arXiv subjects

Lisa Randall

Publications and source records attributed to Lisa Randall.

At least 73 records · Page 4Linked to original sources

arXiv2012

Charming CP Violation and Dipole Operators from RS Flavor Anarchy

Recently the LHCb collaboration reported evidence for direct CP violation in charm decays. The value is sufficiently large that either substantially enhanced Standard Model contributions or non-Standard Model physics is required to explain it. In the latter case only a limited number of possibilities would be consistent with other existing flavor-changing constraints. We show that warped extra dimensional models that explain the quark spectrum through flavor anarchy can naturally give rise to contributions of the size required to explain the the LHCb result. The D meson asymmetry arises through a sizable CP-violating contribution to a chromomagnetic dipole operator. This happens naturally without introducing inconsistencies with existing constraints in the up quark sector. We discuss some subtleties in the loop calculation that are similar to those in Higgs to γγ. Loop-induced dipole operators in warped scenarios and their composite analogs exhibit non-trivial dependence on the Higgs profile, with the contributions monotonically decreasing when the Higgs is pushed away from the IR brane. We show that the size of the dipole operator quickly saturates as the Higgs profile approaches the IR brane, implying small dependence on the precise details of the Higgs profile when it is quasi IR localized. We also explain why the calculation of the coefficient of the lowest dimension 5D operator is guaranteed to be finite. This is true not only in the charm sector but also with other radiative processes such as electric dipole moments, b to sγ, ε'/ε_K and μ to eγ. We furthermore discuss the interpretation of this contribution within the framework of partial compositeness in four dimensions and highlight some qualitative differences between the generic result of composite models and that obtained for dynamics that reproduces the warped scenario.

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arXiv2012

A WIMPy Baryogenesis Miracle

We explore models in which weakly interacting massive particle (WIMP) dark matter annihilation is directly responsible for baryogenesis, thereby connecting dark matter with baryogenesis. We call this process "WIMPy baryogenesis". The dark matter relic density in these models, as with conventional WIMP models, is obtained with only order one couplings and TeV-scale masses according to the WIMP miracle. Thus, WIMPy baryogenesis models naturally accommodate weak-scale dark matter. Furthermore, an extension of the WIMP miracle simultaneously explains the observed baryon asymmetry and the correct dark matter abundance. The models we present have the further feature that they create the baryon number asymmetry at the weak scale, thereby avoiding the problems in some models of baryogenesis associated with high reheat temperatures in supersymmetric theories. Some of these models yield observable consequences in ongoing and future experiments.

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arXiv2012

Light Stops from Seiberg Duality

If low-energy supersymmetry is realized in nature, a seemingly contrived hierarchy in the squark mass spectrum appears to be required. We show that composite supersymmetric theories at the bottom of the conformal window can automatically yield the spectrum that is suggested by experimental data and naturalness. With a non-tuned choice of parameters, the only superpartners below one TeV will be the partners of the Higgs, the electroweak gauge bosons, the left-handed top and bottom, and the right-handed top, which are precisely the particles needed to make weak scale supersymmetry breaking natural. In the model considered here, these correspond to composite (or partially composite) degrees of freedom via Seiberg duality, while the other MSSM fields, with their heavier superpartners, are elementary. The key observation is that at or near the edge of the conformal window, soft supersymmetry breaking scalar and gaugino masses are transmitted only to fundamental particles at leading order. With the potential that arises from the duality, a Higgs with a 125 GeV mass, with nearly SM production rates, is naturally accommodated without tuning. The lightest ordinary superpartner is either the lightest stop or the lightest neutralino. If it is the stop, it is natural for it to be almost degenerate with the top, in which case it decays to top by emitting a very soft gravitino, making it quite difficult to find this mode at the LHC and more challenging to find SUSY in general, yielding a simple realization of the stealth supersymmetry idea. We analyze four benchmark spectra in detail.

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arXiv2011

Emergent Dark Matter, Baryon, and Lepton Numbers

We present a new mechanism for transferring a pre-existing lepton or baryon asymmetry to a dark matter asymmetry that relies on mass mixing which is dynamically induced in the early universe. Such mixing can succeed with only generic scales and operators and can give rise to distinctive relationships between the asymmetries in the two sectors. The mixing eliminates the need for the type of additional higher-dimensional operators that are inherent to many current asymmetric dark matter models. We consider several implementations of this idea. In one model, mass mixing is temporarily induced during a two-stage electroweak phase transition in a two Higgs doublet model. In the other class of models, mass mixing is induced by large field vacuum expectation values at high temperatures - either moduli fields or even more generic kinetic terms. Mass mixing models of this type can readily accommodate asymmetric dark matter masses ranging from 1 GeV to 100 TeV and expand the scope of possible relationships between the dark and visible sectors in such models.

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arXiv2011

Stable Colored Particles R-SUSY Relics or Not?

