Photon-gluon Fusion Process Research Articles

Factorization for J/ψ leptoproduction at small transverse momentum

Nonrelativistic Quantum Chromodynamics (NRQCD) breaks down in the region of low transverse momentum, where the transverse momentum of the produced quarkonium state is sensitive to multiple scattering with the incoming hadron and to soft gluon radiation. In this kinematic regime, the transverse-momentum-dependent (TMD) factorization framework is required, promoting the long-distance matrix elements (LDMEs) of NRQCD to the so-called TMD shape functions (TMDShFs), which encode both the soft gluon radiation and the formation of the heavy-quark bound state. In this work, we apply an effective-field theory approach (combining NRQCD and SCET) to the photon-gluon fusion process in inclusive J/ψ leptoproduction. We derive a factorization theorem for the cross section in terms of TMDShFs, compute these quantities at next-to-leading order, establish their evolution, and study their matching onto the corresponding LDMEs in the high-transverse-momentum region. Our results are particularly relevant to the Electron-Ion Collider, where J/ψ leptoproduction can be used to probe gluon transverse-momentum-dependent parton distribution functions (gluon TMDPDFs).

Results on longitudinal spin physics at COMPASS

The COMPASS experiment at the CERN SPS has taken data on deep inelastic scattering of polarised muons on a polarised NH3 target in 2007 and 2011 and on a polarised LiD target in 2002–2004 and 2006. The new results on the longitudinal double spin asymmetry A 1 p and the spin-dependent structure function g 1 p obtained from the 2011 data set are presented. These results are used in a NLO QCD fit to the world data to obtain the polarised parton distributions. Also an update of the results on the Bjorken sum rule, connecting the integral of the non-singlet spin-dependent structure function with the ratio of the weak coupling constants, will be given.Direct access to the gluon polarisation is possible via the photon gluon fusion process in semi-inclusive deep inelastic scattering. This process is studied using the p T dependence of charged hadron asymmetries. The latest results indicate a positive gluon polarisation in the kinematic region of COMPASS

Results on longitudinal spin physics at COMPASS

The COMPASS experiment at the CERN SPS has taken data for deep inelastic scattering of polarised muons off a polarised NH3 target in 2007 and 2011 and off a polarised 6LiD target in 2002–2004 and 2006.We present our new results on the longitudinal double spin asymmetry A1p and the spin-dependent structure function g1p obtained from the 2011 data set. These results are used in a NLO QCD fit to the world data to obtain the polarised parton distributions. Also an update on our results on the Bjorken sum rule, connecting the integral of the non-singlet spin-dependent structure function with the ratio of the weak coupling constants, is given.The gluon polarisation can be accessed directly via the photon gluon fusion process in semi-inclusive deep inelastic scattering. This process is studied using the pT dependence of charged hadron asymmetries. The latest results indicate a positive gluon polarisation in the kinematic region of COMPASS.

Gluon contribution to the Sivers effect. COMPASS results on deuteron target

Sivers effect for gluons is connected to gluon orbital angular momentum which may be the missing part of the nucleon spin puzzle. We present a method of extraction of Sivers effect for gluons from COMPASS SIDIS data on transversely polarised target. In order to access the Sivers effect for gluons photon-gluon fusion (PGF) process is used. To enhance the fraction of PGF in the sample high-pT hadron pair events are selected. The method is based on a assumption that there are 3 processes contributing to the muon-nucleon scattering: PGF, leading process and QCD Compton process. Then one performs a weighting procedure which enables to extract the asymmetries for the 3 contributing processes simultaneously. In order to do that a neural network trained by a Monte Carlo to assign to each event 3 probabilities corresponding to the 3 processes is needed. Finaly we show results of Sivers effect for gluons extraction on COMPASS data with transversely polarised deuteron target. A PGF sinΦ 2h –Φ S = −0.14 ± 0.15 (stat.) at ‹XG › = 0.126.

