Search for a Low Mass Standard Model Higgs Boson with the ATLAS Detector at the LHC

. In 2011, the ATLAS experiment collected 4.9 fb -1 of proton-proton collision data at √ s = 7 TeV provided by the Large Hadron Collider. This data set allowed the search for a Standard Model Higgs Boson up to 1 TeV. In this paper, the focus is put on the searches of a low mass Higgs in the range (110-150) GeV. Using the 2011 ATLAS dataset, the combination of all the channels showed that a SM Higgs boson was restricted to live in the region (116.6-119.4) GeV and (122.1-129.2) GeV. Importantly, an excess of events has been observed around 126 GeV with a statistical significance of 2.9 standard deviations. The present document describes the final results published by ATLAS based on the 2011 dataset alone.


INTRODUCTION
The Standard Model (SM) of particle physics is the model of elementary particles and fields that describes successfully the results in particle physics produced by several experiments since more than 50 years. Nevertheless, one of the key ingredients of this model, the Higgs mechanism introduced to explain the Electroweak Symmetry breaking [2,3,4,5,6], has not been verified experimentally. The main prediction of this mechanism is the existence of a new spin 0 boson called the SM Higgs boson.
After intensive but inconclusive searches of this new boson at LEP [8,9] and TeVatron [10], the Large Hadron Collider (LHC) at CERN (Geneva) is now the machine at which this research is ongoing [10,11]. In the present paper the search of a low mass SM Higgs boson using the ATLAS detector is reported based on the full 2011 dataset [1].

LHC OPERATIONS AND ATLAS DATA TAKING IN 2011-2012
The ATLAS detector is a multipurpose experiment [12,13,14] installed on the LHC [15] at CERN, Geneva. LHC provides high intensity proton beams that collide at the center of the detector. In 2011, ATLAS collected a dataset of 4.9 fb -1 (after quality selection) at √s = 7 TeV.

LOW MASS HIGGS SEARCH AT ATLAS IN 2011
At the LHC, SM Higgs boson is produced through several production mechanisms (gluon-gluon fusion, vector boson fusion and associated production) and it decays to a large number of final states whose branching ratios depend on the hypothesized Higgs boson mass. The channels sensitive to a low mass SM Higgs boson (110 GeV < m H < 150 GeV), because of a favorable production cross-section times branching ratio (σxBR) and/or a high signal over background ratio (S/B), are given in Table 1. In this paper is presented a combination of the results of the search of a SM Higgs in several channels, using up to 4.9 fb -1 of data taken in 2011. Each channel is analyzed using categories based either on event physical properties or on detector performance considerations (number of jets, localization of leptons/photons in the detector…) to better optimize the analysis sensitivity [1]. Among all those channels, two can provide an accurate mass measurement of a potential Higgs boson candidate, namely the diphoton and four leptons final states, thanks to the precise measurement of the full final state provided by the Inner Detector, the Electromagnetic Calorimeter and the external Muon Spectrometer.
The 4-lepton channel has a large S/B but a small event rate whereas the diphoton final state has a S/B of few percent and a sizable statistics at the integrated luminosity delivered by the LHC up to now. The WW (*) channel is characterized by a large event rate but provides very little information about the mass itself because of the presence of undetected neutrinos in the final state that degrades the WW (*) system mass reconstruction. Other channels are included but are less informative with the analyzed data sample since they have neither a large σxBR nor a very good S/B. In these analyses, backgrounds are measured from data whenever possible or Monte Carlo (MC) based estimated using up-to-date theoretical predictions. The accuracy of MC predictions is usually validated in signal free control regions to access systematics. A huge effort has been put to estimate all the possible systematic effects that could affect the measurement as described in details in [1] including background modeling. Figure 1 presents the key distributions of the two precision channels, namely the diphoton invariant mass (1-a) and the 4-lepton invariant mass (1-b), and exhibits a modest excess of events around m H =125 GeV. The statistical significance of this excess is estimated using a maximum likelihood fit method [1] where the magnitude of the signal yield (µ) in units of SM cross-section is computed for each possible mass.   Figure 3-a shows the expected limits over the full mass range whereas Figure 3-b shows the observed limits in the low mass region. More data are needed to measure with accuracy the physics properties of this new particle, to understand if this is indeed the Higgs boson predicted by the Standard Model.