LEP Collider Research Articles

Open charm and beauty production in photon-photon collisions at L3

Inclusive heavy flavour production in γγ collisions has been studied with the L3 detector at the LEP collider. Preliminary results on charm and beauty production at √ s = 189 GeV are presented. Hadronic final states containing charm particles were identified either by detecting electrons or muons from their semi-leptonic decays or by reconstruction of charged D ∗ meson decays. The cross section for beauty production in γγ collisions has been measured for the first time. Even including real and virtual gluon emission (NLO QCD) the direct process γγ → c c, b b is insufficient to describe charm and beauty production in γγ collisions. The cross section values and event distributions require resolved processes, sensitive to the gluon density function of the photon.

Electroweak baryogenesis in the minimal supersymmetric standard model

We review the baryogenesis scenario in the MSSM at the perturbative level and, in particular, the impact of two-loop corrections on the strength of the phase transition and the amount of generated baryon asymmetry. We confirm the baryogenesis window, where m H ⪅ 115 GeV, for m Q ⪅ a few TeV, and the right-handed stop mass is constrained in the region, 100 GeV ⪅ m t ∼ ⪅ m t . This scenario will be tested at LEP and Tevatron colliders.

Experimental signatures of fermiophobic Higgs bosons

The most general Two Higgs Doublet Model potential without explicit CP violation depends on 10 real independent parameters. Excluding spontaneous CP violation results in two 7 parameter models. Although both models give rise to 5 scalar particles and 2 mixing angles, the resulting phenomenology of the scalar sectors is different. If flavour changing neutral currents at tree level are to be avoided, one has, in both cases, four alternative ways of introducing the fermion couplings. In one of these models the mixing angle of the CP even sector can be chosen in such a way that the fermion couplings to the lightest scalar Higgs boson vanishes. At the same time it is possible to suppress the fermion couplings to the charged and pseudo-scalar Higgs bosons by appropriately choosing the mixing angle of the CP odd sector. We investigate the phenomenology of both models in the fermiophobic limit and present the different branching ratios for the decays of the scalar particles. We use the present experimental results from the LEP collider to constrain the models.

Formula omitted] total cross-section in the dipole picture of BFKL dynamics

The total γ ∗γ ∗ cross-section is derived in the Leading Order QCD dipole picture [1] of BFKL dyamics [2], and compared with the one from 2-gluon exchange. The Double Leading Logarithm approximation of the DGLAP cross-section is found to be small in the phase space studied. Cross sections are calculated for realistic data samples at the e + e − collider LEP and a future high energy linear collider. Next to Leading order corrections to the BFKL evolution have been determined phenomelologically, and are found to give very large corrections to the BFKL cross-section, leading to a reduced sensitivity for observing BFKL effects.

ANOMALOUS HIGGS COUPLINGS

We review the effects of new effective interactions on Higgs-boson phenomenology. New physics in the electroweak bosonic sector is expected to induce additional interactions between the Higgs doublet field and the electroweak gauge bosons, leading to anomalous Higgs couplings as well as anomalous gauge-boson self-interactions. Using a linearly realized SU (2)L× U (1)Y invariant effective Lagrangian to describe the bosonic sector of the Standard Model, we review the effects of the new effective interactions on the Higgs-boson production rates and decay modes. We summarize the results from searches for the new Higgs signatures induced by the anomalous interactions in order to constrain the scale of new physics, in particular at CERN LEP and Fermilab Tevatron colliders.

Formula omitted] total cross section in the dipole picture of BFKL dynamics

The total γ ∗γ ∗ cross section is derived in the leading order QCD dipole picture of BFKL dynamics, and compared with the one from two-gluon exchange. The double leading logarithm approximation of the DGLAP cross section is found to be small in the phase space studied. Cross sections are calculated for realistic data samples at the e + e − collider LEP and a future high energy linear collider. Next to leading order corrections to the BFKL evolution have been determined phenomenologically, and are found to give very large corrections to the BFKL cross section, leading to a reduced sensitivity for observing BFKL effects.