Gluon polarization measurements at COMPASS

One of the missing keys in the present understanding of the spin structure of the nucleon is the contribution from the gluons: the so-called gluon polarization. This quantity can be determined in deep inelastic scattering (DIS) through the photon–gluon fusion process, in which two analysis methods may be used: (i) identifying open charm events or (ii) selecting events with high-transverse-momentum (high-pT) hadrons. The data used in the present work were collected in the COMPASS experiment, where a 160 GeV/c naturally polarized muon beam, impinging on a polarized nucleon fixed target, is used. Preliminary results for the gluon polarization from high-pT and open charm analyses are presented. The gluon polarization result for high-pT hadrons is divided, for the first time, into three statistically independent measurements at leading order (LO) in quantum chromodynamic (QCD). The result from open charm analysis is obtained at LO and next-to-leading order in QCD. In both analyses a new weighted method based on a neural network approach is used.

Spin Physics at COMPASS

The COMPASS experiment is a fixed target experiment at the CERN SPS using muon and hadron beams for the investigation of the spin structure of the nucleon and hadron spectroscopy. The main objective of the muon physics program is the study of the spin of the nucleon in terms of its constituents, quarks and gluons. COMPASS has accumulated data during 6 years scattering polarized muons off longitudinally or transversely polarized deuteron (6LiD) or proton (NH3) targets.Results for the gluon polarization are obtained from longitudinal double spin cross section asymmetries using two different channels, open charm production and high transverse momentum hadron pairs, both proceeding through the photon-gluon fusion process. Also, the longitudinal spin structure functions of the proton and the deuteron were measured in parallel as well as the helicity distributions for the three lightest quark flavours.With a transversely polarized target, results were obtained with proton and deuteron targets for the Collins and Sivers asymmetries for charged hadrons as well as for identified kaons and pions. The Collins asymmetry is sensitive to the transverse spin structure of the nucleon, while the Sivers asymmetry reflects correlations between the quark transverse momentum and the nucleon spin.Recently, a new proposal for the COMPASS II experiment was accepted by the CERN SPS which includes two new topics: Exclusive reactions like DVCS and DVMP using the muon beam and a hydrogen target to study generalized parton distributions and Drell-Yan measurements using a pion beam and a polarized NH3 target to study transverse momentum dependent distributions.

Longitudinal spin physics at COMPASS

The spin structure of the nucleon is studied by the COMPASS collaboration using scattering of muons off polarised protons and deuterons. From the resulting asymmetries spin structure functions are extracted and the flavour decomposition of the quark contribution to the nucleon spin is studied. The gluon polarisation is investigated using the photon–gluon fusion process. Here, two main channels are investigated, open charm production and high pT hadron pair production. Also first measurements of unpolarised cross sections are reported.

Measurements of ΔG at COMPASS

The direct measurement of the gluon polarization in the nucleon at the COMPASS experiment is discussed. Three complementary analyses are considered. All of them study ΔG/G using method based on selection of events originating from the Photon Gluon Fusion process. Most precise preliminary value of ΔG/G is obtained from the high pT pair channel for the Q2 < 1 (GeV/c)2 region: ΔG/G = 0.016 ± 0.058 (stat.) ± 0.055 (syst.).

Determination of the gluon contribution to the nucleon spin with COMPASS

New double-spin asymmetry data from the COMPASS experiment at CERN are presented. The data are used to determine the polarized gluon distribution Δ G / G ( x ) , via three different techniques. Next-to-leading order QCD fits to the nucleon polarized structure functions indicate that the first moment of Δ G / G ( x ) is small (either positive or negative) and compatible with zero. This observation is corroborated by three measurements of Δ G / G ( x ) , based on the photon-gluon fusion process.

Inclusive D*± meson and associated dijet production in deep-inelastic scattering at HERA

Inclusive D*± production is measured in deep-inelastic ep scattering at HERA with the H1 detector. In addition, the production of dijets in events with a D*± meson is investigated. The analysis covers values of photon virtuality 2 ≤ Q2 ≤ 100 GeV2 and of inelasticity 0.05≤y≤0.7. Differential cross sections are measured as a function of Q2 and x and of various D*± meson and jet observables. Within the experimental and theoretical uncertainties all measured cross sections are found to be adequately described by next-to-leading order (NLO) QCD calculations, based on the photon–gluon fusion process and DGLAP evolution, without the need for an additional resolved component of the photon beyond what is included at NLO. A reasonable description of the data is also achieved by a prediction based on the CCFM evolution of partons involving the kT-unintegrated gluon distribution of the proton.

