Cross Section For Dilepton Production Research Articles

Upcgen: A Monte Carlo simulation program for dilepton pair production in ultra-peripheral collisions of heavy ions

Ultra-peripheral collisions (UPCs) of heavy ions can be used as a clean environment to study two-photon induced interactions such as dilepton pair photoproduction. Recently, precise data on lepton pair production in UPCs were obtained by the ATLAS experiment at the LHC where significant deviations, of up to 20%, from available theoretical predictions were observed. In this work, we present a Monte Carlo event generator, Upcgen, that implements a refined treatment of the photon flux allowing us to improve the agreement with ATLAS data at large dilepton rapidities. Besides, the new generator offers a possibility to study photon polarization effects and set arbitrary values of the lepton anomalous magnetic moment that can be used in the future studies of tau g−2 via ditau production measurements in UPCs. Program summaryProgram Title: UpcgenCPC Library link to program files:https://doi.org/10.17632/gbv9s729s9.1Developer's repository link:https://github.com/nburmaso/upcgenLicensing provisions: GPLv3Programming language:▪External routines/libraries: ROOT software toolkit; optionally: Pythia8 and (or) Pythia6 event generator, a compiler with OpenMP support.Nature of problem: In view of deviations between theoretical predictions and new experimental measurements of dilepton production cross sections in ultra-peripheral collisions of heavy ions, a more accurate calculation is needed. Precise predictions also become crucial for the studies related to tau anomalous magnetic moment measurements via ditau pair production in UPCs. We made an attempt to implement a dedicated Monte Carlo event generator with improved calculation of the photon flux that can be used to generate dilepton pairs in UPCs with a possibility to freely change the value of the anomalous magnetic moment.Solution method: Utilizing classes implemented in ROOT, the program calculates the dilepton pair production cross section AA→AA+ℓℓ by folding the elementary γγ→ℓℓ cross section and the two-photon luminosity. Computation of the elementary cross section with an arbitrary value of the anomalous magnetic moment is based on the generalized vertex formalism. The calculation of the two-photon luminosity is based on an improved photon flux treatment based on realistic nuclear form factors. Since the computation of the two-photon luminosity is a time-consuming operation, a corresponding 2D-histogram is cached in a ROOT-file. The dilepton pair production cross section is then used to generate lepton pairs via a Monte Carlo simulation process. For the decay of the taus, Pythia8 or Pythia6 can be used.Additional comments including restrictions and unusual features: The program is aimed on simulation of dilepton pair production in ultra-peripheral collisions of heavy ions in collider experiments. A user can set the energy of the colliding system, change species of incoming nuclei and also tune the anomalous magnetic moment of the lepton to be produced. A possibility to generate a realistic transverse momentum distribution of dilepton pairs is also taken into account.

Leading order corrections to the Bethe-Heitler process in the γp→l+l−p reaction

The ratio of di-lepton production cross sections on a proton, using the $\gamma p\rightarrow l^+ l^- p$ process, above and below di-muon production threshold allows to extract the effective lepton-proton interaction, which is required to be identical for electrons and muons if lepton universality is exact. To test for a scenario of broken universality at the percent level, of the size which could explain the different proton charge radii extracted from electron scattering and from muonic hydrogen spectroscopy, we evaluate all one-loop QED corrections to this process, including the full lepton mass dependencies. We furthermore show that two-photon exchange processes with both photons attached to the proton line vanish after averaging over di-lepton angles, and estimate the relatively small radiation off the proton. We compare the full one-loop calculation with a soft-photon approximation of the same order, and present estimates for a planned experiment.

Spin and model determination of Z′ - boson in lepton pair production at CERN LHC

Many new physics models predict production of heavy resonances in Drell-Yan channel and can be observed at the CERN LHC. If a new resonance is discovered as a peak in the dilepton invariant mass distribution at the LHC, the identification of its spin and couplings can be done by measuring production rates and angular distributions of the decay products. Here we discuss the spin-1 identification of Z′-boson for a set of representative models (SSM, E6, LR, and ALR) against the spin-2 RS graviton resonance and a spin-0 sneutrino resonance with the same mass and producing the same number of events under the resonance peak. We use the center-edge asymmetry for spin identification, as well as the total dilepton production cross section for the distinguishing the considered Z′-boson models from one another.

