We study the inclusive production of isolated prompt photons within the framework of the quasi-multi-Regge-kinematic approach, applying the quark Reggeization hypothesis. We describe accurately and without free parameters the transverse momentum and pseudorapidity spectra of prompt photons in the inclusive photoproduction at the HERA Collider. It is shown that the main mechanism of the inclusive prompt photon production in the gamma p collisions is the fusion of a Reggeized quark (antiquark) from the proton and a collinear antiquark (quark) from the photon into a photon, via the effective Reggeon-quark-gamma vertex. The fragmentation of the quark, which is produced via the gamma-Reggeon-quark and quark-Reggeon-quark vertices, into a photon is strongly suppressed by the isolation cone condition and it gives a significant contribution in the region of a large negative pseudorapidity only. At the stage of numerical calculations we use the Kimber-Martin-Ryskin prescription for unintegrated quark and gluon distribution functions, with the following collinear parton densities as input: MRST for a proton and GRV for a photon.
We study the photoproduction of isolated prompt photons associated with hadron jets in the framework of the parton Reggeization approach. The main improvements with respect to previous studies in the k_T-factorization framework include the application of the Reggeized-quark formalism, the generation of exactly gauge-invariant amplitudes with off-shell initial-state quarks, and the exact treatment of the gamma+R -> gamma+g box contribution with off-shell initial-state gluons. In this proceedings, the new data set, published recently by ZEUS collaboration is analyzed, were the distributions in p…
We study the photoproduction of isolated prompt photons associated with hadron jets in the framework of the parton Reggeization approach. The cross section distributions in the transverse energies and pseudorapidities of the prompt photon and the jet as well as the azimuthal decorrelation variables measured by the H1 and ZEUS collaborations at DESY HERA are nicely described by our predictions. The main improvements with respect to previous studies in the $k_T$-factorization framework include the application of the Reggeized-quark formalism, the generation of exactly gauge-invariant amplitudes …
We study open charm production at high energies in the framework of the quasi-multi-Regge-kinematics approach applying the quark-Reggeization hypothesis implemented with Reggeon-Reggeon-particle and Reggeon-particle-particle effective vertices. Adopting the Kimber-Martin-Ryskin unintegrated quark and gluon distribution functions of the proton and photon, we thus nicely describe the proton structure function F_{2,c} measured at DESY HERA as well as the transverse-momentum distributions of D mesons created by photoproduction at HERA and by hadroproduction at the Fermilab Tevatron.
The B_c photoproduction at HERA has been considered in the frame work of the pQCD. The estimated value of the total production cross section of Bc+, Bc*+, Bc- and Bc*- mesons is about 5 pb. Higher excitations yield about 30% to the total cross section. Thus for integrated HERA luminosity 1 fb-1 about 7000 Bc mesons will be produced.
QED radiative corrections to the cross section of diffractive electro- and photoproduction of vector mesons is calculated at HERA energies. Both semi-analytical and Monte Carlo approaches are discussed and compared.
We review the theoretical framework of jet photoproduction at HERA discussing the conceptual ideas, phenomenological models, and higher order perturbative calculations. Numerically, we study the physically interesting distribution of transverse energy within the observed jet, the real and virtual photon structure, and the proton structure at large x.
The NLO calculation of the DIC process with the isolated photon is presented and the comparison is made with the other NLO calculation and with recent measurement at HERA. The satisfactory agreement was found.
The cross section of the $D^*$-meson diffractive photoproduction at the HERA collider has been calculated in the framework of perturbatively motivated model for the different kinematic regions. The camparison between the different Pomeron models has been performed.
It is discussed how the dijet azimuthal correlations in DIS and real photoproduction at HERA probe the unintegrated gluon distribution in the proton. The correlation function shows a strong dependence on kinematical variables. We discuss a possible interplay of perturbative and nonperturbative effects.
Recent NLO calculations for the photoproduction of one and two jets at HERA are compared to data from H1 and ZEUS. We discuss the physics potential of these measurements for constraining the photon structure, especially the quark density at large $x$, and study remaining hadronization and jet definition uncertainties.
I discuss the impact of color-octet contributions to inclusive $J/ψ$ photoproduction at HERA. Emphasis is put on resolved photon processes and on $J/ψ$ polarization, which will be experimentally accessible at HERA in the near future. Both analyses provide a powerful test of the NRQCD factorization approach to charmonium production.
We discuss the factorization breaking observed in diffractive dijet photoproduction by the H1 and ZEUS collaborations at HERA. By considering the effects of rapidity gap survival, hadronisation, migration and NLO contributions, we find that the observed data are compatible with theoretical expectations.
