The Higgs boson discovery. Kern-und Teilchenphysik II Prof. Nicola Serra Dr. Annapaola de Cosa Dr. Marcin Chrzaszcz

Transcription

1 The Higgs boson discovery Kern-und Teilchenphysik II Prof. Nicola Serra Dr. Annapaola de Cosa Dr. Marcin Chrzaszcz

2 Higgs production at the LHC g H VBF (Vector Boson Fusion) g gg fusion q 2 W/Z q 0 2 H W/Z q H q 1 q 0 1 Production mechanisms: gg fusion, VV fusion, W and Z associated production, tt associated production Gluon fusion is the dominant production mode q V associated production W/Z tt associated production g g t t t t H 2

3 Discovery channels BR [pb] σ τ + τ - VBF H τ + τ s ± WW l νqq = 8TeV LHC HIGGS XS WG WW l νl ν + - ZZ l l qq ZZ l l νν ± WH l νbb tth ttbb + - ZH l l bb γγ ZZ l l l l l = e, µ ν = ν e,ν µ,ν τ q = udscb M H [GeV] Gluon fusion is the dominant production mechanism H bb: highest branching ratio 125 GeV) Inclusive H bb search not feasible due to overwhelming QCD background Instead H ZZ, H γγ has lower BR but very clean signature! 3

4 Observation of the Higgs boson at LHC Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC Observation of a new boson at a mass of 125 GeV with the CMS experiment at the 4

5 Discovery channels H ZZ 4l Clean experimental signature: four isolated leptons Narrow resonance peak in four lepton mass spectrum H γγ Clean experimental signature: 2 high energy and isolated photons Narrow resonance in diphoton mass spectrum over a falling con>nuous background Commonalities Both channels suffer low BR Both allow mass peak reconstruc>on with very good resolu>on 5

6 H ZZ* 4l The golden channel Very clean signature four isolated leptons (e or μ) H ZZ* 2μ2e And very small background Challenges: requires high lepton identification/reconstruction/isolation efficiency 6

7 H ZZ* 4l: Backgrounds and selection Reducible background Z+bb, tt, tt+ jets. Z+jets, WZ+jets Irreducible background ZZ* Event selection Double-lepton trigger Selection of events with 4 identified and isolated leptons Use of impact parameter Constraint on dilepton mass (m Z and m Z* ) Kinematical discrimination (exploits secularity of the Higgs boson) 7

8 H ZZ* 4l: Kinematical discriminant Matrix Element Likelihood Analysis (MELA) uses kinematic inputs for signal to background discrimination {m 1, m 2, θ 1, θ 2, θ*, Φ, Φ 1 } SM Higgs (125 GeV) qqzz 8

9 H ZZ* 4l: Kinematical discriminant 2D analysis using m 4l vs MELA SM Higgs (125 GeV) MELA qqzz 9

10 4 lepton invariant mass spectrum 10

11 H γγ Critical channel for the discovery of the Higgs Small BR (~0.2%) but very clear signature: 2 isolated highly energetic photons Search for a narrow peak over a falling background - very good mass resolution (~1-2%m γγ ) VBF production channel helpful for background rejection 2 forward jets from outgoing quarks VBF production 11

12 H γγ - Backgrounds H γγ Signal Reducible backgrounds Mainly due to jets identified as photons Irreducible backgrounds γγ pair production from non-higgs decays Photons do not couple directly to H because massless Process takes place through a top (or W) loop 12

13 H γγ - Event selection Only events with 2 identified and isolated photons are selected Events are then split in orthogonal categories: defined according to a Multivariate Analysis Discriminant based on the properties of identified photons and on the presence of 2 jets at large rapidity (VBF-like events) 13

14 H γγ - Signal and background modelling Signal is parametrised from MC simulation Crystal Ball + Gaussian Background is estimated by fitting a polynomial function to data in the full mass range Signal only Signal + Bkg Combination of all categories From the fit is clearly visible a nice peak over a falling background 14

