US Cosmic Visions: New Ideas in Dark Matter 2017: Community Report
This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in Dark Matter" held at University of Maryland on March 23-25, 2017.
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arXiv subjects
Natalia Toro
Publications and source records attributed to Natalia Toro.
Dark Sectors 2016 Workshop: Community Report
This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years.
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The Heavy Photon Search Test Detector
The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment's technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e$^+$e$^-$ invariant mass spectrum, above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW0$_{4}$ crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e$^+$e$^-$ pairs requires the first layer of silicon sensors be placed only 10~cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab.
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Testing GeV-Scale Dark Matter with Fixed-Target Missing Momentum Experiments
We describe an approach to detect dark matter and other invisible particles with mass below a GeV, exploiting missing energy-momentum measurements and other kinematic features of fixed-target production. In the case of an invisibly decaying MeV-GeV-scale dark photon, this approach can improve on present constraints by 2-6 orders of magnitude over the entire mass range, reaching sensitivity as low as $ε^2\sim 10^{-14}$. Moreover, the approach can explore essentially all of the viable parameter space for thermal or asymmetric dark matter annihilating through the vector portal.
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Accelerating the Discovery of Light Dark Matter
We analyze the present status of sub-GeV thermal dark matter annihilating through Standard Model mixing and identify a small set of future experiments that can decisively test these scenarios.
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A New Class of Particle in 2+1 Dimensions
In two spatial dimensions, spin characterizes how particle states re-phase under changes of frame that leave their momentum and energy invariant. Massless particles can in principle have non-trivial spin in this sense, but all existing field theories only describe the trivial case. This letter presents a field theory for a massless particle with non-trivial physical spin. These particles are the 2+1-dimensional analogues of "continuous-spin" particles in 3+1 dimensions, but here they have only two real degrees of freedom, related by parity. They can be understood as massless generalizations of anyons, but are simpler in key respects.
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A CSP Field Theory with Helicity Correspondence
We propose the first covariant local action describing the propagation of a single free continuous-spin degree of freedom. The theory is simply formulated as a gauge theory in a "vector superspace", but can also be formulated in terms of a tower of symmetric tensor gauge fields. When the spin invariant $ρ$ vanishes, the helicity correspondence is manifest -- familiar gauge theory actions are recovered and couplings to conserved currents can easily be introduced. For non-zero $ρ$, a tower of tensor currents must be present, of which only the lowest rank is exactly conserved. A paucity of local gauge-invariant operators for non-zero $ρ$ suggests that the equations of motion in any interacting theory should be covariant, not invariant, under a generalization of the free theory's gauge symmetry.
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Physics Motivation for a Pilot Dark Matter Search at Jefferson Laboratory
It has recently been demonstrated that a program of parasitic electron-beam fixed-target experiments would have powerful discovery potential for dark matter and other new weakly-coupled particles in the MeV-GeV mass range. The first stage of this program can be realized at Jefferson Laboratory using an existing plastic-scintillator detector downstream of the Hall D electron beam dump. This paper studies the physics potential of such an experiment and highlights its unique sensitivity to inelastic "exciting" dark matter and leptophilic dark matter scenarios. The first of these is kinematically inaccessible at traditional direct detection experiments and features potential "smoking gun" low-background signatures.
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New Electron Beam-Dump Experiments to Search for MeV to few-GeV Dark Matter
In a broad class of consistent models, MeV to few-GeV dark matter interacts with ordinary matter through weakly coupled GeV-scale mediators. We show that a suitable meter-scale (or smaller) detector situated downstream of an electron beam-dump can sensitively probe dark matter interacting via sub-GeV mediators, while B-factory searches cover the 1-5 GeV range. Combined, such experiments explore a well-motivated and otherwise inaccessible region of dark matter parameter space with sensitivity several orders of magnitude beyond existing direct detection constraints. These experiments would also probe invisibly decaying new gauge bosons ("dark photons") down to kinetic mixing of ε~ 10^{-4}, including the range of parameters relevant for explaining the (g-2)_μ discrepancy. Sensitivity to other long-lived dark sector states and to new milli-charge particles would also be improved.
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On the Theory of Continuous-Spin Particles: Wavefunctions and Soft-Factor Scattering Amplitudes
The most general massless particles allowed by Poincare-invariance are "continuous-spin" particles (CSPs) characterized by a scale ρ, which at ρ=0 reduce to familiar helicity particles. Though known long-range forces are adequately modeled using helicity particles, it is not known whether CSPs can also mediate long-range forces or what consequences such forces might have. We present sharp evidence for consistent interactions of CSPs with matter: new CSP equations of motion, wavefunctions, and covariant radiation amplitudes. In a companion paper, we use these results to resolve old puzzles concerning CSP thermodynamics and exhibit a striking correspondence limit where CSP amplitudes approach helicity-0, 1 or 2 amplitudes.
