arXiv subjects

ZEUS Collaboration

Publications and source records attributed to ZEUS Collaboration.

At least 217 records · Page 12Linked to original sources

arXiv1997

Study of Photon Dissociation in Diffractive Photoproduction at HERA

Diffractive dissociation of quasi-real photons at a photon-proton centre of mass energy of W ~ 200 GeV is studied with the ZEUS detector at HERA. The process under consideration is gamma p -> X N, where X is the diffractively dissociated photon system of mass M_X and N is either a proton or a nucleonic system with mass M_N < 2GeV. The cross section for this process in the interval 3 < M_X < 24 GeV relative to the total photoproduction cross section was measured to be sigma^partial_D / sigma_tot = 6.2 +- 0.2(stat) +- 1.4(syst)%. After extrapolating this result to the mass interval of m_phi^2 < M_X^2 < 0.05 W^2 and correcting it for proton dissociation, the fraction of the total cross section attributed to single diffractive photon dissociation, gamma p -> X p, is found to be sigma_SD / sigma_tot = 13.3 +- 0.5(stat) +- 3.6(syst)%. The mass spectrum of the dissociated photon system in the interval 8 < M_X < 24 GeV can be described by the triple pomeron (PPP) diagram with an effective pomeron intercept of alpha_P(0) = 1.12 +- 0.04(stat) +- 0.08(syst). The cross section for photon dissociation in the range 3 < M_X < 8 GeV is significantly higher than that expected from the triple pomeron amplitude describing the region 8 < M_X < 24 GeV. Assuming that this discrepancy is due to a pomeron-pomeron-reggeon (PPR) term, its contribution to the diffractive cross section in the interval 3 < M_X < 24 GeV is estimated to be f_PPR = 26 +- 3(stat) +- 12(syst)%.

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arXiv1997

Comparison of ZEUS Data with Standard Model Predictions for e^+ p -> e^+ X Scattering at High x and Q^2

Using the ZEUS detector at HERA, we have studied the reaction e^+ p -> e^+ X for Q^2 > 5000 GeV^2 with a 20.1 pb^{-1} data sample collected during the years 1994 to 1996. For Q^2 below 15000 GeV^2, the data are in good agreement with Standard Model expectations. For Q^2 > 35000 Gev^2, two events are observed while 0.145+-0.013 events are expected. A statistical analysis of a large ensemble of simulated Standard Model experiments indicates that with probability 6.0%, an excess at least as unlikely as that observed would occur above some Q^2 cut. For x>0.55 and y>0.25, four events are observed where 0.91+-0.08 events are expected. A statistical analysis of the two-dimensional distribution of the events in x and y yields a probability of 0.72% for the region x>0.55 and y>0.25 and a probability of 7.8% for the entire Q^2 > 5000 GeV^2 data sample. The observed excess above Standard Model expectations is particularly interesting because it occurs in a previously unexplored kinematic region.

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arXiv1996

Measurement of Elastic omega Photoproduction at HERA

The reaction $γp \rightarrow ωp$ $(ω\rightarrow π^+π^-π^0$ and $π^0\rightarrowγγ)$ has been studied in $ep$ interactions using the ZEUS detector at photon-proton centre-of-mass energies between $70$ and $90 GeV$ and $|t| < 0.6 GeV^2$, where $t$ is the squared four momentum transferred at the proton vertex.The elastic \ome photoproduction cross section has been measured to be $σ_{γp\rightarrow ωp} = 1.21\pm 0.12\pm 0.23 μb$. The differential cross section $dσ_{γp\rightarrow ωp} /d|t|$ has an exponential shape $e^{-b |t|}$ with a slope $b = 10.0\pm 1.2\pm 1.3 GeV^{-2}$. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of $ω$ photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the $ρ^0$ and $ϕ$ photoproduction cross sections at HERA show a similar behaviour.

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arXiv1996

Study of Elastic rho0 photoproduction at HERA using the ZEUS Leading Proton Spectrometer

The differential cross section $dσ/dt$ for elastic $ρ^0$ photoproduction, $γp \rightarrow ρ^0 p (ρ^0 \rightarrow π^+ π^-)$, has been measured in $ep$ interactions at HERA. The squared four-momentum exchanged at the proton vertex, $t$, has been determined directly by measuring the momentum of the scattered proton using the ZEUS Leading Proton Spectrometer (LPS), a large scale system of silicon micro-strip detectors operating close to the HERA proton beam. The LPS allows the measurement of the momentum of high energy protons scattered at small angles with accuracies of 0.4% for the longitudinal momentum and 5 MeV for the transverse momentum. Photoproduction of $ρ^0$ mesons has been investigated in the interval $0.073< |t| <0.40 GeV^2$, for photon virtualities $Q^2<1 GeV^2$ and photon-proton centre-of-mass energies $W$ between 50 and 100 GeV. In the measured range, the $t$ distribution exhibits an exponential shape with a slope parameter $b = 9.8 \pm 0.8 (stat.) \pm 1.1 (syst.) GeV^{-2}$. The use of the LPS eliminates the contamination from events with diffractive dissociation of the proton into low mass states.

