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ABSTRACT: Both the robust INTEGRAL 511 keV gamma-ray line and the recent tentative hint
of the 135 GeV gamma-ray line from Fermi-LAT have similar signal morphologies,
and may be produced from the same dark matter annihilation. Motivated by this
observation, we construct a dark matter model to explain both signals and to
accommodate the two required annihilation cross sections that are different by
more than six orders of magnitude. In our model, to generate the low-energy
positrons for INTEGRAL, dark matter particles annihilate into a complex scalar
that couples to photon via a charge-radius operator. The complex scalar
contains an excited state decaying into the ground state plus an off-shell
photon to generate a pair of positron and electron. Two charged particles with
non-degenerate masses are necessary for generating this charge-radius operator.
One charged particle is predicted to be long-lived and have a mass around 3.8
TeV to explain the dark matter thermal relic abundance from its late decay. The
other charged particle is predicted to have a mass below 1 TeV given the ratio
of the two signal cross sections. The 14 TeV LHC will concretely test the main
parameter space of this lighter charged particle.
12/2012;
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ABSTRACT: Recent claims of a line in the Fermi-LAT photon spectrum at 130 GeV are
suggestive of dark matter annihilation in the galactic center and other dark
matter-dominated regions. If the Fermi feature is indeed due to dark matter
annihilation, the best-fit line cross-section, together with the lack of any
corresponding excess in continuum photons, poses an interesting puzzle for
models of thermal dark matter: the line cross-section is too large to be
generated radiatively from open Standard Model annihilation modes, and too
small to provide efficient dark matter annihilation in the early universe. We
discuss two mechanisms to solve this puzzle and illustrate each with a simple
reference model in which the dominant dark matter annihilation channel is
photonic final states. The first mechanism we employ is resonant annihilation,
which enhances the annihilation cross-section during freezeout and allows for a
sufficiently large present-day annihilation cross section. Second, we consider
cascade annihilation, with a hierarchy between p-wave and s-wave processes.
Both mechanisms require mass near-degeneracies and predict states with masses
closely related to the dark matter mass; resonant freezeout in addition
requires new charged particles at the TeV scale.
08/2012;
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ABSTRACT: The main background for the supersymmetric stop direct production search
comes from Standard Model ttbar events. For the single-lepton search channel,
we introduce a few kinematic variables to further suppress this background by
focusing on its dileptonic and semileptonic topologies. All are defined to have
end points in the background, but not signal distributions. They can
substantially improve the stop signal significance and mass reach when combined
with traditional kinematic variables such as the total missing transverse
energy. Among them, our variable M^W_T2 has the best overall performance
because it uses all available kinematic information, including the on-shell
mass of both W's. We see 20%-30% improvement on the discovery significance and
estimate that the 8 TeV LHC run with 20 fb-1 of data would be able to reach an
exclusion limit of 650-700 GeV for direct stop production, as long as the stop
decays dominantly to the top quark and a light stable neutralino. Most of the
mass range required for the supersymmetric solution of the naturalness problem
in the standard scenario can be covered.
03/2012;
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ABSTRACT: The LHC is well on track toward the discovery or exclusion of a light
Standard Model (SM)-like Higgs boson. Such a Higgs has a very small SM width
and can easily have large branching fractions to physics beyond the SM, making
Higgs decays an excellent opportunity to observe new physics. Decays into
collider-invisible particles are particularly interesting as they are
theoretically well motivated and relatively clean experimentally. In this work
we estimate the potential of the 7 and 8 TeV LHC to observe an invisible Higgs
branching fraction. We analyze three channels that can be used to directly
study the invisible Higgs branching ratio at the 7 TeV LHC: an invisible Higgs
produced in association with (i) a hard jet; (ii) a leptonic Z; and (iii)
forward tagging jets. We find that the last channel, where the Higgs is
produced via weak boson fusion, is the most sensitive, allowing branching
fractions as small as 40% to be probed at 20 inverse fb for masses in the range
between 120 and 170 GeV, including in particular the interesting region around
125 GeV. We provide an estimate of the 8 TeV LHC sensitivity to an
invisibly-decaying Higgs produced via weak boson fusion and find that the reach
is comparable to but not better than the reach at the 7 TeV LHC. We further
estimate the discovery potential at the 8 TeV LHC for cases where the Higgs has
substantial branching fractions to both visible and invisible final states.
