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     

Charged particle cross sections in photoproduction and extraction of the gluon density in the photon

Photoproduction data collected with the H1 detector at HERA in 1994 are used to study the cross-sections for inclusive charged particle production and the structure of the photon. The differential cross-sections , for in the HERA laboratory frame, and for GeV/c and GeV/c have been measured. Model calculations of these cross-sections, based on perturbative QCD, indicate that the results are sensitive to the parton densities of the photon as well as to higher order effects, which are phenomenologically treated by multiple interactions. This sensitivity is exploited to determine the leading order distribution of partons in the photon using a new method based on high charged particles. The gluon content of the photon is extracted and found to rise with decreasing . Received: 8 October 1998 / Published online: 8 September 1999     

Experimental check of the Gerasimov-Drell-Hearn sum rule for 1H

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] microb with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.     

Observation of the two photon cascade 3.7 → 3.1 + γγ via an intermediate state Pc

The two photon cascade decay of the 3.7 GeV resonance into the 3.1 GeV resonance has been observed in two nearly independent experiments. The clustering of the photon energies around 160 MeV and 420 MeV observed in the channel 3.7 → (3.1 → μ⁺μ^?) + γγ indicates the existence of at least one intermediate state with even charge conjugation at a mass around 3.52 GeV or 3.26 GeV.     

Production and constraints for a massive dark photon at electron-positron colliders

Dark sector may couple to the Standard Model via one or more mediator particles.We discuss two types of mediators:the dark photon A’and the dark scalar mediatorΦ.The total cross-sections and various differential distributions of the processes e+e-→qqA’and e+e→qqφ(q=u,d,c,s and b quarks)are discussed.We focus on the study of the invisible due to the cleaner background at future e+e-colliders.It is found that the kinematic distributions of the two-jet system could be used to identify(or exclude)the dark photon and the dark scalar mediator,as well as to distinguish between them.We further study the possibility of a search for dark photons at a future CEPC experiment with s1/2=91.2 GeV and 240 GeV.With CEPC running at s1/2=91.2 GeV,it would be possible to perform a decisive measurement of the dark photon(20 GeV相似文献   

Measurement of the low-x behaviour of the photon structure function F_2^\gamma

The photon structure function has been measured using data taken by the OPAL detector at centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in ranges of 1.5–30.0 GeV (LEP1), and 7.0–30.0 GeV (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in at low-x as expected from QCD. Received: 9 June 2000 / Published online: 13 November 2000     

Photon discrimination simulation with the PCA method

The shower shape of n,n,p,p,K+,π+ and photons,generated by JPCIAE code for 5.5 TeV/A 208Pb+208Pb collisions,incident on the ALICE photon spectrometer(PHOS),is analyzed with the principal component analysis(PCA) method.The efficiency dependence of purity for the photon discrimination is simulated for the deposited energy ranges 0.5-2 GeV,2-10 GeV,10-50 GeV and 50-100 GeV.The result shows that in the energy range of 0.5 to 100 GeV,the efficiency of the photon identification can reach 90% with purity of 90%.     

Twist-4 photon helicity-flip amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation

We computed the twist-4 part of the photon spin-flip amplitude in deeply virtual Compton scattering on a nucleon in the Wandzura–Wilczek approximation. We found a factorizable contribution, which arises from photon scattering on quarks with non-zero angular momentum along the collision axis. As the genuine twist-2 amplitude arises at the NLO, for moderate virtualities of the hard photon, GeV, a kinematical twist-4 correction can give a numerically important contribution to the photon helicity-flip amplitude. Received: 2 July 2001 / Revised version: 17 July 2001 / Published online: 31 August 2001     

Experiment at SPring-8

The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. The maximum energy of the photon will be above 3 GeV, and the beam intensity will be 10⁷ photons/sec. Polarization of the photon beam will be 100 % at the maximum energy with fully polarized laser photons. We report the outline of the quark nuclear physics project with this high-quality high-intensity beam.     

