LEP Collider Research Articles

FCC-ee: the synthesis of a long history of e+e- circular colliders

The design of FCC-ee is relying on the accumulated experience of mathrm {e^{+}e^{-}} colliders that have been designed, constructed and operated in the past 40 years. FCC-ee will surpass the 26.7 km long Large Electron Positron collider LEP by a factor 4 in size. Like for LEP the large size is justified by the need to control the synchrotron radiation losses that for both machines reach a few percent per turn. To that end LEP had the first large super-conducting (SC) RF system with around 3.8 GV of accelerating voltage. LEP achieved for the first time very large beam-beam parameters of around 0.08, and it relied on transversely polarized beams to determine accurately the beam energy for the experiments. The DAvarPhi NE collider, together with PEP II and KEKB split the two beams into separate vacuum chambers to reach much higher Ampere-level beam currents. To overcome beam-beam lifetime and performance issues DAvarPhi NE used for the first time the Crab Waist concept for the interaction region (IR) optics. The B-factories, PEP-II and KEKB have verified the double-ring mathrm {e^{+}e^{-}} collider with multi-ampere stored currents for over 1000 bunches, small beta ^*, top-up injection, and achieved then-highest luminosity. KEKB has applied 22-mrad crossing angle at the IP with crab crossing. Both machines inherited accelerator techniques from their predecessors, PEP and TRISTAN, which was a small-scale LEP. Currently the next generation SuperKEKB collider is starting up. It has already achieved some milestones required for FCC-ee such as small beta ^* (0.8 mm) and virtual crab-waist scheme with a large Piwinski angle (>10).

Anomalous constants of three-boson vertices in the process of pair production of W±-bosons in proton collisions at CMS LHC

In this paper, we obtained the expected constraints on the anomalous CP-even constants of three-boson interactions on the basis of cross-section for the pair production of W+-bosons in proton-proton collisions. The constraints were obtained for luminosity and the kinematic constraints on the final states typical for the CMS experiment at the Large Hadron Collider at =13 s TeV. One-dimensional and two-dimensional regions of constraints for the anomalous parameters of three-boson interactions were calculated. When calculating the cross-section, the usual approximations of small quark masses and values of the CKM matrix elements were not used. The expected values of the anomalous constants are almost an order of magnitude less than the constraints found at the LEP collider at = 200 s GeV in the reaction e–e+ → W–W+.

Determination of alpha _{S} beyond NNLO using event shape averages

We consider a method for determining the QCD strong coupling constant using fits of perturbative predictions for event shape averages to data collected at the LEP, PETRA, PEP and TRISTAN colliders. To obtain highest accuracy predictions we use a combination of perturbative {{{mathcal {O}}}}(alpha _{S}^{3}) calculations and estimations of the {{{mathcal {O}}}}(alpha _{S}^{4}) perturbative coefficients from data. We account for non-perturbative effects using modern Monte Carlo event generators and analytic hadronization models. The obtained results show that the total precision of the alpha _{S} determination cannot be improved significantly with the higher-order perturbative QCD corrections alone, but primarily requires a deeper understanding of the non-perturbative effects.

Probing the Glashow resonance at electron–positron colliders

The Glashow resonance is a rapid enhancement of the cross-section for scattering of electron antineutrinos on electrons at the [Formula: see text] boson production threshold due to the [Formula: see text]-channel contribution. This resonance is being searched for at large volume neutrino detectors in which the reaction [Formula: see text] to be initiated by cosmic ray antineutrinos of energies about 6.3 PeV. The relatively small neutrino flux reaching the Earth together with the necessity of analyzing ultra-high-energy final states make such searches challenging. We argue here that the Glashow resonance may contribute to the process [Formula: see text]. By extending the method of the effective (equivalent) particles to the neutral leptons, we relate the distribution of the effective neutrinos in the electron to the total cross-section for [Formula: see text]. Our approach gives a good fit to the existing experimental data measured at the collider LEP at CERN and allows one to interpret these measurements as an observation of the Glashow resonance. We also discuss an advantage of future electron–positron colliders for probing the resonance.

High precision determination of \u03b1s from a global fit of jet rates

We present state-of-the-art extractions of the strong coupling based on N3LO+NNLL accurate predictions for the two-jet rate in the Durham clustering algorithm at e+e− collisions, as well as a simultaneous fit of the two- and three-jet rates taking into account correlations between the two observables. The fits are performed on a large range of data sets collected at the LEP and PETRA colliders, with energies spanning from 35 GeV to 207 GeV. Owing to the high accuracy of the predictions used, the perturbative uncertainty is considerably smaller than that due to hadronization. Our best determination at the Z mass is αs (MZ) = 0.11881 ± 0.00063(exp.) ± 0.00101(hadr.) ± 0.00045(ren.) ± 0.00034(res.), which is in agreement with the latest world average and has a comparable total uncertainty.

