After the completion of data taking at HERA-2, a large data set is at hand to study the photoproduction of charm and beauty quarks in ep collisions. New measurements of charm production based on D^* meson tagging and beauty production based on muon and electron reconstruction test perturbative QCD calculations with improved accuracy. In general, QCD calculations at NLO describe the data well. The scale uncertainties are, however, large and dominate over the experimental uncertainties, calling for more precise calculations.
A measurement of charm and beauty photoproduction at the electron proton collider HERA is presented based on the simultaneous detection of a D*^{\pm} meson and a muon. The correlation between the D* meson and the muon serves to separate the charm and beauty contributions and the analysis provides comparable sensitivity to both. The total and differential experimental cross sections are compared to LO and NLO QCD calculations. The measured charm cross section is in good agreement with QCD predictions including higher order effects while the beauty cross section is higher.
Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of 120pb^-1. The fractions of events containing b quarks, and also of events containing c quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.
Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of 120 pb-1. The fractions of events containing b quarks, and also of events containing c quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.
The latest results of beauty quark production measurements at HERA are presented. New measurements have been obtained both in the photoproduction and the deep inelastic scattering regimes. The results were compared with the NLO QCD calculations.
Beauty photoproduction in dijet events has been measured at HERA with the ZEUS detector using an integrated luminosity of 126 pb-1. Beauty was identified in events with a muon in the final state by using the transverse momentum of the muon relative to the closest jet. Lifetime information from the silicon vertex detector was also used; the impact parameter of the muon with respect to the primary vertex was exploited to discriminate between signal and background. Cross sections for beauty production as a function of the muon and the jet variables as well as dijet correlations are compared to QC…
A measurement of charm and beauty dijet photoproduction cross sections at the ep collider HERA is presented. Events are selected with two or more jets of transverse momentum $p_t^{jet}_{1(2)}>11(8)$ GeV in the central range of pseudo-rapidity $-0.9<η^{jet}_{1(2)}<1.3$. The fractions of events containing charm and beauty quarks are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 central vertex detector. Differential dijet cross sections for charm and beauty, and their relative contributions to the flavour i…
Experimental results on beauty production at HERA are reviewed in the context of similar measurements at other colliders. As a result of a phenomenological study of the QCD scale dependence of many different NLO and NNLO predictions, a modification of the ``default'' scale choice is advocated. Experimental constraints on the photon-quark coupling are also investigated.
The photoproduction of beauty quarks in events with two jets and a muon has been measured with the ZEUS detector at HERA using an integrated luminosity of 110 pb$^{- 1}$. The fraction of jets containing b quarks was extracted from the transverse momentum distribution of the muon relative to the closest jet. Differential cross sections for beauty production as a function of the transverse momentum and pseudorapidity of the muon, of the associated jet and of $x_γ^{jets}$, the fraction of the photon's momentum participating in the hard process, are compared with MC models and QCD predictions made…
The results from a recent analysis on beauty production in deep inelastic scattering at HERA using decays into electrons from the ZEUS collaboration are presented. The fractions of events containing b quarks were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections were measured and compared with next-to-leading-order QCD calculations. The beauty contribution to the proton structure function F_2 was extracted from the double-differential cross sections.
The cross section for ep -> e b\bar{b} X in photoproduction is measured with the H1 detector at the ep-collider HERA. The decay channel b\bar{b} -> ee X' is selected by identifying the semi-electronic decays of the b-quarks. The total production cross section is measured in the kinematic range given by the photon virtuality Q^2 <= 1 GeV^2, the inelasticity 0.05 <= y <= 0.65 and the pseudorapidity of the b-quarks |eta(b)|,|eta(\bar{b})| <= 2. The differential production cross section is measured as a function of the average transverse momentum of the beauty quarks <P_T(b)> down to the threshold…
Recent results on open beauty production in positron-proton collisions at a centre-of-mass energy of 300 GeV are presented. The beauty photoproduction cross section is measured by ZEUS and H1 via the semileptonic decay. Both experiments use the transverse lepton momentum with respect to a jet as an observable. The H1 central silicon vertex detector makes it possible to analyse, in addition, the lepton impact parameter spectrum, thus exploiting lifetime information for the first time at HERA. The combined lifetime and transverse momentum signature is further used to observe beauty production fo…
The measurement of the beauty quark production cross section in deep inelastic scattering with the ZEUS detector at HERA is presented. The cross section for beauty quarks, measured in the kinematic range $Q^2 > 2$ GeV$^2$ and $0.05 < y < 0.7$, and for events with at least one muon and one jet in the Breit frame, is somewhat above the next-to-leading order (NLO) QCD prediction, but in agreement within the experimental and theoretical uncertainties.
