H1 Collaborations Research Articles

Particle Production and Fragmentation at HERA

Recent results from the H1 and ZEUS collaborations on particle production in ep scattering at HERA are presented. These results include measurements of the scaled momentum distribution, the charge asymmetry and the p T spectra of charged hadrons, the measurement of K S 0 production and the study of charm quark fragmentation into D ⁎ ± mesons. The data are compared to Monte Carlo expectations, to predictions of NLO QCD calculations and if possible, to results from e + e − annihilation and p p ¯ collision experiments.

A method for tuning parameters of Monte Carlo generators and its application to the determination of the unintegrated gluon density

A method for tuning parameters in Monte Carlo generators is described and applied to a specific case. The method works in the following way: each observable is generated several times using different values of the parameters to be tuned. The output is then approximated by some analytic form to describe the dependence of the observables on the parameters. This approximation is used to find the values of the parameter that give the best description of the experimental data. This results in significantly faster fitting compared to an approach in which the generator is called iteratively. As an application, we employ this method to fit the parameters of the unintegrated gluon density used in the CASCADE Monte Carlo generator, using inclusive deep inelastic data measured by the H1 Collaboration. We discuss the results of the fit, its limitations, and its strong points.

Deeply virtual Compton scattering at small [formula omitted] and the access to the GPD H

We give a partonic interpretation for the deeply virtual Compton scattering (DVCS) measurements of the H1 and ZEUS Collaborations in the small- x B region in terms of generalized parton distributions. Thereby we have a closer look at the skewness effect, parameterization of the t-dependence, revealing the chromomagnetic pomeron, and at a model-dependent access to the anomalous gravitomagnetic moment of nucleon. We also quantify the reparameterization of generalized parton distributions resulting from the inclusion of radiative corrections up to next-to-next-to-leading order. Beyond the leading order approximation, our findings are compatible with a ‘holographic’ principle that would arise from a (broken) SO ( 2 , 1 ) symmetry. Utilizing our leading-order findings, we also perform a first model-dependent “dispersion relation” fit of HERMES and JLAB DVCS measurements. From that we extract the generalized parton distribution H on its cross-over line and predict the beam charge-spin asymmetry, measurable at COMPASS.

Deep inelastic prompt photon production at HERA in thekT-factorization approach

We investigate the prompt photon production in deep inelastic scattering at HERA in the framework of kt-factorization QCD approach. Our study is based on the off-shell partonic amplitude e q* -> e gamma q, where the photon radiation from the leptons as well as from the quarks is taken into account. The unintegrated quark densities in a proton are determined using the Kimber-Martin-Ryskin prescription. The conservative error analisys is performed. We investigate both inclusive and jet associated prompt photon production rates. Our predictions are compared with the recent experimental data taken by the H1 and ZEUS collaborations. We demonstrate that in the kt-factorization approach the contribution from the quark radiation subprocess (QQ mechanism) is enhanced as compared to the leading-order collinear approximation.

Study of noncollinear parton dynamics in the prompt photon photoproduction at HERA

We investigate the prompt photon photoproduction at HERA within the framework of the ${k}_{T}$-factorization QCD approach. Our consideration is based on the off-shell matrix elements for the underlying partonic subprocesses. The unintegrated parton densities in a proton and in a photon are determined using the Kimber-Martin-Ryskin prescription. Additionally, we use the Ciafaloni-Catani-Fiorani-Marchesini--evolved unintegrated gluon as well as valence and sea quark distributions in a proton. A conservative error analysis is performed. Both inclusive and associated with the hadronic jet production rates are investigated. The theoretical results are compared with the recent experimental data taken by the H1 and ZEUS Collaborations. We study also the specific kinematical properties of the photon-jet system which are strongly sensitive to the transverse momentum of incoming partons. Using the Kimber-Martin-Ryskin scheme, the contribution from the quarks emerging from the earlier steps of the parton evolution is estimated and found to be of 15%--20% approximately.

