Dijet photoproduction at HERA and the structure of the photon

The dijet cross section in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb. The events were required to have a virtuality of the incoming photon, , of less than 1 GeV and a photon-proton centre-of-mass energy in the range GeV. Each event contains at least two jets satisfying transverse-energy requirements of GeV and GeV and pseudorapidity requirements of \mbox{}. The measurements are compared to next-to-leading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested. Received: 21 December 2001 / Published online: 5 April 2002     

Longitudinal double-spin asymmetry and cross section for inclusive jet production in polarized proton collisions at square root of s = 200 GeV

We report a measurement of the longitudinal double-spin asymmetry A(LL) and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at square root of s = 200 GeV. The cross section data cover transverse momenta 5 < pT < 50 GeV/c and agree with next-to-leading order perturbative QCD evaluations. The A(LL) data cover 5 < pT < 17 GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.     

NLO electroweak corrections to Higgs boson production at hadron colliders

Results for the complete NLO electroweak corrections to Standard Model Higgs production via gluon fusion are included in the total cross section for hadronic collisions. Artificially large threshold effects are avoided working in the complex-mass scheme. The numerical impact at LHC (Tevatron) energies is explored for Higgs mass values up to 500 GeV (200 GeV). Assuming a complete factorization of the electroweak corrections, one finds a +5% shift with respect to the NNLO QCD cross section for a Higgs mass of 120 GeV both at the LHC and the Tevatron. Adopting two different factorization schemes for the electroweak effects, an estimate of the corresponding total theoretical uncertainty is computed.     

Measurement and QCD analysis of jet cross sections in deep-inelastic positron-proton collisions at \sqrt{s} of 300 GeV

Jet production is studied in the Breit frame in deep-inelastic positron-proton scattering over a large range of four-momentum transfers and transverse jet energies GeV. The analysis is based on data corresponding to an integrated luminosity of taken in the years 1995–1997 with the H1 detector at HERA at a center-of-mass energy GeV. Dijet and inclusive jet cross sections are measured multi-differentially using and angular ordered jet algorithms. The results are compared to the predictions of perturbative QCD calculations in next-to-leading order in the strong coupling constant . QCD fits are performed in which and the gluon density in the proton are determined separately. The gluon density is found to be in good agreement with results obtained in other analyses using data from different processes. The strong coupling constant is determined to be . In addition an analysis of the data in which both and the gluon density are determined simultaneously is presented.     

Measurement of neutral and charged current cross sections in electron-proton collisions at high Q^2

The inclusive single and double differential cross sections for neutral and charged current processes are measured with the H1 detector at HERA, in the range of four-momentum transfer squared between 150 and 30 000 GeV, and Bjorken x between 0.002 and 0.65. The data were taken in 1998 and 1999 with a centre-of-mass energy of 320 GeV and correspond to an integrated luminosity of 16.4 pb. The data are compared with recent measurements of the inclusive neutral and charged current cross sections. For clear evidence is observed for an asymmetry between and neutral current scattering and the generalised structure function is extracted for the first time at HERA. A fit to the charged current data is used to extract a value for the W boson propagator mass. The data are found to be in good agreement with Standard Model predictions. Received: 21 December 2000 / Published online: 15 March 2001     

Double helicity asymmetry in inclusive midrapidity pi0 production for polarized p+p collisions at square root s = 200 GeV

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 0.27. The measurements are the first in 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 midrapidity, where next-to-leading-order perturbative quantum chromodynamic (NLO pQCD) calculations well describe the unpolarized cross section. The observed asymmetry is small and is compared to a NLO pQCD calculation with a range of polarized gluon distributions.     

Particle multiplicity of unbiased gluon jets from \mathrme^+\mathrme^- three-jet events

The charged particle multiplicities of two- and three-jet events from the reaction ee Z are measured for Z decays to light quark (uds) flavors. Using recent theoretical expressions to account for biases from event selection, results corresponding to unbiased gluon jets are extracted over a range of jet energies from about 11 to 30 GeV. We find consistency between these results and direct measurements of unbiased gluon jet multiplicity from and Z decays. The unbiased gluon jet data including the direct measurements are compared to corresponding results for quark jets. We perform fits based on analytic expressions for particle multiplicity in jets to determine the ratio of multiplicities between gluon and quark jets as a function of energy. We also determine the ratio of slopes, , and of curvatures, , where y specifies the energy scale. At 30 GeV, we find and , where the uncertainties are the statistical and systematic terms added in quadrature. These results are in general agreement with theoretical predictions. In addition, we use the measurements of the energy dependence of and to determine an effective value of the ratio of QCD color factors, . Our result, (total), is consistent with the QCD value of 2.25. Received: 25 October 2001 / Revised version: 20 January 2002 / Published online: 5 April 2002     

HERA分解光子过程中的双喷注产生

在次领头级(NLO)近似下计算了HERA分解光子过程中的双喷注截面.结果表明:在分解光子过程中,NLO修正量(当不变质量大于20GeV时)约为LO的0.5到1.这一结果可解释ZEUS的数据分析结果:“LOQCD理论值比实验值低1.5到2个因子.”当双喷注不变质量M_JJ<30GeV=,快度y_JJ<—1时,光子的胶子分布决定了双喷注截面.利用这一特点可测量光子的胶子分布.当双喷注不变质量较大时(例如:M_JJ>30GeV),-1JJ<0时,光子的夸克分布决定了双喷注截面,利用这一特点可测量光子的夸克分布.     

