arXiv subjects

Kathryn M. Zurek

Publications and source records attributed to Kathryn M. Zurek.

At least 109 records · Page 6Linked to original sources

arXiv2013

Flavor and Collider Signatures of Asymmetric Dark Matter

We consider flavor constraints on, and collider signatures of, Asymmetric Dark Matter (ADM) via higher dimension operators. In the supersymmetric models we consider, R-parity violating (RPV) operators carrying B-L interact with n dark matter (DM) particles X through an interaction of the form W = X^n O_{B-L}, where O_{B-L} = q l d^c, u^c d^c d^c, l l e^c. This interaction ensures that the lightest ordinary supersymmetric particle (LOSP) is unstable to decay into the X sector, leading to a higher multiplicity of final state particles and reduced missing energy at a collider. Flavor-violating processes place constraints on the scale of the higher dimension operator, impacting whether the LOSP decays promptly. While the strongest limitations on RPV from n-\bar{n} oscillations and proton decay do not apply to ADM, we analyze the constraints from meson mixing, mu-e conversion, mu -> 3 e and b -> s l^+ l^-. We show that these flavor constraints, even in the absence of flavor symmetries, allow parameter space for prompt decay to the X sector, with additional jets and leptons in exotic flavor combinations. We study the constraints from existing 8 TeV LHC SUSY searches with (i) 2-6 jets plus missing energy, and (ii) 1-2 leptons, 3-6 jets plus missing energy, comparing the constraints on ADM-extended supersymmetry with the usual supersymmetric simplified models.

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arXiv2013

Beyond Collisionless Dark Matter: Particle Physics Dynamics for Dark Matter Halo Structure

Dark matter (DM) self-interactions have important implications for the formation and evolution of structure, from dwarf galaxies to clusters of galaxies. We study the dynamics of self-interacting DM via a light mediator, focusing on the quantum resonant regime where the scattering cross section has a non-trivial velocity dependence. While there are long-standing indications that observations of small scale structure in the Universe are not in accord with the predictions of collisionless DM, theoretical study and simulations of DM self-interactions have focused on parameter regimes with simple analytic solutions for the scattering cross section, with constant or classical velocity (and no angular) dependence. We devise a method that allows us to explore the velocity and angular dependence of self-scattering more broadly, in the strongly-coupled resonant and classical regimes where many partial modes are necessary for the achieving the result. We map out the entire parameter space of DM self-interactions --- and implications for structure observations --- as a function of the coupling and the DM and mediator masses. We derive a new analytic formula for describing resonant s-wave scattering. Finally, we show that DM self-interactions can be correlated with observations of Sommerfeld enhancements in DM annihilation through indirect detection experiments.

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arXiv2013

Open windows for a light axigluon explanation of the top forward-backward asymmetry

The top forward-backward asymmetry (tAFB) measured at the Tevatron remains one of the most puzzling outstanding collider anomalies. After two years of LHC running, however, few models for tAFB remain consistent with LHC data. In this paper we take a detailed look at the most promising surviving class of models, namely light (m_G' <~ 450 GeV), broad axigluons. We show which models simultaneously satisfy constraints from Tevatron and LHC top measurements, hadronic resonance searches, and LEP precision electroweak (PEW) observables. We consider three flavor structures: flavor-universal; down-type nonuniversal, designed to ease constraints from LHC charge asymmetry measurements; and top-type nonuniversal, designed to ameliorate constraints from PEW. We compute contributions to the PEW observables from states in the minimal UV completion of the axigluon model and demonstrate that new heavy fermions make the constraints universally more stringent, while related contributions from new scalars are much smaller, but act to relax the constraints. Paired dijet searches from ATLAS and CMS rule out all narrow axiglue models, while the LHC charge asymmetry measurement is less constraining than expected due to the high central value measured by ATLAS. Excepting the tension with the CMS charge asymmetry measurement, a broad axigluon is consistent with all data over the entire mass range we consider (50 GeV <~m_G' <~450 GeV) in the flavor-universal and top-type nonuniversal models, while it is consistent for m_G' >~200 GeV in the down-type non-universal model. The LHC charge asymmetry remains the best avenue for excluding, or observing, these models.

