Photoproduction In Ultra-peripheral Collisions Research Articles

Predictions for exclusive varUpsilon photoproduction in ultraperipheral {textrm{Pb}}+{textrm{Pb}} collisions at the LHC at next-to-leading order in perturbative QCD

We present predictions for the rapidity-differential cross sections of exclusive varUpsilon photoproduction in ultraperipheral collisions (UPCs) of lead ions at the Large Hadron Collider (LHC). We work in the framework of collinear factorization at next-to-leading order (NLO) in perturbative QCD, modeling the generalized parton distributions (GPDs) through the Shuvaev transform of nuclear parton distribution functions (nPDFs). Our direct NLO predictions depend significantly on nPDF uncertainties and on the choices of the factorization and renormalization scales, but to a much lesser degree on GPD modeling. To tame the scale dependence and to account for the fact that the NLO calculations generally underpredict the photoproduction measurements on protons, we also present alternative, data-driven predictions. In this approach the underlying photoproduction cross sections on lead are found by combining their nuclear modifications calculated at NLO with the measured photoproduction cross sections on protons. The data-driven strategy reduces the uncertainties associated with the scale choices, and essentially eliminates the effects of GPD modeling thereby leaving the cross sections sensitive mainly to the input nPDFs. Our estimates indicate that the process is measurable in {textrm{Pb}}+{textrm{Pb}} collisions at the LHC.

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.

Latest ALICE results on J/$ψ$ photoproduction in ultra-peripheral collisions at the LHC

Ultra-peripheral collisions (UPCs), studied using the ALICE detector, allow us to investigate the low-\mathbf{x}𝐱 behavior of the gluon distribution of the colliding particles.Two new measurements of coherent J/\mathbf{\psi}𝛙 photoproduction cross section from Pb–Pb UPCs at \mathbf{\sqrt{s_{NN}}}=5.02𝐬𝐍𝐍=5.02 TeV are presented: the first measurement of the \mathbf{|t|}|𝐭|-dependence of the cross section providing a new tool to investigate the transverse gluonic structure at low Bjorken-\mathbf{x}𝐱 and a rapidity-differential measurement at midrapidity allowing us to provide stringent constraints on nuclear gluon shadowing and saturation models.In addition, prospects for heavy vector meson photoproduction measurements in LHC Run 3 and 4 are presented.

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.

Momentum transfer squared dependence of exclusive quarkonia photoproduction in ultraperipheral collisions

In this paper, we study fully differential quarkonia photoproduction observables in ultraperipheral collisions (UPCs) as functions of momentum transfer squared. We employ the dipole picture of the QCD part of the scattering with proton and nucleus targets, with the projectile being a quasi-real photon flux emitted by an incoming hadron. We analyse such observables for ground $J/\psi$, $\Upsilon(1S)$ and excited $\psi'$, $\Upsilon(2S)$ states whose light-front wave functions are obtained in the framework of interquark potential model incorporating the Melosh spin transformation. Two different low-$x$ saturation models, one obtained by solving the Balitsky--Kovchegov equation with the collinearly improved kernel and the other with a Gaussian impact-parameter dependent profile, are used to estimate the underlined theoretical uncertainties of our calculations. The results for the proton target and with charmonium in the final state are in agreement with the available HERA data, while in the case of nucleus target we make predictions for $\gamma A$ and $AA$ differential cross sections at different $W$ and at $\sqrt{s}=5.02$ TeV, respectively.

Dijet Photoproduction in Ultraperipheral Collisions at the LHC and Nuclear PDFs at Small x

Dijet Photoproduction in Ultraperipheral Collisions at the LHC and Nuclear PDFs at Small x

Single-transverse-spin asymmetries in exclusive photo-production of J/ψ in ultra-peripheral collisions in the fixed-target mode at the LHC and in the collider mode at RHIC

We investigate the potentialities offered by the study of J/ψ exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected cross sections in both set-ups. Studying Single-Transverse-Spin Asymmetries (AN) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) Eg(x,ξ,t). We evaluate the expected precision on AN for realistic conditions with the LHCb detector in pH↑ and PbH↑ collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.

