Vector meson cross sections at HERA
英文摘要：
Inelastic and elastic (exclusive) cross section measurements of vector meson production at HERA are discussed.
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Rare D Meson Decays at HERA
A status report on the prospects of measuring rare decays of charmed mesons at HERA is given. Based on actual experience with measuring charm at HERA, the sensitivity on limits of rare decays is estimated.
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New Results on Vector Meson Production at HERA
New results on elastic (exclusive) production of vector mesons are presented, mainly on J/ψand phi mesons with emphasis on an interpretation of the data within pQCD.
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Exclusive vector meson electroproduction at HERA
The latest results on exclusive vector meson electroproduction from HERA are reviewed. In particular, the new high-statistics measurements of the rho^0 electroproduction are presented and compared to several models.
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Vector meson production at HERA
New results on elastic electroproduction and proton-dissociative photoproduction at large t of rho, phi and J/psi mesons are presented. They are interpreted within perturbative QCD.
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Vector Mesons and DVCS at HERA
We present a review of the latest exclusive diffraction results achieved at HERA, discussing in particular the $W$ and $t$ dependence of the measured cross-sections in both DIS and photo-production regimes.
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Exclusive Vector Meson Production at HERA
An extended study of exclusive vector meson production in e,p interactions has been performed by the H1 and the ZEUS collaborations at the HERA collider. Recent measurements are reported and discussed within the framework of the dipole model and pQCD.
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Vector Meson Production at HERA
At the HERA collider the experiments H1 and ZEUS have studied the exclusive production of vector mesons over a wide kinematical range. A selection of recent measurements of the cross sections has been presented at Moriond QCD 2003.
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Vector meson production and inclusive KsKs final state at HERA
Measurements of vector mesons at HERA allow a detailed study of diffractive and non-diffractive production mechanisms, fragmentation and decay branching ratios. Results are presented on hadronic resonance measurements. In addition, the first observation of two meson states at masses around 1500 MeV and 1700 MeV are reported using inclusive KsKs production in deep inelastic ep scattering in ZEUS at HERA.
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Diffractive Vector Meson Production at HERA
The H1 and ZEUS collaborations at HERA report new data on the diffractive production of vector mesons with a many-fold increase of luminosity compared to previous measurements. The new data include photoproduction and electroproduction of $ρ^0,ϕ,J/ψ$. The available data on elastic VM production indicate that the interaction scales as $Q^2+M_{V}^2$.
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Inclusive Photoproduction of rho^0, K^{*0} and phi Mesons at HERA
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.
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Vector Mesons and pQCD at HERA
An overview of recent results from the H1 and ZEUS collaborations on vector meson production in electron-proton collisions at HERA is given. For diffractive vector meson production the energy dependence is discussed. The dependences on different scales, M_VM, Q^2 and |t|, are investigated. The data are compared to pQCD theoretical calculations. In addition new results on inelastic J/Psi electroproduction in a kinematical range 2<Q^2<100 GeV^2 and 50<W<225 GeV are shown. Differential cross sections are compared to theoretical predictions for color singlet and color octet contributions as well a…
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Measurement of diffractive production of D*+-(2010) mesons in deep inelastic scattering at HERA
Diffractive production of D*+-(2010) mesons in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 44.3 pb-1. Diffractive charm production is identified by the presence of a large rapidity gap in the final state of events in which a D*+-(2010) meson is reconstructed in the decay channel D*+ -> (D0 -> K-pi+) pi+ (+ charge conjugate). Differential cross sections when compared with theoretical predictions indicate the importance of gluons in such diffractive interactions.