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arXiv1996

Observation of Events with an Energetic Forward Neutron in Deep Inelastic Scattering at HERA

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken $x$ and $Q^2$ in the range $ 3 \cdot 10^{-4} < \xbj < 6\cdot 10^{-3}$ and $10 < Q^2 <100$~GeV$^2$.

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arXiv1996

Measurement of the F2 structure function in deep inelastic scattering using 1994 data from the ZEUS detector at HERA

We present measurements of the structure function \Ft\ in $e^+p$ scattering at HERA in the range $3.5\;\Gevsq < \qsd < 5000\;\Gevsq$. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At $ \qsd < 35 \;\Gevsq$ the range in $x$ now spans $6.3\cdot 10^{-5} < x < 0.08$ providing overlap with measurements from fixed target experiments. At values of $Q^2$ above 1000 GeV$^2$ the $x$ range extends to 0.5. Systematic errors below 5\perc\ have been achieved for most of the kinematic region. The structure function rises as \x\ decreases; the rise becomes more pronounced as \qsd\ increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations.

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arXiv1996

Study of Charged-Current ep Interactions at Q2 > 200 GeV2 with the ZEUS Detector at HERA

Deep inelastic charged--current reactions have been studied in $e^+p$ and $e^-p$ collisions at a center of mass energy of about $300\,\gev$ in the kinematic region $Q^2\greater200\,\gev^2$ and $x\greater0.006$ using the ZEUS detector at HERA. The integrated cross sections for $Q^2\greater200\,\gev^2$ are found to be $\sigep=30.3\,{}^{+5.5}_{\mns4.2}\,{}^{+1.6}_{\mns2.6}\,{\rm pb}$ and $\sigem=54.7\,{}^{+15.9}_{\mns\chax 9.8}\,{}^{+2.8}_{\mns3.4}\,{\rm pb}$. Differential cross sections have been measured as functions of the variables $x$, $y$ and $Q^2$. From the measured differential cross sections $dσ/dQ^2$, the $W$ boson mass is determined to be $M_W=79\,{}^{+8} _{-7}{}^{+4}_{-4}\,\gev$. Measured jet rates and transverse energy profiles agree with model predictions. A search for charged--current interactions with a large rapidity gap yielded one candidate event, corresponding to a cross section of $\sigep(Q^2\greater200\,\gev^2;η_{\rm max}<2.5)=0.8\,{}_{-0.7}^ {+1.8}\,\pm0.1\,{\rm pb}$.

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arXiv1996

Dijet Angular Distributions in Direct and Resolved Photoproduction at HERA

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 the different spins of the quark and gluon exchanged in the hard subprocess.

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arXiv1996

Measurement of the Reaction $γ^* p \to ϕp$ in Deep Inelastic $e^+p$ Scattering at HERA

The production of $ϕ$ mesons in the reaction $e^{+}p \rightarrow e^{+} ϕp$ ($ϕ\rightarrow K^{+}K^{-}$), for $7 < Q^2 < 25$ GeV$^2$ and for virtual photon-proton centre of mass energies ($W$) in the range 42-134 GeV, has been studied with the ZEUS detector at HERA. When compared to lower energy data at similar $Q^2$, the results show that the $γ^*p \rightarrow \phid p$ cross section rises strongly with $W$. This behaviour is similar to that previously found for the $γ^*p \rightarrow ρ^0 p$ cross section. This strong dependence cannot be explained by production through soft pomeron exchange. It is, however, consistent with perturbative QCD expectations, where it reflects the rise of the gluon momentum density in the proton at small $x$. The ratio of $σ(ϕ) / σ(ρ^0)$, which has previously been determined by ZEUS to be 0.065 $\pm$ 0.013 (stat.) in photoproduction at a mean $W$ of 70 GeV, is measured to be 0.18 $\pm $ 0.05 (stat.) $\pm$ 0.03 (syst.) at a mean $Q^2$ of 12.3 GeV$^2$ and mean $W$ of $\approx$ 100 GeV and is thus approaching at large $Q^2$ the value of 2/9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism.

