Inclusive production of charged hadrons and {\rm K}^0_{\rm S} mesons in photon–photon collisions

The production of charged hadrons and mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in interactions with -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions. Received: 4 June 1998 / Published online: 6 November 1998     

Measurement of high-Q^{2} neutral-current e^+p deep inelastic scattering cross-sections at HERA

The neutral-current deep inelastic scattering differential cross-sections , for GeV, and , for , 2500 and 10000 GeV, have been measured with the ZEUS detector at HERA. The data sample of 47.7 pb was collected at a center-of-mass energy of 300 GeV. The cross-section, , falls by six orders of magnitude between and 40000 GeV. The predictions of the Standard Model are in very good agreement with the data. Complementing the observations of time-like contributions to fermion-antifermion annihilation, the data provide direct evidence for the presence of exchange in the space-like region explored by deep inelastic scattering. Received: 18 May 1999 / Published online: 3 August 1999     

Search for high-mass resonances decaying to emu in pp collisions at square root = 1.69 TeV

We describe a general search for resonances decaying to a neutral emu final state in pp collisions at a center-of-mass energy of 1.96 TeV. Using a data sample representing 344 pb(-1) of integrated luminosity recorded by the Collider Detector at Fermilab II experiment, we compare standard model predictions with the number of observed events for invariant masses between 50 and 800 GeV/c2. Finding no significant excess (5 events observed vs 7.7 +/- 0.8 expected for M(emu) > 100 GeV/c2 ), we set limits on sneutrino and Z' masses as functions of lepton family number violating couplings.     

Search for invisibly decaying Higgs bosons with large decay width using the OPAL detector at LEP

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e⁺e^-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb^-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M_H=60 GeV) to 0.57 pb (M_H=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.     

Measurement of the W-pair cross-section and of the W mass in {\mathrm{e^+ e^-}} interactions at 172 GeV

From a data sample of integrated luminosity, collected by DELPHI at a centre-of-mass energy of 172 GeV, 118 events were selected as W-pair candidates. From these, the branching fraction was measured to be and the cross-section for the doubly resonant process to be pb. The mass of the W boson, obtained from direct reconstruction of the invariant mass of the fermion pairs in the decays and , was determined to be , where “int.” denotes the uncertainty from interconnection effects like colour reconnection and Bose-Einstein interference. Combined with the W mass obtained from the cross-sections measured by DELPHI at threshold, a value of was found. Received: 12 December 1997 / Published online: 10 March 1998     

Search for resonances decaying to e^+-jet in e^+p interactions at HERA

The -jet invariant mass spectrum produced in the reaction has been studied at a center-of-mass energy of 300 GeV. The data were collected using the ZEUS detector operating at the HERA collider, and correspond to an integrated luminosity of pb. The observed mass spectrum is in good agreement with Standard Model expectations up to an -jet mass of GeV. Above this mass, some excess is seen. The angular distribution of these events is typical of high- neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state. Limits are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark or R-parity-violating squark production. Specific leptoquark types are ruled out at 95% confidence level for coupling strength for masses between 150 and 280 GeV. Received: 15 February 2000 / Published online: 18 May 2000     

Search for chargino and neutralino production at \sqrt{s} = 181–184 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57 pb at centre-of-mass energies of 181–184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production has been found. Upper limits on chargino and neutralino pair production (, ) cross-sections are obtained as a function of the chargino mass (), the lightest neutralino mass () and the second lightest neutralino mass (). For large chargino masses the limits have been improved with respect to the previous analyses at lower centre-of-mass energies. Exclusion regions at 95% confidence level (C.L.) of parameters of the Constrained Minimal Supersymmetric Standard Model are determined for the case of a large universal scalar mass, , implying heavy scalar fermions, and for the case of a small resulting in light scalar fermions and giving the worst-case limits. Within this framework and for GeV the 95% C.L. lower limits on for GeV are 90.0 and 90.2 GeV for and 35 respectively. These limits for all (the worst-case) are 69.1 and 65.2 GeV for and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for GeV (24.2 GeV for all ), with implications for experimental searches for the lightest neutralino as a dark matter candidate. Received: 19 August 1998 / Published online: 11 March 1999     

Sensitivity of electromagnetic spectra to equation of state and initial energy density in the Pb + Pb collisions at SPS

We study Pb + Pb collisions at 158 A GeV/c using hydrodynamical approach. We test different equations of state (EoSs) and different initial conditions and show that there are more than one initial state for each EoS which reproduce the observed hadronic spectra. We also find that different equations of state favor different freeze-out temperature. Simultaneously we calculate the thermal dilepton and photon spectra for each EoS and initial state. We compare the dilepton mass spectrum to data measured by the CERES collaboration and find that the differences in spectra obtained using different EoSs and initial states are not resolvable within the current experimental resolution. However, at invariant masses over 2 GeV the difference in the yield due to various initial states is close to an order of magnitude. We also study the rapidity distribution of lepton pairs and find that for masses around 800 MeV the shape of the distribution depends strongly on the EoS.     

QCD studies with {\rm e^+e^-} annihilation data at 172-189 GeV

We have studied hadronic events from annihilation data at centre-of-mass energies of 172, 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb. We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling , the mean charged particle multiplicity and the peak position in the distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of , and are consistent with previous determinations at . Received: 17 December 1999 / Published online: 18 May 2000     

Search for chargino and neutralino production at \sqrt{s} = 189 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production is found. Upper limits on chargino and neutralino pair production (, ) cross-sections are obtained as a function of the chargino mass (), the lightest neutralino mass () and the second lightest neutralino mass (). Within the Constrained Minimal Supersymmetric Standard Model framework, and for GeV, the 95% confidence level lower limits on are 93.6 GeV for and 94.1 GeV for . These limits are obtained assuming a universal scalar mass 500 GeV. The corresponding limits for all are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of are 32.8 GeV for GeV and 31.6 GeV for all . Received: 14 October 1999 / Published online: 6 March 2000     

Search for technicolor with DELPHI

Technicolor represents a viable alternative to the Higgs mechanism for generating gauge boson masses. Searches for technicolor particles and have been performed in the data collected by the DELPHI experiment at LEP at centre-of-mass energies between 192 and 208 GeV corresponding to an integrated luminosity of 452 pb. Good agreement is observed with the SM expectation in all channels studied. This is translated into an excluded region in the plane. The production is excluded for all GeV/c. Assuming a point-like interaction of the with gauge bosons, an absolute lower limit on the charged mass at 95% CL is set at 79.8 GeV/c, independently of other parameters of the technicolor model. Received: 10 August 2001 / Published online: 5 November 2001     

Search for anomalous production of acoplanar di-lepton events in {\rm e^+e^-} collisions at \sqrt{s} = 183 and 189 GeV

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb at centre-of-mass energies of approximately 183 GeV and 189 GeV. The observed numbers of events – 78 at 183 GeV and 301 at 189 GeV – are consistent with the numbers expected from Standard Model processes, which arise predominantly from production with both W bosons decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Discrimination techniques are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent. Upper limits on the production cross-section times branching ratio squared for sleptons and for leptonically decaying charginos and charged Higgs are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay , where is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for GeV and right-handed staus with masses below 81.0 GeV for GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for GeV, within the framework of the Minimal Supersymmetric Standard Model assuming GeV and . Charged Higgs bosons, H, are excluded at 95% CL for masses below 82.8 GeV, assuming a 100% branching ratio for the decay . Received: 6 September 1999 / Published online: 6 April 2000     

Search for unstable heavy and excited leptons at LEP2

Searches for unstable neutral and charged heavy leptons, N and , and for excited states of neutral and charged leptons, , , , and , have been performed in collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58 pb at a centre-of-mass energy of 183 GeV, and about 10 pb each at 161 GeV and 172 GeV. No evidence for new particles was found. Lower limits on the masses of unstable heavy and excited leptons are derived. From the analysis of charged-current, neutral-current, and photonic decays of singly produced excited leptons, upper limits are determined for the ratio of the coupling to the compositeness scale, , for masses up to the kinematic limit. For excited leptons, the limits are established independently of the relative values of the coupling constants f and . Received: 29 November 1999 / Published online: 6 March 2000     

Recent results on nucleon resonance electrocouplings from the studies of π~+π~-p electroproduction with the CLAS detector

Recent results on nucleon resonance studies in π~+π~-p electro- production off protons with the CLAS detector are presented. The analysis of CLAS data allowed us to determine all essential contributing mechanisms, providing a credible separation between resonant and non-resonant parts of the cross sections in a wide kinematical area of invariant masses of the final hadronic system 1.3 < W < 1.8 GeV and photon virtualities 0.2 < Q~2 < 1.5 GeV~2. Electrocouplings of several excited proton states with masses less than 1.8 GeV were obtained for the first time from the analysis of π~+π~-p exclusive electroproduction channel.     

