1. 2 photon physics @ BaBar Trigger Workshop CalTech – 2 nd Dec 2004 Jamie Boyd

2. 2nd Dec 2004Jamie Boyd2 Introduction PEP-II is a: B-Factory / Charm Factory / Tau Factory Also a 2 Photon Physics Factory Little 2 Photon analysis done to date with BaBar data though Trigger and Filter very important for 2 photon physics as Low multiplicity final states Low energy/mass final states

3. 2nd Dec 2004Jamie Boyd3 Overview This is a very short talk with 2 aims Try to show that there is interesting 2 photon physics that can be done at BaBar We need to think about this physics when making trigger/filter decisions Will show simple MC study results for 2 photon -> 2 charged tracks (tagged & untagged) 2 photon -> 2pi0’s (tagged & untagged)

4. 2nd Dec 2004Jamie Boyd4 Motivation BaBar has orders of magnitudes more stats than most of the old experiments where many 2 photon measurements were made – AND has better detector BUT trigger/filters and detector not optimised for this type of physics QCD studies Distinguish between different QCD models – test pQCD Resonance structure Can study resonance parameters and partial widths Glueball / Exotic particle hunting Glueballs shouldn’t couple to photons (much)

5. 2nd Dec 2004Jamie Boyd5 γγ -> 2 tracks - Efficiency pi+ pi- MC (not near threshold – see next slide) Skim inefficiency all from BGF

6. 2nd Dec 2004Jamie Boyd6 γγ -> 2 tracks – L1 Data Eff 2 Track data (Etot<8GeV, Ntrack = 2, Sum of Pt <0.2, track mass set to Kaon mass ) Turn on different in MC and data (only data shown I'm afraid) Other L1 lines 2M&A+ and 2BM&2M contribute to the efficiency but probably have similar Pt turn on W(GeV) Pt ~ sqrt((W/2) 2 - m K 2 )sin θ

7. 2nd Dec 2004Jamie Boyd7 γγ -> 2 tracks - BGFTwoProngFilter CUTAbsolute eff Cosmic Veto: (abs(cosLab0+cosLab1)<0.10.74 Plab of both tracks>1Gev0.64 Highest Pcms track in EMC (cos theta >-0.75)0.64 Pt balance (Pt1+Pt2<0.3)0.74 EMC cluster energies <3GeV0.80 All0.57 Can we edit this or write a new one with better efficiency? Put a total Energy cut on to kill BhaBha’s?

8. 2nd Dec 2004Jamie Boyd8 Tagged 2 tracks Tagged pi+pi- MC where all 3 tracks are reconstructed W(GeV)

9. 2nd Dec 2004Jamie Boyd9 γγ->π 0 π 0 L3 eff low for W<1.8 GeV BGFAN eff lowish after that Can we do pi0 making in L3? & in BGFAN? All 4 gammas reconstructed W(GeV)

10. 2nd Dec 2004Jamie Boyd10 γγ->π 0 π 0 tagged All 4 gammas and tagged electron reconstructed Presence of tag track improves L3 eff but now we don’t pass any BGFilter! W(GeV)

11. 2nd Dec 2004Jamie Boyd11 MC versus Data These plots made using MC Very important that MC models L1 / L3 efficiencies well (even at low Pt, low energy etc..) We know that the Pt turn on for PTD isnt well modelled in current setup (hopefully will be better for ZPD) Will have to carry out dedicated trg studies using the data for L1/L3 versus Pt (tracks) / E (neutrals) For tagged decays the MC/Data comparison for L1/L3/BGF maybe worse for very backwards tracks (the tag)?

12. 2nd Dec 2004Jamie Boyd12 Summary There is lots of good 2 photon physics that should be done at BaBar For data already taken L1 efficiency generally OK L3 ok except for all neutral mode Can we improve 2 prong & AllNeutral BGFilter For new data Can we make L3 more efficient for pi0pi0 When improving L1&L3 trigger algorithms can we think about 2photon physics

13. 2nd Dec 2004Jamie Boyd13 QCD stuff… R = Γ(γγ->π + π - ) / Γ(γγ->π 0 π 0 ) predicted to be very different by different QCD models (pQCD versus ‘handbag model’) γγ->p + p - spectra (as function of W) can also distinguish between models (di-quark / pQCD) Angular distributions for γγ-> h + h - can be predicted by pQCD For tagged decays you can look at how the cross-section scales with Q 2 which is predicted by pQCD (eg. for γγ->π + π - )

14. 2nd Dec 2004Jamie Boyd14 Meson Spectroscopy Look at η C, η C (2S), a 0 (980), f 0 (980), f 0 (1370), f 2 (1270),a 2 (1320),f 2 ’(1525),f 2 (1720), χ c0, χ c2 …. Measure mass, width (Γ) and partial width (Γ γγ ) This can test QCD models (eg. Ratio Γ/ Γ γγ well predicted for η C => can measure α S ) Can also use these resonance's produced by 2photon collisions to study the decay process eg. Gautier is looking at the η C -> KKπ Dalitz plot For untagged can observe any C=+1 J=0,2 For tagged can also see J=1 (Yangs theorem)