arXiv subjects

Andrew J. Long

Publications and source records attributed to Andrew J. Long.

79 records · Page 5Linked to original sources

arXiv2013

The Lee-Wick Standard Model at Finite Temperature

The Lee-Wick Standard Model at temperatures near electroweak scale is considered, with the aim of studying the electroweak phase transition. While Lee-Wick theories possess states of negative norm, they are not pathological but instead are treated by imposing particular boundary conditions and using particular integration contours in the calculation of S-matrix elements. It is not immediately clear how to extend this prescription to formulate the theory at finite temperature; we explore two different pictures of finite-temperature LW theories, and calculate the thermodynamic variables and the (one-loop) thermal effective potential. We apply these results to study the Lee-Wick Standard Model and find that the electroweak phase transition is a continuous crossover, much like in the Standard Model. However, the high-temperature behavior is modified due to cancellations between thermal corrections arising from the negative- and positive-norm states.

hep-ph↗

arXiv2013

Gravitino Leptogenesis

We explore here a new mechanism by which the out of equilibrium decay of heavy gravitinos, followed by possible R-parity violating decays in the Minimal Supersymmetric Standard Model (MSSM) can generate the baryon asymmetry of the universe. In this mechanism, gravitino decay produces a CP-asymmetry that is carried by squarks or sleptons. These particles then decay through R-parity violating operators generating a lepton asymmetry. The lepton asymmetry is converted into a baryon asymmetry by weak sphalerons, as in the familiar case of leptogenesis by Majorana neutrino decays. To ensure that the gravitino decays while the sphaleron is still in equilibrium, we obtain a lower bound on the gravitino mass, $m_{3/2} \gtrsim 10^{8} \GeV$, and therefore our mechanism requires a high scale of SUSY breaking, as well as minimum reheating temperature after inflation of $T\gtrsim 10^{12} \GeV$ in order to for the gravitino density to be sufficiently large to generate the baryon asymmetry today. We consider each of the MSSM's relevant R-parity violating operators in turn, and derive constraints on parameters in order to give rise to a baryon asymmetry comparable to that observed today, consistent with low energy phenomenological bounds on SUSY models.

hep-ph↗

arXiv2013

The 125 GeV Higgs and Electroweak Phase Transition Model Classes

Recently, the ATLAS and CMS detectors have discovered a bosonic particle which, to a reasonable degree of statistical uncertainty, fits the profile of the Standard Model Higgs. One obvious implication is that models which predict a significant departure from Standard Model phenomenology, such as large exotic (e.g., invisible) Higgs decay or mixing with a hidden sector scalar, are already ruled out. This observation threatens the viability of electroweak baryogenesis, which favors, for example, a lighter Higgs and a Higgs coupled to or mixed with light scalars. To assess the broad impact of these constraints, we propose a scheme for classifying models of the electroweak phase transition and impose constraints on a class-by-class basis. We find that models, such as the MSSM, which rely on thermal loop effects are severely constrained by the measurement of a 125 GeV Higgs. Models which rely on tree-level effects from a light singlet are also restricted by invisible decay and mixing constraints. Moreover, we find that the parametric region favored by electroweak baryogenesis often coincides with an enhanced symmetry point with a distinctive phenomenological character. In particular, enhancements arising through an approximate continuous symmetry are phenomenologically disfavored, in contrast with enhancements from discrete symmetries. We also comment on the excess of diphoton events observed by ATLAS and CMS. We note that although Higgs portal models can accommodate both enhanced diphoton decay and a strongly first order electroweak phase transition, the former favors a negative Higgs portal coupling whereas the latter favors a positive one, and therefore these two constraints are at tension with one another.

hep-ph↗

arXiv2012

Strongly First Order Phase Transitions Near an Enhanced Discrete Symmetry Point

We propose a group theoretic condition which may be applied to extensions of the Standard Model in order to locate regions of parameter space in which the electroweak phase transition is strongly first order, such that electroweak baryogenesis may be a viable mechanism for generating the baryon asymmetry of the universe. Specifically, we demonstrate that the viable corners of parameter space may be identified by their proximity to an enhanced discrete symmetry point. At this point, the global symmetry group of the theory is extended by a discrete group under which the scalar sector is non-trivially charged, and the discrete symmetry is spontaneously broken such that the discrete symmetry relates degenerate electroweak preserving and breaking vacua. This idea is used to investigate several specific models of the electroweak symmetry breaking sector. The phase transitions identified through this method suggest implications for other relics such as dark matter and gravitational waves.

hep-ph↗

arXiv2011

Cosmological Constant, Dark Matter, and Electroweak Phase Transition

Accepting the fine tuned cosmological constant hypothesis, we have recently proposed that this hypothesis can be tested if the dark matter freeze out occurs at the electroweak scale and if one were to measure an anomalous shift in the dark matter relic abundance. In this paper, we numerically compute this relic abundance shift in the context of explicit singlet extensions of the Standard Model and explore the properties of the phase transition which would lead to the observationally most favorable scenario. Through the numerical exploration, we explicitly identify a parameter space in a singlet extension of the standard model which gives order unity observable effects. We also clarify the notion of a temperature dependence in the vacuum energy.

astro-ph.CO↗

arXiv2011

Probing the Cosmological Constant and Phase Transitions with Dark Matter

The Standard Model and its extensions predict multiple phase transitions in the early universe. In addition to the electroweak phase transition, one or several of these could occur at energies close to the weak scale. Such phase transitions can leave their imprint on the relic abundance of TeV-scale dark matter. In this paper, we enumerate several physical features of a generic phase transition and parameterize the effect of each on the relic abundance. In particular, we include among these effects the presence of the scalar field vacuum energy and the cosmological constant, which is sensitive to UV physics. Within the context of the Standard Model Higgs sector, we find that the relic abundance of generic TeV-scale dark matter is affected by the vacuum energy at the order of a fraction of a percent. For scalar field sectors with strong first order phase transitions, an order one percent apparent tuning of coupling constants may allow corrections induced by the vacuum energy to be of order unity.

astro-ph.CO↗

arXiv2010

Electroweak Phase Transition in the munuSSM

An extension of the MSSM called the munuSSM does not allow a conventional thermal leptogenesis scenario because of the low scale seesaw that it utilizes. Hence, we investigate the possibility of electroweak baryogenesis. Specifically, we identify a parameter region for which the electroweak phase transition is sufficiently strongly first order to realize electroweak baryogenesis. In addition to transitions that are similar to those in the NMSSM, we find a novel class of phase transitions in which there is a rotation in the singlet vector space.

hep-ph↗