Two Photon Decays of Charmonium at Fermilab E835. Mathew Graham University of Minnesota November 8, 2002

Transcription

1 Two Photon Decays of Charmonium at Fermilab E835 Mathew Graham University of Minnesota November 8, 2002

2 Experiment E835 Mass (MeV) We study charmonium via ppbar annihilations, detecting electromagnetic final states. h 1 c S 0 γγ y e + e- 3 S 1 h c 1 P 1 } γγ γγ 3P J γdata samples χ c2 : 8.4 pb -1 χ c0 : 17.0 pb h c 1 S 0 y 3 S 1 e + e- γγ ,1,2 ++ J PC η c : 18.9 pb -1

3 Experiment E835 E835 is dedicated to measuring the bound states of charmonium via p p annihilations Continuation of E760 and preceded by ISR s R704 Collaboration consists of ~50 members from 2 National Laboratories and 6 Universities: 1. Fermi National Accelerator Laboratory 2. Istituto Nazionale di Fisica Nucleare 3. University of Ferrara 4. University of Genova 5. University of California at Irvine 6. University of Minnesota 7. Northwestern University 8. University of Torino

4 Experiment E835: Method Tunable antiproton beam on Hydrogen gas-jet target Allows us to directly produce all states Direct width+mass measurements M = ± 0.3MeV Γ χ χ 0 0 = 9.8 ± 1.0MeV χ 0 J / ψγ e + e γ

5 Two Photon Selection Requirements Neutral event which satisfies two-body kinematics Exactly 2 in-time clusters in calorimeter Any number of out-of-time or undetermined clusters No π 0 s in event Still a large background from π 0 π 0 and π 0 γ events Feeddown Background Asymmetric π 0 low E photon below calorimeter threshold or outside of acceptance Symmetric π 0 opening angle too small for calorimeter to resolve the two photons

6 Feeddown Background Using MC, calculate expected two photon background events from misidentified π 0 γ and π 0 π 0 events. π 0 π 0 π 0 γ P[ππ(or πγ)] > γ] Calculated γbkg

7 π 0 γsymmetry: A Test of the Feeddown Method π 0 γcandidates π 0 π 0 feedown π 0 γcross section From charge conjugation, the π 0 γ cross section must be symmetric. Subtraction of the π 0 π 0 feeddown from the π 0 γsignal restores the symmetry

8 Feeddown Background II Feeddown background is forward peaked so a cosθ * cut is chosen to maximize signal/background η c : α=0.2 χ c2 : α=0.45 Forward peaking increases with energy cosθ * cut is chosen accordingly

9 η c Resonance Parameters Fit to Breit-Wigner and Feeddown yields: Preliminary M η Γ η c c BR( pp = ± 2.1± 1.0MeV = 20.4 ± 7.2 ± 2.0MeV η c ) BR( η γγ ) = (22.4 ± 3.8 ± 2.0) 10 No evidence of interference c 8

10 η c to γpartial Width Using values from PDG, we get: Γ( γγ ) = 3.8± 1.1± η χ kev

11 χ c2 to γdecay Rate Since χ c2 is so narrow, don t fit to BW but use ratio of γγand J/ψ γrates point-by-point. B( χ c2 B( χ c2 γγ ) = ± J / ψγ e e γ ) M. Ambrogiani el al., Phys. Rev. D 62 (2000)

12 χ c2 to γresults Used for PDG2002 Previous experiments results have been recalculated using BRs from 2002 PDG The systematic difference between e + e - and pbarp expts is decreased with new PDG value for B(χ c2 ->γj/ψ )

13 χ c0 to γdecay Rate cosθ * =0.4 Feeddown says a cut of cosθ * =0.4 is appropriate but, the fit is very poor (χ 2 /NDF=32/15) and asymmetric.

14 γcontinuum We could possibly see interference effects from continuum pp γ From CLEO/VENUS/BELLE Continuum ranges from 100->10 pb for η c to χ c2 up to cosθ*=0.6 Continuum is forward peaked most CS is between cosθ*=0.5 and 0.6

15 γcontinuum at the χ c0 Belle(prelim) Using detailed balance, we can calculate σ( γ pp) from measurement of σ( pp γ) for cosθ * <0.6 E835 χ 0

16 χ c0 to γdecay Rate II Maybe interference? σ Fit to: data = σ 2( Mχ x Γ feed s) 0 χ 0 + Ae i δ *** B x+ i 2 χ 2 /NDF =17/13 Continuum σ=1 pb seems reasonable cosθ * <0.4 *** [Mass fixed to MeV] [Width fixed to 9.8 MeV] S Bagnasco el al., Phys. Lett. B 533 (2002) 237.

17 χ c0 to γdecay Rate III cosθ <0.2 Below cosθ * =0.2, fitted continuum consistent with 0 we will cut here. 0.2<cosθ <0.4

18 χ c0 to γresults BR( χ χ 2 The fit for cosθ * <0.2 yields: 0 pp) BR( χ / NDF = 8.3/15 0 γ ) = [6.52± 1.18( sta) ± 0.55( sys)] Using: BR( χ0 pp) = (2.2 ± 0.5) 10 BR( χ0 γ ) = [2.96± 0.54( sta) ± 0.25( sys) ± 0.67( BR)] 10 4 Preliminary Γ γγ = 2.90 ± 0.59( sta + sys) ± 0.66( BR) ± 0.3( width ) kev

19 χ c0 to γresults II Γ χ 0 γγ Previous experiments results have been recalculated using BRs from 2002 PDG PDG 2002 Result

20 Conclusions We ve made new measurements of the mass and total width of the η c : M η Γ η c c = ± 2.1± 1.0MeV = 20.4 ± 7.2 ± 2.0MeV In addition, we ve measured the product of branching ratios: 8 B( pp η ) B( η γ ) = (22.4 ± 3.8 ± 2.0) 10 B( pp χ B( pp B( pp c c0 χ ) B( χ c2 c c0 γ ) = (6.5± 1.2± 0.6) 10 χc2) B( χc2 γ ) = 0.017± J / ψγ e e γ ) 8

21 Experiment E835: Detector Detects electromagnetic final states Large hadronic background ~10 6 times the charmonium production rate 1280 block Pbglass calorimeter

22 π 0 π 0 and π 0 γcross Sections π 0 π 0 Both cross sections are forward peaked and decrease rapidly with energy π 0 γ

23 η c Resonance Parameters Mass Total Width