Diffractive Photoproduction Research Articles

Hard diffraction in SHERPA

We present the first complete simulation framework for the simulation of jet production in diffractive events at next-to leading order in QCD, matched to the parton shower. We validate the implementation in the SHERPA event generator with data from the H1 and ZEUS experiments for diffractive DIS and diffractive photoproduction. For the latter, we review different models aiming to explain the observed factorisation breaking and we argue that at NLO the direct component must also be suppressed. We provide predictions for diffractive jet production both in DIS and in photoproduction events for the upcoming EIC.

Gluon shadowing in nuclei and the role of the [formula omitted] state in the coherent photoproduction of J/ψ in nucleus-nucleus collisions

In this brief note, we confront our results on diffractive photoproduction of J/ψ mesons with the putative gluon shadowing ratio defined as Rg=σ(γA→J/ψA)/σIA., where σIA is the result in impulse approximation. Building on our earlier description of the process in the color-dipole approach, where we took into account the rescattering of cc¯ states only, we demonstrate that the inclusion of cc¯g-Fock states improves the description at small x commonly associated with gluon shadowing.

Helicity-flip transitions and the t-dependence of exclusive photoproduction of rho meson

We calculate the differential cross section dσ/dt for the diffractive photoproduction process γp → ρp and compare our results with experimental data. Our model is based on two-gluon exchange in the nonperturbative domain. We take into account both helicity conserving and often neglected helicity-flip amplitudes in the γ → V transition, which can contribute at finite t. The shape of the differential cross section as well as the role of helicity-flip processes is strongly related to the dependence of the unintegrated gluon distribution on transverse momenta in the nonperturbative region.

Tomography of ultrarelativistic nuclei with polarized photon-gluon collisions.

A linearly polarized photon can be quantized from the Lorentz-boosted electromagnetic field of a nucleus traveling at ultrarelativistic speed. When two relativistic heavy nuclei pass one another at a distance of a few nuclear radii, the photon from one nucleus may interact through a virtual quark-antiquark pair with gluons from the other nucleus, forming a short-lived vector meson (e.g., ρ0). In this experiment, the polarization was used in diffractive photoproduction to observe a unique spin interference pattern in the angular distribution of ρ0 → π+π- decays. The observed interference is a result of an overlap of two wave functions at a distance an order of magnitude larger than the ρ0 travel distance within its lifetime. The strong-interaction nuclear radii were extracted from these diffractive interactions and found to be 6.53 ± 0.06 fm (197Au) and 7.29 ± 0.08 fm (238U), larger than the nuclear charge radii. The observable is demonstrated to be sensitive to the nuclear geometry and quantum interference of nonidentical particles.

Exclusive production of ρ meson in gamma-proton collisions: dσ/dt and the role of helicity flip processes

We calculate the differential cross section dσ/dt for the diffractive photoproduction process γp→ρp and compare to recent data extracted by the CMS collaboration from ultraperipheral proton-lead collisions. Our model is based on two-gluon exchange in the nonperturbative domain. We take into account both helicity conserving and often neglected helicity-flip amplitudes in the γ→V transition. The latter can contribute at finite t. The shape of the differential cross section as well as the role of helicity flip processes is strongly related to the dependence of the unintegrated gluon distribution on transverse momenta in the nonperturbative region.

J/ψ near threshold in holographic QCD: A and D gravitational form factors

The diffractive photoproduction of $J/\Psi$ on a nucleon, is mostly due to gluonic exchanges at all $\sqrt{s}$. In holographic QCD (large number of colors and strong $^\prime$t Hooft coupling), these exchanges are captured by gravitons near threshold, and their Reggeized Pomeron form asymptotically. We revisit our holographic analysis of the A and D gravitational form factors in light of the new lattice data, and use them to refine our predictions for the photoproduction of $J/\Psi$ near threshold, and the comparison to the GlueX data. We use these results to estimate the scalar and mass radii of the nucleon, and describe the gravitational pressure and shear across a nucleon.

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.

Prospects for diffractive dijet photoproduction at the EIC

We discuss the prospects of diffractive dijet photoproduction at the EIC to distinguish different fits of diffractive proton PDFs, different schemes of factorization breaking, to determine diffractive nuclear PDFs and pion PDFs from leading neutron production.

Coherent photoproduction of $J/\psi$ in nucleus-nucleus collisions \\in the color dipole approach- an update

We discuss the role of c \bar c gcc‾g-Fock states in the diffractive photoproduction of J/\psiJ/ψ-mesons. We build on our earlier description of the process in the color-dipole approach, where we took into account the rescattering of c \bar ccc‾ pairs using a Glauber-Gribov form of the dipole-nucleus amplitude. We compare the results of our calculations to recent data on the photoproduction of J/\psiJ/ψ by the ALICE and LHCb collaborations.

