We present a measurement of pi^+pi^-pi^+pi^- photonuclear production in ultra-peripheral Au-Au collisions at sqrt(s_{NN}) = 200 GeV from the STAR experiment. The pi^+pi^-pi^+pi^- final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The pi^+pi^-pi^+pi^- invariant mass spectrum of the coherent events exhibits a broad peak around 1540 pm 40 MeV/c^2 with a width of 570 pm 60 MeV/c^2, in agreement with the photoproduction data for the rho^0(1700). We do not observe a corresponding peak in the pi^+pi^- final state and measure an upper limit for the ratio of the branching fractions of the rho^0(1700) to pi^+pi^- and pi^+pi^-pi^+pi^- of 2.5 % at 90 % confidence level. The ratio of rho^0(1700) and rho^0(770) coherent production cross sections is measured to be 13.4 pm 0.8 (stat.) pm 4.4 (syst.) %.
Lead nuclei, accelerated at the LHC, are sources of strong electromagnetic fields that can be used to measure photon-induced interactions in a new kinematic regime. These interactions can be studied in ultra-peripheral p-Pb and Pb-Pb collisions where impact parameters are larger than the sum of the nuclear radii and hadronic interactions are strongly suppressed. Heavy quarkonium photoproduction is of particular interest since it is sensitive to the gluon distribution in the target. The ALICE Collaboration has studied J/psi and psi(2S) photoproduction in ultra-peripheral Pb-Pb collisions and ex…
Ultra-peripheral Pb-Pb collisions, in which the two nuclei pass close to each other, but at an impact parameter greater than the sum of their radii, provide information about the initial state of nuclei. In particular, heavy vector meson production, where the particle mass sets a hard scale, proceeds in such collisions by photon-gluon interactions, and gives access to nuclear PDFs. The ALICE collaboration has published measurements of J/psi and psi(2S) photoproduction in ultra-peripheral collisions in LHC Run 1 at forward (J/psi) and mid-rapidity, and has obtained a substantially larger data s…
Vector mesons are copiously produced in ultra-peripheral collisions. In these collisions, the impact parameter is larger than the sum of the radii of two projectiles, implying that electromagnetic processes become dominant. The cross section for this process is sensitive to the gluon distribution and can therefore probe nuclear gluon shadowing (Pb-Pb) and the gluon structure function in the nucleon (p-Pb). The ALICE Collaboration has performed the first measurement of the coherent J/$ψ$ and $ψ$(2S) photoproduction cross section in Pb-Pb collisions and that for exclusive J/$ψ$ photoproduction o…
Ultra-peripheral collisions (UPCs), studied using the ALICE detector, allow us to investigate the low-$x$ behavior of the gluon distribution of the colliding particles. Two new measurements of coherent J/$ψ$ photoproduction cross section from Pb-Pb UPCs at $\sqrt{s_{\rm NN}}=5.02$ TeV are presented: the first measurement of the $|t|$-dependence of the cross section providing a new tool to investigate the transverse gluonic structure at low Bjorken-$x$ and a rapidity-differential measurement at midrapidity allowing us to provide stringent constraints on nuclear gluon shadowing and saturation mo…
The ALICE collaboration performed the first rapidity-differential measurement of coherent J/$ψ$ photoproduction in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. The J/$ψ$ is detected via its dimuon decay in the forward rapidity region ($-4.0 < y < -2.5$) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 $μ$b$^{-1}$. The cross section for coherent J/$ψ$ production is presented in six rapidity bins. The results are compared with theoretic…
We report the first measurement at the LHC of coherent photoproduction of $ρ^0$ mesons in ultra-peripheral Pb-Pb collisions. The invariant mass and transverse momentum distributions for $ρ^0$ production are studied in the $π^+ π^-$ decay channel at mid-rapidity. The production cross section in the rapidity range $|y|<0.5$ is found to be $\mathrm{d}σ/\mathrm{d}y = 425 \pm 10 \, (\mathrm{stat.})$ $^{+42}_{-50} \, (\mathrm{sys.})$ mb. Coherent $ρ^0$ production is studied with and without requirement of nuclear breakup, and the fractional yields for various breakup scenarios are presented. The res…
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $J/ψ$ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. The e$^+$e$^-$ and $μ^+μ^-$ decay channels are used to measure the cross section as a function of the rapidity of the $J/ψ$ in the range $-2.5 < y < 2.7$, corresponding to an energy in the $γ$p centre-of-mass in the interval $40 < W_{γ\mathrm{p}}<550$ GeV. The measurements, which are consistent with a power law dependence of the exclusive $J/ψ$ photoproduc…
Photoproduction of $J/Ψ$ in peripheral Oxygen-Oxygen collisions
Universidade do Estado de Santa Catarina（圣卡塔琳娜州立大学）；Federal University of Grande Dourados, Faculty of Exact Sciences and Technology（大杜拉多斯联邦大学精确科学与技术学院）；Institute of Physics and Mathematics, Federal University of Pelotas (UFPel)（佩洛塔斯联邦大学物理与数学研究所）
Cross sections for the coherent photoproduction of $ρ^{0}$ vector mesons in ultra-peripheral Pb-Pb collisions at $ \sqrt{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 coher…
Probing the neutron-skin thickness through $J/ψ$ photoproduction in ultra-peripheral collisions
Shanghai Institute of Applied Physics, Chinese Academy of Sciences（中国科学院上海应用物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Fudan University（复旦大学）；East China Normal University（华东师范大学）
