Missing transverse energy measurement in ATLAS detector: first LHC data results and importance for physics study

The Large Hadron Collider (LHC) at CERN started its operation at the end of November 2009, first at a centre-of-mass energy of 900 GeV, then, since March 2010, at 7 TeV. During this period the ATLAS experiment has collected a large number of proton-proton collision events, resulting up to now in an integrated luminosity of about 45 pb-1. A very good measurement of the missing transverse energy, ETmiss, is essential for many physics studies in ATLAS both for Standard Model channels, as W, Z bosons decaying to tau leptons or top quark decays, and for discovering channels. Events with large ETmiss are expected to be the key signature for new physics such as supersymmetry and extra dimensions. A good ETmiss measurement in terms of linearity and resolution is crucial for the efficient and accurate reconstruction of the Higgs boson mass when the Higgs boson decays to a pair of tau leptons. This thesis describes the first measurement of ETmiss in ATLAS with real data. The performance of the algorithm for ETmiss reconstruction has been widely tested in minimum bias and di-jet events that are not expected to have genuine ETmiss. In ATLAS ETmiss is calculated from the calorimeter cell energies, from reconstructed muons energy and from an estimation of the energy lost in dead materials. Since it is reconstructed from everything seen in the detector, ETmiss is a complex quantity: particles escaping because of the limited coverage of the de tector, presence of dead regions and different sources of noise can produce fake ETmiss . A lot of effort has been dedicated to understand well the tails in the ETmiss distribution and to apply selection cuts to remove fake ETmiss signals. As the ATLAS calorimeters are not compensating, that is the response to hadronic and electromagnetic particles is different, the calibration of the calorimeter cell energies is crucial. With 900 GeV data the ETmiss performance has been studied and it has been found as expected from simulation at the EM scale. With 7 TeV data the ETmiss calibration has been validated. ETmiss performance using different calibrations algorithms has been tested, confirming the good agreement between data and simulation found at the EM scale. The effect of pileup has also been studied in details. To complete the ETmiss commissioning the absolute scale determination is needed. This will be possible only when enough statistics of events with genuine ETmiss will be available. First, with the already available statistics, using the standard ATLAS W to lepton-neutrino selection, the ETmiss scale can be checked from the shape of the W transverse mass distribution that is calculated from the lepton and ETmiss and has only a few percent of background contamination. This method allows to set the ETmiss scale with a precision of 3% considering only statistical errors and using less than 50 pb-1 of data at 7 TeV. Then Z -->t autau events will be used where a Z boson decays into a couple of tau leptons, then a tau decays into a lepton and neutrinos and the other one into hadrons and a neutrino. It is possible to reconstruct the invariant mass of the two taus using the reconstructed lepton, the hadronic part of the tau (tau -jet) and ETmiss in the so-called collinear approximation. From the invariant mass peak position it is possible to determine the ETmiss scale. A detailed study has been done on simulated data to collect a very pure signal sample with low background contamination. With this method the ETmiss scale can be fixed with a precision of 6% with an integrated luminosity of about 100 pb-1 at 10 TeV. The first chapter of this thesis is dedicated to the physics at LHC: the theoretical motivation is briefly described. The importance of a very good ETmiss measurement is shown for the study of many physics channels, with particular attention to the Standard Model Z --> tau tau channel, the search for supersymmetry and the discovery channel A/H --> tau tau in the Minimal Supersymmetric Standard Model. The second chapter describes LHC and the ATLAS experiment focusing on the detector commissioning done with the first data at LHC. In the third chapter I document the results of ETmiss measurement with first ATLAS data. The description of ETmiss calculation in ATLAS is given detailing the different algorithms available, from the most basic to the mos t refined one. The different ETmiss calibrations are also described. The performance of ETmiss in data is compared to the simulation with careful understanding of the events in tails. The last chapter describes the preliminary results obtained with ATLAS data to determine the ETmiss scale from W --> lnu events and also a detailed Monte Carlo based study to set the ETmiss scale from Z --> tau tau events that will be used when at least 100 pb-1 of data will be available.