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

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2 photon physics @ babar

Trigger Workshop CalTech – 2nd Dec 2004

2 photon physics @ BaBar

Jamie Boyd


  • 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

Jamie Boyd


  • 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)

Jamie Boyd


  • 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)

Jamie Boyd

2 tracks efficiency
γγ -> 2 tracks - Efficiency

pi+ pi- MC (not near threshold – see next slide)

Skim inefficiency

all from BGF

Jamie Boyd

2 tracks l1 data eff
γγ -> 2 tracks – L1 Data Eff

2 Track data (Etot<8GeV, Ntrack = 2, Sum of Pt <0.2, track mass set to Kaon mass )

Pt ~ sqrt((W/2)2 - mK2)sin θ

Turn on different

in MC and data

(only data shown

I'm afraid)

Other L1 lines

2M&A+ and


contribute to the

efficiency but

probably have

similar Pt turn on

Jamie Boyd


2 tracks bgftwoprongfilter
γγ -> 2 tracks - BGFTwoProngFilter

Can we edit this or write a new one with better efficiency?

Put a total Energy cut on to kill BhaBha’s?

Jamie Boyd

Tagged 2 tracks
Tagged 2 tracks

Tagged pi+pi- MC where all 3 tracks are reconstructed

Jamie Boyd



All 4 gammas reconstructed

L3 eff low for

W<1.8 GeV

BGFAN eff lowish

after that

Can we do pi0

making in L3?

& in BGFAN?

Jamie Boyd


0 0 tagged
γγ->π0π0 tagged

All 4 gammas and tagged electron reconstructed

Presence of tag

track improves

L3 eff but now

we don’t pass

any BGFilter!

Jamie Boyd


Mc versus data
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)?

Jamie Boyd


  • 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

Jamie Boyd

Qcd stuff
QCD stuff…

  • R = Γ(γγ->π+π-) / Γ(γγ->π0π0) 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 Q2 which is predicted by pQCD (eg. for γγ->π+π-)

Jamie Boyd

Meson spectroscopy
Meson Spectroscopy

  • Look at ηC, ηC(2S), a0(980), f0(980), f0(1370), f2(1270),a2(1320),f2’(1525),f2(1720), χc0, χc2 ….

    • Measure mass, width (Γ) and partial width (Γγγ)

      • This can test QCD models (eg. Ratio Γ/ Γγγ well predicted for ηC => can measure αS)

    • Can also use these resonance's produced by 2photon collisions to study the decay process eg. Gautier is looking at the ηC -> KKπ Dalitz plot

  • For untagged can observe any C=+1 J=0,2

  • For tagged can also see J=1 (Yangs theorem)

Jamie Boyd