Dear Collaborators, 

I am posting my Ph.D thesis in its final-submission form. It was submitted
to McGill University in early January 2000.  The supervisor was Ken RAGAN.

The title and the abstract of the thesis are given below.


Best regards,

Kostas KORDAS
McGill University
Montreal, Canada


PS:  Note that the analysis corresponds to the last blessed results 
     (Sept. 10, 1999 B meeting), also documented in CDF-Note 5113 version 1.0




            Search for Penguin Decays of B Mesons at CDF
            

Using a data sample of integrated luminosity \int{L dt} = 28.9 +/- 1.2 pb^{-1}
of proton-antiproton collisions at a center-of-mass energy \sqrt{s} = 1.8 TeV
collected with the CDF detector at the Fermilab Tevatron collider,
we searched for ``penguin'' radiative decays of B_{d}^{0} and B_{s}^{0} mesons
which involve the flavor-changing neutral-current transition of a 
b quark into an s quark with the emission of a photon, b -> s gamma.
Specifically, we searched for the decays
B^{0}_{d} -> K^{*0} gamma,  K^{*0} -> K^{+} pi^{-}
and 
B^{0}_{s} -> phi gamma, phi -> K^{+} K^{-},
as well as for the charge conjugate chains.

In order to collect such decays, we designed a specialized trigger which
required information on all the decay products of the B meson decay chain,
the first such trigger in a hadron collider environment.
This ``penguin'' trigger collected  data during the last quarter of the 
1994 - 1996 data-taking period. 
After all selection criteria,  we are left with one candidate 
B^{0}_{d} -> K^{*0} gamma,  K^{*0} -> K^{+} pi^{-} decay and no
B^{0}_{s} -> phi gamma, phi -> K^{+} K^{-} candidates in the entire data 
sample. 
We then proceed to set upper limits on the branching fractions of the 
penguin channels.

We exploit the topological similarity between the
B-bar -> e^{-} D^{0} X, D^{0} -> K^{-} pi^{+} and the penguin decays, 
by forming ratios of branching fractions between the penguin and the 
B-bar ->  e^{-} D^{0} X channels.
Uncertainties associated with the B meson production cross section,
common efficiency corrections and other systematic effects 
are minimal in the ratio of branching fractions. The uncertainty on the
B-bar ->  e^{-} D^{0} X  yield is the biggest contribution to the total 
uncertainty on the penguin branching fraction. 
We assume equal production rates for B_{u}^{+} and B_{d}^{0} mesons,
while the probability of producing B_{s}^{0} mesons relative to 
B_{d}^{0} mesons, f_s/f_d, is taken to be 1/3.
The inferred upper limits on the ratios of branching fractions are 

  Br(B^{0}_{s} -> phi gamma)
  --------------------------     <  3.5 x 10^-3  at 90% CL
  Br(B-bar -> e^{-} D^{0} X) 


  Br(B^{0}_{d} -> K^{*0} gamma)
  -----------------------------  <  1.9 x 10^-3  at 90% CL
   Br(B-bar -> e^{-} D^{0} X)


Relative branching fraction measurements were combined with the branching 
fraction measurement of the 
B-bar -> e^{-} D^{0} X, D^{0} -> K^{-} \pi^{+} decay chain, 
Br(B-bar -> e^{-} D^{0} X) \times Br(D^{0} -> K^{-} \pi^{+}) = 
  (294 +/- 40) \times 10^{-5}, 
to extract the following absolute branching fraction limits

 Br(B^{0}_{s} -> phi gamma)     < 2.8 x 10^-4  at 90% CL

 Br(B^{0}_{d} -> K^{*0} gamma)  < 1.5 x 10^-4  at 90% CL

The upper limit for the B^{0}_{d} -> K^{*0} gamma decay is consistent
with the branching fraction measurement reported by the CLEO collaboration, 
Br(B^{0}_{d} -> K^{*0} gamma) = (4.0 +/- 1.9) x 10^-5,
while the upper limit for the as yet unobserved B^{0}_{s} -> phi gamma decay 
is the most  constraining one set to date. 





Posted to /cdf/pub/thesis/cdf5228_gkstar_thesis_kordas.ps
    http://www-cdf.fnal.gov/thesis/cdf5228_gkstar_thesis_kordas.ps

Posted by kordas@cdfsga.fnal.gov