The coherent photoproduction of $\rm{J/ψ}$ and $\rm{ψ'}$ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $\rm{J/ψ}$ is reconstructed using the dilepton ($l^{+} l^{-}$) and proton-antiproton decay channels, while for the $\rm{ψ'}$, the dilepton and the $l^{+} l^{-} π^{+} π^{-}$ decay channels are studied. The analysis is based on an event sample corresponding to an integrated…
The cross section for coherent J/psi photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, X[n]0[n], is measured with the CMS experiment in ultra-peripheral PbPb collisions at sqrt(s[NN]) = 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 inverse microbarns, collected during the 2011 PbPb run. The J/psi mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is dsigma[coh,X[n]0[n]] / d…
The intense photon fluxes of relativistic nuclei provide a possibility to study photonuclear and two-photon interactions in ultra-peripheral collisions (UPC) where the nuclei do not overlap and no strong nuclear interactions occur. The study of such collisions provides information about the initial state of nuclei. First ALICE results from the LHC Run 2 are presented for forward exclusive \jpsi production in UPC, which is sensitive to the gluon distribution in nuclei. The increased statistics and the higher collision energy allows for a more detailed study at lower values of Bjorken-$x$. The a…
The photoproduction of heavy vector mesons in the electromagnetic interactions of ultra-relativistic nuclei is sensitive to the gluon distribution in the nucleus and thus to cold nuclear matter effects like shadowing or parton saturation. Besides the well known observations of vector meson production in ultra-peripheral collisions, first observations of an excess over the expected hadronic J/$ψ$ production at very low transverse momentum ($p_T < $~0.3 GeV/$c$) in peripheral and semi-central nucleus-nucleus collisions both at LHC and RHIC energies were interpreted as the first sign of coherent …
The ALICE collaboration has made the first measurement at the LHC of J/$ψ$ photoproduction in ultra-peripheral Pb-Pb-collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. The J/$ψ$ is identified via its dimuon decay in the forward rapidity region with the muon spectrometer for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 55 $μ$b$^{-1}$. The cross section for coherent J/$ψ$ production in the rapidity interval $-3.6 < y < -2.6$ is measured to be d$σ$/d$y = 1.00 \pm 0.18$ (stat) $+0.24/-0.26$ (sys…
We present the first measurement at the LHC of exclusive J/$ψ$ photoproduction off protons, in ultra-peripheral proton-lead collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Events are selected with a dimuon pair produced either in the rapidity interval, in the laboratory frame, $2.5<y<4$ (p-Pb) or $-3.6<y<-2.6$ (Pb--p), and no other particles observed in the ALICE acceptance. The measured cross sections $σ(γ+ {\rm p} \rightarrow J/ψ+ {\rm p})$ are 33.2 $\pm$ 2.2 (stat) $\pm$ 3.1 (syst) $\pm$ 0.7 (theo) nb in p-Pb and 284 $\pm$ 36 (stat) $^{+27}_{-32}$ (syst) $\pm$ 26 (theo) nb in Pb-p collisions. W…
The ALICE Collaboration at the LHC has measured the J/$ψ$ and $ψ^{'}$ photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 $μ\rm{b}^{-1}$. The cross section for coherent and incoherent J/$ψ$ production in the rapidity interval $-0.9<y<0.9$, are $\mathrm{d}σ_{{\rm J}/ψ}^{\rm coh}/\mathrm{d}y = 2.38^{+0.34}_{-0.24}\big(\mathrm{sta+sys}\bi…
Exclusive dimuon production and coherent charmonium photoproduction at forward rapidity in ultra-peripheral Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV
European Organization for Nuclear Research（欧洲核子研究中心）
The first measurement of the coherent photoproduction of $ρ^{0}$ vector mesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}= 5.44$ TeV is presented. This result, together with previous HERA $γ$p data and $γ$-Pb measurements from ALICE, describes the atomic number ($A$) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the ${\rm Xe}+{\rm Xe} \to ρ^{0} + {\rm Xe}+{\rm Xe}$ process, measured at midrapidity through the decay channel $ρ^{0}\toπ^…
Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., $J/ψ$ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC $J/ψ$ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to lo…
The $ρ^0$ is copiously photoproduced in ultra-peripheral heavy-ion collisions at the LHC. In this talk, I will present recent results on $ρ$ photoproduction with ALICE, including cross-section measurements from PbPb and XeXe collisions, including a discussion of the production of high-mass final states that decay to $π^+π^-$, and of neutron production that accompanies $ρ$ photoproduction. I will conclude by presenting some prospects for ALICE in Runs 3 and 4.
The cross section for coherent $\mathrm{J/}ψ$ photoproduction in Pb-Pb collisions is the sum of two contributions, one from low-, the other from high-energy photon-nucleus interactions. A novel method to disentangle both contributions allowing one to extract the coherent photo-nuclear cross section for coherent $\mathrm{J/}ψ$ production, $σ_{γ\rm Pb}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultra-peripheral Pb-Pb collisions at the LHC. Applying the proposed method to the available data it is possible to obtain $σ_{γ\rm Pb}$ up to a photon…
Measurement of jet photoproduction in ultra-peripheral Pb+Pb collisions without nuclear breakup at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with the ATLAS detector
EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)（欧洲核子研究中心）
Coherent Photon-Pomeron and Photon-Photon Interactions in Ultra-Peripheral Collisions at RHIC
超周边重离子碰撞涉及在大于两倍核半径的碰撞参量下的长程电磁相互作用，其中不发生核子-核子碰撞。STAR合作组首次观测到了伴随和不伴随核分裂的相干rho^0产生，分别为AuAu->Au*Au*rho^0和AuAu->Au Au rho^0，以及e+e-对产生AuAu->Au*Au*e+e-。横动量谱在低pT处出现峰值，表明与核的相干耦合。在2 π不变质谱中观察到一个清晰的rho^0信号。