R-hadrons are only one of many possible stable colored states that the LHC might produce. All such particles would provide a spectacular, if somewhat unusual, signal at ATLAS and CMS. Produced in large numbers and leaving a characteristic signature throughout all layers of the detector, including the muon chamber, they could be straightforward to discover even with low luminosity. Though such long lived colored particles (LLCPs) can be realized in many extensions of the Standard Model, most analyses of their phenomenology have focused only on R-hadrons. In order to distinguish among the possibilities, fundamental quantum numbers of the new states must be measured. In this paper, we demonstrate how to identify the $SU(3)_C$ charge and spin of such new particles at the LHC.

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arXiv2011

Xogenesis

We present a new paradigm for dark matter in which a dark matter asymmetry is established in the early universe that is then transferred to ordinary matter. We show this scenario can fit naturally into weak scale physics models, with a dark matter candidate mass of this order. We present several natural suppression mechanisms, including bleeding dark matter number density into lepton number, which occurs naturally in models with lepton-violating operators transferring the asymmetry.

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arXiv2011

On the Feasibility and Utility of ISR Tagging

The production of new particles at a hadron collider like the LHC is always accompanied by QCD radiation attributable to the initial state (i.e. ISR). This tends to complicate analyses, so ISR is normally regarded as a nuisance. Nevertheless, we show that ISR can also be valuable, yielding information that can help in the discovery and interpretation of physics beyond the Standard Model. To access this information we will introduce new techniques designed to identify ISR jets on an event-by-event basis, a process we term ISR tagging. As a demonstration of their utility, we will apply these techniques to SUSY di-squark (di-gluino) production to show that they can be used to identify ISR jets in roughly 40% (15%) of the events, with a mistag rate of around 10% (15%). We then show that, through the application of a new method which we will introduce, knowledge of an ISR jet allows us to infer the squark (gluino) mass to within roughly 20% of its true value.

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arXiv2010

Early (and Later) LHC Search Strategies for Broad Dimuon Resonances

Resonance searches generally focus on narrow states that would produce a sharp peak rising over background. Early LHC running will, however, be sensitive primarily to broad resonances. In this paper we demonstrate that statistical methods should suffice to find broad resonances and distinguish them from both background and contact interactions over a large range of previously unexplored parameter space. We furthermore introduce an angular measure we call ellipticity, which measures how forward (or backward) the muon is in eta, and allows for discrimination between models with different parity violation early in the LHC running. We contrast this with existing angular observables and demonstrate that ellipticity is superior for discrimination based on parity violation, while others are better at spin determination.

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arXiv2010

General Analysis of Antideuteron Searches for Dark Matter

Low energy cosmic ray antideuterons provide a unique low background channel for indirect detection of dark matter. We compute the cosmic ray flux of antideuterons from hadronic annihilations of dark matter for various Standard Model final states and determine the mass reach of two future experiments (AMS-02 and GAPS) designed to greatly increase the sensitivity of antideuteron detection over current bounds. We consider generic models of scalar, fermion, and massive vector bosons as thermal dark matter, describe their basic features relevant to direct and indirect detection, and discuss the implications of direct detection bounds on models of dark matter as a thermal relic. We also consider specific dark matter candidates and assess their potential for detection via antideuterons from their hadronic annihilation channels. Since the dark matter mass reach of the GAPS experiment can be well above 100 GeV, we find that antideuterons can be a good indirect detection channel for a variety of thermal relic electroweak scale dark matter candidates, even when the rate for direct detection is highly suppressed.

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arXiv2009

Dynamical compactification from de Sitter space

We show that D-dimensional de Sitter space is unstable to the nucleation of non-singular geometries containing spacetime regions with different numbers of macroscopic dimensions, leading to a dynamical mechanism of compactification. These and other solutions to Einstein gravity with flux and a cosmological constant are constructed by performing a dimensional reduction under the assumption of q-dimensional spherical symmetry in the full D-dimensional geometry. In addition to the familiar black holes, black branes, and compactification solutions we identify a number of new geometries, some of which are completely non-singular. The dynamical compactification mechanism populates lower-dimensional vacua very differently from false vacuum eternal inflation, which occurs entirely within the context of four-dimensions. We outline the phenomenology of the nucleation rates, finding that the dimensionality of the vacuum plays a key role and that among vacua of the same dimensionality, the rate is highest for smaller values of the cosmological constant. We consider the cosmological constant problem and propose a novel model of slow-roll inflation that is triggered by the compactification process.

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arXiv2009

LHC Searches for Non-Chiral Weakly Charged Multiplets

Because the TeV-scale to be probed at the Large Hadron Collider should shed light on the naturalness, hierarchy, and dark matter problems, most searches to date have focused on new physics signatures motivated by possible solutions to these puzzles. In this paper, we consider some candidates for new states that although not well-motivated from this standpoint are obvious possibilities that current search strategies would miss. In particular we consider vector representations of fermions in multiplets of $SU(2)_L$ with a lightest neutral state. Standard search strategies would fail to find such particles because of the expected small one-loop-level splitting between charged and neutral states.

