115160247
Another derivation of the geometrical KPZ relations
We give a physicist's derivation of the geometrical (in the spirit of Duplantier–Sheffield) KPZ relations, via heat kernel methods. It gives a covariant way to define neighborhoods of fractals in 2D quantum gravity, and shows that these relations are in the realm of conformal field theory.
173517385
A Short Introduction to Critical Interfaces in 2D
These notes are meant as a gentle introduction to the concepts of Loewner chains, local growth, Stochastic Loewner Evolutions (SLE) and their relationships to Conformal Field Theories (CFT). These concepts have played an important role in physics and mathematics during the recent years. The first section describes two discrete examples, the exploration process and loop-erased random walks. It can be read almost without any prerequisites. The aim is to show that even for curves defined purely in geometrical terms, it is useful to have a statistical mechanics viewpoint where the measure on curves is derived from a measure on local degrees of freedom of some model. A third model, diffusion-limited aggregation (DLA) is also introduced. The second section introduces Loewner chains and their relevance for the description of growth processes. A prerequisite is a minimal knowledge of complex analysis. The third section contains the derivation of the relevance of SLE in the description of interfaces when two properties, conformal invariance and the domain Markov property, are assumed/proved. The prerequisites are some knowledge of complex analysis and probability theory. The last section outlines the relation with Conformal Field Theory. This relation is via martingales and singular vectors. Again, the reader is assumed to have some basic knowledge of complex analysis and probability theory, and some familiarity with CFT. The discussion is informal. There is little or no claim at originality. We try to give some intuition based on explicit examples.
59058647
COVARIANT DIFFERENTIAL EQUATIONS AND SINGULAR VECTORS IN VIRASORO REPRESENTATIONS
Abstract We give explicit expressions for the singular vectors in highest weight representations of the Virasoro algebra using a precise definition of fusion.
13930079
The recurrent ultra-luminous X-ray transient NGC 253 ULX1
We present the results of ROSAT and XMM-Newton observations of the recurrent ultraluminous X-ray source (ULX) NGC 253 ULX I. This transient is one of the few ULXs that was detected during several outbursts. The luminosity reached 1.4 × 10 39 erg s -1 and 0.5 x 10 39 erg s -1 in the detections by ROSAT and XMM-Newton, respectively, indicating a black hole X-ray binary (BHXRB) with a mass of the compact object of ≥11 M ○. . In the ROSAT detection NGC 253 ULX1 showed significant variability, whereas the luminosity was constant in the detection from XMM-Newton. The XMM-Newton EPIC spectra are well-tit by a hremsstrahlung model (kT = 2.24 keV, N H = 1.74 x 10 20 cm -2 ), which can he used to describe a comptonized plasma. No counterpart was detected in the optical I, R, B, NUV and FUV bands to limits of 22.9, 24.2. 24.3, 22 and 23 mag, respectively, pointing at a XRB with a low mass companion.
118498826
Soft collinear effective theory for heavy WIMP annihilation
A bstractIn a large class of models for Weakly Interacting Massive Particles (WIMPs), the WIMP mass M lies far above the weak scale mW . This work identifies universal Sudakov-type logarithms ∼ α log2(2 M/mW ) that spoil the naive convergence of perturbation theory for annihilation processes. An effective field theory (EFT) framework is presented, allowing the systematic resummation of these logarithms. Another impact of the large separation of scales is that a long-distance wavefunction distortion from electroweak boson exchange leads to observable modifications of the cross section. Careful accounting of momentum regions in the EFT allows the rigorous disentanglement of this so-called Sommerfeld enhancement from the short-distance hard annihilation process. The WIMP is described as a heavy-particle field, while the electroweak gauge bosons are treated as soft and collinear fields. Hard matching coefficients are computed at renormalization scale μ ∼ 2 M , then evolved down to μ ∼ mW , where electroweak symmetry breaking is incorporated and the matching onto the relevant quantum mechanical Hamiltonian is performed. The example of an SU(2)W triplet scalar dark matter candidate annihilating to line photons is used for concreteness, allowing the numerical exploration of the impact of next-to-leading order corrections and log resummation. For M ≃ 3 TeV, the resummed Sommerfeld enhanced cross section is reduced by a factor of ∼ 3 with respect to the treelevel fixed order result.
123530930
EURECA – THE EUROPEAN UNDERGROUND RARE EVENT CALORIMETER ARRAY
18560327
Commissioning run of the CRESST-II dark matter search
Abstract The CRESST cryogenic direct dark matter search at Gran Sasso, searching for WIMPs via nuclear recoil, has been upgraded to CRESST-II by several changes and improvements. The upgrade includes a new detector support structure capable of accommodating 33 modules, the associated multichannel readout with 66 SQUID channels, a neutron shield, a calibration source lift, and the installation of a muon veto. We present the results of a commissioning run carried out in 2007. The basic element of CRESST-II is a detector module consisting of a large ( ∼ 300 g ) CaWO 4 crystal and a very sensitive smaller ( ∼ 2 g ) light detector to detect the scintillation light from the CaWO 4 . The large crystal gives an accurate total energy measurement. The light detector permits a determination of the light yield for an event, allowing an effective separation of nuclear recoils from electron–photon backgrounds. Furthermore, information from light-quenching factor studies allows the definition of a region of the energy-light yield plane which corresponds to tungsten recoils. A neutron test is reported which supports the principle of using the light yield to identify the recoiling nucleus. Data obtained with two detector modules for a total exposure of 48 kg-days are presented. Judging by the rate of events in the “all nuclear recoils” acceptance region the apparatus shows a factor ∼10 improvement with respect to previous results, which we attribute principally to the presence of the neutron shield. In the “tungsten recoils” acceptance region three events are found, corresponding to a rate of 0.063 per kg-day. Standard assumptions on the dark matter flux, coherent or spin independent interactions, then yield a limit for WIMP-nucleon scattering of 4.8 × 10 - 7 pb , at M WIMP ∼ 50 GeV .
119176850
The CRESST II Dark Matter Search
Direct Dark Matter detection with cryodetectors is briefly discussed, with particular mention of the possibility of the identification of the recoil nucleus. Preliminary results from the CRESST II Dark Matter search, with 730 kg-days of data, are presented. Major backgrounds and methods of identifying and dealing with them are indicated.