Title Text. ATLAS Higgs Boson Discovery Potential

Transcription

1 Title Text ATLAS Higgs Boson Discovery Potential Isabelle Wingerter-Seez - LAPP - Annecy isabelle.wingerter@lapp.in2p3.fr Corfu Summer Institute September

2 Title Text ATLAS Standard Model Higgs Boson Discovery Potential Isabelle Wingerter-Seez - LAPP - Annecy isabelle.wingerter@lapp.in2p3.fr Corfu Summer Institute September

3 Where is the SM Higgs Boson? LEP direct search: m H > GeV EW fit favors m H < 163 GeV Recent CDF+D0 direct search combination (4 fb -1 ) excludes range GeV 2

4 How would the SM Higgs boson be produced? gluon fusion Vector-Boson-Fusion (VBF) associated production (W, Z) associated production (ttbar) Typical uncertainlties on total cross-sections gg 10% NNnLO VBF 5% NLO WH, ZH 5% NNLO tth 15% NLO 3

5 How would the Higgs boson decay? Text Width << detector resolution bb! m H < 130 GeV!!! m H < 140 GeV ""! m H < 150 GeV WW! 2m W < m H < 2m Z ZZ! m H > 130 GeV 4

6 Discovery & exclusion N: Number of selected events NB: Number of background events NB: Uncertainty on background S=(N-NB)/ NB Events/2 GeV peak width due to detector resolution σ M (Γ X <<σ M ) To maximize S Increase L (S~ L) X γγ Improve mass resolution σm (S~1/ σm) Increase signal efficiency, background rejection m γγ if S>5 (probability for a stat. fluctuation < 10-7 ) Discovery Otherwise, set an upper limit on Higgs boson cross-section 5

7 A signal emerging with time! Ldt = 0.1 fb -1 (year: 2008/2009) Higgs Pseudo-Data background! Expected Events:! N higgs ~2, N background =96 +/- 9.8! S/!B=0.2! No sensitivity to signal From B. Heinemann 6

8 A signal emerging with time! Ldt = 1 fb -1 (year: ~2009)! Expected Events:! N higgs ~25, N background ~960 +/- 30! S/!B=0.8! Still no sensitivity to signal 7

9 There it is!! Ldt = 30 fb -1 (year: 2011/2012?)! Expected Events:! N higgs ~700, N background = /- 170! S/!B=4.1! Got it!!! 8

10 Reminder: small cross-section Total inelastic cross-section:! ~ 100 mb bb production cross-section:! ~ 100 mb W (!l!) production cross-section:! ~ 10 nb Higgs (m H =150 GeV) cross-section:! ~ 10 pb Total! / Higgs! >

11 H γγ 10

12 H γγ: The Signature θ The difficulties: High background irreducible (2 γ in final state) reducible (π 0, jets) in final state Vertex is not known: pile-up & beam spot 5.6 cm long develop techniques to reconstruct it from the calorimeter use high pt track to select the high Q 2 interaction vertex Photons convert in the tracking detector 11

13 12 H γγ : Mass Resolution Photon conversions in the tracking detector

14 H γγ (10 fb -1 ): splitting the final states inclusive γγ S/B ~ 0.03 γγ + 1 jet S/B ~ 0.08 VBF + gg production γγ + 2 jets S/B ~ 0.4 VBF + gg production γγ+et miss +1lepton S/B ~ 2 tth, WH γγ+et miss S/B ~ 2 ZH, WH 13

