Dijet events in photon-proton collisions in which there is a large pseudorapidity separation Delta eta > 2.5 between the two highest E_T jets are studied with the H1 detector at HERA. The inclusive dijet cross sections are measured as functions of the longitudinal momentum fractions of the proton and photon which participate in the production of the jets, x_pjet and x_gjet respectively, Delta eta, the pseudorapidity separation between the two highest E_T jets, and E_T^gap, the total summed transverse energy between the jets. Rapidity gap events are defined as events in which E_T^gap is less than E_T^cut, for E_T^cut varied between 0.5 and 2.0 GeV. The fraction of dijet events with a rapidity gap is measured differentially in Delta eta, x_pjet and x_gjet. An excess of events with rapidity gaps at low values of E_T^cut is observed above the expectation from standard photoproduction processes. This excess can be explained by the exchange of a strongly interacting colour singlet object between the jets.
The inclusive dijet cross section as a function of X(C=GAMMA).
The inclusive dijet cross section as a function of XP.
The inclusive dijet cross section as a function of the rapidity gap.
The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the pizero mesons at an average photon--proton centre-of-mass energy of 208 GeV and for photon virtualities below Q^2=0.01 GeV^2. The pizero measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of -5.5 in the gamma p centre-of-mass frame. The pizero transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon.
The inclusive PI0 photoproduction cross section in bias of rapidity and PT.Bin centre corrections have been applied and the errors are the quadratic sum o f the statistics and systematics (which dominate).
The inclusive PI0 photoproduction cross section as a function of PT for therapidity range -3.5 to -1.5. Bin centres are given in addition to limits. Error s are the quadratic sum of statistics and systematics.
The inclusive PI0 photoproduction cross section as a function of XL (the Feyman X variable) for the rapidity range -3.5 to -1.5. Bin centres are given as well as limits. Errors are the quadratic sum of statistics and systematics.
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 state and the ratio of the production cross sections for longitudinally and transversely polarised phi mesons is determined. A small but significant violation of s-channel helicity conservation (SCHC) is observed.
The measured ratio of PHI to RHO0 production.
The cross section for elastic PHI meson electro-production calculated by multiplying the PHI/RHO0 cross section ratio by the RHO0 cross section accurately measured in the earlier H1 publication (EPJ C13,371).
The corrected T distribution of elastic PHI meson production for W around 75 GeV and Q2 in the range 2.5 to 15 GeV**2 (mean = 4.8 GeV**2). Statistical error only.
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)+(0.08+-0.17)*t/GeV^2. Models based on perturbative QCD and on pomeron exchange are compared to the data.
The cross sections for the elastic photoproduction of J/PSI particles.
Differential cross section, DSIG/DT, for the elastic photoproduction of J/PSI particles.
The slope of the DSIG/DT distribution.
The production of hard di-jet events in photoproduction at HERA is dominated by resolved photon processes in which a parton in the photon with momentum fraction x_gamma is scattered from a parton in the proton. These processes are sensitive to the quark and gluon content of the photon. The differential di-jet cross-section dsigma/dlog(x_gamma) is presented here, measured in tagged photoproduction at HERA using data taken with the H1 detector, corresponding to an integrated luminosity of 7.2 pb^(-1). Using a restricted data sample at high transverse jet energy, E_(T,jet)>6 GeV, the effective parton density f_gamma,eff(x_gamma) = [q(x_gamma) + bar(q)(x_gamma) +9/4g(x_gamma)] in the photon in leading order QCD is measured down to x_gamma=0.05 from which the gluon density in the photon is derived.
The di-jet photoproduction cross section for ET > 4 GeV.
The di-jet photoproduction cross section for ET > 6 GeV after pedestal energy subtraction.
The first observation of open b production in ep collisions is reported. An event sample containing muons and jets has been selected which is enriched in semileptonic b quark decays. The visible cross section \sigma(ep -> b \bar{b}X -> \mu X') for Q^2 < 1 GeV^2, 0.1 < y < 0.8 is measured to be 0.176+-0.016(stat.)+0.026-0.017(syst.) nb for the muons to be detected in the range 35 deg < \theta^\mu < 130 deg and \pt^\mu > 2.0 GeV in the laboratory frame. The expected visible cross section based on a NLO QCD calculation is 0.104+-0.017 nb. The cross sections for electroproduction with Q^2<1 GeV^2 and photoproduction are derived from the data and found to be \sigma(ep-> e b\bar{b}X) = 7.1+-0.6(stat.)+1.5-1.3(syst.) nb and \sigma(\gamma p-> b\bar{b} X) = 111+-10(stat.)+23-20(syst.) at an average <W_{\gamma p}> \sim 180 GeV, respectively.
