High Transverse Energy Research Articles

Measurement of dijet photoproduction at high transverse energies at HERA

The cross section for dijet photoproduction at high transverse energies is presented as a function of the transverse energies and the pseudorapidities of the jets. The measurement is performed using a sample of ep-interactions corresponding to an integrated luminosity of 6.3 pb $^{-1}$ , recorded by the ZEUS detector. Jets are defined by applying a $k_T$ -clustering algorithm to the hadrons observed in the final state. The measured cross sections are compared to next-to-leading order QCD calculations. In a kinematic regime where theoretical uncertainties are expected to be small, the measured cross sections are higher than these calculations.

Semi-leptonic decays of heavy quarks in Dijet photoproduction at HERA

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.

Search for Nonstandard Higgs Bosons Using High Mass Photon Pairs in pp¯→γγ+2 Jets at √s=1.8 TeV

A search has been carried out for events in the channel $p\overline{p}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}+2\mathrm{jets}$. Such a signature can characterize the production of a nonstandard Higgs boson together with a $W$ or $Z$ boson. We refer to this nonstandard Higgs, having standard model couplings to vector bosons but no coupling to fermions, as a ``bosonic Higgs.'' With the requirement of two high transverse energy photons and two jets, the diphoton mass $({m}_{\ensuremath{\gamma}\ensuremath{\gamma}})$ distribution is consistent with expected background. A 90 (95)% confidence level (C.L.) upper limit on the cross section as a function of mass is calculated, ranging from 0.60 (0.80) pb for ${m}_{\ensuremath{\gamma}\ensuremath{\gamma}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}65\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$ to 0.26 (0.34) pb for ${m}_{\ensuremath{\gamma}\ensuremath{\gamma}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}150\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$, corresponding to a $95%$ C.L. lower limit on the mass of a bosonic Higgs of $78.5\mathrm{GeV}{/c}^{2}$.

Search for Squarks and Gluinos in Single-Photon Events with Jets and Large Missing Transverse Energy inpp¯Collisions at√s=1.8TeV

We search for new physics using events with one high transverse energy photon, two or more jets, and an apparent imbalance in transverse energy, in p{ovr p} collisions at the Fermilab Tevatron at {radical} (s) =1.8 TeV . Such events are predicted for production of supersymmetric particles in some models. No excess is observed beyond expected background. For the parameter space of the minimal supersymmetric standard model with branching fraction B({tilde {chi}}{sup 0}{sub 2}{r_arrow}{gamma}thinsp{tilde {chi}}{sup 0}{sub 1})=1 and m{sub {tilde {chi}}{sup 0}{sub 2 }}{minus}m{sub {tilde {chi}}{sup 0}{sub 1}}{gt}20 GeV , we obtain a 95{percent} confidence level lower mass limit of 310thinspthinspGeV for equal mass squarks and gluinos. The results are also interpreted in models with low energy gauge mediated supersymmetry breaking. {copyright} {ital 1998} {ital The American Physical Society }

Measurement of dijet photoproduction at high transverse energies at HERA

The cross section for dijet photoproduction at high transverse energies is presented as a function of the transverse energies and the pseudorapidities of the jets. The measurement is performed using a sample of ep-interactions corresponding to an integrated luminosity of 6.3 pb^(-1), recorded by the ZEUS detector.Jets are defined by applying a k_T-clustering algorithm to the hadrons observed in the final state. The measured cross sections are compared to next-to-leading order QCD calculations. In a kinematic regime where theoretical uncertainties are expected to be small, the measured cross sections are higher than these calculations.

Jet cross sections at HERA - current issues

Since the start of HERA operation there has been considerable progress in the understanding of jet production in ep collisions. QCD calculations are now able to accommodate the hadronic structure of the virtual photon. The luminosities delivered by HERA are now sufficient to allow studies of final states in which more than two high transverse energy jets are produced. The transition between jet processes in photoproduction and in deep inelastic scattering has been studied in some detail. These advances are highlighted here.

Search for Heavy Pointlike Dirac Monopoles

We have searched for central production of a pair of photons with high transverse energies in p{bar p} collisions at {radical}(s)=1.8 TeV using 70 pb{sup {minus}1} of data collected with the D0 detector at the Fermilab Tevatron in 1994{endash}1996. If they exist, virtual heavy pointlike Dirac monopoles could rescatter pairs of nearly real photons into this final state via a box diagram. We observe no excess of events above background, and set lower 95{percent} C.L. limits of 610, 870, or 1580 GeV/c{sup 2} on the mass of a spin 0, 1/2 , or 1 Dirac monopole. {copyright} {ital 1998} {ital The American Physical Society}

Observation of isolated high-E photons in photoproduction at HERA

Events containing an isolated prompt photon with high transverse energy, together with a balancing jet, have been observed for the first time in photoproduction at HERA.The data were taken with the ZEUS detector, in a $\gamma p$ centre of mass energy range 120--250 GeV. The fraction of the incoming photon energy participating in the production of the prompt photon and the jet, $x_\gamma$, shows a strong peak near unity, consistent with LO QCD Monte Carlo predictions. In the transverse energy and pseudorapidity range $5\le \eTg < 10$ GeV, $-0.7 \le \eta^\gamma < 0.8$, $\eTj\ge 5$ GeV, and $-1.5\le \eta^{jet}\le 1.8$, with $\xgO > 0.8,$ the measured cross section is $15.3 \pm 3.8 \pm 1.8 pb$, in good agreement with a recent NLO calculation.

