Photon-gluon Fusion Research Articles

INTRINSIC CHARM IN PROTON AND J/ψ PHOTOPRODUCTION AT HIGH ENERGIES

J/ψ and open charm photoproduction on charm quarks in a proton via partonic subprocess γc → J/ψc are discussed based on the perturbative theory of quantum chromodynamics and nonrelativistic quark model. It is shown that the value and energy dependence of the cross-section for such process depend remarkably on the choice of charm distribution function in a proton. In the region of small z = EJ/Eγ < 0.2 the contribution of the γc → J/ψc subprocess in the inelastic J/ψ photoproduction spectra is larger than the contribution of the photon-gluon fusion subprocess. At the energy range of HERA collider charm quarks contribution in the total inclusive J/ψ photoproduction cross-section may be equal to 60% of the dominant contribution of photon-gluon fusion mechanism.

Relativistic corrections to photoproduction ofJ/ψ

We consider various models for photoproduction ofJ/ψ on protons, based on the well-known photongluon fusion mechanism. We show that relativistic corrections to theJ/ψ bound state lead to a better description of the EMC and NMC data, especially in the highz region (z>0.8) where the naive (nonrelativistic) colour singlet model breaks down (z isE J/ψ/E γ in the proton rest frame). As most of the events are expected in this domain, these corrections are important for extracting the gluon distribution function from the data. We also discuss diffractiveJ/ψ production and a heuristic model for elastic production.

A measurement of elastic J/ ψ photoproduction cross section at Fermilab E687

Measurements of elastic photoproduction cross sections for the J/ ψ meson from 100 GeV to 375 GeV are presented. The results indicate that the cross section increases slowly in this range. The shape of the energy dependence agrees well with the photon-gluon fusion model prediction.

J/ψ-PRODUCTION MECHANISMS AND DETERMINATION OF THE GLUON DENSITY AT HERA

We discuss photo- and leptoproduction of J/ψ mesons at energies ranging from those of fixed-target experiments up to those of HERA. Elastic and diffractive production as well as various inelastic processes are studied. We investigate the range in which J/ψ production is described by photon-gluon fusion in the color-singlet model. We show how inelastic J/ψ production at HERA can be used to extract the gluon density. We estimate an accessible range of 3×10−4<x<0.1 and discuss sources of errors in the reconstruction of the gluon density at HERA.

Study of charm photoproduction mechanisms

This paper presents results on charm photoproduction in the energy interval 40 to 160 GeV, obtained from the high-statistics charm samples of the NA 14/2 experiment at CERN. We measure the charm cross-section, the distributions inx F andp T2 and various production ratios and charge asymmetries. The total non-diffractive open-charm cross-section per nucleon is measured to be\(\sigma _{(\gamma N \to c\bar cX)} \) at 〈E γ〉 =100 GeV. We discuss the photoproduction of charm in terms of theoretical and phenomenological models. We compare the measuredp T2 andx F distributions with first-order QCD calculations of photon-gluon fusion and obtain a value for the charm-quark mass ofm c =1.5 −0.1+0.2 GeV/c2.

Production of b and [formula omitted] quarks by photon-gluon fusion in heavy-ion collisions

Electromagnetic Higgs production in ultrarelativistic heavy-ion collisions has been proposed as an alternative for detecting Higgs particles in the mass range m Z< m H<2 m W. We consider the fussion of a photon and a gluon into b and b quarks as background to the b b decay of the Higgs boson. This completely hides the Higgs signal. We also discuss the possibility of utilizing photon-gluon fusion into b b and c c as a sensitive tool to determine the gluon distribution of the nucleon inside the nucleus, e.g., at RHIC.

Photon-gluon fusion at HERA: measurement of the fractional momentum of the gluon in lowQ 2 events

We have performed a Monte Carlo study of photon-gluon interactions at theep collider HERA. We show that it is possible to determine the fractional momentum carried by the gluon (xg) from hadronic informations alone. In particular, in the photoproduction region, where thex andQ2 are badly measured, this could be the only global physical variable of interest, together with the hadronic invariant mass. The study of these quantities would allow a direct measurement of the gluon density of the proton in the rangexg=2.5×10−3∶5×10−1.

Photoproduction of the Λ c charmed baryon

In a photoproduction experiment using a mean photon energy of 100 GeV we have observed 29±8 Λ c ( Λ c ) charmed-baryon and antibaryon decays in the pK −π + ( p K +π −) final state. Quasi two-body final states do not contribite significantly to this channel. The mass of the Λ c was measured to be 2281.7±2.7±2.6 MeV/ c 2 and its lifetime 0.18±0.03±0.03 ps. The ratio of Λ c D production, measured in this experiment, is significantly greater than that predicted by photon-gluon fusion and using a Lund model to describe the hadronization. This excess cannot be completely accounted for in this model, even using a Λ c branching fraction in pK π as high as 5%.

Quarkonium production at hera

We have calculated the production cross-sections for ϒ and J/ψ (both the 3S 1 states) via photon-gluon fusion at HERA energies. We find sizeable cross-sections even after phase space cuts needed to guarantee inelasticity are imposed.

Measuring the parton content of the photon at the DESY ep collider HERA.

