Publications (16)80.93 Total impact
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ABSTRACT: Light top superpartners play a key role in stabilizing the electroweak scale in supersymmetric theories. For $R$parity conserved supersymmetric models, traditional searches are not sensitive to the compressed regions. In this paper, we propose a new method targeting this region, with stop and neutralino mass splitting ranging from $m_{\tilde t}  m_\chi \gtrsim m_t$ to about 20 GeV. In particular, we focus on the signal process in which a pair of stops are produced in association with a hard jet, and define a new observable $R_M$ whose distribution has a peak in this compressed region. The position of the peak is closely correlated with $m_{\tilde t}$. We show that for 13 TeV LHC with a luminosity of 3000 fb$^{1}$, this analysis can extend the reach of stop in the compressed region to $m_{\tilde t}$ around 800 GeV.  [Show abstract] [Hide abstract]
ABSTRACT: Effective contact operators provide the simplest parameterization for dark matter searches at the LHC. At the same time, light mediator can change the sensitivity and search strategies in important ways. Considering simple models of mediators is an important next step for collider searches. In this paper, we consider the case of a $t$channel mediator. Its presence opens up new contributions to the monojet$+\displaystyle{\not{\text{E}}}_T$ searches and can change the reach significantly. We also study the complementarity between monojet$+\displaystyle{\not{\text{E}}}_T$ and direct pair production of the mediators. There is a large region of parameter space in which the monojet$+\displaystyle{\not{\text{E}}}_T$ channel provides the stronger limit. We combine the reaches of LHC search and direct detection, and compare it with the requirement from thermal relic abundance. We find that in the Dirac fermion dark matter case, there is no region in the parameter space that reconciles the combined constraint of monojet$+\displaystyle{\not{\text{E}}}_T$ search and direct detection with constraint from not over closing the universe; and in the Majorana fermion dark matter case, the mass of dark matter must be larger than about 200 GeV. If the relic abundance requirement are not satisfied within the simple model, discovery of dark matter at the LHC in monojet$+\displaystyle{\not{\text{E}}}_T$ and dijet $+\displaystyle{\not{\text{E}}}_T$ channels predicts additional new physics.  [Show abstract] [Hide abstract]
ABSTRACT: Light dark matter with mass smaller than about 10 GeV is difficult to probe from direct detection experiments. In order to have the correct thermal relic abundance, the mediator of the interaction between dark matter and the Standard Model (SM) should also be relatively light, $\sim 10^2$ GeV. If such a light mediator couples to charged leptons, it would already be strongly constrained by direct searches at colliders. In this work, we consider the scenario of a leptophobic light $Z'$ vector boson as the mediator, and study the the prospect of searching for it at the 8 TeV Large Hadron Collider (LHC). To improve the reach in the low mass region, we perform a detailed study of the processes that the $Z'$ is produced in association with jet, photon, $W^\pm$ and $Z^0$. We show that in the region where the mass of $Z'$ is between 80 and 400 GeV, the constraint from associated production can be comparable or even stronger than the known monojet and dijet constraints. Searches in these channels can be complementary to the monojet search, in particular if the $Z'$ couplings to quarks ($g_{Z'}$) and dark matter ($g_D$) are different. For $g_D < g_{Z'}$, we show that there is a larger region of parameter space which has correct thermal relic abundance and a light $Z'$, $M_{Z'} \sim 100 $ GeV. This region, which cannot be covered by the monojet search, can be covered by the resonance searches described in this paper.  [Show abstract] [Hide abstract]
ABSTRACT: The ATLAS and CMS collaborations have announced discovery of a ~125 GeV Higgs boson, after a combined analysis of the diphoton and ZZ search channels. This observation has significant impact on lowenergy supersymmetry. First, some finetuning is necessary to accommodate such a Higgs mass in the Minimal Supersymmetric Standard Model (MSSM) because the treelevel mass of the SMlike Higgs boson in the MSSM is relatively small. We study the possibility of lifting the mass of the SMlike Higgs boson by nondecoupling Dterm from an additional U(1) gauge symmetry. In particular, we focus on a gauged PecceiQuinn symmetry which can also be related to a possible solution of the mu problem in the MSSM. In addition to the measurement of the mass of the Higgs, the data also reveals a tantalizing hint of a significantly enhanced diphoton signal rate, 1.56 pm 0.43 and 1.9 pm 0.5 times of the SM prediction in the CMS and ATLAS experiments, respectively. We demonstrate that such an enhancement can be accommodated in this MSSM extension. Anomaly cancellation requires the introduction of charged exotics. If some of them happen to be light and have sizable coupling to the SMlike Higgs boson, the diphoton signal rate can be enhanced significantly. EW precision measurements provide stringent constraints on this model. Taking these into account, we identify two benchmark scenarios. We argue that they are representative of large classes of viable models beyond our current example which can consistently enhance the Higgs to diphoton rate. We also comment on possible signals of such light exotics at the LHC.  [Show abstract] [Hide abstract]