12/2011;
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ABSTRACT: A heavy Standard Model Higgs boson is not only disfavored by electroweak
precision observables but is also excluded by direct searches at the 7 TeV LHC
for a wide range of masses. Here, we examine scenarios where a heavy Higgs
boson can be made consistent with both the indirect constraints and the direct
null searches by adding only one new particle beyond the Standard Model. This
new particle should be a weak multiplet in order to have additional
contributions to the oblique parameters. If it is a color singlet, we find that
a heavy Higgs with an intermediate mass of 200 - 300 GeV can decay into the new
states, suppressing the branching ratios for the standard model modes, and thus
hiding a heavy Higgs at the LHC. If the new particle is also charged under QCD,
the Higgs production cross section from gluon fusion can be reduced
significantly due to the new colored particle one-loop contribution. Current
collider constraints on the new particles allow for viable parameter space to
exist in order to hide a heavy Higgs boson. We categorize the general
signatures of these new particles, identify favored regions of their parameter
space and point out that discovering or excluding them at the LHC can provide
important indirect information for a heavy Higgs. Finally, for a very heavy
Higgs boson, beyond the search limit at the 7 TeV LHC, we discuss three
additional scenarios where models would be consistent with electroweak
precision tests: including an additional vector-like fermion mixing with the
top quark, adding another U(1) gauge boson and modifying triple-gauge boson
couplings.
12/2011;
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ABSTRACT: Many new physics models with strongly interacting sectors predict a mass
hierarchy between the lightest vector meson and the lightest pseudoscalar
mesons. We examine the power of jet substructure tools to extend the 7 TeV LHC
sensitivity to these new states for the case of QCD octet mesons, considering
both two gluon and two b-jet decay modes for the pseudoscalar mesons. We
develop both a simple dijet search using only the jet mass and a more
sophisticated jet substructure analysis, both of which can discover the
composite octets in a dijet-like signature. The reach depends on the mass
hierarchy between the vector and pseudoscalar mesons. We find that for the
pseudoscalar-to-vector meson mass ratio below approximately 0.2 the simple jet
mass analysis provides the best discovery limit; for a ratio between 0.2 and
the QCD-like value of 0.3, the sophisticated jet substructure analysis has the
best discovery potential; for a ratio above approximately 0.3, the standard
four-jet analysis is more suitable.
07/2011;
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ABSTRACT: At the LHC, top quark pairs are dominantly produced from gluons, making it
difficult to measure the top quark forward-backward asymmetry. To improve the
asymmetry measurement, we study variables that can distinguish between top
quarks produced from quarks and those from gluons: the invariant mass of the
top pair, the rapidity of the top-antitop system in the lab frame, the rapidity
of the top quark in the top-antitop rest frame, the top quark polarization and
the top-antitop spin correlation. We combine all the variables in a likelihood
discriminant method to separate quark-initiated events from gluon-initiated
events. We apply our method on models including G-prime's and W-prime's
motivated by the recent observation of a large top quark forward-backward
asymmetry at the Tevatron. We have found that the significance of the asymmetry
measurement can be improved by 10% to 30%. At the same time, the central values
of the asymmetry increase by 40% to 100%. We have also analytically derived the
best spin quantization axes for studying top quark polarization as well as
spin-correlation for the new physics models.
06/2011;
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Daniele Alves,
Nima Arkani-Hamed,
Sanjay Arora, Yang Bai,
Matthew Baumgart,
Joshua Berger,
Matthew Buckley,
Bart Butler,
Spencer Chang,
Hsin-Chia Cheng, [......],
Brooks Thomas,
Scott Thomas,
Natalia Toro,
Tomer Volansky,
Jay Wacker,
Wolfgang Waltenberger,
Itay Yavin,
Felix Yu,
Yue Zhao,
Kathryn Zurek
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ABSTRACT: This document proposes a collection of simplified models relevant to the
design of new-physics searches at the LHC and the characterization of their
results. Both ATLAS and CMS have already presented some results in terms of
simplified models, and we encourage them to continue and expand this effort,
which supplements both signature-based results and benchmark model
interpretations. A simplified model is defined by an effective Lagrangian
describing the interactions of a small number of new particles. Simplified
models can equally well be described by a small number of masses and
cross-sections. These parameters are directly related to collider physics
observables, making simplified models a particularly effective framework for
evaluating searches and a useful starting point for characterizing positive
signals of new physics. This document serves as an official summary of the
results from the "Topologies for Early LHC Searches" workshop, held at SLAC in
September of 2010, the purpose of which was to develop a set of representative
models that can be used to cover all relevant phase space in experimental
searches. Particular emphasis is placed on searches relevant for the first
~50-500 pb-1 of data and those motivated by supersymmetric models. This note
largely summarizes material posted at http://lhcnewphysics.org/, which includes
simplified model definitions, Monte Carlo material, and supporting contacts
within the theory community. We also comment on future developments that may be
useful as more data is gathered and analyzed by the experiments.
