arXiv subjects

Andreas Goudelis

Publications and source records attributed to Andreas Goudelis.

32 records · Page 2Linked to original sources

arXiv2016

One jet to rule them all: monojet constraints and invisible decays of a 750 GeV diphoton resonance

The ATLAS and CMS collaborations recently reported a mild excess in the diphoton final state pointing to a resonance with a mass of around 750 GeV and a potentially large width. We consider the possibility of a scalar resonance being produced via gluon fusion and decaying to electroweak gauge bosons, jets and pairs of invisible particles, stable at collider scales. We compute limits from monojet searches on such a resonance and test their compatibility with the requirement for a large width. We also study whether the stable particle can be a a dark matter candidate and investigate the corresponding relic density constraints along with the collider limits. We show that monojet searches rule out a large part of the available parameter space and point out scenarios where a broad diphoton resonance can be reconciled with monojet constraints.

hep-ph↗

arXiv2016

A light particle solution to the cosmic lithium problem

We point out that the cosmological abundance of ${}^7$Li can be reduced down to observed values if during its formation Big Bang Nucleosynthesis is modified by the presence of light electrically neutral particles $X$ that have substantial interactions with nucleons. We find that the lithium problem can be solved without affecting the precisely measured abundances of deuterium and helium if the following conditions are satisfied: the mass and lifetimes of such particles are bounded by $ 1.6~{\rm MeV}\leq m_X \leq 20~{\rm MeV}$ and $ {\rm few}~100~{\rm s} \lesssim τ_X \lesssim 10^4~{\rm s}$, and the abundance times the absorption cross section by either deuterium or ${}^7$Be are comparable to the Hubble rate, $n_X σ_{\rm abs} v \sim H$, at the time of ${}^7$Be formation. We include $X$-initiated reactions into the primordial nucleosynthesis framework, observe that it leads to a substantial reduction of the freeze-out abundances of ${}^7$Li+${}^7$Be, and find specific model realizations of this scenario. Concentrating on the axion-like-particle case, $X=a$, we show that all these conditions can be satisifed if the coupling to $d$-quarks is in the range of $f_d^{-1} \sim {\rm TeV}^{-1}$, which can be probed at intensity frontier experiments.

hep-ph↗

arXiv2016

Characterising the 750 GeV diphoton excess

We study kinematic distributions that may help characterise the recently observed excess in diphoton events at 750 GeV at the LHC Run 2. Several scenarios are considered, including spin-0 and spin-2 750 GeV resonances that decay directly into photon pairs as well as heavier parent resonances that undergo three-body or cascade decays. We find that combinations of the distributions of the diphoton system and the leading photon can distinguish the topology and mass spectra of the different scenarios, while patterns of QCD radiation can help differentiate the production mechanisms. Moreover, missing energy is a powerful discriminator for the heavy parent scenarios if they involve (effectively) invisible particles. While our study concentrates on the current excess at 750 GeV, the analysis is general and can also be useful for characterising other potential diphoton signals in the future.

hep-ph↗

arXiv2016

Threshold enhancement of diphoton resonances

The data collected by the LHC collaborations at an energy of 13 TeV indicates the presence of an excess in the diphoton spectrum that would correspond to a resonance of a 750 GeV mass. The apparently large production cross section is nevertheless very difficult to explain in minimal models. We consider the possibility that the resonance is a pseudoscalar boson $A$ with a two--photon decay mediated by a charged and uncolored fermion having a mass at the $\frac12 M_A$ threshold and a very small decay width, $\ll 1$ MeV; one can then generate a large enhancement of the $Aγγ$ amplitude which explains the excess without invoking a large multiplicity of particles propagating in the loop, large electric charges and/or very strong Yukawa couplings. The implications of such a threshold enhancement are discussed in two explicit scenarios: i) the Minimal Supersymmetric Standard Model in which the $A$ state is produced via the top quark mediated gluon fusion process and decays into photons predominantly through loops of charginos with masses close to $\frac12 M_A$ and ii) a two Higgs doublet model in which $A$ is again produced by gluon fusion but decays into photons through loops of vector--like charged heavy leptons. We also comment on a minimal scenario in which the $A$ state couples only to photons through a heavy lepton loop and is both produced and decays through this coupling. In all these scenarios, while the mass of the charged fermion has to be adjusted to be extremely close to half of the $A$ resonance mass, the small total widths are naturally obtained if only suppressed three-body decay channels occur. Finally, the implications of some of these scenarios for dark matter are discussed.

