Theoretical Particle Physics and Cosmology Group

London, United Kingdom

Theoretical Particle Physics and Cosmology Group

London, United Kingdom
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Djouadi A.,University Paris - Sud | Maiani L.,University of Rome La Sapienza | Polosa A.,University of Rome La Sapienza | Quevillon J.,Theoretical Particle Physics and Cosmology Group | Riquer V.,University of Rome La Sapienza
Journal of High Energy Physics | Year: 2015

Abstract: In the context of the Minimal Supersymmetric extension of the Standard Model (MSSM), we reanalyze the search for the heavier CP-even H and CP-odd A neutral Higgs bosons at the LHC in their production in the gluon-fusion mechanism and their decays into gauge and lighter h bosons and into top quark pairs. We show that only when considering these processes, that one can fully cover the entire parameter space of the Higgs sector of the model. Indeed, they are sensitive to the low tan β and high Higgs mass ranges, complementing the traditional searches for high mass resonances decaying into τ -lepton pairs which are instead sensitive to the large and moderate tan β regions. The complementarity of the various channels in the probing of the complete [tan β, MA] MSSM parameter space at the previous and upcoming phases of the LHC is illustrated in a recently proposed simple and model independent approach for the Higgs sector, the hMSSM, that we also refine in this paper. © 2015, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Sakurai K.,Durham University
Journal of High Energy Physics | Year: 2016

Abstract: In a recent paper, Tye and Wong (TW) have argued that sphaleron-induced transitions in high-energy proton-proton collisions should be enhanced compared to previous calculations, based on a construction of a Bloch wave function in the periodic sphaleron potential and the corresponding pass band structure. Here we convolute the calculations of TW with parton distribution functions and simulations of final states to explore the signatures of sphaleron transitions at the LHC and possible future colliders. We calculate the increase of sphaleron transition rates in proton-proton collisions at centre-of-mass energies of 13/14/33/100 TeV for different sphaleron barrier heights, while recognising that the rates have large overall uncertainties. We use a simulation to show that LHC searches for microscopic black holes should have good efficiency for detecting sphaleron-induced final states, and discuss their experimental signatures and observability in Run 2 of the LHC and beyond. We recast the early ATLAS Run-2 search for microscopic black holes to constrain the rate of sphaleron transitions at 13 TeV, deriving a significant limit on the sphaleron transition rate for the nominal sphaleron barrier height of 9 TeV. © 2016, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Sanz V.,University of Sussex | You T.,Theoretical Particle Physics and Cosmology Group
Journal of High Energy Physics | Year: 2015

Abstract: We treat the Standard Model as the low-energy limit of an effective field theory that incorporates higher-dimensional operators to capture the effects of decoupled new physics. We consider the constraints imposed on the coefficients of dimension-6 operators by electroweak precision tests (EWPTs), applying a framework for the effects of dimension- 6 operators on electroweak precision tests that is more general than the standard S, T formalism, and use measurements of Higgs couplings and the kinematics of associated Higgs production at the Tevatron and LHC, as well as triple-gauge couplings at the LHC. We highlight the complementarity between EWPTs, Tevatron and LHC measurements in obtaining model-independent limits on the effective Standard Model after LHC Run 1. We illustrate the combined constraints with the example of the two-Higgs doublet model. © 2015, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Ellis S.A.R.,University of Michigan | Quevillon J.,Theoretical Particle Physics and Cosmology Group | And 2 more authors.
Journal of High Energy Physics | Year: 2016

Abstract: We consider interpretations of the recent ∼ 3σ reports by the CMS and ATLAS collaborations of a possible X (∼750 GeV) state decaying into γγ final states. We focus on the possibilities that this is a scalar or pseudoscalar electroweak isoscalar state produced by gluon-gluon fusion mediated by loops of heavy fermions. We consider several models for these fermions, including a single vector-like charge 2/3 T quark, a doublet of vector-like quarks (T, B), and a vector-like generation of quarks, with or without leptons that also contribute to the X → γγ decay amplitude. We also consider the possibility that X (750) is a dark matter mediator, with a neutral vector-like dark matter particle. These scenarios are compatible with the present and prospective direct limits on vector-like fermions from LHC Runs 1 and 2, as well as indirect constraints from electroweak precision measurements, and we show that the required Yukawa-like couplings between the X particle and the heavy vector-like fermions are small enough to be perturbative so long as the X particle has dominant decay modes into gg and γγ. The decays X → ZZ, Zγ and W+W− are interesting prospective signatures that may help distinguish between different vector-like fermion scenarios. © 2016, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Luo F.,CERN | Olive K.A.,University of Minnesota
Journal of High Energy Physics | Year: 2015

