1. 1 Higgs decay into two photons with ATLAS JJC- Angers 22-11-2010 N. Andari (LAL Orsay) Supervisor: L. Fayard

2. 2 Plan  Higgs mechanism briefly  Theoretical and experimental limits on the Higgs mass  Higgs decaying channel into a pair of photons and importance of prompt photon study  Presentation of the ATLAS detector  Analysis and Results quoting a purity of prompt photon signal  Conclusion  Future and To Do List

3. 3 The beginning of the story … L=? 1- Gauge Symmetry  SU(3) C *SU(2) L *U(1) Y 2- Representations of fermions and scalars 3- SSB (relations between parameters)  One scalar with  2 < 0 So the most general renormalizable L : L = L Kin + L SSB +L Yukawa

4. 4 The Higgs mechanism (in fact the Brout-Englert-Guralnik-Hagen-Higgs-Kibble mechanism) From the kinetic term of the Higgs we get the mass for the gauge bosons : The Higgs mass is one of the SM parameter :

5. 5 Constraints on the Higgs mass Theoretical arguments: 1- Unitarity: m H < 700 GeV 2- Triviality: (if SM valid ~10TeV) m H <500 GeV 3- Vacuum stability: (if SM valid ~1TeV) m H >70 GeV Direct and Indirect Experimental limits: 1- LEP (direct search): m H >114.4 GeV at 95%CL 2- CDF & D0 (direct search at TeVatron) excludes the Higgs mass range between 158 and 175 GeV with 6.7fb -1 of integrated luminosity. 3- Indirect limits are derived from the precision measurement of EW observables. The Higgs mass is constrained by fitting the SM parameters {G F,m t,m Z,  QED,  EM }. The most recent probable value from  2 minimization give: m H =85 -28 +39 GeV

6. 6 LHC : Large Hadron Collider A pp collider : circonference 27km, 100 m underground (LEP tunnel) at CERN, designed to operate with nominal energy in the center of mass of 14 TeV. Four big Experiments: ALICE, ATLAS, CMS, LHCb First collisions in 2009 at 900 GeV, c.m. energy increased to 7 TeV in April 2010. Now running with heavy ions.

7. 7 Higgs production mode at the LHC Main process: Gluon fusion Higgs decay mode at the LHC Despite its small branching ratio the H  is a very important channel at low masses 100-150 GeV. It presents a very clean signature and has narrow width, although large associated background.

8. 8 H  channel For 7 TeV and 1 fb -1 we wait for : 20 events of H  with m H =120GeV (resolution~1.4GeV) ~8 500 of diphotons with 100<m  <150GeV 8M of single photons with pT>40GeV With the available statistics, we can start by looking at single photons events

9. 9 Prompt photon study Prompt photons include both « direct » + « fragmentation » photons, QED radiation off quarks. The main background comes from  0  The study of the background, for the moment, constitutes a very important step for preparing the analysis to the Higgs search : 1- study of the rejection of the background 2- study of direct photons important to understand pQCD, the measurement of the cross section could directly constrain the gluon parton distribution function. QED radiation off quarks

10. 10 Way to Reality … Beginning …

11. 11 ATLAS EM Calorimeter Inner Detector Detector

12. 12 Photon Reconstruction Clusters matched with one or two tracks originating from reconstructed conversion vertices in the inner detector = converted photons B = 2T  converted  into e + e - B = 2T Photon reconstruction is seeded by clusters in the EM calorimeter with transverse energies>2.5GeV, measured in the second layer of the calorimeter. Clusters without matching tracks = unconverted photons

13. 13 Importance of the EM calorimeter A Liquid Argon – Lead sampling calorimeter with an ‘accordion' geometry (3 longitudinal layers) The granularity of the 1st sampling  ~ 0.003 is very important to reject the main background for the photons coming from  0  where the photons emitted are close. The opening angle between the two photons of a pi0 of pT=40 GeV is  R>0.007 (larger than strip calo width )

14. 14  0 candidate

15. 15 Photon candidate

16. 16 Analysis and Results

17. 17 Luminosity recorded Luminosity known today to 11% (error dominated by knowledge of beam currents) Peak luminosity in ATLAS L ~ 2.1*10 32 cm -2 s -1 For most of the time an average number of pp interactions per crossing slightly larger than 1  max average 4 interactions per BC

18. 18

19. 19 Only with 15.8  1.7 nb -1

20. 20 But wait! The results are still interesting!