Recent gluon polarization results from COMPASS

One of the main goals of the COMPASS experiment at CERN is the measurement of the gluon polarization in the nucleon, ΔG, by scattering of 160 GeV/c polarized muons on a polarized 6LiD target. This quantity is experimentally accessible via the photon-gluon fusion process, tagged either by charmed mesons production or by high pT hadron pairs production. The status of these two analyses is presented. Preliminary results obtained from the 2002/03 data samples on the D0 and the D∗± channels are shown. The high pT hadron pairs, produced at Q2≶1 (GeV/c)2, were also analysed, and the measured ΔG/G values are presented here.

Determination of the gluon polarisation in COMPASS experiment at CERN

Determining of the gluon polarisation in the nucleon, by measurement of the double spin asymmetry of the photon-gluon fusion process is the main goal of the COMPASS experiment at CERN. Experiment uses 160 GeV polarized muon beam and polarised 6 LiD target. Photon-gluon fusion events are selected using charmed mesons and hadron pairs at large p T . Preliminary results on charm signal extraction, and spin asymmetry A γ ∗ d of the cross sections for high p T hadron pairs production are presented.

Spin asymmetries for events with highpThadrons in DIS and an evaluation of the gluon polarization

We present a measurement of the longitudinal spin cross section asymmetry for deep-inelastic muon-nucleon interactions with two high transverse momentum hadrons in the final state. Two methods of event classification are used to increase the contribution of the photon-gluon fusion process to above 30%. The most effective one, based on a neural network approach, provides the asymmetries AplN→lhhX=0.030±0.057(stat)±0.010(syst) and AdlN→lhhX=0.070±0.076(stat)±0.010(syst). From these values we derive an averaged gluon polarization ΔG/G=−0.20±0.28(stat)±0.10(syst) at an average fraction of nucleon momentum carried by gluons 〈η〉=0.07.Received 31 January 2004DOI:https://doi.org/10.1103/PhysRevD.70.012002©2004 American Physical Society

1- Results from the SMC experiment at CERN. 2- The COMPASS experiment at CERN

SMC : The SMC collaboration has measured the proton and deuteron spin structure function g 1 in the kinematical range 0.00006 < x < 0.8 and 0.02 < Q 2 < 100 GeV 2. A brief summary of the NLO pQCD global analysis of all g 1 world data available in 1998 is given. The first moments of the proton, deuteron and neutron structure function were calculated in two factorization schemes ( MS and AB). They lead to a value of a 0 close to 0.2, which corresponds to ΔΣ= 0.19 in the MS scheme, and 0.38 in the AB one. The Bjorken sum rule is tested and found to agree with the theoretical predictions at the level of 10 %. COMPASS : The first goal of the COMPASS experiment at CERN is the measurement of the contribution of the gluon polarization Δ g to the nucleon spin. The measurement will be done by scattering high energy polarized muons on a polarized proton or deuteron target. The photon gluon fusion process, which is sensitive to the gluon density distribution, will be signed by the presence of charmed particles ( D 0, D 0) or of high p t hadrons pairs of opposite charge. In addition to Δ g, the longitudinal and transversely polarized parton distributions will be measured for the different flavors. A new spectrometer with large acceptance, high rate capability and identification of particles is being commissioned.

Factorial moment analyses in diffractive deep-inelastic ep scattering

It is pointed out this talk that “the colorless objects” in diffractive scattering in the small- χ B region can be probed by measuring the scaled factorial moments of final-state-hadrons and the dependence of their scaling behavior upon the diffractive kinematic variables. The Monte Carlo implementation of RAPGAG and JETSET are discussed as illustrative examples. The results of these model-calculations show in particular that inclusion of the contributions from the photon gluon fusion processes can considerably enlarge the power of the scaled factorial moments. The significant χ B -dependence of intermittency index in small- χ B region in this MC analyses shows the hadronic final states depend very much on the lifetime of the exchanged colorless object c ∗ 0 . The dependence of the intermittency index χ P , Q 2 and β are also discussed.