The visible BQ BQBAR --> MUON X cross section in the stated kinematic range.
The total electroproduction and photoproduction cross sections extrapolated to the full phase space.
The electroproduction of J/psi and psi(2S) mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers 2 < Q^2 < 80 GeV^2 and photon-proton centre of mass energies 25 < W < 180 GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic J/psi production is measured as a function of Q^2 and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic psi(2S) and J/psi meson production is measured as a function of Q^2. The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of J/psi mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions.
Cross section for elastic J/PSI photoproduction in W bins.
Cross section for elastic J/PSI photoproduction in W bins.
Cross section for elastic J/PSI photoproduction in W bins.
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 to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.
Average values of the spin density matrix elements measured for the 1996 data sample.
Spin density matrix elements measured for 3 Q**2 values for the 1996 data sample.
Spin density matrix elements measured for 3 W values for the 1996 data sample.
The triple-differential dijet cross-section, d^3 sigma_{ep}/dQ2 dE_t2 dxgjets, is measured with the H1 detector at HERA as a function of the photon virtuality Q^2, the fraction of the photon's momentum carried by the parton entering the hard scattering, xgjets, and the square of the mean transverse energy, E_t2, of the two highest E_t jets. Jets are found using a longitudinal boost-invariant k_T clustering algorithm in the gamma* p center of mass frame. The measurements cover the ranges 1.6 < Q^2 < 80 GeV$^2 in virtuality and 0.1 < y < 0.7 in inelasticity y. The results are well described by leading order QCD models which include the effects of a resolved component to the virtual photon. Models which treat the photon as point-like fail to describe the data. An effective leading order parton density for the virtual photon is extracted as a function of the photon virtuality, the probing scale and the parton momentum fraction. The x_gamma and probing scale dependences of the parton density show characteristic features of photon structure, and a suppression of this structure with increasing Q^2 is seen.
No description provided.
No description provided.
No description provided.
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 gluon momentum fraction x_g has been obtained from the measured kinematics of the scattered electron and the D* meson in the final state. The results compare well with the gluon distribution obtained from the analysis of scaling violations of the proton structure function F_2.
Total cross section for DIS D*+- production in the specified kinemtaic range.
DIS cross section as a function of the transverse D* momentum in the laboratory frame.
DIS cross section as a function of the transverse D* momentum in the hadronic centre-of-mass frame.
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).
Overall value for photoproduction cross section combining the different decay modes and data sample.. 1994 and 1995 data.. The second systematic error is from the branching ratio uncertainties.
Combined cross section from PSI(3685) --> J/PSI(1S) < E+ E- > PI+ PI- and PSI(3685) --> J/PSI(1S) < MU+ MU- > PI+ PI- modes using both the 1994 and 1995 data.. The second systematic error is from the branching ratio uncertainties.
Cross section from PSI(3685) --> J/PSI(1S) < E+ E- > PI+ PI- mode.
The double-differential inclusive di-jet cross section in photoproduction processes is measured with the H1 detector at HERA. The cross section is determined as a function of the average transverse jet energy E_T^jets for ranges of the fractional energy x_gamma^jets of the parton from the photon side. An effective leading order parton distribution in the photon is determined at large parton fractional energies for scales between 80<p_T^2<1250 GeV^2. The measurement is compatible with the logarithmic scale dependence that is predicted by perturbative QCD.
No description provided.
No description provided.
No description provided.
The transition between photoproduction and deep-inelastic scattering is investigated in jet production at the HERA ep collider, using data collected by the H1 experiment. Measurements of the differential inclusive jet cross-sections dsigep/dEt* and dsigmep/deta*, where Et* and eta* are the transverse energy and the pseudorapidity of the jets in the virtual photon-proton centre of mass frame, are presented for 0 < Q2 < 49 GeV2 and 0.3 < y < 0.6. The interpretation of the results in terms of the structure of the virtual photon is discussed. The data are best described by QCD calculations which include a partonic structure of the virtual photon that evolves with Q2.
The ET differential jet cross section in the virtual-photon CM frame.
The ET differential jet cross section in the virtual-photon CM frame.
The ET differential jet cross section in the virtual-photon CM frame.
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 for Phi and rho meson production is investigated as a function of Q^2.