ABSTRACT: Extending the minimal supersymmetric standard model to explain small neutrino masses via the inverse seesaw mechanism can lead to a new light supersymmetric scalar partner which can play the role of inelastic dark matter (IDM). It is a linear combination of the superpartners of the neutral fermions in the theory (the light lefthanded neutrino and two heavy standard model singlet neutrinos) which can be very light with mass in ~520 GeV range, as suggested by some current direct detection experiments. The IDM in this class of models has keVscale mass splitting, which is intimately connected to the small Majorana masses of neutrinos. We predict the differential scattering rate and annual modulation of the IDM signal which can be testable at future germanium and xenonbased detectors.  [Show abstract] [Hide abstract]
ABSTRACT: Light Dark Matter, $<10$ GeV, with sizable direct detection rate is an interesting and less explored scenario. Collider searches can be very powerful, such as through the channel in which a pair of dark matter particle are produced in association with a jet. It is a generic possibility that the mediator of the interaction between DM and the nucleus will also be accessible at the Tevatron and the LHC. Therefore, collider search of the mediator can provide a more comprehensive probe of the dark matter and its interactions. In this article, to demonstrate the complementarity of these two approaches, we focus on the possibility of the mediator being a new $U(1)'$ gauge boson, which is probably the simplest model which allows a large direct detection cross section for a light dark matter candidate. We combine searches in the monojet+MET channel and dijet resonance search for the mediator. We find that for the mass of $Z'$ between 250 GeV and 4 TeV, resonance searches at the colliders provide stronger constraints on this model than the monojet+MET searches.  [Show abstract] [Hide abstract]
ABSTRACT: We reanalyze the annual modulation data observed by CoGeNT experiment and show that if the annually modulated anomaly detected by CoGeNT is induced by collision between dark matter particle and nucleus, it can be fitted by a $Z'$ model with inelastic dark matter and isospinviolating interaction, and the constraint from XENON100 can be avoided. This $Z'$ model is strongly constrained by collider physics that the upper bound of the mass of $Z'$ is around twice of the mass of dark matter.  [Show abstract] [Hide abstract]
ABSTRACT: In a recent paper, four of the present authors proposed a class of dark matter models where generalized parity symmetry leads to equality of dark matter abundance with baryon asymmetry of the Universe and predicts dark matter mass to be around 5 GeV. In this note we explore how this model can be tested in direct search experiments. In particular, we point out that if the dark matter happens to be the mirror neutron, the direct detection cross section has the unique feature that it increases at low recoil energy unlike the case of conventional WIMPs. It is also interesting to note that the predicted spindependent scattering could make significant contribution to the total direct detection rate, especially for light nucleus. With this scenario, one could explain recent DAMA and CoGeNT results.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a model of asymmetric dark matter (DM) where the dark sector is an identical copy of both forces and matter of the standard model (SM) as in the mirror universe models discussed in literature. In addition to being connected by gravity, the SM and DM sectors are also connected at high temperature by a common set of heavy righthanded Majorana neutrinos via their Yukawa couplings to leptons and Higgs bosons. The lightest nucleon in the dark (mirror) sector is a candidate for dark matter. The out of equilibrium decay of righthanded neutrino produces equal lepton asymmetry in both sectors via resonant leptogenesis which then get converted to baryonic and dark baryonic matter. The dark baryon asymmetry due to higher dark nucleon masses leads to higher dark matter density compared to the familiar baryon density that is observed. The standard model neutrinos in this case acquire masses from the inverse seesaw mechanism. A kinetic mixing between the U(1) gauge fields of the two sectors is introduced to guarantee the success of BigBang Nucleosynthesis. Comment: 16 pages, 1 figure; references added  [Show abstract] [Hide abstract]
ABSTRACT: Using an effective theory approach, we calculate the neutron electric dipole moment (nEDM) in the minimal leftright symmetric model with both explicit and spontaneous CP violations. We integrate out heavy particles to obtain flavorneutral CPviolating effective Lagrangian. We run the Wilson coefficients from the electroweak scale to the hadronic scale using oneloop renormalization group equations. Using the stateoftheart hadronic matrix elements, we obtain the nEDM as a function of righthanded Wboson mass and CPviolating parameters. We use the current limit on nEDM combined with the kaondecay parameter $\epsilon$ to provide the most stringent constraint yet on the leftright symmetric scale $ M_{W_R} > (10 \pm 3) $ TeV.  [Show abstract] [Hide abstract]