Discovery limits for a new contact interaction at future hadronic colliders with polarized beams

The production of high-transverse energy jets in hadron-hadroncollisions is sensitive to the presence of new contact interactions between quarks. If proton polarization were available, the measurement of some parity violating spin asymmetries in one-jet production at large transverse energy would complement the usual search for deviations from the expected QCD cross section. In the same time, a unique information on the chirality structure of the new interaction could be obtained. In this context, we compare the potentialities of various $pp$ and $p\bar p$ colliders that are planned or have been proposed, with the additional requirement of beam polarization.

Central collisions of Au on Au at 150, 250 and 400 A·MeV

Collisions of Au on Au at incident energies of 150, 250 and 400 A·MeV were studied with the FOPI-facility at GSI Darmstadt. Nuclear charge ( Z ⩽ 15) and velocity of the products were detected with full azimuthal acceptance at laboratory angles 1° ⩽ θlab ⩽ 30°. Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6–90°. Central collisions representing about 1% of the reaction cross section were selected by requiring high total transverse energy, but vanishing side flow. The velocity space distributions and yields of the emitted fragments are reported. the data are analysed in terms of a thermal model including radial flow. A comparison with predictions of the quantum molecular model is presented.

Dijet angular distributions in direct and resolved photoproduction at HERA

Jet photoproduction, where the two highest transverse energy (ETjet) jets have ETjet 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.

Inclusive jet production at HERA: Next-to-leading order QCD corrections to the resolved and direct photon contribution

We have calculated high transverse energy single-jet inclusive cross sections for γp-collisions at next-to-leading order superimposing direct and resolved photoproduction. The dependence on the scale and the scheme for the factorization of the photon structure is studied quantitatively. The accuracy of the predictions due to variations of all scales is investigated.

Rapidity gaps between jets in pp-bar collisions at sqrt s =1.8 TeV.

First experimental results are presented from a search for events with a rapidity gap between jets. The D0 detector was used to examine events produced by the Fermilab Tevatron pp¯ collider at √s =1.8 TeV. The fraction of events with an observed rapidity gap between the two highest transverse energy (ET) jets is measured as a function of the pseudorapidity separation between the jet edges (Δηc). An upper limit at the 95% confidence level of 1.1×10−2 is obtained on the fraction of events with non particles between the jets, for events with Δηc>3 and jet ET greater than 30 GeV.Received 28 December 1993DOI:https://doi.org/10.1103/PhysRevLett.72.2332©1994 American Physical Society

Looking for new gluon physics at the Tevatron

CDF data from the 1988–1989 Tevatron run are used to constrain the characteristic scale of physics beyond the Standard Model which couples to gluons. New gluonic physics would cause the inclusive jet cross section to deviate from that of pure QCD at high transverse jet energies. We find that the CDF data place a lower bound of 2.03 TeV at 95% CL on the characteristic scale of new gluon sector physics.

D sigma /dEt in heavy ion collisions and nucleon-nucleon cross section fluctuations.

The prediction of transverse energy distributions, [ital d][sigma]/[ital dE][sub [ital t]], in heavy ion collisions is examined and extended and the evidence for the effect of anomalous cross section fluctuations on the cross section at high transverse energy is critically examined. We calculate the contribution to the transverse energy of rescattering of target nucleons on their unstruck neighbors.

Heavy top-quark searches in the dilepton mode at the Fermilab Tevatron.

We present the results of a detailed study of the effects of $b$-tagging on the heavy top-quark signal and backgrounds for the modes of the di-lepton plus two high transverse energy jets at the Fermilab Tevatron. The general characteristics of the heavy top-quark signal events are also discussed so that a comparison can be made between $b$-tagging and imposing stringent kinematical cuts to eliminate backgrounds.

Transverse energy measurements in proton-nucleus interactions at high energy

The results of two sets of transverse energy measurements, performed with incident proton beams of 200 and 450 GeV/c momentum on several nuclear targets, are presented. The transverse energy cross sections dσ/dET are measured in a pseudorapidity range including the target fragmentation region (−0.1<η<2.9) for both data sets and also in a nearly complete pseudorapidity coverage (−0.1<η<5.5) for the data taken at 200 GeV/c incident momentum. A comparison is made of the transverse energy distributions in the target fragmentation region and in the full η region. We find that the mean value of pseudorapidity of the dET/dη distributions shifts towards the target fragmentation region as the atomic mass number of the target increases or a selection of high transverse energy events is made. A parametrization based on a simple geometrical nucleonnucleon scattering approach was found to be inadequate to describe all features of the transverse energy distributions. Finally, the VENUS model is compared with the experimental data.

One-Loop QCD Corrections to the Inclusive Production Cross-Section for Heavy Quarks inp Collisions at High Transverse Energies

We calculate one-loop QCD corrections to the production cross-section for heavy quarks in protonantiproton collisions. We are interested in the behaviour of the cross-section at high transverse energies. At high PT the quark mass should become an irrelevant parameter. Therefore we work in the framework of massless QCD and absorb all mass singularities in a redefinition of the parton densities and fragmentation functions. Numerical results of the cross-section at √S = 0.63, 1.8 and 10 TeV are presented and the dependence of these results on the factorization scheme is examined.

A search for multiplicity fluctuations in high energy nucleus-nucleus collisions

A search for non-statistical fluctuations was performed in 200 GeV per nucleon oxygen and sulphur ion-emulsion interactions selected by a high transverse energy trigger. No clear signal of dynamical correlations or of unusual fluctuations was found.

First results from the hybrid emulsion experiment on16O-nucleus collisions at 200 GeV/N

First data from the HELIOS hybrid experiment are presented. The set-up and experimental procedure are described, and a study of medium multiplicity events and of high transverse energy, high multiplicity events is reported. A study of unbiased samples of16O-emulsion interactions, showing in particular the importance of electromagnetic dissociation of the projectile, is presented.