David McKeen

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.

Modified Higgs branching ratios versus CP and lepton flavor violation

New physics thresholds which can modify the diphoton and dilepton Higgs branching ratios significantly may also provide new sources of CP and lepton flavor violation. We find that limits on electric dipole moments impose strong constraints on any CP-odd contributions to Higgs diphoton decays unless there are degeneracies in the Higgs sector that enhance CP-violating mixing. We exemplify this point in the language of effective operators and in simple UV-complete models with vector-like fermions. In contrast, we find that electric dipole moments and lepton flavor violating observables provide less stringent constraints on new thresholds contributing to Higgs dilepton decays.

Electric dipole moment signatures of PeV-scale superpartners

A possible supersymmetric interpretation of the new Higgs-like 126 GeV resonance involves a high sfermion mass scale, from tens of TeV to a PeV or above. This scale provides sufficiently large loop corrections to the Higgs mass and can naturally resolve the constraints from flavor-violating observables, even with a generic flavor structure in the sfermion sector. We point out that such high scales could still generate CP-violating electric dipole moments (EDMs) at interesting levels due to the enhancement of left-right (LR) sfermion mixing. We illustrate this by saturating the light fermion mass corrections from the sfermion threshold, leaving the gaugino masses unconstrained. In this framework, we find that the current EDM bounds probe energy scales of 0.1 PeV or higher; this is competitive with the reach of K and more sensitive than other hadronic and leptonic flavor observables. We also consider the sensitivity to higher dimensional supersymmetric operators in this scenario, including those that lead to proton decay.

New Parity-Violating Muonic Forces and the Proton Charge Radius

The recent discrepancy between proton charge radius measurements extracted from electron-proton versus muon-proton systems is suggestive of a new force that differentiates between lepton species. We identify a class of models with gauged right-handed muon number, which contains new vector and scalar force carriers at the ∼100 MeV scale or lighter, that is consistent with observations. Such forces would lead to an enhancement by several orders-of-magnitude of the parity-violating asymmetries in the scattering of low-energy muons on nuclei. The relatively large size of such asymmetries, O(10(-4)), opens up the possibility for new tests of parity violation in neutral currents with existing low-energy muon beams.

Signatures of sub-GeV dark matter beams at neutrino experiments

We study the high-luminosity fixed-target neutrino experiments at MiniBooNE, MINOS and T2K and analyze their sensitivity to light stable states, focusing on MeV--GeV scale dark matter. Thermal relic dark matter scenarios in the sub-GeV mass range require the presence of light mediators, whose coupling to the Standard Model facilitates annihilation in the early universe and allows for the correct thermal relic abundance. The mediators in turn provide a production channel for dark matter at colliders or fixed targets, and as a consequence the neutrino beams generated at fixed targets may contain an additional beam of light dark matter. The signatures of this beam include elastic scattering off electrons or nucleons in the (near-)detector, which closely mimics the neutral current scattering of neutrinos. We determine the event rate at modern fixed target facilities and the ensuing sensitivity to sub-GeV dark matter.

Diphotons from tetraphotons in the decay of a 125 GeV Higgs boson at the LHC

Recently the ATLAS and CMS experiments have presented data hinting at the presence of a Higgs boson at

Leptophobic dark matter at neutrino factories

m_h\simeq125

Low Mass WIMP Searches with a Neutrino Experiment: A Proposal for Further MiniBooNE Running

GeV. The best-fit

Constraints on muon-specific dark forces

h\rightarrow\gamma\gamma

Diphotons, New Vacuum Angles, and Strong CP

rate averaged over the two experiments is approximately