Collider HERA Research Articles

Proton structure and parton distribution functions from HERA

The H1 and ZEUS collaborations at the electron-proton collider HERA collected e ± p scattering data corresponding to an integrated luminosity of about 1 fb −1 . The data were taken at proton beam energies of 920, 820, 575 and 460 GeV and an electron beam energy of 27.5 GeV, with different electric charges and longitudinal polarisation of the electron beam. Using these data inclusive neutral and charged current deep inelastic cross sections were measured over six orders of magnitude in negative four-momentum-transfer squared, Q 2 , and Bjorken x . A combination of all inclusive cross sections, published by the H1 and ZEUS collaborations at HERA, was performed. Using these combined HERA data and the individual H1 and ZEUS data taken using the polarised electron beams, the proton structure functions F 2 , F γZ 2 , x F γZ 3 and F L were obtained, and scaling violations, electroweak unification, and polarisation effects in the charged current process were demonstrated. The combined cross sections were used as a sole input to QCD analyses at leading, next-to-leading and next-to-next-to-leading orders, providing a new set of parton distribution functions, denoted as HERAPDF2.0. An extension of the analysis by including HERA data on charm and jet production allowed a simultaneous determination of parton distributions and the strong coupling.

Open charm meson production at BNL RHIC withinkt-factorization approach and revision of their semileptonic decays

We discuss inclusive production of open charm mesons in proton-proton scattering at the BNL RHIC. The calculation is performed in the framework of $k_t$-factorization approach which effectively includes higher-order pQCD corrections. Different models of unintegrated gluon distributions (UGDF) from the literature are used. We focus on UGDF models favoured by the LHC data and on a new up-to-date parametrizations based on the HERA collider DIS high-precision data. Results of the $k_t$-factorization approach are compared to next-to-leading order collinear predictions. The hadronization of heavy quarks is done by means of fragmentation function technique. The theoretical transverse momentum distributions of charmed mesons are compared with recent experimental data of the STAR collaboration at $\sqrt{s} = 200$ and $500$ GeV. Theoretical uncertainties related to the choice of renormalization and factorization scales as well as due to the quark mass are discussed. Very good description of the measured integrated cross sections and differential distributions is obtained for the Jung setB$0$ CCFM UGDF. Revised charm and bottom theoretical cross sections corresponding to those measured recently by the STAR and PHENIX collaborations for semileptonic decays of $D$ and $B$ mesons are presented. Significant improvement in theoretical description of the non-photonic electrons measurements is clearly obtained with respect to the previous studies within the $k_{t}$-factorization.

Measuring the gluon distribution in nuclei at an electron–ion collider

Despite the successes of the HERA collider, where much information was gained on the structure of the nucleon, data on the structure of the nucleus at moderate-to-small x remains elusive, as only fixed-target high- x data currently exist. The small- x region, however, is of great interest. The nucleon structure in this region is dominated by gluons which show a rapid rise with decreasing x . At low- x , this growth must be tamed and the gluon distribution will be saturated. This saturation phenomena is expected to be universal, appearing in both nucleons and nuclei. A knowledge of this regime is of vital importance to understanding the underlying physics which governs the initial conditions of heavy-ion collisions at both the LHC and RHIC, where particle production is dominated by gluons from this unknown region. However, only tantalising hints of this have been observed so far. Therefore, the construction of an Electron–Ion Collider (EIC), colliding polarised electrons with polarised protons and also a wide variety of nuclei, will allow an exploration of the region of small- x in great detail (with luminosities 100 x that of HERA), answering questions on both the spatial and momentum distributions of gluons and sea quarks in nuclei. In particular, the saturation region is more accessible in nuclei due to the amplification of the saturation scale with nuclear size ( Q S ∝ A 1 / 3 ). In this paper I present the current status of measuring the gluon distribution in nuclei in e + A collisions at an EIC.

Exclusive vector meson production at high energies and gluon saturation

We systematically study exclusiv2 diffractive (photo)production of vector mesons ($J/\ensuremath{\psi}$, $\ensuremath{\psi}(2s)$, $\ensuremath{\phi}$, and $\ensuremath{\rho}$) off protons in high-energy collisions and investigate whether the production is a sensitive probe of gluon saturation. We confront saturation-based results for diffractive $\ensuremath{\psi}(2s)$ and $\ensuremath{\rho}$ production at HERA and $J/\ensuremath{\psi}$ photoproduction with all available data, including recent work from HERA, ALICE, and LHCb, finding good agreement. In particular, we show that the $t$ distribution of differential cross sections of photoproduction of vector mesons offers a unique opportunity to discriminate among saturation and nonsaturation models. This is due to the emergence of a pronounced dip (or multiple dips) in the $t$ distribution of diffractive photoproduction of vector mesons at relatively large, but potentially accessible $|t|$ that can be traced back to the unitarity features of color dipole amplitude in the saturation regime. We show that in saturation models the dips in $t$ distribution recede towards lower $|t|$ with decreasing mass of the vector meson, increasing energy or decreasing Bjorken-$x$, and decreasing virtuality $Q$. We provide various predictions for exclusive (photo)production of different vector mesons including the ratio of $\ensuremath{\psi}(2s)/J/\ensuremath{\psi}$ at HERA, the LHC, and future colliders.

