Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic ep scattering regimes have been measured with the ZEUS detector at HERA using an integrated luminosity of 82.2 pb-1. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies Et(jet) > 17 GeV. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of Et(jet), jet pseudorapidity, the jet-jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of alphas(Mz) of alphas(Mz) = 0.1176 +-0.0009(stat.) -0.0026 +0.0009 (exp.) -0.0072 +0.0091 (th.) was extracted from the measurements of jet shapes in deep inelastic scattering.
Measured mean integrated jet shape corrected to the hadron level in photoproduction with ET(C=JET) > 17 GeV.
Measured mean integrated jet shape corrected to the hadron level in photoproduction with ET(C=JET) > 17 GeV.
Measured mean integrated jet shape corrected to the hadron level in photoproduction with -1 < ETARAP(C=JET) < 2.5.
The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a muon has been measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA using integrated luminosity of 72 pb^-1. The total visible cross section is sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The next-to-leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions: however at low values of Q^2, Bjorken x, and muon transverse momentum, and high values of jet transverse energy and muon pseudorapidity, the prediction is about two standard deviations below the data.
Total visible cross section in the specified kinematic region.
Differential cross section w.r.t. Q**2.
Differential cross section w.r.t. log10(x).
We present a measurement of the double longitudinal spin asymmetry in inclusive pi^0 production in polarized proton-proton collisions at sqrt(s)=200 GeV. The data were taken at the Relativistic Heavy Ion Collider with average beam polarizations of 26%. The measurements are the first of a program to study the longitudinal spin structure of the proton, using strongly interacting probes, at collider energies. The asymmetry is presented for transverse momenta 1-5 GeV/c at mid-rapidity, where next-to-leading order perturbative quantum chromodynamic (NLO pQCD) calculations describe the unpolarized cross section well. The observed asymmetry is small and is compared with a NLO pQCD calculation with a range of polarized gluon distributions.
Double spin asymmetry for the raw signal ($\pi^0 + BG$), for the background ($BG$) and for $\pi^0$ background corrected; single spin asymmetry for $\pi^0$ background corrected; for the four $p_T$ bins.
Single spin raw asymmetry normalized by the beam polarization, $\epsilon$/$P$, as a function of azimuthal angle $\phi$, for forward neutron production.
The measured double spin asymmetry $A^{\pi^0}_{LL}$ versus mean $p_T$ of $\pi^0$s in each bin. Not included in the figure/table: the correlated for all points scale systematic uncertainty of 65% (scales values and stat. uncertainties of points by the same factor).
The exclusive electroproduction of J/psi mesons, ep->epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q^2<0.8 GeV^2 and 2<Q^2<100 GeV^2 using integrated luminosities of 69 pb^-1 and 83 pb^-1, respectively.The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex |t|<1.The cross sections and decay angular distributions are presented as functions of Q^2, W and t. The effective parameters of the Pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma_L/sigma_T, grows with Q^2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.
Cross sections for exclusive J/PSI production as a function of W in the Q**2 region 0.15 to 0.18 GeV**2.
Cross sections for exclusive J/PSI production as a function of W in the Q**2 region 2 to 5 GeV**2.
Cross sections for exclusive J/PSI production as a function of W in the Q**2 region 5 to 10 GeV**2.
The dependence of dijet production on the virtuality of the exchanged photon, Q^2, has been studied by measuring dijet cross sections in the range 0 < Q^2 < 2000 GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb^-1. Dijet cross sections were measured for jets with transverse energy E_T^jet > 7.5 and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range -3 < eta^jet <0. The variable xg^obs, a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The Q^2 dependence of the ratio of low- to high-xg^obs events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-xg^obs contribution relative to that at high xg^obs. Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing Q^2, provide a qualitative description of the data.
The measured dijet DSIG/DQ**2 distributions.
Measured dijet cross sections as a function of Q**2 in two regions of X(C=GAMMA).
The measured dijet DSIG/DQ**2/DET distributions. Here ET(P=4) is the transverse energy of the highest transverse energy jet.
The photoproduction of $D^{*\pm} (2010)$ mesons associated with a leading neutron has been observed with the ZEUS detector in $ep$ collisions at HERA using an integrated luminosity of 80 pb$^{-1}$. The neutron carries a large fraction, {$x_L>0.2$}, of the incoming proton beam energy and is detected at very small production angles, {$\theta_n<0.8$ mrad}, an indication of peripheral scattering. The $D^*$ meson is centrally produced with pseudorapidity {$|\eta|<1.5$}, and has a transverse momentum {$p_{\it T} > 1.9$ GeV}, which is large compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to inclusive $D^*$ production is $8.85\pm 0.93({\rm stat.})^{+0.48}_{-0.61}({\rm syst.})\%$ in the photon-proton center-of-mass energy range {$130 <W<280$ GeV}. The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state.
