Photoproduction Cross Section Research Articles

Photoproduction of η mesons off the proton for 1.2<Eγ<4.7GeV using CLAS at Jefferson Laboratory

Photoproduction cross sections are reported for the reaction $\gamma p\to p\eta$ using energy-tagged photons and the CLAS spectrometer at Jefferson Laboratory. The $\eta$ mesons are detected in their dominant charged decay mode, $\eta\to \pi^+\pi^-\pi^0$, and results on differential cross sections are presented for incident photon energies between 1.2 and 4.7 GeV. These new $\eta$ photoproduction data are consistent with earlier CLAS results but extend the energy range beyond the nucleon resonance region into the Regge regime. The normalized angular distributions are also compared with the experimental results from several other experiments, and with predictions of $\eta$ MAID\,2018 and the latest solution of the Bonn-Gatchina coupled-channel analysis. Differential cross sections $d\sigma/dt$ are presented for incident photon energies $E_\gamma > 2.9$ GeV ($W > 2.5$ GeV), and compared with predictions which are based on Regge trajectories exchange in the $t$-channel (Regge models). The data confirm the expected dominance of $\rho$, $\omega$ vector-meson exchange in an analysis by the Joint Physics Analysis Center.

Exclusive ϒ and ρ(770)0 photoproduction in pPb at [formula omitted] with CMS

The first measurements of exclusive ϒ and ρ(770)0 vector meson photoproduction with the CMS detector at LHC in ultraperipheral proton-lead collisions at sNN=5.02TeV are presented. The ϒ(1S) cross section, which has been measured in the dumuon channel, is consistent with the updated ZEUS ep results at HERA. A fit of function A(Wγp[GeV]/400)δ to the CMS pPb measurement together with HERA ep and LHCb pp data gives the δ value, consistent with J/ψ photoproduction results. The obtained result provides a new insight for low-x gluon distribution in protons in an energyrange previously less constrained. The cross section for exclusive photoproduction of ρ(770)0 has been measured where ρ(770)0 decays to two pions. The obtained pomeron slope parameter, α′ = 0.28 ± 0.11(stat) ± 0.12(syst)GeV−2, is consistent with the ZEUS result and with the Gribov-Regge theory expectation.

Next-to-leading order QCD predictions for dijet photoproduction in lepton-nucleus scattering at the future EIC and at possible LHeC, HE-LHeC, and FCC facilities

We calculate cross sections for inclusive dijet photoproduction in electron-nucleus scattering in the kinematics of the future EIC and the possible LHeC, HE-LHeC, and the FCC using next-to-leading order (NLO) perturbative QCD and nCTEQ15 and EPPS16 nuclear parton density functions (nPDFs). We make predictions for distributions in the dijet average transverse momentum ${\bar p}_T$, the average rapidity $\bar{\eta}$, the observed nuclear momentum fraction $x_A^{\rm obs}$, and the observed photon momentum fraction $x_{\gamma}^{\rm obs}$. Comparing the kinematic reaches of the four colliders, we find that an increase of the collision energy from the EIC to the LHeC and beyond extends the coverage in all four considered variables. Notably, the LHeC and HE-LHeC will allow one to probe the dijet cross section down to $x_A^{\rm obs} \sim 10^{-4}$ (down to $x_A^{\rm obs} \sim 10^{-5}$ at the FCC). The ratio of the dijet cross sections on a nucleus and the proton, $\sigma_A/(A\sigma_p)$, depends on $x_A^{\rm obs}$ in a similar way as the ratio of gluon densities, $g_A(x_A,\mu^2)/[A g_p(x_A,\mu^2)]$, for which current nPDFs predict a strong suppression due to nuclear shadowing in the region $x_A^{\rm obs} < 0.01$. Dijet photoproduction at future lepton-nucleus colliders can therefore be used to test this prediction and considerably reduce the current uncertainties of nPDFs.

Nuclear form factor of xenon from photoproduction of vector mesons in Xe-Xe ultraperipheral collisions at the LHC

Using the Gribov-Glauber model for photon-nucleus scattering and a generalization of the vector meson dominance model for the hadronic structure of the photon, we calculate cross sections of light and heavy vector meson photoproduction in ultraperipheral Xe-Xe collisions at 5.44 TeV at the Large Hadron Collider. Analyzing the momentum transfer distribution in this process, we examine the feasibility to extract the nuclear isoscalar form factor of Xe isotopes, which are needed in searches for dark matter with Xenon-based detectors.

