Elastic and proton-dissociative

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: Discussionmentioning

confidence: 75%

“…The result from ZEUS with proton targets for W γN in the range 55-90 GeV shows that |B/A| varies with the γ-proton momentum tranfer [27]. The average is |B/A| = 0.67 ± 0.02 (stat.)…”

Section: Jhep09(2015)095mentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

“…Other parameterizations of the ρ 0 shape are possible, and as a cross check the invariant mass distribution was also fit to a Ross-Stodolsky function [27,31]:…”

Section: Jhep09(2015)095mentioning

confidence: 99%

mentioning

confidence: 99%

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

Photoproduction of π + π − pairs in a model with tensor-pomeron and vector-odderon exchange

Bolz

Ewerz

Maniatis

et al. 2015

We consider the reaction γp → π + π − p at high energies. Our description includes dipion production via the resonances ρ, ω, ρ and f 2 , and via non-resonant mechanisms. The calculation is based on a model of high energy scattering with the exchanges of photon, pomeron, odderon and reggeons. The pomeron and the C = +1 reggeons are described as effective tensor exchanges, the odderon and the C = −1 reggeons as effective vector exchanges. We obtain a gauge-invariant version of the Drell-Söding mechanism which produces the skewing of the ρ-meson shape. Starting from the explicit formulae for the matrix element for dipion production we construct an event generator which comprises all contributions mentioned above and includes all interference terms. We give examples of total and differential cross sections and discuss asymmetries which are due to interference of C = +1 and C = −1 exchange contributions. These asymmetries can be used to search for odderon effects. Our model is intended to provide all necessary theoretical tools for a detailed experimental analysis of elastic dipion production for which data exist from fixed target experiments, from HERA, and are now being collected by LHC experiments.

Diffractive electroproduction of ρ and ϕ mesons at HERA

Aaron¹,

Martin²,

Alexa³

et al. 2010

115

107

Diffractive electroproduction of ρ and φ mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb −1 . About 10500 ρ and 2000 φ events are analysed in the kinematic range of squared photon virtuality 2.5 ≤ Q 2 ≤ 60 GeV 2 , photon-proton centre of mass energy 35 ≤ W ≤ 180 GeV and squared four-momentum transfer to the proton |t| ≤ 3 GeV 2 . The total, longitudinal and transverse cross sections are measured as a function of Q 2 , W and |t|. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small qq dipoles, for Q 2 larger than 10 to 20 GeV 2 . They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q 2 +M 2 V )/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.