Photon-gluon Fusion 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).

Doubly heavy hadron production in ultraperipheral collision

We study the double heavy baryon ΞQQ′ and tetraquark TQQ production through photon-photon and photon-gluon fusion via ultraperipheral collisions at the LHC and Future Circular Collider (FCC) within the framework of nonrelativistic QCD factorization formalism. Various ion-ion collisions are taken into account, two cc(bb)-diquark configurations ([cc(bb),S31−3¯] and [cc(bb),S10−6]) and four bc-diquark configurations ([bc,S31−3¯], [bc,S31−6], [bc,S10−3¯], and [bc,S10−6]) are considered in the calculation. Numerical results indicate that the [cc,S31−3¯] diquark provides dominant contribution for Ξcc (Tcc) production, and a considerable number of Ξcc (Tcc) can be produced. Because the event topologies for ultraperipheral collision are very clear, the background from various QCD interactions can be suppressed, hence the experimental investigation for Ξcc and Tcc is feasible. The productions for Ξbc/bb are also discussed, leaving only a slight possibility for Ξbc through photon-gluon fusion with ultraperipheral collisions at the FCC. Published by the American Physical Society 2024

A model for $F_L$ structure function at low values of $Q^2$ and $x$ - revisited

A reanalysis of the model for the longitudinal structure function F_L (x,Q^2)FL(x,Q2) at low xx and low Q^2Q2 was undertaken, in view of the advent of the EIC. The model includes the kinematic constraint F_L\sim Q^4FL∼Q4 as Q^2\rightarrowQ2→ 0. It is based on the photon-gluon fusion mechanism suitably extrapolated to the region of low Q^2Q2. Revised model was critically updated, extended to the EIC kinematic region, and e.g. contains new parameterisations of the parton distribution functions.

Longitudinal structure function FL at low Q2 and low x with model for higher twist: An update

A reanalysis of the model for the longitudinal structure function FL(x,Q2) at low x and low Q2 was undertaken, in view of the advent of the EIC. The model is based on the photon-gluon fusion mechanism suitably extrapolated to the region of low Q2. It includes the kinematic constraint FL∼Q4 as Q2→ 0 and higher twist contribution which vanishes as Q2→∞. Revised model was critically updated and compared to the presently available data.

Quarkonium TMD fragmentation functions in NRQCD

We study the transverse-momentum spectrum of quarkonium production from single light-parton fragmentation mechanism. In the case of semi-inclusive deep inelastic scattering, we observe that there are two possible initiating processes, namely photon-gluon fusion and light-quark fragmentation. For the second case we derive the factorization theorem, which involves a new hadronic quantity: the quarkonium transverse-momentum-dependent fragmentation functions in NRQCD. We calculate their matching onto the non-perturbative long distance matrix elements at the lowest order in the strong-coupling constant, mathcal{O}left({alpha}_s^2right) . Focusing on the case of the electron-ion collider, we make a comparative phenomenological study of the two production mechanisms and find the regions of the phase space where one is dominant over the other.

Accessing the gluon Sivers function at a future electron-ion collider

In this work, we present a systematic study on the feasibility of probing the largely unexplored transverse momentum dependent gluon Sivers function (GSF) in open charm production, high p$_T$ charged di-hadron and di-jet production at a future high energy, high luminosity Electron-Ion Collider (EIC). The Sivers function is a measure for the anisotropy of the parton distributions in momentum space inside a transversely polarized nucleon. It is proposed that it can be studied through single spin asymmetries in the photon-gluon fusion subprocess in electron proton collisions at the EIC. Using a well tuned Monte Carlo model for deep inelastic scattering, we estimate the possible constraints of the GSF from the future EIC data. A comparison of all the accessible measurements illustrates that the di-jet channel is the most promising way to constrain the magnitude of the GSF over a wide kinematic range.

Sivers effect in inelastic J/ψ photoproduction in ep↑ collision in color octet model

The prediction of single-spin asymmetry in inelastic photoproduction of $J/\psi$ in $ep^\uparrow$ collision is presented. At next-to-leading order, the dominating process is photon-gluon fusion, $\gamma+g\rightarrow J/\psi+g$ for the production of $J/\psi$ in $e+p^\uparrow\rightarrow J/\psi+X$, which directly probes the gluon Sivers function. Using the non-relativistic QCD based color octet model, the color octet states $\leftidx{^{3}}{S}{_1}^{(8)}$, $\leftidx{^{1}}{S}{_0}^{(8)}$ and $\leftidx{^{3}}{P}{_{J(0,1,2)}}^{(8)}$ contribution to $J/\psi$ production is calculated. Sizable asymmetry is estimated as a function of transverse momentum $P_T$ and energy fraction $z$ of $J/\psi$ in the range $0<P_T\leq 1$ GeV and $0.3<z\leq 0.9$. The unpolarized differential cross section of inelastic $J/\psi$ photoproduction is found to be in good agreement with H1 and ZEUS data.

