Particle Physics

A beautiful understanding of the smallness of the neutrino masses may be obtained via the seesaw mechanism, whereby one takes advantage of the key qualitative distinction between the neutrinos and the other fermions: right-handed neutrinos are gauge singlets, and may therefore have large Majorana masses. The standard seesaw mechanism, however, does not address the apparent lack of hierarchy in the neutrino masses compared to the quarks and charged leptons, nor the large leptonic mixing angles compared to the small angles of the CKM matrix. In this talk, I will show that the singlet nature of the right-handed neutrinos may be taken advantage of in one further way in order to solve these remaining problems: Unlike particles with gauge interactions, whose numbers are constrained by anomaly cancellation, the number of gauge singlet particles is essentially undetermined. If large numbers of gauge singlet fermions are present at high energies - as is suggested, for example, by various string constructions - then the effective low energy neutrino mass matrix may be determined as a sum over many distinct Yukawa couplings, with the largest ones being the most important. This can reduce hierarchy, and lead to large mixing angles. Assuming a statistical distribution of fundamental parameters, we will show that this scenario leads to a good fit to low energy phenomenology, with only a few qualitative assumptions guided by the known quark and lepton masses. The scenario leads to predictions of a normal hierarchy for the neutrino masses, and a value for the |m_ee| mass matrix element of about 1-6 meV.

The IceCube neutrino observatory is the world's largest high-energy neutrino telescope, utilizing the deep Antarctic ice as the Cherenkov detector medium.  In December 2010 the last of the observatory's 86 strings of optical detectors was deployed, completing the approximate cubic-kilometer array.   With the addition of a low-energy extension, called DeepCore, the observatory has very high neutrino detection efficiency for energies ranging from ~10 GeV to a few EeV.  The low-energy threshold establishes the first steps towards precision neutrino measurements in the Antarctic. Discussed will be early results from this emerging particle physics program as well as initial expectations from studies of potential future detector upgrades towards creating a multi-megaton neutrino detector with O(10 MeV) energy threshold.

Direct dark matter (DM) detection experiments almost always focus on Weakly Interacting Massive Particles (WIMPs), which have a mass in the 1--1000 GeV range.  However, what if DM is not a WIMP?  In this talk, new direct detection strategies for DM particles with MeV to GeV mass will be presented.  In this largely unexplored mass range, DM can scatter with electrons, causing ionization of atoms in a detector target material and leading to single- or few-electron events.  I will present the first direct detection limits on DM as light as a few MeV, using XENON10 data.  Theoretically interesting models can already be probed.  Significant improvements in sensitivity should be possible with dedicated experiments, opening up a window to new regions in DM parameter space.  

During the last ten years, a lot of experimental information has been collected from neutrino oscillation experiments  on the masses and and mixings among the three known  neutrino species. This presents  an almost comparable flavor picture for leptons as is already known to exist for quarks. In the talk I would like to discuss what these results imply for physics beyond the standard model since SM predicts zero neutrino mass. In particular, it seems that one can make a compelling case for the idea of grand unified theory of matter and forces as a candidate theory for neutrinos as well as providing a possible route to flavor unification. I will  outline these arguments,  present a candidate theory and discuss  its tests.

Models with right-handed currents have recently attracted attention because of their potential ability to solve the discrepancy in various determinations of |V_{ub}|. We consider a minimal setup with an SU(2)_L x SU(2)_R x U(1)_{B-L} gauge symmetry, for which we perform a simultaneous analysis of the most important constraints from electroweak precision observables, particle anti-particle mixing and the B -> X_{s,d} gamma decays. The main goals of our analysis are the determination of allowed parameter space for the new right-handed mixing matrix and the possible solution of various anomalies in the current flavor data.

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.

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.

Supersymmetry is a popular candidate for the 'model beyond the Standard Model', however minimal versions of it are quite constrained by the first year of data from the LHC. In this talk I will focus on supersymmetry scenarios where the gaugino masses are Dirac rather than Majorana. This seemingly innocuous change has a profound impact on collider bounds -- reducing the bound on (1st and 2nd generation) squark masses by nearly a factor of two. In addition, Dirac gaugino scenarios have amazing flavor properties, smoking gun LHC signals, and cosmological implications.

Observing lepton-number violating processes is a decisive step toward establishing the Majorana nature of the neutrino mass. We explore the prospects searching for Delta L = 2 processes and propose the tests for the three types of the Seesaw mechanisms. Potential signals at the LHC
are studied and correlations to the neutrino oscillation parameters are investigated.