The photoproduction of beauty quarks in ep collisions has been measured using a data sample of 170 pb^-1 collected with the H1 detector at HERA-II in the years 2006 and 2007. Events with two jets and a muon in the final state were investigated, and beauty events were identified using the muon's relative transverse momentum to a jet and its impact parameter. Visible cross sections were measured differentially in the transverse momenta of the highest energy jet (p_T^jet1) and the muon (pt_mu), the pseudorapidity of the muon (eta_mu) and of the photon's momentum fraction x_gamma entering the hard…
Beauty production in events containing two muons in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb-1. A low transverse-momentum threshold for muon identification, in combination with the large rapidity coverage of the ZEUS muon system, gives access to almost the full phase space for beauty production. The total cross section for beauty production in ep collisions at sqrt(s)= 318 GeV has been measured to be sigma_tot(ep -> bbbar X) = 13.9 +- 1.5(stat.) +4.0 -4.3(syst.) nb. Differential cross sections and a measurement of bbbar correlati…
The production of beauty quarks with a D*+- and a muon in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb-1. Low transverse-momentum thresholds for the muon and D* meson allow a measurement of beauty production closer to the production threshold than previous measurements. The beauty signal was extracted using the charge correlations and angular distributions of the muon with respect to the D* meson. Cross sections for photoproduction and deep inelastic scattering are somewhat higher than, but compatible with, next-to-leading-order QCD …
A selection of topics on open heavy quark production at HERA are reviewed here. Measurements of charm fragmentation parameters will be presented together with developments in the study of D* di-jet angular distributions. Charm production in deep inelastic scattering (DIS) is also discussed. Finally we deal with recent measurements of b cross sections using impact parameters in both DIS and photoproduction regimes.
The production of charm and beauty quarks in ep interactions has been measured with the ZEUS detector at HERA for squared four-momentum exchange Q^2>20 GeV^2, using an integrated luminosity of 126 pb^{-1}. Charm and beauty quarks were identified through their decays into muons. Differential cross sections were measured for muon transverse momenta p_T^μ>1.5 GeV and pseudorapidities -1.6<η^μ<2.3, as a function of p_T^μ, η^μ, Q^2 and Bjorken x. The charm and beauty contributions to the proton structure function F_2 were also extracted. The results agree with previous measurements based on indepen…
Measurements of cross sections for beauty and charm events with dijets and a muon in the photoproduction regime at HERA are presented. The data were collected with the H1 detector and correspond to an integrated luminosity of 179 pb^{-1}. Events with dijets of transverse momentum P_T^{jet1} > 7 GeV and P_T^{jet2} > 6 GeV in the pseudorapidity range -1.5< η^{jet} < 2.5 in the laboratory frame are selected in the kinematic region of photon virtuality Q^2 < 2.5 GeV^2 and inelasticity 0.2 < y < 0.8. One of the two selected jets must be associated to a muon with P_T^μ > 2.5 GeV in the pseudorapidit…
Recent results on charm and beauty photoproduction at HERA are discussed. The perturbative QCD calculations are generally smaller than the measured cross sections, particularly in the forward (proton) direction. The study of charm dijet photoproduction is consistent with a significant contribution of charm excitation processes.
Inclusive charm and beauty cross sections are measured in e-p and e+p neutral current collisions at HERA in the kinematic region of photon virtuality 5<Q^2<2000 GeV^2 and Bjorken scaling variable 0.0002<x<0.05. The data were collected with the H1 detector in the years 2006 and 2007 corresponding to an integrated luminosity of 189 pb^-1. The numbers of charm and beauty events are determined using variables reconstructed by the H1 vertex detector including the impact parameter of tracks to the primary vertex and the position of the secondary vertex. The measurements are combined with previous da…
A measurement of the beauty production cross section in ep collisions at a centre-of-mass energy of 319 GeV is presented. The data were collected with the H1 detector at the HERA collider in the years 1999-2000. Events are selected by requiring the presence of jets and muons in the final state. Both the long lifetime and the large mass of b-flavoured hadrons are exploited to identify events containing beauty quarks. Differential cross sections are measured in photoproduction, with photon virtualities Q^2 < 1 GeV^2, and in deep inelastic scattering, where 2 < Q^2 < 100 GeV^2. The results are co…
Selected aspects of photoproduction in ep scattering at the HERA collider, studied with the ZEUS detector, are presented. The results are interpreted in the formalism of Vector Dominance Model, Regge theory and perturbative Quantum Chromodynamics.
