Pomeron Exchange Research Articles

Singlet structure function F_1 in double-logarithmic approximation

The conventional ways to calculate the perturbative component of the DIS singlet structure function F_1 involve approaches based on BFKL which account for the single-logarithmic contributions accompanying the Born factor 1 / x. In contrast, we account for the double-logarithmic (DL) contributions unrelated to 1 / x and because of that they were disregarded as negligibly small. We calculate the singlet F_1 in the double-logarithmic approximation (DLA) and account at the same time for the running alpha _s effects. We start with a total resummation of both quark and gluon DL contributions and obtain the explicit expression for F_1 in DLA. Then, applying the saddle-point method, we calculate the small-x asymptotics of F_1, which proves to be of the Regge form with the leading singularity omega _0 = 1.066. Its large value compensates for the lack of the factor 1 / x in the DLA contributions. Therefore, this Reggeon can be identified as a new Pomeron, which can be quite important for the description of all QCD processes involving the vacuum (Pomeron) exchanges at very high energies. We prove that the expression for the small-x asymptotics of F_1 scales: it depends on a single variable Q^2/x^2 only instead of x and Q^2 separately. Finally, we show that the small-x asymptotics reliably represent F_1 at x le 10^{-6}.

Holographic description of total hadronic cross sections at high energies

We investigate the hadron-hadron total cross sections at high energies in the framework of holographic QCD. In our model setup, the involved strong interaction is described by the Brower-Polchinski-Strassler-Tan Pomeron exchange kernel, and the two form factors are obtained from the bottom-up AdS/QCD models. We show that the resulting nucleon-nucleon total cross section is consistent with the experimental data recently measured by the TOTEM collaboration at the LHC. As examples to see the general versatility of the model, we also present our analysis on the pion-nucleon and pion-pion cases, which can be predicted because all the adjustable parameters are fixed by the nucleon-nucleon data.

ϕ photoproduction on the proton at Eγ=1.5–2.9GeV

Differential cross sections at $t=t_{\text{min}}$ and decay asymmetries for the $\gamma p\rightarrow\phi p$ reaction have been measured using linearly polarized photons in the range 1.5 to 2.9 GeV. These cross sections were used to determine the Pomeron strength factor. The cross sections and decay asymmetries are consistently described by the $t$-channel Pomeron and pseudoscalar exchange model in the $E_{\gamma}$ region above 2.37 GeV. In the lower energy region, an excess over the model prediction is observed in the energy dependence of the differential cross sections at $t=t_{\text{min}}$. This observation suggests that additional processes or interference effects between Pomeron exchange and other processes appear near the threshold region.

Coherent diffractive photoproduction of ρ0 mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider

The STAR Collaboration reports on the photoproduction of $\pi^+\pi^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $\pi^+\pi^-$ invariant mass spectrum with a combination of $\rho$ and $\omega$ resonances and a direct $\pi^+\pi^-$ continuum. This is the first observation of the $\omega$ in ultra-peripheral collisions, and the first measurement of $\rho-\omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $\omega$ amplitude is consistent with the measured $\gamma p\rightarrow \omega p$ cross section, a classical Glauber calculation and the $\omega\rightarrow\pi^+\pi^-$ branching ratio. The $\omega$ phase angle is similar to that observed at much lower energies, showing that the $\rho-\omega$ phase difference does not depend significantly on photon energy. The $\rho^0$ differential cross section $d\sigma/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

Hard pomeron impact on the high-energy elastic scattering of nucleons

The role of the hard Pomeron (HP) exchanges in the high-energy diffractive interaction of nucleons is explored. It is demonstrated that the HP subdominance at available energies and low transferred momenta is due to the extremely low slope of its Regge trajectory.

Photon beam asymmetry Σ in the reaction [formula omitted] for Eγ = 1.152 to 1.876 GeV

Photon beam asymmetry Σ measurements for ω photoproduction in the reaction γ→p→ωp are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between t-channel meson exchange and s- and u-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn–Gatchina partial-wave analysis indicate the Σ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the JP=3/2+ partial wave), as well as the resonant portions of the smaller partial waves with JP=1/2−, 3/2−, and 5/2+.

