Theory of Elastic Vector Meson Production

The elastic production of vector mesons at HERA is discussed from the theoretical point of view. We briefly review different models, their successes and shortcomings. Main emphasis is put on recent issues in perturbative QCD calculations. Models including the vector meson wave function are compared with an approach based on parton-hadron duality. We discuss several refinements of these models in some detail, including the important role of off-diagonal parton distributions.Comment: 2+12 pages, LaTeX, five figures. Talk given at the Ringberg Workshop: New Trends in HERA Physics 199

Measurement of the Neutron Spin Structure Function g1ng_1^n with a Polarized ^3He Target

Results are reported from the HERMES experiment at HERA on a measurement of the neutron spin structure function $g_1^n(x,Q^2)$ in deep inelastic scattering using 27.5 GeV longitudinally polarized positrons incident on a polarized $^3$He internal gas target. The data cover the kinematic range $0.023<x<0.6$ and $1 (GeV/c)^2 < Q^2 <15 (GeV/c)^2$. The integral $\int_{0.023}^{0.6} g_1^n(x) dx$ evaluated at a fixed $Q^2$ of $2.5 (GeV/c)^2$ is $-0.034\pm 0.013(stat.)\pm 0.005(syst.)$. Assuming Regge behavior at low $x$, the first moment $\Gamma_1^n=\int_0^1 g_1^n(x) dx$ is $-0.037\pm 0.013(stat.)\pm 0.005(syst.)\pm 0.006(extrapol.)$.Comment: 4 pages TEX, text available at http://www.krl.caltech.edu/preprints/OAP.htm

In-situ observation of the formation of laser-induced periodic surface structures with extreme spatial and temporal resolution

Irradiation of solid surfaces with intense ultrashort laser pulses represents a unique way of depositing energy into materials. It allows to realize states of extreme electronic excitation and/or very high temperature and pressure, and to drive materials close to and beyond fundamental stability limits. As a consequence, structural changes and phase transitions often occur along unusual pathways and under strongly non-equilibrium conditions. Due to the inherent multiscale nature - both temporally and spatially - of these irreversible processes their direct experimental observation requires techniques that combine high temporal resolution with the appropriate spatial resolution and the capability to obtain good quality data on a single pulse/event basis. In this respect fourth generation light sources, namely short wavelength, short pulse free electron lasers (FELs) are offering new and fascinating possibilities. As an example, this chapter will discuss the results of scattering experiments carried at the FLASH free electron laser at DESY (Hamburg, Germany), which allowed us to resolve laser-induced structure formation at surfaces on the nanometer to sub-micron length scale and in temporal regimes ranging from picoseconds to several nanoseconds with sub-picosecond resolution

Search For Exotic Tau-decays

The Crystal Ball detector at the Doris II storage ring at DESY was used to search for the exotic decay processes tau -&gt; e gamma, tau -&gt; e pi0, tau -&gt; e eta. No signal was observed. We obtained the following 90% CL upper limits on the branching fractions:B(tau -&gt; e gamma)&lt; 2.0x10^(-4),B(tau -&gt; e pi0) &lt; 1.4x10^(-4),B(tau -&gt; e eta) &lt; 2.4x10^(-4)

