CERN Large Electron Positron Collider Research Articles

Radiological clearance of equipment from high-energy electron accelerators: The example of LEP superconducting acceleration system

The lifespan of a particle accelerator includes its final decommissioning and the disposal of potentially activated materials, one of the major challenges faced at accelerator laboratories. This paper discusses the computational and experimental techniques adopted for clearing from regulatory control the superconducting acceleration system of the former CERN Large Electron Positron collider (LEP), consisting of nearly 450 tons of valuable metals. The paper first briefly recalls the major sources of activation at high-energy electron accelerators and then discusses the general process, the computational tools employed, the methods to determine the amount of radionuclides that cannot be measured by γ-spectrometry, the selection of samples representative of the whole equipment and the outcome of the operational phase. The process ended up with the clearance of 95% of the material. The aim of this work is to provide a practical example of an approach that could be applicable to similar projects targeting radiological clearance of high-energy particle accelerator equipment.

Observation of a VHE cosmic-ray flare-signal with the L3+C muon spectrometer

The data collected by the L3+C muon spectrometer at the CERN Large Electron–Positron collider, LEP, have been used to search for short duration signals emitted by cosmic point sources. A sky survey performed from July to November 1999 and from April to November 2000 has revealed one single flux enhancement ( chance probability = 2.6 × 10 - 3 ) between the 17th and 20th of August 2000 from a direction with a galactic longitude of (265.02 ± 0.42)° and latitude of (55.58 ± 0.24)°. The energy of the detected muons was above 15 GeV.

Ending the great drought

With the Large Hadron Collider (LHC) springing to life at the CERN laboratory near Geneva, the great data drought in elementary particle physics is finally about to end. Not since the second phase of CERN's Large Electron–Positron Collider (LEP) began operations in 1996 has the field been able to probe virgin territory and measure truly new and exotic phenomena. And that machine merely doubled the energy reach of electron–positron colliders into regions that had already been partially explored using the Tevatron at Fermilab in the US. Researchers at these colliders – the world's most powerful for over a decade – could only chip away at the outer fringes of the unknown. But the LHC, built by installing thousands of superconducting magnets in the LEP tunnel, will permit physicists to strike deep into its dark heart. There they will almost certainly discover something distinctively different.

Search for Decay of a Fermiophobic Higgs Bosonhf→γγwith the D0 Detector ats=1.96 TeV

We report the results of a search for a narrow resonance decaying into two photons in 1.1 fb;{-1} of data collected by the D0 experiment at the Fermilab Tevatron Collider during the period 2002-2006. We find no evidence for such a resonance and set a lower limit on the mass of a fermiophobic Higgs boson of m_{h_{f}}>100 GeV at the 95% C.L. This exclusion limit exceeds those obtained in previous searches at the Fermilab Tevatron and covers a significant region of the parameter space B(h_{f}-->gammagamma) vs m_{h_{f}} which was not accessible at the CERN Large Electron-Positron Collider.

Constraining Light Colored Particles with Event Shapes

Using recently developed techniques for computing event shapes with soft-collinear effective theory, CERN Large Electron Positron Collider event shape data are used to derive strong model-independent bounds on new colored particles. In the effective field theory computation, colored particles contribute in loops not only to the running of alphas but also to the running of hard, jet, and soft functions. Moreover, the differential distribution in the effective theory explicitly probes many energy scales, so even shapes have a strong sensitivity to new particle thresholds. Using thrust data from ALEPH and OPAL, colored adjoint fermions (such as a gluino) below 51.0 GeV are ruled out to 95% confidence. This is nearly an order-of-magnitude improvement over the previous model-independent bound of 6.3 GeV.

Virtual Hadronic and Leptonic Contributions to Bhabha Scattering

Using dispersion relations, we derive the complete virtual QED contributions to Bhabha scattering due to vacuum polarization effects. We apply our result to hadronic corrections and to heavy lepton and top quark loop insertions. We give the first complete estimate of their net numerical effects for both small and large angle scattering at typical beam energies of meson factories, the CERN Large Electron-Positron Collider, and the International Linear Collider. With a typical amount of 1-3 per mil they are of relevance for precision experiments.