Precise determination of αS( $ m_{Z^0 } $ ) from a global fit of energy-energy correlations to NNLO+NNLL predictions

We present a determination of the strong coupling constant αS( $ m_{Z^0 } $ ) using a global fit of theory predictions in next-to-next-next-leading-order (NNLO) combined with resummed predictions at the next-to-next-leading-log level (NNLL) [bibrR11]. The predictions are compared to distributions of energy-energy correlations measured in e+e−annihilation to hadronic final states by experiments at the e+e−colliders LEP, PETRA, TRISTAN and PEP. The predictions are corrected for hadronisation effects using the modern generator programs Sherpa 2.2.4 and Herwig 7.1.1.

Constraints on a light leptophobic mediator from LEP data

We apply data taken at the e+e− collider LEP in the 1990’s at center-of-mass energy up to 209 GeV to constrain Dark Matter models with a light leptophobic spin−1 mediator R. We assume that the dark sector particle (DSP) is a spin−1/2 fermion χ. This scenario is well studied in the context of LHC searches for mediator mass from 100 GeV to several TeV. Emission of the mediator off a quark or antiquark at LEP gives rise to di-jet plus missing energy and 4−jet signatures, which we use to limit the relevant couplings. We focus on scenarios with 2mχ > mR, which are poorly constrained by LHC data. We recast published searches by the ALEPH collaboration. For mχ ≲ 20 GeV the best bounds result from an analysis at sqrt{s} ≃ MZ of di-jet plus missing energy events. For heavier DSP but mR ≲ 70 GeV meaningful bounds can be derived from a four jet analysis at sqrt{s}=183 GeV. Unfortunately published searches using four jet final states at sqrt{s} ≃ MZ use only a small fraction of the total data sample. Moreover, all published searches for di-jet plus missing energy final states at sqrt{s}ge 130 GeV have poor efficiency for our model; we therefore design new cuts that combine good background rejection with higher efficiency. Re-analyzing the higher energy data using our new cuts, and an analysis of the complete four jet data sample taken at sqrt{s} ≃ MZ, can explore new regions of parameter space.

Effective theory of WIMP dark matter supplemented by simplified models: Singlet-like Majorana fermion case

We enumerate the set of simplified models which match onto the complete set of gauge invariant effective operators up to dimension six describing interactions of a singlet-like Majorana fermion dark matter with the standard model. Tree level matching conditions for each case are worked out in the large mediator mass limit, defining a one to one correspondence between the effective operator coefficients and the simplified model parameters for weakly interacting models. Utilizing such a mapping, we compute the dark matter annihilation rate in the early universe, as well as other low-energy observables like nuclear recoil rates using the effective operators, while the simplified models are used to compute the dark matter production rates at high energy colliders like LEP, LHC and future lepton colliders. Combining all relevant constraints with a profile likelihood analysis, we then discuss the currently allowed parameter regions and prospects for future searches in terms of the effective operator parameters, reducing the model dependence to a minimal level. In the parameter region where such a model-independent analysis is applicable, and leaving aside the special dark matter mass regions where the annihilation proceeds through an s-channel Z or Higgs boson pole, the current constraints allow effective operator suppression scales ($\Lambda$) of the order of a few hundred GeV for dark matter masses $m_\chi >$ 20 GeV at $95\%$ C.L., while the maximum allowed scale is around $3$ TeV for $m_\chi \sim \mathcal{O}(1\,{\rm TeV})$. An estimate of the future reach of ton-scale direct detection experiments and planned electron-positron colliders show that most of the remaining regions can be probed, apart from dark matter masses near half of the Z-boson mass (with $500\,{\rm GeV} < \Lambda < 2\,{\rm TeV} $) and those beyond the kinematic reach of the future lepton colliders.

Interview with Prof. Emmanuel Tsesmelis and Prof. Albert De Roeck from CERN

Professor Emmanuel Tsesmelis: is an experimental particle physicist with a career spanning scientific research, academic teaching, science communication, international relations and management at CERN and at several universities. He is a Senior Physicist and Deputy Head of International Relations in CERN's Director-General Unit and a Visiting Professor in Particle and Accelerator Physics at the University of Oxford. He is an elected Fellow of the Australian Institute of Physics and a supernumerary member of Jesus College, Oxford. Professor Albert De Roeck: is a senior research scientist and staff member at CERN. He is also a professor at the University of Antwerp (Belgium) and a visiting professor at the Institute of Particle Physics and Phenomenology in Durham (UK). He obtained his PhD at the university of Antwerp on an experiment at CERN, studying the multiparticle dynamics in hadron-hadron interactions, by colliding meson beams on protons and nuclear targets. After his PhD, Albert spent 10 years at the German particle physics laboratory, DESY, where he and his team made very precise measurements of the quark and gluon structure of the proton. In the late 90's his interest turned to the possibility to discover new physics, in particular Supersymmetry and Extra Dimensions, and he returned to CERN and joined an experiment at the large electron-positron collider LEP. Albert is now one the leaders of the physics program and preparation for physics analysis at the CMS experiment at the LHC.

