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John Ellis

Publications and source records attributed to John Ellis.

At least 181 records · Page 10Linked to original sources
arXiv2016

Search for Sphalerons in Proton-Proton Collisions

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.

hep-ph↗
arXiv2015

Post-Inflationary Gravitino Production Revisited

We revisit gravitino production following inflation. As a first step, we review the standard calculation of gravitino production in the thermal plasma formed at the end of post-inflationary reheating when the inflaton has completely decayed. Next we consider gravitino production prior to the completion of reheating, assuming that the inflaton decay products thermalize instantaneously while they are still dilute. We then argue that instantaneous thermalization is in general a good approximation, and also show that the contribution of non-thermal gravitino production via the collisions of inflaton decay products prior to thermalization is relatively small. Our final estimate of the gravitino-to-entropy ratio is approximated well by a standard calculation of gravitino production in the post-inflationary thermal plasma assuming total instantaneous decay and thermalization at a time $t \simeq 1.2/Γ_ϕ$. Finally, in light of our calculations, we consider potential implications of upper limits on the gravitino abundance for models of inflation, with particular attention to scenarios for inflaton decays in supersymmetric Starobinsky-like models.

astro-ph.CO↗
arXiv2015

Prospects for Supersymmetry at the LHC & Beyond

Run 1 of the LHC has provided three new motivations for supersymmetry: the need to stabilize the electroweak vacuum, the mass of the Higgs boson, and the fact that its couplings are Standard Model-like (so far). The prospects for discovering (and measuring) supersymmetry during future runs of the LHC are discussed in the frameworks of the constrained MSSM (CMSSM), models with non-universal soft supersymmetry-breaking contributions to Higgs masses (NUHM1,2) and the phenomenological MSSM with 10 arbitrary soft supersymmetry-breaking parameters (pMSSM10). In addition to the classic searches for missing transverse energy, searches for long-lived charged sparticles may also be promising. If supersymmetry does show up at the LHC, there are good prospects for measurements of the spectrum that can be compared with the indirect indications from other experiments. On the other hand, a higher-energy future circular proton-proton collider may be the best option for discovering supersymmetry if it does not appear at the LHC.

hep-ph↗
arXiv2015

Sensitivities of Prospective Future e+e- Colliders to Decoupled New Physics

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 $Λ= \mathcal{O}(10)$~TeV, and FCC-ee EWPT measurements may be sensitive to $Λ= \mathcal{O}(30)$~TeV. The prospective sensitivities of Higgs and TGC measurements at the ILC250 (FCC-ee) are to $Λ= \mathcal{O}(1)$~TeV ($Λ= \mathcal{O}(2)$~TeV).

hep-ph↗
arXiv2015

Scenarios for Gluino Coannihilation

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 $m_χ\lesssim 8$~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.

hep-ph↗
arXiv2015

Beyond the CMSSM without an Accelerator: Proton Decay and Direct Dark Matter Detection

We consider two potential non-accelerator signatures of generalizations of the well-studied constrained minimal supersymmetric standard model (CMSSM). In one generalization, the universality constraints on soft supersymmetry-breaking parameters are applied at some input scale $M_{in}$ below the grand unification (GUT) scale $M_{GUT}$, a scenario referred to as `sub-GUT'. The other generalization we consider is to retain GUT-scale universality for the squark and slepton masses, but to relax universality for the soft supersymmetry-breaking contributions to the masses of the Higgs doublets. As with other CMSSM-like models, the measured Higgs mass requires supersymmetric particle masses near or beyond the TeV scale. Because of these rather heavy sparticle masses, the embedding of these CMSSM-like models in a minimal SU(5) model of grand unification can yield a proton lifetime consistent with current experimental limits, and may be accessible in existing and future proton decay experiments. Another possible signature of these CMSSM-like models is direct detection of supersymmetric dark matter. The direct dark matter scattering rate is typically below the reach of the LUX-ZEPLIN (LZ) experiment if $M_{in}$ is close to $M_{GUT}$, but may lie within its reach if $M_{in} \lesssim 10^{11}$ GeV. Likewise, generalizing the CMSSM to allow non-universal supersymmetry-breaking contributions to the Higgs offers extensive possibilities for models within reach of the LZ experiment that have long proton lifetimes.

hep-ph↗
arXiv2015

Summary and Outlook: 2015 Lepton-Photon Symposium

What life is there after Higgs? Specifically, what physics lies beyond the Standard Model (SM)? These are the biggest questions in particle physics today, and my talk is oriented towards efforts to answer them at the LHC and elsewhere.

hep-ph↗
arXiv2015

Gluino Coannihilation Revisited

Some variants of the MSSM feature a strip in parameter space where the lightest neutralino is identified as the lightest supersymmetric particle (LSP), the gluino is the next-to-lightest supersymmetric particle (NLSP) and is nearly degenerate with the LSP, 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. We find that bound-state effects can increase the maximum LSP mass for which the relic cold dark matter density lies within the range favoured by astrophysics and cosmology by as much as ~ 50% if the squark to gluino mass ratio is 1.1, and that the LSP may weigh up to ~ 8 TeV for a wide range of the squark to gluino mass ratio \lesssim 100.

