Gordan Krnjaic

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

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.

The Galactic Center Excess from the Bottom Up

It has recently been shown that dark-matter annihilation to bottom quarks provides a good fit to the galactic-center gamma-ray excess identified in the Fermi-LAT data. In the favored dark matter mass range m ∼ 30− 40 GeV, achieving the best-fit annihilation rate σv ∼ 5× 10−26 cm s−1 with perturbative couplings requires a sub-TeV mediator particle that interacts with both dark matter and bottom quarks. In this paper, we consider the minimal viable scenarios in which a Standard Model singlet mediates s-channel interactions only between dark matter and bottom quarks, focusing on axial-vector, vector, and pseudoscalar couplings. Using simulations that include on-shell mediator production, we show that existing sbottom searches currently offer the strongest sensitivity over a large region of the favored parameter space explaining the gamma-ray excess, particularly for axialvector interactions. The 13 TeV LHC will be even more sensitive; however, it may not be sufficient to fully cover the favored parameter space, and the pseudoscalar scenario will remain unconstrained by these searches. We also find that direct-detection constraints, induced through loops of bottom quarks, complement collider bounds to disfavor the vector-current interaction when the mediator is heavier than twice the dark matter mass. We also present some simple models that generate pseudoscalar-mediated annihilation predominantly to bottom quarks.

Probing light thermal dark matter with a Higgs portal mediator

We systematically study light (< few GeV) Dark Matter (DM) models that thermalize with visible matter through the Higgs portal and identify the remaining gaps in the viable parameter space. Such models require a comparably light scalar mediator that mixes with the Higgs to avoid DM overproduction and can be classified according to whether this mediator decays (in)visibly. In a representative benchmark model with Dirac fermion DM, we find that, even with conservative assumptions about the DM-mediator coupling and mass ratio, the regime in which the mediator is heavier than the DM is fully ruled out by a combination of collider, rare meson decay, and direct detection limits; future and planned experiments including NA62 can further improve sensitivity to scenarios in which the Higgs portal interaction does not determine the DM abundance. The opposite, regime in which the mediator is lighter than the DM and the latter annihilates to pairs of visibly-decaying mediators is still viable, but much of the parameter space is covered by rare meson decay, supernova cooling, beam dump, and direct detection constraints. Nearly all of these conclusions apply broadly to the simplest variations (e.g. scalar or asymmetric DM). Future experiments including SHiP, NEWS, and Super-CDMS SNOLAB can greatly improve coverage to this class of models.

Gauging the Way to MFV

A bstractWe present a UV complete model with a gauged flavor symmetry which approximately realizes holomorphic Minimal Flavor Violation (MFV) in R-parity violating (RPV) supersymmetry. Previous work has shown that imposing MFV as an ansatz easily evades direct constraints and has interesting collider phenomenology. The model in this work spontaneously breaks the flavor symmetry and features the minimum “exotic” field content needed to cancel anomalies. The flavor gauge bosons exhibit an inverted hierarchy so that those associated with the third generation are the lightest. This allows low energy flavor constraints to be easily satisfied and leaves open the possibility of flavor gauge bosons accessible at the LHC. The usual MSSM RPV operators are all forbidden by the new gauge symmetry, but the model allows a purely exotic operator which violates both R-parity and baryon number. Since the exotic fields mix with MSSM-like right handed quarks, diagonalizing the full mass matrix after flavor-breaking transforms this operator into the trilinear baryon number violating operator

Analyzing the Discovery Potential for Light Dark Matter.

\overline{U}\overline{D}\overline{D}

Testing GeV-Scale Dark Matter with Fixed-Target Missing Momentum Experiments

with flavor coefficients all suppressed by three powers of Yukawa couplings. There is a limit where this model realizes exact MFV; we compute corrections away from MFV, show that they are under theoretical control, and find that the model is viable in large regions of parameter space.

Big Bang Darkleosynthesis

In this Letter, we determine the present status of sub-GeV thermal dark matter annihilating through standard model mixing, with special emphasis on interactions through the vector portal. Within representative simple models, we carry out a complete and precise calculation of the dark matter abundance and of all available constraints. We also introduce a concise framework for comparing different experimental approaches, and use this comparison to identify important ranges of dark matter mass and couplings to better explore in future experiments. The requirement that dark matter be a thermal relic sets a sharp sensitivity target for terrestrial experiments, and so we highlight complementary experimental approaches that can decisively reach this milestone sensitivity over the entire sub-GeV mass range.That dark matter (DM) is a thermal relic from the hot early Universe is an inspiring possibility that motivates nongravitational interactions between dark and ordinary matter. The canonical example involves a heavy particle interacting through the weak force (WIMPs). This scenario has motivated searches for DM scattering in underground detectors, for DM annihilation in the cosmos, and for DM production in high-energy colliders. These efforts achieve broad and powerful sensitivity to DM with mass between a few GeV and the TeV scale. A thermal origin is equally compelling — and, in simple models, predictive — even if DM is not a WIMP. DM with any mass from an MeV to tens of TeV can achieve the correct relic abundance by annihilating directly into Standard Model (SM) matter. However, the lower half of this mass range cannot be fully explored using existing strategies – an unfortunate situation that jeopardizes the legacy of the DM search effort. In particular, DM-nuclear and DM-electron scattering searches lose sensitivity precipitously for DM lighter than a few GeV or DM that scatters inelastically; limits on DM annihilation at low temperatures (most notably from the CMB) are irrelevant to many scenarios; and missing energy searches at high-energy colliders are blind to the interactions responsible for the DM abundance. In this paper, we sharply define the challenge of testing subGeV thermal DM. Below a few GeV, avoiding DM overproduction requires that a comparably light particle mediate DM annihilation [1–7]. We focus on the case where DM annihilates through an s-channel mediator directly into SM states1. We classify annihilation mechanisms arising from dark-sector mixing with SM fields, define minimal models representative of each mechanism, and compute all relevant constraints on each model. We introduce a framework to compare all such constraints to one another and to the milestone provided by thermal DM freeze-out. Finally, we identify a small set of “flagship” experiments with complementary sensitivity — using direct detection, B-factory mono-photon, and fixed-target missing momentum strategies — that together can decisively test light DM annihilating through SM mixing.

Very light axigluons and the top asymmetry

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

Thermal dark matter from a highly decoupled sector

\epsilon^2\sim 10^{-14}

DAEδALUS and dark matter detection

. Moreover, the approach can explore essentially all of the viable parameter space for thermal or asymmetric dark matter annihilating through the vector portal.