Ultraperipheral pPb Collisions Research Articles

Timelike Compton scattering in ultraperipheral pPb collisions

The study of exclusive processes in photon-induced interactions allows us to probe the generalized parton distributions (GPDs), which provide information about the 3-dimensional description of the quark and gluon content of hadrons. In this paper we investigate the timelike Compton scattering (TCS) in ultraperipheral pPb collisions at the LHC. Such process is characterized by the exclusive dilepton production through the subprocess γp→γ⁎p→l+l−p, with the real photon in the initial state being emitted by the nucleus. Assuming a given model for the GPDs, the TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the LHC detectors.

Measurement of exclusive {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}\xa0photoproduction in ultraperipheral pPb collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {Te}text {V}

Exclusive {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0} photoproduction is measured for the first time in ultraperipheral pPb collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 5.02,text {Te}text {V} with the CMS detector. The cross section sigma ({upgamma _{}^{}} mathrm{p}rightarrow {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}mathrm{p}) is 11.0 pm 1.4,text {(stat)} pm 1.0,text {(syst)} mu b at langle W_{{upgamma _{}^{}} mathrm{p}}rangle = 92.6,text {Ge}text {V} for photon–proton centre-of-mass energies W_{{upgamma _{}^{}} mathrm{p}} between 29 and 213,text {Ge}text {V} . The differential cross section mathrm {d}sigma /mathrm {d}|t | is measured in the interval 0.025< |t | < 1,text {Ge}text {V} ^{2} as a function of W_{{upgamma _{}^{}} mathrm{p}}, where t is the squared four-momentum transfer at the proton vertex. The results are compared with previous measurements and theoretical predictions. The measured cross section sigma ({upgamma _{}^{}} mathrm{p}rightarrow {{{uprho _{}^{}} _{}^{}}{{left( {770}right) }{}_{}^{}}} ^{0}mathrm{p}) has a power-law dependence on the photon–proton centre-of-mass, consistent with electron–proton collision measurements performed at HERA. The W_{{upgamma _{}^{}} mathrm{p}} dependence of the exponential slope of the differential cross section mathrm {d}sigma /mathrm {d}|t | is also measured.

Inclusive Photoproduction of Υ(1s) and J/ψ in Ultra-peripheral p-Pb Collisions at TeV**Supported by the National Natural Science Foundation of China under Grant Nos. 11465021, 11065010

With a photon spectrum of high energy charged partons, both differential and total cross sections of J/ψ and Υ(1s) inclusive photoproduction in ultra-peripheral p-Pb Collisions (UPCs) at TeV are given. A direct photon process and a resolved photon process including fragment and non-fragment contributions are considered. The total cross section is compared to the inelastic production results in coherent p-Pb collisions at the same center-of-mass energy. Our results seem non-negligible.

Exclusive Particle Production in pp and pPb Collisions at CMS

Exclusive processes provide a useful method to study a broad range of high energy physics fields from gluon density evolutions to searches for new physics. Three measurements from the Compact Muon Solenoid experiment are reviewed. Exclusive π π production is studied in proton–proton collisions. Low-mass meson resonances are observed in the invariant mass distribution of pion pairs. The total exclusive π + π − cross-section is also measured in the p T ( π ) &gt; 0 . 2 GeV, | y | &lt; 2 region, yielding 26.5 ± 0.3 ( stat ) ± 5.0 ( syst ) ± 1.1 ( lumi ) μ b. The photoproduction of Y ( n S ) mesons is observed in ultraperipheral pPb collisions. The differential cross-sections are measured as a function of | t | and y. The comparison with previous measurements and theoretical models provides a better understanding of the gluon density evolution at low x values. Evidence for the γ γ → W + W − process is shown with a 3.7 σ observed significance. According to the results, limits on anomalous quartic gauge couplings can be provided.

Probing saturation physics in the Real Compton Scattering at ultraperipheral pPb collisions

The Real Compton Scattering in ultraperipheral pPb collisions at RHIC and LHC energies is investigated and predictions for the squared transverse momentum (t) and rapidity (Y) distributions are presented. The scattering amplitude is assumed to be given by the sum of the Reggeon and Pomeron contributions and the Pomeron one is described by the Color Dipole formalism taking into account the non-linear (saturation) effects in the QCD dynamics. We demonstrate that the behavior of the cross sections at large-t and/or Y is dominated by the Pomeron contribution and is strongly affected by the non-linear effects present in the QCD dynamics. These results indicate that a future experimental analysis of this process can be useful to probe the QCD dynamics at high energies.

Rapidity and momentum transfer distributions of coherent J/ψ photoproduction in ultraperipheral pPb collisions at the LHC

Based on accurate calculations of the flux of equivalent photons of the proton and heavy nuclei and the pQCD framework for the gluon distribution in the proton and nuclei, we analyze the rapidity and momentum transfer distributions of coherent J/ψ photoproduction in ultraperipheral proton-Pb collisions at the LHC. We demonstrate that unlike the case of proton-proton UPCs marred by certain theoretical uncertainties and experimental limitations, after a cut excluding the region of small momentum transfers, ultraperipheral proton-Pb collisions offer a clean way to study the gluon distribution in the proton down to x ≈ 10−5. Our analysis of the momentum transfer distributions shows that an interplay of J/ψ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small p t in a definite region of the rapidity y.

Nuclear effects in photoproduction of heavy quarks and vector mesons in ultraperipheral PbPb andpPb collisions at energies available at the CERN Large Hadron Collider

The comparison of photoproduction cross sections for $c\bar{c}$ and b-b(bar) in PbPb and pPb collisions can give sensitivity to nuclear shadowing effects. The photoproduction of vector mesons is even more sensitive to the underlying gluon distributions. In this study we present the cross sections and rapidity dependence of the photoproduction of heavy quarks and exclusive production of vector mesons in ultraperipheral pPb and PbPb collisions at the Large Hadron Collider at sqrt(s_NN)=5 TeV and sqrt(s_NN)=2.76$ TeV, respectively. The potentials of using these processes for constraining nuclear gluon shadowing are explored. It is found that photoproduction of $J/\psi$ and $\Upsilon$ in PbPb collisions in particular exhibit very good sensitivity to gluon shadowing.