Measurements of the QED structure of the photon

The structure of both quasi-real and highly virtual photons is investigated using the reaction , proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at centre-of-mass energies close to the mass of the Z boson. The QED structure function and the differential cross-section for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of ranging from 1.5 to 400 GeV. The differential cross-section for highly virtual photons is measured for GeV and GeV, where and are the negative values of the four-momentum squared of the two photons such that . Based on azimuthal correlations the QED structure functions and for quasi-real photons are determined for an average of 5.4 GeV. Received: 8 February 1999 / Published online: 28 September 1999     

Restudy on Dark Matter Time-Evolution in the Littlest HiggsModel with T-Parity

Following previous study, in the littlest Higgs model (LHM), the heavy photon is supposed to be a possible dark matter candidate and its relic abundance of the heavy photon is estimated in terms of the Boltzman-Lee-Weinberg time-evolution equation. The effect of the T-parity violation is also considered. Our calculations show that when Higgs mass M_H taken to be 300 GeV and do not considering T-parity violation, only two narrow ranges 133<M_AH<135 GeV and 167<M_AH<169 GeV are tolerable with the current astrophysical observation and if 135<M_AH<167 GeV, there must at least exist another species of heavy particle contributing to the cold dark matter. As long as the T-parity can be violated, the heavy photon can decay into regular standard model particles and would affect the dark matter abundance in the universe, we discuss the constraint on the T-parity violation parameter based on the present data. Direct detection prospects are also discussed in some detail.     

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.     

Total hadronic cross-section of photon-photon interactions at LEP

The total hadronic cross-section for the interaction of real photons, hadrons, is measured for centre-of-mass energies GeV. The cross-section is extracted from a measurement of the process hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons ( GeV). The data were taken with the OPAL detector at LEP at centre-of-mass energies , 172$ and 183 GeV. The cross-section is compared with Regge factorisation and with the energy dependence observed in p and pp interactions. The data are also compared to models which predict a faster rise of compared to p and pp interactions due to additional hard interactions not present in hadronic collisions. Received: 3 June 1999 / Published online: 6 April 2000     

A measurement of direct photon production at large pT at the CERN ISR

We have measured direct photon production in pp collisions at the CERN intersecting storage rings for c.m. energies 31 < √s < 63 GeV and transverse momenta up to 9GeV/c, using segmented lead/liquid-argon calorimeters. The ratio of direct photon to π⁰ production is significantly larger than zero, starting at p_T ≈ 4 GeV/c and increasing to values of about 0.4 at 9GeV/c. No significant √s dependence is seen.     

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.     

Inclusive production of charged hadrons and {\rm K}^0_{\rm S} mesons in photon–photon collisions

The production of charged hadrons and mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in interactions with -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions. Received: 4 June 1998 / Published online: 6 November 1998     

Four pion final states with tagged photons at electron positron colliders

A Monte Carlo generator has been constructed to simulate the reaction , where the photon is assumed to be observed in the detector. Only initial state radiation is considered. Additional collinear photon radiation has been incorporated with the technique of structure functions. Predictions are presented for CMS energies of 1 GeV, 3 GeV and 10 GeV, corresponding to the energies of DANE, BEBC and of B-meson factories. The event rates are sufficiently high to allow for a precise measurement of in the region of Q between approximately 1 GeV and 2.5 GeV. For the construction of the program we employ isospin relations between the amplitudes governing decays into four pions and electron positron annihilation into four pions. Estimates of the kinematic breaking of these isospin relations as a consequence of the – mass difference are given. Received: 29 August 2000 / Revised version: 17 November 2000 / Published online: 21 December 2000     

Observation of direct photons in central 158A GeV (208)P(208)b+Pb collisions

A measurement of direct photon production in 208Pb+208Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.51.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.     

A new beam asymmetry measurement from pion photoproduction on the neutron using CLAS

We present a preliminary analysis of the photon beam asymmetry observable (Σ) from the pho-toproduction reaction channel γn→pπ~- in the invariant mass range 1.6-2.3 GeV. The measurement was obtained using the near-4π CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1-2.3 GeV, incident on a liquid deuterium target. The measurement will provide new data to address the poorly established neutron excitation spectrum and will greatly expand the sparse world data-set both in energy and angle.     

Quasifree photoproduction of eta mesons off the neutron

Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.