15 H WW lνlν in a nutshell μ p T 32 GeV e p T 34 GeV Signature 2 high p T leptons (e,μ) large missing E T small Δφ ll and low M ll for low m H no resonance peak E T miss 47 GeV Backgrounds WW: con>nuum S/tW: b-jets W+jets: fake lepton Z/ɣ * : mis-measured ME T W/Z+ɣ (*) : ɣ (*) ll WZ/ZZ: V+jj/νν or missing lepton 15

16 H WW lνlν 2 events / 10 GeV/c data m H =125 GeV H125 W+jets VV Top Z/ γ* WW stat. syst. m ll CMS Preliminary -1 s = 8 TeV, L = 12.1 fb 2 events / 10 GeV/c m T = q 2p ll T Emiss T (1 cos E miss data m H =125 GeV H125 W+jets VV Top Z/ γ* WW stat. syst. m T T CMS Preliminary -1 s = 8 TeV, L = 12.1 fb ll) m ll [GeV/c 2 ] data / MC m ll-e T 2 [GeV/c ] ll-e T m T 2 [GeV/c ] 16

17 H WW lνlν: spin properties o events / data m H =125 GeV H125 W+jets VV Top Z/ γ* WW stat. syst. CMS Preliminary -1 s = 8 TeV, L = 12.1 fb 200 If the Higgs boson has spin 0, the two leptons expected to be emitted in the same direction (small azimuthal opening between the two) φ ll [ o ] 17

18 Statistical interpretation 18

19 Claiming a discovery We can claim discovery if we measure a signal yield sufficiently inconsistent with zero What does it mean sufficiently? How do we quantify it? We can quantify how relevant is the excess by stating what is its significance. Statistical significance = probability p to observe s or larger signal yield in the case of pure background fluctuations Often preferred to quote nσ significance, where: Z 1 p = n It is common habit to claim: 1 p e x2 /2 dx =1 2 Evidence of, if the significance is > 3 σ Observation, if the significance is > 5 σ - which corresponds to probability of background fluctuation = 2.87 x erf Discovery np More details in Statistical methods for Data Analysis in Particle Physics, L.Lista 2 19

20 Significance and corresponding p-value 20

21 What if we do not observe a significant excess? Not always experiments lead to discoveries: The signal may indeed not be there Or the collected dataset is not enough to claim a discovery We can still say something by setting upper limits One possible definition of upper limit: largest value of the signal s for which the probability of a signal under-fluctuation smaller or equal to what has been observed is less than a given level (usually 10% or 5%) Upper limits for an exclusion are set requiring: p<0.05 (95%CL) or p<0.10 (95%CL) In this case p indicates the probability of a signal underfluctuation 21

22 Statistical Interpretation of Higgs searches Null Hypothesis (H 0 ) vs alternative hypothesis (H 1, Existence of Higgs) Need to quantify the level at which each hypothesis is accepted or rejected H 0 H 1 Identify the experimental observables and define a statistical test and the parameters of the model m γγ Compute the confidence level for exclusion or the significance of the excess 22

23 The CLs method Signal strength μ = σ/σ SM test statistic q μ q µ = 2ln L(obs µ, ˆ µ ) L(obs ˆµ, ˆ ) 0 < ˆµ <µ CL s+b (s+b hypothesis) CL s+b = P (q µ q obs µ µ 6= 0) CL b (b-only hypothesis) CL b = P (q µ q obs µ µ = 0) CL s The exclusion of a SM Higgs boson is defined by the following condition: CL s (μ = 1) α at 1-α = 95% confidence level, C.L. We can say that s< s up at 95% C L (or 90% CL) CL s = CL s+b CL b 23

24 Likelihoods L(obs, µ, ) = NY ch L c (obs c µ, ) YN p i ( i i ) c=1 i=1 If we are combining more channels, the overall likelihood is the product of the likelihood functions of each channel (L c ) product of the response function of the measurement associated to the nuisance parameters (systematic uncertainties) In case of a counting experiment L(obs µ, ) =Poisson(n obs,µ s( )+b( )) Inputs: Signal yield, s(θ) Background yield, b(θ) Observed data, n obs Systematic uncertainties on yields, θ 24