Exclusive diffractive photoproduction of heavy vector mesons (V=phi, J/psi and Upsilon) at HERA is studied in a model employing a dipole Pomeron exchange (P) with an inelastic gamma-P-V vertex. The model is fitted to the data on dsigma/dt, B and elastic sigma for Q^2=0 and beyond the threshold region. The elastic cross sections for both phi and J/psi photoproduction show a moderate increase within the HERA energy region. The flattening of the slope B(s) (little or no shrinkage) for J/psi is not correlated with the slope of the Pomeron trajectory. Estimates for Upsilon photoproduction at HERA a…
A measurement of the inclusive cross-section for the photoproduction of protons in the central fragmentation region at HERA is presented. The measured cross-section is compared with the prediction of the PYTHIA MC model. The obtained value of the diquark suppression factor in the Lund string model is different from that obtained in e+e- data. The dependence of the cross-section as a function of the transverse mass M_T is shown to be similar for different hadrons produced in photoproduction at HERA.
We revise and update an earlier model for $J/ψ$ photoproduction based on a dipole Pomeron exchange. We show that the H1 and ZEUS experimental data reported recently can be well fitted by a soft Pomeron alone.
W bosons can be produced in the channels e+- p --> W+- + X at HERA thus allowing anomalous trilinear couplings among the gauge bosons to be probed. We discuss the next-to-leading order (NLO) QCD corrections to the photoproduction of W bosons with finite transverse momentum at HERA. The higher-order QCD corrections reduce the factorization scale dependence significantly and modify the leading-order (LO) cross sections by +- O(10%).
Processes of heavy quark photoproduction at HERA energies and beyond are investigated using the semihard ($k_{\bot}$ factorization) approach. Virtuality and longitudinal polarization of gluons in the photon - gluon subprocess as well as the saturation effects in the gluon distribution function at small $x$ have been taken into account. The total cross sections, rapidity and $p_{\bot}$ distributions of the charm and beauty quark photoproduction have been calculated. The results are compared with ZEUS experimental data for charm photoproduction cross section.
The amplitude for photoproduction gamma p -> Υp is calculated in a pQCD k_\perp-factorization approach. The total cross section for diffractive Υ's is compared to recent HERA data. The amplitude is used to predict the cross section for exclusive ppbar \to ppbar Υ(1S,2S) process in hadronic reactions at Tevatron energies. We also included absorption effects.
We study photoproduction of jets and heavy flavors in a polarized collider mode of HERA at $\sqrt{S}= 298$ GeV. We examine the sensitivity of the cross sections and their asymmetries to the proton's polarized gluon distribution and to the completely unknown parton distributions of longitudinally polarized photons.
We study photoproduction of jets and single-inclusive hadrons in a polarized ep collider mode of HERA at sqrt(s)=300 GeV, examining the sensitivity of the cross sections and their asymmetries to the proton's polarized gluon distribution and to the completely unknown parton distributions of polarized photons. We also present for the first time the NLO corrections to the direct part of polarized single-inclusive hadron photoproduction.
Exclusive photoproduction of heavy quarkonia at HERA is analyzed in the framework of the Regge-eikonal approach together with the nonrelativistic bound state formalism. Total and differential cross-sections for the process $γ+p\to (Q\bar{Q})_{1S}+p$ are calculated. The model predicts cross-sections of Exclusive Double Diffractive Events (EDDE) at TeVatron and LHC.
We apply the MC@NLO formalism to the production of heavy-quark pairs in pointlike photon-hadron collisions. By combining this result with its analogue relevant to hadron-hadron collisions, we obtain NLO predictions matched to parton showers for the photoproduction of QQ^{-} pairs. We compare MC@NLO results to the measurements of c- and b-flavoured hadron observables performed by the H1 and ZEUS collaborations at HERA.
We compare our theoretical results for charmed quark and J/Psi - photoproduction cross sections obtained in the semihard approach of QCD with H1 and ZEUS experimental data.
A new next-to-leading order Monte Carlo program for the calculation of jet cross sections in photoproduction is described. The contributions from both resolved and direct components are included to O(αα_s^2). Properties of the predictions for various inclusive jet and dijet observables are discussed and comparisons with HERA data are presented.
A study of $3+1$ jet event photoproduction at HERA is presented. We define an angular variable which is sensitive to the topology of the final state jets and is therefore able to discriminate between the different contributing subprocesses, and between QCD and an abelian gluon model. We also investigate the contribution from the direct production of light gluinos to the $3+1$ cross section.