The saga of the LEP dynamic aperture

The large electron–positron collider LEP at CERN provides a beautiful example of our conceptual limits concerning the problem of dynamic aperture (short term stability of the transverse oscillations of particle trajectories) in circular machines. For the operation at 45 GeV (Z0 peak) the dynamic aperture did not pose any problem although its measurement gave a value much smaller than that predicted up to the end of 1993. After this date the measurements gave the same result as predicted but it was not possible to trace back the origin of the discrepancy. At high energy (the maximum operating energy foreseen is 100 GeV) the beam emittance increases with the square of the beam energy. Therefore, low emittance optics were proposed. These optics suffer from large anharmonicities because of the increased sextupole strengths. This led to an unexpected limitation of the beam lifetime.

The small angle tile calorimeter in the DELPHI experiment

The Small angle TIle Calorimeter (STIC) provides calorimetric coverage in the very forward region of the DELPHI experiment at the CERN LEP collider. The structure of the calorimeters, built with a so-called “shashlik” technique, gives a perfectly hermetic calorimeter and still allows for the insertion of tracking detectors within the sampling structure to measure the direction of the showering particle. A charged-particle veto system, composed of two scintillator layers, makes it possible to trigger on single photon events and provides e–γ separation. Results are presented from the extensive studies of these detectors in the CERN testbeams prior of installation and of the detector performance at LEP.

New Higgs couplings ate+e−and hadronic colliders

We examine the potentiality of both CERN LEP and Fermilab Tevatron colliders to establish bounds on new couplings involving the bosonic sector of the standard model. We pay particular attention to the anomalous Higgs interactions with $\gamma$, $W^{\pm}$ and $Z^0$. A combined exclusion plot for the coefficients of different anomalous operators is presented. The sensitivity that can be achieved at the Next Linear Collider and at the upgraded Tevatron is briefly discussed.

QCD results from ALEPH at LEP2

Preliminary measurements of properties of hadronic events from e +e − annihilation are presented. The data have been obtained at centre-of-mass energies of E cm = 130, 161, 172 and 183 Gev by the ALEPH detector at the LEP collider from November 1995 through 1997. The quantities investigated include particle momentum spectra, multiplicities and distributions of event-shape variables. These are compared to predictions of Monte Carlo models and QCD formulae. The energy dependence of several quantities is investigated, including the mean multiplicity of charged particles and the strong coupling constant α s.

Spin physics at e+e− colliders

A large number of measurements with polarized beams and/or spin analysis of final state particles has been performed at the e^+e^- colliders LEP and SLC, providing important information on the dynamics of high energy interactions. In this paper three subjects, for which the role of spin studies was particularly relevant, will be covered: the measurements of the electroweak couplings, the study of fragmentation dynamics and the search for physics beyond the Standard Model.

Search for the standard model Higgs boson at the LEP2 Collider near [formula omitted] GeV

During 1997 the ALEPH experiment at LEP gathered 57 pb −1 of data at centre-of-mass energies near 183 GeV. These data are used to look for possible signals from the production of the Standard Model Higgs boson in the reaction e +e −→HZ. No evidence of a signal is found in the data; seven events are selected, in agreement with the expectation of 7.2 events from background processes. This observation results in an improved lower limit on the mass of the Higgs boson: m H >87.9 GeV /c 2 at 95% confidence level.

Search for the standard model Higgs boson at the LEP2 collider near [formula omitted

During 1997 the ALEPH experiment at LEP gathered 57 pb −1 of data at centre-of-mass energies near 183 GeV . These data are used to look for possible signals from the production of the Standard Model Higgs boson in the reaction e +e −→HZ. No evidence of a signal is found in the data; seven events are selected, in agreement with the expectation of 7.2 events from background processes. This observation results in an improved lower limit on the mass of the Higgs boson: m H >87.9 GeV /c 2 at 95% confidence level.

