Scientific Series

The axion provides a solution to the strong CP problem and is a cold dark matter candidate.  I'll briefly review the limits on the axion from particle physics, stellar evolution and cosmology.  The various constraints suggest that the axion mass is in the micro-eV to milli-eV range.  In this window, axions contribute significantly to the energy density of the universe in the form of cold dark matter.  It was recently found that dark matter axions thermalize and form a Bose-Einstein condensate (BEC).  As a result, it may be possible to distinguish axions from other forms of dark matter, such as weakly interacting massive particles (WIMPs), on observational grounds.  Axions accreting onto a galactic halo fall in with net overall rotation because almost all go to the lowest energy available state for given angular momentum.  In contrast, WIMPs accrete onto galactic halos with an irrotational velocity field.  The inner caustics are different in the two cases.  I'll argue that the dark matter is axions because there is observational evidence for the type of inner caustic produced by, and only by, an axion BEC.

I will discuss a wide class of models which realise a bounce in a spatially flat Friedmann universe in standard General Relativity. The key ingredient is a noncanonical, minimally coupled scalar field
belonging to the class of theories with Kinetic Gravity Braiding/Galileon-like self-couplings. In these models, the universe smoothly volves from contraction to expansion, suffering neither from ghosts
nor gradient instabilities around the turning point. The end-point of he evolution can be a standard radiation-domination era or an nflationary phase.
The talk is based on arXiv:1109.1047v2 [hep-th], JCAP11(2011)021.

In this lecture I will describe in simple terms the basic ideas of gauge symmetry in phase space, its consequences in the form of a deeper redefinition of space and time, and some observable manifestations of an extra space and extra time dimensions.

Neutrino physics entered a new era in the last decade. With the discovery of a non-vanishing neutrino rest mass in oscillation experiments a variety of new questions showed up in the context of nuclear and particle physics. One of the crucial questions is the determination of the absolute neutrino mass, which cannot be measured in oscillation experiments. One option is neutrino-less double beta decay, the simultaneous conversion of two neutrons into two protons emitting two electrons. This total lepton number violating process requires that neutrinos are their own antiparticles and is considered to be gold plated. Furthermore, the measured half-life is directly linked with the neutrino mass. Currently, half-life measurements beyond 1025 years are discussed. This requires a large amount of the isotope of interest and a reduction of disturbing background to the smallest possible level. Indeed it is a search for the needle in a haystack. After a general introduction into double beta decay and the related physics, the talk will focus on the status of the experiments GERDA, COBRA and SNO+.

The Large Hadron Collider has been operating for more than a year and delivering exciting results. It has already excluded large parts of the parameter space for supersymmetry. If the hints of a Higgs boson at 125 GeV hold up, the implications for supersymmetry are even more profound. I will explain some of the consequences, including the failure of large classes of models like general gauge mediation to account for such a heavy Higgs. I will also discuss some ideas about how to look for scalar top quarks, which must be present in the low-energy spectrum for supersymmetry to be natural.

The LHC is offering our first glimpses of physics at energies above a TeV, allowing us an unprecedented chance to search for very heavy new particles from electroweak compositeness, new gauge forces, extra dimensions, and supersymmetry.  Some of the most interesting signals involve decays into Standard Model particles that we are used to thinking of as "heavy":  W/Z bosons, top quarks, and perhaps Higgs bosons.  However, at genuinely TeV-scale energies, these SM particles with O(100 GeV) mass are produced with relativistic velocities.  Consequently, their own decay products are Lorentz-boosted into very collimated configurations, and can look uncomfortably similar to the jets of particles that are copiously produced by QCD at hadron colliders.  The past several years have witnessed a surge of ideas for how to uncover these challenging signals by carefully organizing the patterns of radiation observed in the LHC detectors, an approach called Jet Substructure.  I will discuss some of these recent ideas, and show a handful of important applications to searches for new physics.

Based on the joint work with Sergey Bravyi, IBM Watson.   We show that any topologically ordered local stabilizer model of spins in three dimensional lattices that lacks string logical operators can be used as a reliable quantum memory against thermal noise. It is shown that any local process creating a topologically charged particle separated from other particles by distance $R$, must cross an energy barrier of height $c \log R$. This property makes the model glassy. We devise an efficient decoding algorithm that should be used at the final read-out, and prove a lower bound on the memory time until which the fidelity between the outcome of the decoder and the initial state is close to 1. The memory time increases as $L^{\beta}$ where $L$ is the system size and $\beta$ the inverse temperature, as long as $L < L^\star \sim e^\beta$. Hence, the optimal memory time scales as $e^{\beta^2}$. Our bound applies when the system interacts with thermal bath via a Markovian master equation. We give an example of 3D local stabilizer codes that satisfies all of our assumptions. We numerically verify for this example that our bound is tight up to constants.

We introduce an exactly solvable model to test various proposals for the imposition of the spin foam simplicity constraints. This model is a three-dimensional Holst-Plebanski action for the gauge group SO(4), in which the simplicity constraints mimic the situation of the four-dimensional theory. In particular, the canonical analysis reveals the presence of secondary second class constraints conjugated to the primary ones. We perform the spin foam quantization of the theory in the spirit of the BC and EPRL models, and give arguments for modifying the measure over the holonomies in order to account for the presence of the secondary second class constraints.