Development of a high voltage system for the ATLAS Tile calorimeter and hadronic jets improvements for H->bb searches
IDPASC Portugal - PHD Programme 2015
2015 / 2016
Experimental Particle Physics
Faculdade de Ciências - Universidade de Lisboa
Laboratório de Instrumentação e Física Experimental de Partículas
This thesis proposal focuses on the development of a high voltage (HV) system to be used in the ATLAS TileCal for the operation in the future High Luminosity LHC (HL-LHC) environment and on the search for the Higgs boson in the H->bb channel. TileCal, the ATLAS hadron calorimeter in the barrel region is a key sub-detector, essential in the identification of hadronic jets and the measurement of their energy, the measurement of missing transverse energy, and the identification of soft muons. Besides many other physics analyses, these measurements are crucial in the search for the Higgs boson in the b-bbar channel, an experimental priority during the LHC run 2 and one of the subjects of this PhD proposal. Due to the huge QCD background prevailing in the LHC, the first observation of Higgs boson decays to a b-quark pair will rely on associated production channels with a vector boson or a pair of top quarks. This proposal focuses on the associated production of a Higgs boson and a W vector boson, which decays leptonically to electrons, muons and neutrinos, leading to final states containing, in addition to two b-quark initiated jets (b-jets), an electron or muon plus a neutrino. A good measurement of the missing transverse energy is extremely important to identify the neutrino, and the best possible b-jet energy measurement is crucial to obtain a good resolution of the Higgs mass peak, and therefore improve the analysis sensitivity. However, the calibration of jets is normally obtained from an inclusive jet sample, not taking into account the specific characteristics of b-jets. One of these characteristics is the relatively high fraction of b-jets containing a soft muon, from the semileptonic decay of a B meson. We propose to take this muon into account in the measurement of the b-jet energy scale, in cases where it can be identified using the muon chambers and/or the TileCal. To achieve the best possible jet energy measurement, a procedure to refine the calibration is proposed specifically for these b-jets. In addition, and in view of the ATLAS High Luminosity Upgrade, an improved High Voltage system for the TileCal calorimeter will be developed, which will be crucial to the ATLAS physics output in many future analyses. The current HV system is based on a HV distributor located in the detector, which receives a single HV cable per TileCal module as input, and regulates locally the individual voltages applied to each photomultiplier tube (PMT). This concept has been in use in the TileCal since the beginning, and has the important advantage of not requiring a large number of 100 m long cables from the HV regulator to the PMTs. On the other hand, this approach has important drawbacks: mainly the radiation damage to the distributor system and the impossibility of replacing distributor boards if a problem occurs, since access is possible only during LHC shutdowns. For the new HV system, two different approaches will be investigated. The first uses local voltage regulation based in the current system and needs further development in the monitoring and R&D to find radiation hard components. The second HV option uses a remote approach in which the HV boards are located in crates outside the ATLAS cavern. They can use standard electronics components since they are not located inside the detector but have the drawback of requiring long cables, so additional noise tests must be made. This option requires a new control and monitoring system. Both approaches will be investigated by implementing and testing realistic prototypes of the HV distributor boards and their control systems. They will be tested in a calorimeter environment, in a setup similar to the one of the ATLAS detector, using beams of high energy pions, electrons and muons at CERN. These tests will allow assessing the performance of the new TileCal electronics, which are being developed for the HL-LHC. Linearity, stability and noise of the developed system will be measured in the tests with beams of high-energy particles. Both HV options (remote and local) will be tested to evaluate their respective performance and characteristics, thus informing the choice of the best-suited HV system for TileCal in the HL-LHC era.