The COMPASS experiment at CERN

The recently approved COMPASS experiment attempts a measurement of the gluon polarization around η ⋍ 0.1 with a precision of δ(Δ g / g ) ⋍ 0.1. The experiment uses open charm muo-production and large P T hadron pairs to tag the photon-gluon fusion process. COMPASS will also cover a rich spin-physics program in polarized DIS.

Measurement of inelastic J/ $\psi$ photoproduction at HERA

We present a measurement of the inelastic, non diffractive J/$\psi$ photoproduction cross section in the reaction $e^{+} p \to e^{+} {J}/\psi X$ with the ZEUS detector at HERA. The J/$\psi$ was identified using both the $\mu^{+}\mu^{-}$ and $e^{+}e^{-}$ decay channels and events were selected within the range $0.4<z<0.9$ ($0.5<z<0.9$) for the muon (electron) decay mode, where $z$ is the fraction of the photon energy carried by the J/$\psi$ in the proton rest frame. The cross section, the $p^2_T$ and the $z$ distributions, after having subtracted the contributions from resolved photon and diffractive proton dissociative processes, are given for the photon-proton centre of mass energy range $50<W<180$ GeV; $p^2_T$ is the square of the J/$\psi$ transverse momentum with respect to the incoming proton beam direction. In the kinematic range $0.4 < z < 0.9$ and $p^2_T > 1$ GeV$^2$, NLO calculations of the photon-gluon fusion process based on the colour-singlet model are in good agreement with the data. The predictions of a specific leading order colour-octet model, as formulated to describe the CDF data on J/$\psi$ hadroproduction, are not consistent with the data.

Comparison between the various descriptions for charm electroproduction and the HERA-data

We examine the charm component F_{2,c}(x,Q^2,m^2) of the proton structure function F_2(x,Q^2) in three different schemes and compare the results with the data in the x and Q^2 region explored by the HERA experiments. Studied are (1) the three flavour number scheme (TFNS) where the production mechanisms are given by the photon-gluon fusion process and the higher order reactions with three light-flavour parton densities as input (2) the four flavour number scheme (FFNS) where F_{2,c} is expressed in four light flavour densities including one for the charm quark and (3) a variable-flavour number scheme (VFNS) which interpolates between the latter two. Both the VFNS and the TFNS give good descriptions of the experimental data. However one cannot use the FFNS for the description of the data at small Q^2.

Physics with polarized proton and electron collisions at HERA

We describe here some possible measurements at the HERA collider in the scenario where both the electron and the proton beam are polarized. The proposed experiments have been studied during the recent workshop on Future Physics at HERA in the dedicated working group “polarized protons and electrons”.The measurement of the polarized gluon distribution function ΔG(x, Q2) from photongluon fusion processes in electron-proton deep inelastic scattering producing two jets has been investigated. The size of the expected spin asymmetry and corresponding statistical uncertainties have been estimated. The results show that the asymmetry can reach a few percent, and is not washed out by hadronization and higher order processes.Estimates of possible data on spin dependent asymmetries in inclusive deep inelastic ep scattering are presented. By accessing a new kinematic domain in x and Q2 these data would shed light on the behaviour of the polarized structure function of the proton g1p at low x and high Q2. They would reduce substantially the uncertainty on the determination of the polarized gluon distribution ΔG(x, Q2) from QCD evolution equations.Finally, the spin asymmetries from charged current events allow to measure the spin dependent structure function g5 which is related to the polarized distribution functions. The charged current reactions allow to distinguish quark and antiquark flavours. These measurements would provide data on Δuv and Δdv at high x and high Q2 complementary to the fixed target data, with a very good statistical precision. In addition, semi inclusive D-production events could give a good signal for the determination of Δs and Δs.

Hadronic and photonic production of doubly flavored baryons

We consider the production of baryons containing two heavy quarks via gluon-gluon, photongluon and photon-photon fusion processes in the framework of nonrelativistic heavy quark model and perturbative chromodynamics. We calculate the corresponding cross sections for pp, ep and γγ collisions at the energies of present and future colliders and analyse the uncertainties of theoretical estimations.