31 research outputs found
A Study of the Scintillation Induced by Alpha Particles and Gamma Rays in Liquid Xenon in an Electric Field
Scintillation produced in liquid xenon by alpha particles and gamma rays has
been studied as a function of applied electric field. For back scattered gamma
rays with energy of about 200 keV, the number of scintillation photons was
found to decrease by 64+/-2% with increasing field strength. Consequently, the
pulse shape discrimination power between alpha particles and gamma rays is
found to reduce with increasing field, but remaining non-zero at higher fields.Comment: 15 pages, 12 figures, accepted by Nuclear Instruments and Methods in
Physics Research
Measurement of single electron emission in two-phase xenon
We present the first measurements of the electroluminescence response to the
emission of single electrons in a two-phase noble gas detector. Single
ionization electrons generated in liquid xenon are detected in a thin gas layer
during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon
detector for WIMP dark matter searches. Both the pressure dependence and
magnitude of the single-electron response are in agreement with previous
measurements of electroluminescence yield in xenon. We discuss different
photoionization processes as possible cause for the sample of single electrons
studied in this work. This observation may have implications for the design and
operation of future large-scale two-phase systems.Comment: 11 pages, 6 figure
Improved Constraints on Inelastic Dark Matter
We perform an extensive study of the DAMA annual modulation data in the
context of inelastic dark matter. We find that inelastic dark matter with mass
m > 15 GeV is excluded at the 95% confidence level by the combination of DAMA
spectral information and results from other direct detection experiments.
However, at smaller m, inelastic dark matter constitutes a possible solution to
the DAMA puzzle.Comment: 22 pages, 7 figures, Reference added, matches published versio
The Inert Doublet Model and Inelastic Dark Matter
The annual modulation observed by DAMA/NaI and DAMA/Libra may be interpreted
in terms of elastic or inelastic scattering of dark matter particles. In this
paper we confront these two scenarios within the framework of a very simple
extension of the Standard Model, the Inert Doublet Model (IDM). In this model
the dark matter candidate is a scalar, the lightest component of an extra Higgs
doublet. We first revisit the case for the elastic scattering of a light scalar
WIMP, M_DM~10 GeV, a scenario which requires that a fraction of events in DAMA
are channelled. Second we consider the possibility of inelastic Dark Matter
(iDM). This option is technically natural in the IDM, in the sense that the
mass splitting between the lightest and next-to-lightest neutral scalars may be
protected by a Peccei-Quinn (PQ) symmetry. We show that candidates with a mass
M_DM between ~535 GeV and ~50 TeV may reproduce the DAMA data and have a cosmic
abundance in agreement with WMAP. This range may be extended to candidates as
light as ~50 GeV if we exploit the possibility that the approximate PQ symmetry
is effectively conserved and that a primordial asymmetry in the dark sector may
survive until freeze-out.Comment: 16 pages, 7 figures. v2: minor changes and discussion on the
embedding in SO(10) added. v3: matches the published version in JCA
PAMELA, DAMA, INTEGRAL and Signatures of Metastable Excited WIMPs
Models of dark matter with ~ GeV scale force mediators provide attractive
explanations of many high energy anomalies, including PAMELA, ATIC, and the
WMAP haze. At the same time, by exploiting the ~ MeV scale excited states that
are automatically present in such theories, these models naturally explain the
DAMA/LIBRA and INTEGRAL signals through the inelastic dark matter (iDM) and
exciting dark matter (XDM) scenarios, respectively. Interestingly, with only
weak kinetic mixing to hypercharge to mediate decays, the lifetime of excited
states with delta < 2 m_e is longer than the age of the universe. The
fractional relic abundance of these excited states depends on the temperature
of kinetic decoupling, but can be appreciable. There could easily be other
mechanisms for rapid decay, but the consequences of such long-lived states are
intriguing. We find that CDMS constrains the fractional relic population of
~100 keV states to be <~ 10^-2, for a 1 TeV WIMP with sigma_n = 10^-40 cm^2.
