Jet and photon measurements from ATLAS

Di erential cross-section measurements of inclusive-jet and di-jet production provide stringent tests of perturbative QCD predictions and provide inputs for determination of parton density functions. Ratios of jet multiplicities are sensitive to s and have reduced theoretical uncertainties. Measurements of the inclusive isolated-photon and di-photon cross-sections provide a direct probe of short-distance physics, complementary to that from measurements of jets or vector-bosons and are sensitive to the gluon density in the proton. The measurements are compared to next-to-leading-order or higher-order QCD calculationsPeer Reviewe

Calibration and performance of the tile calorimeter of ATLAS with cosmic ray muons

Tese de doutoramento, Física, Universidade de Lisboa, Faculdade de Ciências, 2011The installation of the ATLAS detector in the experimental cavern, took place from 2005 until 2009. During this period, technicians, engineers and physicists have been intensivelyworking on the preparation of the detector for its main objective: probing the new frontiers of high energy physics with the LHC, the particle collider with the largest center of mass energy (14 TeV nominal) and very high luminosities(1034cm−2s−1 nominal). The context of this thesis was this challenging environment that involved all ATLAS members in the preparation of the detector for collisions during the period of the detector commissioning with cosmic ray muons and with calibration and monitoring systems. In 2008 during a short period of time single beam data was available and was used to study the detector response. This large effort was fundamental to prepare the detector for the first collisions at the LHC that started in November 2009. Before collisions started, the only high energy particles available for studieswith the LHC detectors were the muons produced by the interaction of cosmic particles in the atmosphere. These cosmic ray muons are the only detectable particles reaching the earth surface in quantities large enough to study the performance of the different sub-systems of the ATLAS detector. Thework I have developed duringmy PhDand thatwill be detailed in this document is centered on the energy calibration and synchronization of the Tile Calorimeter, the barrel hadronic calorimeter of ATLAS, using cosmic ray muons. The two main topics of study are now summarized: Contribution to the energy calibration of the Tile Calorimeter A electromagnetic energy scale was set in testbeam using high energy particles for 12% of the Tile Calorimeter modules. My contribution was centered in the validation of the global energy scale algorithm and the detector’s energy response uniformity in φ using the TileMuonFitter. The results presented in this document have shown that both the energy scale application, from testbeam to all modules in the experimental cavern, and the energy uniformity in φ are better than 5%. A difference between radial layers A and D of 3% is measured and it is something not completely understood and must be studied later using e.g. isolated muons from collisions. The used data stream and method, still have shown that a full coverage in φ can be achieved for these measurements. These results obtained with an independent method are consistent with an earlier analysis, reported in the readiness paper of the Tile Calorimeter [18]. Calorimeters are not designed and developed for the detection of muons however they play an important role on the commissioning of the LHC detectors and physics program. Before reaching the muon chambers the muons produced in collisions will lose energy in the calorimeter volume. Corrections on the energy loss in the calorimeters are necessary to improve the precision of the muon momentum measurement. This correctionmus be applied to anymuons crossing the calorimeter volume and in particular in fundamental processes used on the final calibration of the detectorwhich includes complex objects as the Z boson decaying to two muons. Lepton isolation techniques are used in the so called golden-channel for the Higgs boson discovery, the decay to four leptons H→ZZ→4l, for the rejection of QCD background. The Tile Calorimeter performance with muons can have an important impact in physics beyond the standard model, such as Super-Symmetry, for instance on the search for stable massive particles, since some of these massive particles are characterized by having an energy loss in the calorimeter similar to muons. The work developed with cosmic muons can also be applied later using muons produced in collisions to monitor the EM scale during the LHC operation. So the work developed with cosmic ray muons is not only important for the commissioning of the detector but can also be relevant for the physics of the LHC to be done with the ATLAS detector. Understanding the response of the Tile Calorimeter to muons as well as to have under control the EM energy scale are fundamental to achieve the best performance of the ATLAS detector. Synchronization of the Tile Calorimeter The Tile Calorimeter synchronization was established during 2008 combining measurements with the laser system and high energy particles: cosmic ray muons and muons from single beam. Thework presented in this thesis uses both types ofmuons, butwith different objectives inmind. Using the single beamdataweremeasured corrections to the velocity of propagation of light in the clear fibers, a parameter used in the laser synchronization. The measured value of 18.5 cm/ns resulted in the update of this parameter in the laser calibration system. The work done with cosmic muons consisted in the determination of the time offsets of the Tile Calorimeter measured both for towers and individual cells. The time offsets were calculated as the residuals after the synchronization made with the laser system. The final results have shown that the cosmic ray muons and single beam data agree within less than 2 ns. The timing is fundamental for the operation of the detector and all systems must be internally synchronized and externally synchronized with the LHC clock ( f = 1 25 ns given by the bunch crossing). The timing plays an important role in the energymeasurement due to the stringent operation conditions of the LHC that require the online signal reconstruction for the Tile Calorimeter channels to be done without iterations. The time of each channel must be known with a precision of the order of a few nanoseconds so that the correct parameters are chosen for the online reconstruction method. Time is also used to select particles that come from p-p collisions, to provide quality factors on the selection of events, and it is the most sensitive quantity for the discovery of slow long lived particles, also called stable massive particles, that are predicted in models beyond the Standard Model. This thesis is divided in 7 chapters. The first is introductory and presents the Large Hadron Collider, the ATLAS detector and its physics goals. In Chapter 2 the Tile Calorimeter is described in some detail presenting the geometry, calibration systems and performance features obtained from the last testbeam results. The following chapters are dedicated to the commissioning of the Tile Calorimeter with cosmic ray muons. The third chapter presents the motivations for the work developed, focusing on the energy scale and synchronization of the Tile Calorimeter. These quantities are of course important in the overall detector performance and have also a larger importance in specific physics channels. Chapter 4 introduces the commissioning and gives a brief overview of the activities during this stage, it is mostly descriptive but also reporting with some detail the activities in which I contributed during the development of my thesis work. The main contributions to the Tile Calorimeter commissioning is included in the next two chapters. Chapter 5 presents the results on the energy scale and uniformity in φ using the TileMuonFitter. Chapter 6 is dedicated to the methods and results for synchronization with cosmic ray muons data. Finally in Chapter 7 conclusions are given.Fundação para a Ciência e Tecnologia (SFRH/BD/27416/2006

