Observation of scaling violations in scaled momentum distributions at HERA

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of x and Q"2 using the ZEUS detector. The evolution of the scaled momentum, x_p, with Q"2, in the range 10 to 1280 GeV"2, has been investigated in the current fragmentation region of the Breit frame. The results show clear evidence, in a single experiment, for scaling violations in scaled momenta as a function of Q"2. (orig.)Available from TIB Hannover: RA 2999(97-183) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of open beauty production in photoproduction at HERA

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process e"+p #-># e"+ + dijet + e"- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb"-"1. Events with photon-proton centre-of-mass energies, W_#gamma#_p, between 134 and 269 GeV and a photon virtuality, Q"2, less than 1 GeV"2 were selected requiring at least two jets of transverse energy E_T"j"e"t"1"("2") > 7(6) GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement: The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, p_T"r"e"l. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum p_T"b > p_T"m"i"n = 5 GeV and pseudorapidity vertical stroke #eta#"b vertical stroke < 2. The extrapolated cross section is 1.6 #+-# 0.4 (stat.)_-_0_._5"+"0"."3(syst.)_-_0_._4"+"0"."2(ext.) nb. The result is compared to a perturbative QCD calculation performed to next-to-leading order. (orig.)Available from TIB Hannover: RA 2999(00-166) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of the proton structure function F_2 and #sigma#"#gamma#""*""p_t_o_t at low Q"2 and very low x at HERA

A small electromagnetic sampling calorimeter, installed in the ZEUS experiment in 1995, significantly enhanced the acceptance for very low x and low Q"2 inelastic neutral current scattering, e"+p #-># e"+X,at HERA. A measurement of the proton structure function F_2 and the total virtual photon-proton (#gamma#"*p) cross-section is presented for 0.11#<=#Q"2#<=#0.65 GeV"2 and 2 x 10"-"6#<=#x#<=#6 x 10"-"5, corresponding to a range in the #gamma#"*p c.m. energy of 100#<=#W#<=#230 GeV. Comparisons with various models are also presented. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of the diffractive cross section in deep inelastic scattering using ZEUS 1994 data

The DIS diffractive cross section, d#sigma#_#gamma#_"*_p_#->#_X_N"d"i"f"f/dM_X, has been measured in the mass range M_X<15 GeV for #gamma#"*p c.m. energies 60 < W < 200 GeV and photon virtualities Q"2=7 to 140 GeV"2. For fixed Q"2 and M_X, the diffractive cross section rises rapidly with W, d#sigma#_#gamma#_"*_p_#->#_X_N"d"i"f"f(M_X,W,Q"2)/dM_X #propor to# W"a""d""i""f""f with a"d"i"f"f=0.507#+-#0.034(stat)_-_0_._0_4_6"+"0"."1"5"5(syst) corresponding to a t-averaged pomeron trajectory of /score#alpha#_P=1.127 #+-#0.009(stat)_-_0_._0_1_2"+"0"."0"3"9(syst) which is larger than anti #alpha#_P observed in hadron-hadron scattering. The W dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function F_2"D"("3") factorizes according to x_PF_2"D"("3")(x_P,#beta#,Q"2)=(x_0/x_P)"nF_2"D"("2")(#beta#,Q"2). They are also consistent with QCD based models which incorporate factorization breaking. The rise of x_PF_2"D"("3") with decreasing x_P and the weak dependence of F_2"D"("2") on Q"2 suggest a substantial contribution from partonic interactions. (orig.)Available from TIB Hannover: RA 2999(98-084) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_#gamma#"O"B"S) and pomeron (#beta#"O"B"S) momentum participating in the production of the dijet system. The observed x_#gamma#"O"B"S dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d#sigma#/d#beta#"O"B"S increases as #beta#"O"B"S increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron. (orig.)Available from TIB Hannover: RA 2999(98-045) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Charged particles and neutral kaons in photoproduced jets at HERA

