Narrow Results

Striking events with isolated charged leptons, large missing transverse momentum and large transverse momentum of the hadronic final state were observed at the electron proton collider HERA in a data sample corresponding to an integrated luminosity of about 130 pb^-1. The H1 collaboration observed 11 events with isolated electrons or muons and with . Only 3.4±0.6 events were expected from Standard Model (SM) processes. Six of these events have , while 1.3±0.3 events were expected. The ZEUS collaboration observed good agreement with the SM. However, ZEUS found two events with a similar event topology, but tau leptons instead of electrons or muons in the final state. Only 0.2±0.05 events were expected from SM processes. For various hypotheses, the compatibility of the experimental results was investigated with respect to the SM and with respect to possible explanations beyond the SM. Prospects for the high-luminosity HERA-II data taking period are given.

Charm production data from HERA may be used to determine the charm quark mass and jet production data from HERA may be used to determine α_s(M_Z). Recent results are summarised.

New and recent results on QCD measurements from the H1 and ZEUS experiments at the HERA ep collider are reviewed. The final results on the combined deep-inelastic neutral and charged current cross-sections are presented and their role in the extractions of parton distribution functions (PDFs) is studied. The PDF fits give insight into the compatibility of QCD evolution and heavy flavor schemes with the data as a function of kinematic variables such as the scale Q². Measurements of jet production cross-sections in ep collisions provide direct proves of QCD and extractions of the strong coupling constants are performed. Charm and beauty cross-section measurements are used for the determination of the heavy quark masses. Their role in PDF fits is investigated. In the regime of diffractive DIS and photoproduction, dijet and prompt photon production cross-sections provide insights into the process of factorization and the nature of the diffractive exchange.

Multijet production rates in neutral current deep inelastic scattering (DIS) have been measured in the range of exchanged boson virtualities 10<Q²<5000 GeV². The data were taken at the ep collider HERA with centre-of-mass energy using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb^-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential multijet cross sections are presented as functions of jet transverse energy , pseudorapidity and Q² with and . Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant α_s(M_Z), determined from the ratio of the trijet to dijet cross sections, is .

Measurements of Charmonium production at the electron proton collider HERA are presented. In inelastic production the charmonium is formed from pairs that are produced in photon gluon fusion. The measurements of the cross sections and helicity distributions are compared to calculations performed in the framework of non-relativistic quantum chromodynamics (NRQCD), which includes both color-singlet and color-octet contributions.

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.

Recent searches for physics beyond the Standard Model at high-energy colliders are reviewed, with emphasis on supersymmetry, additional space dimensions, extra gauge bosons, leptoquarks and model-independent searches. The results reported are based on data samples of up to 0.5 and 2.5 fb^-1 collected at HERA and at the Tevatron, respectively.

The rich experimental landscape of exclusive vector meson production at the high energy electron–proton collider HERA is reviewed, with emphasis on the transition from soft to hard diffraction and QCD interpretations.

The recent, selected results on meson production at HERA electron-proton collider from the H1 and ZEUS experiments are presented and interpreted within the framework of quantum chromodynamics.

The search for squarks in R-parity violating supersymmetry in ep collisions at HERA is reviewed. In the full data sample used by the H1 collaboration corresponding to about 0.5 fb^-1 no deviation from Standard Model predictions was observed and limits were set in phenomenological models. For example, for a R-parity violating coupling of electromagnetic strength λ′_11k = 0.3(k ∈ {1, 2}), the mass bound on the corresponding right-handed down-type squark is . The results are compared to other searches for supersymmetry. In particular, they are put in context with more recent results from the Large Hadron Collider (LHC). An outlook is given on search opportunities at a future ep collider.

We present a review of the recent theoretical and experimental developments related to the field of diffraction, parton saturation, and forward physics. We first discuss our present understanding of the proton structure in terms of quarks and gluons, the degrees of freedom of quantum chromodynamics. We then focus on some of the main results on diffraction at the HERA electron–proton collider in DESY, Germany, at the Tevatron proton–antiproton collider at Fermilab, Batavia, US, and at the CERN Large Hadron Collider (LHC) proton–proton and nucleus–nucleus collider, which is located in Geneva, Switzerland. We also present a selected amount of results on diffraction and photon exchanges that can be done at the LHC experiments and at a future Electron Ion Collider (EIC) to be built in the US at Brookhaven National Laboratory, New York.

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.