[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 di-jet
$+\displaystyle{\not{\text{E}}}_T$ channels predicts additional new physics.
Physical Review D 08/2013; 89(11). DOI:10.1103/PhysRevD.89.115014 · 4.64 Impact Factor
[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 mono-jet search, can be covered by the resonance
searches described in this paper.
Physics of the Dark Universe 12/2012; 2(1). DOI:10.1016/j.dark.2013.03.002 · 8.57 Impact Factor
[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 di-photon and ZZ search channels. This
observation has significant impact on low-energy supersymmetry. First, some
fine-tuning is necessary to accommodate such a Higgs mass in the Minimal
Supersymmetric Standard Model (MSSM) because the tree-level mass of the SM-like
Higgs boson in the MSSM is relatively small. We study the possibility of
lifting the mass of the SM-like Higgs boson by non-decoupling D-term from an
additional U(1) gauge symmetry. In particular, we focus on a gauged
Peccei-Quinn 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
di-photon 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 SM-like Higgs boson, the di-photon
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 di-photon 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 left-handed neutrino and two heavy standard model singlet neutrinos) which can be very light with mass in ~5-20 GeV range, as suggested by some current direct detection experiments. The IDM in this class of models has keV-scale 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 xenon-based 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.
Journal of High Energy Physics 02/2012; 2012(7). DOI:10.1007/JHEP07(2012)182 · 6.11 Impact Factor
[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 isospin-violating 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
spin-dependent 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 right-handed 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 right-handed 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 Big-Bang Nucleosynthesis. Comment: 16 pages, 1 figure; references added
Journal of High Energy Physics 11/2009; 2010(3). DOI:10.1007/JHEP03(2010)124 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using an effective theory approach, we calculate the neutron electric dipole
moment (nEDM) in the minimal left-right symmetric model with both explicit and
spontaneous CP violations. We integrate out heavy particles to obtain
flavor-neutral CP-violating effective Lagrangian. We run the Wilson
coefficients from the electroweak scale to the hadronic scale using one-loop
renormalization group equations. Using the state-of-the-art hadronic matrix
elements, we obtain the nEDM as a function of right-handed W-boson mass and
CP-violating parameters. We use the current limit on nEDM combined with the
kaon-decay parameter $\epsilon$ to provide the most stringent constraint yet on
the left-right symmetric scale $ M_{W_R} > (10 \pm 3) $ TeV.
Journal of High Energy Physics 10/2009; 2010(3). DOI:10.1007/JHEP03(2010)088 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In a class of beyond-standard-model theories, CP-odd observables, such as the neutron electric dipole moment, receive significant contributions from flavor-neutral P-odd and CP-odd four-quark operators. However, considerable uncertainties exist in the hadronic matrix elements of these operators strongly affecting the experimental constraints on CP-violating 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 leading-order QCD evolutions. We then match the four-quark 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 non-perturbative matching coefficients in simple quark models. We finally compare the results for the neutron electric dipole moment and P-odd and CP-odd pion-nucleon 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
Journal of High Energy Physics 08/2009; 2010(2). DOI:10.1007/JHEP02(2010)043 · 6.11 Impact Factor
[Show abstract][Hide abstract] 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 two-phase theory with both spontaneous and explicit CP violations. We present a systematic way to solve the right-handed quark mixing matrix analytically in this model and find that the leading order solution has the same hierarchical structure as the left-handed CKM matrix with one more CP-violating phase coming from the complex Higgs vev. Armed with this explicit right-handed mixing matrix, we explore its implications for flavor changing and conserving processes in detail, low-energy CP-violating 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 flavor-changing neutral Higgs mass is 25 TeV.
Nuclear Physics B 10/2008; 802(1-2-802):247-279. DOI:10.1016/j.nuclphysb.2008.05.019 · 3.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study flavor-changing neutral current and CP violations in the minimal supersymmetric left-right model. We calculate the beyond-standard-model contributions to the neutral kaon mixing ΔMK and ϵ, and find it is possible to have a numerical cancellation between the contributions from the right-handed gauge boson and supersymmetric box diagram. With the cancellation, the right-handed W-boson 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 left-right 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 right-handed 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 left-right 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 10-20 GeV above the MSSM bound if mass of the right-handed 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 right-handed quark mixings in the minimal left-right symmetric model which generally has both explicit and spontaneous CP violations. The leading-order result has the same hierarchical structure as the left-handed CKM mixing, but with additional CP phases originating from a spontaneous CP-violating phase in the Higgs vev. We explore the phenomenology entailed by the new right-handed mixing matrix, particularly the bounds on the mass of $W_R$ and the CP phase of the Higgs vev.