ABSTRACT: In a class of beyondstandardmodel theories, CPodd observables, such as the neutron electric dipole moment, receive significant contributions from flavorneutral Podd and CPodd fourquark operators. However, considerable uncertainties exist in the hadronic matrix elements of these operators strongly affecting the experimental constraints on CPviolating parameters in the theories. Here we study their hadronic matrix elements in combined chiral perturbation theory and nucleon models. We first classify the operators in chiral representations and present the leadingorder QCD evolutions. We then match the fourquark operators to the corresponding ones in chiral hadronic theory, finding symmetry relations among the matrix elements. Although this makes lattice QCD calculations feasible, we choose to estimate the nonperturbative matching coefficients in simple quark models. We finally compare the results for the neutron electric dipole moment and Podd and CPodd pionnucleon couplings with the previous studies using naive factorization and QCD sum rules. Our study shall provide valuable insights on the present hadronic physics uncertainties in these observables. Comment: 40 pages, 7 figures. This is the final version. A discussion of the uncertainty of the calculation is added 
Article: General CP violation in minimal left–right symmetric model and constraints on the righthanded scale
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ABSTRACT: In minimal left–right symmetric theories, the requirement of parity invariance allows only one complex phase in the Higgs potential and one in the Yukawa couplings, leading to a twophase theory with both spontaneous and explicit CP violations. We present a systematic way to solve the righthanded quark mixing matrix analytically in this model and find that the leading order solution has the same hierarchical structure as the lefthanded CKM matrix with one more CPviolating phase coming from the complex Higgs vev. Armed with this explicit righthanded mixing matrix, we explore its implications for flavor changing and conserving processes in detail, lowenergy CPviolating observables in particular. We report an improved lower bound on the WR mass of 2.5 TeV from ΔMK and ΔMB, and a somewhat higher bound (4 TeV) from kaon decay parameters ϵ, ϵ′, and neutron electric dipole moment. The new bound on the flavorchanging neutral Higgs mass is 25 TeV. 
Article: Constraining the righthanded scale through kaon mixing in the supersymmetric leftright model
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ABSTRACT: We study flavorchanging neutral current and CP violations in the minimal supersymmetric leftright model. We calculate the beyondstandardmodel contributions to the neutral kaon mixing ΔMK and ϵ, and find it is possible to have a numerical cancellation between the contributions from the righthanded gauge boson and supersymmetric box diagram. With the cancellation, the righthanded Wboson mass scale can be lowered to about 2 TeV, well within the search limit of LHC.  [Show abstract] [Hide abstract]
ABSTRACT: We show that in supersymmetric leftright models (SUSYLR), the upper bound on the lightest neutral Higgs mass can be appreciably higher than that in the minimal supersymmetric standard model (MSSM). The exact magnitude of the bound depends on the scale of parity restoration and can be 10–20 GeV above the MSSM bound if the mass of the righthanded WR is in the TeV range. An important implication of our result is that since SUSYLR models provide a simple realization of the seesaw mechanism for neutrino masses, measurement of the Higgs boson mass could provide an independent probe of a low seesaw scale.  [Show abstract] [Hide abstract]
ABSTRACT: We show that in supersymmetric leftright models (SUSYLR), the upper bound on the lightest neutral Higgs mass can be appreciably higher than that in minimal supersymmetric standard model (MSSM). The exact magnitude of the bound depends on the scale of parity restoration and can be 1020 GeV above the MSSM bound if mass of the righthanded gauge boson $W_R$ is in the TeV range. An important implication of our result is that since SUSYLR models provide a simple realization of seesaw mechanism for neutrino masses, measurement of the Higgs boson mass could provide an independent probe of a low seesaw scale.  [Show abstract] [Hide abstract]
ABSTRACT: We present a systematic approach to solve analytically for the righthanded quark mixings in the minimal leftright symmetric model which generally has both explicit and spontaneous CP violations. The leadingorder result has the same hierarchical structure as the lefthanded CKM mixing, but with additional CP phases originating from a spontaneous CPviolating phase in the Higgs vev. We explore the phenomenology entailed by the new righthanded mixing matrix, particularly the bounds on the mass of $W_R$ and the CP phase of the Higgs vev.
Publication Stats
573  Citations  
80.93  Total Impact Points  
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Institutions

20072012

University of Maryland, College Park
 Department of Physics
Maryland, United States
