Narrow Results

Recent results for direct searches for physics beyond the Standard Model are reviewed. The results include Tevatron II data up to 1.2 fb^-1 and HERA results up to 350 pb^-1. Searches for Supersymmetry, for compositeness and for large extra dimensions are presented. The excess of events with an isolated lepton and high missing transverse momentum at HERA is discussed.

Multi-jet production has been investigated at HERA using the ZEUS detector. Three- and four-jet final states have been measured¹ in photoproduction (PHP). The three- and four-jet cross sections have been compared with two leading-logarithm parton-shower Monte Carlo models, with and without multi-parton interactions. Two- and three-jet production in deep inelastic (DIS) ep scattering has been measured² for 10 < Q² < 100 GeV² in the region of small Bjorken x (10^-4 < x_Bj < 10^-2) and compared to the predictions based on next-to-leading-order QCD calculations. In the third analysis³ described here, angular correlations between the jets and the proton beam direction were investigated in three-jet production in DIS to separate the contributions from the different color configurations. The measurements are compared to fixed-order perturbative calculations assuming the values of the color factors C_F, C_A and T_F as derived from a variety of gauge groups. The photoproduction analysis used a data sample with a luminosity of 121 pb^-1. Both DIS analyses used 82 pb^-1. Jets were reconstructed with the k_T cluster algorithm in the longitudinally invariant inclusive mode.

Recent results, obtained by the H1 and ZEUS collaborations, are presented on differential cross sections, for inclusive prompt photon production in DIS and for photoproduction of prompt photons accompanied by a hadronic jet. Also presented are cross sections of normalised scaled momentum distribution of charged final state hadrons, measured by H1 in DIS ep collisions at high Q² in the Breit frame of reference.

Highlights are presented mainly from two recent measurements of the diffractive Deep Inelastic Scattering cross section at HERA. In the first, the process ep → eXp is studied by tagging the leading final state proton. In the second, events of this type are selected by requiring a large rapidity gap devoid of hadronic activity in the proton direction. The two measurements are compared in detail and the kinematic dependences are interpreted within the framework of a factorisable diffractive exchange. Diffractive parton distributions are determined from a next-to-leading order QCD analysis of the large rapidity gap data, which can be applied to the prediction of diffractive processes, also at the TEVATRON and the LHC.

Results on recent measurements of leading baryon production in ep scattering at HERA will be presented. Comparisons with newly published theoretical calculations give insight into absorption effects which also play an important role in exclusive production processes at the LHC. Monte Carlo simulation generator studies for leading baryon production are shown and discussed.

Measurements by the H1 and ZEUS collaborations of the cross sections for charged current deep inelastic scattering in e^±p collisions with longitudinally polarised lepton beams are presented. The total cross sections for e^±p charged current deep inelastic scattering are presented at positive and negative values of the longitudinal polarisation of the lepton beam. In addition, single and double-differential cross sections are presented for the kinematic region Q² > 200 GeV². The measured cross sections are compared with the predictions of the Standard Model.

The world's only ep-collider HERA can explore the deep inelastic scattering over a wide kinematic region due to its large center-of-mass energy. Therefore, a unique study of a simultaneous determination of parton distribution functions and electro-weak parameters can be performed. The HERA-II high precision data with longitudinally polarized electron beams brought significant improvements on the sensitivity to both parameters.

We report on searches in deep inelastic ep scattering for narrow baryonic states decaying into Ξ^-π^-, Ξ^-π⁺, and their charge conjugates, at centre-of-mass energies of 300 and 318 GeV. No signal for a new narrow baryonic state is observed in the mass ranges 1600-2100 MeV (Ξπ) and from threshold up to 1700 MeV (). The standard baryon Ξ(1530)⁰ is observed in the decay mode Ξ^-π⁺ and mass dependent upper limits on the ratio of the hypothetical pentaquark states and to the Ξ(1530)⁰ signal are given. Also for the hypothetical strange pentaquark Θ⁺ mass dependent upper limits on σ(ep → eΘ⁺X) × BR(Θ⁺ → K⁰p) are obtained. Finally measurements of the acceptance corrected ratios σ(D*p(3100))/σ(D*) for the electroproduction of the anti-charmed baryon state D*p(3100) decaying into D* and p are presented.

Supersymmetric scenarios with R-parity violation have been investigated by the H1 and ZEUS collaborations at HERA using ep collision data collected in 1994-2000. Constraints have been derived for several models (unconstrained MSSM, mSUGRA, GMSB) and for a wide range of the model parameters. The limits are competitive respect to other colliders and to high-precision low-energy experiments.

Longitudinal polarisation of the lepton beam is a key ingredient in the success of the world's unique e^±p ring collider HERA. This article aims at providing a brief introduction to the physics motivation for deep-inelastic scattering of polarised electrons or positrons off protons, the basic mechanisms to establish lepton polarisation in the high-energy storage ring and to describe briefly the three different polarimeters, which measured both the transverse and the longitudinal polarisation.