Analytic forms for cross sections of di-lepton production from e+e− collisions around the J/Ψ resonance**Supported by National Natural Science Foundation of China (11275211) and Istituto Nazionale di Fisica Nucleare, Italy

A detailed theoretical derivation of the cross sections of e+e− → e+e− and e+e− → μ+μ− around the J/ψ resonance is reported. The resonance and interference parts of the cross sections, related to J/ψ resonance parameters, are calculated. Higher-order corrections for vacuum polarization and initial-state radiation are considered. An arbitrary upper limit of radiative correction integration is involved. Full and simplified versions of analytic formulae are given with precision at the level of 0.1% and 0.2%, respectively. Moreover, the results obtained in the paper can be applied to the case of the ψ(3686) resonance.

Investigation of pionic contribution in the Drell–Yan dilepton production cross section in p-Cu collision based on the shell and Fermi gas models

In this study, the changes in the dilepton production cross section have been investigated by adding quark distribution functions of bound pions beside the quark distribution functions of bound nucleons in the framework of the Shell and Fermi gas models. Similar extracted results from two aforementioned models showed that by considering the pionic contribution the consistency between the theoretical results and the experimental data could be improved about 3–8%.

Dilepton production at SPS and RHIC energies within the PHSD off-shell transport approach

The microscopic parton-hadron-string dynamics (PHSD) transport approach describes the full evolution of relativistic heavy-ion collisions: from the initial hard scatterings to the partonic phase in the early hot and dense reaction region (when the local energy density is above ≈1 GeV/fm3) followed by hadronization and off-shell hadron propagation and interactions. The description of quarks and gluons in PHSD is based on the dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. The DQPM describes QCD properties in terms of single-particle Green's functions and leads to the notion of effective quasiparticles which are massive and have broad spectral functions (due to large interaction rates). Dressing the quark and gluon lines with effective propagators provided by the DQPM, we obtain cross sections of dilepton production in the reactions q + q̄ → l+l− (Born mechanism), q + q̄ → l+l− + g (quark annihilation with the gluon Bremsstrahlung in the final state), and q/q̄ + g → l+l− + q/q̄ (gluon Compton scattering) by off-shell quarks and gluons in the strongly interacting partonic phase. After implementing the off-shell cross sections for the partonic processes into the PHSD transport approach, we address the dilepton production from the parton interactions in the early stage of relativistic heavy-ion collisions at SPS and RHIC energies. Dilepton production in In + In collisions at 158 AGeV and in Au + Au at √s = 200 GeV is calculated. By comparing to the data of the NA60 and PHENIX Collaborations, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant.

Dilepton production by dynamical quasi-particles in the strongly interacting quark–gluon plasma

We address the dilepton production by the constituents of the strongly interacting quark–gluon plasma (sQGP). In order to make quantitative predictions for dilepton rates at experimentally relevant low dilepton mass (O(1GeV)) and strong coupling (αS ∼ 0.5–1), we take into account non-perturbative spectral functions and self-energies of the quarks, antiquarks and gluons. For this purpose, we use parametrizations of the quark and gluon propagators provided by the dynamical quasi-particle model (DQPM) matched to reproduce lattice quantum chromodynamics (QCD) data. The DQPM describes QCD properties in terms of the single-particle Green's functions and leads to the notion of the constituents of the sQGP being effective quasi-particles, which are massive and have broad spectral functions (due to large interaction rates). By ‘dressing’ the quark and gluon lines with the effective propagators, we derive the off-shell cross sections for dilepton production in the reactions (Born mechanism), (quark annihilation with the gluon bremsstrahlung in the final state), (gluon–Compton scattering), and (virtual gluon decay, virtual quark decay). In contrast to previous calculations of these cross sections, we account for virtualities of all the quarks and gluons. We find that finite masses of the effective quasi-particles not only screen the singularities typical of the perturbative cross sections with massless quarks, but also modify the shape of the dilepton production cross sections, especially at low dilepton mass Q and at the edges of the phase space. Finally, we use the calculated mass-dependent cross sections to identify the dependence of the dilepton rates on the spectral function widths of the initial and final quarks and gluons, which has not been estimated so far. The results demonstrate that the multiple partonic scatterings encoded in the broad spectral functions of the dynamical quasi-particles have considerable effect on the dilepton rates.