Upcoming searches at CDMS, as well as xenon, silicon, and argon targets, can
push this limit significantly lower. We also consider the possibility that the
DAMA excitation occurs from a metastable state into the XDM state, which decays
via e+e- emission, which allows lighter states to explain the INTEGRAL signal
due to the small kinetic energies required. Such models yield dramatic signals
from down-scattering, with spectra peaking at high energies, sometimes as high
as ~1 MeV, well outside the usual search windows. Such signals would be visible
at future Ar and Si experiments, and may be visible at Ge and Xe experiments.
We also consider other XDM models involving ~ 500 keV metastable states, and
find they can allow lighter WIMPs to explain INTEGRAL as well.Comment: 22 pages, 7 figure
Non-Abelian Dark Sectors and Their Collider Signatures
Motivated by the recent proliferation of observed astrophysical anomalies,
Arkani-Hamed et al. have proposed a model in which dark matter is charged under
a non-abelian "dark" gauge symmetry that is broken at ~ 1 GeV. In this paper,
we present a survey of concrete models realizing such a scenario, followed by a
largely model-independent study of collider phenomenology relevant to the
Tevatron and the LHC. We address some model building issues that are easily
surmounted to accommodate the astrophysics. While SUSY is not necessary, we
argue that it is theoretically well-motivated because the GeV scale is
automatically generated. Specifically, we propose a novel mechanism by which
mixed D-terms in the dark sector induce either SUSY breaking or a super-Higgs
mechanism precisely at a GeV. Furthermore, we elaborate on the original
proposal of Arkani-Hamed et al. in which the dark matter acts as a messenger of
gauge mediation to the dark sector. In our collider analysis we present
cross-sections for dominant production channels and lifetime estimates for
primary decay modes. We find that dark gauge bosons can be produced at the
Tevatron and the LHC, either through a process analogous to prompt photon
production or through a rare Z decay channel. Dark gauge bosons will decay back
to the SM via "lepton jets" which typically contain >2 and as many as 8
leptons, significantly improving their discovery potential. Since SUSY decays
from the MSSM will eventually cascade down to these lepton jets, the discovery
potential for direct electroweak-ino production may also be improved.
Exploiting the unique kinematics, we find that it is possible to reconstruct
the mass of the MSSM LSP. We also present decay channels with displaced
vertices and multiple leptons with partially correlated impact parameters.Comment: 44 pages, 25 figures, version published in JHE
Inelastic Dark Matter, Non-Standard Halos and the DAMA/LIBRA Results
The DAMA collaboration have claimed to detect particle dark matter (DM) via
an annual modulation in their observed recoil event rate. This appears to be in
strong disagreement with the null results of other experiments if interpreted
in terms of elastic DM scattering, while agreement for a small region of
parameter space is possible for inelastic DM (iDM) due to the altered
kinematics of the collision. To date most analyses assume a simple galactic
halo DM velocity distribution, the Standard Halo Model, but direct experimental
support for the SHM is severely lacking and theoretical studies indicate
possible significant differences. We investigate the dependence of DAMA and the
other direct detection experiments on the local DM velocity distribution,
utilizing the results of the Via Lactea and Dark Disc numerical simulations. We
also investigate effects of varying the solar circular velocity, the DM escape
velocity, and the DAMA quenching factor within experimental limits. Our data
set includes the latest ZEPLIN-III results, as well as full publicly available
data sets. Due to the more sensitive dependence of the inelastic cross section
on the velocity distribution, we find that with Via Lactea the DAMA results can
be consistent with all other experiments over an enlarged region of iDM
parameter space, with higher mass particles being preferred, while Dark Disc
does not lead to an improvement. A definitive test of DAMA for iDM requires
heavy element detectors.Comment: 22 pages, 10 figures, PDFLaTex Additional analysis of Via Lactea
simulation include
The ZEPLIN II dark matter detector: data acquisition system and data reduction
ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching
for WIMP-nucleon interactions. In this paper we describe the data acquisition
system used to record the data from ZEPLIN-II and the reduction procedures
which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure
The ZEPLIN II dark matter detector: data acquisition system and data reduction
ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching
for WIMP-nucleon interactions. In this paper we describe the data acquisition
system used to record the data from ZEPLIN-II and the reduction procedures
which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure
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.Comment: 44 pages, 6 figures, added reference