ATLAS-CONF October 15, 2010

Transcription

1 ATLAS-CONF 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

2 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:

3 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.

4 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)

5 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

6 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

7 Event Breakdown Candidates with Electron Candidates with Muon MC

8 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.

9 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)

10 Determining k W +jets n W +jets

11 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

12 Determining n all Data MC n all = 16 ± 5

13 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

14 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 ± ± ± 0.5 = 25 ± 9 This agrees well with 29 observed events.

15 Systematic Uncertainties

16 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.