Search for SM Higgs Boson at CMS

Transcription

1 Search for SM Higgs Boson at CMS James D. Olsen Princeton University (on behalf of the CMS Collaboration) Higgs Hunting Workshop July 29, 2011 Documentation: HIG (click on Higgs Physics Analysis Summaries)

2 First July 29, 2011 J. Olsen - Higgs Hunting Workshop 2 Dear LHC, Thank You!!! LHC delivered 1.49 fb -1 CMS recorded 1.38 fb -1 (93% operating efficiency) Intense and dynamic luminosity profile: (Apr Jul :: 2e32 2e33) Higgs trigger challenge: Rising lepton thresholds Tightened isolation Addition of cross triggers Higgs searches reported here use fb -1

3 July 29, 2011 J. Olsen - Higgs Hunting Workshop 3 SM Higgs CMS Search in three general regions between GeV Low Mass ( ): H gg, tt, bb, WW(2l2n), ZZ(4l) Medium Mass ( ): H WW(2l2n) High Mass ( ): H ZZ(4l), ZZ(2l2n), ZZ(2l2j), WW(2l2n) Channels included in this combination: Mode Mass L (fb -1 ) Doc Talk gg HIG M. Bluj (Thurs) tt HIG ZZ(2l2n) HIG E. Di Marco (Fri) ZZ(2l2j) HIG ZZ(4l) HIG WW(2l2n) HIG A. Benaglia (Fri) Critical parameters for sensitivity: Signal strength (plot above) Mass resolution Background level and composition Are bkgs controllable from data?

4 July 29, 2011 J. Olsen - Higgs Hunting Workshop 4 Statistical Methods: limit setting CMS uses the CLs method to set limits on μ = σ/σ SM Frequentist approach including systematic error evaluation Likelihood function: Observed Systematics Test statistic: Expected Run toys (lots of toys!), calculate: fix m, vary θ μ vary μ and θ CLs = Then the 95% C.L. on m is the value that gives CLs = 0.05 q μ

5 Statistical Methods: significance July 29, 2011 J. Olsen - Higgs Hunting Workshop 5 Would like to start quantifying observed excesses Use same machinery described on the previous slide, but now try to quantify the probability of the null hypothesis Approximate local p-value: obs q 0 Where is the observed value of q μ in the background-only scenario (m = 0) Significance Z from p-value: So that p ~ 2% gives Z ~ 2σ, etc. q 0

6 July 29, 2011 J. Olsen - Higgs Hunting Workshop 6 Caution: bumps ahead The look-elsewhere effect (LEE) is significant in Higgs searches Higgs mass is unknown a priori Each channel can represent many effective measurements (Trial Factor) If errors are correctly estimated, ~2% of measurements will fluctuate up by 2s or more expect to see bumps! Resolution is the key parameter For a given search window: higher resolution larger LEE Local p-value plot will show features with frequency driven by resolution P-value plot: roadmap Is it consistent with the SM Higgs? (here σ ~ 3 σ SM ) Is there a bump? (here p-value ~ 2%) LEE in modes with high resolution gives trial factors O(20-100)

7 Channel-by-channel Results July 29, 2011 J. Olsen - Higgs Hunting Workshop 7

8 Low Mass: H gg July 29, 2011 J. Olsen - Higgs Hunting Workshop 8 All channels resolution ~ 1-2% No significant excess is found Observed exclusion limits: 2-7 x SM One < 2s excursion near 120 GeV High resolution trial factor ~ 20 Probability to see a fluctuation of this size or larger ~60%

9 July 29, 2011 J. Olsen - Higgs Hunting Workshop 9 t e t h (pp H) Low Mass: H tt t m t h (VBF) resolution ~ 20% No significant excess is found Observed exclusion limits: 10 x SM Many improvements in the pipeline Uniform sensitivity Low resolution trial factor ~ 1

