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.
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.
119748220
New results from hermes
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.
55047377
Optical characterization of n = 1.03 silica aerogel used as radiator in the RICH of HERMES
The optical properties of silica aerogel tiles with a refractive index of 1.03 and dimensions 11]11] 1c m 3, produced by the Matsushita Electric Works (Japan), have been measured in the wavelength range from 200 to 900 nm. The tiles are used as one of the two radiators of the ring imaging Cherenkov counter of the HERMES experiment at DESY-HERA. The transmittance of light has been measured on 200 tiles by means of a double beam spectrophotometer. The light trans#ectance and re#ectance have been measured on one tile by means of a single-beam spectrophotometer and an integrating re#ecting sphere. Typical values of the measured transmittances at a wavelength j"400 nm are around 0.67. The measured trans#ectance increases almost linearly from 0.4 to 0.96 in the interval 200}300 nm, and remains nearly constant at the value 0.95 in the complementary j-range. The measured re#ectance, mostly con"ned below 400 nm, is completely interpretable as backscattering from inside the aerogel, revealing an absence of light re#ection at the aerogel surfaces. The transmittance data have been "tted with the Hunt formula, whose parameters have been used to evaluate the number of unscattered and scattered Cherenkov photons produced by aerogel. For stacks of 5 tiles and quantum
125760852
Diffractive vector meson production at intermediate energies.
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.
15529188
Inflation from a tachyon fluid
Motivated by recent works of Sen [hep-th/0203211, hep-th/0203265] and Gibbons [hep-th/0204008], we study the evolution of a flat and homogeneous universe dominated by tachyon matter. In particular, we analyze the necessary conditions for inflation in the early roll of a single tachyon field. Comparison with string theory reveals that the potential is too steep to explain the fluctuations in the CMBR. We suggest a scenario based on the simultaneous decay of many branes and comment on the issue of reheating of the universe after tachyon rolling.
119253725
Is the Lorentz limiting speed equal to the speed of light? Photons vs neutrino tests
We discuss the possibility that the “limiting speed” climit (cl) appearing in Lorentz equations might be different (i.e. slightly larger) than the observed speed of light cn. We show that such a possibility can be tested by state-of-the-art Michelson–Morley experiments, but also by careful measurement of neutrino speeds. It would indeed suffice to show that cn<cν≤cl. Quite interestingly, current limits from both approaches are competitive, in some circumstances. We also comment on competing tests using gamma-ray burst, assuming a dispersive character for the propagation of light.
120876664
Some aspects of large N(c) at finite temperature
119348282
Sum rules in the CFL phase of QCD at finite density
Abstract We study the asymmetry between the vector current and axial-vector current correlators in the colour–flavour locking (CFL) phase of QCD at finite density. Using Weinberg's sum rules, we compute the decay constant f π of the Goldstone modes and find agreement with previous derivations. Using Das's sum rule, we also estimate the contribution of electromagnetic interactions to the mass of the charged modes. Finally, we comment on low temperature corrections to the effective field theory describing the Goldstone bosons.