Laser-plasma accelerator group

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.

Abstract:

A search for pentaquarks decaying to Ξ-π- (Ξ-π+) and corresponding antiparticles has been performed with the ZEUS detector at HERA. The data sample consists of deep inelastic ep scattering events at centre-of-mass energies of 300 and 318 GeV, and corresponds to 121 pb -1 of integrated luminosity. A clear signal for Ξ0(1530) →Ξ-π+ was observed. However, no signal for any new baryonic state was observed at higher masses in either the Ξ-π- or Ξ-π+ channels. The searches in the antiparticle channels were also negative. Upper limits on the ratio of a possible Ξ3/2 - (Ξ3/20) signal to the Ξ0(1530) signal were set in the mass range 1650-2350 MeV. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Energetic neutrons produced in ep collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon-proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction 0.64

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