hep-ph↗

arXiv2015

Status of the Inert Doublet Model of dark matter after Run-1 of the LHC

The Inert Doublet Model (IDM) is one of the simplest extensions of the Standard Model that can provide a viable dark matter (DM) candidate. Despite its simplicity, it predicts a versatile phenomenology both for cosmology and for the Large Hadron Collider. We briefly summarize the status of searches for IDM dark matter in direct DM detection experiments and the LHC, focusing on the impact of the latter on the model parameter space. In particular, we discuss the consequences of the Higgs boson discovery as well as those of searches for dileptons accompanied by missing transverse energy during the first LHC Run and comment on the prospects of probing some of the hardest to test regions of the IDM parameter space during the 13 TeV Run.

hep-ph↗

arXiv2015

Neutralino dark matter and naturalness of the electroweak scale

If weak scale supersymmetry (SUSY) is to somehow explain the radiative stability of the Higgs boson mass, it is likely that non-minimal variants of SUSY models should be considered. Under the assumption that the dark matter of the universe is comprised of neutralinos, recent limits from direct detection experiments as well as dark matter abundance measurements place stringent bounds on the naturalness of minimal supersymmetric models. We show that even non-minimal models introducing moderately decoupled new physics in order to address the Higgs boson mass problem face similar issues, with one interesting exception.

hep-ph↗

arXiv2015

Dark Matter Benchmark Models for Early LHC Run-2 Searches: Report of the ATLAS/CMS Dark Matter Forum

This document is the final report of the ATLAS-CMS Dark Matter Forum, a forum organized by the ATLAS and CMS collaborations with the participation of experts on theories of Dark Matter, to select a minimal basis set of dark matter simplified models that should support the design of the early LHC Run-2 searches. A prioritized, compact set of benchmark models is proposed, accompanied by studies of the parameter space of these models and a repository of generator implementations. This report also addresses how to apply the Effective Field Theory formalism for collider searches and present the results of such interpretations.

hep-ex↗

arXiv2014

The Dark Side of Electroweak Naturalness Beyond the MSSM

Weak scale supersymmetry (SUSY) remains a prime explanation for the radiative stability of the Higgs field. A natural account of the Higgs boson mass, however, strongly favors extensions of the Minimal Supersymmetric Standard Model (MSSM). A plausible option is to introduce a new supersymmetric sector coupled to the MSSM Higgs fields, whose associated states resolve the little hierarchy problem between the third generation squark masses and the weak scale. SUSY also accomodates a weakly interacting cold dark matter (DM) candidate in the form of a stable neutralino. In minimal realizations, the thus-far null results of direct DM searches, along with the DM relic abundance constraint, introduce a level of fine-tuning as severe as the one due to the SUSY little hierarchy problem. We analyse the generic implications of new SUSY sectors parametrically heavier than the minimal SUSY spectrum, devised to increase the Higgs boson mass, on this little neutralino DM problem. We focus on the SUSY operator of smallest scaling dimension in an effective field theory description, which modifies the Higgs and DM sectors in a correlated manner. Within this framework, we show that recent null results from the LUX experiment imply a tree-level fine-tuning for gaugino DM which is parametrically at least a few times larger than that of the MSSM. Higgsino DM whose relic abundance is generated through a thermal freeze-out mechanism remains also severely fine-tuned, unless the DM lies below the weak boson pair-production threshold. As in the MSSM, well-tempered gaugino-Higgsino DM is strongly disfavored by present direct detection results.

