Nathaniel Craig

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.

Reheating Metastable O'Raifeartaigh Models

In theories with multiple vacua, reheating to a temperature greater than the height of a barrier can stimulate transitions from a desirable metastable vacuum to a lower energy state. We discuss the constraints this places on various theories and demonstrate that in a class of supersymmetric models this transition does not occur even for arbitrarily high reheating temperature.

String photini at the LHC

String theories with topologically complex compactification manifolds suggest the simultaneous presence of many unbroken U(1)s without any light matter charged under them. The gauge bosons associated with these U(1)s do not have direct observational consequences. However, in the presence of low energy supersymmetry the gauge fermions associated with these U(1)s, the photini, mix with the bino and extend the minimal supersymmetric standard model neutralino sector. This leads to novel signatures at the LHC. The lightest ordinary supersymmetric particle (LOSP) can decay to any one of these photini. In turn, photini may transition into each other, leading to high lepton and jet multiplicities. Both the LOSP decays and interphotini transitions can lead to displaced vertices. When the interphotini decays happen outside the detector, the cascades can result in different photini escaping the detector, leading to multiple reconstructed masses for the invisible particle. If the LOSP is charged, it stops in the detector and decays out of time to photini, with the possibility that the produced final photini vary from event to event. Observation of a plenitude of photini at the LHC would be evidence that we live in a string vacuum with a topologically rich compactification manifold.

The goldstini variations

We study the ‘goldstini’ scenario of Cheung, Nomura, and Thaler, in which multiple independent supersymmetry (SUSY) breaking sectors lead to multiple would-be goldstinos, changing collider and cosmological phenomenology. In supergravity, potentially large corrections to the previous prediction of twice the gravitino mass for goldstini masses can arise when their scalar partners are stabilized far from the origin. Considerations arising from the complexity of realistic string compactifications indicate that many of the independent SUSY-breaking sectors should be conformally sequestered or situated in warped Randall-Sundrum-like throats, further changing the predicted goldstini masses. If the sequestered hidden sector is a metastable SUSY-breaking sector of the Intriligator-Seiberg-Shih (ISS) type then multiple goldstini can originate from within a single sector, along with many supplementary ‘modulini’, all with masses of order twice the gravitino mass. These fields can couple to the Supersymmetric Standard Model (SSM) via the ‘Goldstino Portal’. Collider signatures involving SSM sparticle decays can provide strong evidence for warped-or-conformally-sequestered sectors, and of the ISS mechanism of SUSY breaking. Along with axions and photini, the Goldstino Portal gives another potential window to the hidden sectors of string theory.

The fraternal WIMP miracle

We identify and analyze thermal dark matter candidates in the fraternal twin Higgs model and its generalizations. The relic abundance of fraternal twin dark matter is set by twin weak interactions, with a scale tightly tied to the weak scale of the Standard Model by naturalness considerations. As such, the dark matter candidates benefit from a fraternal WIMP miracle, reproducing the observed dark matter abundance for dark matter masses between 50 and 150 GeV. However, the couplings dominantly responsible for dark matter annihilation do not lead to interactions with the visible sector. The direct detection rate is instead set via fermionic Higgs portal interactions, which are likewise constrained by naturalness considerations but parametrically weaker than those leading to dark matter annihilation. The predicted direct detection cross section is close to current LUX bounds and presents an opportunity for the next generation of direct detection experiments.

On the Phenomenology of Strongly Coupled Hidden Sectors

In models of supersymmetry (SUSY) breaking and mediation, strongly coupled SUSY-breaking sectors can play a significant role in determining the low-energy spectrum of the model. For example, strong dynamics may provide a natural solution to both the SUSY flavor problem and the μ/Bμ problem. Recently, it has been suggested that a large class of these models lead to identical boundary conditions at the SUSY breaking scale. These boundary conditions would severely constrain the models viability. We demonstrate that the boundary conditions are instead sensitive to the details of the hidden sector, so that only specific hidden sectors may be ruled out by phenomenological considerations. We determine the high scale boundary conditions using the operator product expansion of the hidden sector. The techniques used to determine the beta functions are generally applicable to the RG flow of any approximately conformal hidden sector. The discrepancy with previously proposed boundary conditions can be traced to the fact that the renormalization group (RG) flow involves multiple fixed points.

Sequestering the Gravitino: Neutralino Dark Matter in Gauge Mediation

In conventional models of gauge-mediated supersymmetry breaking, the lightest supersymmetric particle is invariably the gravitino. However, if the supersymmetry-breaking sector is strongly coupled, conformal sequestering may raise the mass of the gravitino relative to the remaining soft supersymmetry-breaking masses. In this paper, we demonstrate that such conformal dynamics in gauge-mediated theories may give rise to satisfactory neutralino dark matter while simultaneously solving the flavor and {mu}/B{mu} problems.

Disassembling the clockwork mechanism

A bstractThe clockwork mechanism is a means of naturally generating exponential hierarchies in theories without significant hierarchies among fundamental parameters. We emphasize the role of interactions in the clockwork mechanism, demonstrating that clockwork is an intrinsically abelian phenomenon precluded in non-abelian theories such as Yang-Mills, non-linear sigma models, and gravity. We also show that clockwork is not realized in extra-dimensional theories through purely geometric effects, but may be generated by appropriate localization of zero modes.

Single-Sector Supersymmetry Breaking, Chirality, and Unification

Calculable single-sector models provide an elegant framework for generating the flavor textures via compositeness, breaking supersymmetry, and explaining the electroweak scale. Such models may be realized naturally in supersymmetric QCD with additional gauge singlets (SSQCD), though it remains challenging to construct models without a surfeit of light exotic states where the Standard Model index emerges naturally. We classify possible single-sector models based on Sp confining SSQCD according to their Standard Model index and number of composite messengers. This leads to simple, calculable models that spontaneously break supersymmetry, reproduce the fermion flavor hierarchy, and explain the Standard Model index dynamically with little or no additional matter. At low energies these theories realize a more minimal soft spectrum with direct mediation and a gravitino LSP.

Cosmology in Mirror Twin Higgs and Neutrino Masses

A bstractWe explore a simple solution to the cosmological challenges of the original Mirror Twin Higgs (MTH) model that leads to interesting implications for experiment. We consider theories in which both the standard model and mirror neutrinos acquire masses through the familiar seesaw mechanism, but with a low right-handed neutrino mass scale of order a few GeV. In these νMTH models, the right-handed neutrinos leave the thermal bath while still relativistic. As the universe expands, these particles eventually become nonrelativistic, and come to dominate the energy density of the universe before decaying. Decays to standard model states are preferred, with the result that the visible sector is left at a higher temperature than the twin sector. Consequently the contribution of the twin sector to the radiation density in the early universe is suppressed, allowing the current bounds on this scenario to be satisfied. However, the energy density in twin radiation remains large enough to be discovered in future cosmic microwave background experiments. In addition, the twin neutrinos are significantly heavier than their standard model counterparts, resulting in a sizable contribution to the overall mass density in neutrinos that can be detected in upcoming experiments designed to probe the large scale structure of the universe.