Photoproduction Cross Section Research Articles

Towards clarifying the possibility of observation of the LHCb hidden-charm pentaquarks [formula omitted], [formula omitted], [formula omitted] and [formula omitted] in near-threshold charmonium photoproduction off protons and nuclei

We study the near-threshold J/ψ meson photoproduction from protons and nuclei by considering incoherent direct non-resonant (γp→J/ψp, γn→J/ψn) and two-step resonant (γp→Pci+→J/ψp, γn→Pci0→J/ψn, i=1, 2, 3, 4; Pc1+,0=Pc+,0(4312), Pc2+,0=Pc+,0(4337), Pc3+,0=Pc+,0(4440), Pc4+,0=Pc+,0(4457)) charmonium production processes. We calculate the absolute excitation functions, energy and momentum distributions for the non-resonant, resonant and for the combined (non-resonant plus resonant) production of J/ψ mesons on protons as well as, using the nuclear spectral function approach, on carbon and tungsten target nuclei at near-threshold incident photon energies by assuming the spin-parity assignments of the hidden-charm resonances Pc+,0(4312), Pc+,0(4337), Pc+,0(4440) and Pc+,0(4457) as JP=(1/2)−, JP=(1/2)−, JP=(1/2)− and JP=(3/2)− within three different realistic scenarios for the branching ratios of their decays to the J/ψp and J/ψn modes (0.25, 0.5 and 1%). We show that will be very hard to measure the Pci+ pentaquark states through the scan of the J/ψ total photoproduction cross section on a proton target in the near-threshold energy region around the resonant photon energies of 9.44, 9.554, 10.04 and 10.12 GeV if these branching ratios ∼ 1% and less. We also demonstrate that at these photon beam energies the J/ψ energy and momentum combined distributions considered reveal distinct sensitivity to the above scenarios, respectively, at “low” J/ψ total energies and momenta, which implies that they may be an important tool to provide further evidence for the existence of the pentaquark Pci+ and Pci0 resonances and to get valuable information on their decay rates to the J/ψp and J/ψn final states. The measurements of these distributions could be performed in the future at the 12 GeV CEBAF facility.

Exclusive J/ψ photoproduction in ultraperipheral Pb+Pb collisions at the CERN Large Hadron Collider calculated at next-to-leading order perturbative QCD

We present the first next-to-leading-order (NLO) perturbative QCD (pQCD) study of rapidity-differential cross sections of coherent exclusive photoproduction of $J/\psi$ mesons in heavy-ion ultraperipheral collisions (UPCs) at the LHC, $d\sigma/dy(\text{Pb} + \text{Pb} \rightarrow \text{Pb} + J/\psi + \text{Pb})$. For this, we account for the photon-nucleon NLO cross sections at the forward limit, the $t$ dependence using a standard nuclear form factor, and the photon fluxes of the colliding nuclei. Approximating the generalized parton distributions with their forward-limit parton distribution functions (PDFs), we quantify the NLO contributions in the cross sections, show that the real part of the amplitude and quark-PDF contributions must not be neglected, quantify the uncertainties arising from the scale-choice and PDFs, and compare our results with ALICE, CMS and LHCb $J/\psi$ photoproduction data in Pb+Pb UPCs, exclusive $J/\psi$ photoproduction data from HERA, and LHCb data in p+p. The scale dependence in $d\sigma/dy(\text{Pb} + \text{Pb} \rightarrow \text{Pb} + J/\psi + \text{Pb})$ is significant, but we can find a scale-choice that reproduces the Pb+Pb UPC data both at 2.76 and 5.02 TeV collision energies. This process has traditionally been suggested to be a direct probe of nuclear gluon distributions. We show that the situation changes rather dramatically from LO to NLO: the NLO cross sections reflect the nuclear effects of both gluons and quarks in a complicated manner where the relative signs of the LO and NLO terms in the amplitude play a significant role.

