Ultra-peripheral Pb-Pb Collisions Research Articles

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at sqrt{{textrm{s}}_{textrm{NN}}} = 5.02 TeV

The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sqrt{{textrm{s}}_{textrm{NN}}} = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| < 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 < Wγ Pb,n < 920 GeV, where Wγ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10−5 < x < 3.3 × 10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.

Exclusive J/Psi plus jet associated production in ultraperipheral PbPb collisions

The study of exclusive processes in ultraperipheral collisions at the Large Hadron Collider (LHC) has allowed us to test several aspects of the Standard Model and to search for New Physics. In this letter, we investigate the possibility of using these processes to improve our understanding of the quarkonium production mechanism through the study of the exclusive J/Psi plus jet associate production in ultraperipheral PbPb collisions. We estimate the transverse-momentum and rapidity distributions considering that the gamma gamma rightarrow J/Psi + X(X = gamma , , g) subprocess is described by the Non-Relativistic QCD (NRQCD) formalism and present predictions for the rapidity ranges covered by central and forward detectors. The experimental separation of these events is discussed and the results indicate that a future experimental analysis is, in principle, feasible in future runs of the LHC and the Future Circular Collider (FCC).

Double particle production in ultraperipheral PbPb collisions at the Large Hadron Collider and Future Circular Collider

Double particle production in ultraperipheral PbPb collisions at the Large Hadron Collider and Future Circular Collider

Study of exclusive photoproduction of charmonium in ultra-peripheral lead-lead collisions

The cross-sections of exclusive (coherent) photoproduction J/ψ and ψ(2S) mesons in ultra-peripheral PbPb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV are measured using a data sample corresponding to an integrated luminosity of 228 ± 10 μb−1, collected by the LHCb experiment in 2018. The differential cross-sections are measured separately as a function of transverse momentum and rapidity in the nucleus-nucleus centre-of-mass frame for J/ψ and ψ(2S) mesons. The integrated cross-sections are measured to be {sigma}_{J/psi}^{textrm{coh}} = 5.965 ± 0.059 ± 0.232 ± 0.262 mb and {sigma}_{psi (2S)}^{textrm{coh}} = 0.923 ± 0.086 ± 0.028 ± 0.040 mb, where the first listed uncertainty is statistical, the second systematic and the third due to the luminosity determination. The cross-section ratio is measured to be {sigma}_{psi (2S)}^{textrm{coh}}/{sigma}_{J/psi}^{textrm{coh}} = 0.155 ± 0.014 ± 0.003, where the first uncertainty is statistical and the second is systematic. These results are compatible with theoretical predictions.

Vector meson photoproduction in ALICE at the LHC

Ultra-peripheral collisions (UPC) of heavy ions provide unique opportunity to study vector meson (VM) photoproduction in ALICE. The recent results of analyses of coherent ρ0 photoproduction cross sections in Pb-Pb and Xe-Xe UPC at sNN=5.02 and 5.44 TeV, respectively are presented. Moreover, rapidity and |t| dependence of coherent J/ψ and rapidity dependence of ψ′ photoproduction cross section in Pb-Pb UPC at sNN=5.02TeV are shown. The most recent J/ψ photoproduction and exclusive dimuon cross section studies in p-Pb collisions at sNN=8.16TeV as well as perspectives for future measurements in Run 3 are also discussed in this paper.

Associated $$\phi $$ and $$J/\Psi $$ photoproduction in ultraperipheral PbPb collisions at the Large Hadron Collider and Future Circular Collider

In this paper we analyze the associated $\phi$ and $J/\Psi$ photoproduction in ultraperipheral $PbPb$ collisions through the double scattering mechanism for the energies of the Large Hadron Collider (LHC) and Future Circular Collider (FCC). Our results complement a previous analysis for the $\rho\rho$, $J/\Psi J/\Psi$ and $\rho J/\Psi$ production. We present our predictions for the total cross sections and rapidity distributions considering the rapidity ranges covered by the ALICE and LHCb detectors, which indicate that a future experimental analysis of $\phi J/\Psi$ final state is feasible. These results point out that the study of the double vector meson photoproduction in heavy ion collisions can be useful to constrain the double scattering mechanism and improve our understanding of the QCD dynamics at high energies.

Recent heavy-ion results from LHCb

We briefly report on the recent heavy-ion measurements performed by the LHCb collaboration. An interesting variety of results are presented, such as the measurement of promptly produced charged particles in pp and pPb collisions, the cross-section ratio χc2 over χc1 in pPb and the measurement of coherent J/ψ in peripheral and ultra-peripheral PbPb collisions.

