Photon-nucleus Interactions Research Articles

TMD factorisation for diffractive jets in photon-nucleus interactions

Using the colour dipole picture and the colour glass condensate effective theory, we study the diffractive production of two or three jets via coherent photon-nucleus interactions at high energy. We consider the hard regime where the photon virtuality and/or the transverse momenta of the produced jets are much larger than the saturation momentum Qs of the nuclear target. We show that, despite this hardness, the leading-twist contributions are controlled by relatively large parton configurations, with transverse sizes R ~ 1/Qs, which undergo strong scattering and probe gluon saturation. We demonstrate that these leading-twist contributions admit transverse-momentum dependent (TMD) factorisation, in terms of quark and gluon diffractive TMD distribution functions, for which we obtain explicit expressions from first principles. We go beyond our previous work by evaluating the contributions involving the quark diffractive distributions and by establishing that their DGLAP evolution emerges via controlled calculations within the colour dipole picture. We find the same expression for the quark diffractive TMD in two different processes (semi-inclusive diffraction and the diffractive production of quark-gluon dijets), thus demonstrating its universality.

Looking for QGP signatures in ultraperipheral PbPb collisions

Ultraperipheral collisions of relativistic heavy ion beams lead to a diverse set of photon-nucleus (photonuclear) interactions. The measurements of particles and their interaction produced in photonuclear reactions can shed light on the QCD dynamics of these novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Previous studies by ATLAS indicate significant elliptic and triangular flow coefficients in these events [1]. Thus, it is imperative to check these events for other potential QGP signatures including radial flow, strangeness enhancement, and enhanced baryon/meson production. This proceeding presents the measurement of charged hadron yields in photonuclear collisions using 5.02 TeV Pb+Pb data collected in 2018 by ATLAS, with a dedicated photo-nuclear event trigger. The charged hadron yields are presented as a function of pseudorapidity and transverse momentum in different categories of event multiplicity. The results are compared with 5.02 TeV p+Pb data collected in 2016 by ATLAS, at the same event multiplicities. The results are also compared with calculations from DPMJET and hydrodynamic-based models. These comparisons enable detailed characterizations of photonuclear collision properties, including the photon energy distribution, and whether small QGP droplets may be formed.

Exclusive Four-pion Photoproduction in Ultra-peripheral Heavy-ion Collisions at RHIC and LHC Energies

We study the photoproduction of exclusive $2\pi^+2\pi^-$ mesons in ultra-peripheral heavy-ion collisions at RHIC and LHC energies. Predictions in photon-nucleus interactions are calculated for various resonances at central and forward rapidities. The recent H1 preliminary data are utilized to improve the description of the poorly known $\gamma p \to 4\pi^\pm p$ process. We present the comparisons of our results to the available STAR data at RHIC, and made predictions for LHC energies.

Observation of Excess J/ψ Yield at Very Low Transverse Momenta in Au+Au Collisions at sqrt[s_{NN}]=200 GeV and U+U Collisions at sqrt[s_{NN}]=193 GeV.

We report on the first measurements of J/ψ production at very low transverse momentum (p_{T}<0.2 GeV/c) in hadronic Au+Au collisions at sqrt[s_{NN}]=200 GeV and U+U collisions at sqrt[s_{NN}]=193 GeV. Remarkably, the inferred nuclear modification factor of J/ψ at midrapidity in Au+Au (U+U) collisions reaches about 24 (52) for p_{T}<0.05 GeV/c in the 60%-80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the dN/dt distribution of J/ψ for the very low p_{T} range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semicentral collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/ψ yield observed at extremely low p_{T} originates from coherent photon-nucleus interactions. In particular, coherently produced J/ψ's in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.