Corrected PT**2 distribution for RHO production from the proton dissociative sample. Statistical errors only.
Cross sections and ratio of proton dissociative to elastic cross sections.
Cross sections and ratio of proton dissociative to elastic cross sections.
Inclusive $K^0$ and $\Lambda$ 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 predictions of a Monte Carlo model.
Additional systematic uncertainty given above.
Additional systematic uncertainty given above.
Additional systematic uncertainty given above.
A study is presented of the process gamma p -->XY, where there is a large rapidity gap between the systems X and Y. Measurements are made of the differential cross section as a function of the invariant mass mx of the system produced at the photon vertex. Results are presented at centre of mass energies of W_gp = 187 GeV and W_gp = 231 GeV, both where the proton dominantly remains intact and, for the first time, where it dissociates. Both the centre of mass energy and the mx~2 dependence of HERA data and those from a fixed target experiment may simultaneously be described in a triple-Regge model. The low mass photon dissociation process is found to be dominated by diffraction, though a sizable subleading contribution is present at larger masses. The pomeron intercept is extracted and found to be alpha_pom(0) = 1.068 \pm 0.016 (stat.) \pm 0.022 (syst.) \pm 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process gamma p --> Xp with mx~2 / W_gp~2 < 0.05 is measured to be 22.2 \pm 0.6 (stat.) \pm 2.6 (syst.) \pm 1.7 (model) % of the total gamma p cross section at W_gp = 187 GeV.
Data for proton remaining intact.
Data for proton dissociating.
At the electron-proton collider HERA the inclusive $D~{*\pm}$ meson photoproduction cross section has been measured with the H1 detector in two different, but partly overlapping, kinematical regions. For the first, where $\langle W_{\gamma p}\rangle \approx 200$\ GeV and $Q~2 < 0.01\,\gev~2$, the result is $\sigma(\gamma p \rightarrow c \bar{c} X) = (13.2 \pm 2.2 ~{+2.1}_{-1.7}\, ~{+9.9}_{-4.8})\,\mu b$. The second measurement for $Q~2 < 4\,\gev~2$ yields $\sigma(\gamma p \rightarrow c \bar{c} X) = ( 9.3 \pm 2.1 ~{+1.9}_{-1.8}\, ~{+6.9}_ {-3.2} )\,\mu b$ at $\langle W_{\gamma p}\rangle \approx 142$\,GeV and $\sigma(\gamma p \rightarrow c \bar{c} X) = ( 20.6 \pm 5.5 ~ {+4.3}_{-3.9}\, ~{+15.4}_{-7.2})\,\mu b$ at $\langle W_{\gamma p} \rangle \approx 230$\,GeV, respectively. The third error accounts for an additional uncertainty due to the proton and photon parton density parametrizations. Differential cross sections are presented as a function of the $D~{*\pm}$ transverse momentum and rapidity. The results compare reasonably well with next-to-leading order QCD calculations. Evidence for diffractive photoproduction of charm quark is presented.
Tagged events.
Untagged events.
Second systematical errors are due to the dependence on the parton density parametrizations. Tagged events.
The total cross sections for the elastic electroproduction of $\rh0$ and $J/\Psi$ mesons for $Q~2$ $>$ 8 GeV$~2$ and $\langle W \rangle \simeq 90$ GeV/c$~2$ are measured at HERA with the H1 detector. The measurements are for an integrated electron$-$proton luminosity of $\simeq$3pb$~{-1}$. The dependences of the total virtual photon$-$proton ($\gamma~* p$) cross sections on $Q~2$, $W$ and the momentum transfer squared to the proton ($t$), and, for the $\rho$, the dependence on the polar decay angle ($\cos \theta~*$), are presented. The $J/\Psi$ : $\rh0$ cross section ratio is determined. The results are discussed in the light of theoretical models and of the interplay of hard and soft physics processes.
Overall EP cross section for M(PI+PI-) < 1.5 GEV.
Overall EP cross section, taking into account the J/PSI --> LEPTON+ LEPTON - branching fraction 0.12.
Integrated EP cross section.
The cross section for the elastic photoproduction of \r0\ mesons ($\gamma p \rightarrow \rho~0 p$) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187GeV. TheFcenterline lower energy point was measured by observing directly the $\rho~{0}$ decay giving a cross section of $9.1\pm 0.9\,(\stat)\pm 2.5\,(\syst)\;\mu$b. The logarithmic slope parameter of the differential cross section, ${\rm d}\sigma/{\rm d}t$, 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 conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be $13.6\pm 0.8\,(\stat)\pm 2.4\,(\syst)\;\mu$b.
