Quantum Fields and Strings

In the last years, the NA48/2 experiment at the CERN SPS has recorded an unprecedented sample of charged kaon decays. From this, we report very precise measurements of fundamental parameters of Chiral Perturbation Theory (ChPT) and the study of low energy pi-pi scattering. Several rare and very rare decays have been studied. From more than 10^6 K+- -> pi+ pi0 gamma decays, a first measurement of the interference between Bremsstrahlung and Direct Emission amplitudes and a stringent limit on direct CP violation in this channel is presented. For K+- -> pi+ gamma gamma, about 1000 events have been selected, more than 30 times the existing statistics. Also, the first observation of K+- -> pi+ gamma e+ e- is reported with 120 events. For both decays, the branching fraction and the free parameter of O(p4) ChPT has been measured with high accuracy. Finally, we report on a new precise measurement of the branching fraction and form factors of K+- -> pi+- e+ e-, which is highly suppressed by the GIM mechanism. The measurement of the e+ e- spectrum is an important input for ChPT and probes the weak static interaction. The analyses of K+- -> pi+- pi0 pi0 (K3pi) and K+- -> pi+ pi- e+- nu (Ke4) decays give complementary approaches to the study of low energy pi-pi scattering. From data samples of ~90 Millions 3pi and ~1 Million Ke4 decays, precise values of a0 and a2, the Isospin 0 and 2 S-wave PI-PI scattering lengths, can be extracted with an unprecedented experimental precision, allowing accurate tests of Chiral Perturbation Theory predictions. The form factors of the Ke4 decays and their energy dependence are also measured with an improved precision, while the Dalitz plot parameters of the K3PI decays are determined including a new quadratic term.

We study observables in a conformal field theory which are very closely related to the ones used to describe hadronic events at colliders. We focus on the correlation functions of the energies deposited on calorimeters placed at a large distance from the collision. We consider initial states produced by an operator insertion and we study some general properties of the energy correlation functions for conformal field theories. We argue that the small angle singularities of energy correlation functions are controlled by the twist of non-local light-ray operators with a definite spin. We relate the charge two point function to a particular moment of the parton distribution functions appearing in deep inelastic scattering. The one point energy correlation functions are characterized by a few numbers. For ${cal N}=1$ superconformal theories the one point function for states created by the R-current or the stress tensor are determined by the two parameters $a$ and $c$ charac terizing the conformal anomaly. Demanding that the measured energies are positive we get bounds on $a/c$. We also give a prescription for computing the energy and charge correlation functions in theories that have a gravity dual. The prescription amounts to probing the falling string state as it crosses the $AdS$ horizon with gravitational shock waves. In the leading, two derivative, gravity approximation the energy is uniformly distributed on the sphere at infinity, with no fluctuations. We compute the stringy corrections and we show that they lead to small, non-gaussian, fluctuations in the energy distribution. Corrections to the one point functions or antenna patterns are related to higher derivative corrections in the bulk.

Astrophysical evidence indicates that the universe consists to about 25% of non-baryonic, cold Dark Matter, compared to merely ~4% of \'regular\' matter, composed of quarks and electrons. The existence of Dark Matter and Dark Energy is striking evidence for physics beyond the Standard Model, and understanding their nature ranks among the foremost questions in science today. If the bulk of matter in the universe consists of relic massive particles moving at non-relativistic speeds, we may be able to detect these particles in direct searches with low background experiments. This talk will focus on the search for weakly interacting massive particles (WIMPs), predicted in particular by theories invoking supersymmetry. The recoils of target nuclei resulting from elastically scattering WIMPs should be detectable in principle with sensitive detectors. I will review the current status of direct Dark Matter searches, and then elaborate on the XENON suite of experiments. The XENON Dark Matter program was established in 2002, and reached a major milestone with the installation of its first Dark Matter detector, XENON10, at the Gran Sasso underground laboratory in Italy. XENON10 reported the world-best limits on spin-independent WIMP-nucleon cross-sections last year. (CDMS-II has recently improved their limits even further.) Meanwhile, we are building the next generation detector XENON100 at the same location. The new detector will feature ten-fold greater fiducial mass and 100 times improved background. We anticipate first results by the end of the year. I will report on the status of XENON100 and its projected sensitivity, and conclude with an outlook on the prospects of the field.

We present the results from the MiniBooNE neutrino oscillations search in which no significant excess of events is observed above background in the energy range from 475 MeV to 3000 MeV. For lower energies an excess of events that is not consistent with a two neutrino oscillation model is observed. We present recent advances in the understanding of this excess, including a study of muon and electron neutrinos from the nearby NuMI neutrino source. The techniques used in the first oscillation analysis are discussed as well as tose of a recent analysis that combines two different electron neutrino candidate samples with a high statistics muon neutrino sample in the oscillations fit to reduce systematic uncertainties.

The sensitivity of inflationary models to Planck-suppressed operators motivates modeling inflation in string theory. The case of high-scale inflation is particularly interesting both theoretically and observationally. Observationally it yields a gravity wave (B mode polarization) signature, and theoretically it requires a large field excursion which is particularly sensitive to UV physics. I\'ll present a simple mechanism derived recently in collaboration with A. Westphal for obtaining large-field inflation, and hence a gravitational wave signature, from string theory. The simplest version of this mechanism, arising on twisted torus compactifications of string theory, yields an observationally distinctive version of chaotic inflation with a potential proportional to the 2/3 power of the inflaton, falsifiable on the basis of upcoming CMB measurements. This mechanism for extending the field range arises widely in string compactifications, though in all cases it requires sufficient symmetry to control the corrections to the slow-roll parameters. I will finish by describing further developments in this direction.

The atomic hydrogen gas left over from the Big Bang was affected by processes ranging from quantum fluctuations during the early epoch of inflation to irradiation by the first galaxies at late times. Mapping this gas through its resonant 21cm line serves a dual role as a powerful probe of both fundamental physics and astrophysics. Current cosmological data sets (such as galaxy surveys or the microwave background) cover only 0.1% of the comoving volume of the observable Universe. 21cm observations hold the potential of mapping matter through most of the remaining volume. Radio observatories are currently being designed and constructed with this goal in mind. The three-dimensional 21cm maps could potentially set unprecedented statistical constraints on the power spectrum of cosmic density fluctuations and its gravitational growth with cosmic time. The reduced uncertainties could allow for precise measurements of fundamental parameters, such as the mass of the neutrino or the equation of state of the dark energy (from acoustic oscillations in the 21cm power spectrum), and will test generic predictions of cosmic inflation for deviations of the density fluctuations from scale invariance and gaussianity. The measured gravitational growth of the fluctuations with cosmic time would constrain the nature of the dark matter or alternative theories of gravity.