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arXiv2013

Inspecting the Higgs for New Weakly Interacting Particles

We explore new physics scenarios which are optimally probed through precision Higgs measurements rather than direct collider searches. Such theories consist of additional electroweak charged or singlet states which couple directly to or mix with the Higgs boson; particles of this kind may be weakly constrained by direct limits due to their meager production rates and soft decay products. We present a simplified framework which characterizes the effects of these states on Higgs physics by way of tree level mixing (with neutral scalars) and loop level modifications (from electrically charged states), all expressed in terms of three mixing angles and three loop parameters, respectively. The theory parameters are constrained and in some cases even fixed by ratios of Higgs production and decay rates. Our setup is simpler than a general effective operator analysis, in that we discard parameters irrelevant to Higgs observables while retaining complex correlations among measurements that arise due to the underlying mixing and radiative effects. We show that certain correlated observations are forbidden, e.g. a depleted ratio of Higgs production from gluon fusion versus vector boson fusion together with a depleted ratio of Higgs decays to bb versus WW. Moreover, we study the strong correlation between the Higgs decay rate to gamma gamma and WW and how it can be violated in the presence of additional electrically charged particles. Our formalism maps straightforwardly onto a variety of new physics models, such as the NMSSM. We show, for example, that with a Higgsino of mass > 100 GeV and a singlet-Higgs coupling of lambda=0.7, the photon signal strength can deviate from the vector signal strength by up to ~ 40-60% while depleting the vector signal strength by only 5-15% relative to the Standard Model.

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arXiv2012

On Symmetric and Asymmetric Light Dark Matter

We examine cosmological, astrophysical and collider constraints on thermal dark matter (DM) with mass mX in the range 1 MeV to 10 GeV. Cosmic microwave background (CMB) observations, which severely constrain light symmetric DM, can be evaded if the DM relic density is sufficiently asymmetric. CMB constraints require the present anti-DM to DM ratio to be less than 2*10^{-6} (10^{-1}) for DM mass mX = 1 MeV (10 GeV) with ionizing efficiency factor f ~ 1. We determine the minimum annihilation cross section for achieving these asymmetries subject to the relic density constraint; these cross sections are larger than the usual thermal annihilation cross section. On account of collider constraints, such annihilation cross sections can only be obtained by invoking light mediators. These light mediators can give rise to significant DM self-interactions, and we derive a lower bound on the mediator mass from elliptical DM halo shape constraints. We find that halo shapes require a mediator with mass mphi > 4 * 10^{-2} MeV (40 MeV) for mX = 1 MeV (10 GeV). We map all of these constraints to the parameter space of DM-electron and DM-nucleon scattering cross sections for direct detection. For DM-electron scattering, a significant fraction of the parameter space is already ruled out by beam-dump and supernova cooling constraints.

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arXiv2012

Resonant Dark Forces and Small Scale Structure

A dark force can impact the cosmological history of dark matter (DM), both explaining observed cores in dwarf galaxies and setting the DM relic density through annihilation to dark force bosons. For GeV - TeV DM mass, DM self-scattering in dwarf galaxy halos exhibits quantum mechanical resonances, analogous to a Sommerfeld enhancement for annihilation. We show that a simple model of DM with a dark force can accommodate all astrophysical bounds on self-interactions in halos and explain the observed relic density, through a single coupling constant.