How to measure the magnetic field in relativistic heavy-ion collisions

The intense magnetic field created in heavy-ion collisions has often been calculated with classical electrodynamics. This description is thought to be accurate if the occupation number of every field mode is high enough. However, this hypothesis has never been tested. Recently, we have studied a process involving the magnetic field, which we believe to be the simplest: pion production in ultraperipheral collisions. We have started using the classical approximation to study the forward pion production by the magnetic excitation of nucleons through the process N → Δ → N + π. Then, in a subsequent work, we have replaced this classical field by a flux of photons and have calculated the cross section for photoproduction of pions, which is the quantum version of magnetic excitation. We compared the cross sections obtained with both formalisms, which turned out to be compatible with each other. In this note, we review those works, enlarge the discussion and suggest some improvements that can be done to make our results more accurate. We emphasize that these pions could be detected by the ZDC’s installed at ATLAS and ALICE. Since the theoretical uncertainties are under control, this detection could be used to perform an indirect measurement of the magnetic field intensity.

Inclusive and diffractive dijet photoproduction in ultraperipheral heavy ion collisions at the LHC

We calculate the cross section of inclusive dijet photoproduction in ultraperipheral collisions (UPCs) of heavy ions at the CERN Large Hadron Collider using next-to-leading order perturbative QCD and demonstrate that it provides a good description of the ATLAS data. We study the role of this data in constraining nuclear parton distribution functions (nPDFs) using the Bayesian reweighting technique and find that it can reduce current uncertainties of nPDFs at small x by a factor of 2. We also make predictions for diffractive dijet photoproduction in UPCs and examine its potential to shed light on the disputed mechanism of QCD factorization breaking in diffraction.

Measurement of the helicity dependence for single $$\pi ^{0}$$ photoproduction from the deuteron

The helicity-dependent single $\pi^{0}$ photoproduction cross section on the deuteron and the angular dependence of the double polarisation observable $E$ for the quasi-free single $\pi^0$ production off the proton and the neutron have been measured for the first time from the threshold region up to the photon energy 1.4 GeV. The experiment was performed at the tagged photon facility of the MAMI accelerator and used a circularly polarised photon beam and longitudinally polarised deuteron target. The reaction products were detected using the large acceptance Crystal Ball/TAPS calorimeter, which covered 97% of the full solid angle. Comparing the cross section from the deuteron with the sum of free nucleon cross sections provides a quantitative estimate of the effects of the nuclear medium on pion production. In contrast, comparison of $E$ helicity asymmetry data from quasi-free protons off deuterium with data from a free proton target indicates that nuclear effects do not significantly affect this observable. As a consequence, it is deduced that the helicity asymmetry $E$ on a free neutron can be reliably extracted from measurements on a deuteron in quasi-free kinematics.

Hybrid model of proton structure functions

We develop a ``hybrid'' model of the proton inelastic structure functions applicable in a wide region of invariant mass of produced states $W$ and invariant momentum transfer $Q$ including deep inelastic scattering (DIS), nucleon resonance production, as well as the region close to inelastic threshold. DIS is described in terms of the parton distributions together with higher-twist corrections from an available global QCD fit. The resonant part is addressed in terms of the Breit-Wiegner contributions from five states including the $\mathrm{\ensuremath{\Delta}}(1232)$ resonance, the $N(1440)$ Roper resonance, and three effective resonances describing the second and third resonance regions. The couplings of the nucleon resonances to photon are described in terms of helicity amplitudes. The nonresonant background is addressed in terms of DIS structure functions smoothly extrapolated to low-$W$ and low-$Q$ values with the proper behavior at the real photon limit ${Q}^{2}=0$ as well as near the inelastic threshold. We independently treat the transverse ${F}_{T}$ and the longitudinal ${F}_{L}$ structure function and fix the model parameters from a global analysis of the world hydrogen electroproduction and photoproduction cross-section data. We demonstrate a very good performance of the model by comparing our predictions with data on differential cross sections and the structure functions ${F}_{2}$ and $R={F}_{L}/{F}_{T}$.