Development of an accurate DWIA model of coherent π0 —photoproduction to study neutron skins in medium heavy nuclei

Despite decades of studies which have seen the nuclear charge distribution being measured with increasing precision, the neutron distribution remains elusive. The difference between the neutron and proton distributions is often expressed as the difference of their root mean square radii: the neutron skin thickness. Recently, the A2 collaboration at MaMi has measured the skin thickness in lead through coherent pion photoproduction [1] with a very high precision. However, they do not include theoretical uncertainties, which can be significant for this process.A new reaction code in the distorted wave impulse approximation (DWIA) is developed to help the (ongoing) analysis of the recent measurement by the A2 collaboration at MaMi of the coherent pion photoproduction cross section on 116,120,124Sn isotopes [2] and to properly quantify the theoretical uncertainties.

Deciphering the mechanism of near-threshold J/psi photoproduction

The photoproduction of the J/psi off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a J/psi which rescatters with the proton to give the J/psi p final state. In this paper, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the varLambda _c bar{D}^{(*)} intermediate states. In particular, there must be cusp structures at the varLambda _c bar{D}^{(*)} thresholds in the energy dependence of the J/psi photoproduction cross section. The same mechanism also implies the J/psi -nucleon scattering lengths of order 1 mfm. Given this, one expects only a minor contribution of charm quarks to the nucleon mass.

The cos 2\u03d5 azimuthal asymmetry in \u03c10 meson production in ultraperipheral heavy ion collisions

We present a detailed study of vector meson photoproduction in ultraperipheral heavy ion collisions (UPCs). Using the dipole model, we develop a framework for the joint impact parameter and transverse momentum dependent cross sections. We compute the unpolarized cross section and cos 2ϕ azimuthal angular correlation for ρ0 photoproduction with ϕ defined as the angle between the ρ0’s transverse momentum and its decay product pion meson’s transverse momentum. Our result on unpolarized coherent differential cross section gives excellent description to the STAR experimental data. A first compari- son between theoretical calculation and experimental measurement on the cos 2ϕ azimuthal asymmetry, which results from the linearly polarized photons, is performed and reasonable agreement is reached. We find out the characteristic diffractive patterns at both RHIC and LHC energies and predict the impact parameter dependent cos 2ϕ azimuthal asymmetries for ρ0 photoproduction by considering UPCs and peripheral collisions. The future experimental measurements at RHIC and LHC relevant to our calculations will provide a tool to rigorously investigate the coherent and incoherent production of vector meson in UPCs, as well as to probe the nuclear structure in heavy ion collisions.

Possibility of observation of hidden-bottom pentaquark resonances in bottomonium photoproduction on protons and nuclei near threshold

We study the meson photoproduction on protons and nuclei at near-threshold center-of-mass energies below 11.4 GeV (or at the corresponding photon laboratory energies below 68.8 GeV). We calculate the absolute excitation functions for the nonresonant and resonant photoproduction of mesons off protons at incident photon laboratory energies of 63-68 GeV by considering direct ( ) and two-step ( , , ) production channels within different scenarios for the nonresonant total cross section of the elementary reaction and for branching ratios of the decays , , and . We also calculate an analogous function for the photoproduction of mesons on the 12C and 208Pb target nuclei in the near-threshold center-of-mass beam energy region of 9.0-11.4 GeV by considering the respective incoherent direct ( ) and two-step ( , , and , , ) ) production processes using a nuclear spectral function approach. We demonstrate that a detailed scan of the total photoproduction cross section on proton and nuclear targets in the near-threshold energy region in future high-precision experiments at the proposed high-luminosity electron-ion colliders EIC and EicC in the US and China should provide a definite result for or against the existence of the nonstrange hidden-bottom pentaquark states and ( =1, 2, 3) as well as clarify their decay rates.

Exclusive tensor meson photoproduction

We study tensor meson photoproduction outside of the resonance region, at beam energies of few GeVs. We build a model based on Regge theory that includes the leading vector and axial exchanges. We consider two determinations of the unknown helicity couplings, and fit to the recent a2 photoproduction data from CLAS. Both choices give a similar description of the a2 cross section, but result in different predictions for the parity asymmetries and the f2 photoproduction cross section. We conclude that new measurements of f2 photoproduction in the forward region are needed to pin down the correct production mechanism. We also extend our predictions to the 8.5 GeV beam energy, where current experiments are running.

