534 research outputs found
An Electron Fixed Target Experiment to Search for a New Vector Boson A' Decaying to e+e-
We describe an experiment to search for a new vector boson A' with weak
coupling alpha' > 6 x 10^{-8} alpha to electrons (alpha=e^2/4pi) in the mass
range 65 MeV < m_A' < 550 MeV. New vector bosons with such small couplings
arise naturally from a small kinetic mixing of the "dark photon" A' with the
photon -- one of the very few ways in which new forces can couple to the
Standard Model -- and have received considerable attention as an explanation of
various dark matter related anomalies. A' bosons are produced by radiation off
an electron beam, and could appear as narrow resonances with small production
cross-section in the trident e+e- spectrum. We summarize the experimental
approach described in a proposal submitted to Jefferson Laboratory's PAC35,
PR-10-009. This experiment, the A' Experiment (APEX), uses the electron beam of
the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory
(CEBAF) at energies of ~1-4 GeV incident on 0.5-10% radiation length Tungsten
wire mesh targets, and measures the resulting e+e- pairs to search for the A'
using the High Resolution Spectrometer and the septum magnet in Hall A. With a
~1 month run, APEX will achieve very good sensitivity because the statistics of
e+e- pairs will be ~10,000 times larger in the explored mass range than any
previous search for the A' boson. These statistics and the excellent mass
resolution of the spectrometers allow sensitivity to alpha'/alpha one to three
orders of magnitude below current limits, in a region of parameter space of
great theoretical and phenomenological interest. Similar experiments could also
be performed at other facilities, such as the Mainz Microtron.Comment: 19 pages, 12 figures, 2 table
A Hybrid Higgs
We construct composite Higgs models admitting a weakly coupled Seiberg dual
description. We focus on the possibility that only the up-type Higgs is an
elementary field, while the down-type Higgs arises as a composite hadron. The
model, based on a confining SQCD theory, breaks supersymmetry and electroweak
symmetry dynamically and calculably. This simultaneously solves the \mu/B_\mu
problem and explains the smallness of the bottom and tau masses compared to the
top mass. The proposal is then applied to a class of models where the same
confining dynamics is used to generate the Standard Model flavor hierarchy by
quark and lepton compositeness. This provides a unified framework for flavor,
supersymmetry breaking and electroweak physics. The weakly coupled dual is used
to explicitly compute the MSSM parameters in terms of a few microscopic
couplings, giving interesting relations between the electroweak and soft
parameters. The RG evolution down to the TeV scale is obtained and salient
phenomenological predictions of this class of "single-sector" models are
discussed.Comment: 56 pages, 7 figures, v2: discussion on FCNCs and references added,
v3: JHEP versio
Supersymmetry phenomenology beyond the MSSM after 5/fb of LHC data
We briefly review the status of motivated beyond-the-MSSM phenomenology in
the light of the LHC searches to date. In particular, we discuss the conceptual
consequences of the exclusion bounds, of the hint for a Higgs boson at about
125 GeV, and of interpreting the excess of direct CP violation in the charm
sector as a signal of New Physics. We try to go into the various topics in a
compact way while providing a relatively rich list of references, with
particular attention to the most recent developments.Comment: 20 pages + refs. v2: minor modifications, published versio
CoGeNT Interpretations
Recently, the CoGeNT experiment has reported events in excess of expected
background. We analyze dark matter scenarios which can potentially explain this
signal. Under the standard case of spin independent scattering with equal
couplings to protons and neutrons, we find significant tensions with existing
constraints. Consistency with these limits is possible if a large fraction of
the putative signal events is coming from an additional source of experimental
background. In this case, dark matter recoils cannot be said to explain the
excess, but are consistent with it. We also investigate modifications to dark
matter scattering that can evade the null experiments. In particular, we
explore generalized spin independent couplings to protons and neutrons, spin
dependent couplings, momentum dependent scattering, and inelastic interactions.