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Production of excited charm and charm-strange mesons at HERA
The production of excited charm, D_1(2420)^0 and D_2^*(2460)^0, and charm-strange, D_{s1}(2536)^+-, mesons in ep collisions was measured with the ZEUS detector at HERA using an integrated luminosity of 126 pb^-1. Masses, widths and helicity parameters were determined. The measured yields were converted to the rates of c quarks hadronising as a given excited charm meson and to the ratios of the dominant D_2^*(2460)^0 and D_{s1}(2536)^+- branching fractions. A search for the radially excited charm meson, D^{*'}(2640)^+-, was also performed. The results are compared with those measured previously…
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Study of Charm Fragmentation into D^{*\pm} Mesons in Deep-Inelastic Scattering at HERA
The process of charm quark fragmentation is studied using $D^{*\pm}$ meson production in deep-inelastic scattering as measured by the H1 detector at HERA. Two different regions of phase space are investigated defined by the presence or absence of a jet containing the $D^{*\pm}$ meson in the event. The parameters of fragmentation functions are extracted for QCD models based on leading order matrix elements and DGLAP or CCFM evolution of partons together with string fragmentation and particle decays. Additionally, they are determined for a next-to-leading order QCD calculation in the fixed flavo…
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Inelastic Production of J/psi Mesons in Photoproduction and Deep Inelastic Scattering at HERA
A measurement is presented of inelastic photo- and electroproduction of J/psi mesons in ep scattering at HERA. The data were recorded with the H1 detector in the period from 2004 to 2007. Single and double differential cross sections are determined and the helicity distributions of the J/psi mesons are analysed. The results are compared to theoretical predictions in the colour singlet model and in the framework of non-relativistic QCD. Calculations in the colour singlet model using a k_T factorisation ansatz are able to give a good description of the data, while colour singlet model calculatio…
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Recent Results from Investigations of Diffractive Vector Meson Production with the ZEUS Detector at HERA
We present results from recent investigations of diffractive vector meson production using the ZEUS detector at HERA. These consist of measurements of rho production in the region of photon virtuality 0.25 < Q^2 < 0.85 GeV^2, of rho, phi, and J/psi production for values of the momentum transfer to the proton 0.3 < |t| < 4.0 GeV^2, and of elastic J/psi production both in photoproduction and for Q^2 > 2 GeV^2.
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Measurement of elastic electroproduction of phi mesons at HERA
The elastic electroproduction of phi mesons is studied at HERA with the H1 detector for photon virtualities 1 < Q^2 < 15 GeV^2 and hadronic centre of mass energies 40 < W < 130 GeV. The Q^2 and t dependences of the cross section are extracted (t being the square of the four-momentum transfer to the target proton). When plotted as function of (Q^2 + M_V^2) and scaled by the appropriate SU(5) quark charge factor, the phi meson cross section agrees within errors with the cross sections of the vector mesons V = rho, omega and J/psi. A detailed analysis is performed of the phi meson polarisation st…
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Inclusive D* Meson Cross Sections and D* Jet Correlations in Photoproduction at HERA
Differential photoproduction cross sections are measured for events containing D* mesons. The data were taken with the H1 detector at the ep collider HERA and correspond to an integrated luminosity of 51.1 pb-1. The kinematic region covers small photon virtualities Q^2 < 0.01 GeV^2 and photon-proton centre-of-mass energies of 171 < W_gammap < 256 GeV. The details of the heavy quark production process are further investigated in events with one or two jets in addition to the D* meson. Differential cross sections for D* jet production are determined and the correlations between the D* meson and …
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Inclusive Production of D^+, D^0, D_s^+ and D^*+ Mesons in Deep Inelastic Scattering at HERA
Inclusive production cross sections are measured in deep inelastic scattering at HERA for meson states composed of a charm quark and a light antiquark or the charge conjugate. The measurements cover the kinematic region of photon virtuality 2 < Q^2 < 100 GeV^2, inelasticity 0.05 < y < 0.7, D meson transverse momenta p_t(D) > 2.5 GeV and pseudorapidity |eta(D)| < 1.5. The identification of the D-meson decays and the reduction of the combinatorial background profit from the reconstruction of displaced secondary vertices by means of the H1 silicon vertex detector. The production of charmed mesons…
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Exclusive photoproduction of {\bf $Υ$} mesons at HERA
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.
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Photo-production of psi' Mesons at HERA
Quasi-elastic (z >0.95) photo-production of psi' mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The psi' mesons were identified through their decays to l+l- and to J/psi pi+ pi-, where the J/psi subsequently decays to l+l-, the lepton l being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be [18.0 +- 2.8 (stat) +- 3.0(syst)] nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the psi' to J/psi quasi-elastic cross-sections is 0.150 +- 0.027 (stat) +- 0.022 (syst).
