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     

Calibration of centre-of-mass energies at LEP1 for precise measurements of Z properties

The determination of the centre-of-mass energies from the LEP1 data for 1993, 1994 and 1995 is presented. Accurate knowledge of these energies is crucial in the measurement of the Z resonance parameters. The improved understanding of the LEP energy behaviour accumulated during the 1995 energy scan is detailed, while the 1993 and 1994 measurements are revised. For 1993 these supersede the previously published values. Additional instrumentation has allowed the detection of an unexpectedly large energy rise during physics fills. This new effect is accommodated in the modelling of the beam-energy in 1995 and propagated to the 1993 and 1994 energies. New results are reported on the magnet temperature behaviour which constitutes one of the major corrections to the average LEP energy. The 1995 energy scan took place in conditions very different from the previous years. In particular the interaction-point specific corrections to the centre-of-mass energy in 1995 are more complicated than previously: these arise from the modified radiofrequency-system configuration and from opposite-sign vertical dispersion induced by the bunch-train mode of LEP operation. Finally an improved evaluation of the LEP centre-of-mass energy spread is presented. This significantly improves the precision on the Z width. Received: 25 March 1998 / Revised version: 3 August 1998 / Published online: 29 October 1998     

Measurement of the fraction of hadronic Z decays into charm quark pairs

The full statistics of hadronic Z decays collected with the ALEPH detector are analysed to measure, by three methods, the ratio, , of the partial decay width of the Z into quarks to the total hadronic width. One method uses the inclusive and spectra of electrons. The other two use a double-tagging technique to measure independently of the charm-tagging efficiency. In one, the tagging procedure uses the reconstruction of high momentum charmed mesons (, and to detect the charm quarks. In the other, a charm quark is tagged from the 's reconstructed in the decay channel and the other from the slow pion from the decay. The combination of these measurements leads to . Received: 6 March 1998 / Published online: 2 July 1998     

Measurement of the {\rm\bf B_{\bf d}^{\bf 0}- {\bf \bar{B}}_{\bf d}^{\bf 0}} oscillation frequency

Time-dependent mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of meson oscillations is Received: 20 February 1998 / Revised version: 23 March 1998 / Published online: 12 August 1998     

Measurement of the ratio of inclusive cross sections sigma(pp --> Z + b jet)/sigma(pp --> Z + jet) at square root(s) = 1.96 TeV

Using the data collected with the D0 detector at square root(s) = 1.96 TeV, for integrated luminosities of about 180 pb(-1), we have measured the ratio of inclusive cross sections for pp --> Z + b jet to pp --> Z + jet production. The inclusive Z + b-jet reaction is an important background to searches for the Higgs boson in associated ZH production at the Fermilab Tevatron collider. Our measurement is the first of its kind, and relies on the Z --> e+ e- and Z --> mu+ mu- modes. The combined measurement of the ratio yields 0.021+/-0.005 for hadronic jets with transverse momenta pT > 20 GeV/c and pseudorapidities absolute value(eta) < 2.5, consistent with next-to-leading-order predictions of the standard model.     

Superheavy nuclei in deformed mean--field calculations

The ground–state properties of superheavy nuclei are investigated within various parametrisations of relativistic and nonrelativistic nuclear mean–field models. The heaviest known even–even nuclei starting with Z = 98 are used as a benchmark to estimate the predictive power of the models and forces. From that starting point, deformed doubly magic nuclei are searched in the region 100 ≤ Z ≤ 130 and 142 ≤ N ≤ 190. Received: 6 April 1998 / Revised version: 16 June 1998     

QCD corrections to the forward-backward asymmetries of c and b quarks at the Z pole

Measurements of the forward-backward production asymmetry of heavy quarks in Z decays provide a precise determination of . The asymmetries are sensitive to QCD effects, in particular hard gluon radiation. In this paper QCD corrections for and are discussed. The interplay between the experimental techniques used to measure the asymmetries and the QCD effects is investigated using simulated events. A procedure to estimate the correction needed for experimental measurements is proposed, and some specific examples are given. Received: 26 February 1998 / Published online: 2 June 1998     