05/2011;
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ABSTRACT: We examine the implications of the recent CDF measurement of the top-quark
forward-backward asymmetry, focusing on a scenario with a new color octet
vector boson at 1-3 TeV. We study several models, as well as a general
effective field theory, and determine the parameter space which provides the
best simultaneous fit to the CDF asymmetry, the Tevatron top pair production
cross section, and the exclusion regions from LHC dijet resonance and contact
interaction searches. Flavor constraints on these models are more subtle and
less severe than the literature indicates. We find a large region of allowed
parameter space at high axigluon mass and a smaller region at low mass; we
match the latter to an SU(3)xSU(3)/SU(3) coset model with a heavy vector-like
fermion. Our scenario produces discoverable effects at the LHC with only 1-2
inverse femtobarns of luminosity at 7-8 TeV. Lastly, we point out that a
Tevatron measurement of the b-quark forward-backward asymmetry would be very
helpful in characterizing the physics underlying the top-quark asymmetry.
01/2011;
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ABSTRACT: Assuming dark matter particles can be pair-produced at the LHC from cascade decays of heavy particles, we investigate strategies to identify the event topologies based on the kinematic information of final state visible particles. This should be the first step towards measuring the masses and spins of the new particles in the decay chains including the dark matter particle. As a concrete example, we study in detail the final states with 4 jets plus missing energy. This is a particularly challenging scenario because of large experimental smearing effects and no fundamental distinction among the 4 jets. Based on the fact that the invariant mass of particles on the same decay chain has an end point in its distribution, we define several functions which can distinguish different topologies depending on whether they exhibit the end-point structure. We show that all possible topologies (e.g., two jets on each decay chain or three jets on one chain and the other jet on the other chain, and so on) in principle can be identified from the distributions of these functions of the visible particle momenta. We also consider cases with one jet from the initial state radiation as well as off-shell decays. Our studies show that the event topology may be identified with as few as several hundred signal events after basic cuts. The method can be readily generalized to other event topologies. In particular, event topologies including leptons will be easier because the end points are expected to be sharper and there are more distinct invariant mass distributions from different charges. Comment: 34 pages, 18 figures, 8 tables
12/2010;
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ABSTRACT: A long-range "dark force" has recently been proposed to mediate the dark matter (DM) annihilation. If DM particles are copiously produced at the Large Hadron Collider, the light dark force mediator will also be produced through radiation. We demonstrate how and how precise we can utilize this fact to measure the coupling constant of the dark force. The light mediator's mass is measured for the "lepton jet" to which it decays. In addition, the mass of the DM particle is determined using the m(T2) technique. Knowing these quantities is critical for calculating the DM relic density.
Physical Review Letters 07/2009; 103(5):051801. · 7.37 Impact Factor
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ABSTRACT: We propose a dark matter model with standard model singlet extension of the universal extra dimension model to explain the recent observations of ATIC, PPB-BETS, PAMELA, and DAMA. Other than the standard model fields propagating in the bulk of a five-dimensional space, one fermion field and one scalar field are introduced and both are standard model singlets. The zero mode of the new fermion is identified as the right-handed neutrino, while its first Klein-Kaluza (KK) mode is the lightest KK-odd particle and the dark matter candidate. The cosmic ray spectra from ATIC and PPB-BETS determine the dark matter particle mass and hence the fifth dimension compactification scale to be 1.0–1.6 TeV. The zero mode of the singlet scalar field with a mass below 1 GeV provides an attractive force between dark matter particles, which allows a Sommerfeld enhancement to boost the annihilation cross section in the Galactic halo to explain the PAMELA data. The DAMA annual modulation results are explained by coupling the same scalar field to the electron via a higher-dimensional operator. We analyze the model parameter space that can satisfy the dark matter relic abundance and accommodate all the dark matter detection experiments. We also consider constraints from the diffuse extragalactic gamma-ray background, which can be satisfied if the dark matter particle and the first KK mode of the scalar field have highly degenerate masses.