Probing quantum entanglement and collectivity effects in e+p collisions at HERA H1

Charged particle multiplicity spectra are measured at HERA H1 in DIS process at \sqrt{s}s=319 GeV. The hadron entropy determined from the multiplicity spectra is compared to the gluon entropy predicted from a quantum-entanglement model. On the other hand, observed collective behaviours in high multiplicity ppA, pppp and ultra-peripheral PbPb collisions at RHIC and LHC motivate a study in epep collisions. Two- and multi-particle correlations are studied in DIS, as well as photoproduction events with collision energy \left\langle W_{\gamma p} \right \rangle \sim⟨Wγp⟩∼270 GeV.

Recent results from heavy ion collisions at LHCb

LHCb is a fully instrumented spectrometer covering the forward rapidity region at the LHC, which provides unique access to the small Bjorken-xx region inside the nucleus. In this contribution, a selection of recent results from the LHCb heavy-ion program are presented, including a measurement of charged hadron production in proton-proton (pp) and proton-lead (pPb) collisions, \chi_{c2}/\chi_{c1}χc2/χc1 production in pPb collisions, and coherent J/\psiJ/ψ production in peripheral and ultra-peripheral PbPb collisions. Comparisons with calculations and effects from the nuclear PDFs and dense QCD medium are discussed.

Central Diffraction and Ultra-Peripheral Collisions in ALICE in Run 3 and 4

The ALICE experiment at the LHC is undergoing a major upgrade during the Long Shutdown 2 (2019-2021). In particular, the Time Projection Chamber (TPC) is being equipped with new GEM-based readout chambers and the readout electronics of several detectors are being replaced with faster and more flexible technology. This will allow ALICE to read out most of the detectors in the continuous mode and record minimum bias Pb-Pb events at rates of about 50 kHz in Run 3 (2021-2024) and Run 4 (2027-2030). The ALICE Collaboration is also considering the possibility to collect a large sample of proton-proton collisions at interaction rates of about 1 MHz using online and offline preselection of rare events. These goals require a completely new online computing system that will be used to perform fast reconstruction and compression of the data stream. The event selection strategy becomes especially challenging for the case of central diffractive events and ultra-peripheral Pb-Pb collisions characterized by rapidity gaps at forward and backward directions with only few tracks at central rapidity. In this contribution, the motivation for studying central diffractive and ultra-peripheral events is presented, and feasibility studies for their selection in Run 3 and 4 will be given.

How large is the diffractive contribution to inclusive dijet photoproduction in ultraperipheral collisions at the LHC?

Using next-to-leading order (NLO) perturbative QCD, we calculate the diffractive contribution to inclusive dijet photoproduction in Pb-Pb ultraperipheral collisions (UPCs) at the LHC and find that it does not exceed $5-10$% in small-$x_A$ bins in the ATLAS kinematics at $\sqrt{s_{NN}}=5.02$ TeV. Its smallness is a result of the restricted kinematics ($p_{T1} > 20$ GeV and $x_A > 0.001$), the large nuclear suppression of nuclear diffractive parton distribution functions predicted in the leading twist model of nuclear shadowing, and additional suppression due to QCD factorization breaking in diffraction. At the same time, using looser $p_T$ cuts, e.g., $p_{T1} > 10$ GeV and $p_{T i\neq 1} > 5-7$ GeV, we find that $(d\sigma_{\rm diff}/dx_A)/(d\sigma_{\rm inc}/dx_A)$ can reach $10-20$% at $x \approx 5 \times 10^{-4}$. Also, applying our framework to proton-proton UPCs at $\sqrt{s_{NN}}=13$ TeV, we find that the ratio of the diffractive and inclusive cross sections of dijet photoproduction $(d\sigma_{\rm diff}/dx_p)/(d\sigma_{\rm inc}/dx_p)$ is as large as $10-15$% for $x_p \sim 5 \times 10^{-4}$. An account of the contribution of Pomeron-Pomeron scattering, which is not included in our analysis, should make this ratio somewhat larger.

Search for low-mass leptoquarks using the llqq final states in Pb-Pb ultra-peripheral collisions

After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant ≲ 0.1). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in p-p collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, S3, with an electric charge left|qright|=frac{4}{3}e , as example, the mass point of 100 GeV can be excluded at the 95% confidence level using a dataset of 4 pb−1 Pb-Pb ultra-peripheral collisions at sqrt{s} = 5.02 TeV and the performance of the ATLAS detector in Run 2. The proposed method also applies to searching for high-mass LQs in p-p collisions as long as the LQ pair production mechanism dominates.