H1 Grid production tool for large scale Monte Carlo simulation

The H1 Collaboration at HERA has entered the period of high precision analyses based on the final data sample. These analyses require a massive production of simulated Monte Carlo (MC) events. The H1 MC framework (H1MC) is a software for mass MC production on the LCG Grid infrastructure and on a local batch system created by H1 Collaboration. The aim of the tool is a full automatisation of the MC production workflow including management of the MC jobs on the Grid down to copying of the resulting files from the Grid to the H1 mass storage tape device. The H1 MC framework has modular structure, delegating a specific task to each module, including task specific to the H1 experiment: Automatic building of steer and input files, simulation of the H1 detector, reconstruction of particle tracks and post processing calculation. Each module provides data or functionality needed by other modules via a local database. The Grid jobs created for detector simulation and reconstruction from generated MC input files are fully independent and fault-tolerant for 32 and 64-bit LCG Grid architecture and in Grid running state they can be continuously monitored using Relational Grid Monitoring Architecture (R-GMA) service. To monitor the full production chain and detect potential problems, regular checks of the job state are performed using the local database and the Service Availability Monitoring (SAM) framework. The improved stability of the system has resulted in a dramatic increase in the production rate, which exceeded two billion MC events in 2008.

Factorisation breaking in diffractive dijet photoproduction at HERA

We discuss the factorisation 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.

Nonforward Compton scattering in AdS/CFT correspondence

We study the nonforward Compton scattering in particular the deeply virtual Compton scattering from AdS/CFT. We first calculate the contributions from the s-channel and u-channel supergravity diagrams as well as the four point interaction diagram which correspond to the Compton scatterings on a dilaton target in CFT. Furthermore, we study the Compton scattering on a dilatino target. Assuming that protons can be identified as supergravity modes of dilatino, we compare the calculated DVCS cross section to the low-energy experimental data from the H1 and ZEUS collaborations and find good agreement. We also discuss the t-channel graviton exchange contribution and show that it should be dominant in the high-energy limit.

Nonlinear correction to the longitudinal structure function at small x

We computed the longitudinal proton structure function FL, using the nonlinear Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (NLDGLAP) evolution equation approach at small x . For the gluon distribution, the nonlinear effects are related to the longitudinal structure function. As the very small-x behavior of the gluon distribution is obtained by solving the Gribov, Levin, Ryskin, Mueller and Qiu (GLR-MQ) evolution equation with the nonlinear shadowing term incorporated, we show that the strong rise that corresponds to the linear QCD evolution equations can be tamed by screening effects. Consequently, the obtained longitudinal structure function shows a tamed growth at small x . We computed the predictions for all details of the nonlinear longitudinal structure function in the kinematic range where it has been measured by the H1 Collaboration and made comparisons with the computation by Moch, Vermaseren and Vogt at the second order with input data from the MRST QCD fit.

Combined measurement and QCD analysis of the inclusive e ± p scattering cross sections at HERA

A combination is presented of the inclusive deep inelastic cross sections measured by the H1 and ZEUS Collaborations in neutral and charged current unpolarised ep scattering at HERA during the period 1994-2000. The data span six orders of magnitude in negative four-momentum-transfer squared, Q^2, and in Bjorken x. The combination method used takes the correlations of systematic uncertainties into account, resulting in an improved accuracy. The combined data are the sole input in a NLO QCD analysis which determines a new set of parton distributions, HERAPDF1.0, with small experimental uncertainties. This set includes an estimate of the model and parametrisation uncertainties of the fit result.

A precision measurement of the inclusive ep scattering cross section at HERA

A measurement of the inclusive deep-inelastic neutral current e+p scattering cross section is reported in the region of four-momentum transfer squared, 12<=Q^2<=150 GeV^2, and Bjorken x, 2x10^-4<=x<=0.1. The results are based on data collected by the H1 Collaboration at the ep collider HERA at positron and proton beam energies of E_e=27.6 GeV and E_p=920 GeV, respectively. The data are combined with previously published data, taken at E_p=820 GeV. The accuracy of the combined measurement is typically in the range of 1.3-2%. A QCD analysis at next-to-leading order is performed to determine the parton distributions in the proton based on H1 data.