We find that some of these generalizations can explain most of the CoGeNT
events without violation of other constraints. Generalized couplings with some
momentum dependence, allows further consistency with the DAMA modulation
signal, realizing a scenario where both CoGeNT and DAMA signals are coming from
dark matter. A model with dark matter interacting and annihilating into a new
light boson can realize most of the scenarios considered.Comment: 24 pages, 12 figs, v2: published version, some discussions clarifie
Direct Detection of Electroweak-Interacting Dark Matter
Assuming that the lightest neutral component in an SU(2)L gauge multiplet is
the main ingredient of dark matter in the universe, we calculate the elastic
scattering cross section of the dark matter with nucleon, which is an important
quantity for the direct detection experiments. When the dark matter is a real
scalar or a Majorana fermion which has only electroweak gauge interactions, the
scattering with quarks and gluon are induced through one- and two-loop quantum
processes, respectively, and both of them give rise to comparable contributions
to the elastic scattering cross section. We evaluate all of the contributions
at the leading order and find that there is an accidental cancellation among
them. As a result, the spin-independent cross section is found to be
O(10^-(46-48)) cm^2, which is far below the current experimental bounds.Comment: 19 pages, 7 figures, published versio
The kinetic dark-mixing in the light of CoGENT and XENON100
Several string or GUT constructions motivate the existence of a dark U(1)_D
gauge boson which interacts with the Standard Model only through its kinetic
mixing. We compute the dark matter abundance in such scenario and the
constraints in the light of the recent data from CoGENT, CDMSII and XENON100.
We show in particular that a region with relatively light WIMPS, M_{Z_D}< 40
GeV and a kinetic mixing 10^-4 < delta < 10^-3 is not yet excluded by the last
experimental data and seems to give promising signals in a near future. We also
compute the value of the kinetic mixing needed to explain the
DAMA/CoGENT/CRESST excesses and find that for M_{Z_D}< 30 GeV, delta ~ 10^-3 is
sufficient to fit with the data.Comment: 6 pages, 5figure
Stop the Top Background of the Stop Search
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.Comment: 16 pages, 5 figure
A New Era in the Quest for Dark Matter
There is a growing sense of `crisis' in the dark matter community, due to the
absence of evidence for the most popular candidates such as weakly interacting
massive particles, axions, and sterile neutrinos, despite the enormous effort
that has gone into searching for these particles. Here, we discuss what we have
learned about the nature of dark matter from past experiments, and the
implications for planned dark matter searches in the next decade. We argue that
diversifying the experimental effort, incorporating astronomical surveys and
gravitational wave observations, is our best hope to make progress on the dark
matter problem.Comment: Published in Nature, online on 04 Oct 2018. 13 pages, 1 figur
Supersymmetry with Light Stops
Recent LHC data, together with the electroweak naturalness argument, suggest
that the top squarks may be significantly lighter than the other sfermions. We
present supersymmetric models in which such a split spectrum is obtained
through "geometries": being "close to" electroweak symmetry breaking implies
being "away from" supersymmetry breaking, and vice versa. In particular, we
present models in 5D warped spacetime, in which supersymmetry breaking and
Higgs fields are located on the ultraviolet and infrared branes, respectively,
and the top multiplets are localized to the infrared brane. The hierarchy of
the Yukawa matrices can be obtained while keeping near flavor degeneracy
between the first two generation sfermions, avoiding stringent constraints from
flavor and CP violation. Through the AdS/CFT correspondence, the models can be
interpreted as purely 4D theories in which the top and Higgs multiplets are
composites of some strongly interacting sector exhibiting nontrivial dynamics
at a low energy. Because of the compositeness of the Higgs and top multiplets,
Landau pole constraints for the Higgs and top couplings apply only up to the
dynamical scale, allowing for a relatively heavy Higgs boson, including m_h =
125 GeV as suggested by the recent LHC data. We analyze electroweak symmetry
breaking for a well-motivated subset of these models, and find that fine-tuning
in electroweak symmetry breaking is indeed ameliorated. We also discuss a flat
space realization of the scenario in which supersymmetry is broken by boundary
conditions, with the top multiplets localized to a brane while other matter
multiplets delocalized in the bulk.Comment: 27 pages, 7 figure
SUSY, the Third Generation and the LHC
We develop a bottom-up approach to studying SUSY with light stops and
sbottoms, but with other squarks and sleptons heavy and beyond reach of the
LHC. We discuss the range of squark, gaugino and Higgsino masses for which the
electroweak scale is radiatively stable over the "little hierarchy" below ~ 10
TeV. We review and expand on indirect constraints on this scenario, in
particular from flavor and CP tests. We emphasize that in this context,
R-parity violation is very well motivated. The phenomenological differences
between Majorana and Dirac gauginos are also discussed. Finally, we focus on
the light subsystem of stops, sbottom and neutralino with R-parity, in order to
probe the current collider bounds. We find that 1/fb LHC bounds are mild and
large parts of the motivated parameter space remain open, while the 10/fb data
can be much more decisive.Comment: 42 pages, 8 figures, 1 table. V2: minor corrections, references adde
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