Coherent photoproduction of \u03c10 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV

Cross sections for the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV are reported. The measurements, which rely on the π+π− decay channel, are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c2 and a width of about 140 MeV/c2 is reported and compared with similar observations from other experiments.

A phenomenological analysis of the nonperturbative QCD contributions for the photon wave function

The photon – induced interactions, present in ep, eA, pp, pA, AA and e^+ e^- collisions, are expressed within the color dipole approach in terms of the photon wave function, which describes the transition of the photon into a quark – antiquark color dipole. Such quantity is usually calculated using perturbation theory assuming that long distance corrections associated to strong interactions can be neglected. In this paper we investigate the impact of these nonperturbative QCD (npQCD) corrections to the description of the photon wave function for dipoles of large size in several observables measured at HERA, LEP and LHC. We assume a phenomenological ansatz for the treatment of these npQCD corrections and constrain the free parameters of our model using the experimental data for the photoproduction cross section. The predictions for the gamma gamma cross section, exclusive rho production in ep collisions and the rapidity distribution for the rho production in PbPb collisions are compared with the data. We demonstrate that the inclusion of the nonperturbative QCD corrections improves the description of processes that are dominated by large dipoles.

Precision measurement of the neutral pion lifetime.

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π0 into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the [Formula: see text] process. The differential cross sections for π0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding [Formula: see text], where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of [Formula: see text] Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.

D-wave effects in diffractive electroproduction of heavy quarkonia from the photon-like Vrightarrow Q{bar{Q}} transition

We analyse the validity of a commonly used identification between structures of the virtual photon gamma ^*rightarrow Qbar{Q} and vector meson Vrightarrow Q{bar{Q}} transitions. In the existing studies of S-wave vector-meson photoproduction in the literature, such an identification is typically performed in the light-front (LF) frame while the radial component of the meson wave function is rather postulated than computed from the first principles. The massive photon-like Vrightarrow Q{bar{Q}} vertex, besides the S-wave component, also contains an extra D-wave admixture in the Q{bar{Q}} rest frame. However, the relative weight of these contributions cannot be justified by any reasonable nonrelativistic Q{bar{Q}} potential model. In this work, we investigate the relative role of the D-wave contribution starting from the photon-like quarkonium Vrightarrow Q{bar{Q}} transition in both frames: in the Q{bar{Q}} rest frame (with subsequent Melosh spin transform to the LF frame) and in the LF frame (without Melosh transform). In both cases, we employ the same radial wave functions found in the potential approach. We show that the photon-like transition imposed in the Q{bar{Q}} rest frame, with subsequent boosting to the LF frame, leads to significant discrepancies with the experimental data. In the second case, i.e. imposing the photon-like transition straight in the LF frame, we find that the corresponding total J/ psi (1S) photoproduction cross sections are very close to those obtained with the “S-wave only” Vrightarrow Q{bar{Q}} transition, both leading to a good description of the data. However, we find that the “S-wave only” transition leads to a better description of photoproduction data for excited (e.g. psi '(2S)) heavy quarkonium states, which represent a more effective tool for study of D-wave effects. Consequently, the predictions for production of excited states based on the photon-like structure of Vrightarrow Q{bar{Q}} transition should be treated with a great care due to a much stronger sensitivity of the D-wave contribution to the nodal structure of quarkonium wave functions.

Coherent J/ψ photoproduction at forward rapidity in ultra-peripheral Pb–Pb collisions at [formula omitted] TeV

The ALICE collaboration performed the first rapidity-differential measurement of coherent J/ψ photoproduction in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV. The J/ψ is detected via its dimuon decay in the forward rapidity region (−4.0<y<−2.5) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 μb−1. The cross section for coherent J/ψ production is presented in six rapidity bins. The results are compared with theoretical models for coherent J/ψ photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of ψ′ to J/ψ coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons.

Inclusive dijet photoproduction in ultraperipheral heavy ion collisions at the CERN Large Hadron Collider in next-to-leading order QCD

We compute the cross section of inclusive dijet photoproduction in ultraperipheral Pb-Pb collisions at the LHC using next-to-leading order perturbative QCD. We demonstrate that our theoretical calculations provide a good description of various kinematic distributions measured by the ATLAS collaboration. We find that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of parton distribution functions (PDFs) at the level of 10 to 20%. Hence, this process can be used to reduce uncertainties in the determination of these nuclear PDFs, whose current magnitude is comparable to the size of the calculated nuclear modifications of the dijet photoproduction cross section.