Charged particles (h"#+-#) and K"0 mesons have been studied in photoproduced events containing at least one jet of E_T > 8 GeV in a pseudorapidity interval (-0.5, 0.5) in the ZEUS laboratory frame. Distributions are presented in terms of transverse momentum, pseudorapidity and distance of the particle from the axis of a jet. The properties of h"#+-# within the jet are described well using the standard settings of PYTHIA, but the use of the multiparton interaction option improves the description outside the jets. A reasonable overall description of the K"0 behaviour is possible with PYTHIA using a reduced value of the strangeness suppression parameter. The numbers of h"#+-# and K"0 within a jet as defined above are measured to be 3.25#+-#0.02#+-#0.28 and 0.431#+-#0.013#+-#0.088 respectively. Fragmentation functions are presented for h"#+-# and K"0 in photoproduced jets; agreement is found with calculations of Binnewies et al. and, at higher momenta, with pp scattering and with standard PYTHIA. Fragmentation functions in direct photoproduced events are extracted, and at higher momenta give good agreement with data from related processes in e"+e"- annihilation and deep inelastic ep scattering. (orig.)Available from TIB Hannover: RA 2999(97-229) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

High-E_T inclusive jet cross sections in photoproduction at HERA

Inclusive jet differential cross sections for the reaction e"+p#->#e"++jet+X with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134<W<277 GeV and jet pseudorapidity in the range -1<#eta#"j"e"t<2 in the laboratory frame. The results are presented for three cone radii in the #eta#-#phi# plane, R=1.0, 0.7 and 0.5. Measurements of d#sigma#/d#eta#"j"e"t above various jet-transverse-energy thresholds up to 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with R=1.0 differences between data and NLO calculations are seen at high #eta#"j"e"t and low E_T"j"e"t. The measured cross sections for jets defined with R=0.7 are well described by the calculations in the entire measured range of #eta#"j"e"t and E_T"j"e"t. The inclusive jet cross section for E_T"j"e"t>21 GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations. (orig.)Available from TIB Hannover: RA 2999(98-018) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of exclusive #omega# electroproduction at HERA

The exclusive electroproduction of #omega# mesons, ep#-># e#omega#p, has been studied in the kinematic range 3<Q"2<20 GeV"2, 40<W<120 GeV and vertical stroke t vertical stroke <0.6 GeV"2 with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb"-"1. The #omega# mesons were identified via the decay #omega# #-># #pi#"-#pi#"-#pi#"0. The exclusive cross section in the above kinematic region is #sigma#_e_p_#->#_e_#omega#_p=0.108 #+-#0.014(stat.)#+-#0.026(syst.) nb. The reaction ep#->#e#phi#p, #phi##->##pi#"+#pi#"-#pi#"0, has also been measured. The cross sections, as well as the ratios #sigma#_#gamma#_p_#->#_#omega#_p/#sigma#_#gamma#_p_#->#_#rho#_"0_p and #sigma#_#gamma#_p_#->#_#omega#_p/#sigma#_#gamma#_p_#->#_#phi#_p, are presented as a function of W and Q"2. Thus, for the first time, the properties of #omega# electroproduction can be compared to those of #rho#"0, #phi# and J/#psi# electroproduction at high W. (orig.)Available from TIB Hannover: RA 2999(00-084) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of t distribution in diffractive photoproduction at HERA

Photon diffractive dissociation, #gamma#p#->#Xp, has been studied at HERA with the ZEUS detector using ep interactions where the virtuality Q"2 of the exchanged photon is smaller than 0.02 GeV"2. The squared four-momentum t exchanged at the proton vertex was determined in the range 0.073 < vertical stroke t vertical stroke < 0.40 GeV"2 by measuring the scattered proton in the ZEUS leading proton spectrometer. In the photon-proton centre-of-mass energy interval 176<W<225 GeV and for masses of the dissociated photon system 4<M_X<32 GeV, the t distribution has an exponential shape, dN/d vertical stroke t vertical stroke #propor to#exp(-b vertical stroke t vertical stroke), with a slope parameter b=6.8#+-#0.9(stat.)_-_1_._1"+"1"."2(syst.) GeV"-"2. (orig.)Available from TIB Hannover: RA 2999(97-238) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of inelastic J/#psi# photoproduction at HERA

We present a measurement of the inelastic, non diffractive J/#psi# photoproduction cross section in the reaction e"+p#->#e"+J/#psi#X with the ZEUS detector at HERA. The J/#psi# was identified using both the #mu#"+#mu#"- and e"+e"- decay channels and events were selected within the range 0.4<z<0.9 (0.5<z<0.9) for the muon (electron) decay mode, where z is the fraction of the photon energy carried by the J/#psi# in the proton rest frame. The cross section, the p"2_T and the z distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range 50<W<180 GeV; p"2_T is the square of the J/#psi# transverse momentum with respect to the incoming proton beam direction. In the kinematic range 0.4<z<0.9 and p"2_T>1GeV"2, NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/#psi# hadroproduction, are not consistent with the data. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman