DESY HERA Research Articles

Quarkonium production at the Fermilab Tevatron through soft color interactions

The direct charmonium and bottomonium production rate observed at high ${p}_{\ensuremath{\perp}}$ in $p\overline{p}$ collisions at the Fermilab Tevatron is factors of 10 larger than predictions based on conventional perturbative QCD. We show that this excess can be accounted for by our model for soft color interactions, previously introduced to describe in a novel way the large rapidity gap events observed at DESY HERA.

Search for Scalar Leptoquark Pairs Decaying to Electrons and Jets inp¯pCollisions

We have searched for the pair production of first generation scalar leptoquarks in the $\mathrm{eejj}$ channel using the full data set $(123{\mathrm{pb}}^{\ensuremath{-}1})$ collected with the D0 detector at the Fermilab Tevatron during 1992--1996. We observe no candidates with an expected background of approximately 0.4 events. Comparing the experimental 95% confidence level upper limit to theoretical calculations of the cross section with the assumption of a 100% branching fraction of $\mathrm{eq}$, we set a lower limit on the mass of a first generation scalar leptoquark of $225\mathrm{GeV}{/c}^{2}$. The results of this analysis rule out the interpretation of the excess of high ${Q}^{2}$ events at DESY HERA as leptoquarks which decay exclusively to $\mathrm{eq}$.

Large- $p_T$ photoproduction of $D^{*\pm}$ mesons in $ep$ collisions

The cross section for the inclusive photoproduction of large-p_T D^*+- mesons is calculated at next-to-leading order, adopting different approaches to describe the fragmentation of charm quarks into D^*+- mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the photon. We present inclusive single-particle distributions in transverse momentum and rapidity, including the contributions due to both direct and resolved photons. We compare and assess the various implementations of fragmentation. We argue that, in the high-p_T regime, a particularly realistic description can be obtained by convoluting the Altarelli-Parisi-evolved fragmentation functions of Peterson et al. with the hard-scattering cross sections of massless partons where the factorization of the collinear singularities associated with final-state charm quarks is converted to the massive-charm scheme. The predictions thus obtained agree well with recent experimental data by the H1 and ZEUS Collaborations at DESY HERA.

Colour-octet contributions to photoproduction via fragmentation at HERA

We study J Ψ photoproduction via fragmentation at next-to-leading order in the QCD-improved parton model, using the nonrelativistic factorization formalism proposed by Bodwin, Braaten, and Lepage. We consider direct and resolved photoproduction of prompt J Ψ mesons and χ cJ mesons radiatively decaying to J Ψ + γ , taking into account the formation of both colour-singlet and colour-octet c c states. Adopting the values of the long-distance colour-octet matrix elements extracted from fits to prompt- J /Ψ data recently taken at the Fermilab Tevatron, we predict that measurements of J Ψ photoproduction at DESY HERA should show a distinctive excess over the expectation based on the colour-singlet model at small values of the inelasticity variable z . This is complementary to the expected enhancement at z ≲ 1 due to the colour-octet contribution to photon-gluon fusion.

Single-top-quark production viaW-gluon fusion at next-to-leading order

Single-top-quark production via W-gluon fusion at hadron colliders provides an opportunity to directly probe the charged-current interaction of the top quark. We calculate the next-to-leading-order corrections to this process at the Fermilab Tevatron, the CERN Large Hadron Collider, and DESY HERA. Using a b-quark distribution function to sum collinear logarithms, we show that there are two independent corrections, of order 1/[ln(m_t^2/m_b^2)] and alpha_s. This observation is generic to processes involving a perturbatively derived heavy-quark distribution function at an energy scale large compared with the heavy-quark mass.

Much ado about leptoquarks: A comprehensive analysis

We examine the phenomenological implications of a $\ensuremath{\sim}200\mathrm{GeV}$ leptoquark in light of the recent excess of events at DESY HERA. Given the relative predictions of events rates in ${e}^{+}p$ versus ${e}^{\ensuremath{-}}p,$ we demonstrate that classes of leptoquarks may be excluded, including those contained in ${\mathrm{E}}_{6}$ grand unified theory (GUT) models. It is shown that future studies with polarized beams at HERA could reveal the chirality of the leptoquark fermionic coupling and that given sufficient luminosity in each ${e}_{L,R}^{\ifmmode\pm\else\textpm\fi{}}$ channel the leptoquark quantum numbers could be determined. The implications of 200--220 GeV leptoquarks at the Fermilab Tevatron are examined. While present Tevatron data most likely exclude vector leptoquarks and leptogluons in this mass region, it does allow for scalar leptoquarks. We find that while leptoquarks have little influence on Drell-Yan production, further studies at the Main Injector may be possible in the single production channel provided the Yukawa couplings are sufficiently large. We investigate precision electroweak measurements as well as the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}$ at CERN LEP II and find they provide no further restrictions on these leptoquark models. We then ascertain that cross section and polarization asymmetry measurements at the Next Linear Collider (NLC) provide the only direct mechanism to determine the leptoquark's electroweak quantum numbers. The single production of leptoquarks in $\ensuremath{\gamma}e$ collisions by both the back-scattered laser and Weisacker-Williams techniques at the NLC is also discussed. Finally, we demonstrate that we can obtain successful coupling constant unification in models with leptoquarks, both with or without supersymmetry. The supersymmetric case requires the GUT group to be larger than SU(5) such as flipped ${\mathrm{SU}(5)\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)}_{X}.$

Photoproduction of jets at DESY HERA in next-to-leading order QCD

A new next-to-leading order Monte Carlo program for the calculation of jet cross sections in photoproduction is described. The contributions from both resolved and direct components are included to O(\alpha \alpha_s^2). Properties of the predictions for various inclusive jet and dijet observables are discussed and comparisons with HERA data are presented.

