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Search for light Higgs in Y(1S) → gamma lepton-pairs (update)

Search for light Higgs in Y(1S) → gamma lepton-pairs (update). Nasra Sultana & Tomasz Skwarnicki. Motivation.

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Search for light Higgs in Y(1S) → gamma lepton-pairs (update)

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  1. Search for light Higgs in Y(1S)→ gamma lepton-pairs(update) Nasra Sultana & Tomasz Skwarnicki

  2. Motivation • Some NMSSM models (Dermisek, Gunion, McElrath: hep-ph/0612031) predict existence of a new non-SM-like higgs boson a0 (pseudo-scalar) with ma < 2mb to avoid fine-tuning of parameters in electroweak symmetry breaking • Such light higgs avoids the LEP limit mH > 100GeV based on e+e- →ZH(→bb) searches since its mass is below the threshold for decay to bb. • In this scenario also SM-like higgs boson h (scalar) also avoids the LEP lower mass limit since Br(h→ bb) is much smaller than Br(h →a0a0) • The perfect place to search for a0 is in radiative decays of Upsilon meson, Υ → g a0. • Such an a0 decays predominantly into heaviest pair of fermions available (Br(a0→ t+t-)~0.9 for ma>2mt) • We have studied the decay Υ → ga0followed bya0→ t+t-and a0→ m+m- (forma<2mt)

  3. Previous and new approach • Previously we used Y(2S)→p+p-Y(1S) • David McKeen and Jon Rosner performed theoretical calculations which indicated that direct Y(1S) production (e+e- → Y(1S)) will be more effective than Y(2S) →p+p-Y(1S) in setting limits for low mass a0→m+m-. • We have switched to directly produced Y(1S) resonances: • 21M Y(1S); previously 9M Y(2S) * BR(p+p-) = 1M Y(1S) • Keep cuts the same except that don’t need to reconstruct p+p- (gain in efficiency) • Larger background due to continuum e+e-→(g)t+t-,→(g)m+m-, but gain is statistics and efficiency leads to a net gain in sensitivity

  4. Cuts details (a0→ t+t-) • Numbers of charged tracks = 2 • Require at least one of the 2 charged tracks to be an electron or muon candidate: • e: | E/P-1 | < 0.15, DEDX: se< 3 • m: depthmu >1, muqual=0, 0.15< E< 0.45 , DEDX: sm< 3 • Select the highest energy photon in the good barrel part (Eg > 0.06 GeV) which does not make a p0 mass within 3s with any other photon as a candidate for Υ(1S) →ga. The p0 veto suppress t → rn, r→pp0, p0→gg background • Sum up energy of all other photon candidates: Eneutral • Imbalance of total energy: Eg + Echarged + Eneutral – Ecm < -0.5 GeV • Mass of neutrals (except for the highest energy g) plus the 1-prong not required to be a lepton < 2 GeV • cos(1-prong and g )< 0.99 to suppress final state radiation.

  5. Υ (1S)→ ga0, a0→ t+t- signal MonteCarlo 10,000 events for each mass BKH’s fix to MC energy resolution is on Peaks are fitted with a Crystal Ball function Signal MC: Photon Energy distribution for various ma ma = 4 GeV ma = 5 GeV ma = 6 GeV ma = 7 GeV ma = 8 GeV ma = 8.5 GeV ma = 9 GeV ma = 9.15 GeV ma = 9.30 GeV ma = 9.35 GeV

  6. Efficiencyobtained from fits to signal MC and interpolated for the regions in between. Fits to MC data (previous slide) Polynomial fit to interpolate to other photon energies (used in calculation of upper limits on signal BR) new Old (p+p-)

  7. Energy resolutionobtained from fits to signal MC (points) and interpolation to other energies (solid line). Factor from fits to our MC Obtained by BKH and Selina (CBX 02-22) from fits to single g MC (before the MC resolution fix)

  8. Photon spectrum with binning comparable to expected signal width 21M Υ(1S)

  9. p+p- approach

  10. Peak at 203+-3 MeV Peak significance = 3.216 investigate this peak further by varying tt selection criteria Eg (MeV)

  11. Require one of the 2 charged tracks to be an electron and the other a muon candidate R=(Anom/enom) / (Aem/eem) = 0.25 ± 0.24 R should be 1 for the signal Conclusion: inconsistent with the signal, must be a fluctuation

  12. Setting upper limits on signal yield • At each energy fit CB line shape with width determined from MC on top of linear background in the ± Dln(E) = 0.5 range around the peak • Fix signal amplitude at values ³0 minimize with respect to the background parameters, then plot the fit likelihood as a function of the signal amplitude • Determine 90% U.L. on the signal amplitude by integral of the likelihood function 90% Example for ln(E in MeV)=7.5

  13. Upper Limit on product branching ratio Br(Υ(1S)→ ga0)*Br(a0→ t+t-) as function of Eg Br(Υ(1S)→ ga0)*Br(a0→ t+t-) = NULsig /(e * NΥ(2S) )

  14. Comparison with our previous results Previous Results New Results

  15. From Dermisek, Gunion, McElrath: hep-ph/0612031 NMSSM consistent with all previous results CLEO III We have improved ULs by about two orders of magnitude or more. We are constraining NMSSM models. Many models with 2mt<ma<7.5 GeV (represented by red points) ruled out by our results.

  16. a0→ m+m- • Two identified muons: • depthmu >1 • muqual=0 • 0.15< E< 0.45 • DEDX: sm< 3 • RICH: • c2K1-c2m1+c2K2-c2m2 ³ 0 • Balanced energy: | Eg + Echarged – Ecm | < 0.25 GeV J/y

  17. Signal MC: Mass of muon pair Ma=250 MeV Υ (1S)→ ga0, a0→ m+m- signal MonteCarlo 10,000 events for each mass BKH’s fix to MC energy resolution is on Peaks are fitted with a Gaussian function Ma=300 MeV Ma=500MeV Ma=1GeV Ma=2GeV Ma=3GeV

  18. Efficiencyobtained from fits to signal MC and interpolated for the regions in between. Fits to MC data (previous slide)

  19. Mass resolution obtained from fits to signal MC (points) and interpolation to other masses (solid line). Fits to MC data Linear fit to interpolate to other masses

  20. Setting upper limits on signal yield • At each energy fit Gaussion with width determined from MC on top of linear background in the ± Ma = 6s range around the peak • Fix signal amplitude at values ³0 minimize with respect to the background parameters, then plot the fit likelihood as a function of the signal amplitude • Determine 90% U.L. on the signal amplitude by integral of the likelihood function Example for Ma (MeV)=420

  21. Upper Limit on product branching ratio Br(Υ(1S)→ ga0)*Br(a0→ m+m-) as function of Ma X 10-6 J/y

  22. Eliminates all of NMSSM models for ma<2mt (blue points)

  23. Summary and plans • We have substantially improved upper limits on Br(Υ(1S)→ ga0)*Br(a0→ t+t-) and *Br(a0→ m+m-) by switching to directly produced Υ(1S) • The Y(2S) →ppY(1S) analysis presented in preliminary form at CHARM07 will not be finalized • To do list: • Implement TAUOLA simulation of a0→tt for correct tau polarizations (at present we are using phase-space model) • Fold in systematic errors

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