Challenging exclusive top quark pair production at low and high luminosity LHC

The elastic production of top quark pairs in $pp$ collisions at low and high luminosity regimes is investigated in detail. We extend the study performed in Phys. Rev. D102, 074014 (2020) which has demonstrated that the sum of two semi-exclusive $t\bar{t}$ production modes, namely in photon-Pomeron and Pomeron-Pomeron interactions, can be experimentally measured when the $t\bar{t}$ system decays semi-leptonically, $t\bar{t}\rightarrow jjbl\nu_l\bar{b}$, both forward protons are tagged and a low amount of pile-up is present. In this study we focus on separating individual channels and a special attention is paid to the situation at high-luminosity LHC environment. We observe that the separation of the pomeron-pomeron and photon-photon events is a challenging task, especially at high amounts of pile-up even with an optimistic 10~ps resolution of timing detectors. In contrast, the photon-Pomeron signal is relatively well separable from all backgrounds at low levels of pile-up, allowing us to discover the elastic $t\bar{t}$ production and probe, for the first time, the production of a top quark pair in the photon-Pomeron interactions. The diffractive photoproduction of such a complex system as the $t\bar{t}$ pair hence can be used not only to study diffractive properties of the scattering amplitude but also to search for new physics beyond the Standard Model, and consequently to be a solid part of the physics programme of forward proton detectors at LHC.

Diffractive dijet photoproduction at the EIC

AbstracWe present a first, detailed study of diffractive dijet photoproduction at the recently approved electron-ion collider (EIC) at BNL. Apart from establishing the kinematic reaches for various beam types, energies and kinematic cuts, we make precise predictions at next-to-leading order (NLO) of QCD in the most important kinematic variables. We show that the EIC will provide new and more precise information on the diffractive parton density functions (PDFs) in the pomeron than previously obtained at HERA, illuminate the still disputed mechanism of global vs. only resolved-photon factorization breaking, and provide access to a completely new quantity, i.e. nuclear diffractive PDFs.

Diffractive photoproduction of J/ψ and ϒ using holographic QCD: Gravitational form factors and GPD of gluons in the proton

We present a holographic analysis of diffractive photoproducton of charmonium $J/\psi$ and upsilonium $\Upsilon$ on a proton, considered as a bulk Dirac fermion, for all ranges of $\sqrt{s}$, i.e., from near threshold to very high energy. Using the bulk wave functions of the proton and vector mesons, within holographic QCD, and employing Witten diagrams in the bulk, we compute the diffractive photoproduction amplitude of $J/\psi$ and $\Upsilon$. The holographic amplitude shows elements of the strictures of vector meson dominance (VMD). It is dominated by the exchange of a massive graviton or $2^{++}$ glueball resonances near threshold, and its higher spin-j counterparts that reggeize at higher energies. Both the differential and total cross sections are controlled by the gravitational form factor $A(t)$, and compare well to the recent results reported by the GlueX collaboration near threshold and the world data at large $\sqrt{s}$. The holographic gravitational form factors, including the D-term, which is due to the exchange of massive spin-0 glueballs, are in good agreement with lattice simulations. We use it to extract the holographic pressure and shear forces inside the proton. Finally, using a pertinent integral representation of the holographic gravitational form factor $A(t)$ near threshold, and its Pomeron counterpart way above threshold, we extract the generalized parton distribution (GPD) of gluons inside the proton at different resolutions.

BFKL Pomeron loop contribution in diffractive photoproduction and inclusive hadroproduction of J/\u03c8 and \u03d2

We analyze contributions to the heavy vector meson production with large transverse momentum in proton-proton and diffractive photon-proton scattering driven by an exchange of two Balitsky-Fadin-Kuraev-Lipatov Pomerons in the squared amplitudes. The Pomerons couple to a single parton and form a Pomeron loop closed by the vector meson impact factors. For the photon-proton case the diffractive cut of the Pomeron loop contributes, and for the inclusive hadroproduction one finds the loop with two cut Pomerons. We compute both of these Pomeron loop contributions and study in detail their properties. The results are then used to calculate the cross sections for diffractive J/ψ photoproduction with large transverse momentum at HERA and the correlated two Pomeron contribution for inclusive J/ψ and ϒ production cross sections at the LHC. Within a unified approach a good description of the photoproduction data is found, but correlated two Pomeron mechanism gives only a small contribution to hadroproduction of the vector mesons at the LHC.

