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De Romeri, V., Karamitros, D., Lebedev, O., & Toma, T. (2020). Neutrino dark matter and the Higgs portal: improved freeze-in analysis. J. High Energy Phys., 10(10), 137–41pp.
Abstract: Sterile neutrinos are one of the leading dark matter candidates. Their masses may originate from a vacuum expectation value of a scalar field. If the sterile neutrino couplings are very small and their direct coupling to the inflaton is forbidden by the lepton number symmetry, the leading dark matter production mechanism is the freeze-in scenario. We study this possibility in the neutrino mass range up to 1 GeV, taking into account relativistic production rates based on the Bose-Einstein statistics, thermal masses and phase transition effects. The specifics of the production mechanism and the dominant mode depend on the relation between the scalar and sterile neutrino masses as well as on whether or not the scalar is thermalized. We find that the observed dark matter abundance can be produced in all of the cases considered. We also revisit the freeze-in production of a Higgs portal scalar, pointing out the importance of a fusion mode, as well as the thermalization constraints.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Search for resonances in diphoton events at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 09(9), 001–50pp.
Abstract: Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on protonproton collision data corresponding to an integrated luminosity of 3.2 fb(-1) at root s = 13TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall-Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13TeV and 8TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2020). Searches for low-mass dimuon resonances. J. High Energy Phys., 10(10), 156–26pp.
Abstract: Searches are performed for a low-mass dimuon resonance, X, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data sample corresponding to an integrated luminosity of 5.1 fb(-1) and collected with the LHCb detector. The X bosons can either decay promptly or displaced from the proton-proton collision, where in both cases the requirements placed on the event and the assumptions made about the production mechanisms are kept as minimal as possible. The searches for promptly decaying X bosons explore the mass range from near the dimuon threshold up to 60 GeV, with nonnegligible X widths considered above 20 GeV. The searches for displaced X -> μ(+)mu (-) decays consider masses up to 3 GeV. None of the searches finds evidence for a signal and 90% confidence-level exclusion limits are placed on the X -> μ(+)mu (-) cross sections, each with minimal model dependence. In addition, these results are used to place world-leading constraints on GeV-scale bosons in the two-Higgs-doublet and hidden-valley scenarios.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Search for Higgs bosons decaying into new spin-0 or spin-1 particles in four-lepton final states with the ATLAS detector with 139 fb(-1) of pp collision data at root s=13 TeV. J. High Energy Phys., 03(3), 041–64pp.
Abstract: Searches are conducted for new spin-0 or spin-1 bosons using events where a Higgs boson with mass 125 GeV decays into four leptons (l = e, mu). This decay is presumed to occur via an intermediate state which contains two on-shell, promptly decaying bosons: H -> XX/ZX 4l, where the new boson X has a mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 139 fb(-1) at a centre-of-mass energy root s = 13 TeV. The data are found to be consistent with Standard Model expectations. Limits are set on fiducial cross sections and on the branching ratio of the Higgs boson to decay into XX/ZX, improving those from previous publications by a factor between two and four. Limits are also set on mixing parameters relevant in extensions of the Standard Model containing a dark sector where X is interpreted to be a dark boson.
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Sierra, D. A., De Romeri, V., Flores, L. J., & Papoulias, D. K. (2021). Axionlike particles searches in reactor experiments. J. High Energy Phys., 03(3), 294–38pp.
Abstract: Reactor neutrino experiments provide a rich environment for the study of axionlike particles (ALPs). Using the intense photon flux produced in the nuclear reactor core, these experiments have the potential to probe ALPs with masses below 10MeV. We explore the feasibility of these searches by considering ALPs produced through Primakoff and Compton-like processes as well as nuclear transitions. These particles can subsequently interact with the material of a nearby detector via inverse Primakoff and inverse Compton-like scatterings, via axio-electric absorption, or they can decay into photon or electron-positron pairs. We demonstrate that reactor-based neutrino experiments have a high potential to test ALP-photon couplings and masses, currently probed only by cosmological and astrophysical observations, thus providing complementary laboratory-based searches. We furthermore show how reactor facilities will be able to test previously unexplored regions in the similar to MeV ALP mass range and ALP-electron couplings of the order of gaee similar to 10(-8) as well as ALP-nucleon couplings of the order of g (1) ann similar to 10(-9), testing regions beyond TEXONO and Borexino limits.
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de Gouvea, A., De Romeri, V., & Ternes, C. A. (2021). Combined analysis of neutrino decoherence at reactor experiments. J. High Energy Phys., 06(6), 042–12pp.