21. 21 Selection cuts 1- Events are required to have the good data quality. 2- Events are triggered using the first level (L1) calorimeter trigger. 3- Events are required to have a reconstructed primary vertex consistent with the average beam spot position and with at least three associated tracks. 3- The transverse energy of the photon candidate is required to be > 10GeV and the pseudorapidity region is |  |<1.37 and 1.52≤|  |<2.37 Photon Identification Shape variables (computed from lateral and longitudinal energy profiles of the shower in the calorimeter) are used to discriminate signal and background. Two sets of selection criteria (« loose » and « tight ») are defined, each based on independent requirements on several shape variables.

22. 22 Photon Isolation The prompt photons are expected to be more isolated from hadronic activity than the fake backgound from  0 (or other neutral hadrons) The isolation criterion is applied based on the amount of energy in a cone of radius  R=0.4 centered around the photon direction. (The contributions from 5*7 calorimeter cells aroind the photon barycenter are not included) The mean value of the small leakage from the photon outside this region is substracted from the measured isolation energy. This one is further corrected by substracting the estimated contributions from the underlying event. Candidates with reconstructed isolation < 3GeV are considered experimentally isolated.

23. 23 Signal extraction A non-negligible residual contribution of background candidates is expected in the selected photon sample after the application of the identification and isolation requirements. A data-driven method is used to estimate the background contribution and to measure the prompt photon signal yield. (2D sideband method) Two assumptions: The signal contamination in the three background control regions is small The isolation profile is the non-tight regions is the same as of the background in the tight regions. Approximate formula corrected by including signal leakage

24. 24 Photon Purity Only statistical errors included The purity is clearly increasing with transverse energy, as expected from MC.

25. 25 Systematic Uncertainties Combining the different effects, we estimate the total systematic uncertainty on the purity to be between 24 % and 6% as a function of the tranverse energy. Final Results

26. 26 Evidence of converted prompt photons Only converted photons into 2 tracks are considered. Tracks leave at least one hit in the Silicon part of the inner detector before reaching the TRT. Nice Evidence of the presence of prompt photons in the data

27. 27 Conclusion From 15.8 nb -1 of 7 TeV pp collisions collected with the ATLAS dectector, we could successfully exctracted a statistically significant prompt photon signal. For ET>20GeV, a signal yield of prompt photons with a purity of (72  7)% is measured, including statistical and systematic uncertainties. A first inclusive isolated cross section paper is in the pipeline … Evidence of prompt diphoton signal soon public and a first measurement will be ready by spring 2011.

28. 28 Near future analysis

29. 29 Beam back the 21st of February 2011 4-day technical stop every 6 weeks End of run ~ 12th December 200 days proton physics Reasonable numbers: 8 TeV (to be discussed) 936 bunches (75ns) 1.2*10 11 protons/bunch Peak Luminosity: 6.4*10 32 cm -2 s -1 11pb -1 integrated per day 200 days ~2.2fb -1 Ultimate reach: 8 TeV 1 400 bunches (50 ns) 1.5*10 11 protons/bunch Peak Luminosity: 2.2*10 33 cm -2 s -1 38 pb -1 integrated per day 200 days ~ 7.6fb -1 Future plan running of the LHC We will have interesting results

30. 30 The H  channel This figure shows the signal and background contributions to the expected di-photon invariant mass distribution for an integrated luminosity of 1fb -1. The signal contribution is enhanced by a factor 6.3 for a better illustration. 6.3

31. 31 Exclusion Limits at  s=7TeV The SM Higgs cross sections that are expected to be excluded at 95%CL for 1fb -1 and 7TeV are:

32. 32 Exclusion Limits at  s=7TeV: Combined channels The planned 1fb -1 at 7TeV allows a median expectation of exclusion of the SM Higgs boson between 129GeV and 460GeV. In the region near 115GeV, the H  +  - and H  bb channels contribute comparably to the H , however this last is clearly the most sensible.

33. 33 Exclusion Limits at  s=7TeV: Combined channels Left: The lower expected limit reduces by about 7 GeV with each doubling of the integrated luminosity, meeting the LEP bound at 5fb -1. The 2fb -1 line can be taken as indicative of what might be achieved by combining the results of 1fb - 1 analysed by each ATLAS and CMS. Right: 139<m H <180GeV: 1fb -1 would be expected to lead to evidence at this level at 3 . If 2 fb -1 becomes available then this region extends down to 131GeV and in addition there is close to 50% chance of 3  evidence for a H with mass between 200 and 430 GeV.

34. 34 To Be Continued …

35. 35 Thank you for your attention!