Brief history for the search and discovery of the Higgs particle — A personal perspective

In 1964, a new particle was proposed by several groups to answer the question of where the masses of elementary particles come from; this particle is usually referred to as the Higgs particle or the Higgs boson. In July 2012, this Higgs particle was finally found experimentally, a feat accomplished by the ATLAS Collaboration and the CMS Collaboration using the Large Hadron Collider at CERN. It is the purpose of this review to give my personal perspective on a brief history of the experimental search for this particle since the '80s and finally its discovery in 2012. Besides the early searches, those at the LEP collider at CERN, the Tevatron Collider at Fermilab, and the Large Hadron Collider at CERN are described in some detail. This experimental discovery of the Higgs boson is often considered to be one of the most important advances in particle physics in the last half a century, and some of the possible implications are briefly discussed. This review is based on a talk presented by the author at the conference "OCPA8 International Conference on Physics Education and Frontier Physics," the 8th Joint Meeting of Chinese Physicists Worldwide, Nanyang Technological University, Singapore, June 23–27, 2014.

Brief history for the search and discovery of the Higgs particle – A personal perspective

In 1964, a new particle was proposed by several groups to answer the question of where the masses of elementary particles come from; this particle is usually referred to as the Higgs particle or the Higgs boson. In July 2012, this Higgs particle was finally found experimentally, a feat accomplished by the ATLAS Collaboration and the CMS Collaboration using the Large Hadron Collider at CERN. It is the purpose of this review to give my personal perspective on a brief history of the experimental search for this particle since the '80s and finally its discovery in 2012. Besides the early searches, those at the LEP collider at CERN, the Tevatron Collider at Fermilab, and the Large Hadron Collider at CERN are described in some detail. This experimental discovery of the Higgs boson is often considered to be the most important advance in particle physics in the last half a century, and some of the possible implications are briefly discussed. This review is partially based on a talk presented by the author at the conference "Higgs Quo Vadis," Aspen Center for Physics, Aspen, CO, USA, March 10–15, 2013.

Constraints on the Z-Z′ mixing angle from data measured for the process e + e − → W + W − at the LEP2 collider

An analysis of effects induced by new neutral gauge Z′ bosons was performed on the basis of data from the OPAL, DELPHI, ALEPH, and L3 experiments devoted to measuring differential cross sections for the process of the annihilation production of pairs of charged gauge W± bosons at the LEP2 collider. By using these experimental data, constraints on the Z′-boson mass and on the angle of Z-Z′ mixing were obtained for a number of extended gauge models.

Forward–backward asymmetries of the bottom and top quarks in warped extra-dimensional models: LHC predictions from the LEP and Tevatron anomalies

Within the paradigm of warped extra dimensions, third generation quarks are expected to be the most sensitive to effects beyond the Standard Model. The anomalies observed at the LEP and Tevatron colliders in the forward–backward asymmetries of the bottom (AFBb) and top (AFBt) quarks can thus be seen as early signatures of warped extra-dimensional scenarios. We propose a realization of such a scenario, with a gauge custodial symmetry in the bulk, which allows to address simultaneously the AFBb anomaly and the discrepancies observed recently on AFBt at high top quark rapidities and tt¯ invariant masses. We then show that the various phenomenological constraints arising from LEP, Tevatron and LHC can be satisfied within the considered model. The model predicts new features, induced by a Kaluza–Klein excitation of the gluon at a mass ∼1.5–2 TeV, in top quark pair production at the s=7 TeV LHC.

Search for supersymmetry in pp collisions at 7 TeV in events with jets and missing transverse energy

A search for supersymmetry with R-parity conservation in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented. The data correspond to an integrated luminosity of 35 pb−1 collected by the CMS experiment at the LHC. The search is performed in events with jets and significant missing transverse energy, characteristic of the decays of heavy, pair-produced squarks and gluinos. The primary background, from standard model multijet production, is reduced by several orders of magnitude to a negligible level by the application of a set of robust kinematic requirements. With this selection, the data are consistent with the standard model backgrounds, namely tt¯, W+jet and Z+jet production, which are estimated from data control samples. Limits are set on the parameters of the constrained minimal supersymmetric extension of the standard model. These limits extend those set previously by experiments at the Tevatron and LEP colliders.