Probing the diffractive production of aZ-boson pair at forward rapidities at the LHC

In this paper, we present the results from phenomenological analysis of Z boson pair hard diffractive production at the LHC. The calculation is based on the Regge factorization approach. Diffractive parton density functions extracted by the H1 Collaboration at DESY-HERA are used. The multiple Pomeron exchange corrections are considered through the rapidity gap survival probability factor. We give numerical predictions for single diffractive as well as double Pomeron exchange cross sections and compare with the photon-induced and non-diffractive ones. The contributions from quark-anti-quark collision and gluon-gluon fusion are displayed. Various kinematical distributions are presented. We make predictions which could be compared to future measurements at the LHC where forward proton detectors are installed and detector acceptances are considered.

Central production of ρ0 in pp collisions with single proton diffractive dissociation at the LHC

We consider the $pp \to pp \rho^{0} \pi^{0}$ and $pp \to pn \rho^{0} \pi^{+}$ processes at LHC energies. Our description is based on the nonperturbative framework of tensor pomeron and tensor reggeon exchanges. We discuss the Drell-Hiida-Deck type mechanism with centrally produced $\rho^{0}$ meson associated with a very forward/backward $\pi N$ system. The considered processes constitute an inelastic (non-exclusive) background to the $p p \to p p \rho^0$ reaction in the case when only the centrally produced $\rho^{0}$ meson decaying into $\pi^{+} \pi^{-}$ is measured, the final state protons are not observed, and only rapidity-gap conditions are checked experimentally. We compare our results for the $\gamma \pi^{+} \to \rho^{0} \pi^{+}$ reaction with the experimental data obtained by the H1 collaboration at HERA. We present several differential distributions for the $pp \to pn \rho^{0} \pi^{+}$ reaction and estimate the size of the proton dissociative background to the exclusive $p p \to p p \rho^0$ process. The ratio of integrated cross sections for the inelastic $p p \to p N \rho^0 \pi$ processes, where $p N \rho^0 \pi$ stands for $p n \rho^0 \pi^{+}$ plus $p p \rho^0 \pi^{0}$, to the reference reaction $pp \to \ pp \rho^0$ is of order of (7--10)\%. We present also the ratios of the $\rho^0$ rapidity and transverse momentum distributions for the inelastic $p p \to p N \rho^0 \pi$ versus the elastic $pp \to \ pp \rho^0$ reaction. Our results may be used to investigate the $\gamma \pi \to \rho^{0} \pi$ process at LHC energies.

Role of σ exchange in the γp→ϕp process and scaling with the f1 axial vector meson from a Reggeized model

We investigate the role driven by the scalar meson $\sigma$ exchange in the photoproduction of the vector meson $\phi$(1020) off a proton by using a Reggeized model. Based on the $\pi^0(135)+\sigma(500)+f_2(1270)$+Pomeron exchanges, we demonstrate that the $\sigma$ exchange plays the role to reproduce the bump structure at the forward angle in the differential cross section as well as the peaking behavior in the total cross section observed in the CLAS Collaboration. We also discuss the possible observation of the scaled cross section $s^7d\sigma /dt$ at the production angle $\theta=90^{\circ}$ from the CLAS data. It is found that the axial vector meson $f_1(1285)$ exchange with the trajectory $\alpha_{f_1}(t)=0.028\,t+0.9\pm0.2$ arising from the axial anomaly of the QCD vacuum plays the role to clarify the scaling up to 5 GeV.

Diffractive Production of Quarkonium in p-A Collisions at LHC

In the present work, we investigated the exclusive diffractive production of charmonium in pronton-nucleus collisions at the Large Hadron Collider (LHC) energies. Such exclusive production reactions possess a cleaner experimental signal than inclusive production once it is characterized by low multiplicity of particles between final produced state and the incident protons of collider beam. The considered theoretical framework was the perturbative model of Quantum Cromodynamics (QCD) for the Pomeron exchange. In particular, we have used the resolved Pomeron model which depends on the Pomeron flux and Pomeron parton distribution functions. The main goal is to provide predictions for the single diffractive cross section for the J/Psi meson production at the energies of 5.02 TeV and 8.8 TeV. We have shown that a measurement is feasible as the order of magnitude is 400 microbarns even absorption corrections. Furthermore, we analyze the ratio of the diffractive to inclusive cross section which allows theoretical uncertainties to be canceled, mostly those associated to approximations performed in inclusive case.

