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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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Ellis, J., Gomez, M. E., Lola, S., Ruiz de Austri, R., & Shafi, Q. (2020). Confronting grand unification with lepton flavour violation, dark matter and LHC data. J. High Energy Phys., 09(9), 197–29pp.
Abstract: We explore possible signatures for charged lepton flavour violation (LFV), sparticle discovery at the LHC and dark matter (DM) searches in grand unified theories (GUTs) based on SU(5), flipped SU(5) (FSU(5)) and SU(4)(c) x SU(2)(L) x SU(2)(R) (4-2-2). We assume that soft supersymmetry-breaking terms preserve the group symmetry at some high input scale, and focus on the non-universal effects on different matter representations generated by gauge interactions at lower scales, as well as the charged LFV induced in Type-1 see-saw models of neutrino masses. We identify the different mechanisms that control the relic DM density in the various GUT models, and contrast their LFV and LHC signatures. The SU(5) and 4-2-2 models offer good detection prospects both at the LHC and in LFV searches, though with different LSP compositions, and the SU(5) and FSU(5) models offer LFV within the current reach. The 4-2-2 model allows chargino and gluino coannihilations with neutralinos, and the former offer good detection prospects for both the LHC and LFV, while gluino coannihilations lead to lower LFV rates. Our results indicate that LFV is a powerful tool that complements LHC and DM searches, providing significant insights into the sparticle spectra and neutrino mass parameters in different models.
Keywords: Supersymmetry Phenomenology
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Bordes, J., Hong-Mo, C., & Tsun, T. S. (2019). Accommodating three low-scale anomalies (g-2, Lamb shift, and Atomki) in the framed Standard Model. Int. J. Mod. Phys. A, 34(25), 1950140–27pp.
Abstract: The framed Standard Model (FSM) predicts a 0(+) boson with mass around 20 MeV in the “hidden sector,” which mixes at tree level with the standard Higgs hW and hence acquires small couplings to quarks and leptons which can be calculated in the FSM apart from the mixing parameter rho Uh. The exchange of this mixed state U will contribute to g – 2 and to the Lamb shift. By adjusting rho Uh alone, it is found that the FSM can satisfy all present experimental bounds on the g – 2 and Lamb shift anomalies for μand e, and for the latter for both hydrogen and deuterium. The FSM predicts also a 1(-) boson in the “hidden sector” with a mass of 17 MeV, that is, right on top of the Atomki anomaly X. This mixes with the photon at 1-loop level and couples thereby like a dark photon to quarks and leptons. It is however a compound state and is thought likely to possess additional compound couplings to hadrons. By adjusting the mixing parameter and the X's compound coupling to nucleons, the FSM can reproduce the production rate of the X in beryllium decay as well as satisfy all the bounds on X listed so far in the literature. The above two results are consistent in that the U, being 0(+), does not contribute to the Atomki anomaly if parity and angular momentum are conserved, while X, though contributing to g – 2 and Lamb shift, has smaller couplings than U and can, at first instance, be neglected there. Thus, despite the tentative nature of the three anomalies in experiment on the one hand and of the FSM as theory on the other, the accommodation of the former in the latter has strengthened the credibility of both. Indeed, if this FSM interpretation were correct, it would change the whole aspect of the anomalies from just curiosities to windows into a vast hitherto hidden sector comprising at least in part the dark matter which makes up the bulk of our universe.
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Durieux, G., Irles, A., Miralles, V., Peñuelas, A., Perello, M., Poschl, R., et al. (2019). The electro-weak couplings of the top and bottom quarks – Global fit and future prospects. J. High Energy Phys., 12(12), 098–44pp.