Nuclear suppression from coherent J/ψ photoproduction at the Large Hadron Collider

Using the data on coherent J/ψ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) obtained in Runs 1 and 2 at the Large Hadron Collider (LHC), we determined with a good accuracy the nuclear suppression factor of SPb(x) in a wide range of the momentum fraction x, 10−5≤x≤0.04. In the small-x region x<10−3, our χ2 fit favors a flat form of SPb(x)≈0.6 with approximately a 5% accuracy for x=6×10−4−10−3 and a 25% error at x=10−4. At the same time, uncertainties of the fit do not exclude a slow decrease of SPb(x) in the small-x limit. At large x, SPb(x) is constrained to better than 10% precision up to x=0.04 and is also consistent with the value of SPb(x) at 〈x〉=0.042, which we extract from the Fermilab data on the A dependence of the cross section of coherent J/ψ photoproduction on fixed nuclear targets. The resulting uncertainties on SPb(x) are small, which indicates the potential of the LHC data on coherent charmonium photoproduction in Pb-Pb UPCs to provide additional constraints on small-x nPDFs. We explicitly demonstrate this using as an example the EPPS16 and nCTEQ16 nuclear parton distribution functions, whose uncertainties decrease severalfold after the Bayesian reweighting of the discussed UPC data.

Exclusive dilepton production at forward rapidities in PbPb collisions

The dilepton production at forward rapidities in diffractive and exclusive processes present in ultraperipheral PbPb collisions at the LHC is investigated. Predictions for the e^+ e^-, mu ^+ mu ^- and tau ^+ tau ^- cross sections are presented taking into account of realistic cuts that can be implemented by the LHCb Collaboration in a future experimental analysis. Our results indicate that the background associated with the diffractive production can be strongly suppressed and the exclusive processes can be cleanly separated. For the tau ^+ tau ^- production, the semi and purely leptonic decay channels are considered. Our results indicate that a future experimental analysis of the dilepton production at the LHCb is feasible and can be useful to search for BSM physics.

Z production in pPb collisions and charmonium production in PbPb ultra-peripheral collisions at LHCb

This article presents recent results of the Z boson production in the proton-lead collisions and the charmonium production in the ultra-peripheral lead-lead collisions at the LHC collected during 2013 to 2018 by the LHCb detector. The potential heavy ion programs in these topics are also discussed.

Incoherent ρ meson photoproduction in ultraperipheral nuclear collisions at the CERN Large Hadron Collider

Using the Gribov-Glauber model for photon-nucleus scattering and a generalization of the vector meson dominance model for the hadronic structure of the photon, we make predictions for the cross section of incoherent $\rho$ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) in the Large Hadron Collider kinematics. We find that the effect of the inelastic nuclear shadowing is significant and leads to an additional 25% suppression of the incoherent cross section. Comparing our predictions to those of the STARlight Monte Carlo framework, we observe very significant differences.

Dark photons from pions produced in ultraperipheral PbPb collisions

In this letter we propose the search of dark photons in the decay of pions produced by γγ interactions in ultraperipheral PbPb collisions. The cross section is estimated considering an accurate treatment for the absorptive corrections and for the nuclear form factor. Predictions for the event rates are presented considering the expected luminosities for the LHC, High – Luminosity LHC and High – Energy LHC as well as for the Future Circular Collider. Our results indicate that a future experimental analysis of the pion production in ultraperipheral heavy ion collisions can be useful to probe the dark photon production and constrain its properties.

Coherent photoproduction of \u03c10 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV

Cross sections for the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV are reported. The measurements, which rely on the π+π− decay channel, are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c2 and a width of about 140 MeV/c2 is reported and compared with similar observations from other experiments.

Production of axionlike particles in PbPb collisions at the LHC, HE–LHC and FCC: A phenomenological analysis

The production of axionlike particles (ALP) in ultraperipheral PbPb collisions (UPHIC) is investigated considering the energies of the next run of the Large Hadron Collider (LHC) and of the future High Energy–LHC (HE–LHC) and Future Circular Collider (FCC). Assuming four different combinations for the ALP mass and coupling and the typical exclusivity cuts for central and forward detectors, we estimate the cross section and invariant mass, rapidity, transverse momentum and acoplanarity distributions associated to the diphoton final state produced in the γγ→a→γγ subprocesses. A detailed analysis of the backgrounds is performed. We demonstrate that the backgrounds can be strongly reduced by the exclusivity cuts and that a forward detector, as the LHCb, is ideal to probe an ALP with small mass. Finally, our results indicate that a future experimental analysis of the diphoton final state in UPHIC can probe the existence and properties of axionlike particles.

Exclusive and diffractive gamma gamma production in PbPb collisions at the LHC, HE-LHC and FCC

In this paper we present a detailed analysis of the contribution of the Light-by-Light (LbL), Durham and double diffractive processes for the diphoton production in ultraperipheral PbPb collisions at the Large Hadron Collider (LHC), High-Energy LHC (HE-LHC) and Future Circular Collider (FCC). The acceptance of the central and forward LHC detectors is taken into account and predictions for the invariant mass, rapidity, transverse momentum and acoplanarity distributions are presented. Our results indicate that the contribution of the Durham process is negligible and that the double diffractive process can be strongly suppressed by the exclusivity cuts, which will allow to perform a precise analysis of the LbL scattering, as well the search of beyond Standard Model physics in this final state.