A generator of forward neutrons for ultra-peripheral collisions: nOOn

Abstract

The study of photon-induced reactions in collisions of heavy nuclei at RHIC and the LHC has become an important direction of the research program of these facilities in recent years. In particular, the production of vector mesons in ultra-peripheral collisions (UPC) has been intensively studied. Owing to the intense photon fluxes, the two nuclei participating in such processes undergo electromagnetic dissociation producing neutrons at beam rapidities. Here, we introduce the n O O n (pronounced noon) Monte Carlo program, which generates events containing such neutrons. n O O n is a ROOT based program that can be interfaced with existing generators of vector meson production in UPC or with theoretical calculations of such photonuclear processes. n O O n can also be easily integrated with the simulation programs of the experiments at RHIC and the LHC. PROGRAM SUMMARYProgram Title: n O O n Licensing provisions: GNU GPLv3 Programming language: C++ External routines: The generator is based on ROOT Nature of problem: The electromagnetic fields of nuclei at RHIC and the LHC can be described as a flux of quasi-real photons. These photons may interact with one of the nucleus in the opposite beam. There are events of interest where two independent interactions occur, one involving a hard scattering and one from the exchange of soft photons. As a result of the latter, the nucleus get excited and upon de-excitation it may emit neutrons, which are boosted to beam rapidities. The program computes the probability of neutron emission based on existing measurements and some mild modelling; it then generates neutrons in a per-event basis. Solution method: The break-up probabilities are computed using existing data and stored in ROOT objects (graphs and histograms). Photon energies from the accompanying hard process, e.g. vector meson production, are loaded into the program and a catalogue of specific break-up probabilities is constructed. The number of neutrons emitted in the event is generated and the neutrons are produced and boosted into the laboratory frame. The output is a TTree with a TClonesArray of TParticles per event, which can be easily interfaced to the simulation programs of the RHIC and LHC collaborations. Restrictions: At the moment only emission from Pb is available. References: https://github.com/mbroz84/noon and references in this article.

Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}} $$ = 5.02 TeV

“…The measurements of coherent ρ 0 photoproduction are in good agreement both with models following the parton-based colour-dipole approach and with the framework of Gribov-Glauber shadowing based on hadronic degrees of freedom. The models [9,41] of electromagnetic nuclear dissociation accompanying vector meson photoproduction provide a satisfactory description of the measured cross sections for different neutron emission classes. This observation suggests that the method proposed in [42] to decouple the low-photon-energy from the high-photon-energy contribution to the UPC cross section using neutron-differential measurements might also be applicable at forward rapidities, which is specially important in view of the expected data samples to be recorded at the LHC during the Run 3 and 4 [43].…”

“…No The modification of the photon flux due to the emission of the forward neutrons is carried out in the first three models as proposed in [9]. The fourth model uses the n O O n afterburner described in [41]. Figure 5 shows that the lower limit of the GKZ model gives a good description of the 0n0n cross section and underestimates a little bit the 0nXn and XnXn cross sections while the upper limit of the same model overestimates the 0n0n, slightly underestimates the 0nXn and describes the XnXn cross sections.…”

$ρ$ photoproduction in ALICE

“…1.1, the cross-sections for additional photon exchange may be easily calculated given a σ γp . The STARlight neutron calculation is done within the STARlight code [14], while the CCKT simulation used the n 0 0 n afterburner [20]. To the extent that these calculations are based on the same parameterized photoexcitation data, they should give the same relative cross-sections for ρ 0 production accompanied by neutron emission.…”

Mapping the Electromagnetic Fields of Heavy-Ion Collisions with the Breit-Wheeler Process

“…The SuperChic model [57], another common EPA implementation, does not provide specific calculations for any neutron emission scenarios. However, various dedicated models [76,77,78] exist for the sole purpose of computing the full neutron emission spectra and can be used for more detailed calculations.…”