Transverse momentum dependent gluon density from DIS precision data

The combined measurements of proton's structure functions in deeply inelastic scattering at the HERA collider provide high-precision data capable of constraining parton density functions over a wide range of the kinematic variables. We perform fits to these data using transverse momentum dependent QCD factorization and CCFM evolution. The results of the fits to precision measurements are used to make a determination of the nonperturbative transverse momentum dependent gluon density function, including experimental and theoretical uncertainties. We present an application of this density function to vector boson + jet production processes at the LHC.

Partons at small x

We give a brief summary of the work on partons at small Bjorken x pursued by us at Gauhati University in recent years. We use both conventional DGLAP method and not so conventional self-similarity inspired method. In DGLAP approach, we solve the QCD evolution equations approximately at small x by two methods- Lagrange's and method of characteristics. We apply them to both spin dependent and spin independent structure functions. In the second approach, we use the notion of self-similarity of fractal geometry to construct Transverse Momentum Dependent Parton Distributions (TMD's), integrated Parton Densities (PDF's) and determine them from HERA collider data. We then use them to compute fractions of momentum carried by quarks and gluons inside the proton and compare with QCD expectations. In view of topical importance of multi parton interactions at LHC at TeV scale, we also outline the construction of Double Parton Distributions (dPDF's) with self-similarity at small x1, x2.

Searches for the decay D 0 → µ+µ− at hadron colliders

Investigations of rare charmed-meson decays are used to test the Standard Model and to perform searches for new physics beyond it. Advances made in searches for the rare decay D0 → µ+µ− in experiments at the HERA and Tevatron hadron colliders and at the LHC over the last few years are considered.

Measurement of the luminosity in the ZEUS experiment at HERA II

The luminosity in the ZEUS detector was measured using photons from electron bremsstrahlung off protons. In 2001 the HERA collider was upgraded for operation at higher luminosity. At the same time the luminosity-measuring system of the ZEUS experiment was modified to tackle the expected higher photon rate and synchrotron radiation. The existing lead-scintillator calorimeter was equipped with radiation hard scintillator tiles and shielded against synchrotron radiation. In addition, a magnetic spectrometer was installed to measure the luminosity independently using photons converted in the beam-pipe exit window. The redundancy provided a reliable and robust luminosity determination with a systematic uncertainty of 1.7%. The experimental setup, the techniques used for luminosity determination and the estimate of the systematic uncertainty are reported.

Charm and Beauty in Photoproduction at HERA

The photoproduction of beauty and charm quarks at the ep collider HERA are presented. The b-quarks production was investigated in the bb → eeX′ channel and the differential production cross section was measured as a function of the average transverse momentum of the beauty quarks down to the threshold. The cross section of D* meson decaying in the golden channel was determined both inclusively and in D*-tagged dijet events. Exploiting the characteristics of the heavy-flavoured hadron decays, beauty and charm quark cross sections were also measured in dijet events using secondary vertices or semi-muonic decays.

The H1 very forward proton spectrometer at HERA

The very forward proton spectrometer, VFPS, is a component of the H1 detector at the HERA collider. Scattered protons emitted with a polar angle less than 1mrad and carrying a fractional energy 1−xP, 0.008<xP≤0.028 of the incident proton beam energy can be detected by scintillating fiber detectors which are read out by position sensitive photo-multipliers. These detectors are contained in Roman pot stations which are moved close to the circulating proton beam. The structure, operation and performance of the two Roman pot stations located at about 220m from the H1 interaction point in the cryogenic section of the proton ring are described.

Using 1-jettiness to measure 2 jets in DIS 3 ways

We predict cross sections in deep inelastic scattering (DIS) for the production of two jets---one along the proton beam direction created by initial state radiation (ISR) and another created by final state radiation after the hard collision. Our results include fixed order corrections and a summation of large logarithms up to next-to-next-to-leading logarithmic (NNLL) accuracy in resummed perturbation theory. We make predictions for three versions of a DIS event shape 1-jettiness, each of which constrains hadronic final states to be well collimated into two jets along the beam and final-state jet directions, but which differ in their sensitivity to the transverse momentum of the ISR from the proton beam. We use the tools of soft collinear effective theory (SCET) to derive factorization theorems for these three versions of 1-jettiness. The sensitivity to the ISR gives rise to significantly different structures in the corresponding factorization theorems---for example, dependence on either the ordinary or the generalized kperp-dependent beam function. Despite the differences among 1-jettiness definitions, we show that the leading nonperturbative correction that shifts the tail region of their distributions is given by a single universal nonperturbative parameter Omega1, even accounting for hadron mass effects. Finally, we give numerical results for Q^2 and x values explored at the HERA collider, emphasizing that the target of our factorization-based analyses is to open the door for higher-precision jet phenomenology in DIS.

