The study Higgs decaying into tau tau with CMS EPS HEP 2013, - PowerPoint PPT Presentation

The study Higgs decaying into tau tau with CMS EPS HEP 2013, Stockholm, 18-24 July Michał Bluj LLR/École Polytechnique – CNRS/IN2P3 on behalf of the CMS Collaboration Documentation: CMS PAS HIG-13-004, CMS PAS HIG-12-053

Outline ◎ Observation of a new boson with mass around 125 GeV consistent with the standard model scalar driven by the bosonic decay modes: H→ZZ→4l , H→γγ ◎ Observation of the new boson decaying into tau tau is an important test of compatibility with the standard model ◎ Decays into tau leptons interesting also in frame of extended models (e.g. MSSM)* ◎ This talk: overview of the H→ττ search with CMS ○ Search strategy strategy ○ Analysis tools ○ Discussion of results 125 * Not discussed here, see A. Nayak's talk EPS-HEP 2013, Stockholm 2

Analysis overview ◎ 5 channels for ggH and VBF Z→ττ Z→ll ○ μτ h , eτ h , τ h τ h eμ, μμ Embedding: Z→μμ data ● l/jet→τ h fakes with μ replaced by ● Shape from ◎ 3 channels for VH simulated τ simulation (l=e/μ, L=e/μ/τ h ) corrected for yield in visible ○ W(l)H(lτ h /τ h τ h ), Z(ll)H(LL) mass region ◎ Analysis strategy ○ Require well identified and isolated leptons, taus (τ h ) ○ Topological cut to suppress background (e.g. m T against W+jets in eτ h /μτ h ) W+jets ● Jet→τ h fakes ○ Categorisation based on jet multiplicity and p T of τ h /μ ● Suppressed by topological ○ Simultaneous max-likelihood QCD cuts (e.g. m T ) fit of di-tau mass (m ττ ) shape ● Jet→l/τ h fakes ● Simulation (binned) in all channels and ● Suppressed by isolation normalized to categories ● Same-sign data data in (corrected for OS/SS ratio) sideband EPS-HEP 2013, Stockholm 3

Event categories eτ h , μτ h , eμ, μμ No. of jets 1-jet, low-p T 2-jet, VBF 0-jet, low-p T ) μ ● ≥ 2 jets ● Enhancement due to jet / ● Large background τ h ● M jj > 500 GeV ( requirement ● No fit for signal , p T ● Better mass resolution constrain uncertainties ● Δη jj > 3.5 ● Central jet veto 0-jet, high-p T 1-jet, high-p T ● Enhancement due to jet ● Large background ● VBF H signal requirement ● No fit for signal , enhanced ● Better mass resolution constrain uncertainties ● Z→ττ suppressed by high-p T (tau) τ h τ h 1-jet 2-jet, VBF ● p T (ττ) > 140 GeV ● p T (ττ) > 110 GeV ● Better mass resolution ● M jj > 250 GeV ● QCD suppressed ● Δη jj > 2.5 ● Central jet veto EPS-HEP 2013, Stockholm 4

Event categories eτ h , μτ h , eμ, μμ No. of jets 1-jet, low-p T 2-jet, VBF 0-jet, low-p T ) μ ● ≥ 2 jets ● Enhancement due to jet / ● Large background τ h ● M jj > 500 GeV ( requirement ● No fit for signal , p T ● Better mass resolution constrain uncertainties ● Δη jj > 3.5 ● Central jet veto 0-jet, high-p T 1-jet, high-p T ● Enhancement due to jet ● Large background ● VBF H signal requirement No fit for signal Constrain uncertainties ● No fit for signal , enhanced ● Better mass resolution constrain uncertainties ● Z→ττ suppressed by high-p T (tau) τ h τ h 1-jet 2-jet, VBF ● p T (ττ) > 140 GeV ● p T (ττ) > 110 GeV ● Better mass resolution ● M jj > 250 GeV ● QCD suppressed ● Δη jj > 2.5 ● Central jet veto EPS-HEP 2013, Stockholm 5

Identification of τ h ◎ Reconstruction of individual decay modes (Hadron-plus-strip algorithm. HPS) ○ Particles by Particle Flow algorithm used ○ 1-prong, 1-prong+π 0 's, 3-prongs ◎ Isolation ○ Multivariate discriminant based on Σp T of particles in rings around τ h ○ Pileup mitigated with FastJet rho used by isolation MVA ◎ Additional selection to reject leptons EPS-HEP 2013, Stockholm 6

Di-tau mass estimation ◎ Neutrinos present in tau decays ⇒ invisible component ◎ Use maximum likelihood based full di-tau mass (m ττ ) estimate ○ Computed on event-by-event basis using four-momenta of visible decay products, miss , E y miss , expected E T miss resolution E x ◎ 15-20% resolution on reconstructed m ττ ○ better Z/H separation than with the visible mass visible m ττ full m ττ EPS-HEP 2013, Stockholm 7

Mass distributions ● 2-jet VBF ● Best S/B category ● VBF H signal enhanced EPS-HEP 2013, Stockholm 8

Combined mass distribution ● Combine channels and categories weighting each by S/B ● S – expected signal ● B – fitted background ● Both S and B in window around peak at 125 GeV Higgs EPS-HEP 2013, Stockholm 9