15 Discriminating variables HIGGS PROSPECTS FOR THE DISCOVERY OF THE STANDARD MODEL HIGGS BOSON ] fb/ d cosθ* [ dσ / H γγ, 0 jets category (dots: MC, line: PDF) H γγ, 1 jet category (dots: MC, line: PDF) H γγ, 2 jets category (dots: MC, line: PDF) ATLAS 0.01] pb / d cosθ* [ dσ / ATLAS Background, 0 jets category (dots: MC, line: PDF) Background, 1 jet category (dots: MC, line: PDF) cosθ* cosθ* GeV] 1.5 fb / T,H dp dσ / [ ATLAS H γγ, 0 jets category (dots: MC, line: PDF) H γγ, 1 jet category (dots: MC, line: PDF) H γγ, 2 jets category (dots: MC, line: PDF) p [GeV] T,H GeV] 2 fb/ T,H dp dσ / [ ATLAS Background, 0 jets category (dots: MC, line: PDF) Background, 1 jet category (dots: MC, line: PDF) p [GeV] T,H Figure 11: Signal (left) and background (right) distributions of the Higgs boson decay angle, cosθ 14

16 Optimizing the description of the data: regions in the detector of constant resolution 2 η (4) [10.4%] 1.92 GeV (3) [12.4%] (2) [22.6%] 2.06 GeV 1.82 GeV (1) [22.6%] 1.47 GeV (6) [15.1%] 1.79 GeV (5) [10.8%] 3.27 GeV (2) [22.6%] 1.82 GeV (4) [10.4%] 1.92 GeV (7) [3.4%] (5) [10.8%] 3.18 GeV 3.27 GeV (8) [2.8%] 1.38 GeV (7) [3.4%] 3.18 GeV (6) [15.1%] 1.79 GeV η 1 2 η (4) [8.7%] 2.19 GeV (3) [10.2%] (2) [14.1%] 2.27 GeV 2.21 GeV (1) [13.2%] 1.79 GeV (6) [24.3%] 2.17 GeV (5) [14.6%] 3.37 GeV (2) [14.1%] 2.21 GeV (4) [8.7%] 2.19 GeV (7) [8.0%] (5) [14.6%] 3.46 GeV 3.37 GeV (8) [7.0%] 2.00 GeV (7) [8.0%] 3.46 GeV (6) [24.3%] 2.17 GeV η 1 15

17 H γγ: Significance 16

18 H ZZ * 4 leptons 17

19 H ZZ * 4 leptons 18

20 H ZZ * 4 leptons: significance 19

21 VBF H τ + τ - 20

22 H ττ: the reciepe Forward jets (tag) Higgs decay 2 high p T tag jets at large rapidity no color flow between tag jets implies a rapidity gap, thus the central jet veto effective to reduce backgrounds Higgs mass is reconstructed using the collinear approximation and the angle between the two! 21

23 22 Event selection Due to poor Higgs mass resolution for H ττ, inclusive analysis not possible Exclusive (VBF) searches: Reduce QCD backgrounds by using distinct topology of jets in association with Higgs Signal 2 high pt jets from quarks, at large η, no jets in between τ-pair from a resonnance leptonic & hadronic τ decays (ll, lh, hh) ET miss Background irreducible: Z+jets (Z ττ) reducible: W+jets, t-tbar+jets leptonic & hadronic τ decays (ll, lh, hh)

24 Reconstructing VBF events Mass Reconstruction collinear approximation: τ decay products go along the τ direction Mass resolution limited bt ET miss (8-10 GeV) & τreconstruction ( 10 GeV) Significance Counting events in the mass peak using a fit over the mass spectrum 23

25 Overall sensitivity with ATLAS 5σ discovery 20 fb -1 M H (GeV) 5 σ discovery 2 fb -1 : mh 160 GeV 3 fb -1 : 135<mH<190 GeV 20 fb -1 : probe down to mh 115 GeV 95%CL exclusions <2 fb -1 : region mh 2mW ~2 fb -1 : 120 < mh< 460 GeV ~3 fb -1 : probe down to mh 115 GeV 95% CL exclusion Systematic effects & uncertainties taken into account M H (GeV) 24

26 25 Conclusive remarks All possible SM Higgs mass range is well covered by several channels ATLAS has a good sensitivity to the SM Higgs already with the first fb -1 Many aspects not covered in this presentation: How to show that a signal is the SM Higgs boson Measurement of the spin and CP Measurement of the couplings And also the MSSM Higgs searches

27 ATLAS sensitivity to MSSM Higgs boson 300 fb-1 ATLAS 26

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