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Energy dependence of exclusive vector-meson production in ep interactions at HERA
The energy dependence of exclusive vector meson (V) production is studied using the ZEUS data. The SU(4) universality of V cross sections in the $Q^2+M_V^2$ scale is tested. The energy dependence of the ratio of the cross sections for V production to the total $γ^*p$ cross section is compared with expectations based on pQCD and Regge approaches.
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Exclusive electroproduction of $ϕ$ mesons at HERA
Exclusive electroproduction of $ϕ$ mesons has been studied in $e^\pm p$ collisions at $\sqrt{s}=318 \gev$ with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb$^{-1}$. The $γ^*p$ cross section is presented in the kinematic range $2<Q^2<70 \gev^2$, $35<W<145 \gev$ and ${|t|}<0.6 \gev^2$. The cross sections as functions of $Q^2$, $W$, $t$ and helicity angle $θ_h$ are compared to cross sections for other vector mesons. The ratios $R$ of the cross sections for longitudinally and transversely polarized virtual photons are presented as functions of $Q^2$ and $W$. The data are also…
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Study of $D^{*+/-}$ Meson Production in Diffractive $ep$ Scattering at HERA
$D^{*+/-}$ production in the diffractive interactions was measured with the ZEUS detector at HERA using integrated luminosities of 44 pb$^{-1}$ in deep inelastic scattering and 38 pb$^{-1}$ in photoproduction processes. Diffractive interactions were identified by a large gap in the rapidity distribution of final states. $D^{*+/-}$ mesons were reconstructed from the decay $D^{*+} -> D^0π_s^+$ with {$D^0 -> K^-π^+$} (+c.c.). Integrated and differential cross sections were compared to theoretical expectations.
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Topical Results on Vector-Meson Production from the HERA Collider Experiments
The HERA collider experiments H1 and ZEUS have established extensive measurement programs for diffractive vector-meson production processes during the first six years of their operation. The results provide stringent phenomenological tests of quantum chromodynamical descriptions of hard diffraction. We discuss recent topical results on Upsilon photoproduction, on decay-angle analyses of rho^0, phi and J/psi electroproduction and on rho^0 and phi photoproduction at high momentum transfer.
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Elastic Vector Meson Production at HERA
The H1 and ZEUS Collaborations report new results on elastic VM cross-section and trajectory determination. Elastic VM production appears to be independent of the photon polarization. The presence of non-zero shrinkage in the photoproduction of J/psi indicates the presence of soft physics. Accumulated data do not accommodate a universal Pomeron trajectory. The steepness of the x dependence of elastic VM production with changing Q^2+M_V^2 is similar to that of F_2 with changing Q^2.
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Production of vector mesons and measurement of the hadronic component of photon light-cone wave function at HERA
A detailed study of vector meson production ($ϕ$ and $J/ψ$) in $e^\pm p$ collisions at HERA with the ZEUS and H1 detector has been performed. The cross sections are measured as a function of $Q^2$, $W$ and $t$. In this contribution, the results are summarised, compared to theoretical calculations and the dynamical picture emerging in perturbative QCD is highlighted. The measurement of the hadronic component of the photon light-cone wave function in the exclusive production of di-pions, $ep\to e π^+π^- p$ is also reported.
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Measurements of DVCS and High |t| Vector Meson Production at HERA
Measurements of Deeply Virtual Compton Scattering (DVCS) are presented using data collected by the H1 and ZEUS experiments at HERA. The transition from a virtual photon to an on-shell particle is expected to necessitate the use of skewed parton distributions for a theoretical description. The results are compared to theoretical predictions. Diffractive vector meson production measurements from H1 and ZEUS are also presented in which the magnitude of the four-momentum transfer squared |t| at the proton vertex is large. A large |t| is expected to make it possible to perform a perturbative calcul…
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Proton Dissociative Rho and Elastic Phi Electroproduction at HERA
The electroproduction of rho mesons with proton diffractive dissociation for Q^2 > 7 GeV^2 and the elastic electroproduction of Phi mesons for Q^2 > 6 Gev^2 are studied in e^+ p collisions at HERA with the H1 detector, for an integrated luminosity of 2.8 pb-1. The dependence of the cross sections on P_t^2 and Q^2 is measured, and the vector meson polarisation obtained. The cross section ratio between proton dissociative and elastic production of rho mesons is measured and discussed in the framework of the factorisation hypothesis of diffractive vertices. The ratio of the elastic cross section …
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Inelastic Leptoproduction of J/Psi Mesons at HERA
The leptoproduction of J/psi mesons is studied in inelastic reactions for four momentum transfers 2<Q^2<100GeV^2. The data were taken with the H1 detector at the electron proton collider HERA and correspond to an integrated luminosity of 77 pb-1. Single differential and double differential cross sections are measured with increased precision compared with previous analyses. New leading order calculations within the non-relativistic QCD factorisation approach including colour octet and colour singlet contributions are compared with the data and are found to give a reasonable description of most…
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Measurement of Inclusive and Dijet D* Meson Cross Sections in Photoproduction at HERA
The inclusive photoproduction of D\ast mesons and of D\ast-tagged dijets is investigated with the H1 detector at the ep collider HERA. The kinematic region covers small photon virtualities Q2 < 2 GeV2 and photon-proton centre-of-mass energies of 100 < Wgp < 285 GeV. Inclusive D\ast meson differential cross sections are measured for central rapidities |eta(D\ast)| < 1.5 and transverse momenta pT (D\ast) > 1.8 GeV. The heavy quark production process is further investigated in events with at least two jets with transverse momentum pT (jet) > 3.5 GeV each, one containing the D\ast meson. Different…
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Production of J/psi Mesons at HERA
Inelastic and diffractive production of J/psi mesons at HERA is reviewed. The data on inelastic photoproduction are described well within errors by the Colour Singlet Model in next-to-leading order. A search for colour octet processes predicted within the NRQCD/factorisation approach is conducted in many regions of phase space. No unambiguous evidence has been found to date. Diffractive elastic production of J/psi mesons has been measured in the limit of photoproduction up to the highest photon proton center of mass energies. The increase of the cross section is described by pQCD models. At la…
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Forward Pi0-Meson Production at HERA
High transverse momentum pi0-mesons have been measured with the H1 detector at HERA in deep-inelastic ep scattering events at low Bjorken-x, down to x <~ 4.10^{-5}. The measurement is performed in a region of small angles with respect to the proton remnant in the laboratory frame of reference, namely the forward region, and corresponds to central rapidity in the centre of mass system of the virtual photon and proton. This region is expected to be particularly sensitive to QCD effects in hadronic final states. Differential cross-sections for inclusive pi0-meson production are presented as a fun…
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Elastic Electroproduction of $\rh0$ and $J/Ψ$ Mesons at large $Q^2$ at HERA
The total cross sections for the elastic electroproduction of $\rh0$ and $J/Ψ$ mesons for $Q^2$ $>$ 8 GeV$^2$ and $<W > \simeq 90$ GeV/c$^2$ are measured at HERA with the H1 detector. The measurements are for an integrated electron$-$proton luminosity of $\simeq$~3~pb$^{-1}$. The dependences of the total virtual photon$-$proton ($γ^* p$) cross sections on $Q^2$, $W$ and the momentum transfer squared to the proton ($t$), and, for the $ρ$, the dependence on the polar decay angle ($\cos θ^*$), are presented. The $J/Ψ$ : $\rh0$ cross section ratio is determined. The results are discussed in the li…
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Measurement of D^{*\pm} Meson Production and Determination of F_2^{ccbar} at low Q2 in Deep-Inelastic Scattering at HERA
Inclusive production of D* mesons in deep-inelastic ep scattering at HERA is studied in the range 5 < Q^2 <100 GeV^2 of the photon virtuality and 0.02 < y < 0.7 of the inelasticity of the scattering process. The observed phase space for the D* meson is p_T(D*) > 1.25 GeV and |eta(D*)| < 1.8. The data sample corresponds to an integrated luminosity of 348 pb^{-1} collected with the H1 detector. Single and double differential cross sections are measured and the charm contribution F_2^{ccbar} to the proton structure function F_2 is determined. The results are compared to perturbative QCD predictio…
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Observation of the strange sea in the proton via inclusive Phi-meson production in neutral current deep inelastic scattering at HERA
Inclusive phi-meson production in neutral current deep inelastic e+p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb^{-1}. The phi mesons were studied in the range 10<Q2<100 GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, p_T, pseudorapidity, eta, and the scaled momentum in the Breit frame, x_p. Monte Carlo models with the strangeness-suppression factor as determined by analyses of e+e- annihilation events overestimate the cross sections. A smaller value of the strangen…
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A measurement of the t dependence of the helicity structure of diffractive rho meson electroproduction at HERA
The helicity structure of the diffractive electroproduction of rho mesons, e + p -> e + rho + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 < t' < 3 GeV^2, where t' = |t| - |t|_min. The data used are collected with the H1 detector at HERA in the kinematic domain 2.5 < Q^2 < 60 GeV^2, 40 < W < 120 GeV. No t dependence of the r^04_00 spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the rho meson is observed for the spin density matrix element combinations r^5_0…
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Heavy Quark Production and Spectroscopy at HERA
Production of final states containing open charm and beauty quarks at HERA is reviewed. Photoproduction (PHP) of the charm meson resonances D^*, D^0 and D_s, as well as D^* production in the deep inelastic scattering (DIS) regime, are measured and compared to QCD predictions. The excited charm mesons D^0_1(2420), D^{*0}_2(2460) and D^{+-}_{s1}(2536) have been observed and the rates of charm quarks hadronising to these mesons were extracted. A search for radially excited charm mesons has been performed. PHP and DIS beauty cross sections are higher than expected in next-to-leading order QCD.