Zinātnisko darbu kopa

Medicīniskajā diagnostikā un terapijā luminiscences nanomateriāli ir daudzsološi marķieri optiskajā attēlveidošanā un luminiscējošā iezīmēšanā, kas ļauj izmantot jaunas neinvazīvas diagnostikas metodes un kompleksas vitālās funkcijas in vivo novērošanu. Šajā darbā ir izveidots aktuālo nanoizmēru luminiscences oksīdu (LaPO4:Ce,Tb; YVO4:Eu; Y3Al5O12:Ce, ZnWO4; NiWO4) sistemātiska spektroskopiska raksturojuma kopsavilkums ar mērķi, izmantojot ierosinājumu luminiscences spektroskopiju, izanalizēt elektronu relaksācijas. Eksperimenti ir veikti Eiropas sinhrotronu centros: DESY (Vācijā) un MAX IV (Zviedrijā). Pētījumi ir fokusēti uz enerģijas pārneses procesiem retzemju jonu optiskajās pārejās. Ar nanodaļiņu virsmu saistīti zudumi centri ir atbildīgi par būtiskām izmaiņām nanooksīdu luminiscences īpašībās. Atslēgvārdi: Sinhrotrona starojums; Nanofosfori; Luminiscences spektroskopijaLuminescent nanomaterials are promising tags for optical imaging and fluorescent labeling to allow for novel techniques of non-invasive diagnosis and in vivo observation of complex vital functions. The current work is a summary of a systematic spectroscopic characterization of actual nanosized oxides (LaPO4:Ce,Tb; YVO4:Eu; Y3Al5O12:Ce; ZnWO4; NiWO4) with an attempt to analyse the electronic relaxation performed by means of excitation luminescence spectroscopy. The experiments carried out at European synchrotron centres: DESY (Germany) and MAX IV Lab (Sweden). The study focused on the energy transfer processes from host lattice to impurity ions as well as within intrinsic centres leading to the optical transitions. Nanoparticle’s surface related loss centres are responsible for significant changes in luminescence properties of nano-oxides. Keywords: Synchrotron radiation; Nanophosphors; Luminescence spectroscop

Measurement of dijet photoproduction for events with a leading neutron at HERA

Differential cross sections for dijet photoproduction and this process in association with a leading neutron, e+ + p -> e+ + jet + jet + X (+ n), have been measured with the ZEUS detector at HERA using an integrated luminosity of 40 pb-1. The fraction of dijet events with a leading neutron was studied as a function of different jet and event variables. Single- and double-differential cross sections are presented as a function of the longitudinal fraction of the proton momentum carried by the leading neutron, xL, and of its transverse momentum squared, pT^2. The dijet data are compared to inclusive DIS and photoproduction results; they are all consistent with a simple pion-exchange model. The neutron yield as a function of xL was found to depend only on the fraction of the proton beam energy going into the forward region, independent of the hard process. No firm conclusion can be drawn on the presence of rescattering effects.Comment: 40 pages, 18 figure

Pion transition form factor in k_T factorization

It has been pointed out that the recent BaBar data on the pi gamma^* -> gamma transition form factor F_{pi gamma}(Q^2) at low (high) momentum transfer squared Q^2 indicate an asymptotic (flat) pion distribution amplitude. These seemingly contradictory observations can be reconciled in the k_T factorization theorem: the increase of the measured Q^2F_{pi gamma}(Q^2) for Q^2 > 10 GeV^2 is explained by convoluting a k_T dependent hard kernel with a flat pion distribution amplitude, k_T being a parton transverse momentum. The low Q^2 data are accommodated by including the resummation of alpha_s ln^2x, x being a parton momentum fraction, which provides a stronger suppression at the endpoints of x. The next-to-leading-order correction to the pion transition form factor is found to be less than 20% in the considered range of Q^2.Comment: 7 pages, 4 figures, REVTeX

Measurement of Longitudinal Spin Transfer to Lambda Hyperons in Deep-Inelastic Lepton Scattering

Spin transfer in deep-inelastic Lambda electroproduction has been studied with the HERMES detector using the 27.6 GeV polarized positron beam in the HERA storage ring. For an average fractional energy transfer = 0.45, the longitudinal spin transfer from the virtual photon to the Lambda has been extracted. The spin transfer along the Lambda momentum direction is found to be 0.11 +/- 0.17 (stat) +/- 0.03 (sys); similar values are found for other possible choices for the longitudinal spin direction of the Lambda. This result is the most precise value obtained to date from deep-inelastic scattering with charged lepton beams, and is sensitive to polarized up quark fragmentation to hyperon states. The experimental result is found to be in general agreement with various models of the Lambda spin content, and is consistent with the assumption of helicity conservation in the fragmentation process.Comment: 8 pages, 3 figures; new version has an expanded discussion and small format change