Studies and Prospects of Higgs searches

Transcription

1 Studies and Prospects of Higgs searches Luca Malgeri First Joint METU-IPM Conference on LHC Physics ICTP-ECAR - Izmir - Turkey 29/9/2015-3/10/2015 L. Malgeri - METU-IPM Higgs results and prospects 1

2 A new star is born Birth date: July 4th 2012 Birth place: Geneva (LHC) Parents (SM) and child are in perfect shape L. Malgeri - METU-IPM Higgs results and prospects 2

3 And everybody was waiting for it From Inspire: total # of papers with Higgs in title: # paper from CMS: 58 # paper from ATLAS: 74 LHC starts Discovery L. Malgeri - METU-IPM Higgs results and prospects 3

4 since a long time.. Last edited Dec 2011 L. Malgeri - METU-IPM Higgs results and prospects 4

5 A more recent history: LHC Run 1 PRD 89 (2014) ,EPJC 74 (2014) 3076, EPJC 75 (2015) 212 From PDG Summer 2011: nothing yet End of 2011: some hint Summer 2012: discovery! End of 2012: confirmation! L. Malgeri - METU-IPM Higgs results and prospects 5

6 σ [pb] Production Cross Section, Higgs is part of the SM measurements now July 2015 W+jet W 1j 2j 3j 4j Z 1j Z+jet 2j 3j All results at: 4j Wγ di-boson Zγ WW WZ ZZ EW γγ EW WVγ tt 1j 2j 3j t qqz WW ssww t-ch tw t s-ch W lν, Z ll, l=e,µ EW qqw CMS 95%CL limit CMS Preliminary ttγ ttw ttz -1 ) -1 7 TeV CMS measurement (L 5.0 fb 8 TeV CMS measurement (L 19.6 fb ) 7 TeV Theory prediction 8 TeV Theory prediction tt+jet t +V Higgs ggh VBF VH tth qqh Th. σ H in exp. σ L. Malgeri - METU-IPM Higgs results and prospects 6

7 Outline Why Higgs Recent highlights: ATLAS+CMS mass combinations ATLAS+CMS couplings combination Other results: Spin/CP studies Invisible Higgs Width Differential cross sections Future Higgs (Run2, Run3, HL-LHC) Conclusions For BSM Higgs studies: A. Nikitenko talk For specific ATLAS Higgs studies: T. Cuhadar talk L. Malgeri - METU-IPM Higgs results and prospects 7

8 Why were/are we so obsessed by the Higgs? Self consistency of SM theory if MH would be 300 GeV, p=3x10-5 Vacuum structure of the Universe Its relation to Physics BSM L. Malgeri - METU-IPM Higgs results and prospects 8

9 Higgs, Top and vacuum stability arxiv: Higgs potential at large If there is the desert between EW scale and Planck scale, it s life on the edge (of metastability). Why so? Current WA L. Malgeri - METU-IPM Higgs results and prospects 9

10 Higgs and naturalness We know that the SM is an effective low energy theory that breaks down at Planck scale. In practical terms: Corrections for elementary scalar are quadratic in new-physics cutoff Λ Main loop corrections from strongest coupled particles: top, W, Z and Higgs self-couplings: m 2 H = v 2 (4m2 t 2m 2 W 4m 2 Z m 2 H) 2 Need a really fine tuning of the parameters to get the Higgs mass so low (quantum correction akes it diverge!) 10% fine tuning If naturalness is taken as a criterium, a light Higgs (125 GeV) suggests that new physics should be maximum at the level of few TeV to keep fine tuning at 1%. If Λ is Mpl, tuning becomes 10-34!!!! 1% fine tuning L. Malgeri - METU-IPM Higgs results and prospects 10

11 A very short reminder/legenda SM Higgs production SM Higgs decays ggh VBF VH tth mh=125.09±0.24 GeV (see later) L. Malgeri - METU-IPM Higgs results and prospects 11