Integrated cross section. The first DSYS error includes the uncertainty in the luminosity and the second DSYS error is due to the knowledge of the branching ratios.
No description provided.
No description provided.
Bose-Einstein correlations of identically charged pion pairs were measured by the PHENIX experiment at mid-rapidity in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. The Bertsch-Pratt radius parameters were determined as a function of the transverse momentum of the pair and as a function of the centrality of the collision. Using the \it{full} Coulomb correction, the ratio $R_{\rm out}/R_{\rm side}$ is smaller than unity for $<k_{\rm T}>$ from 0.25 to 1.2 GeV/c and for all measured centralities. However, using recently developed partial Coulomb correction methods, we find that $R_{\rm out}/R_{\rm side}$ is 0.8-1.1 for the measured $<k_{\rm T}>$ range, and approximately constant at unity with the number of participants.
Panel (a) and (b) show one-dimensional correlation functions for $\pi^+\pi^+$ and $\pi^-\pi^-$. The bottom figures show the three-dimensional correlation function for $\pi^-\pi^-$ with the full Coulomb (open circle) and without Coulomb (filled triangle) corrections for 0.2 < $k_T$ < 2.0 GeV/$c$ for 0-30% centrality. The projection of the 3-D correlation functions are averaged over the lowest 40 MeV in the orthogonal directions. The error bars are statistical only. The lines overlaid on the open circles (filled triangles) correspond to fits to Eq. 1 (Eq. 2) over the entire distribution. Panel (c) shows the one-dimensional correlation function of unlike-signed pions for 0.2 < $k_T$ < 2.0 GeV/$c$. The two overlaid histograms show calculations for the full (dashed) and the 50% partial (solid) Coulomb corrections. $<k_T>$ ~ 0.45 ($\pm$0.17) GeV/$c$ and $<N_{part}>$ ~ 281 ($\pm$4).
The $k_T$ dependence of the Bertsch-Pratt radius parameters and $\lambda$ for charged pions for 0-30% centrality. Filled triangles show the results from fits to a core-halo structure by Eq. 2, with statistical error bars and systematic error bands. Open circles and squares show the results from the full (Eq. 1) and 50% partial (Eq. 3) Coulomb corrections with statistical error bars, respectively. Results at 130 GeV by PHENIX are given by filled circles.
The $k_T$ dependence of the Bertsch-Pratt radius parameters and $\lambda$ for charged pions for 0-30% centrality. Filled triangles show the results from fits to a core-halo structure by Eq. 2, with statistical error bars and systematic error bands. Open circles and squares show the results from the full (Eq. 1) and 50% partial (Eq. 3) Coulomb corrections with statistical error bars, respectively. Results at 130 GeV by PHENIX are given by filled circles.
The photoproduction of beauty quarks in events with two jets and a muon has been measured with the ZEUS detector at HERA using an integrated luminosity of 110 pb$^{- 1}$. The fraction of jets containing b quarks was extracted from the transverse momentum distribution of the muon relative to the closest jet. Differential cross sections for beauty production as a function of the transverse momentum and pseudorapidity of the muon, of the associated jet and of $x_{\gamma}^{jets}$, the fraction of the photon's momentum participating in the hard process, are compared with MC models and QCD predictions made at next-to-leading order. The latter give a good description of the data.
No description provided.
No description provided.
No description provided.
Cross sections for e^+p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of sqrt{s}=318 GeV with the ZEUS detector at HERA using an integrated luminosity of 63.2 pb^-1. The double-differential cross section, d^2sigma/dxdQ^2, is presented for 200 GeV^2 < Q^2 < 30000 GeV^2 and for 0.005 < x < 0.65. The single-differential cross-sections dsigma/dQ^2, dsigma/dx and dsigma/dy are presented for Q^2 > 200 GeV^2. The effect of Z-boson exchange is seen in dsigma/dx measured for Q^2 > 10000 GeV^2. The data presented here were combined with ZEUS e^+p neutral current data taken at sqrt{s}=300 GeV and the structure function F_2^{em} was extracted. All results agree well with the predictions of the Standard Model.
The single differential DSIG/DQ**2 cross section corrected to the electroweak Born level. See next table for a breakdown of the systematic errors.
Systematic errors with bin to bin correlations for the cross section DSIG/DQ**2.
Single differential cross section DSIG/DX for a Q**2 cut of 200 GeV**2 corrected to the electroweak Born level.
We show that Σ+ hyperons produced by 800 GeV/c protons on targets of Be and Cu have significant polarizations (15–20%). These polarizations persist at values of pt≈2 GeV/c and a wide range of xF. The polarizations from the Cu target are consistently less than from Be. The average ratio of the Σ+ polarization from Cu to that from Be is 0.68±0.08.
Measured values of the SIGMA+ polarization as functions of PT and XL for proton CU collisions.
Measured values of the SIGMA+ polarization as functions of PT and XL for proton BE collisions.
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