heavy mediator $m_{med}\sim 10$ TeV. These models can also accommodate stop masses far from the TeV scale, at the expense of weakening the explanation of the $\Omega_{B}-\Omega_{DM}$ coincidence.

Must cosmological perturbations remain nonadiabatic after multifield inflation?

Even if nonadiabatic perturbations are generated in multifield inflation, the perturbations will become adiabatic if the Universe after inflation enters an era of local thermal equilibrium, with no nonzero conserved quantities, and will remain adiabatic as long as the wavelength is outside the horizon, even when local thermal equilibrium no longer applies. Small initial nonadiabatic perturbations associated with imperfect local thermal equilibrium remain small when baryons are created from out-of-equilibrium decay of massive particles, or when dark matter particles go out of local thermal equilibrium.

Cascade nucleosynthesis in the early universe

We describe a calculation of electromagnetic cascading in radiation and matter in the early universe initiated by the decay of massive particles or by some other process. We have used a combination of Monte Carlo and numerical techniques which enables us to use exact cross sections, where known, for all the relevant processes. In cascades initiated after the epoch of big bang nucleosynthesis γ -rays in the cascades will photodisintegrate 4 He, producing 3 He and deuterium. Using the observed 3 He and deuterium abundances we are able to place constraints on the cascade energy deposition as a function of cosmic time. In the case of the decay of massive primordial particles, we place limits on their density as a function of their mean decay time.

Electromagnetic cascades and cascade nucleosynthesis in the early Universe.

We describe a calculation of electromagnetic cascading in radiation and matter in the early universe initiated by the decay of massive particles or by some other process. We have used a combination of Monte Carlo and numerical techniques which enables us to use exact cross sections, where known, for all the relevant processes. In cascades initiated after the epoch of big bang nucleosynthesis $\gamma$-rays in the cascades will photodisintegrate $^4$He, producing $^3$He and deuterium. Using the observed $^3$He and deuterium abundances we are able to place constraints on the cascade energy deposition as a function of cosmic time. In the case of the decay of massive primordial particles, we place limits on the density of massive primordial particles as a function of their mean decay time, and on the expected intensity of decay neutrinos.

Neutrino mass effects in a minimally extended supersymmetric standard model

We consider an extension of the supersymmetric standard model which includes singlet Higgs superfield representations (in three generations) to generate neutrino masses via the see-saw mechanism. The resulting theory may then exhibit R-parity violation in the couplings of the singlets, inducing R-parity violating effective interactions among the standard model superfields, as well as inducing decay of the lightest neutralino, which otherwise would compose a stable LSP. We compute the rates for the resulting neutralino decays, depending on the particular superpotential couplings responsible for the violation of R-parity. We compare to astrophysical constraints on the decay of massive particles.

Decay of massive particles in Robertson-Walker universes with statically bounded expansion laws

The authors study mutually interacting quantum fields in 3-flat Robertson-Walker universes. Using the interaction picture and an in-out approach based on an S matrix, they give an exact calculation of the decay of a massive scalar Phi particle into two massless scalar Psi particles (both conformally coupled to the background) in lowest order of the ( lambda /a( eta )) Phi Psi 2 interaction. The cosmic scale factor a( eta ) is thereby specified (i) as the step-expansion law, and (ii) as the tanh-expansion law which represents the prototype of a smoothed-out step law. These a( eta ) are typical representatives of monotonic expansions with asymptotically static in and out regions. The characteristic modifications of the Minkowskian decay spectrum like the double-resonance structure are discussed. Comparison of amplitudes and decay probabilities for (i) and (ii) demonstrates that smoothing-out the step introduces a gravitational influence of non-vanishing duration, which leads essentially to finite corrections of the step results. Additionally, it can be shown that the massive particles which are gravitationally created out of the background, decay as well and contribute to the number of outgoing massless particles. From the results obtained, the general structure of amplitudes, probabilities, and decay rates holding for all statically bounded monotonic expansion laws can be read off.