CDF Logo Limit on the rare decay W -> π γ in ppbar Collisions at √s=1.96 TeV CDF Logo



Measuring rare particle interactions and decays test the Standard Model at the limit of our understanding. A number of rare radiative decays of the heavy W and Z bosons to a photon and charged or neutral meson are predicted to occur in the standard model but remain unobserved despite the continuous production of these particles at high energy collider experiments. These processes are sensitive to beyond the standard model physics, especially involving anomalous coupling of the photon to the W and Z bosons, but are supressed by meson formation at energy scales around Mw/z.

Among the rare W and Z decays, the radiative decay W ->πγ is an experimentally interesting channel because it results in a clean final state in the detector. Existing theoretical calculations of the decay predict the standard model branching fraction BR(W->πγ)/BR(W->eν) to be very small ( 10-6 -10-8). Previous analysis at the UA1 and UA2 experiments and the Collider Detector at Fermilab (CDF) Run I experiment at the Tevatron p+p- collider have set progressively lower upper limits on the branching ratio. We present a measurement of the normalized branching ratio, BR(W->πγ)/BR(W->eν) at CDF using \luminosity\ of \ppbar\ collisions at 1.96 TeV.

Feynman Diagrams for two contributing processes to W ->πγ decay

Summary of Results

We select candidate W-> πγ from the PHOTON_25_ISO Level 3 Trigger at CDF. Candidate πγ must contain a central photon (Et>25 GeV, ) passing standard CDF photon selection and a central charged track(pt>25 GeV) pointing to a calorimeter energy cluster passing pion selection developed and optimized in this analysis. We extract the limit from 4 bins around the W-mass in the π-γ invariant mass distrubtion, where our backgrond is estimated in a fit from the sidebands.

BR(W->πγ)/BR(W->eν) < 6.4 x 10-5 at 95% confidence

GIF or EPS Observed π-γ candidate events and
expected background fit from side-bands
GIF or EPS Fit residuals scaled by statistical bin error
TEX or GIF Background expectations
observed events in the W signal region.

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