Tau lepton production in ep collisions at HERA

The production of tau leptons in ep collisions is investigated using data recorded by the H1 detector at HERA in the period 1994–2000. Tau leptons are identified by detecting their decay products, using leptonic and hadronic decay modes. The cross section for the production of tau lepton pairs is measured for the first time at HERA. Furthermore, a search for events with an energetic isolated tau lepton and with large missing transverse momentum is performed. The results are found to be in agreement with the Standard Model predictions.     

Improved measurements of the neutral current from hadron and lepton production at LEP

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine ⁺ e ^– collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ ⁰ peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ ⁰ parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.     

Isolated lepton events at HERA: SUSY R-parity violation?

Events with an isolated high p_T lepton, a hadron jet and missing energy as observed in the H1 experiment at HERA, are potentially associated with R-parity violation in supersymmetric theories. However, stringent kinematic constraints must be fulfilled if the production of supersymmetric particles in R-parity violating scenarios were the correct path for explaining these puzzling events. A reference point ℝ/   is specified for which these constraints are illustrated and implications of the supersymmetric interpretation for new classes of multi-lepton events are indicated.     

Search for lepton flavour violation in ep collisions at HERA

A search for the lepton flavour violating processes ep→μX and ep→τX is performed with the H1 experiment at HERA. Final states with a muon or tau and a hadronic jet are searched for in a data sample corresponding to an integrated luminosity of 66.5 pb^-1 for e⁺p collisions and 13.7 pb^-1 for e^-p collisions at a centre-of-mass energy of 319 GeV. No evidence for lepton flavour violation is found. Limits are derived on the mass and the couplings of leptoquarks inducing lepton flavour violation in an extension of the Buchmüller–Rückl–Wyler effective model. Leptoquarks produced in ep collisions with a coupling strength of λ=0.3 and decaying with the same coupling strength to a muon–quark pair or a tau–quark pair are excluded at 95% confidence level up to masses of 459 GeV and 379 GeV, respectively.     

Bottonium production at LEP

We investigate the production of Υ(³ S ₁) and η _b (¹ S ₀) with two gluons from Z decay. We find that at LEP luminosities experimental detection will hardly be feasible.     

Radiative corrections with two Higgs doublets at LEP/SLC and HERA

Formulae for the radiative corrections toe ⁺ e ^?→f \(\bar f\) andep→eX,v _e X′ are given for two Higgs doublets inSU(2)×U(1). The magnitude of deviations from the minimal model is discussed for theM _W ?M _Z mass correlation, thee ⁺ e ^? forward-backward and polarization asymmetries and σ(e ⁺ e ^? → hadrons) at LEP/SLC, and for σ(NC)/σ(CC), charge and polarization asymmetries in deep inelasticep scattering at HERA. Precision experiments can restrict the mass splitting between neutral and charged Higgs bosons to ?100 GeV. In the supersymmetric Higgs model the additional corrections remain unobservably small.     

Hunting for contact interactions at HERA with polarized lepton and proton beams

We explore the discovery and analysis potentials of the HERA collider, with and without polarized beams, searching for electron-quark contact interactions in the neutral current channel. We find that the sensitivity to contact interactions when both beams are polarized is similar to the unpolarized case, and is better than in the case where one has only lepton polarization. We emphasize that the measurement of spin asymmetries in such a polarized context could give some crucial information on the chiral structure of these postulated new interactions. The experimental conditions are carefully taken into account. Received: 8 September 1998 / Published online: 22 March 1999     

Inclusive particle production at HERA

We calculate in next-to-leading order inclusive cross sections of single-particle production via both direct and resolved photons inep collisions at HERA. Transversemomentum and rapidity distributions are presented and the dependences on renormalization and factorization scales and subtraction schemes are investigated.Supported by the Bundesministerium für Forschung und Technologie under Contract 05 6 HH 93P (5), Bonn, Germany     

Inclusive particle production at HERA

We calculate in next-to-leading order inclusive cross sections of single-particle production via resolved photons inep collisions at HERA. Transverse-momentum and rapidity distributions are presented and the scale dependence is studied. The results are compared with first experimental data from the H1 Collaboration at HERA.     

Heavy flavour production at HERA

The production of heavy flavours at HERA has been investigated with a new Monte Carlo generator, which describes the heavy quark production ine–p scattering via boson-gluon fusion. The program has been developed in the framework of the HERWIG Monte Carlo and, different from the existing generator AROMA from the Lund package, takes account of the initial state radiation associated with higher order QCD corrections. The production at HERA energies has been studied in the two different approaches. As expected, large differences for the particle and energy flows have been found.     

Chargino production at LEP 200

We study chargino production and decay in the energy region of LEP 200, exploring the relevant supersymmetry parameter space. We give numerical predictions for the cross sections and for the signaturesl ⁺+l ^?+p′, l+jet+p′, jet+jet+p′. New formulae are presented relating chargino masses and couplings to the supersymmetry parameters.     

Higgs production and background at LEP

We calculate the total cross section for the process at tree level for LEP energies. We take into account the Higgs production subprocess, background processes with two intermediate vector bosons and their interference. A Higgs lighter than 50GeV could be observed at LEP 1. In this case the background can easily be eliminated. For a heavier Higgs we need LEP 200 energies. Here we will get a significant background from doubleZ production.     

Measurement of hot spots inside the proton at HERA and LEP/LHC

In this paper we discuss the measurement of regions in the proton where the density of small-x partons is large, called hot spots, by means of an associated jet analysis. An analytical estimate of the cross section is presented and the jet kinematics is discussed in the HERA and LEP-LHC frame. A Monte Carlo estimate shows that the number of jets produced in deep inelastic scattering events at HERA, suitable for this analysis, amounts to a few 1000 jcts for a data sample with an integrated luminosity of about 10 pb^–1.supported in part by FONDECYT 0367/90     

Diffractive production of dijets at HERA

We investigate the production of a quark-antiquark pair in diffractive photon-proton scattering, approximating soft pomeron exchange by the exchange of two nonperturbative gluons. In deep inelastic scattering at HERA, events with two jets and the scattered proton in the final state are predicted to be observable, with an important contribution from charm production. For photoproduction of light quark jets with high transverse momentum we find that both exchanged gluons must have a large invariant mass, so that the cross section is very small, whereas for charm quarks it is quite appreciable. From our calculation we also extract the quark structure function of the pomeron for the scaling variablez no too close to 0 or 1, finding a strong flavour dependence and a behaviour somewhat harder thanz(1?z) for light quarks.     

Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ^LT for the diffractive production of open charm (cc) from longitudinally and transversely polarised photons (of virtuality Q2) incident at high energy (√S) on a proton target. We study both the Q ² and M ² dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ^T, as well as for σ^L, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the square of the gluon density g(x, K²), and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for x = (Q ² + M ²)/s and scale K2 = (M ²c + )(1 + Q ²/M ²) where the average quark transverse momentum squared 〈k ₂ ^t ~ (M ²c . As compared to diffractive J/#x03C8; production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/#x03C8; wave function. We estimate the fraction of diffractive events that arise from cc¯ production.     

Semi-exclusive production of photons at HERA

We study the feasibility of measuring the semi-exclusive photon production at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts simply be expressed in terms of hard PQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of . The subprocesses of lowest order in are suppressed at low recoil masses , compared to higher order gluon exchange, i.e. BFKL contributions. The distinct -dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate. Received: 27 March 2000 / Revised version: 6 June 2000 / Published online: 9 August 2000