Γp Interactions Research Articles

Two-particle azimuthal correlations in γp interactions using pPb collisions at [formula omitted

The first measurements of the Fourier coefficients (VnΔ) of the azimuthal distributions of charged hadrons emitted from photon-proton (γp) interactions are presented. The data are extracted from 68.8 nb−1 of ultra-peripheral proton-lead (pPb) collisions at sNN=8.16TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This γp system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The VnΔ coefficients are presented in ranges of event multiplicity and transverse momentum (pT) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean pT of charged particles is smaller in γp than in pPb collisions. For both the γp and pPb samples, V1Δ is negative, V2Δ is positive, and V3Δ consistent with 0. For each multiplicity and pT range, V2Δ is larger for γp events. The γp data are consistent with model predictions that have no collective effects.

Study of the Nature of the Confinement in the GlueX Experiment

Confinement is a fundamental property of quantum chromodynamics (QCD) associated with the unique role of the gluonic field responsible for binding quarks in hadrons. Understanding the role of gluons in the confinement of quarks is one of the most tantalizing topics in modern particle physics to be explored. The new experiment GlueX has been recently constructed at Jefferson Lab [1] . The experiment was designed to search for hybrid mesons with exotic quantum numbers using a beam of linearly polarized photons incident on a liquid hydrogen target. The spectrum of these states and their mass splitting from normal mesons may yield information on confinement. In addition, these observations in combination with detailed chromodynamics calculations such as on the Lattice can provide important tests for our understanding of the role of gluons. The production of exotic mesons is expected to be enhanced in γp interactions, where the experimental data is very limited. We present the description of the GlueX detector, beam line, and first results of the commissioning with photon beam.

The FP420 R&D project: Higgs and New Physics with forward protons at the LHC

We present the FP420 R&D project, which has been studying the key aspects of the development and installation of a silicon tracker and fast-timing detectors in the LHC tunnel at 420 m from the interaction points of the ATLAS and CMS experiments. These detectors would measure precisely very forward protons in conjunction with the corresponding central detectors as a means to study Standard Model (SM) physics, and to search for and characterise new physics signals. This report includes a detailed description of the physics case for the detector and, in particular, for the measurement of Central Exclusive Production, pp→p+ϕ+p, in which the outgoing protons remain intact and the central system ϕ may be a single particle such as a SM or MSSM Higgs boson. Other physics topics discussed are γγ and γp interactions, and diffractive processes. The report includes a detailed study of the trigger strategy, acceptance, reconstruction efficiencies, and expected yields for a particular pp→pHp measurement with Higgs boson decay in the bb̄ mode. The document also describes the detector acceptance as given by the LHC beam optics between the interaction points and the FP420 location, the machine backgrounds, the new proposed connection cryostat and the moving (``Hamburg'') beam-pipe at 420 m, and the radio-frequency impact of the design on the LHC. The last part of the document is devoted to a description of the 3D silicon sensors and associated tracking performances, the design of two fast-timing detectors capable of accurate vertex reconstruction for background rejection at high-luminosities, and the detector alignment and calibration strategy.

Scaling violations and determination of αs from jet production in γp interactions at HERA

Differential cross sections for jet photoproduction in the reaction ep→e jet X have been measured with the ZEUS detector at HERA using 82.2 pb−1 of integrated luminosity. Inclusive jet cross sections are presented as a function of the jet transverse energy, ETjet, for jets with ETjet>17 GeV and pseudorapidity −1<ηjet<2.5, in the γp centre-of-mass-energy range 142<Wγp<293 GeV. Scaled jet invariant cross sections are presented as a function of the dimensionless variable xT≡2ETjet/Wγp for 〈Wγp〉=180 and 255 GeV. Next-to-leading-order QCD calculations give a good description of the measured differential cross sections in both magnitude and shape. The ratio of scaled jet invariant cross sections at the two 〈Wγp〉 values shows clear non-scaling behaviour. A value for the strong coupling constant of αs(MZ)=0.1224±0.0001(stat.)+0.0022−0.0019(exp.)+0.0054−0.0042(th.) has been extracted from a QCD analysis of the measured dσ/dETjet. The variation of αs with ETjet is in good agreement with the running of αs as predicted by QCD.