Coherent photoproduction of heavy quarkonia on nuclei

The differential cross section of coherent photo-production of heavy quarkonia on nuclear targets is calculated within the QCD color dipole formalism. The higher-twist nuclear shadowing corresponding to the $\bar QQ$ Fock component of the photon, is calculated including the correlation between dipole orientation $\vec r$ and impact parameter of a collision $\vec b$, which is related to the transverse momentum transfer via Fourier transform. We also included the leading twist gluon shadowing corresponding to higher Fock components of the photon containing gluons, which have specifically short coherence time, especially for multi-gluon components, even at very high energies. The contribution of such fluctuating gluonic dipole is calculated employing the path-integral technique. Our results are in good agreement with recent ALICE data on charmonium production in ultra-peripheral nuclear collisions.

Quarkonia as probes of the QGP and of the initial stages of the heavy-ion collision with ALICE

Studies of quarkonium production in heavy-ion collisions can be used for probing QGP properties. The suppression and regeneration of bound quarkonium states is sensitive to the medium properties. Modifications of the quarkonium polarization in Pb–Pb collisions with respect to pp collisions may give further insight into the suppression and regeneration mechanisms in the QGP. Quarkonia are also sensitive to the initial stages of heavy-ion collisions, and measurements in photonuclear collisions may help constrain the nuclear gluon-distribution at low Bjorken-x. In this work, recent quarkonium measurements performed by ALICE in Pb–Pb collisions at √SNN = 5.02 TeV will be discussed. Preliminary measurements of the inclusive J/ψ RAA measured at both forward and central rapidity will be presented. The J/ψ polarization measured for the first time in Pb–Pb collisions, as well as preliminary measurements of the coherent J/ψ photoproduction cross section, will be discussed.

Coherent pion photoproduction on spin-zero nuclei

We study the possibility of building a universal model describing pion scattering and photoproduction on spin-zero nuclei within the same framework. We show our prediction for photoproduction cross section based on parameters taken from our fit to pion-12C scattering data and demonstrate the importance of the second-order effects arising from the multiple scattering theory.

How large is the diffractive contribution to inclusive dijet photoproduction in ultraperipheral collisions at the LHC?

Using next-to-leading order (NLO) perturbative QCD, we calculate the diffractive contribution to inclusive dijet photoproduction in Pb-Pb ultraperipheral collisions (UPCs) at the LHC and find that it does not exceed $5-10$% in small-$x_A$ bins in the ATLAS kinematics at $\sqrt{s_{NN}}=5.02$ TeV. Its smallness is a result of the restricted kinematics ($p_{T1} > 20$ GeV and $x_A > 0.001$), the large nuclear suppression of nuclear diffractive parton distribution functions predicted in the leading twist model of nuclear shadowing, and additional suppression due to QCD factorization breaking in diffraction. At the same time, using looser $p_T$ cuts, e.g., $p_{T1} > 10$ GeV and $p_{T i\neq 1} > 5-7$ GeV, we find that $(d\sigma_{\rm diff}/dx_A)/(d\sigma_{\rm inc}/dx_A)$ can reach $10-20$% at $x \approx 5 \times 10^{-4}$. Also, applying our framework to proton-proton UPCs at $\sqrt{s_{NN}}=13$ TeV, we find that the ratio of the diffractive and inclusive cross sections of dijet photoproduction $(d\sigma_{\rm diff}/dx_p)/(d\sigma_{\rm inc}/dx_p)$ is as large as $10-15$% for $x_p \sim 5 \times 10^{-4}$. An account of the contribution of Pomeron-Pomeron scattering, which is not included in our analysis, should make this ratio somewhat larger.

Total hadronic and photonic cross sections and the nuclear configurational entropy concept

In the framework of eikonalized mini-jet model, contributions from gluon recombination effects are studied for the free proton in pp and \(\bar{p} p\) reactions. The nuclear configurational entropy is then used to calculate the main parameters to estimate the total proton cross sections for the recent data LHC, also corroborating to previous developments. Photoproduction cross sections with vector meson dominance and saturation effects for both total hadronic and photonic cross sections at a high-energy regime are investigated. Extreme points of the configurational entropy can shed new light on some features of specific models of nuclear reactions, which are described by the corresponding reaction cross section, providing the latter by the necessary model parameters within an optimal root-mean-square deviation.

