511 research outputs found
Energy dependence of dissociative J/ψ photoproduction as a signature of gluon saturation at the LHC
No full textWe have developed a model in which the quantum fluctuations of the proton structure are characterised by hot spots, whose number grows with decreasing Bjorken-x. Our model reproduces the F2(x,Q2) data from HERA at the relevant scale, as well as the exclusive and dissociative J/ψ photoproduction data from H1 and ALICE. Our model predicts that for Wγp≈500 GeV, the dissociative J/ψ cross section reaches a maximum and then decreases steeply with energy, which is in qualitatively good agreement to a recent observation that the dissociative J/ψ background in the exclusive J/ψ sample measured in photoproduction by ALICE decreases as energy increases. Our prediction provides a clear signature for gluon saturation at LHC energies
Energy dependence of dissociative J/ψ photoproduction as a signature of gluon saturation at the LHC
No full textWe have developed a model in which the quantum fluctuations of the proton structure are characterised by hot spots, whose number grows with decreasing Bjorken-x. Our model reproduces the F2(x,Q2) data from HERA at the relevant scale, as well as the exclusive and dissociative J/ψ photoproduction data from H1 and ALICE. Our model predicts that for Wγp≈500GeV, the dissociative J/ψ cross section reaches a maximum and then decreases steeply with energy, which is in qualitatively good agreement to a recent observation that the dissociative J/ψ background in the exclusive J/ψ sample measured in photoproduction by ALICE decreases as energy increases. Our prediction provides a clear signature for gluon saturation at LHC energies
Incoherent J/ψ production at large |t| identifies the onset of saturation at the LHC
No full textWe predict that the onset of gluon saturation can be uniquely identified using incoherent J/ψ production in Pb–Pb collisions at currently accessible energies of the LHC. The diffractive incoherent photo-production of a J/ψ vector meson off a hadron provides information on the partonic structure of the hadron. Within the Good-Walker approach it specifically measures the variance over possible target configurations of the hadronic colour field. For this process then, gluon saturation sets in when the cross section reaches a maximum, as a function of the centre-of-mass energy of the photon-hadron system (W), and then decreases. We benchmark the energy-dependent hot-spot model against data from HERA and the LHC and demonstrate a good description of the available data. We show that the study of the energy dependence of the incoherent production of J/ψ allows us to pinpoint the onset of saturation effects by selecting the region of Mandelstam-t around 1 GeV2 where the contribution of hot spots is dominant. We predict the onset of saturation in a Pb target to occur for W around a few hundred GeV. This can be measured with current data in ultra-peripheral Pb–Pb collisions at the LHC
Photonuclear production at the LHC: Proton-based versus nuclear dipole scattering amplitudes
No full textThe coherent photonuclear production of a vector meson at the LHC has been computed using two different sets of solutions of the impact-parameter dependent Balitsky-Kovchegov equation. The nuclear dipole scattering amplitudes are obtained either from () solutions for this process off proton targets coupled with a Glauber-Gribov prescription, or () from solutions obtained with an initial condition representing the nucleus. These approaches predict different cross sections, which are compared with existing data from ultra-peripheral collisions at the LHC. The latter approach seems to better describe current measurements. Future LHC data should be precise enough to select one of the two approaches as the correct one.The coherent photonuclear production of a J/ψ vector meson at the LHC has been computed using two different sets of solutions of the impact-parameter dependent Balitsky-Kovchegov equation. The nuclear dipole scattering amplitudes are obtained either from ( i ) solutions for this process off proton targets coupled with a Glauber-Gribov prescription, or ( ii ) from solutions obtained with an initial condition representing the nucleus. These approaches predict different cross sections, which are compared with existing data from ultra-peripheral collisions at the LHC. The latter approach seems to better describe current measurements. Future LHC data should be precise enough to select one of the two approaches as the correct one
The onset of spin rotation effects in electroproduction of heavy quarkonia
No full textWe discuss the Melosh spin rotation effects in diffractive photo- and electroproduction of heavy quarkonia off a nucleon target in the dipole picture. The quarkonium light-front wave functions are obtained in the Schroedinger equation based formalism using the realistic interquark interaction potentials. A strong onset of spin rotation effects predicted in the case of J/ψ(1S) photoproduction enables us to reach a reasonable agreement with the available data. In the case of radially excited 2S and 3S states these effects are even stronger as a direct manifestation of the nodal structure of their wave functions. In particular, the spin effects enhance the Ψ0(2S) cross section by a factor of 2 − 3 and therefore cause a substantial increase of the Ψ0(2S)-to-J/Ψ(1S) ratio improving the description of experimental data
Incoherent production at large identifies the onset of saturation at the LHC
No full textWe predict that the onset of gluon saturation can be uniquely identified
using incoherent production in Pb\unicode{x2013}Pb collisions
at currently accessible energies of the LHC. The diffractive incoherent
photo-production of a vector meson off a hadron provides
information on the partonic structure of the hadron. Within the Good-Walker
approach it specifically measures the variance over possible target
configurations of the hadronic colour field. For this process then, gluon
saturation sets in when the cross section reaches a maximum, as a function of