The Q2 dependence of dijet cross sections in γp interactions at HERA

The dependence of the photon structure on the photon virtuality, Q 2, is studied by measuring the reaction e + p→ e ++jet+jet+X at photon-proton centre-of-mass energies 134< W<223 GeV. Events have been selected in the Q 2 ranges ≈ 0 GeV 2, 0.1–0.55 GeV 2, and 1.5–4.5 GeV 2, having two jets with transverse energy E T jet>5.5 GeV in the final state. The dijet cross section has been measured as a function of the fractional momentum of the photon participating in the hard process, x γ OBS. The ratio of the dijet cross section with x γ OBS<0.75 to that with x γ OBS>0.75 decreases as Q 2 increases. The data are compared with the predictions of NLO pQCD and leading-order Monte Carlo programs using various parton distribution functions of the photon. The measurements can be interpreted in terms of a resolved photon component that falls with Q 2 but remains present at values of Q 2 up to 4.5 GeV 2. However, none of the models considered gives a good description of the data.

Inclusive hard processes in γγ and γp interactions

Measurements of jet, prompt photon, high- p T hadron and heavy quark production in photon-induced processes provide tests of QCD and are sensitive to the photon parton densities. A review of the latest experimental results in γγ and γp interactions is presented. Next-to-leading-order QCD calculations for these measurements are discussed.

Dijet cross sections in γp interactions using real and virtual photons at HERA

We present a study of the dependence of the photon structure on the photon virtuality, Q 2, by measuring the reaction e + p → e + + jet +jet + X at photon-proton centre-of-mass energy 134 < W < 223 GeV. Samples of events in the Q 2 ranges 1.5 – 4.5 GeV 2, 0.1 – 0.55 GeV 2, and Q 2 ≈ 0 GeV 2 having two jets in the final state have been selected using the K T algorithm. The ratio of dijet cross section with ξ γ 0BS < 0.75 to that with ξ γ 0BS > 0.75 decreases as Q 2 increases. The data are compared with the predictions of leading order Monte Carlo programs for various parton distribution functions of the photon. These comparisons shows evidence for resolved photon processes at value of Q 2 up to 4.5 GeV 2, and that these processes are suppressed relative to direct processes as Q 2 increases.

Observation of hard processes in rapidity gap events in γp interactions at HERA

Events with no hadronic energy flow in a large interval of pseudo-rapidity in the proton direction are observed in photon-proton interactions at an average centre of mass energy 〈√s γp〉 of 200 GeV These events are interpreted as photon diffractive dissociation. Evidence for hard scattering in photon diffractive dissociation is demonstrated using inclusive single particle spectra, thrust as a function of transverse energy, and the observation of jet production. The data can be described by a Monte Carlo calculation including hard photon-pomeron scattering.

QCD COHERENCE IN DIRECT COMPTON SCATTERING

The color coherence effects are studied for direct processes of γp interactions at high energies using PYTHIA Monte-Carlo simulation and perturbative QCD approach. Sub-processes of QED and QCD Compton scattering on quarks leading to jet topology of photoproduction events are considered. It is shown that the coherence leads to drag phenomenon in the interjet region.

Intrinsic charm in pp and γp interactions

In the Parton Fusion Model (PFM) the intrinsic charm (IC) mechanism can induce a significant contribution to the differential cross-sections of charmed hadrons at large Feynman-x in pp collisions. The possibility to include the intrinsic charm component in the Quark-Gluon String Model (QGSM) is also presented and the results from the two models are compared with each other and with existing experimental data on D, $$\bar D$$ and Λc hadroproduction. In connection with HERA experiments, the effects of intrinsic charm as predicted by PFM in high-energy photoproduction are carefully investigated.

Towards a complete description of high-energy photoproduction

A real photon has a dual nature, as a pointlike elementary particle and as a q q fluctuation, in the latter case often with hadron-like properties. Therefore high-energy γp interactions have a rich and not yet well explored structure. In this paper, we propose a consistent formulation, which is based on a subdivision of the total cross section into three distinct event classes: direct, VMD and anomalous. The VMD class, in its turn, contains the subclasses familiar from hadron-hadron physics. Within this approach, we calculate all the partial cross sections. Many other aspects of the event classes are also considered, such as proton and photon structure functions, jet rates and beam remnant structures. A major rôle is played by two distinct transverse momentum cut-off scales, which separate the classes. When all the ingredients are put together, we obtain a detailed model for photoproduction events. In order to test this approach, we also propose several alternative scenarios, each of which emphasizes different aspects of the photon. Predictions are presented for HERA events.

