Higgs searches in CMS Mario Pelliccioni Istituto Nazionale di Fisica Nucleare Torino Miami 2012 17/12/12
Introduction Why do we even bother to look for the Higgs? An Higgs boson naturally emerges from the EWSB mechanism in the SM Has a few nice perks: Origin of masses of elementary particles Unitarization of WW scattering amplitudes Screening of gauge-bosons self-energies A window on BSM physics: Vacuum instability? Higgs mass and couplings? Additional Higgs bosons? 2
Higgs at LHC Higgs boson couplings proportional to particle masses: Higgs production via couplings to W/Z bosons or top-quarks Production at hadron colliders: Decay channels for Higgs boson of moderate mass (mh < 300 GeV) 3
Where to look for a Higgs Nice synergy of CMS and ATLAS with the LHC Higgs Cross Section Working Group Main forum for information exchange between theoreticians and experimentalists 4
CMS detector 5
Higgs searches 6
Main decay modes Most important decay channels Channel Lumi Range Comments H γγ 5+5 fb-1 110-150 Good mass resolution, clear signature H ZZ 4l 5+12 fb-1 110-1000 Good mass resolution, clear signature H bb 5+12 fb-1 110-135 High BR, experimentally challenging H ττ 5+12 fb-1 110-145 High BR, experimentally challenging H WW 2l2ν 5+12 fb-1 110-600 No mass peak, need good bkg knowledge 7
Expected sensitivities 8
H γγ Clean peak in diphoton invariant mass Large continuous background Split in different photon categories to exploit different S/B VBF topology tag 9
H ZZ 4l Clear peak in 4l invariant mass Can exploit angular information to better separate irreducible background High mass CPS + ggzz-interference treatment allows to extend search to 1 TeV 10
H ZZ 4l results Excess around 126 now with significance of 4.5σ (expected 5.0σ) 11
H WW 2l2ν Two isolated high pt lepton with small opening angle (exploit helicity correlations) Large missing ET Analyze exlusive jet multiplicity categories Dedicated VBF selection Consider different flavor (DF) and same flavor (SF) samples separately 0-jet DF has higher sensitivity Cut & Count and shape analysis (DF 8 TeV) Excess of 3.1σ observed (expected 4.1σ) @ 125 GeV 12
H ττ Consider 5 di-τ final states τμτμ / τeτμ / τeτh / τμτh / τhτh Separate sample into different categories Calculate Higgs candidate invariant mass with maximum likelihood technique Neutrinos momentum free parameter of the fit Includes tau decay phase-space and tau transverse momentum probability density Resolution of ~ 20% at 125 GeV 13
H ττ results With b-tag category, can extend to MSSM search Includes VH Excess slowly building up (1.50σ observed) Still, compatible with both SM Higgs and bkg only 14
VH Vbb Largest BR at low mass, large background from jets Consider 5 categories (2x pt,v categories) Multivariate analysis to separate signal 15
VH Vbb results Small excess visible in invariant mass for 2.2σ (2.2σ expected) 16
UL and p-values combination 17
Measurement of the new boson properties 18
Mass measurement Use H ZZ 4l and H γγ Good resolution, significant excess 1D: assume the two processes to be independent 2D: model dependent, signal strength vs mh m H =125.8±0.4( stat.)±0.4( syst.) GeV 19
Parity measurement Exploit good track resolution in H ZZ 4l Kinematics completely described by 5 variables Build a kinematic discriminant (KD) to separate 0+ and 0 Fit to {m4l,kd} (assume m = 126 GeV) H 0- compatible with 2.4% CLs We reject the 0- scenario with more than 95% CL Sensitivity to spin 2 hypothesis very limited with current statistics 20
Signal strength Extract μ = σ/σsm All channels seem compatible with SM within current errors Combined signal strength is μ=0.88±0.21 (stat. + syst) 21
Measurement of couplings Event yields related to partial and total widths Γtot kept independent (sum of ΓSM and ΓBSM) Modify couplings introducing scale factors κ Variation of factors κ wrt unity indicates possible BSM Statistics imposes some constraints when combining Parameters either fixed to SM or profiled in the likelihood 22
Measurement of vector and fermion couplings Test of custodial symmetry Measure λ WZ =κw /κ Z Couplings to vectors and fermions All channels sensitive only to κ2 but for γγ, which prefers (+,-) 23
Test of presence of BSM particles in loops Test of BSM particles Expected to alter loops (like H γγ and gg H) Measure BRBSM = ΓBSM/Γtot (κg and κγ profiled) 24
Fermion couplings and 2HDM In 2HDMs, fermion couplings are modified between up- and down-quarks (more SUSY like) or between leptons and quarks (more general 2HDMs) 25
Conclusions Wrt July 4th, lots of updated results! In all channels, the significance has increased with the additional statistics A few low sensitivity channels are catching up, but are not (and are not expected) showing an evidence at 126 GeV yet Detailed study of the properties of the 126 GeV Excluded the pseudoscalar hypothesis @ 95% CL Signal strength fully compatible with SM expectations Analysis of couplings fully compatible with SM scenario 26