Studies of Higgs Potential (Higgs Boson Self Coupling Measurements)

Transcription

1 Studies of Higgs Potential (Higgs Boson Self Coupling Measurements) Theory of Higgs boson pair production Searches for Higgs boson pair production Constraint on the Higgs boson self coupling Jianming Qian ( 钱剑明 ) University of Michigan Workshop on Physics Beyond the Standard Model Shanghai Jiao Tong University, July 1 3, 2018

2 The Higgs Potential Everything we know about the Higgs boson is based on this potential Electroweak symmetry breaking: h 2 1 V m h h h h mass term self interactions 2

3 Higgs self coupling After Electroweak Symmetry Breaking: 1 V mhh h h Measured! hhh production hh production The Higgs self coupling can be probed by hh production at the LHC Non resonant hh production 2 m h 2 In SM: =

4 Higgs boson pair production gg hh dominates arxiv: and LHC Higgs XS group At s 13 TeV for m 125 GeV h 5% Th. SM 4.3% hh fb gg % scale 2.3% S 2.1% PDF Compared with scale PDF+ SM gg h pb S 4

5 Higgs boson pair production Negative interference between the two diagrams: fb t NLO m t t t For 1, the cross section is minimum at 2.46 t SM arxiv: arxiv:

6 Beyond Standard Model Extensions to the SM Higgs sector will result in new Higgs boson pair production through resonant processes Singlet model addition of a scalar singlet S m 2 3 V S S S 2 2 Resonant hh production Two Higgs doublet model 2HDM addition another Higgs doublet V m m m , These models predict more than one CP even neutral Higgs bosons. The heavier one H can decay to a pair of light one h : H hh Other SM extensions such as Randall Sundrum graviton models predict Higgs boson pair production through Kaluza Klein gravitons: GKK hh 6

7 Higgs boson pair final states arxiv: Final states with the largest statistics tend to be most challenging due to trigger limitation large backgrounds 7

8 (X )hh bbbb Pros: BRhh bbbb33.6% high rate two hbb resonances, X hh resonance in BSM full X hhbbbb reconstruction Cons: full hadronic final states challenging triggers large multijet continuum backgrounds estimate from data Key to the analysis: triggers performance, b jet identification, di b jets mass resolution, background estimate 8

9 (X )hh bbbb High trigger thresholds due to large multijet backgrounds efficient for high jets and high mass events. p T h bb decay could be highly boosted: 2mh Rbb h pt h For pt 2 mh, the 2 b jets are often reconstructed as a single large radius jet. development of both resolved and boosted analysis strategy Resolved: 4 small R jets For m 1 TeV X j Boosted: 2 large R jets For m 1 TeV X J 9

10 (X )hh bbbb Resolved analysis: 4 R0.4 b jets with pt 40 GeV 2 pairs with mbb consistent with m based on R a nd m bb bb h Boosted analysis: 2 R1.0 jets with pt GeV and mj consistent with mh 4 R 0.2 track jets for b tagging 3 categories: 2/3/4 b tagged track jets Use data control region and sideband to estimate multijet background ATLAS: arxiv: Side band Control region Signal region 10

11 (X )hh bbbb Results Resolved Fit the m hh distribution to extract the potential signal Limit on the SM hh production observed expected Limits on a narrow scalar S hh Boosted ATLAS: arxiv:

12 (X )hh bb Pros: BRhh bb 7.3% modest rate Multiple resonances: hbb and h, X hh in BSM decay signatures facilitate triggers Cons: undetected neutrinos from decays partial X hhbb reconstructions reconstruction and identification, background estimates,... Three distinct h decay final states: h 4 (12%) h h 3 (46%) h 2 (42%) h h final state suffers from the large Drell Yan background, reconstructing hadronic decay is a key: low charge multiplicity collimated energy deposits 12

13 (X )hh bb 2 b jets and 2 's to form respective hbb and h candidates 2 final states studied: lep had and had had calculate mass using Missing Mass Calculator (MMC) main backgrounds: fake 's, Z and tt use BDT to improve signal background separation m Typical 15% m 13

14 (X )hh bb No significant excess was observed. Fit the BDT distributions to set upper limits on hh production. lep-had had-had For the SM non resonant hh production: hh bb fb In progress: constraints on the Higgs self coupling best limit 14

15 (X )hh bb Pros: clean signature with 2 photons trigger signature and 2 b jets full X hhbb decay reconstruction resonances in h and hbb, good m resolution X hh resonance in BSM Cons: small BR hh bb 0.26% ATLAS: arxiv:

16 (X )hh bb 1 and 2 b jets signal categories, use 0 b jet events to validate background estimates overlapping tight and loose kinematic selections, choice based expected sensitivity Non resonant: require 80 mjj 140 GeV, fit m distribution Resonant: cut on both m and m, fit the m distribution Non resonant jj jj Resonant m m jj 16

17 (X )hh bb Limits on the SM non resonant hh production gg hh % CL SM Limits on the self coupling multiplier observed: 8.2, 13.2, expected: 8.3,13.2 BR BR (my estimate) 5.4 SM SM gg hh X vs Limit on B X hh vs m X 17

18 Non Resonant hh Production bbbb, bb, bb are the most sensitive with comparable sensitivities. Small improvements can be expected from other final states Combination and limits on the self couplings are in progress 18

19 CMS Self Coupling Results CMS: arxiv: hh bb Limits on non resonant hh production: hh bb 75.4 fb Limit on self coupling read off the plot : for 1 t SM 10 (my estimate) CMS: arxiv: hh bb Limit on non resonant hh production: observed expected Limit on the self coupling: SM SM 7 (my estimate) No public results from hh bbbb 19

20 hh bb Projection Expect to see an 1.5 signal of SM hh production with 3000 fb Constraining the Higgs self coupling in the range of % CL SM 1 ATLAS ITK (my estimate) SM 20

21 hh bbbb Projection The hh bbbb analysis will be systematics limited at HL LHC. 1 Expected limits for 3000 fb : SM SM with without the current level systematics ATL PHYS PUB SM ATLAS ITK

22 Expectation from LHC Three final states hh bbbb, bb, bb of comparable sensitivities with the current dataset: 510 IF at HL LHC, each of them can achieve a sensitivity of SM 1.5, their combination will result in: 09. Combining the two experiments: sensitive final states, a measurement of 0.5 SM is possible from the HL LHC. SM SM SM 0.6. Including other less Snowmass Higgs Report: arxiv: This projection is worse than the estimation of the US Snowmass study in

23 e + e Colliders Self coupling s m 2 m GeV Z h ILC + CLIC Projections of Snowmass studies in 2013 arxiv:

24 Summary Higgs boson self coupling measurement is difficult. Final states of hh bbbb, bb, bb are found to be most sensitive final states The precision of the current measurement per final state is for 6 final states SM SM and the expected precision from the HL LHC is 0.5 SM Future colliders are needed to improve the precision. For example, it is estimated that FCC hh can measure with a 3% precision! 24

25 Additional Slides

26 (X )hh bb Pros: clean signature with 2 photons trigger signature and 2 b-jets full X hhbb decay reconstruction resonances in h and hbb, good m resolution; X hh resonance in BSM Cons: small BR hh bb 0.26% CMS: arxiv:

27 CMS (X )hh bb Background dominated by the +jets continuum production. Use MVA to improve signal-background separation. High and medium purity categories based on MVA output, low and high mass regions based on High mass: MVA 0.97 (high purity), 0.6 MVA 0.97 (low purity) m jj Low mass: M High mass: M X X 350 GeV, 350 GeV CMS: arxiv: M m m m m m X jj h h jj 27

28 CMS (X )hh bb 2D fits to the m m distribution to extract the potential signal. m bb : GeV, m : GeV bb Model signal with double-sided Crystal Ball function and the +jets continuum background using Polynomials CMS: arxiv: Example: high-mass region and high-purity category 28

29 CMS (X )hh bb Fix 1 SM, varying t Limit on the SM pp hh production observed expected Limit on the Higgs boson self-coupling 11 95% CL SM 29

30 CMS (X )hh bb Limits on non-resonant hh production hh bb 75.4 fb Limit on the self-coupling from the plot: for 1 t Limits in t plane: allow both top-yukawa and Higgs self-coupling to vary Limits from hh bb analysis alone CMS: arxiv:

31 CMS (X )hh bb 31

32 Resonant hh Production 32

33 H Analysis: ditau Mass Ditau mass reconstruction using the Missing Mass Calculator: Solving the unconstrained system by assuming neutrinos are in the direction of the visible tau decay products; Weight solutions based on Etmiss resolution and decay topologies; Return the most probably ditau mass value Typical mass resolution 15%. 33

34 34 ATLAS and CMS Run 1 Summary 2 2 2, 2 2, 2 f Hff V HVV f Hff F V V HVV m g m g m g m g ATLAS & CMS: JHEP 08 (2016) 04 Measured couplings are very Standard Model like