The ratio R = dσL/dσT in heavy-quark pair leptoproduction as a probe of linearly polarized gluons in unpolarized proton

We study the Callan–Gross ratio R=dσL/dσT in heavy-quark pair leptoproduction, lN→l′QQ¯X, as a probe of linearly polarized gluons inside unpolarized proton, where dσT (dσL) is the differential cross section of the γ⁎N→QQ¯X process initiated by a transverse (longitudinal) virtual photon. Note first that the maximal value for the quantity R allowed by the photon-gluon fusion with unpolarized gluons is large, about 2. We calculate the contribution of the transverse-momentum dependent gluonic counterpart of the Boer–Mulders function, h1⊥g, describing the linear polarization of gluons inside unpolarized proton. Our analysis shows that the maximum value of the ratio R depends strongly on the gluon polarization; it varies from 0 to Q24m2 depending on h1⊥g. We conclude that the Callan–Gross ratio in heavy-quark pair leptoproduction is predicted to be large and very sensitive to the contribution of linearly polarized gluons. For this reason, future measurements of the longitudinal and transverse components of the charm and bottom production cross sections at the proposed EIC and LHeC colliders seem to be very promising for determination of the linear polarization of gluons inside unpolarized proton.

How to measure the linear polarization of gluons in unpolarized proton using the heavy-quark pair leptoproduction

We study the azimuthal cos⁡φ and cos⁡2φ asymmetries in heavy-quark pair leptoproduction, lN→l′QQ¯X, as probes of linearly polarized gluons inside unpolarized proton, where the azimuth φ is the angle between the lepton scattering plane (l,l′) and the heavy quark production plane (N,Q). First, we determine the maximal values for the cos⁡φ and cos⁡2φ asymmetries allowed by the photon–gluon fusion with unpolarized gluons; these predictions are large, (3−1)/2 and 1/3, respectively. Then we calculate the contribution of the transverse-momentum dependent gluonic counterpart of the Boer–Mulders function, h1⊥g, describing the linear polarization of gluons inside unpolarized proton. Our analysis shows that the maximum values of the azimuthal distributions depend strongly on the gluon polarization; they vary from 0 to 1 depending on h1⊥g. We conclude that the azimuthal cos⁡φ and cos⁡2φ asymmetries in heavy-quark pair leptoproduction are predicted to be large and very sensitive to the contribution of linearly polarized gluons. For this reason, future measurements of the azimuthal distributions in charm and bottom production at the proposed EIC and LHeC colliders seem to be very promising for determination of the linear polarization of gluons inside unpolarized proton.

Measuring Gluon Sivers Function at a Future Electron-Ion Collider

In this work, we present a systematic study on the feasibility of probing the largely unexplored gluon Sivers function (GSF) based on the open charm production, charged dihadron and dijet method at a future high energy, high luminosity Electron-Ion Collider (EIC). Sivers function describes the anisotropy of parton distributions inside a transversely polarized nucleon in the momentum space and provides us a complete picture of the 2+1D structure of the nucleons. It is proposed that the GSF can be studied through the single spin asymmetry (SSA) measurement in the photon-gluon fusion channel with electron proton collisions at the EIC. Using a well tuned Monte Carlo model for deep inelastic scatterings, we estimate the possible constraints of the gluon Sivers effect one can draw from the future EIC data. Comparisons of all the possible measurements further illustrate that the dijet method is the most promising way to demonstrate the presence of GSF and pin down its evolution effect.

First measurement of the Sivers asymmetry for gluons using SIDIS data

The Sivers function describes the correlation between the transverse spin of a nucleon and the transverse motion of its partons. For quarks, it was studied in previous measurements of the azimuthal asymmetry of hadrons produced in semi-inclusive deep inelastic scattering of leptons off transversely polarised nucleon targets, and it was found to be non-zero. In this letter the evaluation of the Sivers asymmetry for gluons is presented. The contribution of the photon–gluon fusion subprocess is enhanced by requiring two high transverse-momentum hadrons. The analysis method is based on a Monte Carlo simulation that includes three hard processes: photon–gluon fusion, QCD Compton scattering and the leading-order virtual-photon absorption process. The Sivers asymmetries of the three processes are simultaneously extracted using the LEPTO event generator and a neural network approach. The method is applied to samples of events containing at least two hadrons with large transverse momentum from the COMPASS data taken with a 160 GeV/c muon beam scattered off transversely polarised deuterons and protons. With a significance of about two standard deviations, a negative value is obtained for the gluon Sivers asymmetry. The result of a similar analysis for a Collins-like asymmetry for gluons is consistent with zero.

