Searches for New Physics in quarkonium decays at BaBar/Belle

Transcription

1 1 Searches for New Physics in quarkonium decays at BaBar/Belle Lucas Winstrom University of California Santa Cruz for the BaBar Collaboration Presented at QWG08 in Nara, Japan December 5, 2008

2 2 Outline Search for the invisible decay of the at Belle O. Tajima et al. PRL 98, (2007) Run at BaBar Search for Invisible Decays of a Light Scalar in Radiative Transitions arxiv: (SLAC-PUB-13328) Search for the Lepton-Flavor Violating Decays (SLAC-PUB-13480)

3 3 Search for the invisible decay of the at Belle New Physics Low mass dark matter Event Selection - finding invisible particles Tag pions from Backgrounds Peaking Non-Peaking Results Pion Pair 3S 1S Invisible Product

4 4 New Physics Standard Model Invisible decays of Y(1S) come only from neutrinos These have a predicted BF of ~1x10-5 Predictions of decay to dark matter candidates with mass lower than the b quark at a level of up to 6x10-3 Study done on 11 Million Y(3S) mesons at Belle Previous best measurement of BF <23x10-3 [90%CL] from Argus and < 50x10-3 [95%CL] from CLEO

5 5 Finding the Invisible Particles Look for the Y(3S) π + π - Y(1S) by looking at the mass the two pions recoil against Check modeling by looking at relevant variables of the pions for the case where Y(1S) decays to two muons Invariant mass of pions Recoil mass of pions

6 6 Backgrounds Non-peaking Mostly 2 photon processes e + e - e + e - X, X decays to states which are identified as 2 pions Fisher used to discriminate signal and background signal BG Peaking Caused when Y(1S) decays into SM particles that escape detection Indistinguishable from signal Use model to predict yield

7 7 Results Use maximum likelihood fit to find yield 38±39 events Fitted Curve.6% Signal Curve Background Limit of 2.5x10-3 set on Y(1S) invisible branching fraction at 90%CL Order of magnitude better than previous measurements

8 8 Run at BaBar

9 9 Search for Invisible Decays of a Light Scalar in Radiative Transitions. New Physics Potential Scalar Higgs State Light Dark Matter Event Selection Signature Looking for a Monochromatic Peak in Photon Spectrum in Center of Mass Energy

10 10 New Physics Next to Minimal Supersymmetric Model (NMSSM) introduces a singlet Higgs field A linear combination of this and a member of the electroweak doublet produces a CP-odd Higgs state A 0 In certain NMSSM scenarios, the dominant decay mode of A 0 may be invisible, to a pair of LSP particles. A 0 LSP Pair

11 11 New Physics A 0 typically constrained at m A 0 < 2m b making decays an ideal place to search, with predicted branching fractions as high as 10-4 with current experimental limits between 10-4 and 10-5 Detectable through the single photon involved in the decay and a large amount of missing energy Blue points - m A 0 < 2m τ Red points - 2m τ < m A 0 < 7.5 GeV Green points < m A 0 < 8.8 GeV Black points < m A 0 < 9.2 GeV PRL 95:041801,2005 and PRD 76:051105,2007

12 12 Event Selection Split photons into two energy groups: CM Energy between 3.2 and 5.5 GeV Dominated by e + e - ->γγ background CM Energy between 2.2 and 3.7 GeV Dominated by Bhabha and e + e - ->γγ tail Correspond to different mass ranges as m 2 A 0 = m2 Y(3S) -2E γ * m Y(3S) (E γ * is photon center of mass energy Make cuts on missing energy Total extra energy must be less than 100 MeV (high energy region) or 220 MeV (low energy Region) Signal Like Event, Actually an e + e - ->γγ: Very little extra activity in the calorimeter, but muon system detects back to back photons - catches photons in cracks of EMC Photon Muon System Activity

13 13 Spectra of Missing Mass High Energy Region Low Energy Region Non Peaking Background e + e - ->γγ Signal Model Unbinned ML Fit on missing mass squared is done with signal model moved in steps Most Significant peaks are: High Energy Region - m A 0 = 5.2 GeV Yield of 37 ± 15 - Statistical Significance of 2.6σ High Energy Region - m A 0 = 7.3 GeV Yield of 119 ± 71 - Statistical Significance of 1.7σ

14 14 Limit on Branching Fraction x10-6 Above - Yields in mass spectrum Left - Upper limit on branching fraction as a function of A 0 mass No excess is observed, limiting BF between 10-5 and 10-6 with NMSSM predictions going up to 10-4 Improves previous limits by an order of magnitude

15 15 Search for the Lepton-Flavor Violating Decays In Standard Model without neutrino masses, LFV decays of the are forbidden With neutrino masses included, these are suppressed by [(ΔM 2 ν ij )/(M W ) 2 ] 2 <10-48 Therefore, any evidence of lepton flavor violation in the system is a signature of New Physics If there is LFV and the mechanism is in the Higgs sector, it will preferentially couple to bottomonium

16 16 Selecting Events Require a high center of mass momentum e±/µ± with lepton CM momentum close to beam energy Select events with 2 tracks with an opening angle > 90 in the CM frame The total visible mass is less than the collision energy, since the τ always decays into states containing a ν τ 4 signal channels:

17 17 Backgrounds electron signal BF set to 10-4 Bhabha events muon signal BF set to 10-4 µ pair events Measure CM momentum of lepton/energy of beam Non peaking tau pair production (blue and green above) Signal peaks at 0.97 Peaking backgrounds peak close to 1 Bhabha background - one electron misidentified µ pair background - one µ is misidentified or decays in flight, or an electron is generated in material interaction

18 18 Fitting Green - τ pair background Magenta - peaking (dimuons and Bhabha events) - peaks at ~1 Red - signal - peaks at ~.97 Plots of CM momentum of primary (electron or muon) track over beam energy Maximum likelihood fit performed to find signal peaks

19 19 Results We find no evidence of Lepton-Flavor Violation and set limits at the 90% confidence level on the decays: First such measurement Factor of ~4 better than previous best limit (CLEO) Red - Statistical uncertainties only Blue - Both Statistical and Systematic Black line - 90% of positive integral of total likelihood curve (Shaded in Green) Effective Field Theory gives*: In the strong coupling limit of α N =1, 90% confidence limits on the scale of new physics are at Λ (eτ) >1.4 TeV and Λ (µτ) >1.5 TeV *

20 20 Conclusion Invisible branching fraction of Y(1S) improved by an order of magnitude Invisibly Decaying CP-Odd Higgs Branching Fraction Set Constrains NMSSM parameters LFV Decays of Bottomonium Constrained Probes physics up to 1.5 TeV scale More exciting exotic searches on their way from the BaBar and Belle datasets (1S from Belle and 2S and 3S from BaBar).

21 21 Backup Slides:

22 22 Signal Like Event, Actually an e + e - ->γγ Very little extra activity in detector, but muon system detects back to back photons Photon Muon System Activity

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