Laser-plasma accelerator group

Abstract:

The cross sections for charged and neutral current deep inelastic scattering in e ⁺ p collisions with a longitudinally polarised positron beam have been measured using the ZEUS detector at HERA. The results, based on data corresponding to an integrated luminosity of 23.8 pb ^-1 at sqrt(s) = 318   GeV, are given for both e ⁺ p charged current and neutral current deep inelastic scattering for both positive and negative values of the longitudinal polarisation of the positron beam. Single differential cross sections are presented for the kinematic region Q ² > 200   GeV ². The measured cross sections are compared to the predictions of the Standard Model. A fit to the data yields σ ^CC ( P e = - 1 ) = 7.4 ± 3.9 ( stat. ) ± 1.2 ( syst. )   pb, which is consistent within two standard deviations with the absence of right-handed charged currents in the Standard Model. © 2006 Elsevier B.V. All rights reserved.

Abstract:

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO2 laser beam. © 2006 The Royal Society.

Abstract:

Differential inclusive jet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector using an integrated luminosity of 38.7 pb-1. The jets have been identified using the kT cluster algorithm in the longitudinally invariant inclusive mode in the laboratory frame; they have been selected with jet transverse energy, ETjet above 6 GeV and jet pseudorapidity, ηjet, between -1 and 3. Measurements of cross sections as functions of ETjet, Björken x and the photon virtuality, Q2, are presented. Three phase-space regions have been selected in order to study parton dynamics from the most global to the most restrictive region of forward-going (close to the proton-beam direction) jets at low x, where the effects of BFKL evolution might be present. The measurements have been compared to the predictions of leading-logarithm parton-shower Monte Carlo models and fixed-order perturbative QCD calculations. In the forward region, O (αs1) QCD calculations underestimate the data up to an order of magnitude at low x. An improved description of the data in this region is obtained by including O (αs2) QCD corrections, which account for the lowest-order t̂-channel gluon-exchange diagrams, highlighting the importance of such terms in the parton dynamics at low x. © 2005 Elsevier B.V. All rights reserved.