Cross-sections and leptonic forward-backward asymmetries from the Z^0 running of LEP

During 1993 and 1995 LEP was run at 3 energies near the Z peak in order to give improved measurements of the mass and width of the resonance. During 1994, LEP operated only at the Z peak. In total DELPHI accumulated data corresponding to an integrated luminosity of approximately 116 pb. Analyses of the hadronic cross-sections and of the cross-sections and forward-backward asymmetries in the leptonic channels used the most precise evaluations of the LEP energies. In the dimuon channel, events with a photon radiated from the initial state have been used to probe the cross-sections and asymmetries down to PETRA energies. Model independent fits to all DELPHI lineshape and asymmetry data from 1990 to 1995 have been carried out giving values of the resonance parameters: These values are significantly more precise than those previously published. The results are interpreted in terms of the Standard Model. Received: 4 February 2000 / Revised version: 7 March 2000 / Published online: 8 June 2000     

Calibration of centre-of-mass energies at LEP 2for a precise measurement of the W boson mass

The determination of the centre-of-mass energies for all LEP 2 running is presented. Accurate knowledge of these energies is of primary importance to set the absolute energy scale for the measurement of the W boson mass. The beam energy between 80 and 104 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is defined in the NMR model, which is calibrated against precise measurements of the average beam energy between 41 and 61 GeV made using the resonant depolarisation technique. The validity of the NMR model is verified by three independent methods: the flux-loop, which is sensitive to the bending field of all the dipoles of LEP; the spectrometer, which determines the energy through measurements of the deflection of the beam in a magnet of known integrated field; and an analysis of the variation of the synchrotron tune with the total RF voltage. To obtain the centre-of-mass energies, corrections are then applied to account for sources of bending field external to the dipoles, and variations in the local beam energy at each interaction point. The relative error on the centre-of-mass energy determination for the majority of LEP 2 running is 1.2 x 10^-4, which is sufficiently precise so as not to introduce a dominant uncertainty on the W mass measurement. Received: 4 August 2004, Revised: 15 December 2004, Published online: 21 January 2005     

Evaluation of the LEP centre-of-mass energy above the W-pair production threshold

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of . This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined. Received: 14 December 1998 / Published online: 3 November 1999     

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 130–172 GeV at LEP

Production of events with hadronic and leptonic final states has been measured in collisions at centre-of-mass energies of 130–172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z events, and compared to Standard Model expectations. The ratio of the cross-section for production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of -Z interference. The energy dependence of has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for -channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair. Received: 30 July 1997 / Published online: 10 March 1998     

Measurement of tau branching ratios to five charged hadrons

The branching ratios of the decay of the lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at centre-of-mass energies close to the resonance. The branching ratios are measured to be where the first error is statistical and the second systematic. Received: 4 June 1998 / Published online: 11 March 1999     

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     

Accurate determination of the LEP beam energy by resonant depolarization

To improve the measurements of the Z boson mass and resonance width, the 1993 Large Electron Positron Collider (LEP) run was devoted to a three point beam energy scan, with one point close to the peak of the Z resonance and two points roughly 880 MeV below and above the peak. Operational energy calibration by resonant depolarization was successfully commissioned for all three beam energies. 24 energy calibrations were performed at the end of physics fills. The accuracy of each calibration is better than 1 MeV. About one third of the total integrated luminosity was recorded in calibrated fills below and above the resonance and a regular tracking of the beam energies throughout the scan was possible. The evolution of the beam energies in the course of the year showed a large variation of up to 20 MeV. Results from the energy calibrations will be presented and possible explanations for the changes of the beam energy during the year will be described.     

Measurement of αs with radiative hadronic events

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling α_s is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of α_s is studied. Our results are consistent with the running of α_s as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives α_s(M_Z)=0.1182±0.0015(stat.)±0.0101(syst.).     

\mathrm{W^+W^-} production and triple gauge boson couplings at LEP energies up to 183 GeV

A study of W-pair production in annihilations at LEP is presented, based on candidates corresponding to an integrated luminosity of 57 pb at GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be %. The number of W-pair candidates and the angular distributions for each final state (, are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain , and , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be . All these measurements are consistent with the Standard Model expectations. Received: 23 October 1998 / Published online: 1 March 1999     

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb at an centre-of-mass energy of 183 GeV and about 20 pb equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/ and 90 GeV/, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermion to its mass () as a function of the mass. Received: 30 October 1998 / Published online: 1 March 1999     

Comparison of deep-inelastic electron-photon scattering data with the HERWIG and PHOJET Monte Carlo Models

Deep-inelastic electron-photon scattering is studied in the range from 1.2 to 30 GeV using the LEP1 data taken with the ALEPH, L3 and OPAL detectors at centre-of-mass energies close to the mass of the Z boson. Distributions of the measured hadronic final state are corrected to the hadron level and compared to the predictions of the HERWIG and PHOJET Monte Carlo models. For large regions in most of the distributions studied the results of the different experiments agree with one another. However, significant differences are found between the data and the models. Therefore the combined LEP data serve as an important input to improve on the Monte Carlo models. Received: 5 July 2000 / Published online: 15 February 2002     

Measurement of event shape and inclusive distributions at ${sqrt s} = 130$ and 136 GeV

Inclusive charged particle and event shape distributions are measured using 321 hadronic events collected with the DELPHI experiment at LEP at effective centre of mass energies of 130 to 136 GeV. These distributions are presented and compared to data at lower energies, in particular to the precise Z data. Fragmentation models describe the observed changes of the distributions well. The energy dependence of the means of the event shape variables can also be described using second order QCD plus power terms. A method independent of fragmentation model corrections is used to determine α_s from the energy dependence of the mean thrust and heavy jet mass. It is measured to be: $$←pha _s(133 {? GeV})={0.116}pm {0.007}_{exp-0.004theo}^{+0.005}$$ from the high energy data.     

