This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and lowcost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere.

We discuss the sensitivity of neutrino experiments at the luminosity frontier to generic hidden sectors containing new (sub)-GeV neutral states. The weak interaction of these states with the Standard Model can be efficiently probed through all of the allowed renormalizable 'portals' (in the Higgs, vector, and neutrino sectors) at fixed target proton beam facilities, with complementary sensitivity to colliders. We concentrate on the kinetic-mixing vector portal, and show that certain regions of the parameter space for a new U(1) S gauge sector with long-lived sub-GeV mass states decaying to Standard Model leptons are already severely constrained by the datasets at LSND, MiniBooNE, and NuMI/MINOS. Furthermore, scenarios in which portals allow access to stable neutral particles, such as MeV-scale dark matter, generally predict that the neutrino beam is accompanied by a 'dark matter beam', observable through neutral-current-like interactions in the detector. As a consequence, we show that the LSND electron recoil event sample currently provides the most stringent direct constraint on MeV-scale dark matter models.

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of standard model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the µm scale up to the Big Bang Nucleosynthesis limit of ∼10 7 m. Neutral LLPs with lifetimes above ∼ 100 m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

A secluded U(1) S gauge field, kinetically mixed with Standard Model hypercharge, provides a 'portal' mediating interactions with a hidden sector at the renormalizable level, as recently exploited in the context of WIMP dark matter. The U(1) S symmetry-breaking scale may naturally be suppressed relative to the weak scale, and so this sector is efficiently probed by medium energy e + e − colliders. We study the collider signatures of the minimal U(1) S model, focusing on the reach of B-factory experiments such as BaBar and BELLE. In particular, we show that Higgs-strahlung in the secluded sector can lead to multi-lepton signatures which probe the natural range for the kinetic mixing angle κ ∼ 10 −2 − 10 −3 over a large portion of the kinematically accessible parameter space.

We present new constraints on sub-GeV dark matter and dark photons from the electron beamdump experiment E137 conducted at SLAC in [1980][1981][1982]. Dark matter interacting with electrons (e.g., via a dark photon) could have been produced in the electron-target collisions and scattered off electrons in the E137 detector, producing the striking, zero-background signature of a high-energy electromagnetic shower that points back to the beam dump. E137 probes new and significant ranges of parameter space, and constrains the well-motivated possibility that dark photons that decay to light dark-sector particles can explain the ∼ 3.6σ discrepancy between the measured and SM value of the muon anomalous magnetic moment. It also restricts the parameter space in which the relic density of dark matter in these models is obtained from thermal freeze-out. E137 also convincingly demonstrates that (cosmic) backgrounds can be controlled and thus serves as a powerful proof-ofprinciple for future beam-dump searches for sub-GeV dark-sector particles scattering off electrons in the detector.INTRODUCTION. Dark matter (DM) with mass below ∼ 1 GeV and interacting with Standard Model (SM) particles through a light mediator is a viable and natural possibility consistent with all known data (see e.g. [1][2][3][4][5][6][7][8][9][10][11][12]). High-intensity fixed-target experiments have impressive sensitivity to such light DM [3]. The basic experimental strategy begins with the production of a relativistic DM beam out of electron or proton collisions with a fixed target, followed by detection via DM scattering in a detector positioned downstream of the target. The prospects of proton fixed-target experiments, including several ongoing neutrino oscillation experiments, have been investigated in [3,[13][14][15][16], and the MiniBooNE experiment at FNAL is presently conducting the first dedicated search [17]. More recently, the potential of electron beam-dump experiments has been explored [18][19][20] [15]. These proposals complement the ongoing efforts to probe sub-GeV DM with low-energy e + e − colliders [21] and direct detection experiments via DM-electron scattering [4,9,22], as well as broader efforts to search for low-mass dark sectors that are weakly coupled to the SM [23,24]. MODELS. We focus on a motivated class of DM models based on a new 'dark' gauge symmetry, U(1) D [25-27], although our discussion applies to any scenario in which DM interacts with electrons. In this framework, the DM χ is charged under U(1) D , which is kinetically mixed with the SM hypercharge, U(1) Y , allowing for DM interactions with the SM [28,29]. If the U(1) D is spontaneously broken, its gauge boson (the 'dark photon' A ) is massive. The low energy effective Lagrangian is

Dark matter candidates comprising several sub-states separated by a small mass gap ∆m, and coupled to the Standard Model by (sub-)GeV force carriers, can exhibit nontrivial scattering interactions in direct detection experiments. We analyze the secluded U(1) S -mediated WIMP scenario, and calculate the elastic and inelastic cross sections for multi-component WIMP scattering off nuclei. We find that second-order elastic scattering, mediated by virtual excited states, provides strong sensitivity to the parameters of the model for a wide range of mass splittings, while for small ∆m the WIMP excited states have lifetimes exceeding the age of the universe, and generically have a fractional relative abundance above 0.1%. This generates even stronger constraints for ∆m < ∼ 200 keV due to exothermic deexcitation events in detectors.

Simple models of weakly interacting massive particles (WIMPs) predict dark matter annihilations into pairs of electroweak gauge bosons, Higgses or tops, which through their subsequent cascade decays produce a spectrum of gamma rays. Intriguingly, an excess in gamma rays coming from near the Galactic center has been consistently observed in Fermi data. A recent analysis by the Fermi collaboration confirms these earlier results. Taking into account the systematic uncertainties in the modelling of the gamma ray backgrounds, we show for the first time that this excess can be well fit by these final states. In particular, for annihilations to (W W , ZZ, hh, tt), dark matter with mass between threshold and approximately (165, 190, 280, 310) GeV gives an acceptable fit. The fit range for bb is also enlarged to 35 GeV m χ 165 GeV. These are to be compared to previous fits that concluded only much lighter dark matter annihilating into b, τ , and light quark final states could describe the excess. We demonstrate that simple, well-motivated models of WIMP dark matter including a thermal-relic neutralino of the MSSM, Higgs portal models, as well as other simplified models can explain the excess.