12 Recent highlights from Higgs analyses The task of compiling a consistent list of recent results on Higgs studies has been made easy by ATLAS and CMS: two major milestones recently made public. Mass combination (PhysRevLett ) Couplings combination (ATLAS-CONF , CMS-PAS-HIG ) Based on legacy results from the two collaborations: CMS, mass and couplings: Eur. Phys. J. C 75 (2015) 212 ATLAS: Mass: Phys. Rev. Le+. 114, Couplings: arxiv: L. Malgeri - METU-IPM Higgs results and prospects 12

13 Higgs mass Most precise channels used: H γγ and H ZZ fully reconstructed mass peaks high resolution enough sensitivity with Run 1 statistics EPJC 74, 3076 PRD 90, L. Malgeri - METU-IPM Higgs results and prospects 13

14 Higgs mass: inputs In general, couplings analyses are used as starting point. ATLAS H γγ has been designed specifically for the mass measurement. H γγ event categorizations based on quality of photons event vertex chosen according to MVA techniques sgn+bkg shapes: Gauss/CB + data driven H ZZ event categories based on final states (4e, 4µ, 2e2µ, 2µ2e) dominant bkg: diboson production lepton calibration systematics negligible w.r.t. statistics Systematics: dominant energy scale for e,µ and γ subdominant Jet energy scale negligible theory JINST 10 P08010 Phys. Rev. D. 90, L. Malgeri - METU-IPM Higgs results and prospects 14

15 Higgs mass: systematics Theory systematics Other exp systematics Lepton momentum scale/resolutions L. Malgeri - METU-IPM Higgs results and prospects 15

16 Higgs mass: results The ATLAS+CMS combination technique is based on profile likelihood ratio: Cross-section signal modifiers are left to vary independently Nuisance parameters (~300 in total) take into account all systematics and their correlations. Note: correlation among experiments as well as internal! L. Malgeri - METU-IPM Higgs results and prospects 16

17 Higgs mass: results mh= ± 0.24(±0.21 ± 0.11)GeV Full consistency: between experiments inside experiments between decay channels L. Malgeri - METU-IPM Higgs results and prospects 17

18 Higgs signal strength and couplings The experienced gained in the mass combination paved the way for a larger step: couplings! Use as input the published results from ATLAS and CMS (in some cases improved with latest greatest analyses) The mass is fixed to the combined mass value (improves SM couplings measurements) L. Malgeri - METU-IPM Higgs results and prospects 18

19 Higgs strength and couplings The inputs from the different ATLAS and CMS analyses are used in a global fit (profile-likelihood ratio) Signal strength modifiers used both for production and decays: Note: other production modes are included in the fit but not explicitly tagged (i.e. are part of a category above) L. Malgeri - METU-IPM Higgs results and prospects 19

20 Higgs strength and couplings Inputs to the fit: most sensitive final discriminants distributions, usually: - mass distribution for fully reconstructed channels - BDT output for more complex topologies Systematics Four main categories: finite MC statistics (uncorrelated between experiments) Signal theory normalization uncertainties (largely correlated) Bkg theory uncertainty (largely correlated) Experimental uncertainty (largely uncorrelated) Total of 4200 nuisance parameters used in the global fit L. Malgeri - METU-IPM Higgs results and prospects 20

21 Higgs strength and couplings Signal strength A single parameter fit assuming either BR or cross section SM-like, or two signal strength modifiers for bosons and fermions productions mechanism. µ f =1 µi=1 L. Malgeri - METU-IPM Higgs results and prospects 21

22 Higgs strength and couplings Testing compatibility with µi and µ f = 0 Combining the two experiments we reach 5σ in VBF production and H ττ decay mode. We also see a tth excess of 2.3σ w.r.t. SM (and a deficit in H bb) L. Malgeri - METU-IPM Higgs results and prospects 22