FITTING OF Z' PARAMETERS

The paper deals with an approach to the model-independent searching for the Z' gauge boson as a virtual state in scattering processes. The relations between the Z' couplings to fermions covering a wide class of models beyond the Standard Model are found and used. They reduce in an essential way the number of parameters to be fitted in experiments. Special observables which uniquely pick out the Z' at energies of LEP and ILC colliders in different leptonic processes are introduced and the data of LEP experiments are analyzed. The Z' couplings to leptons and quarks are estimated at 95% confidence level. At this level, the LEP data are compatible with the existence of the Z' with the mass mZ′~ 1–1.2 TeV . These estimates may serve as a guide for experiments at the Tevatron and/or LHC. A comparison with other approaches and results is given.

Phenomenology of a three-family model with gauge symmetry SU(3)c ⊗ SU(4)L ⊗ U(1)X

We study an extension of the gauge group SU(3)c ⊗ SU(2)L ⊗ U(1)Y of the standard model to the symmetry group SU(3)c ⊗ SU(4)L ⊗ U(1)X (3-4-1 for short). This extension provides an interesting attempt to answer the question of family replication in the sense that models for the electroweak interaction can be constructed so that anomaly cancellation is achieved by an interplay between generations, all of them under the condition that the number of families must be divisible by the number of colours of SU(3)c. This method of anomaly cancellation requires a family of quarks transforming differently from the other two, thus leading to tree-level flavour changing neutral currents (FCNC) transmitted by the two extra neutral gauge bosons Z′ and Z′′ predicted by the model. In a version of the 3-4-1 extension, which does not contain particles with exotic electric charges, we study the fermion mass spectrum and some aspects of the phenomenology of the neutral gauge boson sector. In particular, we impose limits on the Z − Z′ mixing angle and on the mass scale of the corresponding physical new neutral gauge boson Z2, and establish a lower bound on the mass of the additional new neutral gauge boson Z′′ ≡ Z3. For the analysis we use updated precision electroweak data at the Z-pole from the CERN LEP and SLAC Linear Collider, and atomic parity violation data. The mass scale of the additional new neutral gauge boson Z3 is constrained by using updated experimental inputs from neutral meson mixing in the analysis of the sources of FCNC in the model. The data constrain the Z − Z′ mixing angle to a very small value of , and the lower bounds on and are found to be of and , respectively.

Z′boson detection in the minimal quiver standard model

We undertake a phenomenological study of the extra neutral ${Z}^{\ensuremath{'}}$ boson in the minimal quiver standard model and discuss limits on the model's parameters from previous precision electroweak experiments, as well as detection prospects at the Large Hadron Collider at CERN. We find that masses lower than around 700 GeV are excluded by the $Z$-pole data from the CERN LEP collider, and below 620 GeV by experimental data from di-electron events at the Fermilab Tevatron collider. We also find that at a mass of 1 TeV the LHC cross section would show a small peak in the di-lepton and top pair channel.

Particle-Accelerator Constraints on Isotropic Modifications of the Speed of Light

The absence of vacuum Cherenkov radiation for 104.5 GeV electrons and positrons at the LEP collider at CERN combined with the observed stability of 300 GeV photons at the Tevatron constrains deviations of the speed of light relative to the maximal attainable speed of electrons. Within the standard-model extension, the limit -5.8x10(-12)<or=kappatr-4/3ce00<or=1.2x10(-11) is extracted, which sharpens previous bounds by more than 3 orders of magnitude. The potential for further refinements of this limit with terrestrial experiments and astrophysical observations is discussed.

Brane vector phenomenology

Local oscillations of the brane world are manifested as massive vector fields. Their coupling to the Standard Model can be obtained using the method of nonlinear realizations of the spontaneously broken higher-dimensional space–time symmetries, and to an extent, are model independent. Phenomenological limits on these vector field parameters are obtained using LEP collider data and dark matter constraints.

Inclusive production of the [formula omitted] meson in hadronic decays of the Z boson

The inclusive production rate of the ρ±(770) vector meson in hadronic Z decays is measured with the ALEPH detector at the LEP collider. A total of 3.2 million hadronic events are selected from data recorded between 1991 and 1995. Decays of ρ±→π0+π± are reconstructed for xE>0.05 and xp>0.05 where xp=pρ/pbeam and xE=Eρ/Ebeam. The average ρ± multiplicity per hadronic event is evaluated to be N(ρ±)=2.59±0.03±0.15±0.04 where the first error is statistical and the second systematic. The third error is from the uncertainty in the extrapolation to xp=xE=0. The rates and differential cross-section are compared with Monte Carlo model predictions and OPAL measurements. Residual Bose–Einstein correlations are found to be an important component in the analysis.