The hadronic properties of the photon in γp interactions

We propose a consistent formulation of high-energy γp interactions, in which these are built up as a sum of three contributions. A significant role is played by the hadronic structure of the photon. From the total cross section at low energy we determine the relative sizes of the hadronic contribution and of the components that make a photon distinctly different from a vector-meson, i.e. direct and anomalous photon components. We then obtain predictions for γp collisions at higher energies. Special attention is paid to semi-hard cross sections (mini-jets) and the origin of constituents in the photon.

Search for higher twist in γp interactions

Search for higher twist in γp interactions

Photoproduction of multiparticle states in the beam fragmentation region for photon energies in the range 50–70 GeV

Forward production of hadrons in γp interactions at about 11 GeV centre of mass energy has been analysed in terms of single particle spectra. Comparisons withK+p andK−p data and with deep inelastic scattering data at similar energies confirms the universality of global properties. As the minimump2T is increased the data show features which are consistent with a 2-jet structure in the beam fragmentation region. A small subsample of these events is consistent with a special topology in which a jet is replaced by a single high-pT pion.

Inclusive production of lambdas and antilambdas in γp interactions, for photon energies between 25 and 70 GeV

Results are presented on the inclusive photoproduction of λ and λ for incident photon energies between 25 and 70 GeV. The slope parameter of the p T 2 distribution is found to be 2.83±0.1 GeV −2 for λ and 3.28±0.25 GeV −2 for λ . The x F distributions, measured in the range −0.2 to 0.7, show that while λ are produced centrally, λ production extends to more negative values of x F; the shapes show no energy dependence and are similar to those in pion-induced reactions. The polarization of the produced λ is less than 10%. The results are discussed in terms of vector dominance and quark fusion models.

Inclusive ϱ 0 production in γp interactions at 2.8, 4.7, and 9.3 GeV

Inclusive ϱ 0 production in γp → ϱ 0 + anything is studied at 2.8, 4.7, and 9.3 GeV, using the SLAC linearly polarized backscattered laser photon beam and the 82 inch hydrogen bubble chamber. Over this energy range the inclusive inelastic ϱ 0 cross section rises from 6.0 μb to 20.5 μb. The multiplicity, i.e. the average number of μ 0 mesons per inelastic hadronic event, has an energy dependence consistent with 1n s. The inclusive cross section is studied as a function of Feynman x, c.m. rapidity, and p T 2 variables, and is also broken down into exclusive channels. At 9.3 GeV a forward inelastic peak is observed in the x distribution, containing mainly polarized ϱ 0 mesons. The cross section for this inealastic diffractive component is 2.7 ± 0.6 μb. The p T 2 distributions are exponential with a slope of 3 − 4 (GeV/ c) −2, similar to that found in inclusive ϱ 0 production in pp and μp reactions.

Inelasticity of multipion production from a quark model

The maximum observed multiplicity in πp, pp and γp interactions differs dramatically from what might reasonably be expected. The energy dependence of the maximum multiplicity suggests a quark model with near massless quarks which serve to partition the energy and momentum available for interaction in the incident particles.

Density matrix elements and Donohue-Høgaasen parameters of resonances produced in π+p and γp reactions

We present, in the first part of this paper, density matrix elements for several quasi two-body processes produced in π +p interactions at 8 GeV/ c. In the second part, we study the decay of vector mesons and of Δ ++(1236) isobar in terms of the Donohue-Høgaasen (DH) spin population parameters α, β and γ and the rotation angle Θ. The data used are from π +p scattering at 4 and 8 GeV/ c and from γp interactions in the photon energy region below 5.8 GeV. It is found that the DH parameters have the tendency to be less dependent on momentum transfer than the density matrix elements in the Jackson or in the helicity systems. The stronger variation of the matrix elements in the latter two systems is shown to be a kinematic consequence of the rotation of the frames. This effect is particularly well seen in the reaction γp → Δ ++ π − where the rotation angle strongly depends on the photon energy. The suggestion of Donohue and Høgaasen, that the DH reference system is more natural frame than the others is well confirmed by our experimental results.