Measurement of the inclusive ep scattering cross section at low Q2 and x at HERA

A measurement of the inclusive ep scattering cross section is presented in the region of low momentum transfers, 0.2 GeV^2 < Q^2 < 12 GeV^2, and low Bjorken x, 5x10^-6 < x < 0.02. The result is based on two data sets collected in dedicated runs by the H1 Collaboration at HERA at beam energies of 27.6 GeV and 920 GeV for positrons and protons, respectively. A combination with data previously published by H1 leads to a cross section measurement of a few percent accuracy. A kinematic reconstruction method exploiting radiative ep events extends the measurement to lower Q^2 and larger x. The data are compared with theoretical models which apply to the transition region from photoproduction to deep inelastic scattering.

Leading baryons at HERA

Recent results on leading baryon production in ep scattering from the ZEUS and H1 collaborations are presented and compared with different production models.

Factorisation issues in diffraction

The high centre-of-mass energies of the HERA ep collider and of the Tevatron collider allow us to study the diffractive interactions in the presence of a hard scale and to describe them in terms of perturbative QCD. In QCD, the diffractive exchange is described in terms of partons, and the factorisation theorem states that the cross section for hard interactions can be expressed as convolution of universal diffractive parton distribution functions and process-dependent coefficients, perturbatively calculable. The recent experimental data from the H1 and ZEUS Collaborations at HERA are presented for diffractive dijet and D ∗ meson production, in both deep-inelastic scattering (DIS) and the photoproduction regimes and are compared to next-to-leading order QCD predictions using diffractive parton distributions. While good agreement is found for dijets in DIS and for D ∗ production in both DIS and photoproduction, the dijet photoproduction data are overestimated by NLO predictions at lower transverse momentum of the jets, indicating the breaking of QCD factorisation.

Vector mesons and DVCS

Recent experimental results on diffractive vector meson (VM) production and deeply virtual Compton scattering (DVCS) from the H1 and ZEUS collaborations are reviewed. The data are compared with theoretical predictions.

Jet substructure at HERA

A review is presented of jet substructure measurements from the H1 and ZEUS Collaborations at HERA. The results presented include tests of perturbative QCD, comparison of the properties of quark and gluon jets, the comparison of the pattern of QCD radiation in different hard scattering processes, determination of the strong coupling and studies of the underlying subprocesses dynamics and of the pattern of parton radiation.

A few highlights of Heavy flavor measurements at HERA

Selected results from the H1 and Zeus Collaborations relating to heavy flavor physics in deep inelastic scattering are presented. Results from three different experimental techniques, reconstruction of charmed mesons, search for secondary vertices and inclusive search for displaced tracks are discussed. The high statistics sample of HERA II together with the different and complementary techniques used to analyzed the data promise many beautiful and charming results to come in the near future.

Direct measurement of the longitudinal proton structure function at HERA

The first direct measurements of the longitudinal proton structure function F L ( x , Q 2 ) at HERA are reported. The measurements were performed by the H1 and ZEUS collaborations. They are based on inclusive deep inelastic e + p scattering cross sections with a positron beam energy of 27.5 GeV and proton beam energies of 920, 575 and 460 GeV. The F L values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA.

Resonance Search and Particle Production at HERA

Searches for exotic strange baryon resonances decaying into K s 0 p , K s 0 p ¯ , and Ξ π were performed by the H1 collaboration in deep inelastic scattering (DIS) at HERA. Upper limits are given for the production rates. Deuterons and anti-deuterons were observed by the ZEUS collaboration and their production rates in DIS were compared to those of p ( p ¯ ) . Strange neutral hadrons were produced and cross-sections, baryon to meson ratios and relative yields of strange to light mesons were measured by ZEUS. Excited charm non-strange and strange mesons were produced and the fraction of charm quark fragmenting to these mesons was measured by ZEUS.

Multijets in Photoproduction and Minijets in DIS at HERA

Two measurements based on electron-proton collision data collected at HERA are presented. From the ZEUS collaboration measurements are presented on 3- and 4-jet cross sections in photoproduction. H1 collaboration is for the first time dedicating an analysis to search for multiple interactions in DIS. Both analyses show results which are pointing out the relevance of contributions from the underlying event.