Kai Schmidt-Hoberg

Resolving astrophysical uncertainties in dark matter direct detection

We study the impact of the assumed velocity distribution of galactic dark matter particles on the interpretation of results from nuclear recoil detectors. By converting experimental data to variables that make the astrophysical unknowns explicit, different experiments can be compared without implicit assumptions concerning the dark matter halo. We extend this framework to include the annual modulation signal, as well as multiple target elements. Recent results from DAMA, CoGeNT and CRESST-II can be brought into agreement if the velocity distribution is very anisotropic and thus allows a large modulation fraction. However constraints from CDMS and XENON cannot be evaded by appealing to such astrophysical uncertainties alone.

Discrete R symmetries for the MSSM and its singlet extensions

In this book we have studied supersymmetric extensions of the Standard Model, including the MSSM, and supersymmetric orbifold GUTs in 4 and higher dimensions. Although they provide a framework for solving the hierarchy problem, they introduce additional problems associated with the μ term, flavor problems and new effective operators which violate baryon and lepton number, leading to proton decay. We have discussed particular mechanisms which can ameliorate each of these problems. The μ problem can be solved by incorporating a symmetry which forbids the μ term at the tree level, but is broken spontaneously by a SUSY breaking VEV, i.e. the so-called Giudice-Masiero mechanism [117], or by a Peccei-Quinn symmetry breaking VEV which results in a μ term and an invisible QCD axion, the so-called Kim-Nilles mechanism [118]. Minimal flavor violation and/or heavy scalars can protect against large flavor violation. This, of course, depends on the SUSY breaking mechanisms.

The fine-tuning of the generalised NMSSM

Abstract We determine the degree of fine-tuning needed in a generalised version of the NMSSM that follows from an underlying Z 4 or Z 8 R-symmetry. We find that it is significantly less than is found in the MSSM or NMSSM and extends the range of Higgs mass that have acceptable fine-tuning. Remarkably the minimal fine-tuning is achieved for Higgs masses of around 130 GeV.

Implications of unitarity and gauge invariance for simplified dark matter models

A bstractWe show that simplified models used to describe the interactions of dark matter with Standard Model particles do not in general respect gauge invariance and that perturbative unitarity may be violated in large regions of the parameter space. The modifications necessary to cure these inconsistencies may imply a much richer phenomenology and lead to stringent constraints on the model. We illustrate these observations by considering the simplified model of a fermionic dark matter particle and a vector mediator. Imposing gauge invariance then leads to strong constraints from dilepton resonance searches and electroweak precision tests. Furthermore, the new states required to restore perturbative unitarity can mix with Standard Model states and mediate interactions between the dark and the visible sector, leading to new experimental signatures such as invisible Higgs decays. The resulting constraints are typically stronger than the ‘classic’ constraints on DM simplified models such as monojet searches and make it difficult to avoid thermal overproduction of dark matter.

Tricking Landau–Yang: How to obtain the diphoton excess from a vector resonance ☆

Abstract We show that contrary to naive expectations the recently observed diphoton excess can be explained by a vector resonance, which decays to a photon and a light scalar s , followed by a decay of the scalar into two photons: Z ′ → γ s → 3 γ . As the two photons from the scalar decay are highly boosted, the experimental signature is an apparent diphoton final state. In fact all the necessary ingredients are naturally present in Z ′ models: Additional fermions with electroweak quantum numbers are required in order to render the theory anomaly free and naturally induce the required effective couplings, while the hidden Higgs which gives mass to the Z ′ can be very light. In particular no new coloured states are required in this framework. We also show that in such a setup the width of the resonance can be rather large, while all couplings remain perturbative.

A unique Z4R symmetry for the MSSM

Abstract We consider the possible anomaly free Abelian discrete symmetries of the MSSM that forbid the μ -term at perturbative order. Allowing for anomaly cancellation via the Green–Schwarz mechanism we identify discrete R -symmetries as the only possibility and prove that there is a unique Z 4 R symmetry that commutes with SO ( 10 ) . We argue that non-perturbative effects will generate a μ -term of electroweak order thus solving the μ -problem. The non-perturbative effects break the Z 4 R symmetry leaving an exact Z 2 matter parity. As a result dimension four baryon- and lepton-number violating operators are absent while, at the non-perturbative level, dimension five baryon- and lepton-number violating operators get induced but are highly suppressed so that the nucleon decay rate is well within present bounds.

Constraining Dark Sectors with Monojets and Dijets

A bstractWe consider dark sector particles (DSPs) that obtain sizeable interactions with Standard Model fermions from a new mediator. While these particles can avoid observation in direct detection experiments, they are strongly constrained by LHC measurements. We demonstrate that there is an important complementarity between searches for DSP production and searches for the mediator itself, in particular bounds on (broad) dijet resonances. This observation is crucial not only in the case where the DSP is all of the dark matter but whenever — precisely due to its sizeable interactions with the visible sector — the DSP annihilates away so efficiently that it only forms a dark matter subcomponent. To highlight the different roles of DSP direct detection and LHC monojet and dijet searches, as well as perturbativity constraints, we first analyse the exemplary case of an axial-vector mediator and then generalise our results. We find important implications for the interpretation of LHC dark matter searches in terms of simplified models.

Colliding clusters and dark matter self-interactions

When a dark matter halo moves through a background of dark matter particles, self-interactions can lead to both deceleration and evaporation of the halo and thus shift its centroid relative to the collisionless stars and galaxies. We study the magnitude and time evolution of this shift for two classes of dark matter self-interactions, viz. frequent self-interactions with small momentum transfer (e.g. due to long-range interactions) and rare self-interactions with large momentum transfer (e.g. contact interactions), and find important differences between the two cases. We find that neither effect can be strong enough to completely separate the dark matter halo from the galaxies, if we impose conservative bounds on the self-interaction cross-section. The majority of both populations remain bound to the same gravitational potential and the peaks of their distributions are therefore always coincident. Consequently any apparent separation is mainly due to particles which are leaving the gravitational potential, so will be largest shortly after the collision but not observable in evolved systems. Nevertheless the fraction of collisions with large momentum transfer is an important characteristic of self-interactions, which can potentially be extracted from observational data and provide an important clue as to the nature of dark matter.

A taste of dark matter: flavour constraints on pseudoscalar mediators

A bstractDark matter interacting via the exchange of a light pseudoscalar can induce observable signals in indirect detection experiments and experience large self-interactions while evading the strong bounds from direct dark matter searches. The pseudoscalar mediator will however induce flavour-changing interactions in the Standard Model, providing a promising alternative way to test these models. We investigate in detail the constraints arising from rare meson decays and fixed target experiments for different coupling structures between the pseudoscalar and Standard Model fermions. The resulting bounds are highly complementary to the information inferred from the dark matter relic density and the constraints from primordial nucleosynthesis. We discuss the implications of our findings for the dark matter self-interaction cross section and the prospects of probing dark matter coupled to a light pseudoscalar with direct or indirect detection experiments. In particular, we find that a pseudoscalar mediator can only explain the Galactic Centre excess if its mass is above that of the B mesons, and that it is impossible to obtain a sufficiently large direct detection cross section to account for the DAMA modulation.

Large Hierarchies from Approximate R Symmetries

We show that hierarchically small vacuum expectation values of the superpotential in supersymmetric theories can be a consequence of an approximate R symmetry. We briefly discuss the role of such small constants in moduli stabilization and understanding the huge hierarchy between the Planck and electroweak scales.