Laser-plasma accelerator group

Abstract:

A method for producing non-uniformly spaced (chirped) trains of ultrafast pulses is demonstrated, using an acousto-optic programmable dispersive filter (AOPDF). Programmable pulse trains of this type may find applications in quasi-phase matching of high-harmonic generation. © 2009 Optical Society of America.

Abstract:

Synchrotrons and free-electron lasers are the most powerful sources of X-ray radiation. They constitute invaluable tools for a broad range of research 1 ; however, their dependence on large-scale radiofrequency electron accelerators means that only a few of these sources exist worldwide. Laser-driven plasma-wave ccelerators 2-10 provide markedly increased accelerating fields and hence offer the potential to shrink the size and cost of these X-ray sources to the niversity-laboratory scale. Here, we demonstrate the generation of soft-X-ray undulator radiation with laser-plasma-accelerated electron beams. The well-collimated beams deliver soft-X-ray pulses with an expected pulse duration of ∼ 10 fs (inferred from plasma-accelerator physics). Our source draws on a 30-cm-long undulator and a 1.5-cm-long accelerator delivering stable electron beams with energies of ∼ 210 MeV. The spectrum of the generated undulator radiation typically consists of a main peak centred at a wavelength of ∼ 18 nm (fundamental), a second peak near ∼ 9 nm (second harmonic) and a high-energy cutoff at ∼ 7 nm. Magnetic quadrupole lenses ensure efficient electron-beam transport and demonstrate an enabling technology for reproducible generation of tunable undulator radiation. The source is scalable to shorter wavelengths by increasing the electron energy. Our results open the prospect of tunable, brilliant, ultrashort-pulsed X-ray sources for small-scale laboratories. © 2009 Macmillan Publishers Limited. All rights reserved.

Abstract:

The semi-inclusive reaction e+p→e+Xp was studied with the ZEUS detector at HERA with an integrated luminosity of 12.8 pb -1. The final-state proton, which was detected with the ZEUS leading proton spectrometer, carried a large fraction of the incoming proton energy, xL > 0.32, and its transverse momentum squared satisfied p 2T < 0.5GeV2; the exchanged photon virtuality, Q2, was greater than 3GeV2 and the range of the masses of the photon-proton system was 45 < W < 225GeV. The leading proton production cross section and rates are presented as a function of xL, p2 T , Q2 and the Bjorken scaling variable, x. © SISSA 2009.

Abstract:

Beauty photoproduction in dijet events has been measured at HERA with the ZEUS detector using an integrated luminosity of 126 pb-1. Beauty was identified in events with a muon in the final state by using the transverse momentum of the muon relative to the closest jet. Lifetime information from the silicon vertex detector was also used; the impact parameter of the muon with respect to the primary vertex was exploited to discriminate between signal and background. Cross sections for beauty production as a function of the muon and the jet variables as well as dijet correlations are compared to QCD predictions and to previous measurements. The data are well described by predictions from next-to-leading-order QCD. © SISSA 2009.

Abstract:

The charm fragmentation function has been measured in D* photoproduction with the ZEUS detector at HERA using an integrated luminosity of 120 pb-1. The fragmentation function is measured versus z = (E +p||)D* /2Ejet, where E is the energy of the D * meson and p|| is the longitudinal momentum of the D * meson relative to the axis of the associated jet of energy Ejet. Jets were reconstructed using the kT clustering algorithm and required to have transverse energy larger than 9GeV. The D* meson associated with the jet was required to have a transverse momentum larger than 2GeV. The measured function is compared to different fragmentation models incorporated in leading-logarithm Monte Carlo simulations and in a next-to-leading-order QCD calculation. The free parameters in each fragmentation model are fitted to the data. The extracted parameters and the function itself are compared to measurements from e+e-experiments. © SISSA 2009.