Hadron beam test of a scintillating fibre tracker system for elastic scattering and luminosity measurement in ATLAS

A scintillating fibre tracker is proposed to measure elastic proton scattering at very small angles in the ATLAS experiment at CERN. The tracker will be located in so-called Roman Pot units at a distance of 240 m on each side of the ATLAS interaction point. An initial validation of the design choices was achieved in a beam test at DESY in a relatively low energy electron beam and using slow off-the-shelf electronics. Here we report on the results from a second beam test experiment carried out at CERN, where new detector prototypes were tested in a high energy hadron beam, using the first version of the custom designed front-end electronics. The results show an adequate tracking performance under conditions which are similar to the situation at the LHC. In addition, the alignment method using so-called overlap detectors was studied and shown to have the expected precision.Comment: 12 pages, 8 figures. Submitted to Journal of Instrumentation (JINST

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

Measurement of D*+/- meson production in jets from pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

This paper reports a measurement of D*+/- meson production in jets from proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV at the CERN Large Hadron Collider. The measurement is based on a data sample recorded with the ATLAS detector with an integrated luminosity of 0.30 pb^-1 for jets with transverse momentum between 25 and 70 GeV in the pseudorapidity range |eta| < 2.5. D*+/- mesons found in jets are fully reconstructed in the decay chain: D*+ -> D0pi+, D0 -> K-pi+, and its charge conjugate. The production rate is found to be N(D*+/-)/N(jet) = 0.025 +/- 0.001(stat.) +/- 0.004(syst.) for D*+/- mesons that carry a fraction z of the jet momentum in the range 0.3 < z < 1. Monte Carlo predictions fail to describe the data at small values of z, and this is most marked at low jet transverse momentum.Comment: 10 pages plus author list (22 pages total), 5 figures, 1 table, matches published version in Physical Review

Search for supersymmetry in final states with jets, missing transverse momentum and one isolated lepton in sqrt{s} = 7 TeV pp collisions using 1 fb-1 of ATLAS data

We present an update of a search for supersymmetry in final states containing jets, missing transverse momentum, and one isolated electron or muon, using 1.04 fb^-1 of proton-proton collision data at sqrt{s} = 7 TeV recorded by the ATLAS experiment at the LHC in the first half of 2011. The analysis is carried out in four distinct signal regions with either three or four jets and variations on the (missing) transverse momentum cuts, resulting in optimized limits for various supersymmetry models. No excess above the standard model background expectation is observed. Limits are set on the visible cross-section of new physics within the kinematic requirements of the search. The results are interpreted as limits on the parameters of the minimal supergravity framework, limits on cross-sections of simplified models with specific squark and gluino decay modes, and limits on parameters of a model with bilinear R-parity violation.Comment: 18 pages plus author list (30 pages total), 9 figures, 4 tables, final version to appear in Physical Review

Reducing heterotic M-theory to five dimensional supergravity on a manifold with boundary

This paper constructs the reduction of heterotic $M$-theory in eleven dimensions to a supergravity model on a manifold with boundary in five dimensions using a Calabi-Yau three-fold. New results are presented for the boundary terms in the action and for the boundary conditions on the bulk fields. Some general features of dualisation on a manifold with boundary are used to explain the origin of some topological terms in the action. The effect of gaugino condensation on the fermion boundary conditions leads to a `twist' in the chirality of the gravitino which can provide an uplifting mechanism in the vacuum energy to cancel the cosmological constant after moduli stabilisation.Comment: 16 pages, RevTe

Measurement of tau polarization in W->taunu decays with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV

In this paper, a measurement of tau polarization in W->taunu decays is presented. It is measured from the energies of the decay products in hadronic tau decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb^-1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the tau polarization is Ptau = -1.06 +/- 0.04 (stat) + 0.05 (syst) - 0.07 (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95% CL interval [-1,-0.91]. Measurements of tau polarization have not previously been made at hadron colliders.Comment: 10 pages plus author list (25 pages total), 4 figures, 4 tables, revised author list, matches published EPJC versio