10 High Mass: H ZZ 2l2n July 29, 2011 J. Olsen - Higgs Hunting Workshop 10 2m2n 2e2n resolution ~ 15% No significant excess is found Observed exclusion limits: 1-4 x SM One 2s excursion near 300 Gev Medium resolution moderate LEE Would prefer s > 2s SM if real

11 High Mass: H ZZ 2l2j July 29, 2011 J. Olsen - Higgs Hunting Workshop 11 0 b-tags 2 b-tags resolution ~ 10% No significant excess is found Observed exclusion limits: ~2-10 x SM One 2s excursion near 500 GeV Medium resolution moderate LEE Would prefer s ~ 3s SM if real

12 High and Low Mass: H ZZ 4l July 29, 2011 J. Olsen - Higgs Hunting Workshop events selected: 3 in 4e 6 in 4m 6 in 2e2m 14.4 ± 0.6 expected For M 4l < 2M Z : expect 1.9 ± 0.1 and see 6 resolution ~ 1-2% No significant excess is found Observed exclusion limits: x SM Three ~ 2s excursions for M 4l < 2M Z High resolution trial factor O100 Bumps at 120/160 prefer s >> s SM Higgs at ~140? More data will tell

13 All Masses(!): H WW 2l2n July 29, 2011 J. Olsen - Higgs Hunting Workshop 13 em (0 jets) MVA analysis: Signal at 160 excluded by eye Good agreement in shape for data vs. MC background 0, 1, 2-jet bins resolution ~ 20% No significant excess is found Observed exclusion: GeV One broad ~2s excursion: Low resolution trial factor ~ 3 Bumps smeared by effective resolution Inconsistent with SM Higgs < 120 (> 150)

14 Summary: individual results by channel July 29, 2011 J. Olsen - Higgs Hunting Workshop 14 Low mass: Driven by gg and WW, assisted by tt Medium mass: WW alone High mass: Driven by ZZ, assisted by WW M H (GeV)

15 Combined Results July 29, 2011 J. Olsen - Higgs Hunting Workshop 15

16 Combined Results: s/s 95% CL Good agreement between Bayesian and CLs limits Excess at low mass driven by WW, with a wink from ZZ(4l) Expected exclusion region: Gev Observed exclusion regions: [ ] and [ ] July 29, 2011 J. Olsen - Higgs Hunting Workshop 16

17 Summary of p-values and s/s SM July 29, 2011 J. Olsen - Higgs Hunting Workshop 17

18 July 29, 2011 J. Olsen - Higgs Hunting Workshop 18 Limits on Heavy 4 th Generation SM Two more heavy quarks in the loop Enhances s up to an order of magnitude over the SM Assuming 4 fermion generations, we exclude the SM4 Higgs boson over a large mass range: % C.L. For details, including caveats, see LHC Higgs Cross Section WG: LHCPhysics/SM4At7TeV

19 Summary July 29, 2011 J. Olsen - Higgs Hunting Workshop 19 CMS has searched for the SM Higgs boson over the mass range GeV No significant excesses are observed 95% C.L. exclusion regions are obtained in the medium mass range [ GeV], and in the high-mass range [ GeV] Reinterpreted in a model with a 4 th heavy fermion generation, a large region [ GeV] is excluded at 95 C.L. A few moderate excursions from the backgroundonly hypothesis are seen; more data is necessary to fully interpret these results (LHC is helping!) Combined LHC limit expected at Lepton-Photon

20 Backup July 29, 2011 J. Olsen - Higgs Hunting Workshop 20

21 CLs Values vs. M H 90% C.L. exclusion: GeV July 29, 2011 J. Olsen - Higgs Hunting Workshop 21

22 Systematic Uncertainties (I) July 29, 2011 J. Olsen - Higgs Hunting Workshop 22 A. Korytov (EPS 2011)

23 July 29, 2011 J. Olsen - Higgs Hunting Workshop 23 Systematic Uncertainties (II) List of Correlated Systematic Uncertainties