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     

On the photon production in nucleus-nucleus collisions at high energy

A modified Landau hydrodynamical model is applied to study hard thermal photon production in central heavy-ion collisions at LHC, RHIC and SPS energies. It is shown that the phase transition from quark-gluon plasma into hadrons in consequence of the thermodynamical expansion is close to the second order phase transition if a resonance production is taken into account. Hard direct photon emission is also investigated with consideration of nuclear shadowing effect on structure function of quarks and gluons. Also π^π photon background is investigated. It is demonstrated that at the LHC energy photon yield from the quark-gluon plasma in the photon transversal momentumk _⊥ range from 5 to 25 GeV/c exceeds both the background and the direct photon yield. This conclusion may be important for the quark-gluon plasma diagnostic aims. It is also shown that for the LHC energy the thermal photon yield in the present model essentially exceeds this yield obtained in the Bjorken scaling model.     

A new scaling law for analyzing power in hadron production by transversely polarized baryons

Experimental data on analyzing power for inclusive meson and baryon production in collisions of transversely polarized protons and antiprotons with protons and light nuclei have been analyzed. It is found that the existing data can be described by a simple function of collision energy (), transverse momentum () and a new scaling variable . At beam energies above 40 GeV and above 1.0 GeV/c the analyzing power is described by a function of and only () for both polarized proton fragmentation and central regions of proton-hadron collision. Comparison of data from Fermilab and new IHEP data measured using 40 GeV/c polarized proton beam was most decisive for the revelation of the above regularities. This new scaling law allows one to predict the analyzing powers for kinematic regions, not yet explored in experiments and constrains models of strong interactions. The new scaling law allows one also to use some reactions as polarimeters for experiments with a polarized beam. Received: 19 October 1999 / Revised version: 26 January 2000 / Published online: 6 March 2000     

Polarization measurements in high-energy deuteron photodisintegration

We present measurements of the recoil proton polarization for the d(gamma-->,p-->)n reaction at straight theta(c.m.) = 90 degrees for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization p(y) vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer Cx does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(gamma,p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.     

Influence of shear viscosity of quark-gluon plasma on elliptic flow in ultrarelativistic heavy-ion collisions

We investigate the influence of a temperature-dependent shear viscosity over entropy density ratio η/s on the transverse momentum spectra and elliptic flow of hadrons in ultrarelativistic heavy-ion collisions. We find that the elliptic flow in √S(NN)=200 GeV Au+Au collisions at RHIC is dominated by the viscosity in the hadronic phase and in the phase transition region, but largely insensitive to the viscosity of the quark-gluon plasma (QGP). At the highest LHC energy, the elliptic flow becomes sensitive to the QGP viscosity and insensitive to the hadronic viscosity.     

Measurement of dijet cross sections in photoproduction at HERA

Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises data with an integrated luminosity of 34.9 pb. Jets are selected using the inclusive algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction and photon longitudinal momentum fraction in the ranges and . The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range. Received: 4 January 2002 / Published online: 12 July 2002     

Measurements of transverse energy flow in deep-inelastic scattering at HERA

Measurements of transverse energy flow are presented for neutral current deep-inelastic scattering events produced in positron-proton collisions at HERA. The kinematic range covers squared momentum transfers from 3.2 to 2 200 GeV, the Bjorken scaling variable x from to 0.11 and the hadronic mass W from 66 to 233 GeV. The transverse energy flow is measured in the hadronic centre of mass frame and is studied as a function of , x, W and pseudorapidity. A comparison is made with QCD-based models. The behaviour of the mean transverse energy in the central pseudorapidity region and an interval corresponding to the photon fragmentation region are analysed as a function of and W. Received: 15 July 1999 / Published online: 27 January 2000     

Pion Interferometry of square root of (s(NN)) =130 GeV Au + Au collisions at RHIC

Two-pion correlation functions in Au+Au collisions at square root of [s(NN)] = 130 GeV have been measured by the STAR (solenoidal tracker at RHIC) detector. The source size extracted by fitting the correlations grows with event multiplicity and decreases with transverse momentum. Anomalously large sizes or emission durations, which have been suggested as signals of quark-gluon plasma formation and rehadronization, are not observed. The Hanbury Brown-Twiss parameters display a weak energy dependence over a broad range in square root of [s(NN)].     

Measurement of inclusive D^{\ast\pm} and associated dijet cross sections in photoproduction at HERA

Inclusive photoproduction of mesons has been measured for photon-proton centre-of-mass energies in the range GeV and photon virtuality 1 GeV. The data sample used corresponds to an integrated luminosity of 37 pb. Total and differential cross sections as functions of the transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant. Received: 9 July 1998 / Published online: 22 October 1998     

Energy dependence of transverse momentum invariant distribution of pions and neutral kaons in K−p interactions between 14.3 and 70 GeV/c

The energy dependence of the transverse momentum invariant distribution of pions and neutral kaons is studied in K^?p interactions between 14.3 and 70 GeV/c. The large P_T part of the distributions violates the Feynman scaling and, above P_T ? 1.5 GeV/c, appears to be reasonably described by hard scattering models. The variation of the average transverse momentum is also studied as a function of the c.m. reduced longitudinal momentum, and its behaviour is compared to the data obtained via the hadronic shower produced in lepton-hadron interactions.     