Radiation levels in the CERN Large Electron Positron collider during the LEP 2 phase (68–105 GeV)

The CERN Large Electron Positron (LEP) collider was in operation from 1989 to 2000. At the end of 1995 the LEP 2 phase began, with the progressive upgrade of the collider energy above the W pair production threshold, until the final energy of 105 GeV per beam. During the 11-year operation an extensive radiation survey program monitored the dose levels inside and around the installation. The radiation levels monitored in the underground areas and on the surface during 1989–1995 (LEP 1 phase) were discussed in a previous paper. The aim of this paper is to complete the data reported earlier. This paper first gives an overview of the radiation levels in the LEP tunnel and in other underground areas, and then discusses measurements of the photon radiation performed in the machine tunnel at each energy increase. An estimate of neutron sources, measurements of radiation streaming through ducts and shafts and some results of measurements of synchrotron radiation from the wigglers are given next. Residual dose rates are then briefly addressed. Finally, an overview is provided of the radiation levels recorded on ground surface during operation, both at the LEP access points and at some reference areas in the French and Swiss countryside.

Lower limits on R-parity-violating couplings in supersymmetric models with light squarks.

We interpret the results of searches for strongly interacting massive particles to place absolute lower limits on R-parity-violating couplings for squarks with mass (m(q) below 100 GeV. Recent searches for anomalous isotopes require that there be a baryon-number-violating or lepton-number-violating coupling larger than 10(-22)-10(-21) if m(q)>18 GeV. Using data from searches for stable particles at the CERN Large Electron Positron Collider (LEP) we demonstrate that this lower limit increases by 14 orders of magnitude, to an R-parity-violating coupling larger than 10(-8)-10(-7) for any squarks of mass less than 90 GeV. In the presence of an R-parity-violating coupling of this magnitude, neutralinos cannot explain the dark matter density in the Universe.

Investigation of induced radioactivity in the CERN Large Electron Positron collider for its decommissioning

The future installation of the Large Hadron Collider in the tunnel formerly housing the Large Electron Positron collider (LEP) required the dismantling of the latter after 11-year operation. As required by the French legislation, an extensive theoretical study was conducted before decommissioning to establish the possible activation paths both in the accelerator and in the four experiments (L3, ALEPH, OPAL and DELPHI) installed around the ring. The aim was to define which areas may contain activated material and which ones would be completely free of activation. The four major sources of activation in LEP, i.e., distributed and localized beam losses, synchrotron radiation and the super-conducting RF cavities, were investigated. Conversion coefficients from unit lost beam power to induced specific activity were established for a number of materials. A similar study was conducted for the four experiments, evaluating the four potential sources of induced radioactivity, namely e +e − annihilation events, two-photon events, e +e − Bhabha scattering events and beam-related radiation, i.e., synchrotron radiation and off-momentum beam particles. Decommissioning started early in 2001 and was completed by February 2002. Every single piece removed from the machine tunnel and experimental areas was double-checked for induced radioactivity, with complementary gamma-spectrometry measurements performed on samples of the various materials. The measurements have to a large extent confirmed the predictions. The overall intent of this paper is to provide data and computational guidelines for assessing induced radioactivity in high-energy electron accelerators, which could be of use in future decommissioning.

Very high intensity positron beam generated by LEP2 synchrotron radiation

This year (1996), the CERN large electron-positron collider (LEP) will produce electron positron collisions from accelerated beams with energies up to 80 GeV per beam, thus providing the highest energy, intense electron beam in the world. At LEP, a 4 T superconducting wiggler could be used to produce synchrotron radiation [1]. For an electron beam with an intensity of 9 mA, this unique γ-beam would contain photons in the energy range 1–100 MeV at an intensity up to 10 16 γ/s mrad MeV. The laminar shape of the synchrotron beam can be used to build a converter that preserves some of the initial (at the production) forward collimation of the positron beam. The converter consists of 200 targets with individual thicknesses of 2 · 10 −3 X 0, where X 0 is the radiation length, spaced by 10 cm for a total length of 20 m. The 20 m target is contained in a solenoid to focus the positron beam. The e + beam is transported by a solenoid and is energy dispersed by a pair of dipole magnets. The positron yield in the energy interval 1–2 MeV is N e + ∼ 10 13 positron/s MeV at the production level. The flux of positrons that can be obtained at LEP2 is two orders of magnitude larger than that predicted for future projects [2].

Higgs boson production via the Bjorken process e+e--->H0 micro+ micro- at high energy e+e- colliders.

