Laser-plasma accelerator group

Abstract:

We give on overview of recent advances in collisionally pumped optical field-ionization soft X-ray lasers developed at LOA. Saturated amplification has been achieved on the 5d-5p transition in Xe8+ at 41.8 nm, and on the 4d-4p transition in Kr8+ at 32.8 nm. We demonstrate a significant increase of the energy output from the Xe8+ laser driven within two types of wave-guide. Finally, we present results of a pioneering work aimed to set up and characterize the first true soft X-ray laser chain.

Abstract:

The ZEUS inclusive differential cross-section data from HERA, for charged and neutral current processes taken with e ⁺ and e ^- beams, together with differential cross-section data on inclusive jet production in e ⁺ p scattering and dijet production in γp scattering, have been used in a new NLO QCD analysis to extract the parton distribution functions of the proton. The input of jet-production data constrains the gluon and allows an accurate extraction of alpha s(MZ) at NLO; alphas = 0.1183±0.0028(exp.)±0.0008(model). An additional uncertainty from the choice of scales is estimated as ±0.005. This is the first extraction of alphas(MZ) from HERA data alone. © Springer-Verlag/Società Italiana de Fisica 2005.

Abstract:

We present calculations of the small-signal gain coefficient, gain length, and output energy of a recombination laser in Ar⁷⁺ driven by optical field ionization. Simulations are presented for both 400 and 800 nm pump radiation, and for two targets containing mixtures of argon and hydrogen: a gas cell, and a gas-filled capillary discharge waveguide. Extremely high values for the small-signal gain coefficient are calculated for the 4s-3p transition at 23.2 nm using a pump wavelength of 400 nm for both the gas cell and waveguide. Operation in the waveguide is predicted to greatly increase the XUV laser output owing to a large increase of the gain length. The calculations also show that use of the waveguide allows significant single-pass gain to be achieved even with pump radiation of 800 nm wavelength. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Deep inelastic scattering and its diffractive component, ep → e′ γ* p → e′ X N, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb^-1. The measurement covers a wide range in the γ* p c.m. energy W (37-245 GeV), photon virtuality Q² (2.2-80 GeV²) and mass MX (0.28-35 GeV). The diffractive cross section for MX > 2 GeV rises strongly with W; the rise is steeper with increasing Q². The latter observation excludes the description of diffractive deep inelastic scattering in terms of the exchange of a single pomeron. The ratio of diffractive to total cross section is constant as a function of W, in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to γ* p scattering. Above MX of 8 GeV, the ratio is flat with Q², indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, F2^D(3)(β, xℙ, Q²), of the proton. For fixed β, the Q² dependence of xℙF2 ^D(3) changes with xℙ in violation of Regge factorisation. For fixed xℙ, xℙ F2^D(3) rises as β → 0, the rise accelerating with increasing Q². These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section. © 2005 Elsevier B.V. All rights reserved.