hep-ph↗
arXiv2015

Flipped GUT Inflation

We analyse the prospects for constructing hybrid models of inflation that provide a dynamical realisation of the apparent closeness between the supersymmetric GUT scale and the possible scale of cosmological inflation. In the first place, we consider models based on the flipped SU(5)$\times$U(1) gauge group, which has no magnetic monopoles. In one model, the inflaton is identified with a sneutrino field, and in the other model it is a gauge singlet. In both cases we find regions of the model parameter spaces that are compatible with the experimental magnitudes of the scalar perturbations, $A_s$, and the tilt in the scalar perturbation spectrum, $n_s$, as well as with an indicative upper limit on the tensor-to-scalar perturbation ratio, $r$. We also discuss embeddings of these models into SO(10), which is broken at a higher scale so that its monopoles are inflated away.

hep-ph↗
arXiv2015

A Provably, Linear Time, In-place and Stable Merge Algorithm via the Perfect Shuffle

We reconsider a recently published algorithm (Dalkilic et al.) for merging lists by way of the perfect shuffle. The original publication gave only experimental results which, although consistent with linear execution time on the samples tested, provided no analysis. Here we prove that the time complexity, in the average case, is indeed linear, although there is an Omega(n^2) worst case. This is then the first provably linear time merge algorithm based on the use of the perfect shuffle. We provide a proof of correctness, extend the algorithm to the general case where the lists are of unequal length and show how it can be made stable, all aspects not included in the original presentation and we give a much more concise definition of the algorithm.

cs.DS↗
arXiv2015

No-Scale Inflation

Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s \sim 0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favoured by Planck and other data on the cosmic microwave background (CMB). Detailed measurements of the CMB may provide insights into the embedding of inflation within string theory as well as its links to collider physics.

hep-ph↗
arXiv2015

Calculations of Inflaton Decays and Reheating: with Applications to No-Scale Inflation Models

We discuss inflaton decays and reheating in no-scale Starobinsky-like models of inflation, calculating the effective equation-of-state parameter, $w$, during the epoch of inflaton decay, the reheating temperature, $T_{\rm reh}$, and the number of inflationary e-folds, $N_*$, comparing analytical approximations with numerical calculations. We then illustrate these results with applications to models based on no-scale supergravity and motivated by generic string compactifications, including scenarios where the inflaton is identified as an untwisted-sector matter field with direct Yukawa couplings to MSSM fields, and where the inflaton decays via gravitational-strength interactions. Finally, we use our results to discuss the constraints on these models imposed by present measurements of the scalar spectral index $n_s$ and the tensor-to-scalar perturbation ratio $r$, converting them into constraints on $N_*$, the inflaton decay rate and other parameters of specific no-scale inflationary models.

hep-ph↗
arXiv2015

An Updated Historical Profile of the Higgs Boson

The Higgs boson was postulated in 1964, and phenomenological studies of its possible production and decays started in the early 1970s, followed by studies of its possible production in electron-positron, antiproton-proton and proton-proton collisions, in particular. Until recently, the most sensitive searches for the Higgs boson were at LEP between 1989 and 2000, which were complemented by searches at the Fermilab Tevatron. Then the LHC experiments ATLAS and CMS entered the hunt, announcing on July 4, 2012 the discovery of a "Higgs-like" particle with a mass of about 125~GeV. This identification has been supported by subsequent measurements of its spin, parity and coupling properties. It was widely anticipated that the Higgs boson would be accompanied by supersymmetry, although other options, like compositeness, were not completely excluded. So far there are no signs any new physics, and the measured properties of the Higgs boson are consistent with the predictions of the minimal Standard Model. This article reviews some of the key historical developments in Higgs physics over the past half-century.

hep-ph↗
arXiv2015

Phenomenological Aspects of No-Scale Inflation Models

We discuss phenomenological aspects of no-scale supergravity inflationary models motivated by compactified string models, in which the inflaton may be identified either as a Kähler modulus or an untwisted matter field, focusing on models that make predictions for the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$ that are similar to the Starobinsky model. We discuss possible patterns of soft supersymmetry breaking, exhibiting examples of the pure no-scale type $m_0 = B_0 = A_0 = 0$, of the CMSSM type with universal $A_0$ and $m_0 \ne 0$ at a high scale, and of the mSUGRA type with $A_0 = B_0 + m_0$ boundary conditions at the high input scale. These may be combined with a non-trivial gauge kinetic function that generates gaugino masses $m_{1/2} \ne 0$, or one may have a pure gravity mediation scenario where trilinear terms and gaugino masses are generated through anomalies. We also discuss inflaton decays and reheating, showing possible decay channels for the inflaton when it is either an untwisted matter field or a Kähler modulus. Reheating is very efficient if a matter field inflaton is directly coupled to MSSM fields, and both candidates lead to sufficient reheating in the presence of a non-trivial gauge kinetic function.

hep-ph↗
arXiv2015

Discrete Glimpses of the Physics Landscape after the Higgs Discovery

What is the Higgs boson telling us? What else is there? How do we find it? This talk discusses these current topics in particle physics in the wake of the Higgs discovery, with particular emphasis on the discrete symmetries CP and R-parity, not forgetting flavour physics and dark matter, and finishing with some remarks about possible future colliders.

hep-ph↗
arXiv2014

New Higgs Inflation in a No-Scale Supersymmetric SU(5) GUT

Higgs inflation is attractive because it identifies the inflaton with the electroweak Higgs boson. In this work, we construct a new class of supersymmetric Higgs inflationary models in the no-scale supergravity with an SU(5) GUT group. Extending the no-scale Kahler potential and SU(5) GUT superpotential, we derive a generic potential for Higgs inflation that includes the quadratic monomial potential and a Starobinsky-type potential as special limits. This type of models can accommodate a wide range of the tensor-to-scalar ratio $r = O(10^{-3}-10^{-1})$, as well as a scalar spectral index $n_s \sim 0.96$.

hep-ph↗