the centre-of-mass energy of the photon-hadron system (), and then
decreases. We benchmark the energy-dependent hot-spot model against data from
HERA and the LHC and demonstrate a good description of the available data. We
show that the study of the energy dependence of the incoherent production of
allows us to pinpoint the onset of saturation effects by
selecting the region of Mandelstam- around 1 GeV where the contribution
of hot spots is dominant. We predict the onset of saturation in a Pb target to
occur for around a few hundred GeV. This can be measured with current data
in ultra-peripheral Pb\unicode{x2013}Pb collisions at the LHC.Comment: 18 pages, 3 figure
Vector meson production using the Balitsky-Kovchegov equation including the dipole orientation
No full textIn this proceedings a solution of the target-rapidity Balitsky-Kovchegov (BK) equation is presented considering the complete impact-parameter dependence, including the orientation of the dipole with respect to the impact-parameter vector. The target-rapidity formulation of the BK equation introduces non-locality in rapidity. Three different prescriptions are considered to take into account the rapidities preceding the initial condition. The solutions are used to compute the structure functions of the proton and the diffractive photo- and electro-production of off protons. The predictions agree well with HERA data, confirming that the target-rapidity Balitsky-Kovchegov equation with the full impact-parameter dependence is a viable tool to study the small Bjorken- limit of perturbative QCD at current facilities like RHIC and LHC as well as in future colliders like the EIC
Calculation of Direct photon production in nuclear collisions
No full textPrompt photons produced in a hard reaction are not expected to be accompanied by any final state interaction, either energy loss or absorption and one should not expect any nuclear effects at high pT . However, data from the PHENIX experiment indicates large-pT suppression in d+Au and central Au+Au collisions that cannot be accompanied by coherent phenomena. We propose a mechanism based on the energy sharing problem at large pT near the kinematic limit that is induced by multiple initial state interactions and that improves the agreement of calculations with PHENIX data. We calculate inclusive direct photon production cross sections in p+p collisions at RHIC and LHC energies using the color dipole approach without any additional parameter. Our predictions are in good agreement with the available data. Within the same framework, we calculate direct photon production rates in d+A and A+A collisions at RHIC energy. We also provide predictions for the same process in p+A collisions at LHC energy. Since the kinematic region where the expected suppression manifests can be achieved also at forward rapidity, we present a comparison of forward rapidity to midrapidity behaviour. We also include and analyze the contribution of gluon shadowing as a leading twist shadowing correction that modifies nuclear effects especially at small pT
Solutions to the Balitsky-Kovchegov equation including the dipole orientation
No full textSolutions of the target-rapidity Balitsky-Kovchegov (BK) equation are studied
considering, for the first time, the complete impact-parameter dependence,
including the orientation of the dipole with respect to the impact-parameter
vector. In our previous work, it has been demonstrated that the spurious
Coulomb tails could be tamed using the collinearly-improved kernel and an
appropriate initial condition in the projectile-rapidity BK equation.
Introducing a different interpretation of the evolution variable, the
target-rapidity formulation of the BK equation brings non-locality in rapidity
and a kernel modification, removing the term that previously helped to suppress
the Coulomb tails. To address this newly emerged non-locality, three different
prescriptions are explored here to take into account the rapidities preceding
the initial condition. Two of these approaches induce mild Coulomb tails, while
the other is free from this effect within the studied rapidity range. The range
is chosen to correspond to that of interest for existing and future
experiments. To demonstrate that this set up can be used for phenomenological
studies, the obtained solutions are used to compute the F2 structure function
of the proton and the diffractive photo- and electro-production of J/{\psi} off
protons. The predictions agree well with HERA data, confirming that the
target-rapidity Balitsky-Kovchegov equation with the full impact-parameter
dependence is a viable tool to study the small Bjorken-x limit of perturbative
QCD at current facilities like RHIC and LHC as well as in future colliders like
the EIC
Direct photons at large pT : from RHIC to LHC
No full textUsing the color dipole formalism we study the production of direct photons in proton-nucleus and nucleus-nucleus collisions at energies corresponding to RHIC and LHC experiments. Prompt photons produced in a hard reaction are not accompanied with any final state interaction, either energy loss or absorption. Therefore, in the RHIC energy range besides small isotopic corrections one should not expect any nuclear effects at large pT . However, data from the PHENIX experiment indicates a significant large-pT suppression in d+Au and central Au+Au collisions that cannot be accompanied by coherent phenomena. We demonstrate that such an unexpected result is subject to the energy sharing problem universally induced by multiple initial state interactions (ISI) at large pT and/or at forward rapidities. In the LHC kinematic region ISI corrections are irrelevant at mid rapidities but cause rather strong suppression at forward rapidities. We present for the first time predictions for expected nuclear effects at large pT in p + Pb and Pb + Pb collisions at different rapidities. We include and analyze also a contribution of coherent effects associated with gluon shadowing modifying nuclear effects predominantly at small and medium-high pT
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