Photoproduction of J/ψ in non-single-diffractive p+p collisions * * Supported by the National Natural Science Foundation of China (11775213, 11505180, 11675168), the U.S. DOE Office of Science (DE-SC0012704) and MOST (2016YFE0104800)

Significant enhancements of production at very low transverse momenta were recently observed by the ALICE and STAR collaborations in peripheral hadronic A+A collisions. The anomalous excess points to coherent photon-nucleus interactions in violent hadronic heavy-ion collisions, which were conventionally studied only in ultra-peripheral collisions. Assuming that the coherent photoproduction is the underlying mechanism responsible for the excess observed in peripheral A+A collisions, its contribution in p+p collisions with nuclear overlap, i.e. non-single-diffractive collisions, is of particular interest. In this paper, we perform a calculation of exclusive photoproduction in non-single-diffractive p+p collisions at the RHIC and LHC energies based on the pQCD motivated parametrization using the world-wide experimental data, which could be further employed to improve the precision of the phenomenological calculations for photoproduction in A+A collisions. The differential rapidity and transverse momentum distributions of from photoproduction are presented. In comparison with the production from hadronic interactions, we find that the contribution of photoproduction is negligible.

Coherent J/ψ photoproduction in hadronic heavy-ion collisions

Significant excesses of $J/\ensuremath{\psi}$ yield at very low transverse momentum (${p}_{T}&lt;0.3$ GeV/$c$) were observed by the ALICE and STAR collaborations in peripheral hadronic $A+A$ collisions. This is a sign of coherent photoproduction of $J/\ensuremath{\psi}$ in violent hadronic interactions. Theoretically, the photoproduction of $J/\ensuremath{\psi}$ in hadronic collisions raises questions about how spectator and nonspectator nucleons participate in the coherent reaction. We argue that the strong interactions in the overlapping region of incoming nuclei may disturb the coherent production, leaving room for different coupling assumptions. The destructive interference between photoproduction on ions moving in opposite directions also needs to be included. This paper presents calculations of $J/\ensuremath{\psi}$ production from coherent photon-nucleus ($\ensuremath{\gamma}+A\ensuremath{\rightarrow}J/\ensuremath{\psi}+A$) interactions in hadronic $A+A$ collisions at BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider energies with both nucleus and spectator coupling hypotheses. The integrated yield of coherent $J/\ensuremath{\psi}$ as a function of centrality is found to be significantly different, especially towards central collisions, for different coupling scenarios. Differential distributions as a function of transverse momentum, azimuthal angle, and rapidity in different centrality bins are also shown, and found to be more sensitive to the Pomeron coupling than to the photon coupling. These predictions call for future experimental measurements to help better understand the coherent interaction in hadronic heavy-ion collisions.

Parton saturation scaling function for exclusive production of vector mesons and deeply virtual Compton scattering

We provide a universal expression of cross sections for the exclusive vector meson production and Deeply Virtual Compton Scattering (DVCS) in photon-proton and photon-nucleus interactions based on the geometric scaling phenomenon. The theoretical parameterization based on the scaling property depends only on the single variable $\tau_A = Q^2/Q_{\mathrm{sat}}^2$, where the saturation scale, $Q_{\mathrm{sat}}$, drives the energy dependence and the corresponding nuclear effects. This phenomenological result describes all available data from DESY-HERA for $\rho,\,\phi,\,J/\psi$ production and DVCS measurements. A discussion on the size of nuclear shadowing corrections on photon-nucleus interaction is also done.

Gluon shadowing at small x from coherent J/ψ photoproduction data at energies available at the CERN Large Hadron Collider

The cross section for coherent $\mathrm{J/}\psi$ photoproduction in Pb-Pb collisions is the sum of two contributions, one from low-, the other from high-energy photon-nucleus interactions. A novel method to disentangle both contributions allowing one to extract the coherent photo-nuclear cross section for coherent $\mathrm{J/}\psi$ production, $\sigma_{\gamma \rm Pb}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultra-peripheral Pb-Pb collisions at the LHC. Applying the proposed method to the available data it is possible to obtain $\sigma_{\gamma \rm Pb}$ up to a photon-lead centre-of-mass energy of 470 GeV, which corresponds to $x$ of 4.4x10$^{-5}$. To illustrate a possible use of the extracted photo-nuclear cross sections, the corresponding nuclear suppression factors are computed and they are compared to predictions of gluon shadowing calculated in the leading-twist approximation approach.