hep-ph↗

arXiv2014

En-gauging Naturalness

The discovery of a 125.5 GeV Higgs with standard model-like couplings and naturalness considerations motivate gauge extensions of the MSSM. We analyse two variants of such an extension and carry out a phenomenological study of regions of the parameter space satisfying current direct and indirect constraints, employing state-of-the art two-loop RGE evolution and GMSB boundary conditions. We find that due to the appearance of non-decoupled D-terms it is possible to obtain a 125.5 GeV Higgs with stops below 2 TeV, while the uncolored sparticles could still lie within reach of the LHC. We compare the contributions of the stop sector and the non-decoupled D-terms to the Higgs mass, and study their effect on the Higgs couplings. We further investigate the nature of the next-to lightest supersymmetric particle, in light of the GMSB motivated searches currently being pursued by ATLAS and CMS.

hep-ph↗

arXiv2012

Higgs-induced lepton flavor violation

Due to the smallness of the lepton Yukawa couplings, higher-dimensional operators can give a significant contribution to the lepton masses. In this case, the lepton mass matrix and the matrix of lepton-Higgs couplings are misaligned leading to lepton flavor violation (LFV) mediated by the Standard Model Higgs boson. We derive model-independent bounds on the Higgs flavor violating couplings and quantify LFV in decays of leptons and electric dipole moments for a class of lepton-Higgs operators contributing to lepton masses. We find significant Higgs-mediated LFV effects at both 1-loop and 2-loop levels.

hep-ph↗

arXiv2011

Neutralino dark matter with a Light Higgs

We examine the neutralino dark matter phenomenology in supersymmetric scenarios with nonuniversal Higgs masses at the gauge coupling unification scale that can acommodate a light Higgs boson, where the correct relic density is obtained mostly through the annihilation into a pseudoscalar A. Our analysis shows that most part of the A pole region can produce detectable gamma-ray and antiproton signals. We further focus on uncertainties influencing the results in indirect and mainly direct detection.

hep-ph↗

arXiv2011

Detection of WIMP-like dark matter in some extensions of the Standard Model

One of the most popular classes of candidates for dark matter are Weakly Interacting Massive Particles (WIMPs), i.e. particles possessing masses and couplings falling roughly within the electroweak scale. Apart from offering a natural mechanism for explaining dark matter, WIMPs also provide us with a hope that they could be detected through their non-gravitational interactions. In such an eventuality, we shall certainly get closer to the goal of identifying the true nature of dark matter. After introducing some formalism and elements of theory, we study the phenomenology of WIMP-type dark matter both in a model-independent manner as well as in the framework of specific models, focusing on the prospects for detection in running or oncoming direct and indirect detection experiments.

hep-ph↗

arXiv2010

Exploring SUSY light Higgs boson scenarios via dark matter experiments

We examine the dark matter phenomenology in supersymmetric light higgs boson scenarios, adapting nonuniversal Higgs masses at the gauge coupling unification scale. The correct relic density is obtained mostly through the annihilation into a pseudoscalar $A$, which gives high values for the self-annihilation cross-section at present times. Our analysis shows that most part of the $A$ pole region can produce detectable gamma-rays and antiproton signals, and still be compatible with with recent direct detection data from XENON100 and CDMS-II.

hep-ph↗

arXiv2010

Dark matter detection in the BMSSM

The addition of non-renormalizable terms involving the Higgs fields to the MSSM (BMSSM) ameliorates the little hierarchy problem of the MSSM. For neutralino dark matter, new regions for which the relic abundance of the LSP is consistent with WMAP (as the bulk region and the stop coannihilation region) are now permitted. In this framework, we analyze in detail the direct dark matter detection prospects in a Xenon-like experiment. On the other hand, we study the capability of detecting gamma-rays, antiprotons and positrons produced in the annihilation of neutralino LSPs in the Fermi and oncoming AMS-02 experiments.

hep-ph↗