Search for the Higgs boson in the H→WW→lνjj decay channel in pp collisions at √s=7 TeV with the ATLAS detector

A search for a Higgs boson has been performed in the H→WW→?νjj channel in 1.04 fb(-1) of pp collision data at √s=7 TeV recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 240 GeV相似文献   

Inclusive photoproduction of neutral pions in the photon hemisphere at HERA

The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below GeV. The measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of in the centre-of-mass frame. The transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon. Received: 14 June 2000 / Published online: 27 November 2000     

Transverse momentum and total cross section of e(+)e(-) pairs in the Z-boson region from p&pmacr; collisions at sqrt

The transverse momentum and total cross section of e(+)e(-) pairs in the Z-boson region of 66相似文献   

Photoproduction of Vector Meson ø off Deuteron in QCD Inspired Model

Based on the quark-gluon contents of nucleon and strongly believing that the force mediators, Pomeron and its counterpart in the conventional approach of Regge theory, for high energy diffractive process would be the tensor glueball and Odderonrespectively, we discuss photo-production of vector meson ø off the deuteron at energy less than 3 GeV in the QCD inspired model in which the quark gluon degrees of freedom and glueball, Odderon exchange are taken into account. A calculation isperformed for γ+D → ø+D, and the theoretical predictions of the differential cross section dσ ^γD/dt, are presented and compared with available experimental data. Our QCD inspired model reproduces data quite well in the whole range of the experimental measurements up to |t|≌ 0.4~GeV. Our results can be used toextract γn →øn data, which cannot be measured in experiment.     

A measurement of the photonuclear interactions of 180 GeV muons in iron

The energy spectrum and the cross section of photonuclear interactions of 180 GeV muons in iron were measured at the CERN SPS using prototype modules of the ATLAS hadron calorimeter. The differential cross section for a muon fractional energy loss was measured in the range 0.1&lt;v&lt;1. The integrated cross section is cm²g^-1 in agreement with the theoretical prediction of cm²g^-1. The best adjustment of the data to the theory is achieved for the value of of the photon-nucleon cross section for photons with energies in the range from 18 to 180 GeV. Received: 22 January 2003 / Published online: 5 May 2003     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

Diffractive jet production in deep-inelastic e^+p collisions at HERA

A measurement is presented of dijet and 3-jet cross sections in low- diffractive deep-inelastic scattering interactions of the type , where the system X is separated by a large rapidity gap from a low-mass baryonic system Y. Data taken with the H1 detector at HERA, corresponding to an integrated luminosity of 18.0 pb, are used to measure hadron level single and double differential cross sections for and . The energy flow not attributed to jets is also investigated. The measurements are consistent with a factorising diffractive exchange with trajectory intercept close to 1.2 and tightly constrain the dominating diffractive gluon distribution. Viewed in terms of the diffractive scattering of partonic fluctuations of the photon, the data require the dominance of over states. Soft colour neutralisation models in their present form cannot simultaneously reproduce the shapes and the normalisations of the differential cross sections. Models based on 2-gluon exchange are able to reproduce the shapes of the cross sections at low values. Received: 21 December 2000 / Published online: 6 April 2001     

Resolved Photoproduction of J/ψ at HERA

The J/ψ photoproduction via resolved photon process at HERA is studied. It turns out that the J/ψ production is dominated by the leading order production process in low pt range (p_t < 4 GeV), while in the high pt range (p_t > 4 GeV), it is dominated by the fragmentation processes. The difference of J/ψ cross section caused by using GRV and DO photon structure functions is large, especially in the large negative rapidity range. Hence it is possible to use the J/ψ photoproduction process to probe the parton distribution of photon and proton, as well as to test the gluon and heavy quark fragmentation of J/ψ from the perturbative QCD.     

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 with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb. Events with photon-proton centre-of-mass energies, , between 134 and 269 GeV and a photon virtuality, , less than 1 GeV were selected requiring at least two jets of transverse energy 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, . 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 5 GeV and pseudorapidity 2. The extrapolated cross section is . The result is compared to a perturbative QCD calculation performed to next-to-leading order. Received: 22 November 2000 / Published online: 5 February 2001     

Elastic electroproduction of \rho mesons at HERA

The elastic electroproduction of mesons is studied at HERA with the H1 detector for a photon virtuality in the range and for a hadronic centre of mass energy in the range GeV. The shape of the () mass distribution in the resonance region is measured as a function of . The full set of spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on , W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large , the cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron–hadron cross sections. Received: 22 February 1999 / Published online: 25 February 2000     

Elastic and proton-dissociative \rho^0 photoproduction at HERA

Elastic and proton-dissociative photoproduction (, , respectively, with ) has been studied in interactions at HERA for photon-proton centre-of-mass energies in the range GeV and for GeV, where is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range . For the elastic process, the invariant mass spectrum has been investigated as a function of . As in fixed target experiments, the resonance shape is asymmetric; this asymmetry decreases with increasing , as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant production. The cross section has been studied as a function of ; a fit to the resonant part with the form gives . The resonant part of the cross section is b at GeV. The dependence of the cross section can be described by a function of the type with (stat.) (syst.) GeV and (stat.) (syst.) GeV. The dependence has also been studied as a function of and a value of the slope of the pomeron trajectory GeV has been deduced. The spin density matrix elements , and have been measured and found to be consistent with expectations based on -channel helicity conservation. For proton-dissociative photoproduction in the mass range, the distributions of the two-pion invariant mass, and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The distribution has been fitted to an exponential function with a slope parameter GeV. The ratio of the elastic to proton-dissociative photoproduction cross section is . Received: 3 December 1997 / Online publication: February 26, 1998