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

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

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

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

Limits on spin-dependent WIMP-nucleon cross-sections from the first ZEPLIN-II data

The first underground data run of the ZEPLIN-II experiment has set a limit on the nuclear recoil rate in the two-phase xenon detector for direct dark matter searches. In this paper the results from this run are converted into the limits on spin-dependent WIMP-proton and WIMP-neutron cross-sections. The minimum of the curve for WIMP-neutron cross-section corresponds to 0.07 pb at a WIMP mass of around 65 GeV.Comment: 12 pages, 2 figures, to be published in Physics Letters