Coherent mathrm{J}/psi \xa0and {uppsi '}\xa0photoproduction at midrapidity in ultra-peripheral Pb\u2013Pb \xa0collisions at sqrt{s_{mathrm {NN}}}~=~5.02\xa0TeV

The coherent photoproduction of mathrm{J}/psi and {uppsi '} mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy sqrt{s_{mathrm {NN}}}~=~5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The mathrm{J}/psi is reconstructed using the dilepton (l^{+} l^{-}) and proton–antiproton decay channels, while for the {uppsi '} the dilepton and the l^{+} l^{-} pi ^{+} pi ^{-} decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 mu {mathrm{b}}^{-1}. The results are compared with theoretical models for coherent mathrm{J}/psi and {uppsi '} photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6times 10^{-4}, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent mathrm{J}/psi cross section including ALICE measurements at forward rapidity. The ratio of {uppsi '} to mathrm{J}/psi coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.

First measurement of coherent ρ0 photoproduction in ultra-peripheral Xe–Xe collisions at [formula omitted] TeV

The first measurement of the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Xe–Xe collisions at sNN=5.44 TeV is presented. This result, together with previous HERA γp data and γ–Pb measurements from ALICE, describes the atomic number (A) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+Xe→ρ0+Xe+Xe process, measured at midrapidity through the decay channel ρ0→π+π−, is found to be dσ/dy=131.5±5.6(stat.)−16.9+17.5(syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent ρ0 photoproduction at a centre-of-mass energy per nucleon of the γA system of WγA,n=65 GeV is found to be consistent with a power-law behaviour σ(γA→ρ0A)∝Aα with a slope α=0.96±0.02(syst.). This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.

Exclusive J/Ψ and Ψ(2s) photo-production as a probe of QCD low x evolution equations

We investigate photo-production of vector mesons J/Psi and Psi(2s), based on both HERA and LHC data, using 2 fits of unintegrated gluon distributions. The latter are subject to non-linear Balitsky-Kovchegov evolution (Kutak-Sapeta gluon; KS) and linear next-to-leading order Balitsky-Kuraev-Fadin-Lipatov evolution (Hentschinski-Sabio Vera-Salas; HSS gluon) respectively. Apart from extending previous studies to the case of radially excited charmonium Psi(2s), we further use an improved set of charmonium wave functions and provide an estimate of the uncertainties associated with the HSS gluon. While we observe that the difference between linear and non-linear evolution somehow diminishes and a clear distinction between both HSS and KS gluon is no longer possible using the currently available data-set, we find that the differences between both gluon distributions are enhanced for the ratio of the photo-production cross-sections of Psi(2s) and J/Psi vector mesons.

$$J/\psi $$-p scattering length from the total and differential photoproduction cross sections

The $$J/\psi $$ -p scattering length, $$\alpha _{J/\psi p}$$ , can be extracted from the $$J/\psi $$ photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction $$\gamma p \rightarrow J/\psi p$$ to $$J/\psi p \rightarrow J/\psi p$$ . Such estimates based on experimental data result in values for $$|\alpha _{J/\psi p}|$$ , which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample, such as the near threshold $$J/\psi $$ photoproduction dataset. We estimate a range for $$|\alpha _{J/\psi p}|$$ of 0.003–0.025 fm as extracted from experimental data within the VMD model and discuss possible reasons for such lower values compared to the theoretical results.

Photoproduction of strange hidden-charm and hidden-bottom states

Recently BESIII collaboration discovered a charged strange hidden-charm state Z_{cs}(3985) in the D_s^-D^{*0} + D_s^{*-}D^{0} spectrum. A higher Z'_{cs} state coupling to {bar{D}}_s^{*-}D^{*0} is expected by SU(3)-flavor symmetry, and their bottom partners are anticipated by heavy quark flavor symmetry. Here we study the photoproduction of these exotic states and investigate carefully the background from Pomeron exchange. Our results indicate that the maximal photoproduction cross section of strange partner is around 1–2 orders of magnitude smaller than that of the corresponding non-strange states. The possibility of searching for them in future electron-ion colliders (EIC) is briefly discussed.

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.