Dilepton production in nucleon-nucleon collisions around 1 GeV/nucleon: A theoretical update

We present a fully relativistic and gauge invariant framework for calculating the cross sections of dilepton production in nucleon-nucleon (NN) collisions which is based on the meson-exchange approximation for the NN scattering amplitudes. Predictions of our model are compared with those of other covariant models that have been used to describe this reaction. We discuss the comparison of our calculations with the old DLS and the recent HADES data.

A–dependence in dilepton rapidity distributions: parton model and dipole approach analysis

We calculate dilepton production cross section at RHIC using the improved parton model and color dipole approach in p–A collisions at backward rapidities. Our results are given as nuclear modification factors. We test three nuclear parton distribution functions: EKS, EPS08 and EPS09; we found no changes in the results. In the color dipole approach, we use GBW, DHJ, and BUW color dipole cross section models; again, no relevant difference is found in the nuclear modification factor, in spite of that DHJ breaks geometric scaling and that the p–p cross sections show relevant differences. The presence of intrinsic transverse momentum in the improved parton model is investigated, showing that nuclear effects are sensitive to this assumption. The color dipole approach lacks this intrinsic k T and thus nuclear effects can discriminate among both analysis. We extend our improved parton model results to forward rapidities and compare with CGC results, obtaining agreement between the models.

Spin and model identification ofZ′bosons at the LHC

Heavy resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via measuring production rates and angular distributions of the decay products. We discuss the discrimination of the spin-1 of ${Z}^{\ensuremath{'}}$ representative models (${Z}_{\mathrm{SSM}}^{\ensuremath{'}}$, ${Z}_{\ensuremath{\psi}}^{\ensuremath{'}}$, ${Z}_{\ensuremath{\eta}}^{\ensuremath{'}}$, ${Z}_{\ensuremath{\chi}}^{\ensuremath{'}}$, ${Z}_{\mathrm{LR}}^{\ensuremath{'}}$, and ${Z}_{\mathrm{ALR}}^{\ensuremath{'}}$) against the Randall-Sundrum graviton resonance (spin-2) and a spin-0 resonance (sneutrino) with the same mass and producing the same number of events under the observed peak. To assess the range of the ${Z}^{\ensuremath{'}}$ mass where the spin determination can be performed to a given confidence level, we focus on the angular distributions of the Drell-Yan leptons, in particular, we use as a basic observable an angular-integrated center-edge asymmetry, ${A}_{\mathrm{CE}}$. The spin of a heavy ${Z}^{\ensuremath{'}}$ gauge boson can be established with ${A}_{\mathrm{CE}}$ up to ${M}_{{Z}^{\ensuremath{'}}}\ensuremath{\simeq}3.0\text{ }\text{ }\mathrm{TeV}$, for an integrated luminosity of $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, or minimal number of events around 110. We also examine the distinguishability of the considered ${Z}^{\ensuremath{'}}$ models from one another, once the spin-1 has been established, using the total dilepton production cross section. With some assumption, one might be able to distinguish among these ${Z}^{\ensuremath{'}}$ models at 95% C.L. up to ${M}_{{Z}^{\ensuremath{'}}}\ensuremath{\simeq}2.1\text{ }\text{ }\mathrm{TeV}$.