A search for leptoquark bosons and lepton flavor violation in e^+ p collisions at HERA

A search for new bosons possessing couplings to lepton-quark pairs is performed in the H1 experiment at HERA using 1994 to 1997 data corresponding to an integrated luminosity of pb. First generation leptoquarks (LQs) are searched in very high neutral (NC) and charged (CC) current data samples. The measurements are compared to Standard Model (SM) expectations from deep-inelastic scattering (DIS). A deviation in the spectrum previously observed in the 1994 to 1996 dataset at GeV remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at GeV which is not observed with the 1997 dataset alone. The NC DIS data is used to constrain the Yukawa couplings of first generation scalar and vector LQs in the Buchmüller–Rückl–Wyler effective model. Scalar LQs are excluded for masses up to 275 GeV for a coupling of electromagnetic strength, . A sensitivity to coupling values is established for masses up to 400 GeV for any LQ type. The NC and CC DIS data are combined to constrain for arbitrary branching ratios of the LQ into eq in a generic model. For a decay branching ratio into pairs as small as 10%, LQ masses up to 260 GeV are ruled out for . LQs possessing couplings to mixed fermion generations, which could lead to signals of lepton flavor violation (LFV), are searched in events with a high transverse momentum or . No or event candidate is found that is compatible with LQ kinematics. Constraints are set on the Yukawa coupling involving the and lepton in a yet unexplored mass range. Received: 2 July 1999 / Published online: 28 September 1999     

Charged particle cross sections in photoproduction and extraction of the gluon density in the photon

Photoproduction data collected with the H1 detector at HERA in 1994 are used to study the cross-sections for inclusive charged particle production and the structure of the photon. The differential cross-sections , for in the HERA laboratory frame, and for GeV/c and GeV/c have been measured. Model calculations of these cross-sections, based on perturbative QCD, indicate that the results are sensitive to the parton densities of the photon as well as to higher order effects, which are phenomenologically treated by multiple interactions. This sensitivity is exploited to determine the leading order distribution of partons in the photon using a new method based on high charged particles. The gluon content of the photon is extracted and found to rise with decreasing . Received: 8 October 1998 / Published online: 8 September 1999     

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.     

Measurement and interpretation of fermion-pair production at LEP energies from 130 to 172 GeV

The data collected with the DELPHI detector at centre-of-mass energies between 130 and 172 GeV, during LEP operation in 1995 and 1996, have been used to determine the hadronic and leptonic cross-sections and leptonic forward–backward asymmetries. In addition, the cross-section ratios and forward–backward asymmetries for flavour-tagged samples of light (uds), c and b quarks have been measured. No significant deviations from the Standard Model expectations are found. The results are interpreted by performing S-matrix fits to these data and to the data collected previously at the energies near the resonance peak (88-93 GeV). The results are also interpreted in terms of physics beyond the Standard Model: contact interactions, R-parity violating SUSY particle exchange and of possible Z bosons. Received: 9 February 1999 / Published online: 14 October 1999     

Photoproduction of positive pions from hydrogen at large angles in the energy range 0.3–2.0 GeV

The differential cross-section for the reaction γp↦π ⁺ n was measured using the bremsstrahlung beam of the Bonn 2.5 GeV electron synchrotron. The pions were detected and momentum analysed in a multichannel magnet spectrometer. Data reduction resulted in 1278 measured cross-sections which are presented as energy distributions at six laboratory angles between 180° and 95°. The range of laboratory photon energies extended from 0.3 to 2.1 GeV. The statistical accuracy is better than 3 percent, the systematic error is about 5 percent. The data are compared with other experimental results and predictions of a phenomenological analysis. These cross-sections are the result of a measurement program which was started in the seventies. Further results will be reported in forthcoming publications. Received: 2 February 2000 / Accepted: 14 August 2001