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On the Theory of Continuous-Spin Particles: Helicity Correspondence in Radiation and Forces
We have recently shown that continuous-spin particles (CSPs) have covariant single-emission amplitudes with the requisite properties to mediate long-range forces. CSPs, the most general massless particle type consistent with Lorentz symmetry, are characterized by a scale ρ. Here, we demonstrate a helicity correspondence at CSP energies larger than ρ, in which these amplitudes are well approximated by the familiar ones for particles of helicity 0, 1, or 2. These properties follow from Lorentz invariance. We also construct tree-level multi-emission and CSP-exchange amplitudes that are unitary, appropriately analytic, and consistent with helicity-0 correspondence. We propose sewing rules from which these amplitudes and others can be obtained. We also exhibit a candidate CSP-graviton matrix element, which shows that the Weinberg-Witten theorem does not apply to CSPs. These results raise the surprising possibility that the known long-range forces might be mediated by CSPs with very small ρ, rather than by helicity 1 and 2 particles.
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A Gauge Field Theory of Continuous-Spin Particles
We propose and quantize a local, covariant gauge-field action that unifies the description of all free helicity and continuous-spin degrees of freedom in a simple manner. This is the first field-theory action of any kind for continuous spin particles; it is consistent as a quantum theory and generalizes to any number of dimensions. The fields live on the null cone of an internal four-vector "spin-space"; in D dimensions a linearized gauge invariance reduces their physical content to a single function on a Euclidean (D-2)-plane, on which the little group E(D-2) acts naturally. A projective version of the action further reduces the physical content to S^{D-3}, enabling a new local description of particles with any spin structure, and in particular a tower of all integer-helicity particles for D=4. Gauge-invariant interactions with a background current are added in a straightforward manner.
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Sommerfeld-Enhanced Annihilation in Dark Matter Substructure: Consequences for Constraints on Cosmic-Ray Excesses
In models of dark matter (DM) with Sommerfeld-enhanced annihilation, where the annihilation rate scales as the inverse velocity, N-body simulations of DM structure formation suggest that the local annihilation signal may be dominated by small, dense, cold subhalos. This contrasts with the usual assumption of a signal originating from the smooth DM halo, with much higher velocity dispersion. Accounting for local substructure modifies the parameter space for which Sommerfeld-enhanced annihilating DM can explain the PAMELA and Fermi excesses. Limits from the inner galaxy and the cosmic microwave background are weakened, without introducing new tension with substructure-dependent limits, such as from dwarf galaxies or isotropic gamma-ray studies. With substructure, previously excluded parameter regions with mediators of mass ~1-200 MeV are now easily allowed. For O(MeV) mediators, subhalos in a specific range of host halo masses may be evaporated, further suppressing diffuse signals without affecting substructure in the Milky Way.
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Multiphotons and Photon-Jets
We discuss an extension of the Standard Model with a new vector-boson decaying predominantly into a multi-photon final state through intermediate light degrees of freedom. The model has a distinctive phase in which the photons are collimated. As such, they would fail the isolation requirements of standard multi-photon searches, but group naturally into a novel object, the photon-jet. Once defined, the photon-jet object facilitates more inclusive searches for similar phenomena. We present a concrete model, discuss photon-jets more generally, and outline some strategies that may prove useful when searching for such objects.
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Study of LHC Searches for a Lepton and Many Jets
Searches for new physics in high-multiplicity events with little or no missing energy are an important component of the LHC program, complementary to analyses that rely on missing energy. We consider the potential reach of searches for events with a lepton and six or more jets, and show they can provide increased sensitivity to many supersymmetric and exotic models that would not be detected through standard missing-energy analyses. Among these are supersymmetric models with gauge mediation, R-parity violation, and light hidden sectors. Moreover, ATLAS and CMS measurements suggest the primary background in this channel is from t-tbar, rather than W+jets or QCD, which reduces the complexity of background modeling necessary for such a search. We also comment on related searches where the lepton is replaced with another visible object, such as a Z boson.
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Simplified Models for LHC New Physics Searches
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.
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High Energy Electron Signals from Dark Matter Annihilation in the Sun
In this paper we discuss two mechanisms by which high energy electrons resulting from dark matter annihilations in or near the Sun can arrive at the Earth. Specifically, electrons can escape the sun if DM annihilates into long-lived states, or if dark matter scatters inelastically, which would leave a halo of dark matter outside of the sun. Such a localized source of electrons may affect the spectra observed by experiments with narrower fields of view oriented towards the sun, such as ATIC, differently from those with larger fields of view such as Fermi. We suggest a simple test of these possibilities with existing Fermi data that is more sensitive than limits from final state radiation. If observed, such a signal will constitute an unequivocal signature of dark matter.
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Discovering New Light States at Neutrino Experiments
Experiments designed to measure neutrino oscillations also provide major opportunities for discovering very weakly coupled states. In order to produce neutrinos, experiments such as LSND collide thousands of Coulombs of protons into fixed targets, while MINOS and MiniBooNE also focus and then dump beams of muons. The neutrino detectors beyond these beam dumps are therefore an excellent arena in which to look for long-lived pseudoscalars or for vector bosons that kinetically mix with the photon. We show that these experiments have significant sensitivity beyond previous beam dumps, and are able to partially close the gap between laboratory experiments and supernovae constraints on pseudoscalars. Future upgrades to the NuMI beamline and Project X will lead to even greater opportunities for discovery. We also discuss thin target experiments with muon beams, such as those available in COMPASS, and show that they constitute a powerful probe for leptophilic PNGBs.
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