Dilepton production in nucleon-nucleon collisions reexamined

We present a fully relativistic and gauge-invariant framework for calculating the cross sections of dilepton production in nucleon-nucleon ($\mathit{NN}$) collisions that is based on the meson-exchange approximation for the $\mathit{NN}$-scattering amplitudes. Predictions of our model are compared with those of other covariant models that have been used earlier to describe this reaction. Our results are also compared with those of the semiclassical models that are employed to get the input elementary cross sections in the transport model calculations of the dilepton production in nucleus-nucleus collisions. It is found that cross sections obtained within the semiclassical and quantum mechanical models differ noticeably from each other.

Dilepton production from the color glass condensate

We consider dilepton production in high energy proton-nucleus (and very forward nucleus-nucleus) collisions. Treating the target nucleus as a Color Glass Condensate and describing the projectile proton (nucleus) as a collection of quarks and gluons as in the parton model, we calculate the differential cross section for dilepton production in quark-nucleus scattering and show that it is very sensitive to the saturation scale characterizing the target nucleus.

Constraints for the nuclear sea quark distributions from the Drell–Yan process at the SPS

Nuclear modifications to the Drell-Yan dilepton production cross sections in p+A and A+A collisions in the leading twist approximation are caused by nuclear effects in the parton distributions of bound nucleons. For non-isoscalar nuclei, isospin corrections must also be considered. We calculate these effects for p+A and Pb+Pb collisions at CERN SPS energies. Our goal is to place constraints on nuclear effects in sea quark distributions in the region x > 0.2. We show that the net nuclear corrections remain small for p+A collisions at E_lab=450 GeV. However, in Pb+Pb collisions at E_lab=158 AGeV, effects of > 20% are predicted at large M. The data collected by the NA50 collaboration could thus be used to constrain the nuclear effects in the sea quark distributions in the region of the EMC effect, x > 0.3.

Matter-inducedρ-δmixing: A source of dileptons

We study the possibility of $\rho-\delta$ mixing via N-N excitations in dense nuclear matter. This mixing induces a peak in the dilepton spectra at an invariant mass equal to that of the $\delta$. We calculate the cross section for dilepton production through the mixing process and we compare its size with that of $\pi-\pi$ annihilation. In-medium masses and mixing angles are also calculated.

The dilepton-production cross section in principal value resummation

Using a recent calculation of the perturbative hard part for dilepton production that sums large threshold corrections to all orders in perturbative QCD, we compute the corresponding cross sections. The hard part has been evaluated using principal value resummation and contains all singular momentum-dependent corrections. We also include a resummation of large Sudakov terms, which are independent of parton momenta. We give predictions for the dilepton-mass distribution, the rapidity distribution and the rapidity-integrated K-factor at fixed-target energies and compare with various experimental results in several kinematic regimes. We find that principal value resummation produces cross sections that are finite and well-behaved. For both protons and anti-protons on fixed targets, the resummed cross sections are, in general, in excellent agreement with the data.

HOW MUCH CAN BE LEARNT ABOUT PION WAVE FUNCTION FROM THE DRELL–YAN PROCESS?

Contribution of the high twist Feynman diagrams to the dilepton production cross section in pion–hadron collisions is investigated. In calculations the asymptotic, Chernyak–Zhitnitsky and two other pion model wave functions P2, P3 are used. It is shown that this contribution depends on the choice of wave functions and for large dilepton transverse momentum pT and invariant mass Q in the limit [Formula: see text] exceeds the leading one by an order of magnitude. Perspectives of the Drell–Yan process in the context of the pion wave function investigation are discussed.

Dilepton as a probe of pion dynamics in heavy-ion collisions

Modeling heavy-ion collisions by an expanding fireball, we calculate the dilepton production cross section including contributions from both the nucleon-nucleon interactions and the pion-pion annihilations. For the pion, we take into account the medium polarization effect on its dispersion relation. We find that dilepton production at large invariant masses is dominated by the latter process and thus is a viable probe to the pion dynamics in heavy-ion collisions.

The nuclear size dependence of large momentum transfer scattering and quark interaction dynamics

A simple dynamical picture for hadron interactions is used to discuss the A dependence of large p ⊥ scattering and dilepton production. Observations in large p ⊥ experiments are explained. Important corrections to dilepton production cross sections are found, leading to acceptable agreement between colored-quark theory and experiment.