Phys. Rev. D. 05/2009; 79(9).
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ABSTRACT: A long-range ``dark force'' has recently been proposed to mediate the dark matter (DM) annihilation. If DM particles are copiously produced at the Large Hadron Collider (LHC), the light dark force mediator will also be produced through radiation. We demonstrate how and how precise we can utilize this fact to measure the coupling constant of the dark force. The light mediator's mass is measured from the ``lepton jet'' it decays to. In addition, the mass of the DM particle is determined using the MT2 technique. Knowing these quantities is critical for calculating the DM relic density. Comment: 4 pages, 4 figures; final version for PRL
02/2009;
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ABSTRACT: We propose a dark matter model with standard model singlet extension of the universal extra dimension model (sUED) to explain the recent observations of ATIC, PPB-BETS, PAMELA and DAMA. Other than the standard model fields propagating in the bulk of a 5-dimensional space, one fermion field and one scalar field are introduced and both are standard model singlets. The zero mode of the new fermion is identified as the right-handed neutrino, while its first KK mode is the lightest KK-odd particle and the dark matter candidate. The cosmic ray spectra from ATIC and PPB-BETS determine the dark matter particle mass and hence the fifth dimension compactification scale to be 1.0-1.6 TeV. The zero mode of the singlet scalar field with a mass below 1 GeV provides an attractive force between dark matter particles, which allows a Sommerfeld enhancement to boost the annihilation cross section in the Galactic halo to explain the PAMELA data. The DAMA annual modulation results are explained by coupling the same scalar field to the electron via a higher-dimensional operator. We analyze the model parameter space that can satisfy the dark matter relic abundance and accommodate all the dark matter detection experiments. We also consider constraints from the diffuse extragalactic gamma-ray background, which can be satisfied if the dark matter particle and the first KK-mode of the scalar field have highly degenerate masses. Comment: minor changes; final version for PRD
11/2008;
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ABSTRACT: We perform a model-independent study for top-antitop and top-top resonances in the dilepton channel at the Large Hadtron Collider. In this channel, we can solve the kinematic system to obtain the momenta of all particles including the two neutrinos, and hence the resonance mass and spin. For discovering top-antitop resonances, the dilepton channel is competitive to the semileptonic channel because of the good resolution of lepton momentum measurement and small standard model backgrounds. Moreover, the charges of the two leptons can be identified, which makes the dilepton channel advantageous for discovering top-top resonances and for distinguishing resonance spins. We discuss and provide resolutions for difficulties associated with heavy resonances and highly boosted top quarks.
10/2008;
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ABSTRACT: We construct a calculable model of electroweak symmetry breaking in which the Higgs doublet emerges from the metastable SUSY breaking sector as a pseudo Nambu-Goldstone boson. The Higgs boson mass is further protected by the little Higgs mechanism, and naturally suppressed by a two-loop factor from the SUSY breaking scale of 10 TeV. Gaugino and sfermion masses arise from standard gauge mediation, but the Higgsino obtains a tree-level mass at the SUSY breaking scale. At 1 TeV, aside from new gauge bosons and fermions similar to other little Higgs models and their superpartners, our model predicts additional electroweak triplets and doublets from the SUSY breaking sector.
Phys. Rev. D. 09/2007; 76(6).
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ABSTRACT: We construct a calculable model of electroweak symmetry breaking in which the Higgs doublet emerges from the meta-stable SUSY breaking sector as a pseudo Nambu-Goldstone boson. The Higgs boson mass is further protected by the little Higgs mechanism, and naturally suppressed by a two-loop factor from the SUSY breaking scale of 10 TeV. Gaugino and sfermion masses arise from standard gauge mediation, but the Higgsino obtains a tree-level mass at the SUSY breaking scale. At 1 TeV, aside from new gauge bosons and fermions similar to other little Higgs models and their superpartners, our model predicts additional electroweak triplets and doublets from the SUSY breaking sector.
07/2007;
