Particle Physics

Astrophysical observations suggest that the majority of matter in the Universe is made up of novel Weakly Interacting Massive Particles (WIMPs).  Such WIMPs are often predicted by extensions to the Standard Model.  Efforts have been underway for more than two decades to detect WIMPs directly in detectors on earth.  The challenge is great because of the small energies involved and the low interaction rates. The field has been driven by progress in detectors able to identify radioactive backgrounds. I will review how recent enthusiasm for low-mass WIMPs, which was generated by tantalizing hints seen in several experiments, has waned.  Lastly, I will discuss various ideas to check the longstanding DAMA WIMP-detection claim, including the feasibility of alkali-halide cryogenic detectors.

Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental, particle nature of the dark matter.
I will discuss some of the recent puzzling findings in astro-particle and astronomical observations that might be related to signals from dark matter. I will first review the status of explanations to the cosmic-ray positron excess, emphasizing how we might be able to discriminate between astrophysical sources and dark matter. 
I will then discuss the evidence for an X-ray line at 3.5 keV, and present new results on systematic effects and on the role of previously underestimated astrophysical lines.
Finally, I will discuss a reported excess of gamma rays from the central regions of the Galaxy.  I will address the question of whether we are possibly observing a signal from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.

 

Astrophysical observations of the structure of galaxies and clusters are no longer simply proving the existence of DM, but have sharpened into a discovery tool probing the particle physics of dark matter.  I discuss small scale structure anomalies for cold dark matter and their possible implications for dark matter physics, such as the existence of forces in the dark sector.   New results on cluster scales provide a new important handle for constraining dark matter's particle interactions.

As an experimentalist involved in the search for physics beyond the Standard Model at the LHC, one must choose carefully which signatures to pursue. While theoretical guidance in identifying well motivated gaps in the coverage of “natural” BSM extensions is an important ingredient in this choice, unexplored territory is often unexplored for a reason, namely that there are likely non-trivial (“tricky”) experimental difficulties. One must thus consider the risk (time) vs. reward (discovery) in deciding what to pursue. In this talk, I will take you through my though process as I was confronted with this optimization problem in Run 1 of the LHC, using the CMS search for disappearing tracks (sensitive to AMSB supersymmetric scenarios) as an example of what non-trivial experimental difficulties are encountered in such an analysis. I will conclude with a discussion of how my thinking on this topic has evolved for Run 2 of the LHC that has just recently begun.

The presence of an instability in the Standard Model Higgs potential may have important implications for inflation and the viability of our Universe. In particular, if the Hubble scale during inflation is comparable to (or larger than) the instability scale of the potential, quantum fluctuations in the Higgs field will lead to the Higgs sampling the unstable part of the potential during inflation. However, to correctly study transitions to the unstable regime and determine the significance for the resulting universe requires addressing a number of subtleties. I will discuss these subtleties and a variety of possible approaches to studying Higgs evolution during inflation. By considering both (1) the evolution of Higgs fluctuations in the Gaussian approximation and (2) a perturbative calculation of the fluctuation two-point correlation function, I aim to elucidate how to address these issues in a consistent, physical way. The insight provided by these approaches will set the scene for studying Higgs fluctuations via the Fokker-Planck (FP) approach, which captures the non-Gaussian nature of the field. As such, it provides information about the rare—but, in terms of the fate of our Universe, potentially extremely important—patches that experience particularly large fluctuations.

With the increase of the center-of-mass energy from 8 TeV
to 13 TeV for LHC Run 2, the probability for boosted topologies will
become even higher than in Run 1. This also comes with a large
increase in pileup from the increased luminosity. This talk
investigates the state of the art of boosted algorithms and grooming
techniques, addresses shortcomings and possible improvements, and
discusses hot-topic items that will be interesting early on in Run 2.

I will talk about the physics of models in which dark matter consists of composite bound states carrying a large conserved dark “nucleon” number. The properties of sufficiently large dark nuclei may obey simple scaling laws, and this scaling can determine the number distribution of nuclei resulting from Big Bang Dark Nucleosynthesis. For plausible models of asymmetric dark matter, dark nuclei of large nucleon number, e.g. >~ 10^8, may be synthesised, with the number distribution taking one of two characteristic forms, which interestingly are broadly independent of initial conditions. A possible consequence of these scenarios is alterations to direct detection signals, which may be coherently enhanced (relative to collider signals), and could be modified by new momentum-dependent form factors. Inelastic interactions between dark matter states might also be important in astrophysical settings.

It is not known how to explain the excess of matter over antimatter with the Standard Model.  This matter asymmetry can be accounted for in certain extensions of the Standard Model through the mechanism of electroweak baryogenesis (EWBG), in which the extra baryons are created in the early Universe during the electroweak phase transition.  In this talk I will review EWBG, connect it to theories of new physics beyond the Standard Model, and show that in many cases the new particles and interactions required for efficient EWBG can be discovered using existing and expected data from the LHC.

I will describe a new collider object we have termed emerging jets.
These can arise when there is a confining dark sector connected to the
Standard Model by a TeV scale mediator, a scenario that is well
motivated by dark matter considerations. The signature of an emerging
jet is O(10) displaced vertices inside the jet each with different
impact parameter, and a small number of prompt tracks. I will describe
strategies that can be used to discover emerging jets even if they
have very small cross sections.

We introduce and systematically study an expansive class of "orbifold Higgs" theories in which the weak scale is protected by accidental symmetries arising from the orbifold reduction of continuous symmetries. The protection mechanism eliminates quadratic sensitivity of the Higgs mass to higher scales at one loop (or more) and does not involve any new states charged under the Standard Model.

The Higgs boson was discovered at the LHC more than two years ago.
So far, the LHC data is consistent with the Standard Model (SM)
predictions. Given its increased rate in the next run of the LHC
with a center-of-mass energy of 14 TeV, double Higgs production will
become an important channel in the search for deviations from the SM
due to new heavy particles. The study of double Higgs production is
also important for understanding the structure of the scalar potential.
In this talk, I will review the production mechanism of double Higgs
production in the SM and consider a model with an additional singlet scalar.
I'll discuss the size of the corrections to single and double Higgs production
in this model and the search for regions of parameter space where double
Higgs production is enhanced relative to the SM prediction.

The Higgs couplings to fermions are known parameters within the Standard Model. Deviations from these
expectations would be clear signals of new physics and are thus important target measurements for the LHC program. 
In this talk I shall discuss ways to extra information about the coupling of the Higgs boson to the charm quark with
emphasis on methods applicable with the available LHC data set. A novel method based on the current ATLAS and CMS
Hbb measurement will be presented and compared to our knowledge so far.  Future projections and the even more
challenging case of light-quark Yukawa couplings will be discussed.