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Elastic Photoproduction of J/psi and Upsilon Mesons at HERA
Cross sections for elastic photoproduction of J/Psi and Upsilon mesons are presented. For J/Psi mesons the dependence on the photon-proton centre-of-mass energy W_gammap is analysed in an extended range with respect to previous measurements of 26<=W_gammap<= 285 GeV. The measured energy dependence is parameterized as sigma_gammap proportional W_gammap^delta with delta=0.83+-0.07. The differential cross section dsigma/dt for J/Psi mesons is derived, its dependence on W_gammap and on t is analysed and the effective trajectory (in terms of Regge theory) is determined to be alpha(t)=(1.27+-0.05)+(…
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Measurement of D^(*+-) Meson Production and F_2^c in Deep-Inelastic Scattering at HERA
The inclusive production of D^{*+-}(2010) mesons in deep-inelastic scattering is studied with the H1 detector at HERA. In the kinematic region 1<Q^2<100 GeV^2 and 0.05<y<0.7 an e^+p cross section for inclusive D^(*+-) meson production of 8.50+- 0.42 (stat.)^(+1.21)_(-1.00) (syst.) nb is measured in the visible range p_(tD^*)>1.5 GeV and |η_(D^*)|<1.5. Single and double differential inclusive D^(*+-) meson cross sections are compared to perturbative QCD calculations in two different evolution schemes. The charm contribution to the proton structure, F_2^c(x,Q^2), is determined by extrapolating t…
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Measurement of the D* Meson Production Cross Section and F_2^{ccbar}, at High Q^2, in ep Scattering at HERA
The inclusive production of D*(2010) mesons in deep-inelastic ep scattering is measured in the kinematic region of photon virtuality 100 < Q^2 < 1000 GeV^2 and inelasticity 0.02 < y < 0.7. Single and double differential cross sections for inclusive D* meson production are measured in the visible range defined by |eta(D*)| < 1.5 and p_T(D*) > 1.5 GeV. The data were collected by the H1 experiment during the period from 2004 to 2007 and correspond to an integrated luminosity of 351 pb^{-1}. The charm contribution, F_2^{ccbar}, to the proton structure function F_2 is determined. The measurements a…
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Production of D* Mesons with Dijets in Deep-Inelastic Scattering at HERA
Inclusive D* production is measured in deep-inelastic ep scattering at HERA with the H1 detector. In addition, the production of dijets in events with a D* meson is investigated. The analysis covers values of photon virtuality 2< Q^2 <=100 GeV^2 and of inelasticity 0.05<= y <= 0.7. Differential cross sections are measured as a function of Q^2 and x and of various D* meson and jet observables. Within the experimental and theoretical uncertainties all measured cross sections are found to be adequately described by next-to-leading order (NLO) QCD calculations, based on the photon-gluon fusion pro…
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Measurement of the $e^+$ and $e^-$ induced charged current cross sections at HERA
The cross sections for the charged current processes ${e^{-}p}\rightarrow{ν_e+hadrons}$ and, for the first time, ${e^{+}p}\rightarrow{\overlineν_e+hadrons}$ are measured at HERA for transverse momenta larger than 25 GeV.