Total hadronic cross-section of photon-photon interactions at LEP

The total hadronic cross-section for the interaction of real photons, hadrons, is measured for centre-of-mass energies GeV. The cross-section is extracted from a measurement of the process hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons ( GeV). The data were taken with the OPAL detector at LEP at centre-of-mass energies , 172$ and 183 GeV. The cross-section is compared with Regge factorisation and with the energy dependence observed in p and pp interactions. The data are also compared to models which predict a faster rise of compared to p and pp interactions due to additional hard interactions not present in hadronic collisions. Received: 3 June 1999 / Published online: 6 April 2000     

A search for neutral Higgs bosons in the MSSM and models with two scalar field doublets

A search is described for the neutral Higgs bosons and predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the and production channels is based on data corresponding to an integrated luminosity of 25 pb from collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the resonance to set limits on and in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for , minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits GeV and GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan. Received: 20 February 1998 / Published online: 13 July 1998     

Search for extra dimensions in boson and fermion pair production in ee interactions at LEP

Extra spatial dimensions are proposed by recent theories that postulate the scale of gravity to be of the same order as the electroweak scale. A sizeable interaction between gravitons and Standard Model particles is then predicted. Effects of these new interactions in boson and fermion pair production are searched for in the data sample collected at centre-of-mass energies above the Z pole by the L3 detector at LEP. In addition, the direct production of a graviton associated with a Z boson is investigated. No statistically significant hints for the existence of these effects are found and lower limits in excess of 1 TeV are derived on the scale of this new theory of gravity.     

Measurement of the strange quark forward-backward asymmetry around the Z^{0} peak

A precise measurement of the strange quark forward-backward asymmetry used 3.2M multihadronic events around the Z peak collected by the DELPHI experiment from 1992 to 1995. The ring imaging Cherenkov detectors in the barrel and end-cap regions identify high energy charged kaons which tag the s quark. The s quark asymmetry was measured at different centre-of-mass energies; in particular at the Z pole taking the expected d and u quark asymmetries from the Standard Model. The quark flavour fractions are assumed from the Standard Model and the fragmentation process is modelled by JETSET. From the s quark pole asymmetry the electroweak mixing angle was determined: The parity violating coupling of the s quark to the Z was determined to be: Received: 8 October 1999 / Revised version: 23 February 2000 / Published online: 18 May 2000     

A precise measurement of the \tau polarisation at LEP-1

The polarisation has been studied with the data collected by the DELPHI detector at LEP in 1993, 1994 and 1995 around the Z resonance firstly through the exclusive decay channels , , , and and secondly with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision. The results have been combined with those previously published on 1990 to 1992 DELPHI data, to produce results which reflect the full LEP-1 statistics. The fit of the polarisation dependence on the production angle yielded the polarisation parameters and . From these results the ratio of the vector and axial-vector effective couplings and have been derived, compatible with e universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons was obtained, implying a value of the effective weak mixing angle . Received: 1 December 1999 / Revised version: 2 February 2000 / Published online: 14 April 2000     

Searches for supersymmetric particles and anomalous four-jet production at} $sqrt{s}=130$ and 136 GeV at LEP

A search for supersymmetric particles (charged scalar leptons, charginos decaying semileptonically, scalar top quarks) and for the associated production of two massive particles decaying into quarks has been performed using a data sample of more than 5 pb^?1 at centre-of-mass energies of $sqrt{s}=130$ and 136 GeV collected with the OPAL detector at LEP during November 1995. No evidence for production of any of these particles has been observed in the data. Limits are presented on the production of charged scalar leptons, semileptonically decaying charginos, scalar top quarks and on the associated production of two massive particles leading to four-jet final states.     

Studies of QCD at ${\rm e}^ + {\rm e}^-$ centre-of-mass energies between 91 and 209 GeV

The hadronic final states observed with the ALEPH detector at LEP in annihilation are analysed using 730 pb^-1 of data collected between 91 and 209 GeV in the framework of QCD. In particular event-shape variables and inclusive charged particle spectra are measured. The energy evolution of quantities derived from these measurements is compared to analytic QCD predictions. The mean charged particle multiplicity, the charged particle momentum spectrum and its peak position are compared to predictions of the modified-leading-logarithmic approximation. The strong coupling constant is determined from a fit of the QCD prediction to distributions of six event-shape variables at eight centre-of-mass energies. A study of non-perturbative power law corrections is presented.Received: 18 May 2004, Published online: 23 June 2004     

Isotopic dependence of fusion cross-sections – linear relationships

A systematic study of isotopic dependence of fusion cross-section is carried out by adding neutrons gradually to N=Z colliding nuclei. We find that fusion barrier position increases and height decreases, both linearly with the increase of N/Z ratio of the compound system. The increase in barrier position is larger compared to decrease in barrier height. In terms of these linear relationships, a parameterized form of fusion cross-sections is given for the neutron-rich colliding nuclei. The fusion cross-sections are also enhanced linearly with the N/Z ratio, and this enhancement is larger for lower incident centre-of-mass energies and independent of the choice of reaction partners. Experimental data and other theoretical studies are called for to verify these results. Received: 27 February 1998 / Revised version: 22 May 1998