Production of the standard Higgs boson via the Bjorken process, [ital e][sup +][ital e][sup [minus]][r arrow][ital H][sup 0][mu][sup +][mu][sup [minus]], at the center-of-mass energy [radical][ital s] [approx]200 GeV (i.e., for the CERN Large Electron-Positron Collider LEP II) is investigated in detail for a Higgs boson mass ranging from 60 GeV to about 120 GeV. We focus on physical quantities which have not been emphasized so far, including the differential cross sections in the various [mu][sup +] (or [mu][sup [minus]]) scattering angles (with the remaining angles integrated out completely) and the anticipated charge asymmetry. Numerical results on these quantities, in addition to the total production cross section, are described.

One-loop corrections to the Higgs sector in minimal supergravity models.

We study the Higgs spectrum of the minimal supersymmetric standard model in the context of supergravity models with radiative gauge-symmetry breaking, including the leading one-loop corrections. We find that, unlike in more general scenarios, these corrections cannot change the ordering of the Higgs-boson masses as predicted by tree-level expressions. Moreover, the upper bound on the mass of the lightest Higgs boson is reduced compared to the general case; however, even in the most restrictive supergravity model the Higgs sector might escape detection at the CERN Large Electron-Positron Collider LEP 200. We also present an expression for the one-loop-corrected charged-Higgs-boson mass, and describe an improved calculation of the bound on the lightest Higgs boson in general global supersymmetry scenarios.

The L3 database system

The database system developed for the L3 experiment at the CERN large electron-positron (LEP) collider is described. The solution chosen has two components: a FORTRAN package (DBL3) and a system of distributed servers and data (L3DBSM). DBL3 provides basic database functions supplemented by facilities specific to high-energy physics experiments. The L3DBSM system, operational since the start of LEP data taking, has handled efficiently several hundred Mbytes of data over three different systems (VAX/VMS, IBM VM/CMS, Apollo Aegis SR10.x) and at eight different sites.

Particle physics after a year of LEP

The Z° boson has changed in the last year from being an exotic rarity to being the tool for a wide range of research. CERN's large electron-positron collider, LEP, produces enough Z° particles to allow many predictions of particle physics theory and cosmology to be tested.

The design, construction and commissioning of the CERN large Electron–Positron collider

Abstract A description is given of the most important parameters considered in the design of the CERN Large Electron–Positron collider. It is shown how these parameters affect the collider performance and how they have been optimised with respect to the cost of the project. The functioning of each major subsystem is described with respect to its role as part of the collider. Finally, the planning, testing and initial commissioning of LEP is described and possible future developments are outlined.

Determination of the number of light neutrino species

The cross-section for e +e − → hadrons in the vicinity of the Z boson peak has been measured with the ALEPH detector at the CERN Large Electron Positron collider, LEP. Measurements of the Z mass, M z = (91.174±0.070) GeV, the Z width Γ z=(2.68±0.15) GeV, and of the peak hadronic cross-section, σ had peak=(29.3±1.2) nb, are presented. With the constraints of the standard electroweak model, the number of light neutrino species is found to be N v=3.27±0.30. this results rules out of the possibility of a fourth type of light neutrino at 98% CL.

Some crucial predictions of compositeness for the experiments at 100-GeV e+e- colliders.

Under the assumption that the quarks and the Higgs boson are composites, it has been argued that the third $\ensuremath{\tau}$ family is expected, on rather general grounds, to have an inverse size of order only 1 TeV. We note that the study of ${e}^{\ensuremath{-}}{e}^{+}\ensuremath{\rightarrow}t\ensuremath{-}\mathrm{quarkonium}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{\ensuremath{-}}{\ensuremath{\tau}}^{+}$ would be especially suitable to observe this compositeness already at energies of the CERN large electron-positron collider ring (LEP), assuming that ${m}_{\mathrm{top}}\ensuremath{\lesssim}50$ GeV.

FASTBUS Software in Europe

A large part of the European High-Energy Physics community, particularly that engaged with LEP, the future CERN Large Electron Positron Collider, has adopted FASTBUS as an instrumentation standard. Therefore, the focus of long-term FASTBUS support for hardware and software is at CERN, with end-users located both on-site and at collaborating institutes. The variety of Host machines and FASTBUS interfaces used, and the required mobility of the systems between development in home laboratories and ultimate incorporation at the experiment, has necessitated the maximum exploitation of standards wherever possible.