Search for Higgs bosons in {\rm e^+e^-} collisions at 183 GeV

The data collected by the OPAL experiment at GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54 pb. None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, GeV and GeV are obtained for , no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range is excluded for minimal scalar top mixing and GeV. More general scans of the MSSM parameter space are also considered. Received: 27 October 1998 / Published online: 19 February 1999     

Search for neutral Higgs bosons in Z0 decays using the OPAL detector at LEP

A search for neutral Higgs bosons has been performed using the full sample of Z⁰ decays collected by the OPAL detector at LEP up to 1995. The data were taken at centre-of-mass energies between 88 GeV and 95 GeV and correspond to an integrated luminosity of approximately 160 pb^?1. The present search addresses the processes Z⁰→H⁰Z* and h⁰Z*, where H⁰ is the Higgs boson predicted by the Standard Model and h⁰ the lightest neutral scalar Higgs boson predicted in the framework of the Minimal Supersymmetric Standard Model. For the virtual Z⁰ boson, Z*, the following decay channels are considered: Z*→vv?, e⁺e^? and μ⁺μ^?. Two candidate events have been found in the vv?H⁰ channel and one in the μ⁺μ^?H⁰ channel. Combined with earlier searches, the present search excludes the SM Higgs boson, at the 95% confidence level (CL), from the mass range below 59.6 GeV. In the framework of the Minimal Supersymmetric Standard Model, allowing a wide range of variation for most relevant model parameters, a 95% CL lower limit of 44.3 GeV is obtained for the mass of the h⁰ boson. Combined with earlier direct searches for the Higgs boson pair production process Z⁰→h⁰A⁰ and with measurements of the Z⁰ line shape, a 95% CL lower limit of 23.5 GeV is obtained for the mass of the pseudoscalar Higgs boson A⁰, assuming tan β≥ 1.     

Determination of {\rm |V_{ub}|} from the measurement of the inclusive charmless semileptonic branching ratio of b hadrons

From a study of the kinematic properties of the final state produced in the semileptonic decays , the inclusive charmless semileptonic branching ratio of b hadrons is measured. With a sample of 3.6 million hadronic Z decays recorded between 1992 and 1995 with the ALEPH detector at LEP, the value is determined to be , where represents any charmless hadronic state and b is a mixture of b hadrons weighted by their production rates. This measurement yields the result where the last error comes from the conversion of the branching ratio to the CKM matrix element squared. Received: 6 May 1998 / Published online: 14 December 1998     

Are there anomalous Z fermion couplings?

The couplings of the fermions to the Z boson are of great importance in establishing the validity of the Standard Model and in looking for physics beyond it. The couplings of the b-quark to the Z boson have been the subject of much experimental study and theoretical interpretation. The apparent excess in the value of , the ratio of the partial width of the Z boson to to its total hadronic width, above the Standard Model expectation reported a few years ago has now become much less significant. However, the measurements of the pole forward-backward asymmetry for b-quarks at the Z pole and of the polarisation parameter , obtained using a polarised electron beam, have improved considerably in accuracy. The latest data are examined and values of the vector and axial-vector b-quark and c-quark couplings to the Z are extracted. The left and right handed couplings are also extracted. It is found that whereas the c-quark couplings are compatible with the Standard Model, those of the b-quark data are only compatible with the Standard Model at about the 1% level. In addition, the individual lepton couplings are extracted and the degree to which the data support the hypothesis of lepton universality is discussed. The sensitivity of the limits from electroweak fits to the Higgs boson mass to these data is examined. Received: 21 December 1998 / Revised version: 12 February 1999 / Published online: 15 April 1999     

