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Measurements of the Standard Model Higgs parameters at ATLAS. Lidija Živković Weizmann Institute Israel. Measurements of the Standard Model Higgs parameters. If there is a Higgs boson , it would be found at ATLAS (> 5 s ) over the full mass range after a few months of running.
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Measurements of the Standard ModelHiggs parameters at ATLAS Lidija Živković Weizmann Institute Israel
Measurements of the Standard Model Higgs parameters • If there is a Higgs boson, it would be found at ATLAS (> 5s) over the full mass range after a few months of running. • Precision measurement of Higgs parameters will become a major goal • Necessary for better understanding of Standard Model symmetry-breaking mechanism • Refernces for this talk: • Physics TDR • M. Hohlfeld, ATL-PHYS-2001-004 • D. Zeppenfeld, et al., hep-ph/0002036 • M. Dührssen, et al., note in preparation • E. Gross, L. Živković, A. Ito, note in prepeartion • C. P. Buszello, et al, SN-ATLAS-2003-025 (spin and CP) L. Živković Weizmann Institute
The Higgs Mechanism • Essential ingredient of the Standard Model: • complex scalar field with potential • Used to break the el. weak symmetry...... • ..... and to generate fermion masses: • Search for the Higgs Boson is a key issue for experiments at future colliders • Experimental challenges: • Higgs boson discovery • Measurement of Higgs boson parameters (couplings to bosons and fermions) • Measurment of the Higgs self coupling Higgs potential • Mass limits: lower 114.4 GeV/c2and upper up to 1TeV form theory and ~200 GeV from precision measurement U f2 f1 L. Živković Weizmann Institute
Production … The highest cross-section Two high-pT forward jets gg → H qq → WH qq → qqH High pT lepton for trigger gg,qq → ttH One ortwohigh pT leptons for trigger qq → ZH Reconstruction of one or two top quarks L. Živković Weizmann Institute
2mZ LEP excluded bb WW ZZ tt cc tt gg gg … and Decays Light Higgs L. Živković Weizmann Institute
SM Higgs Physics TDR ATLAS-PHYS-2003-005 L. Živković Weizmann Institute
Search channels - mass range100 – 1000 GeV L. Živković Weizmann Institute
Measurements of Higgs Mass • Mass: • Direct in the channels where Higgs mass can be reconstructed: • H → gg, • H → bb, • H → ZZ(*)→ 4l • indirect elsewhere: • H → WW → lnln • WH → WWW → lnlnln • VBF channels (t final states) • Errors: statistical, systematic on absolute energy scale (assumed 0.1% for channels with leptons and photons and 1% for jets; goal 0.02% for channels with leptons and photons) • Precission from 0.1% to 1% Physics TDR and ATL-PHYS-2001-004 L. Živković Weizmann Institute
Measurements of Higgs Rates • The cross section times branching ratio: where Є is efficency and h is acceptance. • Erorrs: 5% on background where it can’t be subtracted, neglected otherwise, 5% or 10% on luminosity’ 2% on Є∙h • Relative error: ATL-PHYS-2001-004 ttH gg fusion WH L. Živković Weizmann Institute
Measurements of Higgs Couplings • Couplings and branching ratios • Can be obtained from rate measurements if is known from the theory; • Otherwise: measure ratios of rates for different channels: (errors on the) cross sections, L, G cancel get ratios of couplings • Couplings to bosons (mH < 190 GeV considered so far): • directly from and the same production mode for mH > 150 GeV • indirectly using and • the last one can be checked via with the same production mode (VBF, ttH) L. Živković Weizmann Institute
Measurements of Higgs Couplings • Couplings to fermions: • ttH from • bbH from and • ttH from and L. Živković Weizmann Institute
Measurements of Higgs Couplings • Errors: • K-factors - 20% gluon fusion, 5% VBF • proportionality constant for Higgs decay to gg or gg via loop – 10% • theoretical uncertainties from comparison of two different production modes ATL-PHYS-2001-004 L. Živković Weizmann Institute
Relative branching ratios: Theoretical assumption : only one Higgs-Boson Relative couplings: Additional assumption: only particles of Standard-Model Measurements of Higgs Couplings New results from Physics Workshop in Athens: M. Dührssen L. Živković Weizmann Institute
Measurements of Higgs Width • Width: • direct H → ZZ(*)→ 4l just for mH > 200 GeV because total width GH < GDetector, detector resolution. • Errors: statistical, systematic (detector energy and momentum resolution) ~ 1.5% Physics TDR L. Živković Weizmann Institute
Measurements of Higgs Width • Width: • Indirect measurement at lower masses mH < 200 GeV. • Method proposed by D. Zeppenfeld et al., hep-ph/0002036: • The total width is dominated by bb, cc, WW, ZZ, gg, gg, tt→ most of them partially detectable in this mass range and the part that can’t be measured taken into account by the parameter e: • The H → ZZ* and H → WW* partial widths are related by SU(2): • The ratio of b to t partial widths is connected to their mass ratio: L. Živković Weizmann Institute
Measurements of Higgs Width • Method proposed by D. Zeppenfeld et al., hep-ph/0002036 (continued): • Let us defined: • Then provides lower limit on G(H→WW(*)) = GW • Then • With a knowledge of GW and G, it is easy to obtain the other partial widths H → WW H → tt H → gg gg → H H → WW VBF or VH (V = Z or W) L. Živković Weizmann Institute
Measurements of Higgs Width ATLAS + CMS 100 + 100 fb-1 • Correct assumptions (small e, SM ratios) • Just prediction for VBF processes: D. Zeppenfeld et al., hep-ph/0002036 WW bb ZZ tt L. Živković Weizmann Institute
WW bb ZZ tt Measurements of Higgs Width M. Dührssen • But H→bb can’t really be measured at mH ~ 150 GeV enot so small (~20%) b to t width ratio (y) can’t really be checked • Assumption: BRvisible = 1 – e, e from Standard Model L. Živković Weizmann Institute
Preliminary L = 30 fb-1 Hbb gttH/gttH HW+W- 2030% H+- MH [GeV/c2] MH [GeV/c2] top-Yukawa coupling • ttH → tt tt not included yet • Can improve measurement of top Yukawa coupling M. Dührssen E. Gross, et al. L. Živković Weizmann Institute
f- angle between decay planes in the rest frame of the Higgs. q - angle between leptons and the momentum of the Z in the rest frame of the Z. Azimuthal angle f: Polar angle q: Measurements of Spin SN-ATLAS-2003-025 L. Živković Weizmann Institute
Theoretical prediction Results after background subtraction For mH > 250 GeV all non Standard Model particles are excluded Measurements of Spin mH = 200 GeV SN-ATLAS-2003-025 L. Živković Weizmann Institute
Measurements of Spin • Exclusion of non Standard Model particles through polar angle for higher Higgs masses. • For mH = 200 GeVpseudoscalar is excluded even at 100 fb-1. • The spin 1, CP even can be excluded at 300 fb-1 with significance 6.4s, while for the CP odd case significance is only 3.9s. SN-ATLAS-2003-025 L. Živković Weizmann Institute
Conclusions • Precision measurements: • masses 0.1% - 1% • production rates 10% - 20% • relative branching ratios 15% – 45% • relative coupling ratios 10% - 20% • width 10% - 30% for mH > 150 GeV • For mH > 200 (230) GeV a Spin-0 CP-odd and a Spin-1 (CP-even and CP-odd) Higgs-Boson can be ruled out at 2s(5s) –Level with 100 fb-1 L. Živković Weizmann Institute