The HERA-B experiment at DESY has acquired a data set of approximately 300,000 decays J/psi -> l+l- during its 2002/2003 data-taking period. These data are used to analyze the production of heavy quarks in proton-nucleus interactions at a center-of-mass energy of 41.6 GeV. In this article, preliminary results of two measurements are discussed, a measurement of nuclear effects in the production of J/psi mesons and a measurement of the b-bbar production cross section.
Selected new results from the H1 and ZEUS collaborations on $ep$ interactions at 300 - 318 GeV centre-of-mass energy are presented. The full pre-upgrade integrated luminosity of HERA of 110 pb$^{-1}$ is used. Charm cross sections are measured up to high values of $x_B$ and $Q^2$ and are found to be well described by NLO QCD in the 3 flavour scheme. Orbitally excited $D$ mesons are observed; radial excitations are searched for, but are not seen. The first $b$ cross section measurement is confirmed with a lifetime based method, establishing the excess over NLO QCD.
A procedure of implementing QCD corrections in Monte Carlo programs by a reweighting method is described for the photoproduction of W bosons at HERA. Tables for W boson production in LO and NLO are given in bins of the transverse momentum of the W boson and its rapidity.
The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process $e^+p -> e^+ + {dijet} + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 ${\rm pb^{-1}}$. Events with photon-proton centre-of-mass energies, $W_{γp}$, between 134 and 269 GeV and a photon virtuality, Q^2, less than 1 ${\rm GeV^2}$ were selected requiring at least two jets of transverse energy $E_T^{\rm jet1(2)} >7(6)$ GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of bea…
The energy dependence of the total photon-proton cross-section is determined from data collected with the ZEUS detector at HERA with two different proton beam energies.
Recent results on jet production in neutral current deep inelastic scattering and high energy photoproduction at the HERA electron-proton-collider are briefly reviewed. The results are compared to QCD expectations in NLO and $α_s$ determinations using these data are summarized.
The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a muon has been measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA using integrated luminosity of 72 pb^-1. The total visible cross section is sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The next-to-leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions; however at low values of Q^2, Bjorken x…
Recent measurements of beauty production in ep collisions at HERA are presented. The results were obtained using data from the experiments H1 and ZEUS from the years 1996 to 2000 (HERA I). An outlook is given for analysis on data from HERA II. Differential measurements using muons and jets in deep inelastic scattering and photoproduction are compared to NLO QCD predictions. The beauty contribution to the proton structure function F_2 was determined at large Q^2 (Q^2 > 150 GeV^2) and is compared with predictions.
Measurements of cross sections for events with charm and beauty jets in deep inelastic scattering at HERA are presented. Events with jets of transverse energy E_T^jet > 6 GeV and pseudorapidity -1.0 < eta^jet < 1.5 in the laboratory frame are selected in the kinematic region of photon virtuality Q^2 > 6 GeV^2 and inelasticity variable 0.07 < y < 0.625. Measurements are also made requiring a jet in the Breit frame with E_T^*jet > 6 GeV. The data were collected with the H1 detector in the years 2006 and 2007 corresponding to an integrated luminosity of 189 pb^-1. The numbers of charm and beauty …
The production of prompt photons in photoproduction reactions in the ZEUS experiment is discussed. Cross sections for inclusive prompt photons and for prompt photons accompanied by jets are compared with theory, and the latter are used to determine the effective transverse momentum of the quarks in the proton.
Studies on the structure of the photon are presented by means of the extraction of a leading order effective parton distribution in the photon and measurements of inclusive jet differential cross sections in photoproduction. Measurements of the internal structure of jets have been performed and are also presented as a function of the transverse energy and pseudorapidity of the jets.
The import of HERA data on heavy quark production for LHC experiments is discussed. Knowlegde of all aspects of the beauty and charm production process, viz. the parton density functions of colliding hadrons, the hard scatter, and the fragmentation of the quarks into hadrons, can aid LHC experimentation. This short write-up concentrates on possible influences HERA data can have and on the current status (and history) of beauty production from both HERA and Tevatron experiments. In general, next-to-leading order QCD gives a reasonable description of beauty production although some regions of ph…
Recent measurements of Charm and Beauty production at HERA are described. The charm results are well described by NLO QCD calculations with a somewhat better description in the CCFM than in the DGLAP evolution schemes. However, the Beauty results in both photoproduction and in deep inelastic scattering (DIS) are poorly described by such calculations.