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Energy dependence of the total photoproduction cross section at HERA
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.
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Resonance searches at HERA
Inclusive production of K0S K0S in ep collisions was studied with the ZEUS detector. Significant production of JPC=2++ tensor mesons and of the 0++ glueball candidate f0(1710) was seen. Masses and widths were compared with previous experiments. The H1 Collaboration saw a charm pentaquark candidate in the D*p spectrum at 3.1 GeV, which was not confirmed by a ZEUS higher statistics search. With the full HERA data, H1 did not see a signal in this region. Masses, widths and helicity parameters of excited charm and charm-strange mesons were measured by ZEUS. Rates of c-quarks hadronising into these…
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Elastic and Proton-Dissociative Photoproduction of J/psi Mesons at HERA
Cross sections for elastic and proton-dissociative photoproduction of J/psi mesons are measured with the H1 detector in positron-proton collisions at HERA. The data were collected at $ep$ centre-of-mass energies sqrt{s} approx 318 GeV and sqrt{s} approx 225 GeV, corresponding to integrated luminosities of L = 130 pb^{-1} and L = 10.8 pb^{-1}, respectively. The cross sections are measured as a function of the photon-proton centre-of-mass energy in the range 25< Wgp < 110 GeV. Differential cross sections $\mathrm{d}σ/ \mathrm{d}t$, where $t$ is the squared four-momentum transfer at the proton ve…
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Structure Functions and Large Q^2 Cross Sections at HERA
The data collected with the H1 and ZEUS detectors during the running period 1994-99 are used to give an experimental review on the proton structure functions and the neutral current and charged current large Q^2 cross sections in e^\pm p scattering at HERA.
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Inclusive high Q2 cross sections and QCD and EW fits at HERA
The latest measurements of the cross sections for neutral and charged current deep inelastic scattering in e+/-p collisions at HERA with longitudinally polarised lepton beams are presented. The measured cross sections are compared with the predictions of the Standard Model. Next-to-leading-order QCD analyses of HERA data are also presented. The analyses yield the parton distribution functions of the proton and are extended to also extract electroweak parameters. The determinations from HERA are compared with those from LEP and the Tevatron.
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Measurement of the Spin-Density Matrix Elements in Exclusive Electroproduction of rho^0 Mesons at HERA
Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the azimuthal angle between the positron scattering plane and the rho^0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) rho^0 mesons from…
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Structure Function Measurements and Polarised Cross Section Measurements from HERA
Recent measurements of inclusive and semi-inclusive measurements from the HERA collaborations are presented. The measurements include neutral current structure functions $F_2$, $F_L$ and $xF_3$; the charged current cross section including first measurements of the dependence on electron polarisation; and measurements of the heavy quark structure functions $F_2^{c\bar{c}}$ and $F_2^{b\bar{b}}$.
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Measurement of D* Meson Cross Sections at HERA and Determination of the Gluon Density in the Proton using NLO QCD
With the H1 detector at the ep collider HERA, D* meson production cross sections have been measured in deep inelastic scattering with four-momentum transfers Q^2>2 GeV2 and in photoproduction at energies around W(gamma p)~ 88 GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe the differential cross sections within theoretical and experimental uncertainties. Using these calculations, the NLO gluon momentum distribution in the proton, x_g g(x_g), has been extracted in the momentum fraction range 7.5x10^{-4}< x_g <4x10^{-2} at average scales mu^2 =25 to 50 GeV2. The glu…
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Measurement of D mesons Production in Deep Inelastic Scattering at HERA
Charm production in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb^{-1}. Charm has been tagged by reconstructing D^{*+}, D^0, D^{+} and D_s^+ (+ c.c.) charm mesons. The charm hadrons were measured in the kinematic range p_T(D^{*+},D^0,D^{+}) > 3 GeV, p_T(D_s^+)>2 GeV and |η(D)| < 1.6 for 1.5 < Q^2 < 1000 GeV^2 and 0.02 < y < 0.7. The production cross sections were used to extract charm fragmentation ratios and the fraction of c quarks hadronising into a particular charm meson in the kinematic range considered. The cross sectio…
