125736234
Recent HERMES Results on Inclusive Scattering and Exclusive Reactions
122512363
Exclusive meson production at HERMES
Abstract.Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π+, ρ0 and pion pair production will be presented.
125760852
Diffractive vector meson production at intermediate energies.
119748220
New results from hermes
202096488
Particle identification with the HERMES RICH
Abstract In early 1998 a dual-radiator RICH detector was installed into the HERMES spectrometer. It is designed to allow hadron identification over the HERMES kinematic range (2–20 GeV) using both an aerogel and C 4 F 10 gas radiator. Three different approaches that are currently pursued for particle identification are presented.
13891794
Vector meson production at HERMES.
Diiractive production of , , ! and J== mesons is observed at the Hermes experiment at DESY. Hermes scatters the 27.5 GeV longitudinally polarised positrons of the HERA storage ring oo an internal gas storage cell target. Polarised and unpolarised gas targets of diierent types are used. The exclusive production from diierent nuclear targets (H, D, 3 He and 14 N) shows coherent and incoherent contributions that depend on the atomic mass A and Q 2. For 3 He the incoherent contribution is smaller than for H and D. The nuclear transparency T has been extracted as a function of Q 2 and and shows no signiicant signal of color transparency eeects. The polari-sation of beam and target at Hermes will allow for the world's rst measurement of double-spin asymmetries in combination with decay angle information. This will allow a nearly complete characterization of the angular momentum structure of the reaction. First results for the longitudinal and linear polarisation of-mesons and for the ratio of the longitudinal versus transverse cross section of-mesons as a function of Q 2 are presented. J== mesons are observed at Hermes in the muonic and the electronic decay channel. As they are produced by photon-gluon fusion processes, the observed spin asymmetry from polarised target data contains information about the gluon spin distribution in the nucleon. As the available statistics is marginal for a determination of G, Hermes currently performs an upgrade of the spectrometer. A ring image Cerenkov detector, a muon detector and a small angle spectrometer will be installed in 1998 and an additional vertex detector in 1999. The upgrade will improve the identiication of charmed and strange mesons, the detection of heavy decaying and slow recoil particles and the detection of the scattered positron in low-Q 2 (photoproduction) events.
119211034
WIMP dark matter, Higgs exchange and DAMA
In the WIMP scenario, there is a one-to-one relation between the dark matter (DM) relic density and spin-independent direct detection rate if both the annihilation of DM and its elastic scattering on nuclei go dominantly through Higgs exchange. In particular, for DM masses much smaller than the Higgs boson mass, the ratio of the relevant cross sections depends only on the DM mass. Assuming DM mass and direct detection rate within the ranges allowed by the recent DAMA collaboration results—taking account of the channelling effect on energy threshold and the null results of the other direct detection experiments—gives a definite range for the relic density. For scalar DM models, like the Higgs portal models or the inert doublet model, the relic density range turns out to be in agreement with WMAP. This scenario implies that the Higgs boson has a large branching ratio to pairs of DM particles, a prediction which might challenge its search at the LHC.
16746073
Generation of the baryon assymetry of the Universe within the left-right symmetric model
Abstract Fermions scattering off first-order phase transition bubbles, in the framework of SU(2) L ⊗ SU(2) R ⊗ U(1) models, may generate the Baryon Assymetry of the Universe (BAU), either at the LR-symmetry-breaking scale, or at the weak scale. In the latter case, the baryon assymetry of the Universe is related to CP violation in the K 0 − K 0 system.
119388048
Effective operators and extended symmetry
In this note we expand on our previous study of the implications of LEP1 results for future colliders. We extend the effective operator-based analysis of De Rujula et al. to a larger symmetry group, and show at which cost their expectations can be relaxed. Of particular interest to experiment is a rephrasing of our previous results in terms of the Renard et al. parametrization for the gauge boson self-couplings (slightly extended to include δgγ). We suggest the use of a (δgγ, δgZ) plot to confront the expectations of various models.
51157
Excitations in hot non-commutative theories
We study the dispersion relation for scalar excitations in supersymmetric, non-commutative theories at finite temperature. In = 4 Yang-Mills the low momentum modes have superluminous group velocity. In the massless Wess-Zumino model the minimum of the dispersion relation is at non zero momentum for temperatures above T0 ≈ (gθ)−1/2. We briefly comment on = 2 Yang-Mills at finite density.
14469784
Effective field theory approach to N=4 supersymmetric Yang-Mills at finite temperature
We study the perturbation expansion of the free energy of N = 4 supersymmetric SU(N) Yang-Mills at finite temperature in powers of ’t Hooft’s coupling g2N in the large N limit. Infrared divergences are controlled by constructing a hierarchy of two 3 dimensional effective field theories. This procedure is applied to the calculation of the free energy to order (g2N)3/2, but it can be extended to higher order corrections. Agustin.Nieto@cern.ch mtytgat@ulb.ac.be In recent years some light has been shed on understanding the relation between string theory and gauge theories [1]. A particularly interesting system is N coincident, parallel D3-branes. This system realizes N = 4 supersymmetric U(N) Yang-Mills in its world volume (4 dimensions). In the large N limit, the 3-brane system becomes a black brane whose Bekenstein-Hawking entropy can be obtained by considering Ramond-Ramond charged 3brane classical solutions [2]. It is therefore interesting to compare the 3-brane thermodynamics with that of N = 4 supersymmetric U(N) Yang-Mills. The free energy density of N = 4 supersymmetric U(N) Yang-Mills in the ideal gas approximation is Fideal = −π2N2T /6. On the other hand, the free energy of a black 3-brane was found to be [2] FBH = (3/4) ×Fideal. Maldacena’s conjecture [3] helps to understand the relative factor 3/4 between Fideal and FBH. The conjecture relates type IIB superstrings on AdS5 × S to N = 4 supersymmetric SU(N) Yang-Mills in 4 dimensions and allows to compute the next-to-leading order correction to FBH in the strong coupling limit [4], FBH Fideal = 3 4 + 45ζ(3)