Elliptic flow of thermal photons in relativistic nuclear collisions

We predict the transverse momentum (p(T)) dependence of elliptic flow of thermal photons for Au + Au collisions at the BNL Relativistic Heavy Ion Collider. We model the system hydrodynamically, with a thermalized quark-gluon plasma at early times followed by hadronization and decoupling. Photons are emitted throughout the expansion history. Contrary to hadron elliptic flow, which increases monotonically with p(T), the elliptic flow nu2(p(T)) of thermal photons is predicted to first rise and then fall again. Photon elliptic flow at high p(T) reflects the quark momentum anisotropy at early times when it is small, while at low p(T) it mirrors the large pion momentum anisotropy during the late hadronic emission stage. An interesting structure is predicted at intermediate p(T) approximately 0.4 GeV/c, where photon elliptic flow reflects the momenta and the (compared to pions) reduced nu2 of heavy vector mesons in the late hadronic phase.     

Multiplicity distribution and spectra of negatively charged hadrons in Au+Au collisions at square root of (sNN) = 130 GeV

The minimum-bias multiplicity distribution and the transverse momentum and pseudorapidity distributions for central collisions have been measured for negative hadrons ( h(-)) in Au+Au interactions at square root of ([s(NN)]) = 130 GeV. The multiplicity density at midrapidity for the 5% most central interactions is dN(h(-))/d(eta)/(eta = 0) = 280+/-1(stat)+/-20(syst), an increase per participant of 38% relative to pp collisions at the same energy. The mean transverse momentum is 0.508+/-0.012 GeV/c and is larger than in central Pb+Pb collisions at lower energies. The scaling of the h(-) yield per participant is a strong function of p( perpendicular). The pseudorapidity distribution is almost constant within /eta/<1.     

Search for excited and exotic electrons in the egamma decay channel in pp collisions at sqrt[s] = 1.96 TeV

We present a search for excited and exotic electrons (e(*)) decaying to an electron and a photon, both with high transverse momentum. We use 202 pb(-1) of data collected in pp collisions at sqrt[s] = 1.96 TeV with the Collider Detector at Fermilab II detector. No signal above standard model expectation is seen for associated ee(*) production. We discuss the e(*) sensitivity in the parameter space of the excited electron mass M(e(*)) and the compositeness energy scale Lambda. In the contact interaction model, we exclude 132 GeV/c(2)相似文献   

Scaling properties of azimuthal anisotropy in Au+Au and Cu+Cu Collisions at sqrt[s NN]=200 GeV

Differential measurements of elliptic flow (v2) for Au+Au and Cu+Cu collisions at sqrt[sNN]=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v2 with eccentricity, system size, and transverse kinetic energy (KE T). For KE T identical with mT-m up to approximately 1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KE T mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v2 for both mesons and baryons over the full KE T range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KE T, rather than transverse momentum.     

Measurement of prompt photons with associated jets in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb^-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_γ ^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the k_T-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the k_T-factorisation calculations are in good agreement with the data.     

Single source emission of proton projectile fragments in nucleus-emulsion interactions

The projected angular distribution and transverse momentum distribution of proton projectile fragments produced in 3.7A GeV 16O,500A MeV 56Fe,and 1.7A GeV 84Kr induced by different kinds of emulsion target(H,CNO,and AgBr) interactions are investigated.It is found that the projected angular distribution and transverse momentum distribution can be well represented by a single Gaussian distribution.Comparison of transverse momentum distribution with the Maxwell-Boltzmann distribution reveals that proton projectile fragments are emitted from a single-temperature emission source.The temperature is different for different colliding systems,and linearly depends on the target size.     

Charged particle productions at 90° in the center of mass in very high energy proton-proton collisions

The transverse momentum distribution at 90° of pions, protons and antiprotons have been measured at the CERN intersecting storage rings for C.M. energies between 23.2 and 52.7 GeV. In this energy range, the pion and proton distributions are almost energy independent. The antiproton production rises by a factor of two between 23.2 and 52.7 GeV.     

Coherent photo- and lepto-production of vector mesons from deuterium

We discuss the coherent photo- and lepto-production of vector mesons from deuterium at intermediate (virtual) photon energies, 3 GeV ≲ ν ≲ 30 GeV. These processes provide several options to explore the space-time evolution of small size quark-gluon configurations. Furthermore, we study the dependence of the production cross section on the energy and momentum transfer t due to variations of the finite longitudinal interaction length. Kinematic regions are determined where the production cross section is most sensitive to the final state interaction of the initially produced hadronic wave packet. For unpolarized deuteron targets this double scattering contribution can be investigated mainly at large values of the momentum transfer t. For polarized targets kinematic windows sensitive to double scattering are available also at moderate t. We suggest several methods for an investigation of color coherence effects at intermediate energies.     

Conformal approach to fluctuations in hadroproduction

We study two-photon final-states atep-colliders. In particular, we investigate the process γ+p →γ+γ+X at HERA and LEP/LHC energies with the intention of determining its measurability at these accelerators. We find that the transverse momentum distribution will be measured top _T=25 GeV/c at HERA andp _T=50 GeV/c at LEP/LHC. We find that the cross section of this process is too small for it to be a sensitive, direct probe of the gluon content of the proton and the photon. However, we find that it will supplement deep inelastic structure function investigations of the quark distributions within the proton and photon. Further, we predict that at very low transverse momenta, this process will allow the measurement of the box diagramg+g→γ+γ.