Coherent and Incoherent Elastic Photoproduction of π0 Meson on Light Nuclei

The paper presents calculations of the cross sections of the elastic photoproduction for π0 meson on 6Li, 7Li and 9Be nuclei in terms of the impulse approximation using the intermediate coupling p-shell model. Measurements are performed in the excitation energy region of Δ(1232)3/2+ resonance. The algorithm of the cross section selection is proposed for γ7Li → π07Li reaction. These calculation results are in good agreement with the experimental data on the elastic photoproduction cross sections for π0 meson on 7Li nucleus obtained by other authors [2].

Constraints on nuclear parton distributions from dijet photoproduction at the LHC

Using QCD calculations of the cross section of inclusive dijet photoproduction in Pb–Pb ultraperipheral collisions in the LHC kinematics as pseudo-data, we study the effect of including these data using the Bayesian reweighting technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear parton distribution functions (nPDFs). We find that, depending on the assumed error of the pseudo-data, it leads to a significant reduction of the nPDF uncertainties at small values of the momentum fraction x_A. Taking the error to be 5%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs reduces approximately by a factor of two at x_A=10^{-3}. At the same time, the reweighting effect on EPPS16 nPDFs is much smaller due to the higher value of the tolerance and a more flexible parametrization form.

Energy dependence of exclusive mathrm {J}/psi photoproduction off protons in ultra-peripheral p\u2013Pb collisions at sqrt{s_{mathrm {scriptscriptstyle NN}}} = 5.02 TeV

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of mathrm {J}/psi vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair sqrt{s_mathrm{NN}} = 5.02 TeV. The e^+e^- and mu ^+mu ^- decay channels are used to measure the cross section as a function of the rapidity of the mathrm {J}/psi in the range -2.5< y < 2.7, corresponding to an energy in the gamma p centre-of-mass in the interval 40< W_{gamma mathrm {p}}<550 GeV. The measurements, which are consistent with a power law dependence of the exclusive mathrm {J}/psi photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.

Measurement of exclusive mathrm {Upsilon } photoproduction from protons in mathrm {p}Pb collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {TeV}

The exclusive photoproduction of mathrm {Upsilon }mathrm {(nS)} meson states from protons, gamma mathrm {p} rightarrow mathrm {Upsilon }mathrm {(nS)} ,mathrm {p} (with mathrm {n}=1,2,3), is studied in ultraperipheral mathrm {p}Pb collisions at a centre-of-mass energy per nucleon pair of sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {TeV} . The measurement is performed using the mathrm {Upsilon }mathrm {(nS)} rightarrow mu ^+mu ^- decay mode, with data collected by the CMS experiment corresponding to an integrated luminosity of 32.6,text {nb}^{-1}. Differential cross sections as functions of the mathrm {Upsilon }mathrm {(nS)} transverse momentum squared p_{mathrm {T}} ^2, and rapidity y, are presented. The mathrm {Upsilon (1S)} photoproduction cross section is extracted in the rapidity range |y |< 2.2, which corresponds to photon–proton centre-of-mass energies in the range 91<W_{gamma mathrm {p}} <826,text {GeV} . The data are compared to theoretical predictions based on perturbative quantum chromodynamics and to previous measurements.

Spin rotation effects in diffractive electroproduction of heavy quarkonia

In this work we present for the first time the comprehensive study of the Melosh spin rotation effects in diffractive electroproduction of S-wave heavy quarkonia off a nucleon target. Such a study has been performed within the color dipole approach using, as an example and a reference point, two popular parametrizations of the dipole cross section and two potentials describing the interaction between Q and {bar{Q}} and entering in the Schrödinger equation based formalism for determination of the quarkonia wave functions. We find a strong onset of spin rotation effects in 1S charmonium photoproduction which is obviously neglected in present calculations of corresponding cross sections. For photoproduction of radially excited psi '(2S) these effects are even stronger leading to an increase of the photoproduction cross section by a factor of 2div 3 depending on the photon energy. Even in production of radially excited Upsilon '(2S) and Upsilon ''(3S) they can not be neglected and cause the 20–30% enhancement of the photoproduction cross section. Finally, we predict that the spin effects vanish gradually with photon virtuality Q^2 following universality properties in production of different heavy quarkonia as a function of Q^2 + M_V^2.