The influence of train leakage currents on the LEP dipole field

The determination of the mass and the width of the Z boson at CERN’s LEP accelerator, an e + e − storage ring with a circumference of approximately 27 km, imposes heavy demands on the knowledge of the LEP counter-rotating electron and positron beam energies. The precision required is of the order of 1 MeV or ≈ 20 ppm. Due to its size, the LEP collider is influenced by various macroscopic and regional factors such as the position of the moon or seasonal changes of the rainfall in the area, as reported earlier. A new and not less surprising effect on the LEP energy was observed in 1995: railroad trains in the Geneva region perturb the dipole field. A parasitic flow of electricity, originating from the trains, travels along the LEP vacuum chamber, affecting the LEP dipole field. An account of the phenomenon with its explanation substantiated by dedicated measurements is presented.

Electroweak baryogenesis and Higgs and stop searches at LEP and the Tevatron

It has been recently shown that the observed baryon number may originate at the electroweak phase transition, provided that the Higgs boson and the lightest stop are sufficiently light. In this work, we perform a detailed analysis, including all dominant two-loop finite-temperature corrections to the Higgs effective potential, as well as the non-trivial effects proceeding from the mixing in the stop sector, to define the region of parameter space for which electroweak baryogenesis can happen. The limits on the stop and Higgs masses are obtained by taking into account the experimental bounds on these quantities, as well as those coming from the requirement of avoiding dangerous color breaking minima. We find for the Higgs mass m h ⪷ 105 GeV , while the stop mass may be close to the present experimental bound and must be smaller than, or of the order of, the top quark mass. These results provide a very strong motivation for further non-perturbative analysis of the electroweak phase transition, as well as for the search for Higgs and stop particles at the LEP and Tevatron colliders.

Measurements of the rising photon-photon total cross section at the CERN LEP

Using vector meson dominance we calculate the {gamma}{gamma} total cross section from data on pp, p{bar p} and {gamma}p total cross sections. Our result agrees with recent high energy measurements performed by the L3 experiment at the CERN LEP collider, not with the conflicting preliminary data obtained by OPAL. {copyright} {ital 1998} {ital The American Physical Society}

Search for Higgs particles at LEP

Searches for Higgs bosons at the e +e − collider LEP at centre-of-mass energies above the Z 0 resonance are reviewed. No evidence for Higgs boson production has been found so far. Preliminary results obtained by the four LEP experiments, ALEPH, DELPHI, L3 and OPAL, are presented.

The Z lineshape at LEP

The e +e − collider LEP at CERN was operated in the years 1989–1995 at center-of-mass energies around 91 GeV. More than 16 million Z boson decays have been recorded by the four experiments ALEPH, DELPHI, L3 and OPAL. This vast amount of data allows a precise determination of the Standard Model parameters. An interpretation of the measurements within the Standard Model leads to a prediction of the top quark mass which is in agreement with the measurements at Tevatron. In addition the Higgs boson mass is constrained.

Measurement of B mixing and oscillations at LEP

This paper reviews the most recent measurements on time dependent B mixing and oscillations performed at LEP collider. A precise determination of the mass difference of the B d 0 − B d 0 system has been possible in several channels, providing the result Δ m d = 0.466 ± 0.019 ps −1. In addition, a search for B s 0 − B s 0 oscillation has been done with no signal found. The combined limit for the mass difference Δ m s is found to be Δ m s > 8.0 ps −1 at 95% CL. The LEP average for Δ m d provides also the fraction of weakly decaying B hadrons in Z → b b events. f B + = f B d 0 = 0.394 −0.020 +0.016 and f B s 0 = 0.105 ± 0.016. Interesting improvements are still expected by fully exploiting the LEP 1 statistics and new methods.

Effective QCD Coupling and Power Corrections to Photon-Photon Scattering

The scattering of two off-shell photons is an infrared-safe process in QCD. For photon virtualities $Q$ in the range of a few $\mathrm{GeV}$, accessible at the CERN ${e}^{+}{e}^{\ensuremath{-}}$ collider LEP II, power-behaved contributions in $({\ensuremath{\Lambda}}_{\mathrm{QCD}}/Q{)}^{n}$ to the total cross section may become non-negligible. Based on a dispersion relation for the running coupling, we discuss these contributions and calculate the coefficients of the leading power correction for transversely polarized and longitudinally polarized virtual photons.