Processes of hevy quark photoproduction at HERA energies and beyond are investigated using the semihard ($k_{\bot}$ factorization) approach. The virtuality and longitudinal polarization of gluons in the photon - gluon subprocess as well as the saturation effects in the gluon distribution function at small $x$ have been taken into account. The total cross sections, rapidity and $p_{\bot}$ distributions of the charm and beauty quark photoproduction have been calculated. We obtained the some differences between the predictions of the standard parton model and the semihard approach used here.
Recent $HERA$ data on the photoproduction of vector mesons are analysed within a "soft", dipole pomeron model. We argued that the data on $σ^{γ\rightarrow J/ψ}_{el}$ is consistent with a soft pomeron, the apparent rapid increase resulting from the non-asymptotic effects due to the delayed asymptotics of $σ_{el}$ with respect to $σ_{tot}$.
We estimate the cross section for gamma p -> Upsilon p by two independent methods. First, by studying the corrections to the naive leading-order QCD formula and, second, by using parton-hadron duality. The estimates are in good agreement with each other and with the recent measurements of the cross section at HERA.
In the framework of the vector dominance model (VDM) new contribution into the cross-section of $D^*$-meson photoproduction at HERA has been estimated. This contribution is due to the interaction of the virtual vector $c\bar c$-mesons from photon with the proton. It has been shown that the mechanism under discussion plays the essential role for all values of transverse momentum of $D^*$-mesons at HERA. Taking into account for the contribution of this mechanism improves the description of the experimental cross section distribution.
We discuss the inclusive jet production at HERA in the next-to-leading logarithm approximation. Theoretical uncertainties are considered in some details. We show the importance of the jet rapidity distribution to constrain the parton densities in the photon. A comparison is made with the recent H1 data.
A study of the $\J$-meson photoproduction at large transverse momentum at HERA energies via the charm quark excitation in the photon is presented. Based on perturbative QCD, nonrelativistic quark model and photon structure function formalism, our calculation demonstrates that the charm content of the photon may be very important for the $\J$ photoproduction at the large transverse momentum. It is shown that at HERA energies $\J$ production via the subprocess $\cg$ dominates over the resolved photon contribution via the gluon-gluon fusion at $\pt\ge 2$ GeV/c and over the leading order direct ph…
The next-to-leading order (NLO) QCD calculation for the isolated photon and isolated photon plus jet photoproduction at the ep collider DESY HERA is presented. The predictions for the isolated photon with no restrictions imposed on the jet are compared with the previous ones obtained in the small cone approximation, and the differences are found to be below 2%. The theoretical uncertainties in the cross section of the photoproduction of the photon plus jet are discussed. A short comparison with the new preliminary H1 data and with the NLO predictions of Fontannaz et al. is also presented.
The diffractive photoproduction of vector mesons is usually described considering the two-gluon (Pomeron) exchange, non-diagonal parton distributions and the contribution of the real part to the cross section. In this letter we analyze the diffractive photoproduction of the $Υ$ at HERA using an alternative model, the Color Evaporation Model (CEM), where the cross section is simply determined by the boson-gluon cross section and an assumption for the production of the colorless state. We verify that, similarly to the $J/Ψ$ case, the HERA data for this process can be well described by the CEM. M…
The precision of new HERA data on jet photoproduction opens up the possibility to discriminate between different models of the photon structure. This requires equally precise theoretical predictions from perturbative QCD calculations. In the past years, next-to-leading order calculations for the photoproduction of jets at HERA have become available. Using the kinematic cuts of recent ZEUS analyses, we compare the predictions of three calculations for different dijet and three-jet distributions. We find that in general all three calculations agree within the statistical accuracy of the Monte Ca…
The cross section for photoproduction of an isolated prompt photon in association with a jet is studied in Next-to-Leading Order. The kinematics are those appropriate for the DESY $ep$ collider HERA. The effects on the cross section of various experimental cuts including isolation cuts on the photon is examined. Comparisons with the ZEUS preliminary data using two parametrizations of the photon structure function is made, and good agreement is found. The data is not yet precise enough to make a distinction between various models for the photon structure function.
We present calculations of the prompt photon photoproduction at HERA collider in the k_T-factorization approach. Both direct and resolved contributions are taken into account. The conservative error analisys is performed. The unintegrated parton densities in a proton and in a photon are determined using the Kimber-Martin-Ryskin prescription. We investigate both inclusive and associated with jet prompt photon photoproduction rates. In particular, we study the angular correlations between produced photon and hadronic jet in the transverse momentum plane which can provide a unique information abo…
Decay widths, branching ratios and production dynamics of some recently discovered $J^{PC} = 1^{-+}, 0^{-+}$,$1^{--}$ and $2^{-+}$ mesons are found to be in remarkable agreement with the predicted properties of hybrid mesons. We propose tests for this new dynamics, emphasise the critical role of $πb_1$ or $πh_1$ decay channels in discriminating hybrids from conventional states, and suggest that photoproduction may offer special opportunities for isolation and confirmation of hybrids.