D* meson production cross sections in e − p and e + p scattering processes at the HERA collider

The cause of the D* meson production cross sections in e − p and e + p deep inelastic scattering processes in the ZEUS experiment at the HERA collider that are anomalously different is determined.

Charm quark mass dependence in a global QCD analysis

We study the effect of the charm quark mass in the CTEQ global analysis of parton distribution functions (PDFs) of the proton. Constraints on the $\bar{\rm MS}$ mass of the charm quark are examined at the next-to-next-to-leading order (NNLO) accuracy in the S-ACOT-$\chi$ heavy-quark factorization scheme. The value of the charm quark mass from the hadronic scattering data in the CT10 NNLO fit, including semiinclusive charm production in DIS at HERA collider, is found to agree with the world average value. Various approaches for constraining $m_{c}$ in the global analysis and impact on LHC cross sections are reviewed.

Leading Hadron Production at HERA

Data from the recent measurements of very forward baryon and photon production with the H1 and ZEUS detectors at electron-proton collider HERA are presented and compared to the theoretical calculations and Monte Carlo models.Results are presented of the production of leading protons, neutrons and photons in deep inelastic scattering ( e p → e ' p X , e p → e ' n X , e p → e ' γ X ) as well as the leading neutron production in the photoproduction of dijets ( e p → e j j X n ). The forward baryon and photon results from the H1 and ZEUS Experiments are compared also with the models of the hadronic interactions of high energy Cosmic Rays. The sensitivity of the HERA data to the differences between the models is demonstrated.

Determination of the integrated luminosity at HERA using elastic QED Compton events

A measurement of the integrated luminosity at the ep collider HERA is presented, exploiting the elastic QED Compton process ep \rightarrow ep. The electron and the photon are detected in the backward calorimeter of the H1 experiment. The integrated luminosity of the data recorded in 2003 to 2007 is determined with a precision of 2.3%. The measurement is found to be compatible with the corresponding result obtained using the Bethe-Heitler process.

Measurement of inclusive and dijet D∗ meson cross sections in photoproduction at HERA

AbstractThe inclusive photoproduction of D∗ mesons and of D∗-tagged dijets is investigated with the H1 detector at the ep collider HERA. The kinematic region covers small photon virtualities Q2&lt;2 GeV2 and photon–proton centre-of-mass energies of 100&lt;Wγp&lt;285 GeV. Inclusive D∗ meson differential cross sections are measured for central rapidities |η(D∗)|&lt;1.5 and transverse momenta pT(D∗)&gt;1.8 GeV. The heavy quark production process is further investigated in events with at least two jets with transverse momentum pT(jet)&gt;3.5 GeV each, one containing the D∗ meson. Differential cross sections for D∗-tagged dijet production and for correlations between the jets are measured in the range |η(D∗)|&lt;1.5 and pT(D∗)&gt;2.1 GeV. The results are compared with predictions from Monte Carlo simulations and next-to-leading order perturbative QCD calculations.

H100 - A Centralised Analysis Framework for the H1 Experiment

The H1 experiment recorded an integrated luminosity of almost 0.5pb−1 (0.3G events in HERA-I, 0.8G events in HERA-II) at the ep collider HERA in the years 1992 through 2007. The goal of the H100 project is to provide a data storage and analysis environment and in particular standardised and fast event selection facilities that improve the overall H1 physics analysis efficiency and performance. The software is based on and embedded in the ROOT framework, which provides a generic collection of tools for high-performance I/O jobs, as well as for the interactive analysis environment and for the event display. In this coherent approach all different physics analyses employ the same data and MC samples, which provide the users with the best possible knowledge of the H1 collaboration. The analysis files are produced centrally in a very efficient semi-automatic procedure, allowing a very short turn-around time when installing new reconstruction and analysis software versions.

Factorisation Breaking in Diffractive Dijets Photoproduction at HERA

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider are analysed with an emphasis on QCD factorisation breaking effects. The possible sources of the different conclusions of H1 and ZEUS collaborations are studied.

Factorisation breaking in diffractive dijet photoproduction at HERA?

Recent experimental data on dijet cross sections in diffractive photoproduction at HERA collider are analysed with an emphasis on QCD factorisation breaking effects. The possible sources of the different conclusions of H1 and ZEUS collaborations are studied.

Heavy-quark deep-inelastic scattering with a running mass

We study the production of heavy quarks in deep-inelastic scattering within perturbative QCD. As a new result, we employ for the first time the running mass definition in the MS¯ scheme for deep-inelastic charm and bottom production. We observe an improved stability of the perturbative expansion and a reduced theoretical uncertainty due to variations of the renormalization and factorization scales. As our best estimate we extract from a global fit to fixed-target and HERA collider data for the charm-quark an MS¯ mass of mc(mc)=1.01±0.09(exp)±0.03(th)GeV.