arXiv subjects

Arvind Rajaraman

Publications and source records attributed to Arvind Rajaraman.

At least 55 records · Page 3Linked to original sources

arXiv2013

Particle Physics Implications and Constraints on Dark Matter Interpretations of the CDMS Signal

Recently the CDMS collaboration has reported an excess of events in the signal region of a search for dark matter scattering with Silicon nuclei. Three events on an expected background of 0.4 have a significance of about 2 sigma, and it is premature to conclude that this is a signal of dark matter. Nonetheless, it is important to examine the space of particle theories capable of explaining this excess, to see what theories are capable of explaining it, and how one might exclude it or find corroborating evidence in other channels. We examine a simplified model containing a scalar mediator particle, and find regions consistent with the CDMS observations. Bounds from colliders put important restrictions on the theory, but viable points, including points leading to the observed thermal relic density, survive.

hep-ph↗

arXiv2013

Collisions of Jets of Particles from Active Galactic Nuclei with Neutralino Dark Matter

We examine the possibility that energetic Standard Model particles contained in the jets produced by active galactic nuclei (AGN) may scatter off of the dark matter halo which is expected to surround the AGN. In particular, if there are nearby states in the dark sector which can appear resonantly in the scattering, the cross section can be enhanced and a distinctive edge feature in the energy spectrum may appear. We examine bounds on supersymmetric models which may be obtained from the Fermi Gamma-ray Space Telescope observation of the nearby AGN Centaurus A.

hep-ph↗

arXiv2012

Effective Theories of Gamma-ray Lines from Dark Matter Annihilation

We explore theories of dark matter in which dark matter annihilations produce mono-energetic gamma rays ("lines") in the context of effective field theory, which captures the physics for cases in which the particles mediating the interaction are somewhat heavier than the dark matter particle itself. Building on earlier work, we explore the generic signature resulting from SU(2)xU(1) gauge invariance that two (or more) lines are generically expected, and determine the expected relative intensities, including the possibility of interference between operators.

hep-ph↗

arXiv2012

Strange Couplings to the Higgs

We explored the coupling of strange quark to the state of mass close to 126 GeV recently observed by the ATLAS and CMS experiments at the LHC. An enhanced coupling relative to the expectations for a SM Higgs has the effect of increasing both the inclusive production cross section and the partial decay width into jets. For very large modifications, the latter dominates and the net rate into non-jet decay modes such as diphotons is suppressed, with the result that one can use observations of the diphoton decay mode to place an upper limit on the strange quark coupling. We find that the current observations of the diphoton decay mode imply that the coupling of the new resonance to strange quark can be at most ~ 50 times the SM expectation at the 95 % C.L., if one assumes at most a O(1) modification of the coupling to gluons.

hep-ph↗

arXiv2012

Spin Determination for Fermiophobic Bosons

We discuss the prospects for production and detection of fermiophobic bosons (exotic bosons decaying only to standard model gauge bosons) at the LHC, and describe simple methods for determining spin. We consider two complementary approaches to spin determination: the search for decays in the diphoton channel, and the comparison of events with no extra spectator jets to those with one extra jet. We show that these approaches can together allow the fermiophobic bosons spin to be determined over a wide range of parameter space. We study both even and odd parity states.

hep-ph↗

arXiv2012

Two Lines or Not Two Lines? That is the Question of Gamma Ray Spectra

Lines in the spectrum of cosmic gamma rays are considered one of the more robust signatures of dark matter annihilation. We consider such processes from an effective field theory vantage, and find that generically, two or more lines are expected, providing an interesting feature that can be exploited for searches and reveal details about the underlying theory of dark matter. Using the 130 GeV feature recently reported in the Fermi-LAT data as an example, we analyze the energy spectrum in the multi-line context and find the data to be consistent with a single $γγ$ line, a single $γZ$ line or both a $γγ$ and a $γZ$ line.

hep-ph↗

arXiv2012

On the Decay of Massive Fields in de Sitter

Interacting massive fields with m > d H/2 in d+1 dimensional de Sitter space are fundamentally unstable. Scalar fields in this mass range can decay to themselves. This process (which is kinematically forbidden in Minkowski space) can lead to an important change to the propagator and the physics of these fields. We compute this decay rate by doing a 1-loop computation for a massive scalar field with a cubic interaction. We resum the 1-loop result by consistently solving the Schwinger-Dyson equations. We also perform an explicit resummation of all chain graphs in the case of the retarded propagator. The decay rate is exponentially suppressed for large m/H and the flat space answer (vanishing decay rate) is reproduced in that limit.

hep-th↗

arXiv2012

Searching for $Z'$ bosons decaying to gluons

The production and decay of a new heavy vector boson, a chromophilic $Z'$ vector boson, is described. The chromophilic $Z'$ couples only to two gluons, but its two-body decays are absent, leading to a dominant decay mode of $Z'\rightarrow q\bar{q}g$. The unusual nature of the interaction predicts a cross-section which grows with $m_{Z'}$ for a fixed coupling and an accompanying gluon with a coupling that rises with its energy. We study the $t\bar{t}g$ decay mode, proposing distinct reconstruction techniques for the observation of an excess and for the measurement of $m_{Z'}$. We estimate the sensitivity of current experimental datasets.

hep-ph↗

arXiv2012

Collider Searches for Fermiophobic Gauge Bosons

We explore the phenomenology of an extra U(1) gauge boson which primarily couples to standard model gauge bosons. We classify all possible parity-odd couplings up to dimension 6 operators. We then study the prospects for the detection of such a boson at the LHC and show that the electroweak decay channels lead to very clean signals, allowing us to probe couplings well into the TeV scale.