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arXiv2012

Three Exceptions for Thermal Dark Matter with Enhanced Annihilation to Gamma Gamma

Recently, there have been hints for dark matter (DM) annihilation in the galactic center to one or more photon lines. In order to achieve the observed photon line flux, DM must have a relatively large effective coupling to photons, typically generated radiatively from large couplings to charged particles. When kinematically accessible, direct annihilation of DM to these charged particles is far too large to accommodate both the DM relic density and constraints from the observed flux of continuum photons from the galactic center, halo and dwarf galaxies. We discuss three exceptions to these obstacles, generating the observed line signal while providing the correct relic density and evading photon continuum constraints. The exceptions are (i) coannihilation, where the DM density is set by interactions with a heavier state that is not populated today, (ii) forbidden channels, where DM annihilates to heavier states that are kinematically blocked today, but open in the early Universe, and (iii) asymmetric DM, where the relic density is set by a primordial asymmetry. We build simple models to realize these scenarios.

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arXiv2012

Neutrino Phenomenology in a 3+1+1 Framework

Evidence continues to grow in the MiniBooNE (MB) antineutrino mode supporting a low-energy excess compatible with the MB neutrino mode and possibly also confirming the results of the LSND experiment. At least one sterile neutrino is required to explain the anomalies consistent with the observations of other experiments. At the same time, there is a strong tension between the positive signals of LSND and MB and the null results of nu_e and nu_mu disappearance experiments. We explore a scenario, first proposed in \cite{Nelson:2010hz}, where the presence of an additional heavy sterile neutrino (with mass well above an eV) can alleviate tension between LSND, MB and the null results of disappearance experiments. We compare and contrast this 3+1+1 scenario with the more standard 3+1 scenario and carry out global fits to all oscillation data including new 2011 MB anti-nu data. We find that the tension can be somewhat alleviated and that a phenomenologically viable window for the heavy neutrino, consistent with rare decays and BBN constraints, can be found if the fifth neutrino has a mass of order 0.3 - 10 GeV. We also find, however, that the 2011 MB anti-nu data exacerbates the tension with null experiments in both the 3+1 and 3+1+1 models when the lowest energy bins are included, resulting in little improvement in the global fit. We also discuss the implications of an additional neutrino for the reactor and gallium anomalies, and show that an oscillation explanation of the anomalies is disfavored by cosmological considerations, direct searches, and precision electroweak tests.

hep-ph↗

arXiv2012

The Dark Matter Inverse Problem: Extracting Particle Physics from Scattering Events

The primary observable in dark matter direct detection is the spectrum of scattering events. We simulate multiple positive direct detection signals (on germanium, xenon, and argon targets) to explore the extent to which the underlying particle physics, manifested in the momentum dependence of the operator mediating the scattering, can be extracted. Taking into account realization (Poisson) noise, a single target nucleus with 300 events has limited power to discriminate operators with momentum dependence differing by q^\pm2 for a wide range of dark matter masses from 10 GeV to 1 TeV. With the inclusion of multiple targets (or a factor of several more events on a single target), the discrimination of operators with different momentum dependence becomes very strong at the 95% C.L. for dark matter candidates of mass 50 GeV and above. On the other hand, operator discrimination remains poor for 10 GeV candidates until multiple experiments each collect 1000 or more events.

hep-ph↗

arXiv2012

Affleck-Dine Cogenesis

We propose a novel framework in which the observed baryon and dark matter abundances are simultaneously generated via the Affleck-Dine mechanism. In its simplest realization, Affleck-Dine cogenesis is accomplished by a single superpotential operator and its A-term counterpart. These operators explicitly break B-L and X, the dark matter number, to the diagonal B-L+X. In the early universe these operators stabilize supersymmetric flat directions carrying non-zero B-L and X, and impart the requisite CP violation for asymmetry generation. Because B-L+X is preserved, the resulting B-L and X asymmetries are equal and opposite, though this precise relation may be relaxed if B-L and X are violated separately by additional operators. Our dark matter candidate is stabilized by R-parity and acquires an asymmetric abundance due to its non-zero X number. For a dark matter mass of order a few GeV, one naturally obtains the observed ratio of energy densities today, Omega_{DM}/Omega_B \sim 5. These theories typically predict macroscopic lifetimes for the lightest observable supersymmetric particle as it decays to the dark matter.