PI+ PI- cross section.
RHO0 cross section by selecting Mpipi to lie between 2Mpi and Mrho + 5width0.
No description provided.
Photoproduction of 2-jet events is studied with the H1 detector at HERA. Parton cross sections are extracted from the data by an unfolding method using leading order parton-jet correlations of a QCD generator. The gluon distribution in the photon is derived in the fractional momentum range $0.04\le x_\gamma \le 1$ at the average factorization scale $75$ GeV$~2$.
No description provided.
No description provided.
Gluon density of the photon (alpha is the fine structure constant).
We present a study of J ψ meson production in collisions of 26.7 GeV electrons with 820 GeV protons, performed with the H1-detector at the HERA collider at DESY. The J ψ mesons are detected via their leptonic decays both to electrons and muons. Requiring exactly two particles in the detector, a cross section of σ(ep → J ψ X) = (8.8±2.0±2.2) nb is determined for 30 GeV ≤ W γp ≤ 180 GeV and Q 2 ≲ 4 GeV 2 . Using the flux of quasi-real photons with Q 2 ≲ 4 GeV 2 , a total production cross section of σ ( γp → J / ψX ) = (56±13±14) nb is derived at an average W γp =90 GeV. The distribution of the squared momentum transfer t from the proton to the J ψ can be fitted using an exponential exp(− b ∥ t ∥) below a ∥ t ∥ of 0.75 GeV 2 yielding a slope parameter of b = (4.7±1.9) GeV −2 .
No description provided.
No description provided.
QED background subtracted.
The process γγ→π+π−π+π− has been investigated in reactions of the typee+e−→e+e−π+π−π+π− in the single tag mode. The range of the four momentum squared of one of the virtual photons was 0.28 GeV2/c2≦Q2≦3.6 GeV2/c2, the average being 〈Q2〉=0.92 GeV2/c2; the other photon was quasi real. The reaction is mainly described by the channels γγ→ρ0ρ0 and γγ→4π (phase space), occuring with about equal probability. TheQ2-dependence of the cross section is in agreement with the ρ form factor.
Data read from graph.. Additional overall systematic error 25%.
Data read from graph.. Additional overall systematic error 25%.. The Q**2 approx 0 datum is deduced from the earlier TASSO paper, Brandelik et al, Phys. Lett. 97B(1980)448, (<a href=http://durpdg.dur.ac.uk/scripts/reacsearch.csh/TESTREAC/red+1151> RED = 1151 </a>) on rho0 rho0 production.
We present a measurement of the photon structure functionF2γ in the reactionee→eeX forQ2 in the range 0.2<Q2<7 GeV2, using 9,200 multihadron events obtained with the TPC/Two-Gamma detector at PEP. The data have been corrected for detector effects using a regularized unfolding procedure and are presented as a function ofx andQ2. The structure function shows scaling in the region 0.3<Q2<1.6 GeV2,x<0.3 and rises for higherQ2. AtQ2=5.1 GeV2 the results are compared with QCD and, within the scheme of Antoniadis and Grunberg, rather conservative bounds for the QCD scale parameter of 133±50<\(\Lambda _{\overline {{\rm M}S} } \)<268±98 MeV are obtained. A study of the final state structure shows that the rise ofF2γ is consistent with being entirely due to the pointlike component of the photon.
No description provided.
No description provided.
No description provided.
The structure function F2γ for a quasireal photon has been measured in the reaction ee→eeX for Q2 in the range 0.2<Q2<7 GeV2, by use of 9200 multihadron events obtained with the TPC/Two-Gamma detector at the SLAC storage ring PEP. The data have been corrected for detector effects by a regularized unfolding procedure and are presented as F2γ(x,Q2). The structure function shows scaling in the region 0.3<Q2<1.6 GeV2, x<0.3, and rises for higher Q2 and x>0.1. Below Q2=0.3 GeV2, scaling breaks down in accordance with the finite cross-section bound for real photons.
Data read from graph.
Data read from graph.
Data read from graph.
The reactione+e−→e+e− A2 (1320) has been observed by detecting the decayA2→π+,π-π0. The two-photon width of theA2 has been measured to be Г(A2→γγ)=(0.09±0.27 (stat)±0.16 (syst)) keV. The cross section σ(γγ→π+,π-π0 has been determined outside theA2 resonance region.
Data read off a graph.
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