Reggeon and pion contributions in semiexclusive diffractive processes at DESY HERA

A detailed analysis of semi-exclusive diffractive processes in ep DIS at HERA, with the diffractive final states in the forward direction is presented. The contributions of the subleading f2, !, a2, � reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the (a2,�) reggeons are entirely responsible for the forward neutron production

Diffractive mass spectra at DESY HERA in the interacting gluon model

We have successfully applied the interacting gluon model to calculate diffractive mass spectra measured recently in $e\ensuremath{-}p$ collisions at DESY HERA. We show that it is possible to treat them in terms of gluon-gluon collisions in the same way as was done before for hadronic collisions. An analysis of available data is performed.

Has the substructure of quarks and leptons been found also by the H1 and ZEUS detectors at DESY HERA?

The significant excess of events recently found by the H1 and ZEUS Collaborations at HERA in the deep-inelastic e^+ p scattering for high momentum-transfers squared Q^2>15000 GeV^2 over the expectation of the standard model can be explained by either one of the following possible consequences of the substructure of quarks and leptons: 1) production of leptoquarks (Phi), 2) exchange of an excited Z boson (Z'), 3) intrinsic form factors of quarks (or leptons), 4) intrinsic anomalous magnetic moments of quarks, 5) production of excited quarks (q^*), and 6) that of excited positrons (e^*+). The masses of these new particles are estimated to be m(Phi) = 280-440 GeV, m(Z') = 210-250 GeV, m(q^*) = 120-140 GeV, and m(e^*+) = 300-370 GeV, although the possibilities 2) and 5) are excluded by the currently available experimental constraints.

Like-sign dileptons at the Fermilab Tevatron reexamined in the light of the DESY HERA high-Q2anomaly

We reexamine like-sign dilepton signals at the Fermilab Tevatron assuming that the excess high-${Q}^{2}$ events recently seen at the DESY HERA are due to scalar resonances such as squarks of $R$-parity-violating supersymmetry. For gluinos in the mass range of 200--350 GeV, the like-sign dilepton signal can help to make the crucial distinction between the most favored squark explanation and other proposed solutions.

Phenomenology of forward hadrons in deep inelastic scattering: Fracture functions and itsQ2evolution

We analyze recent data on the production of forward neutrons in deep inelastic scattering at DESY HERA in the framework of a perturbative QCD description for semi-inclusive processes, which includes fracture functions. Using a model estimate for the nonperturbative piece of the fragmentation process, in fairly good agreement with the available data, we analyze the ${Q}^{2}$ dependence of the resulting fracture functions, which is driven by nonhomogeneous evolution equations. We also propose a measurement of the pion production cross section in the target fragmentation region as a new test of perturbative QCD, which in this case predicts also a different ${Q}^{2}$ evolution with respect to the one given by the usual Altarelli-Parisi equations.

D*production frome+e−toepcollisions in NLO QCD

Fragmentation functions for $D$ mesons, based on the convolution of a perturbative part, related to the heavy quark perturbative showering, and a nonperturbative model for its hadronization into the meson, are used to describe ${D}^{*}$ production in ${e}^{+}{e}^{\ensuremath{-}}$ and $\mathrm{ep}$ collisions. The nonperturbative part is determined by fitting the ${e}^{+}{e}^{\ensuremath{-}}$ data taken by ARGUS and OPAL at 10.6 and 91.2 GeV, respectively. When fitting with a nonperturbative Peterson fragmentation function and using next-to-leading evolution for the perturbative part, we find an \ensuremath{\epsilon} parameter significantly different from the one commonly used, which is instead found with a leading order fit. The use of this new value is shown to increase considerably the cross section for ${D}^{*}$ production at DESY HERA, suggesting a possible reconciliation between the next-to-leading order theoretical predictions and the experimental data.

New measurements of proton polarized structure functions in charged current processes at DESY HERA

Estimates for longitudinal spin asymmetries which single out new polarized nucleon structure functions in deeply inelastic charged current interactions at HERA energies are given, exploiting their interpretation in terms of polarized quark distributions. These asymmetries turn out to be large and allow a measurement of the new polarized structure functions $g_1^{W}$ and $g_5^{W}$, which would add valuable tests and information on the spin content of quarks inside a polarized proton. We also show that single spin asymmetries in neutral current interactions are very small.