hep-ph↗

arXiv2012

Higgs and Dark Matter Hints of an Oasis in the Desert

Recent LHC results suggest a standard model (SM)-like Higgs boson in the vicinity of 125 GeV with no clear indications yet of physics beyond the SM. At the same time, the SM is incomplete, since additional dynamics are required to accommodate cosmological dark matter (DM). In this paper we show that interactions between weak scale DM and the Higgs which are strong enough to yield a thermal relic abundance consistent with observation can easily destabilize the electroweak vacuum or drive the theory into a non-perturbative regime at a low scale. As a consequence, new physics--beyond the DM itself--must enter at a cutoff well below the Planck scale and in some cases as low as O(10 - 1000 TeV), a range relevant to indirect probes of flavor and CP violation. In addition, this cutoff is correlated with the DM mass and scattering cross-section in a parameter space which will be probed experimentally in the near term. Specifically, we consider the SM plus additional spin 0 or 1/2 states with singlet, triplet, or doublet electroweak quantum numbers and quartic or Yukawa couplings to the Higgs boson. We derive explicit expressions for the full two-loop RGEs and one-loop threshold corrections for these theories.

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arXiv2012

Confronting Top AFB with Parity Violation Constraints

We consider the implications of low-energy precision tests of parity violation on t-channel mediator models explaining the top AFB excess measured by CDF and D0. Flavor-violating u-t or d-t couplings of new scalar or vector mediators generate at one-loop an anomalous contribution to the nuclear weak charge. As a result, atomic parity violation constraints disfavor at >3 sigma t-channel models that give rise to a greater than 20% AFB at the parton level for M_tt > 450 GeV while not producing too large a top cross-section. Even stronger constraints are expected through future measurements of the proton weak charge by the Q-Weak experiment.

hep-ph↗

arXiv2012

Oscillating Asymmetric Dark Matter

We study the dynamics of dark matter (DM) particle-antiparticle oscillations within the context of asymmetric DM. Oscillations arise due to small DM number-violating Majorana-type mass terms, and can lead to recoupling of annihilation after freeze-out and washout of the DM density. We derive the density matrix equations for DM oscillations and freeze-out from first principles using nonequilibrium field theory, and our results are qualitatively different than in previous studies. DM dynamics exhibits particle-vs-antiparticle "flavor" effects, depending on the interaction type, analogous to neutrino oscillations in a medium. "Flavor-sensitive" DM interactions include scattering or annihilation through a new vector boson, while "flavor-blind" interactions include scattering or s-channel annihilation through a new scalar boson, or annihilation to pairs of bosons. In particular, we find that flavor-sensitive annihilation does not recouple when coherent oscillations begin, and that flavor-blind scattering does not lead to decoherence.

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arXiv2012

Constraints on Scalar Asymmetric Dark Matter from Black Hole Formation in Neutron Stars

We consider possibly observable effects of asymmetric dark matter (ADM) in neutron stars. Since dark matter does not self-annihilate in the ADM scenario, dark matter accumulates in neutron stars, eventually reaching the Chandrasekhar limit and forming a black hole. We focus on the case of scalar ADM, where the constraints from Bose-Einstein condensation and subsequent black hole formation are most severe due to the absence of Fermi degeneracy pressure. We also note that in some portions of this constrained parameter space, non-trivial effects from Hawking radiation can modify our limits. We find that for scalar ADM with mass between 100 keV and 10^5 GeV, the constraint from pulsars in globular clusters on the scattering cross-section with neutrons ranges from σ_n < 10^{-45} cm^2 to 10^{-52} cm}^2. In particular, for scalar ADM with mass between 1 GeV and 1 TeV (in the case where black hole evaporation due to Hawking radiation is unimportant), the constraint on the scattering cross-section is below what is reachable with ton scale direct detection experiments.