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Elastic and Inelastic Photoproduction of \jpsi Mesons at HERA
Results on \jpsi production in $e p$ interactionsin the H1 experiment at HERA are presented. The \jpsi mesons are produced by almost real photons ($Q^2\approx 0$) and detected via their leptonic decays. The data have been taken in 1994 and correspond to an integrated luminosity of $2.7 {pb}^{-1}$. The $γp$ cross section for elastic \jpsi production is observed to increase strongly with the \cm energy. The cross section for diffractive $J/ψ$ production with proton dissociation is found to be of similar magnitude as the elastic cross section. Distributions of transverse momentum and decay angle …
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Production of the excited charm mesons D_1 and D^*_2 at HERA
The production of the excited charm mesons D_1(2420) and D^*_2(2460) in ep collisions has been measured with the ZEUS detector at HERA using an integrated luminosity of 373 pb^{-1}. The masses of the neutral and charged states, the widths of the neutral states, and the helicity parameter of D_1(2420)^{0} were determined and compared with other measurements and with theoretical expectations. The measured helicity parameter of the D^0_1 allows for some mixing of S- and D-waves in its decay to D^{*\pm}π^{\mp}. The result is also consistent with a pure D-wave decay. Ratios of branching fractions o…
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Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA
Elastic and proton-dissociative photoproduction of $ρ^0$, $ϕ$ and $J/ψ$ vector mesons ($γp\to Vp$, $γp\to VN$, respectively) have been measured in e^+p interactions at HERA up to -t=3 GeV$^2$, where t is the four-momentum transfer squared at the photon-vector meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, Q^2, to values less than 0.01 GeV$^2$, and selects a $γp$ average center-of-mass energy of <W> = 94 GeV. Results for the differentia…
hep-ex
Elastic Electroproduction of rho Mesons at HERA
The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 < Q^2 < 60 GeV^2 and for a hadronic centre of mass energy in the range 30 < W < 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared…
hep-ex
Diffractive Photoproduction of Psi(2S) Mesons at HERA
Results on diffractive photoproduction of psi(2S) mesons are presented using data collected between 1996 and 2000 with the H1 detector at the HERA ep collider. The data correspond to an integrated luminosity of 77 pb^(-1). The energy dependence of the diffractive psi(2S) cross section is found to be similar to or possibly somewhat steeper than that for J/psi mesons. The dependences of the elastic and proton dissociative psi(2S) photoproduction cross sections on the squared momentum transfer t at the proton vertex are measured. The t-dependence of the elastic channel, parametrised as e^(bt), yi…
hep-ex
Elastic Photoproduction of $ρ^{0}$ Mesons at HERA
The cross section for the elastic photoproduction of \r0 mesons ($γp \to ρ^0 p$) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. TheFcenterline lower energy point was measured by observing directly the $ρ^{0}$ decay giving a cross section of $9.1\pm 0.9 (\stat)\pm 2.5 (\syst) μ$b. The logarithmic slope parameter of the differential cross section, ${\rm d}σ/{\rm d}t$, is found to be $10.9 \pm 2.4 (\stat) \pm 1.1 (\syst) $GeV$^{-2}$. The \r0 decay polar angular distribution is found to be consistent with s-channel helicity c…
hep-ex
Forward pi^0-meson production at HERA
The production of high transverse momentum piz^0-mesons has been measured in deep-inelastic e-p scattering events at low Bjorken-$x$ taken with the H1 detector at HERA. The production of high p_T particles is strongly correlated to the emission of hard partons in QCD and is therefore sensitive to the dynamics of the strong interaction. For the first time the measurement of single particles has been extended to the region of small angles w.r.t. the proton remnant (forward region) and down to very low values of x ~ 5x10^-5. This region is expected to be particularly sensitive to QCD evolution ef…
hep-ex
Measurement of proton-dissociative diffractive photoproduction of vector mesons at large momentum transfer at HERA
Diffractive photoproduction of vector mesons, gamma p --> V Y, where Y is a proton-dissociative system, has been measured in ep interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb^-1. The differential cross section, ds/dt, is presented for -t<12 GeV^2, where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, ds/dt ~ (-t)^{-n}. The slope of the Pomeron trajectory, measured from the W dependence of the rho^0…
hep-ex
