Letters in High Energy Physics http://journals.andromedapublisher.com/index.php/LHEP Andromeda Publishing And Academic Services LTD en-US Letters in High Energy Physics 2632-2714 <p align="justify">Letters in High Energy Physics (LHEP) is an open access journal published by Andromeda Publishing and Education Services. The articles in LHEP are distributed according to the terms of <a href="http://creativecommons.org/licenses/by/4.0" target="_blank" rel="noopener">the creative commons license CC-BY 4.0</a>. Under the terms of this license, copyright is retained by the author while use, distribution and reproduction in any medium are permitted provided proper credit is given to original authors and sources.</p> <h2 style="margin-bottom: 0px; margin-top: 0px;">Terms of Submission</h2> <p style="margin-top: 0px; margin-bottom: 0px;" align="justify">By submitting an article for publication in LHEP, the submitting author asserts that:</p> <p style="margin-top: 0px; margin-bottom: 0px;" align="justify">1. The article presents original contributions by the author(s) which have not been published previously in a peer-reviewed medium and are not subject to copyright protection.</p> <p style="margin-top: 0px; margin-bottom: 0px;" align="justify">2. The co-authors of the article, if any, as well as any institution whose approval is required, agree to the publication of the article in LHEP.</p> The Thermodynamics of Relativistic Multi-Fluid Systems http://journals.andromedapublisher.com/index.php/LHEP/article/view/141 <p>This article extends the single-fluid relativistic irreversible thermodynamics theory of Müller, Israel and<br>Stewart (hereafter the MIS theory) to a multi-fluid system with inherent species interactions. This is illustrated<br>in a two-fluid toy-model where an effective complex 4-velocity plays the role of a primary dynamical<br>parameter. We find that an observer who resides in the real-part of this universe will notice that their<br>knowledge of the universe parametrized using real, rather than imaginary, quantities are insufficient to fully<br>determine properties such as the total energy density, pressure or entropy, In fact, such an observer will<br>deduce the existence of some negative energy that affects the expansion of their perceived real universe.</p> Bob Osano ##submission.copyrightStatement## http://creativecommons.org/licenses/by/4.0 2020-04-01 2020-04-01 10.31526/lhep.2020.141 Applying New Physics to the Problems of the \(\eta′\to\pi^0\gamma\gamma\) Decay http://journals.andromedapublisher.com/index.php/LHEP/article/view/156 <p>Rare decays of light mesons may be a discovery window for a new weakly coupled forces hidden at a low-energy QCD scale. BES-III Collaboration reported an observation of the rare decay \(\eta'&nbsp;\to\pi^0\gamma\gamma\). The observed decay width disagrees with the preliminary theoretical estimates. We show that this tension may be attributed to New Physics, presumably Dark Photon. For completeness, we consider a possible influence of New Physics on a similar well-measured decay \(\eta \to\pi^0\gamma\gamma\) and a recently measured one, \(\eta'&nbsp;\to\eta\gamma\gamma\), showing that the impact of the hypothetical Dark Photon may be also present in these decays.</p> Yaroslav Balytskyi ##submission.copyrightStatement## http://creativecommons.org/licenses/by/4.0 2020-05-26 2020-05-26 10.31526/lhep.2020.156 Effects of an H-μ-τ coupling in quarkonium lepton flavor violation decays http://journals.andromedapublisher.com/index.php/LHEP/article/view/152 <p>In this work we study the consistency of a possible non-vanishing coupling Hμτ smaller than 0.25% as pointed by the CMS collaborations [1], with measured lepton flavor violation processes involving quarkonium. We show that the most promising channel to confirm this excess is to look for the lepton flavor tau decay into a f<sub>0</sub> and μ where the experimental limit could strongly improved with the new B factories as Belle II.</p> David Delepine Mauro Napsuciale Eduardo Peinado ##submission.copyrightStatement## http://creativecommons.org/licenses/by/4.0 2020-06-19 2020-06-19 10.31526/lhep.2020.152