We discuss the possibility to measure the parton, and especially gluon, content of the photon at the forthcoming DESY ep collider HERA. We first reemphasize that the production of two jets with high transverse momentum ${p}_{T}$ is dominated by events where the photon is resolved into quarks and gluons for ${p}_{T}$\ensuremath{\lesssim}40 GeV. We then show that these ``resolved'' contributions, and in particular those that are initiated by a gluon from the photon, are especially important at large jet rapidities. Rapidity distributions also allow us to isolate the direct photon-gluon fusion contributions to bb\ifmmode\bar\else\textasciimacron\fi{} production. This makes it possible to use this process to determine the gluon content of the proton. However, a direct measurement of the gluon content of the photon will only be possible if the spectator jet from the photon can be detected. We find that this jet typically has an energy of 10--12 GeV and estimate its opening angle to be around 15\ifmmode^\circ\else\textdegree\fi{}, which should allow for its detection.

Spin-dependent parton distributions in polarized deep inelastic lepton nucleon scattering

We analyze the recently measured first moment Г p (Q 2) of the structure functiong 1 (x,Q 2) obtained in deep inelastic scattering of polarized electrons from polarized protons. It is shown that the recent experimental result Г p (〈Q 2〉≃10 GeV2)≃ 0.113 implies a polarized strange quark sea whose total polarization Δs(μ2)≃−0.22 is comparable to Δμ(μ2)≃0.74 and Δd(μ2)≃−0.50 already at a typically hadronic scale μ=0(1 GeV). Most remarkably, the total polarization due to quark distributions Δμ(μ2)+Δd(μ2)+Δs(μ2) practically vanishes which implies that the spin of the proton is carried by the gluons, ΔG(μ2), and/or the angular momentum of the partons. This surprising conjecture can be experimentally tested with heavy quark production in longitudinally polarized lepton-nucleon scattering. In particular the (real) photon-gluon fusion process γg→Q $$\bar Q$$ provides a rather unique measure of ΔG(x,Q 2).

Production of charged Higgs bosons in ep collisions

The photon-gluon fusion mechanism for charged Higgs boson production in ep collisions is discussed. It is shown that this mechanism is the most promising one at HERA and higher energies. Even at HERA one could get up to 10 3 events/yr provided both the top quark and the charged Higgs boson are light.

Photon gluon fusion cross sections at HERA energy

We present cross sections for heavy flavour production through photon gluon fusion in electron proton collisions at HERA energy. The electron photon vertex is taken into account explicitly, and theQ 2 of the exchanged photon ranges from nearly zero (almost real photon) to the kinematically allowed maximum. In our approach the scale is set by the mass of the produced quarks. Our formalism is also applicable to the production of light quarks as long as the invariant mass of the pair is sufficiently high, so we also give cross sections for $$u\bar u,d\bar d$$ and $$s\bar s$$ production.

Measurement ofJ/? and ?? real photoproduction on lithium-6 at a mean energy of 90 GeV

Inelastic and elasticJ/ψ (3097) photoproduction on Li6 are measured at a mean γ energy of 90 GeV in an open spectrometer. TheJ/ψ are identified by their decays intoμ+μ− ore+e−. A signal of ψ′(3685) intoμ+μ− andJ/ψπ+π− is also seen. The inelastic cross-section withZ=Eψ/Eγ<0.9 is compared in shape and magnitude with the colour singlet model of photon-gluon fusion.

The production of heavy E 6 fermions in ep collisions

We have studied the production and decay of exotic E 6 quarks and leptons in a model in which E 6 breaks to give a new low energy SU(2) I gauge group with conventional and exotic fermions in the same doublets. We give the cross section for the process ep→ Eh+ X and find that in general W I exchange gives a larger cross section for h-quark production than photon-gluon fusion.

Charm production in deep inelastic muon-iron interactions at 200 GeV/c

Dimuon and trimuon events have been studied in deep inelastic muon scattering on an iron target at an incident muon energy of 200 GeV. The events are shown to originate mainly from charm production. Comparison of the measured cross sections with data taken at higher muon energies shows that charm production originates predominantly from transverse virtual photons. Within the framework of the photon gluon fusion model this indicates that the parity of the gluon is odd.

Cross sections and some features of charm photoproduction at γ energies of 20–70 GeV

We report some results on charm photoproduction at γ energies ranging from 20 to 70 GeV. 36 events with pairs of charmed particles have been found in emulsions. The computed total cross section is (230±57)nb/nucleon. Frequencies of different production channels and some distributions are presented and compared with the predictions from a photon-gluon fusion model with string fragmentation.

Lattice contribution to charm photoproduction momentum distributions

Using the lattice Hamiltonian QCD theory of Drell and his collaborators, we compute the lattice contribution to the momentum distributions for charmed hadrons produced in γ+p→c\(\bar c\)+X. When this lattice effect is added to the usual perturbative photon-gluon fusion results for γ+p→c\(\bar c\)+X, there is no obvious disagreement between the corresponding theoretical predictions and the limited observation which now exists.

A nonperturbative component in muoproduction of charm

A nonperturbative C C component in the quark fragmentation function, as suggested by the neutrino-induced like-sign dimuon data, contributes appreciably to muoproduction of charm. It is comparable in size to the perturbative QCD (photon-gluon fusion) contribution; and its inclusion gives a better fit to the charm muoproduction (muon-induced dimuon) data.

The Monte Carlo program and the pattern recognition for the microvertex detector of the E687 experiment at fnal

A Monte Carlo program and a pattern recognition have been developed in Milan to design and study the performance of microvertex detector to be used in a photoproduction experiment at the Tevatron. The aim of the experiment is the study of the heavy flavoured states (lifetimes, production mechanism, branching ratios, …). The present program has been written to simulate the photoproduction mechanism (according to the vector-meson and the photon-gluon fusion models), the tracking of the charged particles inside the detector and to study the pattern recognition performance of the whole system for various geometrical configuration of the microvertex as well as for different physical conditions.