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arXiv2011

Prospects for Detecting Dark Matter Halo Substructure with Pulsar Timing

One of the open questions of modern cosmology is the nature and properties of the Dark Matter halo and its substructures. In this work we study the gravitational effect of dark matter substructures on pulsar timing observations. Since millisecond pulsars are stable and accurate emitters, they have been proposed as plausible astrophysical tools to probe the gravitational effects of dark matter structures. We study this effect on pulsar timing through Shapiro time delay (or Integrated Sachs-Wolfe (ISW) effect) and Doppler effects statistically, showing that the latter dominates the signal. For this task, we relate the power spectrum of pulsar frequency change to the matter power spectrum on small scales, which we compute using the stable clustering hypothesis. We compare this power spectrum with the reach of current and future observations of pulsar timing designed for gravitational wave (GW) detection. Our results show that while current observations are unable to detect these signals, the sensitivity of the upcoming Square Kilometer Array (SKA) is only a factor of few weaker than our optimistic predictions.

astro-ph.CO↗

arXiv2011

Tevatron Top $A_{FB}$ Versus LHC Top Physics

We carry out a comprehensive analysis of models for top A_{FB} at CDF in light of new top data arriving from the LHC. We begin with a careful Tevatron analysis, considering in general which sets of effective vertices give rise to a large forward-backward asymmetry while suppressing the contribution to the total t tbar cross-section. We show on general grounds that scalar models struggle to produce sufficient asymmetries consistent with CDF observations, while vector models can produce a large asymmetry with a less significant tension in the total cross-section and $t\bar{t}$ invariant mass distribution at the Tevatron. We examine the essential observables of these models for top physics at LHC7 with 1 fb^{-1} of data, including the total cross-section, invariant mass distribution and number of additional jets in t tbar events. In the case of t-channel mediators, the LHC total cross-section places a strong constraint on light mediators, while the Tevatron invariant mass distributions place strong constraints on heavy mediators that are able to produce the asymmetry. Heavy axigluons are becoming increasingly squeezed by LHC7 t tbar and dijet resonance searches. We conclude that LHC7 top analyses are rapidly closing the window for viable models of the CDF top A_{FB}.

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arXiv2011

A Theory for Maximal Flavor Violation

Theories of flavor violation beyond the Standard Model typically suppose that the new contributions must be small, for example suppressed by Yukawa couplings, as in Minimal Flavor Violation. We show that this need not be true, presenting a case for flavor violation which maximally mixes the first and third generation flavors. As an example, we realize this scenario via new right-handed gauge bosons, which couple predominantly to the combinations (u,b)_R and (t,d)_R. We show that this new flavor violation could be responsible for several anomalies, focusing in particular on the B_d and B_s systems, and the Tevatron top forward-backward asymmetry.

hep-ph↗

arXiv2011

On Models of New Physics for the Tevatron Top A_FB

CDF has observed a top forward-backward asymmetry discrepant with the Standard Model prediction at 3.4 σ. We analyze models that could generate the asymmetry, including flavor-violating W's, horizontal Z'_Hs, triplet and sextet diquarks, and axigluons. We consider the detailed predictions of these models for the invariant mass and rapidity distributions of the asymmetry at the parton level, comparing against the unfolded parton-level CDF results. While all models can reproduce the asymmetry with the appropriate choice of mass and couplings, it appears at first examination that the extracted parton-level invariant mass distribution for all models are in conflict with Tevatron observations. We show on closer examination, however, that t tbar events in Z'_H and W' models have considerably lower selection efficiencies in high invariant mass bins as compared to the Standard Model, so that W', Z'_H, and axigluon models can generate the observed asymmetry while being consistent with the total cross-section and invariant mass spectrum. Triplet and sextet models have greater difficulty producing the observed asymmetry while remaining consistent with the total cross-section and invariant mass distribution. To more directly match the models and the CDF results, we proceed to decay and reconstruct the tops, comparing our results against the "raw" CDF asymmetry and invariant mass distributions. We find that the models that successfully generate the corrected CDF asymmetry at the parton level reproduce very well the more finely binned uncorrected asymmetry. Finally, we discuss the early LHC reach for discovery of these models, based on our previous analysis [arXiv:1102.0018].

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