Prompt photons, together with an accompanying jet, have been studied in the photoproduction regime of ep scattering with the ZEUS detector at HERA. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD underestimate the gamma+jet cross sections for transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data in this region.
The photoproduction reaction gamma p -> mu+ mu- p has been studied in ep interactions using the ZEUS detector at HERA. The data sample corresponds to an integrated luminosity of 43.2 pb^{-1}. The Upsilon meson has been observed in photoproduction for the first time. The sum of the products of the elastic Upsilon(1S), Upsilon(2S), Upsilon(3S) photoproduction cross sections with their respective branching ratios is determined to be 13.3 +- 6.0(stat.)^{+2.7}_{-2.3}(syst.) pb at a mean photon-proton centre of mass energy of 120 GeV. The cross section is above the prediction of a perturbative QCD m…
Differential cross sections, $dσ/d x_γ^{\rm OBS}$, for dijet photoproduction events with a tagged $D^*$ meson are presented, where $x_γ^{\rm OBS}$ is the fraction of the photon energy contributing to the two highest transverse energy jets. Results on open beauty cross sections (from semileptonic decays) compared to LO Monte Carlo predictions are also presented.
The scaled momentum distributions of charged particles in jets have been measured for dijet photoproduction with the ZEUS detector at HERA using an integrated luminosity of 359 pb-1. The distributions are compared to predictions based on perturbative QCD carried out in the framework of the modified leading-logarithmic approximation (MLLA) and assuming local parton-hadron duality (LPHD). The universal MLLA scale, Lambda_eff, and the LPHD parameter, kappa^ch, are extracted.
Selected recent ZEUS and H1 results on open charm and beauty production in high energy ep collisions are reviewed. Measurements of differential D* cross sections, charm meson production ratios and semi-leptonic beauty decays are discussed and compared with QCD predictions.
We review measurements of open beauty production at HERA, with emphasis on recent results based on lifetime signatures. The beauty cross sections in photoproduction and deep-inelastic scattering are found to be higher than expected in QCD at next-to-leading order. The discussion includes new results on beauty production in e+e-, gamma-gamma and p-bar-p interactions. An outlook on the potential for measurements with the upgraded HERA collider and experiments is also given.
The proton-dissociative diffractive photoproduction of J/psi mesons has been studied in ep collisions with the ZEUS detector at HERA using an integrated luminosity of 112 pb^-1. The cross section is presented as a function of the photon-proton centre-of-mass energy and of the squared four-momentum transfer at the proton vertex. The results are compared to perturbative QCD calculations.
The $J/ψ$ decay angular distributions have been measured in inelastic photoproduction in $e p$ collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 $pb^{-1}$. The range in photon-proton centre-of-mass energy, $W$, was 50 $< W <$ 180 $GeV$. The $J/ψ$ mesons were identified through their decay into muon pairs. The polar and azimuthal angles of the $μ^+$ were measured in the $J/ψ$ rest frame and compared to theoretical predictions at leading and next-to-leading order in QCD.
Recent results from the \emph{ep} collier HERA are presented. Inclusive dijet cross sections have been measured in neutral current deep inelastic scattering, for virtualities of the exchanged boson in the range $125 < Q^{2} < 20\,000\; GeV^{2}$ and in photoproduction, $Q^{2} \sim 0 GeV^{2}$. The measurements are compared to perturbative QCD calculations at next-to-leading order.
The exclusive photoproduction reaction gamma p --> Upsilon p has been studied with the ZEUS experiment in ep collisions at HERA using an integrated luminosity of 468 pb^{-1}. The measurement covers the kinematic range 60<W<220 GeV and Q^2<1 GeV^2, where W is the photon-proton centre-of-mass energy and Q^2 is the photon virtuality. These results, which represent the analysis of the full ZEUS data sample for this channel, are compared to predictions based on perturbative QCD.
Inclusive non-diffractive photoproduction of rho(770)^0, K^*(892)^0 and phi(1020) mesons is investigated with the H1 detector in ep collisions at HERA. The corresponding average γp centre-of-mass energy is 210 GeV. The mesons are measured in the transverse momentum range 0.5<p_T<7 GeV and the rapidity range |y_{lab}|<1. Differential cross sections are presented as a function of transverse momentum and rapidity, and are compared to the predictions of hadroproduction models.