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arXiv2009

Extremal limits and black hole entropy

Taking the extremal limit of a non-extremal Reissner-Nordström black hole (by externally varying the mass or charge), the region between the inner and outer event horizons experiences an interesting fate -- while this region is absent in the extremal case, it does not disappear in the extremal limit but rather approaches a patch of $AdS_2\times S^2$. In other words, the approach to extremality is not continuous, as the non-extremal Reissner-Nordström solution splits into two spacetimes at extremality: an extremal black hole and a disconnected $AdS$ space. We suggest that the unusual nature of this limit may help in understanding the entropy of extremal black holes.

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arXiv2009

Candidates for Inelastic Dark Matter

Although we have yet to determine whether the DAMA data represents a true discovery of new physics, among such interpretations inelastic dark matter (IDM) can match the energy spectrum of DAMA very well while not contradicting the results of other direct detection searches. In this paper we investigate the general properties that a viable IDM candidate must have and search for simple models that realize these properties in natural ways. We begin by determining the regions of IDM parameter space that are allowed by direct detection searches including DAMA, paying special attention to larger IDM masses. We observe that an inelastic dark matter candidate with electroweak interactions can naturally satisfy observational constraints while simultaneously yielding the correct thermal relic abundance. We comment on several other proposed dark matter explanations for the DAMA signal and demonstrate that one of the proposed alternatives -- elastic scattering of dark matter off electrons -- is strongly disfavored when the modulated and unmodulated DAMA spectral data are taken into account. We then outline the general essential features of IDM models in which inelastic scattering off nuclei is mediated by the exchange of a massive gauge boson, and construct natural models in the context of a warped extra dimension and supersymmetry.

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arXiv2009

Quark and Lepton Flavor Physics from F-Theory

Recent work on local F-theory models shows the potential for new categories of flavor models. In this paper we investigate the perturbative effective theory interpretation of this result. We also show how to extend the model to the neutrino sector.

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arXiv2009

Natural Neutrino Masses and Mixings from Warped Geometry

We demonstrate that flavor symmetries in warped geometry can provide a natural explanation for large mixing angles and economically explain the distinction between the quark and lepton flavor sectors. We show how to naturally generate Majorana neutrino masses assuming a gauged a U(1)_{B-L} symmetry broken in the UV that generates see-saw masses of the right size. This model requires lepton minimal flavor violation (LMFV) in which only Yukawa matrices (present on the IR brane) break the flavor symmetries. The symmetry-breaking is transmitted to charged lepton bulk mass parameters as well to generate the hierarchy of charged lepton masses. With LMFV, a GIM-like mechanism prevents dangerous flavor-changing processes for charged leptons and permits flavor-changing processes only in the presence of the neutrino Yukawa interaction and are therefore suppressed when the overall scale for the neutrino Yukawa matrix is slightly smaller than one in units of the curvature. In this case the theory can be consistent with a cutoff of 10 TeV and 3 TeV Kaluza-Klein masses.

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arXiv2009

TeV Scale Singlet Dark Matter

It is well known that stable weak scale particles are viable dark matter candidates since the annihilation cross section is naturally about the right magnitude to leave the correct thermal residual abundance. Many dark matter searches have focused on relatively light dark matter consistent with weak couplings to the Standard Model. However, in a strongly coupled theory, or even if the coupling is just a few times bigger than the Standard Model couplings, dark matter can have TeV-scale mass with the correct thermal relic abundance. Here we consider neutral TeV-mass scalar dark matter, its necessary interactions, and potential signals. We consider signals both with and without higher-dimension operators generated by strong coupling at the TeV scale, as might happen for example in an RS scenario. We find some potential for detection in high energy photons that depends on the dark matter distribution. Detection in positrons at lower energies, such as those PAMELA probes, would be difficult though a higher energy positron signal could in principle be detectable over background. However, a light dark matter particle with higher-dimensional interactions consistent with a TeV cutoff can in principle match PAMELA data.

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arXiv2008

Exotic Implications of Electron and Photon Final States

New resonances with masses of order a few ${\rm TeV}$ might be discovered at the LHC. We show that no resonance that couples to electrons only through Standard Model interactions can decay to both $e^+e^-$and $γγ$ with significant branching ratios. This means that finding both electron-positron and two-photon final states is evidence that electrons couple directly to the new physics associated with the resonance and furthermore that the resonance is not spin-1. The least fine-tuned such examples involve electron compositeness. One such example, Kaluza Klein excitations of the graviton in the version of the Randall Sundrum Model where Standard Model matter is located on the ${\rm TeV}$ brane, can be distinguished from other possibilities by its predicted branching fractions into the two modes.

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arXiv2008

Dijet Searches for Supersymmetry at the LHC

We present several strategies for searching for supersymmetry in dijet channels that do not explicitly invoke missing energy. Preliminary investigations suggest that signal-to-background ratios of at least 4--5 should be achievable at the LHC, with discovery possible for squarks as heavy as ~ 1.7 TeV.

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