hep-ph↗

arXiv2011

Higher Representations and Multi-Jet Resonances at the LHC

The CMS collaboration has recently conducted a search for trijet resonances in multi-jet events at the LHC. Motivated in part by this analysis, we examine the phenomenology of exotic particles transforming under higher representations of SU(3) color, focusing on those representations which intrinsically prohibit decays to fewer than three jets. We determine the LHC discovery reach for a particle transforming in a representation of this sort and discuss several additional theoretical and phenomenological constraints which apply to such a particle. Furthermore, we demonstrate that such a particle can provide a consistent explanation for a trijet excess (an invariant-mass peak of roughly 375 GeV) observed in the recent CMS study.

hep-ph↗

arXiv2011

Does Supersymmetry Require Two Higgs Doublets?

We discuss a new class of low energy supersymmetric models in which the Higgs sector includes a single doublet, for example H_u, but not H_d. Chiral gauge anomalies are canceled against new electroweak-charged states. We discuss the main challenges in building such models, and present several models where these issues are addressed. The resulting phenomenology can be distinguished from that of the MSSM in a number of ways, most notably in physics related to down-type quarks and charged leptons. As a first step toward a chiral Higgs sector, we discuss the scenario of an inert H_d doublet. We show that a UV completion of such model naturally includes dark matter with novel, flavorful couplings to SM quarks.

hep-ph↗

arXiv2011

Dark Matter Jets at the LHC

We argue that dark matter particles which have strong interactions with the Standard Model particles are not excluded by current astrophysical constraints. These dark matter particles have unique signatures at colliders; instead of missing energy, the dark matter particles produce jets. We propose a new search strategy for such strongly interacting particles by looking for a signal of two trackless jets. We show that suitable cuts can plausibly allow us to find these signals at the LHC even in early data.

hep-ph↗

arXiv2011

LHC Bounds on Interactions of Dark Matter

We derive limits on the interactions of dark matter with quarks from ATLAS null searches for jets + missing energy based on ~1 fb^-1 of integrated luminosity, using a model-insensitive effective theory framework. We find that the new limits from the LHC significantly extend limits previously derived from CDF data at the Tevatron. Translated into the parameter space of direct searches, these limits are particularly effective for ~GeV mass WIMPs. Our limits indicate tension with isospin violating models satisfying minimal flavor violation which attempt to reconcile the purported CoGeNT excess with Xenon-100, indicating that either a light mediator or nontrivial flavor structure for the dark sector is necessary for a viable reconciliation of CoGeNT with Xenon.

hep-ph↗

arXiv2011

A New Method for Resolving Combinatorial Ambiguities at Hadron Colliders

We present a new method for resolving combinatorial ambiguities that arise in multi-particle decay chains at hadron colliders where the assignment of visible particles to the different decay chains has ambiguities. Our method, based on selection cuts favoring high transverse momentum and low invariant mass pairings, is shown to be significantly superior to the more traditional hemisphere method for a large class of decay chains, producing an increase in signal retention of up to a factor of 2. This new method can thus greatly reduce the combinatorial ambiguities of decay chain assignments.

hep-ph↗

arXiv2011

Asymmetric Leptons for Asymmetric Tops

We propose an efficient method to explore models which which produce like-sign tops at the LHC, using the total charge asymmetry of single lepton events instead of like-sign dileptons. As an example, the method is implemented on a Z' Model, which can explain the top pair forward-backward asymmetry at Tevatron. We show that a large region of the parameter space of this model can be reached using the existing data set at the LHC.

hep-ph↗

arXiv2011

Constraints on Dark Matter from Colliders

We show that colliders can impose strong constraints on models of dark matter, in particular when the dark matter is light. We analyze models where the dark matter is a fermion or scalar interacting with quarks and/or gluons through an effective theory containing higher dimensional operators which represent heavier states that have been integrated out of the effective field theory. We determine bounds from existing Tevatron searches for monojets as well as expected LHC reaches for a discovery. We find that colliders can provide information which is complementary or in some cases even superior to experiments searching for direct detection of dark matter through its scattering with nuclei. In particular, both the Tevatron and the LHC can outperform spin dependent searches by an order of magnitude or better over much of parameter space, and if the dark matter couples mainly to gluons, the LHC can place bounds superior to any spin independent search.

hep-ph↗

arXiv2010

Gamma Ray Line Constraints on Effective Theories of Dark Matter

A monochromatic gamma ray line results when dark matter particles in the galactic halo annihilate to produce a two body final state which includes a photon. Such a signal is very distinctive from astrophysical backgrounds, and thus represents an incisive probe of theories of dark matter. We compare the recent null results of searches for gamma ray lines in the galactic center and other regions of the sky with the predictions of effective theories describing the interactions of dark matter particles with the Standard Model. We find that the null results of these searches provide constraints on the nature of dark matter interactions with ordinary matter which are complementary to constraints from other observables, and stronger than collider constraints in some cases.

hep-ph↗

arXiv2010

Searches for Fourth Generation Charged Leptons

We study the production and decay of fourth generation leptons at the Large Hadron Collider (LHC).We find that for charged leptons with masses under a few hundred GeV, the dominant collider signal comes from the production through a W-boson of a charged and neutral fourth generation lepton. We present a sensitivity study for this process in events with two like-sign charged leptons and at least two associated jets. We show that with sqrts = 7 TeV and 1 inverse fb of data, the LHC can exclude fourth generation charged leptons with masses up to 250 GeV.

hep-ph↗