Fixed-target experiments are ideally suited for discovering new MeV--GeV mass $U(1)$ gauge bosons through their kinetic mixing with the photon. In this paper, we identify the production and decay properties of new light gauge bosons that dictate fixed-target search strategies. We summarize existing limits and suggest five new experimental approaches that we anticipate can cover most of the natural parameter space, using currently operating GeV-energy beams and well-established detection methods. Such experiments are particularly timely in light of recent terrestrial and astrophysical anomalies (PAMELA, Fermi, DAMA/LIBRA, etc.) consistent with dark matter charged under a new gauge force.

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New Fixed-Target Experiments to Search for Dark Gauge Forces

We present an overview of scenarios where the observed Dark Matter (DM) abundance consists of Feebly Interacting Massive Particles (FIMPs), produced nonthermally by the so-called freeze-in mechanism. In contrast to the usual freeze-out scenario, frozen-in FIMP DM interacts very weakly with the particles in the visible sector and never attained thermal equilibrium with the baryon–photon fluid in the early Universe. Instead of being determined by its annihilation strength, the DM abundance depends on the decay and annihilation strengths of particles in equilibrium with the baryon–photon fluid, as well as couplings in the DM sector. This makes frozen-in DM very difficult but not impossible to test. In this review, we present the freeze-in mechanism and its variations considered in the literature (dark freeze-out and reannihilation), compare them to the standard DM freeze-out scenario, discuss several aspects of model building, and pay particular attention to observational properties and general testability o...

The dawn of FIMP Dark Matter : A review of models and constraints

Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the a → πππ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order (0.01 − 1) TeV−1, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → γγ and a → l+l− branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.

/pdf/collider-probes-of-axion-like-particles-1je1gfzke8.pdf

Collider probes of axion-like particles

We perform an extensive survey of nonstandard Higgs decays that are consistent with the 125 GeV Higgs-like resonance. Our aim is to motivate a large set of new experimental analyses on the existing and forthcoming data from the Large Hadron Collider (LHC). The explicit search for exotic Higgs decays presents a largely untapped discovery opportunity for the LHC collaborations, as such decays may be easily missed by other searches. We emphasize that the Higgs is uniquely sensitive to the potential existence of new weakly coupled particles and provide a unified discussion of a large class of both simplified and complete models that give rise to characteristic patterns of exotic Higgs decays. We assess the status of exotic Higgs decays after LHC run I. In many cases we are able to set new nontrivial constraints by reinterpreting existing experimental analyses. We point out that improvements are possible with dedicated analyses and perform some preliminary collider studies. We prioritize the analyses according to their theoretical motivation and their experimental feasibility. This document is accompanied by a Web site that will be continuously updated with further information [ http://exotichiggs.physics.sunysb.edu ].

/pdf/exotic-decays-of-the-125-gev-higgs-boson-4dbqt1eusd.pdf

Exotic decays of the 125 GeV Higgs boson

We explore constraints on gauge bosons of a weakly coupled U(1)B − L, $$ \mathrm{U}{(1)}_{L_{\mu }-{L}_e},\kern0.5em \mathrm{U}{(1)}_{L_e-{L}_{\tau }}\kern0.5em \mathrm{and}\kern0.5em \mathrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$
 . To do so we apply the full constraining power of experimental bounds derived for a hidden photon of a secluded U(1)X and translate them to the considered gauge groups. In contrast to the secluded hidden photon that acquires universal couplings to charged Standard Model particles through kinetic mixing with the photon, for these gauge groups the couplings to the different Standard Model particles can vary widely. We take finite, computable loop-induced kinetic mixing effects into account, which provide additional sensitivity in a range of experiments. In addition, we collect and extend limits from neutrino experiments as well as astrophysical and cosmological observations and include new constraints from white dwarf cooling. We discuss the reach of future experiments in searching for these gauge bosons.

/pdf/hunting-all-the-hidden-photons-wh0ynbn3nq.pdf

Hunting All the Hidden Photons

A search was made for neutral objects which might be produced by 20-GeV electrons incident on the SLAC beam dump, penetrate the downstream natural shielding, and decay upstream of an electromagnetic shower calorimeter. With about 30 C of electrons dumped, no candidate events were found above an energy of \ensuremath{\sim}2 GeV. The 95%-confidence-level limit on the product of mass and lifetime of light axionlike bosons decaying primarily into two photons is determined to be greater than 1.4 keV sec. Limits on photino parameters are also given.

Search for Neutral Metastable Penetrating Particles Produced in the SLAC Beam Dump

We have measured ${\ensuremath{\pi}}^{+}p$, ${\ensuremath{\pi}}^{\ensuremath{-}}p$, and $\mathrm{pp}$ elastic scattering at an incident-beam momentum of 200 GeV/c in the region of $\ensuremath{-}t$, four-momentum transfer squared, from 0.021 to 0.665 (${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. The data allow an investigation of the $t$ dependence of the logarithmic forward slope parameter $b\ensuremath{\equiv}(\frac{d}{\mathrm{dt}})(\mathrm{ln}\frac{d\ensuremath{\sigma}}{\mathrm{dt}})$. In addition to standard parametrization, we use functional forms suggested by the additive quark model to fit the measured $\frac{d\ensuremath{\sigma}}{\mathrm{dt}}$ distributions. Within the context of this model we estimate the size of the clothed quark in the pion and proton. Limits on the elastic-scattering amplitude derived from unitarity bounds are checked, and no violations are observed.

High-statistics study of π + p , π − p , and pp elastic scattering at 200 GeV/ c

We have measured the elastic cross section for $\mathrm{pp}$, $\overline{p}p$, ${\ensuremath{\pi}}^{+}p$, ${\ensuremath{\pi}}^{\ensuremath{-}}p$, ${K}^{+}p$, and ${K}^{\ensuremath{-}}p$ scattering at incident momenta of 70, 100, 125, 150, 175, and 200 GeV/c. The range of the four-momentum transfer squared $t$ varied with the beam momentum from $0.0016\ensuremath{\le}\ensuremath{-}t\ensuremath{\le}0.36$ (${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ at 200 GeV/c to $0.0018\ensuremath{\le}\ensuremath{-}t\ensuremath{\le}0.0625$ (${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ at 70 GeV/c. The conventional parametrization of the $t$ dependence of the nuclear amplitude by a simple exponential in $t$ was found to be inadequate. An excellent fit to the data was obtained by a parametrization motivated by the additive quark model. Using this parametrization we determined the ratio of the real to the imaginary part of the nuclear amplitude by the Coulomb-interference method.

Real part of the forward elastic nuclear amplitude for pp, p»p, +p, -p, K+p, and K-p scattering between 70 and 200 GeV/c

Hadron-nucleus elastic scattering is measured for ..pi../sup + -/, K/sup + -/, p, and p-bar scattering from Be, C, Al, Cu, Sn, and Pb targets at incident beam momenta of 70 and 175 GeV/c and for ..pi../sup +/, K/sup +/, and p scattering from Be, Al, and Pb targets at an incident beam momentum of 125 GeV/c. Parametrizations of dsigma/dt in the forward direction for the reactions are presented.

S. Ecklund

Papers

Search for Neutral Metastable Penetrating Particles Produced in the SLAC Beam Dump

High-statistics study of π + p , π − p , and pp elastic scattering at 200 GeV/ c

Real part of the forward elastic nuclear amplitude for pp, p»p, +p, -p, K+p, and K-p scattering between 70 and 200 GeV/c

Hadron-nucleus elastic scattering at 70, 125, and 175 GeV/c