Collider tests of (composite) diphoton resonances

We analyze the Large Hadron Collider sensitivity to new pseudoscalar resonances decaying into diphoton with masses up to scales of few TeVs. We focus on minimal scenarios where the production mechanisms involve either photon or top-mediated gluon fusion, partially motivated by the tantalizing excess around 750 GeV reported by ATLAS and CMS. The two scenarios lead respectively to a narrow and a wide resonance. We first provide a model-independent analysis via effective operators and then introduce minimal models of composite dynamics where the diphoton channel is characterized by their topological sector. The relevant state here is the pseudoscalar associated with the axial anomaly of the new composite dynamics. If the Standard Model top mass is generated via four-fermion operators the coupling of this state to the top remarkably explains the wide-width resonance reported by ATLAS. Beyond the excess, our analysis paves the way to test dynamical electroweak symmetry breaking via topological sectors.

The Gluon Contribution to the Sivers Effect COMPASS results

The Sivers effect describes the correlation between the spin of the nucleon and the orbital motion of partons. It can be measured via Semi-Inclusive Deep Inelastic Scattering of lepton on a transversely polarised proton and deuteron targets by determining the azimuthal asymmetry related to the modulation in the Sivers angle ϕSiv. In the paper a method of obtaining the Sivers asymmetry for gluons is presented. It is based on the model of lepton nucleon interactions via three single-photon-exchange processes: photon-gluon fusion (PGF), QCD Compton (QCDC) and leading process (LP). A method of simultaneous extraction of the Sivers asymmetries of the three processes with the use of Monte Carlo (MC) and neural networks (NN) approach is presented. The method has been applied to COMPASS data taken with 160GeV/c muon beam scattered off transversely polarised deuteron and transversely polarised proton target. For each target a data sample of events containing at least two hadrons with large transverse momentum has been selected. Finally the results for gluon Sivers asymmetry were obtained to be: Adg = −0.14 ± 0.15(stat.) ± 0.06(syst.) at 〈xg〉 = 0.13 and Apg = −0.26 ± 0.09(stat.) ± 0.08(syst.) at 〈xg〉 = 0.15.

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.

Next-to-leading order transverse momentum-weighted Sivers asymmetry in semi-inclusive deep inelastic scattering: The role of the three-gluon correlator

We study the Sivers asymmetry in semi-inclusive hadron production in deep inelastic scattering. We concentrate on the contribution from the photon-gluon fusion channel at ${\cal O}(\alpha_{em}^2 \alpha_s)$, where three-gluon correlation functions play a major role within the twist-3 collinear factorization formalism. We establish the correspondence between such a formalism with three gluon correlation functions and the usual transverse momentum dependent (TMD) factorization formalism at moderate hadron transverse momenta. We derive the coefficient functions used in the usual TMD evolution formalism related to the quark Sivers function expansion in terms of the three-gluon correlation functions. We further perform the next-to-leading order calculation for the transverse-momentum-weighted spin-dependent differential cross section, and identify the off-diagonal contribution from the three-gluon correlation functions to the QCD collinear evolution of the twist-3 Qiu-Sterman function.

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.

COMPASS experiment at CERN: hadron spectroscopy and open charm results

COmmon Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) is a fixed target experiment at CERN dedicated to studies of the spin structure of the nucleon and of the spectroscopy of hadrons. During the years 2002–2004 and 2006–2007, the COMPASS collaboration has collected a large amount of data by scattering polarized 160 GeV/c muons on polarized 6LiD and NH3 targets. These data were used to evaluate the gluon contribution to the nucleon spin. The gluon polarization Δgg was directly measured from the cross-section helicity asymmetry of D0 mesons production in the photon-gluon fusion reaction.During 2008 and 2009, the world leading data sets were collected with hadron beams which are currently being analyzed using Partial Wave Analysis (PWA) technique. COMPASS is performing a search for JPC-exotic mesons, glueballs and hybrids, through light hadron spectroscopy in high energy 190 GeV/c hadron-proton reactions using both centrally produced and diffractive events. Preliminary results from these searches are discussed.

D ∗ and D meson production in muon nucleon interactions at 160 GeV/c

The production of D* and D mesons in inelastic scattering of 160 GeV/c muons off a ^6LiD target has been investigated with the COMPASS spectrometer at CERN for 0.003 (GeV/c)^2 < Q^2 < 10 (GeV/c)^2 and 3x10^-5< x_Bj < 0.1. The study is based on 8100 events where a D^0 or anti D^0 is detected subsequently to a D*+ or D*- decay, and on 34000 events, where only a D^0 or anti D^0 is detected. Kinematic distributions of D*, D and K*_2 are given as a function of their energy E, transverse momentum p_T, energy fraction z, and of the virtual photon variables nu, Q^2 and x_Bj. Semi-inclusive differential D* production cross-sections are compared with theoretical predictions for D* production via photon-gluon fusion into open charm. The total observed production cross-section for D*+/- mesons with laboratory energies between 22 and 86 GeV is 1.9 nb. Significant cross-section asymmetries are observed between D*+ and D*- production for nu<40 GeV and z>0.6.

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.