23 Higgs couplings and new physics The LHC Higgs cross section working group proposed a framework to test deviations from SM by modifying the couplings by a multiplicative modifier k: In this context BRBSM is either non-sm decay or undetected decays Loops can be resolved in effective k s: k s that are not accessible are just assumed to be : kc=kt, kµ=k τ,ks=kb To first order we are not sensitive to sign(k), but we can access the sign through interference. For example: * all interference terms contain top couplings. Its sign its arbitrary in the fit (set >0 from now on) L. Malgeri - METU-IPM Higgs results and prospects 23

24 Higgs couplings to fermions and bosons Assuming universal coupling modifiers for fermions and bosons Global Per decay channel Negative kf disfavored L. Malgeri - METU-IPM Higgs results and prospects 24

25 Higgs couplings to fermions and bosons Or simply test scaling of coupling with mass (Yukawa coupling behaviour) Assuming only SM particles in loop Just a different (and more appealing) way to plot the same L. Malgeri - METU-IPM Higgs results and prospects 25

26 Does the Higgs couples to BSM? How much room we have for beyond SM Higgs decay? use effective couplings for γ and g, i.e. allow new particles in the loop allow BRBSM to increase the Higgs width Given that we really measure only σxbr, some assumption should be made on ΓH (or BRBSM) kv 1 (e.g. 2HDM) BRBSM=0 SM H H(~apple )= apple2 H (~apple ) 1 BR BSM i BR f = i (~apple ) f (~apple ) H(~apple ) BR BSM < 0.34 at 95% C.L. L. Malgeri - METU-IPM Higgs results and prospects 26

27 Many more studies than just mass and couplings! L. Malgeri - METU-IPM Higgs results and prospects 27

28 Higgs spin/cp fits Bosonic channels are the most sensitive: ZZ is fully reconstructed, polarisation derived from kinematics of decay products γγ fully reconstructed but polarisation is not accessible (pt and cosϑ* give some sensitivity) WW not fully reconstructed but kinematic variables (Mll, Δφ and MT) sensitive to Higgs spin/cp Global fits are are usually based on comparisons of kinematic variables vs matrix elements calculations L. Malgeri - METU-IPM Higgs results and prospects 28

29 Higgs spin/cp fits The SM hypothesis (0+) is tested against many alternative ones. J P =0 -, 1 ±,2 ± excluded at more than 3σ level CP mixture and alternative tensor structure also tested w/o any sign of non-sm behaviour CMS: PRD 92 (2015) ATLAS: arxiv: L. Malgeri - METU-IPM Higgs results and prospects 29

30 Higgs width - 1 Direct limits from H γγ and H ZZ are limited by detector resolutions and not sensitive to the O(MeV) region: ATLAS: 2.6 GeV (6.2 GeV exp) in ZZ 5.0 GeV (6.2 Gev exp) in γγ) CMS: 1.7 GeV (2.3 GeV exp) (γγ and ZZ combined) But ingenuity came to rescue: off-shell vs on-shell production is sensitive to Higgs total width (under certain assumptions/sm dependence) Their ratio depends directly on ΓH, provided couplings ratio are unchanged on-shell and off-shell. Theory Exp. JHEP (2012), PRD (2013) JHEP (2014) PRD 92 (2015) PLB 736 (2014) 64 Limits: CMS: 22 (33) MeV ATLAS: 23 (33) MeV L. Malgeri - METU-IPM Higgs results and prospects 30

31 Higgs width - 2 (an interesting curiosity)..ingenuity calls more ingenuity.. Recently few theorists have also realised that we had already a very large sample of off-shell Higgs, even though not at high mass but a low mass: LEP! arxiv: From a simplified fit: R h = H SM 7.7 MeV 3 MeV L. Malgeri - METU-IPM Higgs results and prospects 31