Exclusive coherent production of heavy vector mesons in nucleus-nucleus collisions at energies available at the CERN Large Hadron Collider

Heavy nuclei at collider energies are a source of high energy Weizsaecker-Williams photons. This photon flux may be utilized to study high energy photon-nucleus interactions. Here we concentrate on the coherent diffractive production of heavy vector mesons on nuclear targets and show how it probes the unintegrated glue of the nucleus in the saturation domain. We present predictions for rapidity distributions of exclusive coherent J/Psi and Upsilon mesons which can be measured by the ALICE experiment at the LHC.

Exclusive production of heavy vector mesons in nucleus-nucleus collisions

Heavy nuclei at collider energies are a source of high energy Weizsäcker-Williams photons. This photon flux may be utilized to study high energy photon-nucleus interactions. Here we concentrate on the coherent diffractive production of heavy vector mesons on nuclear targets and show how it probes the unintegrated glue of the nucleus in the saturation domain.

Hypernuclear production by the (γ,K+) reaction within a relativistic model

Within a fully covariant model based on an effective Lagrangian picture, we investigate the hypernuclear production in a photon-nucleus interaction on an $^{16}\mathrm{O}$ target. The explicit kaon production vertex is described via creation, propagation, and decay into relevant channels of ${N}^{*}(1650)$, ${N}^{*}(1710)$, and ${N}^{*}(1720)$ intermediate baryonic resonance states in the initial interaction of the incident photon with one of the target protons. Bound state nucleon and hyperon wave functions are obtained by solving the Dirac equation. Using vertex parameters determined in the previous studies, contributions of the ${N}^{*}(1710)$ baryonic resonance dominate the total production cross sections which are found to peak at photon energies below 1 GeV. The results show that photoproduction is the most appropriate means for studying the unnatural parity hypernuclear states, thus accessing the spin dependence of the hyperon-nucleon interaction.

Laser-enhanced radioactive decay and selective transmutation of nuclei revisited

We have investigated intense narrow-band electromagnetic radiation sources – ranging from visible to X-ray and gamma-ray region – and their application in direct photon–nucleus interactions. In particular, we discuss means of selective excitation of nuclear resonance states by narrow-band photons. In the relaxation process, an excited mother nucleus may return to its initial state, end up to another isomeric state, or decay into a new daughter nucleus. In the latter case, the mother nucleus is transmuted into a daughter which may have beneficial properties, for instance regarding its radioactivity. One potential application could be the destruction of long-lived nuclear waste isotopes into faster decaying ones. The essential presumption is that the excitation process is both selective and efficient enough compared to parasitic background phenomena. The paper consists of (1) a brief discussion of the generation of short wavelength narrow-band light sources, (2) an exploration of nuclear states excitable by induced photon absorption and of their decay channels, and (3) an assessment of the feasibility of this method. According to our findings, the method may be promising for basic nuclear physics studies but still calls for more efficient narrow-band sources. For processing macroscopic amounts of nuclides, no foreseeable improvements in technology appear sufficient to make the method practical.

Role of in-medium hadrons in photon–nucleus reactions: shadowing and dilepton spectrum

We study the effect of in-medium hadronic properties in photon–nucleus interactions in the context of shadowing as well as the dilepton spectrum for incident photon energies in the range 1.1–3 GeV. A reasonable agreement with the experimental data for shadowing is obtained in a scenario of downward spectral shift of the hadrons. We show that distinguishable features for in-medium changes of the hadronic properties can be observed experimentally through the dilepton spectrum by judicious choice of target nuclei and incident energy of photons.

PHOTON EXCHANGE IN NUCLEUS–NUCLEUS COLLISIONS

The strong electromagnetic fields in peripheral heavy ion collisions give rise to photon–photon and photon–nucleus interactions. I present a general survey of the photon–photon and photon-hadron physics accessible in these collisions. Among these processes I discuss the nuclear fragmentation through the excitation of giant resonances, the Coulomb dissociation method for application in nuclear astrophysics, and the production of particles.

Shadowing in photoabsorption: Role of in-medium hadrons

We study the effects of in-medium hadronic properties on shadowing in photon-nucleus interactions in Glauber model as well as in the multiple scattering approach. A reasonable agreement with the experimental data is obtained in a scenario of downward spectral shift of the hadrons. Shadowing is found to be insensitive to the broadening of the spectral functions. An impact parameter dependent analysis of shadowing might shed more light on the role of in-medium properties of hadrons.