Abstract: Reactor experiments are well suited to probe the possible loss of coherence of neutrino oscillations due to wave-packets separation. We combine data from the short-baseline experiments Daya Bay and the Reactor Experiment for Neutrino Oscillation (RENO) and from the long baseline reactor experiment KamLAND to obtain the best current limit on the reactor antineutrino wave-packet width, sigma > 2.1 x 10(-4) nm at 90% CL. We also find that the determination of standard oscillation parameters is robust, i.e., it is mostly insensitive to the presence of hypothetical decoherence effects once one combines the results of the different reactor neutrino experiments.
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Agarwalla, S. K., Prakash, S., & Sankar, S. U. (2013). Resolving the octant of theta(23) with T2K and NOvA. J. High Energy Phys., 07(7), 131–24pp.
Abstract: Preliminary results of MINOS experiment indicate that theta(23) is not maximal. Global fits to world neutrino data suggest two nearly degenerate solutions for theta(23): one in the lower octant (LO: theta(23) < 45 degrees) and the other in the higher octant (HO: theta(23) > 45 degrees). v(mu) -> v(e) oscillations in superbeam experiments are sensitive to the octant and are capable of resolving this degeneracy. We study the prospects of this resolution by the current T2K and upcoming NOvA experiments. Because of the hierarchy-delta(CP) degeneracy and the octant delta(CP) degeneracy, the impact of hierarchy on octant resolution has to be taken into account. As in the case of hierarchy determination, there exist favorable (unfavorable) values of delta(CP) for which octant resolution is easy (challenging). However, for octant resolution the unfavorable delta(CP) values of the neutrino data are favorable for the anti-neutrino data and vice-verse. This is in contrast to the case of hierarchy determination. In this paper, we compute the combined sensitivity of T2K and NOvA to resolve the octant ambiguity. If sin(2)theta(23) – 0.41, then NOvA can rule out all the values of theta(23) in HO at 2 sigma C.L., irrespective of the hierarchy and delta(CP). Addition of T2K data improves the octant sensitivity. If T2K were to have equal neutrino and anti-neutrino runs of 2.5 years each, a 2 sigma resolution of the octant becomes possible provided sin(2) theta(23) <= 0.43 or >= 0.58 for any value of delta(CP).
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Falkowski, A., Gonzalez-Alonso, M., & Naviliat-Cuncic, O. (2021). Comprehensive analysis of beta decays within and beyond the Standard Model. J. High Energy Phys., 04(4), 126–36pp.
Abstract: Precision measurements in allowed nuclear beta decays and neutron decay are reviewed and analyzed both within the Standard Model and looking for new physics. The analysis incorporates the most recent experimental and theoretical developments. The results are interpreted in terms of Wilson coefficients describing the effective interactions between leptons and nucleons (or quarks) that are responsible for beta decay. New global fits are performed incorporating a comprehensive list of precision measurements in neutron decay, superallowed 0(+)-> 0(+) transitions, and other nuclear decays that include, for the first time, data from mirror beta transitions. The results confirm the V-A character of the interaction and translate into updated values for V-ud and g(A) at the 10(-4) level. We also place new stringent limits on exotic couplings involving left-handed and right-handed neutrinos, which benefit significantly from the inclusion of mirror decays in the analysis.
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Pich, A. (2014). Precision tau physics. Prog. Part. Nucl. Phys., 75, 41–85.
Abstract: Precise measurements of the lepton properties provide stringent tests of the Standard Model and accurate determinations of its parameters. We overview the present status of tau physics, highlighting the most recent developments, and discuss the prospects for future improvements. The leptonic decays of the tau lepton probe the structure of the weak currents and the universality of their couplings to the W boson. The universality of the leptonic Z couplings has also been tested through Z -> l(+)l(-) decays. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interaction in very clean conditions. Accurate determinations of the QCD coupling and the Cabibbo mixing V-us have been obtained with tau data. The large mass of the tau opens the possibility to study many kinematically-allowed exclusive decay modes and extract relevant dynamical information. Violations of flavour and CP conservation laws can also be searched for with tau decays. Related subjects such as μdecays, the electron and muon anomalous magnetic moments, neutrino mixing and B-meson decays into tau leptons are briefly covered. Being one the fermions most strongly coupled to the scalar sector, the tau lepton is playing now a very important role at the LHC as a tool to test the Higgs properties and search for new physics at higher scales.
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Alimena, J. et al, Hirsch, M., Mamuzic, J., Mitsou, V. A., & Santra, A. (2020). Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider. J. Phys. G, 47(9), 090501–226pp.
Abstract: Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments-as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER-to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity 'dark showers', highlighting opportunities for expanding the LHC reach for these signals.
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