I discuss few selected topics in heavy flavour photoproduction at HERA which require large integrated luminosity in order to be experimentally investigated. I present phenomenological predictions for bottom production. As a possible application of measurements involving double-tagged charm events, I outline a method for the direct measurement of the gluon density in the proton. The possibility of using charm data in polarized electron-proton collisions to constrain the polarized gluon density in the proton is also discussed.
Cross sections for the diffractive photoproduction of the Upsilon-family at HERA energies, within the framework of the analysis by Frankfurt, Koepf and Strikman, are presented. They compare well with the recent preliminary data from ZEUS and H1. Two novel effects lead to a significant enhancement of the original calculation: the non-diagonal (or skewed) kinematics, calculated to leading-log(Q^2) accuracy, and the large magnitude of the real part of the amplitude. A considerably stronger rise in energy is predicted than that found in J/Psi-production.
Predictions from a new next-to-leading order (NLO) calculation for direct and resolved photoproduction of one and two jets are compared to simulated HERA data. We propose a method to match experimental and theoretical jet definitions and observe a reduced dependence on jet definitions and hadronization corrections at larger transverse energies. From the irreducible uncertainty, we estimate the maximum benefit that can be obtained from increased luminosity to constrain the structure of the photon and the proton.
We study the photoproduction of an isolated photon and a jet based on a code of partonic event generator type which includes the full set of next-to-leading order corrections. We compare our results to a recent ZEUS analysis in which an effective k_T of the incoming partons has been determined. We find that no additional intrinsic k_T is needed to describe the data.
The cross section for photoproduction of a prompt photon in association with a jet is studied in Next-to-Leading Order at the DESY $ep$ collider HERA. The effect of various cuts imposed on the cross section by the ZEUS collaboration including isolation cuts on the photon is examined. Comparisons with the ZEUS preliminary data using various parametrizations of the photon structure function is made, and good agreement is found. The preliminary data is not yet precise enough to make a distinction between various models for the photon structure function.
The high energy photoproduction of jets is being observed at the ep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets per ep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gl…
We reconsider the prompt photon photoproduction at HERA in the framework of the $k_T$-factorization QCD approach. The proposed method is based on the ${\cal O}(α^2α_s)$ amplitudes for $γq\toγgq$ and $γg^*\toγq\bar q$ partonic subprocesses. Additionally, we take into account the ${\cal O}(α^2α_s^2)$ box contributions $γg\toγg$ to the production cross sections. The unintegrated (or transverse momentum dependent) parton densities in the proton are determined using Kimber-Martin-Ryskin (KMR) prescription. Our consideration covers both inclusive and jet associated prompt photon photoproduction rate…
We investigate in detail the effect of a direct pomeron coupling to quarks on the production of jets in $ep$ scattering with almost real photons. Jet production via a direct pomeron coupling is compared with the resolved--pomeron mechanism. We consider both direct and resolved photoproduction. Rapidity and transverse momentum distributions are calculated and compared with preliminary H1 and ZEUS data.
The forward photoproduction amplitude for gamma p to Upsilon p is calculated in a pQCD k_t-factorization approach with an unintegrated gluon distribution constrained by inclusive deep-inelastic structure functions. The total cross section for diffractive Upsilons is compared with a recent HERA data. We also discuss the 2S/1S ratio in diffractive Upsilon-production. The amplitude is used to predict the cross section for exclusive Upsilon production in hadronic reactions. Differential distributions for the exclusive p pbar to p Upsilon pbar process are calculated for Tevatron energies. We also s…
We consider the photoproduction of D* mesons associated with two hadron jets at HERA collider in the framework of the kt-factorization approach. The unintegrated gluon densities in a proton are obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber-Martin-Ryskin prescription. Resolved photon contributions are reproduced by the initial-state gluon radiation. We investigate different production rates and make comparison with the recent experimental data taken by the ZEUS collaboration. Special attention is put on the specific dijet correlations which …
We calculate the cross section of charm photoproduction at HERA collider in the framework of the kt-factorization QCD approach. Our analysis cover the inclusive charm production as well as charm and associated jet production processes. Both photon-gluon and gluon-gluon fusion mechanisms are taken into account. The unintegrated gluon densities in a proton and in a photon obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber-Martin-Ryskin prescription are used. Our theoretical results are compared with the recent experimental data taken by the H1 and…
We investigate the inclusive photoproduction of J/Psi mesons at HERA within the framework of the kt-factorization QCD approach. Our consideration is based on the color singlet model supplemented with the relevant off-shell matrix elements and the CCFM and KMR unintegrated gluon densities in a proton and in a photon. Both the direct and resolved photon contributions are taken into account. Our predictions are compared with the recent experimental data taken by the H1 and ZEUS collaborations. Special attention is put on the J/Psi polarization parameters lambda and nu which are sensitive to the p…
The photoproduction of Drell-Yan lepton pairs at HERA is studied. We show that in the backward rapidity region the cross section is strongly sensitive to the small-$x$ behaviour of the quark distributions in the proton, and that with sufficient luminosity it should be possible to distinguish singular $xq \sim x^{-1/2}$ behaviour from standard Regge $xq \sim x^0$ behaviour. The effect on the event rate of experimental acceptance cuts is studied with the help of a Monte Carlo event generator.
