arXiv subjects

Yanou Cui

Publications and source records attributed to Yanou Cui.

68 records · Page 4Linked to original sources

arXiv2013

Natural Islands for a 125 GeV Higgs in the scale-invariant NMSSM

We study whether a 125 GeV standard model-like Higgs boson can be accommodated within the scale-invariant NMSSM in a way that is natural in all respects, i.e., not only is the stop mass and hence its loop contribution to Higgs mass of natural size, but we do not allow significant tuning of NMSSM parameters as well. We pursue as much as possible an analytic approach which gives clear insights on various ways to accommodate such a Higgs mass, while conducting complementary numerical analyses. We consider both scenarios with singlet-like state being heavier and lighter than SM-like Higgs. With A-terms being small, we find for the NMSSM to be perturbative up to GUT scale, it is not possible to get 125 GeV Higgs mass, which is true even if we tune parameters of NMSSM. If we allow some of the couplings to become non-perturbative below the GUT scale, then the non-tuned option implies that the singlet self-coupling, kappa, is larger than the singlet-Higgs coupling, lambda, which itself is order 1. This leads to a Landau pole for these couplings close to the weak scale, in particular below ~10^4 TeV. In both the perturbative and non-perturbative NMSSM, allowing large A_lambda, A_kappa gives "more room" to accommodate a 125 GeV Higgs, but a tuning of these A-terms may be needed. In our analysis we also conduct a careful study of the constraints on the parameter space from requiring global stability of the desired vacuum fitting a 125 GeV Higgs, which is complementary to existing literature. In particular, as the singlet-Higgs coupling lambda increases, vacuum stability becomes more serious of an issue.

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arXiv2012

Baryogenesis for WIMPs

We propose a robust, unified framework, in which the similar baryon and dark matter cosmic abundances both arise from the physics of weakly interacting massive particles (WIMPs), with the rough quantitative success of the so-called "WIMP miracle". In particular the baryon asymmetry arises from the decay of a meta-stable WIMP after its thermal freezeout at or below the weak scale. A minimal model and its embedding in R-parity violating (RPV) SUSY are studied as examples. The new mechanism saves RPV SUSY from the potential crisis of washing out primordial baryon asymmetry. Phenomenological implications for the LHC and precision tests are discussed.

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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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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

Top condensation as a motivated explanation of the top forward-backward asymmetry

Models of top condensation can provide both a compelling solution to the hierarchy problem as well as an explanation of why the top-quark mass is large. The spectrum of such models, in particular topcolor-assisted technicolor, includes top-pions, top-rhos and the top-Higgs, all of which can easily have large top-charm or top-up couplings. Large top-up couplings in particular would lead to a top forward-backward asymmetry through $t$-channel exchange, easily consistent with the Tevatron measurements. Intriguingly, there is destructive interference between the top-mesons and the standard model which conspire to make the overall top pair production rate consistent with the standard model. The rate for same-sign top production is also small due to destructive interference between the neutral top-pion and the top-Higgs. Flavor physics is under control because new physics is mostly confined to the top quark. In this way, top condensation can explain the asymmetry and be consistent with all experimental bounds. There are many additional signatures of topcolor with large tu mixing, such as top(s)+jet(s) events, in which a top and a jet reconstruct a resonance mass, which make these models easily testable at the LHC.

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arXiv2011

W-jet Tagging: Optimizing the Identification of Boosted Hadronically-Decaying W Bosons

A method is proposed for distinguishing highly boosted hadronically decaying W's (W-jets) from QCD-jets using jet substructure. Previous methods, such as the filtering/mass-drop method, can give a factor of ~2 improvement in S/sqrt(B) for jet pT > 200 GeV. In contrast, a multivariate approach including new discriminants such as R-cores, which characterize the shape of the W-jet, subjet planar flow, and grooming-sensitivities is shown to provide a much larger factor of ~5 improvement in S/sqrt(B). For longitudinally polarized W's, such as those coming from many new physics models, the discrimination is even better. Comparing different Monte Carlo simulations, we observe a sensitivity of some variables to the underlying event; however, even with a conservative estimates, the multivariate approach is very powerful. Applications to semileptonic WW resonance searches and all-hadronic W+jet searches at the LHC are also discussed. Code implementing our W-jet tagging algorithm is publicly available at http://jets.physics.harvard.edu/wtag

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arXiv2010

Fermion Masses in Emergent Electroweak Symmetry Breaking

We consider the generation of fermion masses in an emergent model of electroweak symmetry breaking with composite $W,Z$ gauge bosons. A universal bulk fermion profile in a warped extra dimension is used for all fermion flavors. Electroweak symmetry is broken at the UV (or Planck) scale where boundary mass terms are added to generate the fermion flavor structure. This leads to flavor-dependent nonuniversality in the gauge couplings. The effects are suppressed for the light fermion generations but are enhanced for the top quark where the $Zt{\bar t}$ and $Wt{\bar b}$ couplings can deviate at the $10-20%$ level in the minimal setup. By the AdS/CFT correspondence our model implies that electroweak symmetry is not a fundamental gauge symmetry. Instead the Standard Model with massive fermions and $W,Z$ gauge bosons is an effective chiral Lagrangian for some underlying confining strong dynamics at the TeV scale, where mass is generated without a Higgs mechanism.