Photoproduction off nuclei: Particle and jet production

High energy multiparticle photoproduction off nuclear targets is studied. The photon is assumed to interact in direct scattering processes or as a resolved $q\overline{q}$ state according to the generalized vector dominance model. In the description of resolved interactions multiple soft and hard processes in each $q\overline{q}$-nucleon interaction are taken into consideration. The model, formulated within the framework of the two-component dual parton model, is shown to agree with hadron production data from real and weakly virtual photon-nucleus interactions. Differences between multiparticle production in photon-nucleus and hadron-nucleus interactions and features of jet production in photon-nucleus collisions at DESY HERA energies are discussed.

Higher-order processes in the photon-nucleus interaction

Some discrepancies between theory and experiment in photon-nucleus interactions, and in nuclear excitations induced by positrons, are reviewed in terms of our current understanding. The problems associated with reconciling the experimental results with theory have inspired several predictions of new higher-order processes in photon-nucleus interactions. Some of the general features of these processes will be discussed.

Effect of the large intermediate masses on the hadronic properties of the photon

The theory describing the hadron-mediated photon-nucleus interaction is generalized with the introduction of large-mass intermediate states. It is shown that if the VDM is valid for the photoproduction of large masses, the hadron-mediated photon-nucleus cross-section is proportional to the hadron-nucleus total cross-section also in the presence of large intermediate masses. The theory is applied to the available experiment of photon-nucleus total cross-section between 20 and 185 GeV, and explains the apparent increase of the transparency of the nucleus with the energy.

Pre-equilibrium particle decay in the photonuclear reactions

Calculations of particle energy spectra resulting from the photonuclear reactions at energies below the meson production threshold have been carried out in the framework of combining the pre-equilibrium exciton model and the quasideuteron model. A 2p-2h initial state in the exciton model is assumed because the quasideuteron absorption is the dominant process in the energy region above giant resonance. With this combined model the subsequent secondary interactions of the emerging particle with the rest of the nucleus following the initial photon-nucleus interaction are appropriately taken into account. The experimental difference energy spectra of fast photoneutrons from several elements (Al, Cu, In, Sn, Ta, Pb, Bi, and U) at bremsstrahlung energies of 55 and 85 MeV were compared with the theoretical predictions. General agreements in both spectral shapes and cross sections are obtained. The relative yields of the reactions ($\ensuremath{\gamma},xn$) resulting from monoenergetic photons on $^{127}\mathrm{I}$ at 50, 100, and 150 MeV are also predicted reasonably well by the combined model together with the conventional evaporation theory.NUCLEAR REACTIONS $^{27}\mathrm{Al}(\ensuremath{\gamma},x)$, $^{63}\mathrm{Cu}(\ensuremath{\gamma},x)$, $^{115}\mathrm{In}(\ensuremath{\gamma},x)$, $^{118}\mathrm{Sn}(\ensuremath{\gamma},x)$, $^{181}\mathrm{Ta}(\ensuremath{\gamma},x)$, $^{207}\mathrm{Pb}(\ensuremath{\gamma},x)$, $^{209}\mathrm{Bi}(\ensuremath{\gamma},x)$, $^{235}\mathrm{U}(\ensuremath{\gamma},x)$, ${E}_{\ensuremath{\gamma}}=55 \mathrm{and} 85$ MeV bremsstrahlung. $^{12}\mathrm{C}(\ensuremath{\gamma},x)$, ${E}_{\ensuremath{\gamma}}=110$ MeV bremsstrahlung. $^{127}\mathrm{I}(\ensuremath{\gamma},xn)$, ${E}_{\ensuremath{\gamma}}=50, 100, \mathrm{and} 150$ MeV. Pre-equilibrium exciton, quasideuteron, and statistical compound nucleus models analyses.

Comment on the point interaction in photon shadowing

We show that the inclusion of heavy vector mesons in the vector-meson-dominance picture of photon-nucleus interactions cannot explain the observed lack of shadowing. Hence we conclude that the "point interaction" in photon interactions cannot be associated with the existence of heavy vector mesons.