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1. Electroweak phase transition with spontaneous Z2-breaking

   https://doi.org/10.1007/JHEP08(2020)107  

   Carena, Marcela; Liu, Zhen; Wang, Yikun (August 2020, Journal of High Energy Physics)

   null (Ed.)

   A bstract This work investigates a simple, representative extension of the Standard Model with a real scalar singlet and spontaneous Z 2 breaking, which allows for a strongly first-order phase transition, as required by electroweak baryogenesis. We perform analytical and numerical calculations that systematically include one-loop thermal effects, Coleman-Weinberg corrections, and daisy resummation, as well as evaluation of bubble nucleation. We study the rich thermal history and identify the conditions for a strongly first-order electroweak phase transition with nearly degenerate extrema at zero temperature. This requires a light scalar with mass below 50 GeV. Exotic Higgs decays, as well as Higgs coupling precision measurements at the LHC and future collider facilities, will test this model. Additional information may be obtained from future collider constraints on the Higgs self-coupling. Gravitational-wave signals are typically too low to be probed by future gravitational wave experiments. 

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2. Nucleation is more than critical: A case study of the electroweak phase transition in the NMSSM

   https://doi.org/10.1007/JHEP03(2021)055  

   Baum, Sebastian; Carena, Marcela; Shah, Nausheen R.; Wagner, Carlos E.; Wang, Yikun (March 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract Electroweak baryogenesis is an attractive mechanism to generate the baryon asymmetry of the Universe via a strong first order electroweak phase transition. We compare the phase transition patterns suggested by the vacuum structure at the critical temperatures , at which local minima are degenerate, with those obtained from computing the probability for nucleation via tunneling through the barrier separating local minima. Heuristically, nucleation becomes difficult if the barrier between the local minima is too high, or if the distance (in field space) between the minima is too large. As an example of a model exhibiting such behavior, we study the Next-to-Minimal Supersymmetric Standard Model, whose scalar sector contains two SU(2) doublets and one gauge singlet. We find that the calculation of the nucleation probabilities prefers different regions of parameter space for a strong first order electroweak phase transition than the calculation based solely on the critical temperatures. Our results demonstrate that analyzing only the vacuum structure via the critical temperatures can provide a misleading picture of the phase transition patterns, and, in turn, of the parameter space suitable for electroweak baryogenesis. 

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3. Triple Higgs boson production and electroweak phase transition in the two-real-singlet model

   https://doi.org/10.1007/JHEP11(2024)077  

   Karkout, Osama; Papaefstathiou, Andreas; Postma, Marieke; Tetlalmatzi-Xolocotzi, Gilberto; van_de_Vis, Jorinde; du_Pree, Tristan (November 2024, Journal of High Energy Physics)

   The production of three Higgs bosons at hadron colliders can be enhanced by a double-resonant effect in the ℤ2-symmetric two-real-singlet extension of the Standard Model, making it potentially observable in future LHC runs. The production rate is maximized for large scalar couplings, which prompts us to carefully reconsider the perturbativity constraints on the theory. This leads us to construct a new set of 140 benchmark points that have a triple Higgs boson production cross-section at least 100 times larger than the SM value. Furthermore, we study the dynamics of the electroweak phase transition, both analytically at leading order, and numerically without the high-temperature expansion. Both analyses indicate that a first-order phase transition is incompatible with the requirement that both singlets have a non-zero vev in the present-day vacuum, as required by doubly-enhanced triple Higgs boson production. Allowing instead one of the singlets to remain at zero field value opens up the possibility of a first-order phase transition, while di-Higgs boson production can still be enhanced by a (single) resonance. 

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4. The muon Smasher’s guide

   https://doi.org/10.1088/1361-6633/ac6678  

   Al Ali, Hind; Arkani-Hamed, Nima; Banta, Ian; Benevedes, Sean; Buttazzo, Dario; Cai, Tianji; Cheng, Junyi; Cohen, Timothy; Craig, Nathaniel; Ekhterachian, Majid; et al (July 2022, Reports on Progress in Physics)

   Abstract We lay out a comprehensive physics case for a future high-energy muon collider, exploring a range of collision energies (from 1 to 100 TeV) and luminosities. We highlight the advantages of such a collider over proposed alternatives. We show how one can leverage both the point-like nature of the muons themselves as well as the cloud of electroweak radiation that surrounds the beam to blur the dichotomy between energy and precision in the search for new physics. The physics case is buttressed by a range of studies with applications to electroweak symmetry breaking, dark matter, and the naturalness of the weak scale. Furthermore, we make sharp connections with complementary experiments that are probing new physics effects using electric dipole moments, flavor violation, and gravitational waves. An extensive appendix provides cross section predictions as a function of the center-of-mass energy for many canonical simplified models. 

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5. Machine learning techniques for intermediate mass gap lepton partner searches at the large hadron collider

   https://doi.org/10.1103/PhysRevD.109.075018  

   Dutta, Bhaskar; Ghosh, Tathagata; Horne, Alyssa; Kumar, Jason; Palmer, Sean; Sandick, Pearl; Snedeker, Marcus; Stengel, Patrick; Walker, Joel W (April 2024, Physical Review D)

   We consider machine learning techniques associated with the application of a boosted decision tree (BDT) to searches at the Large Hadron Collider (LHC) for pair-produced lepton partners which decay to leptons and invisible particles. This scenario can arise in the minimal supersymmetric Standard Model (MSSM), but can be realized in many other extensions of the Standard Model (SM). We focus on the case of intermediate mass splitting ( $\sim 30\mathrm{GeV}$) between the dark matter (DM) and the scalar. For these mass splittings, the LHC has made little improvement over LEP due to large electroweak backgrounds. We find that the use of machine learning techniques can push the LHC well past discovery sensitivity for a benchmark model with a lepton partner mass of $\sim 110\mathrm{GeV}$, for an integrated luminosity of $300{\mathrm{fb}}^{-1}$, with a signal-to-background ratio of $\sim 0.3$. The LHC could exclude models with a lepton partner mass as large as $\sim 160\mathrm{GeV}$with the same luminosity. The use of machine learning techniques in searches for scalar lepton partners at the LHC could thus definitively probe the parameter space of the MSSM in which scalar muon mediated interactions between SM muons and Majorana singlet DM can both deplete the relic density through dark matter annihilation and satisfy the recently measured anomalous magnetic moment of the muon. We identify several machine learning techniques which can be useful in other LHC searches involving large and complex backgrounds. Published by the American Physical Society2024 

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