We present predictions for single inclusive distributions of charmed mesons, relevant to the HERA experiments. Our results are based upon a computation that correctly incorporates mass effects up to the next-to-leading order level, and the resummation of transverse momentum logarithms up to next-to-leading-logarithmic level. We apply the same acceptance cuts as the H1 and Zeus experiments, and compare our results to their data. We perform a study of the sensitivity of our predictions on the charm mass, $\LambdaQCD$, factorization scale, renormalization scale, and fragmentation parameters.
We compute various kinematical distributions for one-jet and two-jet inclusive photoproduction at HERA. Our results are accurate to next-to-leading order in QCD. We use the subtraction method for the cancellation of infrared singularities. We perform a thorough study of the reliability of QCD predictions; in particular, we consider the scale dependence of our results and discuss the cases when the perturbative expansion might break down. We also deal with the problem of the experimental definition of the pointlike and hadronic components of the incident photon, and briefly discuss the sensitiv…
We present results from NLO QCD calculations on the production of jets in ep collisions at HERA in a continuous range of photon virtuality Q^2. Special attention is given to the collinear singularities from the virtual photon and the resolved virtual photon component of the cross sections. Comparisons with dijet data from H1 are shown and the infrared sensitivity of dijet cross sections is discussed.
We study various conceivable photoproduction reactions in a polarized ep collider mode of HERA with respect to their sensitivity to the proton's polarized gluon distribution. A special emphasis is put on the `resolved' part of the cross sections which in principle opens the possibility to determine for the first time also the completely unknown parton content of longitudinally polarized photons. In the very promising case of dijet production we also investigate the impact of parton showering, hadronization and jet finding on the parton level results.
We present calculations of the beauty photoproduction at HERA collider in the framework of the kt-factorization approach. Both direct and resolved photon contributions are taken into account. The unintegrated gluon densities in a proton and in a photon are obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber-Martin-Ryskin prescription. We investigate different production rates (both inclusive and associated with hadronic jets) and compare our theoretical predictions with the recent experimental data taken by the H1 and ZEUS collaborations. Special…
The measurement of the inelastic photoproduction of charmonium at HERA seems to have ignited a new charmonium crisis. The (already discredited) color singlet model fits the data for large charmonium energy fraction z, where the NRQCD model qualitatively fails. We here point out that by the straightforward inclusion of color singlet and octet processes in the soft color (color evaporation) scheme, the HERA data can be accommodated for all z. We anticipate that the color singlet model will fail at low z, as it does in hadroproduction.
We explore QCD calculations for the process gamma p -> V X where V is a vector meson, in the region s >> -t and -t >> Lambda_QCD^2. We compare our calculations for the J/psi, phi and rho mesons with data from the ZEUS Collaboration at HERA and demonstrate that the BFKL approach is consistent with the data even for light mesons, whereas the two-gluon exchange approach is inadequate. We also predict the differential cross-sections for the Upsilon and omega for which no data are currently available.
We study inclusive photoproduction of hadrons at HERA via both direct and resolved photons to next-to-leading order (NLO) in the QCD-improved parton model. We predict transverse-momentum ($\pt$) and rapidity ($\ylab$) distributions and compare them with recent data by the H1 Collaboration. To estimate the theoretical error of the predictions, we investigate their dependence on the choice of renormalization and factorization scales, subtraction scheme, and parton density functions (PDF). We also introduce a new NLO set of fragmentation functions for charged pions and kaons, which is extracted f…