The cross section for the photoproduction of events containing three jets with a three-jet invariant mass of M_3J > 50 GeV has been measured with the ZEUS detector at HERA. The three-jet angular distributions are inconsistent with a uniform population of the available phase space but are well described by parton shower models and O(alpha alpha_s^2) pQCD calculations. Comparisons with the parton shower model indicate a strong contribution from initial state radiation as well as a sensitivity to the effects of colour coherence.
Dijet cross sections from the original 1994 ZEUS data analysis are shown compared to NLO calculations. New 1995 cross sections are shown compared to Monte Carlo predictions. The fraction of dijet events with a rapidity gap inbetween the jets is also presented for the new 1995 data.
Jet photoproduction, where the two highest transverse energy ($\ETJ$) jets have $\ETJ$ above 6 GeV and a jet-jet invariant mass above 23~GeV, has been studied with the ZEUS detector at the HERA $ep$ collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect…
Results on open charm and beauty production and on the search for top production in high-energy electron-proton collisions at HERA are reviewed. This includes a discussion of relevant theoretical aspects, a summary of the available measurements and measurement techniques, and their impact on improved understanding of QCD and its parameters, such as parton density functions and charm- and beauty-quark masses. The impact of these results on measurements at the LHC and elsewhere is also addressed.
Inclusive $K^0$ and $Λ$ photoproduction has been investigated at HERA with the H1 detector at an average photon-proton center of mass energy of 200 GeV in the transverse momentum range 0.5 <p_t <5 GeV. The production rates as a function of $p_t$ and center of mass rapidity are compared to those obtained in deep inelastic scattering at $\av{Q^2}=23 GeV^2$. A similar comparison is made of the rapidity spectra of charged particles. The rate of strangeness photoproduction is compared with $p\bar p$ measurements. The observations are also compared with next-to-leading order QCD calculations and the…
First inclusive measurements of isolated prompt photons in photoproduction at HERA have been made with the ZEUS detector. Cross sections are given as a function of the pseudorapidity and the transverse energy of the photon, for E_T^gamma > 5 GeV in the gamma-p centre-of-mass energy range 134-285 GeV. Comparisons are made with predictions from LO Monte Carlo models and NLO QCD calculations. For forward eta^gamma (proton direction) good agreement is found, but in the rear direction all predictions fall below the data.
We present results for the photoproduction of inclusive prompt photons and for prompt photons accompanied by jets, measured with the ZEUS detector at HERA. Cross sections as a function of pseudorapidity and transverse energy are presented for 5 < E_T^γ< 10 GeV, E_T^{jet} > 5 GeV in the centre of mass energy range 120-270 GeV. Comparisons are made with predictions from leading logarithm parton shower Monte Carlos and next-to-leading order QCD calculations using currently available models of the photon structure. NLO QCD calculations describe the shape and magnitude of the measurements reasonabl…
The production of heavy quarks has been studied in photoproduction processes with the ZEUS detector at HERA using an integrated luminosity of $36.9 pb^{-1}$. Events with a photon virtuality, $Q^2<1 GeV^2$, were selected with two jets of high transverse energy and an electron in the final state. Consideration of the distribution in $p_T^{rel}$ - the momentum of the electron transverse to the axis of the jet to which the electron is closest - allows a measurement of the beauty cross-section in a restricted region of phase space.
An overview over the recent heavy flavour results of the H1 and ZEUS collaborations is presented. Various techniques to tag the heavy quark, which allow to explore different phase space regions, are employed. Predictions of pertubative QCD are compared to the charm and beauty production data. Charm and beauty fractions of the proton structure function $F_2$ are
Dijet angular distributions of photoproduction events in which a $D^{*\pm}$ meson is produced in association with one of two energetic jets have been measured with the ZEUS detector at HERA, using an integrated luminosity of 120 pb$^{-1}$. Differential cross sections as a function of the angle between the charm-jet and the proton-beam direction in the dijet rest frame have been measured for samples enriched in direct or resolved photon events. The results are compared with predictions from leading-order parton-shower Monte Carlo models and with next-to-leading-order QCD calculations. The angul…
We report the latest results of the ZEUS collaboration on the photoproduction of D* mesons in a low W range. Differential cross sections as function of pt(D*) and eta(D*) are measured and compared with several recent NLO pQCD calculations. The differential cross-sections in a restricted kinematical region are higher than the NLO calculations, in particular in the forward (proton) direction. A recent pQCD model (BKL) describes the data reasonably well.