Inclusive diffractive heavy quarkonium photoproduction in pp, pA and AA collisions

The inclusive J/Ψ production by direct and resolved photoproduction in the γp scattering is calculated based on the nonrelativistic quantum chromodynamics (NRQCD) factorization formalism, which is in good agreement with the experimental data of total cross section distribution of heavy quarkonium production at HERA. Then we extend the formalism including the direct and resolved photoproduction processes to resolved pomeron model to study the heavy quarkonium photoproduction at the LHC energies. We present the predictions of rapidity and transverse momentum distributions of the inclusive diffractive J/Ψ, Ψ(2S) and ϒ photoproduction in pp, pPb and PbPb collisions at the LHC energies. Our numerical results indicate that the resolved photoproduction processes play an important role in the heavy quarkonium production. Especially for pp collisions, the contribution of resolved photoproduction processes is the largest, which can reach to 28%, 13% and 44% for the rapidity distributions of J/Ψ, Ψ(2S) and ϒ inclusive diffractive photoproduction, respectively.

Searching for the Odderon in the diffractive f_2(1270) photoproduction at pA collisions

Although the Odderon is an unambiguous prediction of Quantum Chromodynamics, its existence still was not confirmed experimentally. One of the processes where the Odderon contribution is expected to be dominant is the diffractive photoproduction of tensor mesons. In this paper we study the diffractive f_2(1270) photoproduction in pA collisions at LHC and RHIC considering the model of the stochastic vacuum to treat the non-perturbative process. We demonstrate that this process dominates the f_2 production at mid-rapidities, and large values for the cross section are predicted. Our results indicate that the experimental analysis of this final state is, in principle, feasible and can be used to probe the Odderon.

Incoherent diffractive photoproduction of J/ψ and Υ on heavy nuclei in the color dipole approach

We calculate cross sections and transverse momentum distributions for the incoherent diffractive production of vector mesons $J/\psi$ and $\Upsilon$ on heavy nuclei. In distinction to coherent diffraction, the nucleus is allowed to break up, but except for the vector meson no new particles are produced in the reaction. Within the color dipole approach, we derive the multiple scattering expansion of the incoherent diffractive cross section as an expansion over quasielastic scatterings of the color dipole. We also compare our results to the measurement of the ALICE collaboration for incoherent $J/\psi$ production at $\sqrt{s_{\rm NN}} = 2.76 \, \rm {TeV}$ and show predictions at $\sqrt{s_{\rm NN}} = 5.02 \, \rm{TeV}$. We also briefly discuss a possible contribution to $J/\psi$ production in peripheral collisions in the $70 \div 90 \%$ centrality class.

Diffractive quarkonium photoproduction in pp and pA collisions at the LHC: Predictions of the resolved Pomeron model for the Run 2 energies

The inclusive diffractive quarkonium photoproduction in $pp$ and $pA$ collisions is investigated considering the Resolved Pomeron Model to describe the diffractive interaction. We estimate the rapidity and transverse momentum distributions for the $J/\Psi$, $\Psi(2S)$ and $\Upsilon$ photoproduction in hadronic collisions at the LHC and present our estimate for the total cross sections at the Run 2 energies. A comparison with the predictions associated to the exclusive production also is presented. Our results indicate that the inclusive diffractive production is a factor $\gtrsim 10$ smaller than the exclusive one in the kinematical range probed by the LHC.

Studies of the diffractive photoproduction of isolated photons at HERA

The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges 5 < Eγ T < 15 GeV and −0.7 < ηγ < 0.9, inclusively, and also with a jet with transverse energy and pseudorapidity in the ranges 4 < Ejet T < 35 GeV and −1.5 < ηjet < 1.8, using a total integrated electronproton luminosity of 456 pb−1. A number of kinematic variables were studied and compared to predictions from the RAPGAP Monte Carlo model. An excess of data is observed above the RAPGAP predictions for zmeas P > 0.9, where zmeas P is the fraction of the longitudinal momentum of the colorless “Pomeron” exchange that is transferred to the photon-jet final state, giving evidence for direct Pomeron interactions.

New HERA Results on Diffraction

Four new measurements are presented in the area of diffractive and exclusive production at HERA. Preliminary results are available for isolated photons in diffractive photoproduction from ZEUS and open charm cross section in diffractive deep-inelastic scattering (DIS) regime from H1. ZEUS Collaboration has also measured the cross-section ratio σψ(2S)/σJ/ψ(1S) in exclusive DIS using full HERA data statistics. Finally, H1 Collaboration for the first time studied exclusive ρ0 meson photoproduction associated with a leading neutron at HERA.

Color fluctuations in high energy hadron- and photon-nucleus collisions

We explain that coherence of high energy QED and QCD processes implies existence of new kind of phenomena which are beyond a framework based on Regge poles (cuts). New phenomena emerge as the consequence of compositeness of the bound states and the Lorentz slowing down of interaction. We focus on the color fluctuations phenomena predicted earlier for [Formula: see text] collisions within QCD and recent evidence for this phenomenon from [Formula: see text] LHC run, significant modification of nuclear shadowing phenomenon in the diffractive photoproduction of vector mesons observed recently in the ultra peripheral collisions at LHC. We outlined briefly general properties of color fluctuations phenomena and perspectives of future studies of this phenomenon in electron (photon) collisions with nuclei.