ATLAS-CONF-2010-096 October 15, 2010 Data-driven background estimation for the H τ + τ τ h search at 7 TeV with the ATLAS detector Ian Howley 7 December 2010 1
Motivation One of the primary LHC physics goals is to discover the Higgs boson. Higgs production through vector boson fusion, decaying into two taus τ + τ is a leading channel for discovery. τ + τ τ h accounts for 46% of the total branching ratio for tau-tau decay, and was the only decay considered for this study. A single tau decay is: τ ± ν τ ± ν e The primary hadronic tau decays are:
Data The data set is from March-July 2010 at s =7 TeV All ATLAS subdetector systems were operational comprising an integrated luminosity of 310nb -1.
Topology and Background The signal topology is a single electron/muon, low track multiplicity jet (from the hadronic tau decay), missing transverse energy neutrinos). (from three The study itself is an understanding of backgrounds to - QCD Jets H ττ. They are: - W and Z in association with jets - Top quark pairs - Single top quarks E miss T - Pairs of electroweak bosons (WW, WZ, ZZ)
Object Reconstruction and Trigger There are 5 principle objects to reconstruct for this analysis. Along with their requirements they are: - Electrons: p T > 10 GeV. Isolation requirement of the deposited energy in the EM calorimeter in a cone R=.2 to be less than 10% of the p T. - Muons: p T > 8 GeV and η <2.5. Similar isolation requirements as e - - Hadronic tau: p T > 20 GeV and η <2.5. Two algorithms optimized by MC samples were used. With more data a new identification and validation of taus will be implemented. Clusters in the EM and Hadronic Calorimeters Presence of charged tracks. - Jets: reconstructed using the anti-k T algorithm. R=0.4 - Missing E T : Based on energy deposited in EM Calorimter, η <4.5 An electron/muon trigger was used for the eτ/μτ final states. E T or p T > 10 GeV
Event Selection There are 4 requirements for event selection: - One Lepton: p T > 15 GeV. This suppresses - One Hadronic tau: Must have opposite sign to the lepton - Missing E T : > 20 GeV, for rejection of QCD jets and E miss T Z ee, µµ Z ee, µµ, t t, W t - Transverse mass: m T < 30 GeV to reduce W+ jets, and m T = 2p lep T Emiss (1 cos φ) T Δφ is the angle between the lepton and E Miss T in transverse direction The trigger efficiency with respect to this event selection is nearly 100% t t
Event Breakdown Candidates with Electron Candidates with Muon MC
Background Estimation Goal: to predict mass distributions by summing all background processes, using both data and MC. The dominant terms are QCD jets, and W+ jets. Tau id is difficult. Often the id has the wrong sign. The mass distributions should be independent of this sign. It is convenient to define: r = n OS n where OS (opposite sign) and (same sign) relate to the lepton, and hadronic tau candidate.
Background Estimation We can now calculate the number of expected OS events in the signal region n OS (m vis )=r QCD n QCD We can define the relative excess k W+jets and k other as r W +jets = (m vis)+r W +jets n W +jets (m vis )+r other n other (m vis ) nw +jets OS n W +jets =1+k W +jets r other = nother OS n other Assuming r QCD is 1 we find: =1+k other n OS (m vis )=n all (m vis)+k W +jets n W +jets + k other n other (m vis)
Determining k W +jets n W +jets
Determining k W +jets n W +jets Using the transverse mass distribution with a requirement of m T > 50GeV the number of OS an events are counted. The small background (n OS/ ) in this region is estimated using MC. n W +jets is determined by: where the final term is to consider the yield after the E T Miss requirement. k W +jets n W +jets =7.6 ± 7.8
Determining n all Data MC n all = 16 ± 5
Determining k other n other These are events from the remaining background, i.e. Z+jet, di-boson, and top quark single and pair production. It is entirely estimated using MC. The value is: k other n other =1.9 ± 0.5
Results Summing the 3 quantities we just calculated we have n OS (m vis )=n all (m vis)+k W +jets n W +jets + k other n other (m vis) 16 ± 5+7.6 ± 7.8+1.9 ± 0.5 = 25 ± 9 This agrees well with 29 observed events.
Systematic Uncertainties
Summary 12 Electron and 17 muon candidates were found in data with p T >20GeV Data driven background estimates were consistent at 25±9. This search provides a starting point for future Higgs searches in the H τ + τ τ h channel.