25 Likelihoods L(obs, µ, ) = NY ch L c (obs c µ, ) YN p i ( i i ) c=1 i=1 If we are combining more channels, the overall likelihood is the product of the likelihood functions of each channel (L c ) product of the response function of the measurement associated to the nuisance parameters (systematic uncertainties) Including informations from the shape Inputs: Signal yield and shape, s(θ) and f s (x, θ) Background yield and shape, b(θ) and f b (x, θ) Observed data, n obs Systematic uncertainties on yield and shape, θ L(obs µ, ) =P oisson(n obs,µ s( )+b( )) ny obs i=1 f(x i µ, ) =µs( )f s (x i, )+b( )f b (x i, ) f(x i µ, ) 25

26 Combining all channels together Combining together results from Higgs boson searches in 5 decay modes: H γγ, H ZZ, H WW, H bb, H ττ Early Summer 2012: sufficient statistics collected to claim the observation of a new boson Observed excess of 7 σ around 125 GeV Clear excess in the upper limit plot ~ 125 GeV 26

27 Mass measurement of the new boson High resolution channels, H γγ and H ZZ 4l, allowed already in early 2012 to get a good estimate of Higgs boson mass H 4l H γγ Hγγ Measurement of the observed boson mass: m H = ± 0.4 (stat) ± 0.4 (syst) GeV 27

28 Look-elsewhere effect When searching for a signal over a wide range of parameters, such as the Higgs boson mass, we should be careful in stating the significance of the observation Let's say that we observe an excess of events for m H = X the probability that this excess is due to a background fluctuation, showing up exactly at m H = X, is given by the p-value (local significance) However, such an excess could have appeared everywhere in the range considered This is what is called the look-elsewhere effect We need to consider not just what is the probability to have a background fluctuation at m H = X, but more in general what is the probability to observe such a fluctuation everywhere in our range (global significance) The effect can be evaluated with brute-force Toy Monte Carlo Run N experiments with background-only, find the largest local significance over the whole search range, and get its distribution to determine overall significance 28

29 OK, we have discovered a new particle but is this the Higgs boson predicted by the SM? 29

30 Where do we stand now Latest invariant mass distributions produced with the statistics 13 TeV Events / 2 GeV CMS Preliminary H 4l fb (13 TeV) Data H(125) qq ZZ, Zγ* gg ZZ, Zγ* Z+X H γγ m 4l (GeV) 30

31 Cross section measurements H ZZ 4l cross section as a function of s H γγ cross section as a function of the di-photon p T [fb] σ fid fb (7 TeV), 19.7 fb (8 TeV), 35.9 fb (13 TeV) CMS Preliminary Data (stat. sys. unc.) Systematic uncertainty 3 Standard model (m = 125 GeV, N LO gg H) H fb 2.72 ± 0.14 fb (SM) pp (H 4l) + X s (TeV) 31

32 Signal strength and Higgs boson mass μ = (stat.) (syst.) mh = ± 0.20 (stat.) ± 0.08 (syst.) GeV untagged VBF-1jet tagged VBF-2jet tagged µ = 1.17 µ = 0.97 µ = CMS Preliminary m H fb (13 TeV) H ZZ* 4l = GeV µ = 1.05 comb lnl CMS Preliminary m' 4l m' 4l, D' mass kin m' 4l, D' mass, Dbkg kin m' 4l, D' mass, Dbkg fb (13 TeV) (stat. only) VH-hadronic tagged µ = VH-leptonic tagged VH-MET tagged tth tagged µ = 0.00 µ = 0.00 µ = µ m H (GeV) 32

33 Couplings Coupling to gluons and photons, kg vs. kγ (left), and coupling to fermions and bosons, kf vs. kv (right) 33

34 In conclusion, the newly discovered particle really looks like a the Higgs Boson predicted by the SM :) 34