Hadron diffractive production at ultrahigh energies and shadow effects

Shadow effects at collisions of hadrons with light nuclei at high energies were subject of scientific interest of V.N. Gribov, first, we mean his study of the hadron-deuteron scattering, see Sov. Phys. JETP 29, 483 (1969) [Zh. Eksp. Teor. Fiz. 56, 892 (1969)] and discovery of the reinforcement of shadowing due to inelastic diffractive rescatterings. It turns out that the similar effect exists on hadron level though at ultrahigh energies. Diffractive production is considered in the ultrahigh energy region where pomeron exchange amplitudes are transformed into black disk ones due to rescattering corrections. The corresponding corrections in hadron reactions [Formula: see text] with small momenta transferred ([Formula: see text], [Formula: see text]) are calculated in terms of the [Formula: see text]-matrix technique modified for ultrahigh energies. Small values of the momenta transferred are crucial for introducing equations for amplitudes. The three-body equation for hadron diffractive production reaction [Formula: see text] is written and solved precisely in the eikonal approach. In the black disk regime final state scattering processes do not change the shapes of amplitudes principally but dump amplitudes by a factor [Formula: see text]; initial state rescatterings result in additional factor [Formula: see text]. In the resonant disk regime initial and final state scatterings damp strongly the production amplitude that corresponds to [Formula: see text] at [Formula: see text] in this mode.

The practical Pomeron for high energy proton collimation

We present a model which describes proton scattering data from ISR to Tevatron energies, and which can be applied to collimation n high energy accelerators, such as the LHC and FCC. Collimators remove beam halo particles, so that they do not impinge on vulnerable regions of the machine, such as the superconducting magnets and the experimental areas. In simulating the effect of the collimator jaws it is crucial to model the scattering of protons at small momentum transfer~$t$,as these protons can subsequently survive several turns of the ring before being lost. At high energies these soft processes are well described by Pomeron exchange models. We study the behaviour of elastic and single-diffractive dissociation cross sections over a wide range of energy, and show that the model can be used as a global description of the wide variety of high energy elastic and diffractive data presently available. In particular it models low mass diffraction dissociation, where a rich resonance structure is present, and thus predicts the differential and integrated cross sections in the kinematical range appropriate to the LHC.We incorporate the physics of this model into the beam tracking code MERLIN and use it to simulate the resulting loss maps of the beam halo lost in the collimators in the LHC.

Midrapidity inclusive densities in high energy pp collisions in additive quark model

High energy (CERN SPS and LHC) inelastic $pp$ ($p\bar p$) scattering is treated in the framework of additive quark model together with Pomeron exchange theory. We extract the midrapidity inclusive density of the charged secondaries produced in a single quark-quark collision and investigate its energy dependence. Predictions for the $\pi p$ collisions are presented.

Interference Effect betweenϕandΛ(1520)Production Channels in theγp→K+K−pReaction near Threshold

The $\phi$-$\Lambda(1520)$ interference effect in the $\gamma p\to K^+K^-p$ reaction has been measured for the first time in the energy range from 1.673 to 2.173 GeV. The relative phases between $\phi$ and $\Lambda(1520)$ production amplitudes were obtained in the kinematic region where the two resonances overlap. The measurement results support strong constructive interference when $K^+K^-$ pairs are observed at forward angles, but destructive interference for proton emission at forward angles. Furthermore, the observed interference effect does not account for the $\sqrt{s}=2.1$ GeV bump structure in forward differential cross sections for $\phi$ photoproduction. This fact suggests possible exotic structures such as a hidden-strangeness pentaquark state, a new Pomeron exchange and rescattering processes via other hyperon states.

Soft Pomeron in holographic QCD

We study the graviton Regge trajectory in Holographic QCD as a model for high energy scattering processes dominated by soft pomeron exchange. This is done by considering spin J fields from the closed string sector that are dual to glueball states of even spin and parity. In particular, we construct a model that governs the analytic continuation of the spin J field equation to the region of real J < 2, which includes the scattering domain of negative Maldelstam variable t. The model leads to approximately linear Regge trajectories and is compatible with the measured values of 1.08 for the intercept and 0.25 GeV$^{-2}$ for the slope of the soft pomeron. The intercept of the secondary pomeron trajectory is in the same region of the subleading trajectories, made of mesons, proposed by Donnachie and Landshoff, and should therefore be taken into account.

