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

Abstract

The 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.