Abstract: We evaluate the implications of LHC and LEP/SLC measurements for the electro-weak couplings of the top and bottom quarks. We derive global bounds on the Wilson coefficients of ten two-fermion operators in an effective field theory description. The combination of hadron collider data with Z -pole measurements is found to yield tight limits on the operator coefficients that modify the left-handed couplings of the bottom and top quark to the Z boson. We also present projections for the high-luminosity phase of the LHC and for future electron-positron colliders. The bounds on the operator coefficients are expected to improve substantially during the remaining LHC programme, by factors of 1 to 5 if systematic uncertainties are scaled as statistical ones. The operation of an e(+)e(-) collider at a center-of-mass energy above the top-quark pair production threshold is expected to further improve the bounds by one to two orders of magnitude. The combination of measurements in pp and e(+)e(-) collisions allows for a percent-level determination of the top-quark Yukawa coupling, that is robust in a global fit.
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Courtoy, A., Noguera, S., & Scopetta, S. (2019). Double parton distributions in the pion in the Nambu-Jona-Lasinio model. J. High Energy Phys., 12(12), 045–26pp.
Abstract: Two-parton correlations in the pion, a non perturbative information encoded in double parton distribution functions, are investigated in the Nambu-Jona-Lasinio model. It is found that double parton distribution functions expose novel dynamical information on the structure of the pion, not accessible through one-body parton distributions, as it happens in several estimates for the proton target and in a previous evaluation for the pion, in a light-cone framework. Expressions and predictions are given for double parton distributions corresponding to leading-twist Dirac operators in the quark vertices, and to different regularization methods for the Nambu-Jona-Lasinio model. These results are particularly relevant in view of forthcoming lattice data.
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Ghosh, P., Lara, I., Lopez-Fogliani, D. E., Muñoz, C., & Ruiz de Austri, R. (2018). Searching for left sneutrino LSP at the LHC. Int. J. Mod. Phys. A, 33(18-19), 1850110–62pp.
Abstract: We analyze relevant signals expected at the LHC for a left sneutrino as the lightest supersymmetric particle (LSP). The discussion is carried out in the “mu from nu” supersymmetric standard model (mu nu SSM), where the presence of R-parity breaking couplings involving right-handed neutrinos solves the μproblem and reproduces neutrino data. The sneutrinos are pair produced via a virtual W, Z or gamma in the s channel. From the prompt decay of a pair of left sneutrinos LSPs of any family, a significant diphoton signal plus missing transverse energy (MET) from neutrinos can be present in the mass range 118-132 GeV, with 13 TeV center-of-mass energy and an integrated luminosity of 100 fb(-1). In addition, in the case of a pair of tau left sneutrinos LSPs, given the large value of the tau Yukawa coupling diphoton plus leptons and/or multileptons can appear. We find that the number of expected events for the multilepton signal, together with properly adopted search strategies, is sufficient to give a significant evidence for a sneutrino of mass in the range 130-310 GeV, even with the integrated luminosity of 20 fb(-1). In the case of the signal producing diphoton plus leptons, an integrated luminosity of 100 fb(-1) is needed to give a significant evidence in the mass range 95-145 GeV. Finally, we discuss briefly the presence of displaced vertices and the associated range of masses.
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Gomez, M. E., Lola, S., Ruiz de Austri, R., & Shafi, Q. (2018). Dark matter, sparticle spectroscopy and muon (g-2) in SU(4)(c) x SU(2)(L) x SU(2)(R). J. High Energy Phys., 10(10), 062–24pp.
Abstract: We explore the sparticle mass spectra including LSP dark matter within the framework of supersymmetric SU(4)(c) x SU(2)(L) x SU(2)(R) (422) models, taking into account the constraints from extensive LHC and cold dark matter searches. The soft supersymmetry-breaking parameters at M-GUT can be non-universal, but consistent with the 422 symmetry. We identify a variety of coannihilation scenarios compatible with LSP dark matter, and study the implications for future supersymmetry searches and the ongoing muon g-2 experiment.
Keywords: Supersymmetry Phenomenology
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Bordes, J., Hong-Mo, C., & Tsun, T. S. (2018). The Z boson in the framed standard model. Int. J. Mod. Phys. A, 33(32), 1850190–19pp.
