arXiv subjects

Spencer Chang

Publications and source records attributed to Spencer Chang.

At least 37 records · Page 2Linked to original sources

arXiv2014

A Model Independent Approach to Inelastic Dark Matter Scattering

We present a model independent analysis of inelastic dark matter transitions at direct detection experiments by modifying the elastic methodology of Fitzpatrick, et al. By analyzing the kinematics of inelastic transitions, we find the relevant variables to describe these scattering processes, the primary change being a modification of the $\vec{v}^\perp$ variable. Taking this into account, we list the relevant scattering matrix elements and modify the Mathematica package of Anand, et al. to calculate the necessary form factors. As an application, we determine the matrix elements of inelastic scattering for spin transitions between a fermion to fermion, scalar to vector, and scalar to scalar. Finally, we consider fits to the DAMA/LIBRA annual modulation signal for the magnetic inelastic dark matter scenario as well as a model independent scan over relativistic operators, constraining them with limits from direct detection experiments. In the magnetic inelastic dark matter scenario or if the dark matter couples through relativistic operators involving only protons, we find that experiments with xenon and germanium targets can have consistently small rates. However, limits from iodine experiments are much more constraining, leaving small regions of allowed parameter space. We point out that existing uncertainties in the iodine quenching factor strongly affects the constraints, motivating further study to pin down the correct values.

hep-ph↗

arXiv2014

Leptophilic Effective WIMPs

Effective WIMP models are minimal extensions of the standard model that explain the relic density of dark matter by the ``WIMP miracle.'' In this paper we consider the phenomenology of effective WIMPs with trilinear couplings to leptons and a new ``lepton partner'' particle. The observed relic abundance fixes the strength of the cubic coupling, so the parameters of the models are defined by the masses of the WIMP and lepton partner particles. This gives a simple parameter space where collider and direct detection experiments can be compared under well-defined physical minimality assumptions. The most sensitive collider probe is the search for leptons + MET, while the most sensitive direct detection channel is scattering from nuclei arising from loop diagrams. Collider and direct detection searches are highly complementary: colliders give the only meaningful constraint when dark matter is its own antiparticle, while direct detection is generally more sensitive if the dark matter is not its own antiparticle.

hep-ph↗

arXiv2013

Effective WIMPs

The 'WIMP miracle' for the relic abundance of thermal dark matter motivates weak scale dark matter with renormalizable couplings to standard model particles. We study minimal models with such couplings that explain dark matter as a thermal relic. The models contain a singlet dark matter particle with cubic renormalizable couplings between standard model particles and 'partner' particles with the same gauge quantum numbers as the standard model particle. The dark matter has spin 0, 1/2, or 1, and may or may not be its own antiparticle. Each model has 3 parameters: the masses of the dark matter and standard model partners, and the cubic coupling. Requiring the correct relic abundance gives a 2-dimensional parameter space where collider and direct detection constraints can be directly compared. We focus on the case of dark matter interactions with colored particles. We find that collider and direct detection searches are remarkably complementary for these models. Direct detection limits for the cases where the dark matter is not its own antiparticle require dark matter masses to be in the multi-TeV range, where they are extremely difficult to probe in collider experiments. The models where dark matter is its own antiparticle are strongly constrained by collider searches for monojet and jets + MET signals. These models are constrained by direct detection mainly near the limit where the dark matter and partner masses are nearly degenerate, where collider searches become more difficult.

hep-ph↗

arXiv2013

Discovering Nonstandard Higgs bosons in the H -> ZA Channel Decay to Multileptons

In this article we consider the possibility of observing nonstandard Higgs bosons in the H -> Z A -> Z tau+ tau- channel. We present three benchmark scenarios in the NMSSM where H -> ZA is the dominant decay mode for one of the nonstandard Higgs bosons while the lightest CP-even Higgs is Standard Model like. Using the latest CMS multilepton analysis based on 7 TeV LHC data, we put limits on the signal cross-section, which constrain leptophilic scenarios. Projecting to future LHC analyses with improvements in background modeling, we show that with O(30) fb^{-1} of data, such a multilepton analysis is very close to constraining our NMSSM benchmarks. As we illustrate with a toy model, for light A masses, the large boost of the A makes it inefficient to select two hadronic taus, since isolation and the transverse momenta are in tension. This efficiency could be improved by including boosted di-tau jets as an object in future multilepton analyses. We also discuss different methods to confirm this scenario by reconstruction of the m_H and m_A masses. In particular we consider the transverse mass distribution, collinear mass distribution and an analytical solution using trial masses.

