A Study of J/psi Production at the LEP e+e−e^{+} e{-} Collider; and the Implementation of the DELPHI Slow Controls System

This thesis describes two separate areas of work conducted for the DELPHI detector at LEP. The first concerns the Slow Controls of the DELPHI detector, which enable a single operator to oversee the proper functioning of the apparatus and to diagnose faults as they occur. The hardware and software of this system, as well as their interface to the experiment and the operator, are described. Some conclusions are drawn from seven years' design work and the initial six years' operation of DELPHI. Secondly, a study is made of the production, at e+e- collision centre of mass energies close to the Z0 resonance, of J/psi mesons, decaying to mu+ mu-. J/psi mesons produced via a B-hadron are used to measure the mean B lifetime, tau_B = (1.53 +- 0.11 (stat.) +- 0.06 (syst.)) ps A measurement is also made of the fraction of J/psis produced promptly at the e+e- collision point, N(Z0 -> prompt J/psi X) / N(Z0 -> J/psi X) = (9.6 +- 3.2 (stat.) +- 1.2 (syst.))%. This method is largely model-independent

A Linear Iterative Unfolding Method

A frequently faced task in experimental physics is to measure the probability distribution of some quantity. Often this quantity to be measured is smeared by a non-ideal detector response or by some physical process. The procedure of removing this smearing effect from the measured distribution is called unfolding, and is a delicate problem in signal processing, due to the well-known numerical ill behavior of this task. Various methods were invented which, given some assumptions on the initial probability distribution, try to regularize the unfolding problem. Most of these methods definitely introduce bias into the estimate of the initial probability distribution. We propose a linear iterative method, which has the advantage that no assumptions on the initial probability distribution is needed, and the only regularization parameter is the stopping order of the iteration, which can be used to choose the best compromise between the introduced bias and the propagated statistical and systematic errors. The method is consistent: "binwise" convergence to the initial probability distribution is proved in absence of measurement errors under a quite general condition on the response function. This condition holds for practical applications such as convolutions, calorimeter response functions, momentum reconstruction response functions based on tracking in magnetic field etc. In presence of measurement errors, explicit formulae for the propagation of the three important error terms is provided: bias error, statistical error, and systematic error. A trade-off between these three error terms can be used to define an optimal iteration stopping criterion, and the errors can be estimated there. We provide a numerical C library for the implementation of the method, which incorporates automatic statistical error propagation as well.Comment: Proceedings of ACAT-2011 conference (Uxbridge, United Kingdom), 9 pages, 5 figures, changes of corrigendum include

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at root s = 8 TeV with the ATLAS detector

A search for heavy leptons decaying to a Z boson and an electron or a muon is presented. The search is based on pp collision data taken at √s = 8 TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb⁻¹. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a Z boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114–176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100–468 GeV are excluded.G. Aad ... P. Jackson ... L. Lee ... A. Petridis ... N. Soni ... M.J. White ... et al. (ATLAS Collaboration

Search for new phenomena in dijet angular distributions in proton-proton collisions at root s =8 TeV measured with the ATLAS detector

A search for new phenomena in LHC proton-proton collisions at a center-of-mass energy of sqrt[s]=8 TeV was performed with the ATLAS detector using an integrated luminosity of 17.3 fb^{-1}. The angular distributions are studied in events with at least two jets; the highest dijet mass observed is 5.5 TeV. All angular distributions are consistent with the predictions of the standard model. In a benchmark model of quark contact interactions, a compositeness scale below 8.1 TeV in a destructive interference scenario and 12.0 TeV in a constructive interference scenario is excluded at 95% C.L.; median expected limits are 8.9 TeV for the destructive interference scenario and 14.1 TeV for the constructive interference scenario.G. Aad … P. Jackson … L. Lee … A. Petridis … N. Soni ... M. White ... et al. (The ATLAS Collaboration

Measurements of the nuclear modification factor for jets in Pb+Pb collisions at root sNN =2.76 TeV with the ATLAS detector

Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at √(s)NN=2.76  TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and 0.14  nb(-1), respectively. The jets are identified with the anti-k(t) algorithm with R=0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32<p(T)<500  GeV and absolute rapidity |y|<2.1 and as a function of collision centrality. The nuclear modification factor R(AA) is evaluated, and jets are found to be suppressed by approximately a factor of 2 in central collisions compared to pp collisions. The R(AA) shows a slight increase with p(T) and no significant variation with rapidity.G. Aad … P. Jackson … L. Lee … A. Petridis … N. Soni ... M. White ... et al. (The ATLAS Collaboration

Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at root s = 8 TeV with the ATLAS detector

The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/Rc = 950 GeV for a cut-off scale times radius (ΛRc) of approximately 30.G. Aad … P. Jackson … L. Lee … A. Petridis … N. Soni ... M. White ... et al. (The ATLAS Collaboration

Search for the standard model higgs boson produced in association with top quarks and decaying into bb in pp collisions at root s = 8 TeV with the ATLAS detector

A search for the Standard Model Higgs boson produced in association with a top-quark pair, t ¯tH, is presented. The analysis uses 20.3 fb−1 √ of pp collision data at s = 8 TeV, collected with theATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H → b¯b decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t ¯t+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t ¯t+b¯b background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the StandardModel cross section is obtained at 95% confidence level. The ratio of the measured t ¯tH signal cross section to the StandardModel expectation is found to be μ = 1.5±1.1 assuming a Higgs boson mass of 125 GeV .G. Aad ... P. Jackson ... L. Lee ... A. Petridis ... N. Soni ... M.J. White ... et al. (ATLAS Collaboration