A measurement of the proton structure function F_2(x,Q^2) is presented in the kinematic range 0.045 GeV^2 < Q^2 < 0.65 GeV^2 and 6*10^{-7} < x < 1*10^{-3}. The results were obtained using a data sample corresponding to an integrated luminosity of 3.9pb^-1 in e^+p reactions recorded with the ZEUS detector at HERA. Information from a silicon-strip tracking detector, installed in front of the small electromagnetic calorimeter used to measure the energy of the final-state positron at small scattering angles, together with an enhanced simulation of the hadronic final state, has permitted the extension of the kinematic range beyond that of previous measurements. The uncertainties in F_2 are typically less than 4%. At the low Q^2 values of the present measurement, the rise of F_2 at low x is slower than observed in HERA data at higher Q^2 and can be described by Regge theory with a constant logarithmic slope. The dependence of F_2 on Q^2 is stronger than at higher Q^2 values, approaching, at the lowest Q^2 values of this measurement, a region where F_2 becomes nearly proportional to Q^2.
Measured values of F2 at Q**2 = 0.045 GeV**2 as a function of X.
Measured values of F2 at Q**2 = 0.065 GeV**2 as a function of X.
Measured values of F2 at Q**2 = 0.085 GeV**2 as a function of X.
The cross section for $\phi$ meson photoproduction on the proton has been measured for the first time up to a four-momentum transfer -t = 4 GeV^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low four-momentum transfer, the differential cross section is well described by Pomeron exchange. At large four-momentum transfer, above -t = 1.8 GeV^2, the data support a model where the Pomeron is resolved into its simplest component, two gluons, which may couple to any quark in the proton and in the $\phi$.
The differential PHI photoproduction cross section. The errors shown are the quadratic sum of the statistics and the systematic uncertainties which include 3 PCT for normalization, 5 PCT for acceptance and 5-15 PCT for background subtraction.
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 |t|<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 (ep -> e omega p) cross section in the above kinematic region is 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 cross-section ratios omega/rho and omega/phi, 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.
The cross sections for OMEGA and PHI electroproduction.
The corresponding photoproduction cross sections of OMEGA and PHI mesons. The RHO0 data is taken from a previous ZEUS publication (EPJ C6,603).
Ratio of the photoproduction cross sections.
We present the first measurement of pseudorapidity densities of primary charged particles near mid-rapidity in Au+Au collisions at $\sqrt{s} =$ 56 and 130 AGeV. For the most central collisions, we find the charged particle pseudorapidity density to be $dN/d\eta |_{|\eta|<1} = 408 \pm 12 {(stat)} \pm 30 {(syst)}$ at 56 AGeV and $555 \pm 12 {(stat)} \pm 35 {(syst)}$ at 130 AGeV, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than 40% at the higher energy.
No description provided.
Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e^+ + p --> e^+ + n + jet + jet + X_r have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb^{-1}. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies E^{jet}_T > 6 GeV, neutron energy E_n > 400 GeV, and neutron production angle theta_n < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model.
The differential dijet cross section as a function of ET for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ET for the neutron-tagged data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ETARAP for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeterenergy scale.
We present a measurement of the polarization observed for bottomonium states produced in p-Cu collisions at sqrt(s)=38.8 GeV. The angular distribution of the decay dimuons of the Upsilon(1S) state show no polarization at small xF and pT but significant positive transverse production polarization for either pT > 1.8 GeV/c or for xF > 0.35. The Upsilon(2S+3S) unresolved states show a large transverse production polarization at all values of xF and pT measured. These observations are compared with an NRQCD calculation that predicts a transverse polarization in bottomonium production arising from quark-antiquark fusion and gluon-gluon fusion diagrams.
The observed polarization in the Drell-Yan sideband region as a function of PT. There is an additional systematic uncertainty of 0.02 in the polarization measurements.
The observed polarization in the Drell-Yan sideband region as a function of XL. There is an additional systematic uncertainty of 0.02 in the polarization measurements.
The observed polarization in the UPSILON(1S) region as a function of PT. There is an additional systematic error of 0.06 in the polarization measurements.
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 ${\rm pb^{-1}}$. 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 ${\rm GeV^2}$ were selected requiring at least two jets of transverse energy $E_T^{\rm jet1(2)} >7(6)$ 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, $p_T^{\rm rel}$. 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 $p_T^{b} > p_T^{\rm min} =$ 5 GeV and pseudorapidity $|\eta^{b}| <$ 2. The extrapolated cross section is $1.6 \pm 0.4 (stat.)^{+0.3}_{-0.5} (syst.) ^{+0.2}_{-0.4} (ext.) {nb}$. The result is compared to a perturbative QCD calculation performed to next-to-leading order.
The differential distribution of PT(C=REL) for heavy quark decays. The second DSYS error is due to the energy scale uncertainty.
The differential distribution of X(C=GAMMA,OBS), the fraction of the photons momentum contributing to the production of the two highest transverse energy jets. The second DSYS error is due to the energy scale uncertainty.
Cross section for beauty production with a prompt electron in the restricted kinetic region.
We present results for the charged-particle multiplicity distribution at mid-rapidity in Au - Au collisions at sqrt(s_NN)=130 GeV measured with the PHENIX detector at RHIC. For the 5% most central collisions we find $dN_{ch}/d\eta_{|\eta=0} = 622 \pm 1 (stat) \pm 41 (syst)$. The results, analyzed as a function of centrality, show a steady rise of the particle density per participating nucleon with centrality.
130 GeV is sqrt(S) per nucleon-nucleon collision. N(C=N_NUCLEONS) and N(C=N_COLLISONS) are the number of participating nucleons and binary collisions. The statistical errors are negligible and only systematic errors are quoted. COL(NAME=CENTRALITY) is centrality.
We studied the exclusive reaction e p --> e' p' phi using the phi --> K^+ K^- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CLAS detector at Jefferson Lab. Our experiment covers the range in Q^2 from 0.7 to 2.2 GeV^2, and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of phi production on the proton. Our measurement shows the expected decrease of the t-slope with the vector meson formation time c Delta tau below 2 fm. At = 0.6 fm, we measure b_phi = 2.27 +- 0.42 GeV^-2. The cross section dependence on W as W^{0.2+-0.1} at Q^2 = 1.3 GeV^2 was determined by comparison with phi production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction.
Slope of the DSIG/DT distribution in different Q**2 regions.
Cross section as a function of Q**2 and W.
The differential cross section for exclusive PHI electroproduction off the photon, (TP=T-TMIN).
Dijet production has been studied in neutral current deep inelastic e+p scattering for 470 < Q**2 < 20000 GeV**2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb**{-1}. Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q**2 allows both a precise determination of alpha_s(M_Z) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of alpha_s(M_Z), as determined from the QCD fit, is alpha_s(M_Z) = 0.1166 +- 0.0019 (stat.) {+ 0.0024}_{-0.0033} (exp.)} {+ 0.0057}_{- 0.0044} (th.).
The differential dijet cross section dsig/dZP1.
The differential dijet cross section dsig/dlog10(x).
The differential dijet cross section dsig/dlog10(xi).
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