hep-ph↗

arXiv2012

Enhanced Gamma Ray Signals in Cosmic Proton-Wimp Collisions Due to Hadronization

In this paper, we investigate the gamma ray signal produced from dark matter collisions with high energy cosmic protons. Notably, we extend past results by including important hadronization effects. Showering and hadronization produces a high multiplicity of photons from the decays of hadrons, whose rate is not suppressed by the fine structure constant. Notably, proton remnants that do not participate in hard scattering, can produce a large rate of photons in the forward direction. These effects significantly enhance the photon rate and alter the energy and angular distributions compared to previous results which used only parton level calculations. Due to this modification, the gamma ray signal from the nearby active galactic nuclei Centaurus A is potentially testable in future Fermi-LAT and HESS measurements, for a dark matter mass and coupling consistent with current XENON100 bounds.

astro-ph.HE↗

arXiv2012

Early Higgs Hints for Non-Minimal Supersymmetry

We discuss the role that Higgs coupling measurements can play in differentiating supersymmetric extensions of the Standard Model. Fitting current LHC data to the Higgs couplings, we find that the likelihood fit shows a preference in the direction of suppressed (enhanced) bottom (top) quark couplings. In the minimal supersymmetric Standard Model, we demonstrate that for tan beta > 1, there is tension in achieving such fermion couplings due to the structure of the Higgs quartic couplings. In anticipation of interpreting supersymmetric models with future data, we determine a single straightforward condition required to access the region of coupling space preferred by current data.

hep-ph↗

arXiv2012

Revisiting Theories with Enhanced Higgs Couplings to Weak Gauge Bosons

Based on recent LHC Higgs analyses and in anticipation of future results we revisit theories where Higgs bosons can couple to weak gauge bosons with enhanced strength relative to the Standard Model value. Specifically, we look at the Georgi-Machacek model and its generalizations where higher "spin" representations of SU(2)_L break electroweak symmetry while maintaining custodial SU(2). In these theories, there is not only a Higgs-like boson but partner Higgs scalars transforming under representations of custodial SU(2), leading to a rich phenomenology. These theories serve as a consistent theoretical and experimental framework to explain enhanced couplings to gauge bosons, including fermiophobic Higgses. We focus on the phenomenology of a neutral scalar partner to the Higgs, which is determined once the Higgs couplings are specified. Depending on the parameter space, this partner could have i) enhanced fermion and gauge boson couplings and should be searched for at high mass (> 600 GeV), ii) have suppressed couplings and could be searched for at lower masses, where the Standard Model Higgs has already been ruled out, and iii) have fermiophilic couplings, where it can be searched for in heavy Higgs and top resonance searches. In the first two regions, the partner also has substantial decay rates into a pair of Higgs bosons. We touch briefly on the more model-dependent effects of the nontrivial SU(2)_C multiplets, which have exotic signals, such as a doubly-charged Higgs. We also discuss how the loop induced effects of these scalars tend to reduce the Higgs decay rate to photons, adding an additional uncertainty when extracting the couplings for the Higgs boson.

hep-ph↗

arXiv2012

Statistical Tests of Noise and Harmony in Dark Matter Modulation Signals

The aim of the current work is a detailed time-series analysis of the data from Dark Matter direct detection experiments as well as related datasets. We examine recent claims that the cosmic ray muon flux can be responsible for generating the modulation signals seen by DAMA and, more recently, by the CoGeNT collaboration. We find no evidence for such a strong correlation and show that the two phenomena differ in their power spectrum, phase, and possibly in amplitude. In addition, we investigate in more detail, the time dependence of Dark Matter scattering. Since the signal is periodic with period of a year (due to the Earth's motion around the Sun), the presence of higher harmonics can be expected. We show that the higher harmonics generically have similar phase to the annual modulation and the biannual mode in particular could provide another handle in searching for Dark Matter in the laboratory.

hep-ph↗

arXiv2011

Early Higgs Boson Discovery in Non-minimal Higgs Sectors

Particle physics models with more than one Higgs boson occur in many frameworks for physics beyond the standard model, including supersymmetry, technicolor, composite Higgs, and "little Higgs" models. If the Higgs sector contains couplings stronger than electroweak gauge couplings, there will be heavy Higgs particles that decay to lighter Higgs particles plus heavy particles such as $W$, $Z$, and $t$. This motivates searches for final states involving multiple $W$, $Z$, $t$, and $\bar{b}b$ pairs. A two Higgs doublet model with custodial symmetry is a useful simplified model to describe many of these signals. The model can be parameterized by the physical Higgs masses and the mixing angles $\al$ and $\be$, so discovery or exclusion in this parameter space has a straightforward physical interpretation. We illustrate this with a detailed analysis of the process $gg \to A$ followed by $A \to h Z$ and $h \to WW$. For $m_{A} \simeq 330\GeV$, $m_{h} \simeq 200\GeV$ we can get a $4.5\si$ signal with 1 fb${}^{-1}$ of integrated luminosity at the Large Hadron Collider.

hep-ph↗

arXiv2011

Charge Asymmetric Cosmic Ray Signals From Dark Matter Decay

The PAMELA and Fermi measurements of the cosmic-ray electron and positron spectra have generated much interest over the past two years, because they are consistent with a significant component of the electron and positron fluxes between 20 GeV and 1 TeV being produced through dark matter annihilation or decay. However, since the measurements are also consistent with astrophysical interpretations, the message is unclear. In this paper, we point out that dark matter can have a more distinct signal in cosmic rays, that of a charge asymmetry. Such charge asymmetry can result if the dark matter's abundance is due to a relic asymmetry, allowing its decay to generate an asymmetry in positrons and electrons. This is analogous to the baryon asymmetry, where decaying neutrons produce electrons and not positrons. We explore benchmark scenarios where the dark matter decays into a leptophilic charged Higgs boson or electroweak gauge bosons. These models have observable signals in gamma rays and neutrinos, which can be tested by Fermi and IceCube. The most powerful test will be at AMS-02, given its ability to distinguish electron and positron charge above 100 GeV. Specifically, an asymmetry favoring positrons typically predicts a larger positron ratio and a harder (softer) high energy spectrum for positrons (electrons) than charge symmetric sources. We end with a brief discussion on how such scenarios differ from the leading astrophysical explanations.

hep-ph↗

arXiv2011

Measuring Top Squark Interactions With The Standard Model Through Associated Production

A new particle's interactions can be measured at colliders, by observing its associated production with Standard Model particles. We focus on the case of a collider-stable right-handed top squark and study the LHC sensitivities to its couplings to the photon, Z, and the Higgs boson. Such measurements determine the top squark's charge, mixing angle and coupling to the electroweak symmetry breaking sector. Determining these couplings can provide strong evidence for the supersymmetric solution to the hierarchy problem. Our analysis shows that the LHC has great prospects for measuring the photon and Higgs couplings, but will require a very high luminosity to measure the Z coupling.

hep-ph↗

arXiv2011

Simplified Models for LHC New Physics Searches

This document proposes a collection of simplified models relevant to the design of new-physics searches at the LHC and the characterization of their results. Both ATLAS and CMS have already presented some results in terms of simplified models, and we encourage them to continue and expand this effort, which supplements both signature-based results and benchmark model interpretations. A simplified model is defined by an effective Lagrangian describing the interactions of a small number of new particles. Simplified models can equally well be described by a small number of masses and cross-sections. These parameters are directly related to collider physics observables, making simplified models a particularly effective framework for evaluating searches and a useful starting point for characterizing positive signals of new physics. This document serves as an official summary of the results from the "Topologies for Early LHC Searches" workshop, held at SLAC in September of 2010, the purpose of which was to develop a set of representative models that can be used to cover all relevant phase space in experimental searches. Particular emphasis is placed on searches relevant for the first ~50-500 pb-1 of data and those motivated by supersymmetric models. This note largely summarizes material posted at http://lhcnewphysics.org/, which includes simplified model definitions, Monte Carlo material, and supporting contacts within the theory community. We also comment on future developments that may be useful as more data is gathered and analyzed by the experiments.

hep-ph↗

arXiv2010

Magnetic Inelastic Dark Matter

Iodine is distinguished from other elements used in dark matter direct detection experiments both by its large mass as well as its large magnetic moment. Inelastic dark matter utilizes the large mass of iodine to allay tensions between the DAMA annual modulation signature and the null results from other experiments. We explore models of inelastic dark matter that also take advantage of the second distinct property of iodine, namely its large magnetic moment. In such models the couplings are augmented by magnetic, rather than merely electric, interactions. These models provide simple examples where the DAMA signal is compatible with all existing limits. We consider dipole moments for the WIMP, through conventional magnetism as well as "dark" magnetism, including both magnetic-magnetic and magnetic-electric scattering. We find XENON100 and CRESST should generically see a signal, although suppressed compared with electric inelastic dark matter models, while KIMS should see a modulated signal comparable to or larger than that of DAMA. In a large portion of parameter space, de-excitation occurs promptly, producing a ~ 100 keV photon inside large xenon experiments alongside the nuclear recoil. This effect could be searched for, but if not properly considered may cause nuclear recoil events to fail standard cuts.

hep-ph↗

arXiv2010

Impure Thoughts on Inelastic Dark Matter

The inelastic dark matter scenario was proposed to reconcile the DAMA annual modulation with null results from other experiments. In this scenario, WIMPs scatter into an excited state, split from the ground state by an energy delta comparable to the available kinetic energy of a Galactic WIMP. We note that for large splittings delta, the dominant scattering at DAMA can occur off of thallium nuclei, with A~205, which are present as a dopant at the 10^-3 level in NaI(Tl) crystals. For a WIMP mass m~100GeV and delta~200keV, we find a region in delta-m-parameter space which is consistent with all experiments. These parameters in particular can be probed in experiments with thallium in their targets, such as KIMS, but are inaccessible to lighter target experiments. Depending on the tail of the WIMP velocity distribution, a highly modulated signal may or may not appear at CRESST-II.

hep-ph↗

arXiv2010

A New Approach to Searching for Dark Matter Signals in Fermi-LAT Gamma Rays

Several cosmic ray experiments have measured excesses in electrons and positrons, relative to standard backgrounds, for energies from ~ 10 GeV - 1 TeV. These excesses could be due to new astrophysical sources, but an explanation in which the electrons and positrons are dark matter annihilation or decay products is also consistent. Fortunately, the Fermi-LAT diffuse gamma ray measurements can further test these models, since the electrons and positrons produce gamma rays in their interactions in the interstellar medium. Although the dark matter gamma ray signal consistent with the local electron and positron measurements should be quite large, as we review, there are substantial uncertainties in the modeling of diffuse backgrounds and, additionally, experimental uncertainties that make it difficult to claim a dark matter discovery. In this paper, we introduce an alternative method for understanding the diffuse gamma ray spectrum in which we take the intensity ratio in each energy bin of two different regions of the sky, thereby canceling common systematic uncertainties. For many spectra, this ratio fits well to a power law with a single break in energy. The two measured exponent indices are a robust discriminant between candidate models, and we demonstrate that dark matter annihilation scenarios can predict index values that require "extreme" parameters for background-only explanations.

astro-ph.HE↗

arXiv2010

Momentum Dependent Dark Matter Scattering

It is usually assumed that WIMPs interact through spin-independent and spin-dependent interactions. Interactions which carry additional powers of the momentum transfer, q^2, are assumed to be too small to be relevant. In theories with new particles at the ~ GeV scale, however, these q^2-dependent interactions can be large, and, in some cases dominate over the standard interactions. This leads to new phenomenology in direct detection experiments. Recoil spectra peak at non-zero energies, and the relative strengths of different experiments can be significantly altered. We present a simple parameterization for models of this type which captures much of the interesting phenomenology and allows a comparison between experiments. As an application, we find that dark matter with momentum dependent interactions coupling to the spin of the proton can reconcile the DAMA annual modulation result with other experiments.

hep-ph↗

arXiv2010

CoGeNT Interpretations

Recently, the CoGeNT experiment has reported events in excess of expected background. We analyze dark matter scenarios which can potentially explain this signal. Under the standard case of spin independent scattering with equal couplings to protons and neutrons, we find significant tensions with existing constraints. Consistency with these limits is possible if a large fraction of the putative signal events is coming from an additional source of experimental background. In this case, dark matter recoils cannot be said to explain the excess, but are consistent with it. We also investigate modifications to dark matter scattering that can evade the null experiments. In particular, we explore generalized spin independent couplings to protons and neutrons, spin dependent couplings, momentum dependent scattering, and inelastic interactions. We find that some of these generalizations can explain most of the CoGeNT events without violation of other constraints. Generalized couplings with some momentum dependence, allows further consistency with the DAMA modulation signal, realizing a scenario where both CoGeNT and DAMA signals are coming from dark matter. A model with dark matter interacting and annihilating into a new light boson can realize most of the scenarios considered.

hep-ph↗

arXiv2009

Displaced Dark Matter at Colliders

Models in which the dark matter is very weakly coupled to the observable sector may explain the observed dark matter density, either as a "superWIMP" or as "asymmetric dark matter." Both types of models predict displaced vertices at colliders, with a rich variety of possible phenomenology. We classify the cases in which the decays can naturally occur inside particle detectors at the LHC, with particular focus on the nontrivial scenarios where the decaying particle is invisible. Identification of the position and timing of these invisible displaced vertices significantly improves the prospects of reconstructing the new physics in models such as supersymmetry. In many cases, reconstruction of the visible products of the displaced decay can determine the dark matter mass, allowing the dark matter density to be predicted from collider data.

hep-ph↗