Heavy Vector Meson Photoproduction in Ultra-peripheral Collisions at the LHC

Heavy Vector Meson Photoproduction in Ultra-peripheral Collisions at the LHC

Photoproduction of heavy vector mesons in ultra-peripheral Pb–Pb collisions

Ultra-peripheral Pb-Pb collisions, in which the two nuclei pass close to each other, but at an impact parameter greater than the sum of their radii, provide information about the initial state of nuclei. In particular, heavy vector meson production, where the particle mass sets a hard scale, proceeds in such collisions by photon-gluon interactions, and gives access to nuclear PDFs. The ALICE collaboration has published measurements of J/ψ and ψ(2S) photoproduction in ultra-peripheral collisions in LHC Run 1 at forward (J/ψ) and mid-rapidity, and has obtained a substantially larger data set in 2015 from LHC Run 2, allowing much more detailed studies of the production mechanism to be performed. In particular, the increased energy and more detailed measurements in the forward region in Run 2 give access to significantly lower values of Bjorken-x than in previous studies. In this talk, the latest available results from Run 2 will be given.

Diffractive dijet photoproduction in ultraperipheral collisions at the LHC in next-to-leading order QCD

We make predictions for the cross sections of diffractive dijet photoproduction in $pp$, $pA$ and $AA$ ultraperipheral collisions (UPCs) at the LHC during Runs 1 and 2 using next-to-leading perturbative QCD. We find that the resulting cross sections are sufficiently large and, compared to lepton-proton scattering at HERA, have an enhanced sensitivity to small observed momentum fractions in the diffractive exchange, commonly denoted $z_{P}^{\rm jets}$, and an unprecedented reach in the invariant mass of the photon-nucleon system $W$. We examine two competing schemes of diffractive QCD factorization breaking, which assume either a global suppression factor or a suppression for resolved photons only and demonstrate that the two scenarios can be distinguished by the nuclear dependence of the distributions in the observed parton momentum fraction in the photon $x_{\gamma}^{\rm jets}$.

Evidence of subnucleonic degrees of freedom inJ/ψphotoproduction in ultraperipheral collisions at energies available at the CERN Large Hadron Collider

We present calculations for the incoherent photoproduction of J/ψ vector mesons in ultraperipheral heavy ion collisions (UPCs) in terms of hadronic interactions. This study was carried out using the recently developed Monte Carlo model CRISP extended to include UPCs at energies available at the CERN Large Hadron Collider. A careful study of rescattering and destruction of the J/ψ particles is presented for Pb+ Pb collisions at √ sNN = 2.76 TeV. We have also compared our method to Au+Au collisions at√sNN = 200 GeV measured at the BNL Relativistic Heavy Ion Collider.

Inelastic heavy quark and quarkonium ultra-incoherent photoproduction in ultra-peripheral collisions

The inelastic heavy quark and quarkonium photoproduction through the ultra-incoherent photon channel in ultra-peripheral collisions is studied. Considering the ultra-relativistic hadrons as a beam of freely moving elementary constituents, the inclusive photoproduction cross sections for charm quark, bottom quark, $J/\ensuremath{\psi}$, and $\mathrm{\ensuremath{\Upsilon}}(1S)$ are calculated by using the incoherent photon spectrum from the individual quarks in the nucleus. Comparing with the predictions of the coherent photon channel in the literatures, it is shown that the ultra-incoherent photon channel provides non-negligible contributions to the heavy quarks and heavy quarkonia photoproduction in ultra-peripheral collisions.

Diffractive Higgs boson photoproduction in ultraperipheral collisions at LHC

A new production mechanism for the Standard Model Higgs boson in Ultraperipheral Collisions at LHC, which allows Central Exclusive Diffractive production by Double Pomeron Exchange in photon-proton processes, is presented. The Higgs boson is centrally produced by gluon fusion with two Large Rapidity Gaps emerging in the final state, being the main experimental signature for this process. As already studied for Pomeron-Pomeron and two-photon processes, the Higgs boson photoproduction is studied within this new mechanism in proton-proton ($pp$) and proton-nucleus ($pA$) collisions, where each system has a different dynamics to be taken into account. As a result, this mechanism predicts a production cross section for $pp$ collisions of about 1.8 fb, which is similar to that obtained in Pomeron-Pomeron processes. Besides, in $pPb$ collisions the cross sections have increased to about 0.6 pb, being comparable with the results of two-photon processes in $pAu$ collisions. Therefore, as the Rapidity Gap Survival Probability is an open question in high-energy Physics, an analysis for different values of this probability shows how competitive are the mechanisms in the LHC kinematical regime.