Muon anomaly and dark parity violation

The muon anomalous magnetic moment exhibits a 3.6σ discrepancy between experiment and theory. One explanation requires the existence of a light vector boson, Z_{d} (the dark Z), with mass 10-500?MeV that couples weakly to the electromagnetic current through kinetic mixing. Support for such a solution also comes from astrophysics conjectures regarding the utility of a U(1)_{d} gauge symmetry in the dark matter sector. In that scenario, we show that mass mixing between the Z_{d} and ordinary Z boson introduces a new source of "dark" parity violation, which is potentially observable in atomic and polarized electron scattering experiments. Restrictive bounds on the mixing (m_{Z_{d}}/m_{Z})δ are found from existing atomic parity violation results, δ^{2}<2×10^{-5}. Combined with future planned and proposed polarized electron scattering experiments, a sensitivity of δ^{2}～10^{-6} is expected to be reached, thereby complementing direct searches for the Z_{d} boson.     

Improved search for a Higgs boson produced in association with Z → l+ l- in pp collisions sqrt[s] = 1.96 TeV

We search for the standard model Higgs boson produced with a Z boson in 4.1 fb(-1) of integrated luminosity collected with the CDF II detector at the Tevatron. In events consistent with the decay of the Higgs boson to a bottom-quark pair and the Z boson to electrons or muons, we set 95% credibility level upper limits on the ZH production cross section multiplied by the H → bb branching ratio. Improved analysis methods enhance signal sensitivity by 20% relative to previous searches. At a Higgs boson mass of 115 GeV/c2 we set a limit of 5.9 times the standard model cross section.     

Recent results from the D? detector

The D? Experiment started taking data in August 1992. We present preliminary results on inclusive jet production, direct photon production, W/Z production and decays, b Physics and first searches fort t production and new particles beyond the Standard Model.     

Observation of doubly-charmed B decays at LEP

A search for doubly-charmed B decays with both charmed mesons reconstructed is performed, using about 3.8 million hadronic Z decays recorded with the ALEPH detector at LEP. A clear signal is observed in the channels and (where D can be either a D, a D or a D), providing the first direct evidence for doubly-charmed B decays involving no production. Evidence for associated and production in the decays is also presented and some candidates for completely reconstructed decays , and are observed. Furthermore, candidates for the two-body Cabibbo suppressed decays and are also observed. Measurements of the corresponding branching fractions are extracted. Received: 6 March 1998 / Published online: 23 June 1998     

Trapped radioactive isotopes for fundamental symmetry investigations

Discrete symmetries tested in high precision atomic physics experiments provide guidance to model building beyond the Standard Model (SM). Here experimental opportunities arise for searches for permanent electric dipole moments (EDMs) and measurements of atomic parity violation (APV). Heavy atoms are favorable for such experiments since symmetry violating effects in atoms increase faster than the third power of the nuclear charge Z. Of special interest are isotopes of the heavy alkaline earth element radium (Z=88) since they offer large enhancement factors for EDMs and provide a new experimental road towards high precision measurements of atomic parity violation. These opportunities are exploited at the TRIμP facility at KVI, Groningen.     

Recent results from the DØ detector

The DØ Experiment started taking data in August 1992. We present preliminary results on inclusive jet production, direct photon production, W/Z production and decays, b Physics and first searches fort t production and new particles beyond the Standard Model.     

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 the b baryon lifetime and branching fractions in Z decays

Using approximately 4 million hadronic Z decays recorded with the aleph detector from 1991 through 1995, the lifetime of the b baryon is measured with three independent methods. From the impact parameter distribution of candidate leptons in 1063 events with combinations, the average b baryon lifetime is measured to be ps. From a sample of 193 fully reconstructed candidates correlated with a lepton and a sample of 46 combinations, the lifetime is measured to be ps. The product branching fractions to these final states are % for the first sample and % for the second and third samples combined. Received: 15 August 1997 / Online publication: March 10, 1998     

Investigation of the splitting of quark and gluon jets

The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the Delphi detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation . The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution , with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is . Due to non-perturbative effects, the data are below the expectation at small . The transition from the perturbative to the non-perturbative domain appears at smaller for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets. Received: 18 February 1998 / Published online: 24 April 1998