<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>ρ</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math>photoproduction in AuAu collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>62.4</mml:mn></mml:mrow></mml:math>GeV measured with the STAR detector

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Abstract: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 dσ/dy = 425 ± 10 (stat.) +42 −50 (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 results are compared with those from lower energies and with model predictions. Keywords: Hadron-Hadron ScatteringArXiv ePrint: 1503.09177Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP 3 .doi:10.1007/JHEP09(2015)095JHEP09 (2015)095 The ALICE collaboration 20 IntroductionCharged particle beams at the LHC generate an electromagnetic field which can be regarded as a beam of quasi-real photons; thus at the LHC, besides hadronic interactions, also photonuclear and photon-photon interactions occur. Collisions in which the impact parameter exceeds the sum of the radii of the incoming beam particles are called ultraperipheral collisions (UPC). In UPC the cross section for hadronic processes is strongly suppressed, while the cross sections for two-photon and photonuclear interactions remain large. This is particularly the case for heavy ions, because the intensity of the photon flux grows with the square of the ion charge, Z. A number of reviews of UPC are available; e.g., [1,2]. The ALICE Collaboration has previously studied exclusive photoproduction of J/ψ in ultra-peripheral Pb-Pb and p-Pb collisions [3][4][5]. Exclusive photoproduction of ρ 0 vector mesons, Pb + Pb → Pb + Pb + ρ 0 , can be described as the fluctuation of a quasi-real photon into a virtual ρ 0 vector meson, which then scatters elastically off the target nucleus. Two cases can be distinguished. When the interaction involves the complete target nucleus, the process is called coherent. In this case the target nucleus normally remains intact. If the virtual ρ 0 vector meson scatters off only one of the nucleons in the target, then the process is called incoherent and in this case the target nucleus normally breaks up, emitting neutrons at very forward rapidities. For coherent processes, the size of the lead ion restricts the mean transverse momentum of the vector meson to be about 60 MeV/c corresponding to a de Broglie wavelength of the nuclear size, while it is of the order of 500 MeV/c for incoherent processes.Because of the strong electromagnetic fields in ultra-peripheral collisions of heavy ions, multiple photons may be exchanged in a single event. The additional photons can lead to excitation of the nuclei. The dominant process is the excitation to a Giant Dipole Resonance [6]. As these photonuclear processes occur on a different time scale, they are -1 - JHEP09(2015)095assumed to be independent, so the probabilities factorize. The excited nucleus typically decays b...

Section: Jhep09(2015)095contrasting

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confidence: 58%

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See 3 more Smart Citations

Coherent ρ 0 photoproduction in ultra-peripheral Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Adam¹,

Adamová²,

Aggarwal³

et al. 2015

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Diffractive dijet photoproduction in ultraperipheral collisions at the LHC in next-to-leading order QCD

Guzey

Klasen

2016

Abstract:We make predictions for the cross sections of diffractive dijet photoproduction in pp, pA and AA ultraperipheral collisions (UPCs) at the LHC during Runs 1 and 2 using next-to-leading perturbative QCD. We find that the resulting cross sections are sufficiently large and, compared to lepton-proton scattering at HERA, have an enhanced sensitivity to small observed momentum fractions in the diffractive exchange, commonly denoted z jets P , and an unprecedented reach in the invariant mass of the photon-nucleon system W . We examine two competing schemes of diffractive QCD factorization breaking, which assume either a global suppression factor or a suppression for resolved photons only and demonstrate that the two scenarios can be distinguished by the nuclear dependence of the distributions in the observed parton momentum fraction in the photon x jets γ .

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

Acharya

Adamová

Adler

et al. 2020

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Cross sections for the coherent photoproduction of ρ 0 vector mesons in ultraperipheral Pb-Pb collisions at √ s 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 nuclearbreakup 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/c 2 and a width of about 140 MeV/c 2 is reported and compared with similar observations from other experiments.