Hadronic structure of the photon at smallxin holographic QCD

We present our analysis on the photon structure functions at small Bjorken variable x in the framework of the holographic QCD. In the kinematic region, a photon can fluctuate into vector mesons and behaves like a hadron rather than a pointlike particle. Assuming the Pomeron exchange dominance, the dominant hadronic contribution to the structure functions is computed by convoluting the probe and target photon density distributions obtained from the wave functions of the U(1) vector field in the five-dimensional AdS space and the Brower-Polchinski-Strassler-Tan Pomeron exchange kernel. Our calculations are in agreement with both the experimental data from OPAL collaboration at LEP and those calculated from the parton distribution functions of the photon proposed by Gl\"uck, Reya, and Schienbein. The predictions presented here will be tested at future linear colliders, such as the planned International Linear Collider.

Central Exclusive Production in pp collisions at LHCb

Central Exclusive Production (CEP) is a unique process at hadron machines in which particles are produced via colourless propagators, and several measurements at the LHC are directly comparable with past and future electron-ion colliders. LHCb have measured the cross-sections for the CEP of vector mesons, J /ψ,ψ (2S ),ϒ(1S ), ϒ(2S ) and ϒ(3S ), which are photo-produced. In the double Pomeron exchange process, preliminary measurements have been made of χ c 0 , χ c 1 , χ c 2 meson production while the first observations of the CEP of pairs of charmonia, J /ψJ /ψ and J/ψψ (2S ), have been made and limits obtained on the pair production of other charmonia.

Exclusive Centralπ+π−Production in Proton Antiproton Collisions at the CDF

Exclusive π + π − production in proton-antiproton collisions at √ s = 0.9 and 1.96 TeV in the Collider Detector at Fermilab has been measured. We select events with two particles with opposite charge in pseudorapidity region −1.3 <η< 1.3 with no other particles detected in −5.9 <η< 5.9. Particles are assumed to be pions. The π + π − system is required to have rapidity −1.0 <y< 1.0. The data are expected to be dominated by the double pomeron exchange mechanism. Therefore, the quantum numbers of the cen- tral state are constrained. The data extend up to dipion mass M(π + π − ) = 5000 MeV/c 2 . Resonance structures consistent with f0 and f2(1270) mesons are visible. The results are valuable for light hadron spectroscopy and for providing information about the nature of the pomeron in a region between non-perturbative and perturbative quantum chromody- namics.

Central Exclusive Production in Proton-Proton Collisions with the STAR Experiment at RHIC

We shall describe the physics program with tagged forward protons, focusing on Central Exclusive Production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC), with the STAR detector at √ s = 200 GeV. Preliminary results in CEP of two oppositely charged pions and kaons produced in the processes pp → ppπ + π − and pp → ppK + K − shall be presented. Those Double Pomeron Exchange (DPE) processes, allow the final states to be dominated by gluonic exchanges. Silicon strip detectors placed in Roman Pots were used for measuring forward protons. The preliminary results are based on the measurement of the recoil system of charged particles in the STAR experiment’s Time Projection Chamber (TPC). Ionization energy loss, dE/dx, of charged particles was used for particle identification (PID).

Photon structure functions at small x in holographic QCD

We investigate the photon structure functions at small Bjorken variable x in the framework of the holographic QCD, assuming dominance of the Pomeron exchange. The quasi-real photon structure functions are expressed as convolution of the Brower–Polchinski–Strassler–Tan (BPST) Pomeron kernel and the known wave functions of the U(1) vector field in the five-dimensional AdS space, in which the involved parameters in the BPST kernel have been fixed in previous studies of the nucleon structure functions. The predicted photon structure functions, as confronted with data, provide a clean test of the BPST kernel. The agreement between theoretical predictions and data is demonstrated, which supports applications of holographic QCD to hadronic processes in the nonperturbative region. Our results are also consistent with those derived from the parton distribution functions of the photon proposed by Glück, Reya, and Schienbein, implying realization of the vector meson dominance in the present model setup.