32 Higgs prospects Guiding principle: Stirling plot, cross sections increase by factor /14 TeV (up to a factor of 4 for tth) 1fb -1 equivalent reach to Run1 2 fb -1 equivalent reach to Run1 10 fb -1 equivalent reach to Run1 L. Malgeri - METU-IPM Higgs results and prospects 32

33 Higgs prospects LHC timeline in terms of integrated luminosity L. Malgeri - METU-IPM Higgs results and prospects 33

34 Higgs 300 fb -1 Higgs Physics expectations ~5 M Higgs events produced ~50 K events useful for precision measurements (x 40 w.r.t. now) Physics subjects Higgs precision measurement - Mass (100 MeV reachable?) - Cross-sections - Couplings H μμ might be measured at 30% level Possible (but very difficult) W L W L scattering? CMS and ATLAS white papers: arxiv: and Assumptions on systematic uncertainties: Scenario 1: no change Scenario 2: theory unc. / 2, rest goes like 1/ L L. Malgeri - METU-IPM Higgs results and prospects 34

35 Higgs 3000 fb -1 Higgs Physics expectations 50 M Higgs events produced 500 K events useful for precision measurements (x 400 w.r.t. now) Physics subjects Higgs precision measurement - Mass (100 MeV syst. limited?) - Cross-sections - Couplings H μμ might be measured at 10% level? - di-higgs (bbγγ and bbττ) up to 2σ Possible (but very difficult) W L W L scattering? CMS and ATLAS white papers: arxiv: and Assumptions on systematic uncertainties: Scenario 1: no change Scenario 2: theory unc. / 2, rest goes like 1/ L L. Malgeri - METU-IPM Higgs results and prospects 35

36 Conclusions - 1 The last three years have been a fantastic ride for Higgs physics at LHC: mass accuracy signal strength accuracy Yukawa couplings behaviour verified at percent accuracy It was also an exemplary exercise where both experiments managed to work together and delivered combined results in record time! Many other studies performed and on-going, all of them showing that this particle is indeed the SM Higgs boson: spin/cp, width, differential xsec, etc. The Run2 and Run3 Higgs studies are expected to bring another order of magnitude in accuracy L. Malgeri - METU-IPM Higgs results and prospects 36

37 Conclusions - 2 The prospects for (future) Higgs studies are more than optimistic despite early warnings : From S. Gori talk at LHCP15 L. Malgeri - METU-IPM Higgs results and prospects 37

38 Additional material L. Malgeri - METU-IPM Higgs results and prospects 38

39 Higgs couplings: generic parametrizations Generic parametrisations are based on ratios of parameters that are less affected by theoretical uncertainties: ratio of cross sections and BR ratio of coupling modifiers in both cases the gg H ZZ production/decay is used as reference (cleanest channel) reference L. Malgeri - METU-IPM Higgs results and prospects 39

40 Higgs couplings: generic parametrizations 2.4σ discrepancy in BR bb /BR ZZ driven by large ZH and tth Double minima seen in λtg and λwz L. Malgeri - METU-IPM Higgs results and prospects 40

41 Invisible Higgs searches Several channels and production mechanisms used by both ATLAS and CMS: VBF production gives largest sensitivity (clean signature): Large Δη(jj), m(jj) and large missing energy ATLAS arxiv: CMS- PAS- HIG ATLAS: BR(H - > inv.) < 25% (27% exp.) CMS: BR(H - > inv.) < 47% (35% expected) L. Malgeri - METU-IPM Higgs results and prospects 41

42 Differential distributions/xsec ATLAS:arXiv: CMS: PAS-HIG Statistics is not yet really discriminating but we are already starting to go differentials: Fiducial region defined to minimise acceptance corrections (unfolding applied for final results) Kinematics sensitive to PDF, production mode (i.e. VBF), QCD effects Affected by spin/cp and possible BSM Extrapolation to full phase space (first real Xsec measurements) L. Malgeri - METU-IPM Higgs results and prospects 42