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

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

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**Search for light Higgs in Y(1S)→ gamma lepton-pairs**Nasra Sultana & Tomasz Skwarnicki**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-(or a0→ m+m- for ma<2mt)**Previous results are very old**• ARGUS: Phys.Lett.B154:452,1985 Υ(2S) → p+p-Υ(1S), Υ(1S)→ g (t+t-) • CUSB: Phys.Rev.D35:2883,1987 Υ(1S)→ g X**Method (a0→ t+t-)**• To study the decay Υ (1S)→ ga0, a0→ t+t-we tag Υ(1S)viaΥ(2S) → p+p-Υ(1S). • By tagging the Υ(1S) we eliminate events with photon coming from initial state radiation in tau pair production (e+e-→gt+t-), a serious background for the reactions e+e-→ Υ(1S)→ga0. • The channel a0 → t+t- is selected by using 1-prongt decays,requiring missing energy (neutrinos!) and at least one leptonic t decay: t→ mnn or t →enn.**Cuts details**• Numbers of charged tracks = 4 • -0.015 < Recoil-Mass(p+p-) – M(Y(1S)) < +0.015 GeV • Require at least one of the remaining 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.**p+p- Recoil mass – Υ(1S) mass**Signal region Side band**Photon Energy distributionin the rest frame of Υ(1S)**9.4 M Υ(2S) decays Scaled side bands (non Y(1S) background)**Photon Energy distributionin the rest frame of Υ(1S) after**side band subtraction Sideband-subtracted data e+e- → p+p-Υ(1S), Υ(1S)→ l+l- MCs scaled by PDG BRs Data above 200 MeV saturated by e+e- → p+p-Υ(1S),Υ(1S)→ t+t- Within errors all data well described by Υ(1S)→ l+l- We used Υ(1S)→ t+t- MC to optimize our data selection procedure. + → e+e- MC + → m+m- MC Υ(1S)→ t+t- MC**e+e- → p+p-Υ(1S)**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 ma = 9.41 GeV**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) Plotted efficiencies based on phase-space MC Multiply them by 0.91 to account for 1+cos2θgdistribution for Υ→ga**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)**Photon spectrum with binning comparable to expected signal**width**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**Upper Limit on product branching ratio Br(Υ(1S)→**ga0)*Br(a0→ t+t-) as function of ma Br(Υ(1S)→ ga0)*Br(a0→ t+t-) = Ns /(e * NΥ(2S) * Br(Υ(2S)→ p+p-Υ(1S) ) Br(Υ(2S)→ p+p-Υ(1S))=18.8 % PDG’06 Upper limits are loose at low photon energies (Eg<150 MeV) since our analysis was optimized for intermediate and high energies.**From**Dermisek, Gunion, McElrath: hep-ph/0612031 NMSSM consistent with all previous results CLEO III We have improved ULs by about an order 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. Switch to a0→ m+m- for ma<2mt (blue points) - see next!**a0→ m+m-**• Two identified muons: • depthmu >1, muqual=0, 0.15< E< 0.45 , DEDX: sm< 3 • RICH: c2K1-c2m1+c2K2-c2m2³0 • | Eg + Echarged – Ecm |< 0.25 GeV Data (sideband subtraction very small) e+e- → p+p-Υ(1S), Υ(1S)→ m+m- MC scaled by PDG BRs ( includes tiny Υ(1S)→ t+t- contribution)**a0→ m+m-**Data Signal MC ma=3 GeV 0.5 GeV**Using e=5.4% Br(Υ(1S)→ ga0)*Br(a0→ m+m-) < 2.5 x**10-5 (90% C.L.) Eliminates most of NMSSM models for ma<2mt (blue points) Concerns about ability of our MC to correctly predict tracking efficiency for very small ma (no opening angle between tracks) Do not intend to show any a0→ m+m- results in public at this point**Two identified kaons:**• RICH: c2K-c2p<0 • DEDX: sK< 3 • depthmu <1 • | Eg + Echarged – Ecm | < 0.25 GeV a0→ K+K- • Using e=4.3% Br(Υ(1S)→ ga0)*Br(a0→ K+K-) < 3.2 x 10-5 (90% C.L.) • Same concerns about MC as for m+m- Data Signal MC ma=4 GeV 2.0 GeV**Summary and plans**• We have obtained meaningful limits on Br(Υ(1S)→ ga0)*Br(a→ t+t-) and *Br(a0→ m+m-) • Future work: • Study effects of angular correlations in MC to reduce systematic error • Try separate set of cuts to optimize for high a0masses (Eg< 150 MeV) ? • Study track reconstruction efficiency for low a0 masses in a0→m+m- with e+e-→gg followed by g conversion (g→e+e-) • 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 will investigate this with CLEO data and MC.**From**Dermisek, Gunion, McElrath: hep-ph/0612031 NMSSM consistent with all previous results CLEO III We have improved ULs by about an order 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. Switch to a→ m+m- for ma<2mt (blue points) - see next!

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