Abstract: Some variants of the MSSM feature a strip in parameter space where the lightest neutralino χ is identified as the lightest supersymmetric particle (LSP), the gluino g˜ is the next-to-lightest supersymmetric particle (NLSP) and is nearly degenerate with χ, and the relic cold dark matter density is brought into the range allowed by astrophysics and cosmology by coannihilation with the gluino NLSP. We calculate the relic density along this gluino coannihilation strip in the MSSM, including the effects of gluino-gluino bound states and initial-state Sommerfeld enhancement, and taking into account the decoupling of the gluino and LSP densities that occurs for large values of the squark mass mq˜. We find that bound-state effects can increase the maximum mχ for which the relic cold dark matter density lies within the range favoured by astrophysics and cosmology by as much as ∼ 50% if mq˜/mg˜=1.1, and that the LSP may weigh up to ∼ 8 TeV for a wide range of mq˜/mg˜≲100$. © 2015, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Evans J.L.,University of Minnesota | Luo F.,CERN | Olive K.A.,University of Minnesota
Journal of High Energy Physics | Year: 2016

Abstract: We study supersymmetric scenarios in which the gluino is the next-to-lightest supersymmetric particle (NLSP), with a mass sufficiently close to that of the lightest supersymmetric particle (LSP) that gluino coannihilation becomes important. One of these scenarios is the MSSM with soft supersymmetry-breaking squark and slepton masses that are universal at an input GUT renormalization scale, but with non-universal gaugino masses. The other scenario is an extension of the MSSM to include vector-like supermultiplets. In both scenarios, we identify the regions of parameter space where gluino coannihilation is important, and discuss their relations to other regions of parameter space where other mechanisms bring the dark matter density into the range allowed by cosmology. In the case of the non-universal MSSM scenario, we find that the allowed range of parameter space is constrained by the requirement of electroweak symmetry breaking, the avoidance of a charged LSP and the measured mass of the Higgs boson, in particular, as well as the appearance of other dark matter (co)annihilation processes. Nevertheless, LSP masses mχ ≲ 8 TeV with the correct dark matter density are quite possible. In the case of pure gravity mediation with additional vector-like supermultiplets, changes to the anomaly- mediated gluino mass and the threshold effects associated with these states can make the gluino almost degenerate with the LSP, and we find a similar upper bound. © 2016, The Author(s).


Drozd A.,Theoretical Particle Physics and Cosmology Group | Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | Quevillon J.,Theoretical Particle Physics and Cosmology Group | You T.,Theoretical Particle Physics and Cosmology Group
Journal of High Energy Physics | Year: 2015

Abstract: We develop a universal approach to the one-loop effective field theory (EFT) using the Covariant Derivative Expansion (CDE) method. We generalise previous results to include broader classes of UV models, showing how expressions previously obtained assuming degenerate heavy-particle masses can be extended to non-degenerate cases. We apply our method to the general MSSM with non-degenerate stop squarks, illustrating our approach with calculations of the coefficients of dimension-6 operators contributing to the hgg and hγγ couplings, and comparing with exact calculations of one-loop Feynman diagrams. We then use present and projected future sensitivities to these operator coefficients to obtain present and possible future indirect constraints on stop masses. The current sensitivity is already comparable to that of direct LHC searches, and future FCC-ee measurements could be sensitive to stop masses above a TeV. The universality of our one-loop EFT approach facilitates extending these constraints to a broader class of UV models. © 2015, The Author(s).


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN
International Journal of Modern Physics D | Year: 2016

The plethora of recent and forthcoming data on the cosmic microwave background (CMB) data are stimulating a new wave of inflationary model-building. Naturalness suggests that the appropriate framework for models of inflation is supersymmetry. This should be combined with gravity in a supergravity theory, whose specific no-scale version has much to commend it, e.g. its derivation from string theory and the flat directions in its effective potential. Simple no-scale supergravity models yield predictions similar to those of the Starobinsky (Formula presented.) model, though some string-motivated versions make alternative predictions. Data are beginning to provide interesting constraints on the rate of inflaton decay into Standard Model particles. In parallel, LHC and other data provide significant constraints on no-scale supergravity models, which suggest that some sparticles might have masses close to present experimental limits. © 2016 World Scientific Publishing Company


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN
International Journal of Modern Physics D | Year: 2016

Cosmology and particle physics have long been dominated by theoretical paradigms: Einstein’s general theory of relativity in cosmology and the standard model of particle physics. The time may have come for paradigm shifts. Does cosmological inflation require a modification of Einstein’s gravity? Have experiments at the LHC discovered a new particle beyond the Standard Model? It is premature to answer these questions, but we theorists can dream about the possibilities. © 2016 World Scientific Publishing Company


Ellis J.,Theoretical Particle Physics and Cosmology Group | Ellis J.,CERN | You T.,Theoretical Particle Physics and Cosmology Group | You T.,University of Cambridge
Journal of High Energy Physics | Year: 2016

Abstract: We explore the indirect sensitivities to decoupled new physics of prospective precision electroweak measurements, triple-gauge-coupling measurements and Higgs physics at future e+e− colliders, with emphasis on the ILC250 and FCC-ee. The Standard Model effective field theory (SM EFT) is adopted as a model-independent approach for relating experimental precision projections to the scale of new physics, and we present prospective constraints on the Wilson coefficients of dimension-6 operators. We find that in a marginalised fit ILC250 EWPT measurements may be sensitive to new physics scales (Formula presented.) TeV, and FCC-ee EWPT measurements may be sensitive to (Formula presented.) TeV. The prospective sensitivities of Higgs and TGC measurements at the ILC250 (FCC-ee) are to (Formula presented.) TeV (Formula presented.) TeV). © 2016, The Author(s).

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