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arXiv1996

Measurement of the Diffractive Cross Section in Deep Inelastic Scattering

Diffractive scattering of $γ^* p \to X + N$, where $N$ is either a proton or a nucleonic system with $M_N~<~4$~GeV has been measured in deep inelastic scattering (DIS) at HERA. The cross section was determined by a novel method as a function of the $γ^* p$ c.m. energy $W$ between 60 and 245~GeV and of the mass $M_X$ of the system $X$ up to 15~GeV at average $Q^2$ values of 14 and 31~GeV$^2$. The diffractive cross section $dσ^{diff} /dM_X$ is, within errors, found to rise linearly with $W$. Parameterizing the $W$ dependence by the form $dσ^{diff}/dM_X \propto (W^2)^{(2\overline{\mbox{$α_{_{I\hspace{-0.2em}P}}$}} -2)}$ the DIS data yield for the pomeron trajectory $\overline{\mbox{$α_{_{I\hspace{-0.2em}P}}$}} = 1.23 \pm 0.02(stat) \pm 0.04 (syst)$ averaged over $t$ in the measured kinematic range assuming the longitudinal photon contribution to be zero. This value for the pomeron trajectory is substantially larger than $\overline{\mbox{$α_{_{I\hspace{-0.2em}P}}$}}$ extracted from soft interactions. The value of $\overline{\mbox{$α_{_{I\hspace{-0.2em}P}}$}}$ measured in this analysis suggests that a substantial part of the diffractive DIS cross section originates from processes which can be described by perturbative QCD. From the measured diffractive cross sections the diffractive structure function of the proton $F^{D(3)}_2(β,Q^2, \mbox{$x_{_{I\hspace{-0.2em}P}}$})$ has been determined, where $β$ is the momentum fraction of the struck quark in the pomeron. The form $F^{D(3)}_2 = constant \cdot (1/ \mbox{$x_{_{I\hspace{-0.2em}P}}$})^a$ gives a good fit to the data in all $β$ and $Q^2$ intervals with $a = 1.46 \pm 0.04 (stat) \pm

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arXiv1996

Measurement of Elastic $ϕ$ Photoproduction at HERA

The production of $ϕ$ mesons in the reaction $e^{+}p \rightarrow e^{+} ϕp$ ($ϕ\rightarrow K^{+}K^{-}$) at a median $Q^{2}$ of $10^{-4} \ \rm{GeV^2}$ has been studied with the ZEUS detector at HERA. The differential $ϕ$ photoproduction cross section $dσ/dt$ has an exponential shape and has been determined in the kinematic range $0.1<|t|<0.5 \ \rm{GeV^2}$ and $60 < W < 80 \ \rm{GeV}$. An integrated cross section of $σ_{γp \rightarrow ϕp} = 0.96 \pm 0.19^{+0.21}_{-0.18}$ $\rm{μb}$ has been obtained by extrapolating to {\it t} = 0. When compared to lower energy data, the results show a weak energy dependence of both $σ_{γp \rightarrow ϕp}$ and the slope of the $t$ distribution. The $ϕ$ decay angular distributions are consistent with $s$-channel helicity conservation. From lower energies to HERA energies, the features of $ϕ$ photoproduction are compatible with those of a soft diffractive process.

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arXiv1995

Exclusive $ρ^0$ production in deep inelastic electron-proton scattering at HERA

The exclusive production of $ρ^0$ mesons in deep inelastic electron-proton scattering has been studied using the ZEUS detector. Cross sections have been measured in the range $7 < Q^2 < 25$ GeV$^2$ for $γ^*p$ centre of mass (c.m.) energies from 40 to 130 GeV. The $γ^*p \to ρ^0 p$ cross section exhibits a $Q^{-(4.2 \pm 0.8 ^{+1.4}_{-0.5})}$ dependence and both longitudinally and transversely polarised $ρ^0$'s are observed. The $γ^*p \to ρ^0 p$ cross section rises strongly with increasing c.m. energy, when compared with NMC data at lower energy, which cannot be explained by production through soft pomeron exchange. The data are compared with perturbative QCD calculations where the rise in the cross section reflects the increase in the gluon density at low $x$. the gluon density at low $x$.

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arXiv1995

Diffractive Hard Photoproduction at HERA and Evidence for the Gluon Content of the Pomeron

Inclusive jet cross sections for events with a large rapidity gap with respect to the proton direction from the reaction $ep \rightarrow jet \; + \; X$ with quasi-real photons have been measured with the ZEUS detector. The cross sections refer to jets with transverse energies $E_T^{jet}>8$~GeV. The data show the characteristics of a diffractive process mediated by pomeron exchange. Assuming that the events are due to the exchange of a pomeron with partonic structure, the quark and gluon content of the pomeron is probed at a scale $\sim (E_T^{jet})^2$. A comparison of the measurements with model predictions based on QCD plus Regge phenomenology requires a contribution of partons with a hard momentum density in the pomeron. A combined analysis of the jet cross sections and recent ZEUS measurements of the diffractive structure function in deep inelastic scattering gives the first experimental evidence for the gluon content of the pomeron in diffractive hard scattering processes. The data indicate that between 30\% and 80\% of the momentum of the pomeron carried by partons is due to hard gluons.