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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

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

Emergent Electroweak Symmetry Breaking with Composite W, Z Bosons

We present a model of electroweak symmetry breaking in a warped extra dimension where electroweak symmetry is broken at the UV (or Planck) scale. An underlying conformal symmetry is broken at the IR (or TeV) scale generating masses for the electroweak gauge bosons without invoking a Higgs mechanism. By the AdS/CFT correspondence the W,Z bosons are identified as composite states of a strongly-coupled gauge theory, suggesting that electroweak symmetry breaking is an emergent phenomenon at the IR scale. The model satisfies electroweak precision tests with reasonable fits to the S and T parameter. In particular the T parameter is sufficiently suppressed since the model naturally admits a custodial SU(2) symmetry. The composite nature of the W,Z-bosons provide a novel possibility of unitarizing WW scattering via form factor suppression. Constraints from LEP and the Tevatron as well as discovery opportunities at the LHC are discussed for these composite electroweak gauge bosons.

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arXiv2008

Non-Thermal Dark Matter from Cosmic Strings

Cosmic strings can be created in the early universe during symmetry-breaking phase transitions, such as might arise if the gauge structure of the standard model is extended by additional U(1) factors at high energies. Cosmic strings present in the early universe form a network of long horizon-length segments, as well as a population of closed string loops. The closed loops are unstable against decay, and can be a source of non-thermal particle production. In this work we compute the density of WIMP dark matter formed by the decay of gauge theory cosmic string loops derived from a network of long strings in the scaling regime or under the influence of frictional forces. We find that for symmetry breaking scales larger than 10^10 GeV, this mechanism has the potential to account for the observed relic density of dark matter. For symmetry breaking scales lower than this, the density of dark matter created by loop decays from a scaling string network lies below the observed value. In particular, the cosmic strings originating from a U(1) gauge symmetry broken near the electroweak scale, that could lead to a massive Z' gauge boson observable at the LHC, produce a negligibly small dark matter relic density by this mechanism.

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arXiv2008

Cosmic Strings from Supersymmetric Flat Directions

Flat directions are a generic feature of the scalar potential in supersymmetric gauge field theories. They can arise, for example, from D-terms associated with an extra abelian gauge symmetry. Even when supersymmetry is broken softly, there often remain directions in the scalar field space along which the potential is almost flat. Upon breaking a gauge symmetry along one of these almost flat directions, cosmic strings may form. Relative to the standard cosmic string picture based on the abelian Higgs model, these flat-direction cosmic strings have the extreme Type-I properties of a thin gauge core surrounded by a much wider scalar field profile. We perform a comprehensive study of the microscopic, macroscopic, and observational characteristics of this class of strings. We find many differences from the standard string scenario, including stable higher winding mode strings, the dynamical formation of higher mode strings from lower ones, and a resultant multi-tension scaling string network in the early universe. These strings are only moderately constrained by current observations, and their gravitational wave signatures may be detectable at future gravity wave detectors. Furthermore, there is the interesting but speculative prospect that the decays of cosmic string loops in the early universe could be a source of ultra-high energy cosmic rays or non-thermal dark matter. We also compare the observational signatures of flat-direction cosmic strings with those of ordinary cosmic strings as well as (p,q) cosmic strings motivated by superstring theory.

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arXiv2007

Narrow Trans-TeV Higgs Bosons and $H\to hh$ Decays: Two LHC Search Paths for a Hidden Sector Higgs Boson

We consider the addition of a condensing singlet scalar field to the Standard Model. Such a scenario may be motivated by any number of theoretical ideas, including the common result in string-inspired model building of singlet scalar fields charged under some hidden sector gauge symmetry. For concreteness, we specify an example model of this type, and consider the relevant constraints on Higgs physics, such as triviality, perturbative unitarity and precision electroweak analysis. We then show that there are two unique features of the phenomenology that present opportunities for discovery at the Large Hadron Collider (LHC). First, it is possible to identify and discover a narrow trans-TeV Higgs boson in this scenario -- a mass scale that is well above the scale at which it is meaningful to discuss a SM Higgs boson. Second, the decays of the heavier scalar state into the lighter Higgs bosons can proceed at a high rate and may be the first discovery mode in the Higgs sector.

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arXiv2006

Adjoint Chiral Supermultiplets and Their Phenomenology

Matter fields in the MSSM are chiral supermultiplets in fundamental (or singlet) representations of the standard model gauge group. In this paper we introduce chiral superfields in the adjoint representation of $SU(3)_C$ and study the effective field theory and phenomenology of them. These states are well motivated by intersecting D-brane models in which additional massless adjoint chiral supermultiplets appear generically in the low energy spectrum. Although it has been pointed out that the existence of these additional fields may make it difficult to obtain asymptotic freedom, we demonstrate that this consideration does not rule out the existence of adjoints. The QCD gauge coupling can be perturbative up to a sufficiently high scale, and therefore a perturbative description for a D-brane model is valid. The full supersymmetric and soft SUSY breaking Lagrangians and the resulting renormalization group equations are given. Phenomenological aspects of the adjoint matter are also studied, including the decay and production processes. The similarity in gauge interaction between the adjoint fermion and gluino facilitates our study on these aspects. It is found that these adjoint multiplets can give detectable signals at colliders and satisfy the constraints from cosmology.

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