Abstract: The framed standard model (FSM), constructed initially for explaining the existence of three fermion generations and the hierarchical mass and mixing patterns of quarks and leptons,(1,2) suggests also a “hidden sector” of particles(3) including some dark matter candidates. It predicts in addition a new vector boson G, with mass of order TeV, which mixes with the gamma and Z of the standard model yielding deviations from the standard mixing scheme, all calculable in terms of a single unknown parameter mG. Given that standard mixing has been tested already to great accuracy by experiment, this could lead to contradictions, but it is shown here that for the three crucial and testable cases so far studied (i) m(Z) – m(W), (ii) Gamma(Z -> l(+)l(-)), (iii) Gamma(Z -> hadrons), the deviations are all within the present stringent experimental bounds provided m(G) > 1 TeV, but should soon be detectable if experimental accuracy improves. This comes about because of some subtle cancellations, which might have a deeper reason that is not yet understood. By virtue of mixing, G can be produced at the LHC and appear as a l(+)l(-) anomaly. If found, it will be of interest not only for its own sake but serve also as a window on to the “hidden sector” into which it will mostly decay, with dark matter candidates as most likely products.
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Chala, M., Delgado, A., Nardini, G., & Quiros, M. (2017). A light sneutrino rescues the light stop. J. High Energy Phys., 04(4), 097–22pp.
Abstract: Stop searches in supersymmetric frameworks with R-parity conservation usually assume the lightest neutralino to be the lightest supersymmetric particle. In this paper we consider an alternative scenario in which the left-handed tau sneutrino is lighter than neutralinos and stable at collider scales, but possibly unstable at cosmological scales. Moreover the (mostly right-handed) stop (t) over tilde is lighter than all electroweakinos, and heavier than the scalars of the third generation lepton doublet, whose charged component, (T) over tilde, is heavier than the neutral one, (v) over tilde. The remaining supersymmetric particles are decoupled from the stop phenomenology. In most of the parameter space, the relevant stop decays are only into t (T) over tildeT, t (v) over tildev and b (v) over tildeT via off-shell electroweakinos. We constrain the branching ratios of these decays by recasting the most sensitive stop searches. Due to the “double invisible” kinematics of the (t) over tilde -> t (v) over tildev process, and the low efficiency in tagging the t (T) over tildeT decay products, light stops are generically allowed. In the minimal supersymmetric standard model with similar to 100 GeV sneutrinos, stops with masses as small as similar to 350 GeV turn out to be allowed at 95% CL.
Keywords: Supersymmetry Phenomenology
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Ayala, C., Cvetic, G., & Kogerler, R. (2017). Lattice-motivated holomorphic nearly perturbative QCD. J. Phys. G, 44(7), 075001–30pp.
Abstract: Newer lattice results indicate that, in the Landau gauge at low spacelike momenta, the gluon propagator and the ghost dressing function are finite non-zero. This leads to a definition of the QCD running coupling, in a specific scheme, that goes to zero at low spacelike momenta. We construct a running coupling which fulfills these conditions, and at the same time reproduces to a high precision the perturbative behavior at high momenta. The coupling is constructed in such a way that it reflects qualitatively correctly the holomorphic (analytic) behavior of spacelike observables in the complex plane of the squared momenta, as dictated by the general principles of quantum field theories. Further, we require the coupling to reproduce correctly the nonstrange semihadronic decay rate of tau lepton which is the best measured low-momentum QCD observable with small higher-twist effects. Subsequent application of the Borel sum rules to the V + A spectral functions of tau lepton decays, as measured by OPAL Collaboration, determines the values of the gluon condensate and of the V + A six-dimensional condensate, and reproduces the data to a significantly higher precision than the usual (MS) over bar running coupling.
Keywords: perturbative QCD; lattice QCD; QCD phenomenology; resummation
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