Small x resummations confronted with F2( x, Q2) data

It has been observed recently that a consistent LO BFKL gluon evolution leads to a steep growth of F 2( x, Q 2) for x → 0 almost independently of Q 2. We show that current data from the DESY HERA collider are precise enough to finally rule out a pure BFKL behaviour in the accessible small x region. Several attempts have been made by other groups to treat the BFKL type small x resummations instead as additions to the conventional anomalous dimensions of the successful renormalization group “Altarelli-Parisi” equations. We demonstrate that all presently available F 2 data, in particular at lower values of Q 2, can not be described using the presently known NLO (two-loop consistent) small x resummations. Finally we comment on the common reason for the failure of these BFKL inspired methods which result, in general, in too steep >x-dependencies as x → 0.

Subleading Reggeons in deep inelastic diffractive scattering at DESY HERA

The contribution of subleading Reggeons to the diffractive structure function dF{sub 2}{sup D}/dx{sub P}dt is estimated from soft physics data. This contribution leads in a natural way to the violation of the factorization property of the diffractive structure function. {copyright} {ital 1997} {ital The American Physical Society}

Forward Jet Production at DESY HERA in the LowxRegime in Next-to-Leading Order

The production of forward jets of transverse momentum ${p}_{T}(j)\ensuremath{\approx}Q$ and large momentum fraction ${x}_{\mathrm{jet}}\ensuremath{\gg}x$ probes the onset of Balitsky-Fadin-Kuraev-Lipatov (BFKL) dynamics at DESY HERA. A full $O({\ensuremath{\alpha}}_{s}^{2})$ calculation of the inclusive forward jet cross section is presented and compared to the expected BFKL cross section. The kinematical region populated by these events and the scale dependence of the fixed order perturbative QCD cross sections are discussed.

Q2 dependence of the hard diffractive photoproduction of a vector meson or photon and the range of PQCD validity.

We consider two coupled problems. We study the dependence on the photon virtuality ${Q}^{2}$ for the semihard quasi-elastic photoproduction of neutral vector mesons on a quark, gluon, or real photon [at $s\ensuremath{\gg}{p}_{\ensuremath{\perp}}^{2}, {Q}^{2}$; ${p}_{\ensuremath{\perp}}^{2}\ensuremath{\gg}{\ensuremath{\mu}}^{2}\ensuremath{\approx}{(0.3\mathrm{GeV})}^{2}$]. To this end we calculate the corresponding amplitudes (in an analytical form) in the lowest nontrivial approximation of perturbative QCD. It is shown that the amplitude for the production of a light meson varies very rapidly with the photon virtuality near ${Q}^{2}=0$. We estimate the bound of the PQCD validity region for such processes. For a real incident photon the obtained bound for $\ensuremath{\rho}$ meson production is very high. This bound decreases fast with the increase of ${Q}^{2}$, and we expect that the virtual photoproduction at DESY HERA will give us the opportunity to test the PQCD results. The signature of this region is discussed. For the hard Compton effect PQCD should work well at not too high ${p}_{\ensuremath{\perp}}$, and this effect seems measurable at HERA.

Deep-inelastic final states in a space-time description of shower development and hadronization.

We extend a quantum kinetic approach to the description of hadronic showers in space, time, and momentum space to deep-inelastic $\mathrm{ep}$ collisions, with particular reference to experiments at DESY HERA. We follow the history of hard scattering events back to the initial hadronic state and forward to the formation of color-singlet prehadronic clusters and their decays into hadrons. The time evolution of the spacelike initial-state shower and the timelike secondary partons are treated similarly, and cluster formation is treated using a spatial criterion motivated by confinement and a nonperturbative model for hadronization. We calculate the time evolution of particle distributions in rapidity, transverse, and longitudinal space. We also compare the transverse hadronic energy flow and the distribution of observed hadronic masses with experimental data from HERA, finding encouraging results, and discuss the background to large-rapidity-gap events. The techniques developed in this paper may be applied in the future to more complicated processes such as $\mathrm{eA}$, $\mathrm{pp}$, $\mathrm{pA}$, and $\mathrm{AA}$ collisions.

Proton and deuteron structure functions in muon scattering at 470 GeV.

The proton and deuteron structure functions F2p and F2d are measured in inelastic muon scattering with an average beam energy of 470 GeV. The data were taken at Fermilab experiment E665 during 1991 and 1992 using liquid hydrogen and deuterium targets. The F2 measurements are reported in the range 0.0008<x<0.6 and 0.2<Q2<75 GeV2. These are the first precise measurements of F2 in the low x and Q2 range of the data. In the high x range of the data where they overlap in x and Q2 with the measurements from NMC, the two measurements are in agreement. The E665 data also overlap in x with the DESY HERA data, and there is a smooth connection in Q2 between the two data sets. At high Q2 the E665 measurements are consistent with QCD-evolved leading twist structure function models. The data are qualitatively described by structure function models incorporating the hadronic nature of the photon at low Q2. The Q2 and the W dependence of the data measure the transition in the nature of the photon between a pointlike probe at high Q2 and a hadronic object at low Q2.Received 9 February 1996DOI:https://doi.org/10.1103/PhysRevD.54.3006©1996 American Physical Society