Main systematics Z→ττ ● 8% tau-Id efficiency Z→ll ● Category efficiency 0-8% ● Normalization ● Tau energy scale 3% (shape) 20%/30% for ee/μμ W+jets ● Normalization 10-20% ● Includes QCD ● Statistical unc. In ● Normalization 10% control region ● Unc. of extrapolation ● Shape (bin-by-bin) factor uncertainty in low stat ● Unc. of MC bias of categories topological variable EPS-HEP 2013, Stockholm 10

VH analysis WH channel ZH channel W(l)H(lτ h ) W(l)H(τ h τ h ) Z(ll)H(LL) ● Four final states ● Main background ● Main when one or two ● eτ h , μτ h , τ h τ h , eμ irreducible WZ fake τ h (reducible) ● Require Z→ll candidate ● Same-sign of two ● Z→ll+2(jet→τ h ) fakes ● Irreducible background leptons to suppress reduced by vetoing from ZZ, reducible from Z→ll+(jet→τ h fake) events with 2 nd lepton Z→ll+2jets All VH channels: Reducible backgrounds with jet→l/τ h fakes measured from data using fake-rate method EPS-HEP 2013, Stockholm 11

VH results WH channel ZH channel Combined VH limit ● Sensitivity of ~3xSM ● Small excess consistent with both H(125) signal injected SM Higgs at 125 GeV and background EPS-HEP 2013, Stockholm 12

Limits ◎ Combining all channels (VH included) ◎ Observed exclusion limit of 1.81 x SM at 125 GeV, while 0.76 x SM expected ◎ Result consistent with SM Higgs expectation H(125) signal injected EPS-HEP 2013, Stockholm 13

Signal cross section ◎ Signal strength (σ/σ SM ) by channel (left) and by category (right) at 125 GeV ◎ Combined signal strength σ/σ SM = 1.1 ± 0.4 ◎ Consistent between channels and categories EPS-HEP 2013, Stockholm EPS-HEP 2013, Stockholm 14

Significance & mass ◎ Observe broad excess over whole probed mass range ◎ Consistent with SM Higgs expectation ◎ Max. significance of 2.93σ at 120 GeV ◎ 2.85σ obs. (2.62σ exp.) at 125 GeV ◎ Most favoured mass of the observed excess ○ From likelihood fit ◎ m H = 120 +9 -7 (stat+sys) GeV ○ Consistent with m H (4l) = 125.8 ± 0.5 GeV EPS-HEP 2013, Stockholm EPS-HEP 2013, Stockholm 15

Conclusions ◎ Search for H→ττ with the full 2011+2012 dataset collected by CMS was shown ◎ A wide excess compatible with SM Higgs is observed ○ Obs. limit of 1.81xSM at 125 GeV, while 0.76xSM exp. ○ Signal strength of 1.1 ± 0.4 ○ Significance of 2.85σ obs. (2.62 exp.) at 125 GeV ◎ Strong indication that the new particle decays to taus! EPS-HEP 2013, Stockholm 16

VBF H→μτ h event candidate Thank you! EPS-HEP 2013, Stockholm 17

Additional material EPS-HEP 2013, Stockholm 18

Luminosity After high quality selection ● 4.9/fb at 7 TeV (2011) ● 19.4/fb at 8 TeV (2012) EPS-HEP 2013, Stockholm 19

VBF H→μτ h event candidate EPS-HEP 2013, Stockholm 20

Systematic uncertainties (exp) EPS-HEP 2013, Stockholm 21

Systematic uncertainties (th) EPS-HEP 2013, Stockholm 22

Sensitivity break-down by channel by category ● Expected limits for full 2011+2012 dataset ● Sensitivity of 0.76 x SM at 125 GeV ● Sensitivity lead by the μτ h channel EPS-HEP 2013, Stockholm 23

Combined mass distribution ● Combine channels and categories weighting each by S/B ● S – expected signal ● B – fitted background ● Both S and B in window around peak at 125 GeV Higgs EPS-HEP 2013, Stockholm 24

Consistency with SM H(125) H(125) signal injected H(125) as background ● Result consistent with background + SM Higgs m H = 125 GeV EPS-HEP 2013, Stockholm 25

Event yields and efficiency μτ h eτ h τ h τ h eμ EPS-HEP 2013, Stockholm 26

Mass distributions (1-jet) ● 1-jet, high-p T : second most sensitive category ● ggH dominated ● Jet and high-p T requirements improve resolution and S/B EPS-HEP 2013, Stockholm 27

Mass distributions (0-jet) ● 0-jet : ● Low S/B ● Background fit only ● To constrain uncertainties in signal sensitive categories EPS-HEP 2013, Stockholm 28

Extension to low mass ● 8 TeV (2012) only : ● Limit extended down to 90 GeV ● Combining the eτ h , μτ h , eμ and τ h τ h channels. The μμ and VH channels not included. H(125) as background EPS-HEP 2013, Stockholm 29

Jets ◎ Used for event categorisation ○ Specially important as tags of VBF H ◎ “Fake” jets from pileup ○ High-E T jets from overlapping pileup jets ◎ Discriminate “fake” jets with MVA using ○ Track-vertex association ○ Jet shape ◎ Reduces background in VBF category by a factor of ~2 EPS-HEP 2013, Stockholm 30