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arXiv1995

Neutral strange particle production in deep inelastic scattering at HERA

This paper presents measurements of \k\ and \lam\ production in neutral current, deep inelastic scattering of 26.7 GeV electrons and 820 GeV protons in the kinematic range $ 10 < Q^{2} < 640 $ GeV$^2$, $0.0003 < x < 0.01$, and $y > 0.04$. Average multiplicities for \k\ and \lam\ production are determined for transverse momenta \ \ptr\ $> 0.5 $ GeV and pseudorapidities $\left| η\right| < 1.3$. The multiplicities favour a stronger strange to light quark suppression in the fragmentation chain than found in $e^+ e^-$ experiments. The production properties of \k's in events with and without a large rapidity gap with respect to the proton direction are compared. The ratio of neutral \k's to charged particles per event in the measured kinematic range is, within the present statistics, the same in both samples.

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arXiv1995

Measurement of the diffractive structure function in deep inelastic scattering at HERA

This paper presents an analysis of the inclusive properties of diffractive deep inelastic scattering events produced in $ep$ interactions at HERA. The events are characterised by a rapidity gap between the outgoing proton system and the remaining hadronic system. Inclusive distributions are presented and compared with Monte Carlo models for diffractive processes. The data are consistent with models where the pomeron structure function has a hard and a soft contribution. The diffractive structure function is measured as a function of $\xpom$, the momentum fraction lost by the proton, of $β$, the momentum fraction of the struck quark with respect to $\xpom$, and of $Q^2$. The $\xpom$ dependence is consistent with the form \xpoma where $a~=~1.30~\pm~0.08~(stat)~^{+~0.08}_{-~0.14}~(sys)$ in all bins of $β$ and $Q^2$. In the measured $Q^2$ range, the diffractive structure function approximately scales with $Q^2$ at fixed $β$. In an Ingelman-Schlein type model, where commonly used pomeron flux factor normalisations are assumed, it is found that the quarks within the pomeron do not saturate the momentum sum rule.

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arXiv1995

Study of the photon remnant in resolved photoproduction at HERA

Photoproduction at HERA is studied in $ep$ collisions, with the ZEUS detector, for $γp$ centre-of-mass energies ranging from 130-270 GeV. A sample of events with two high-$p_T$ jets ($p_T > 6$ GeV, $η<1.6$) and a third cluster in the approximate direction of the electron beam is isolated using a clustering algorithm. These events are mostly due to resolved photoproduction. The third cluster is identified as the photon remnant. Its properties, such as the transverse and longitudinal energy flows around the axis of the cluster, are consistent with those commonly attributed to jets, and in particular with those found for the two jets in these events. The mean value of the photon remnant $p_T$ with respect to the beam axis is measured to be $2.1 \pm 0.2$ GeV, which demonstrates substantial mean transverse momenta for the photon remnant.

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arXiv1995

Measurement of Charged and Neutral Current $e^-p$ Deep Inelastic Scattering Cross Sections at High $Q^2$

Deep inelastic $e^-p$ scattering has been studied in both the charged-current (CC) and neutral-current (NC) reactions at momentum transfers squared, $Q^2$, between 400 GeV$^2$ and the kinematic limit of 87500 GeV$^2$ using the ZEUS detector at the HERA $ep$ collider. The CC and NC total cross sections, the NC to CC cross section ratio, and the differential cross sections, $ dσ/dQ^2 $, are presented. For $Q^2 \simeq M_W^2$, where $M_W$ is the mass of the $W$ boson, the CC and NC cross sections have comparable magnitudes, demonstrating the equal strengths of the weak and electromagnetic interactions at high $Q^2$. The $Q^2$ dependence of the CC cross section determines the mass term in the CC propagator to be $M_{W} = 76 \pm 16 \pm 13$~GeV.

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arXiv1995

Inclusive transverse momentum distributions of charged particles in diffractive and non--diffractive photoproduction at HERA

Inclusive transverse momentum spectra of charged particles in photoproduction events in the laboratory pseudorapidity range $-1.2<η<1.4$ have been measured up to $p_{T}=8\GeV $ using the ZEUS detector. Diffractive and non--diffractive reactions have been selected with an average $γp$ centre of mass (c.m.) energy of $\langle W \rangle = 180\GeV$. For diffractive reactions, the $p_{T}$ spectra of the photon dissociation events have been measured in two intervals of the dissociated photon mass with mean values $\langle M_{X} \rangle = 5$ GeV and $10$ GeV. The inclusive transverse momentum spectra fall exponentially in the low $p_{T}$ region. The non--diffractive data show a pronounced high $p_{T}$ tail departing from the exponential shape. The $p_{T}$ distributions are compared to lower energy photoproduction data and to hadron--hadron collisions at a similar c.m. energy. The data are also compared to the results of a next--to--leading order QCD calculation.

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