Hi all,
I started investigating the possibility of using the Q2 variable. I just
looked at possible advantages in the statistical error (no theoretical
error was taken into account) adding a cut on Q2.
First, these are the Q2 vs Mx plots for the signal and the background:
http://www.slac.stanford.edu/~daniele/vub/Q2vsMx_vub.ps signal
http://www.slac.stanford.edu/~daniele/vub/Q2vsMx_vcb.ps vcb bkg
Then I studied the dependence of the statistical significance as a
function of a cut on Q2 (for different cuts on Mx). This is the result for
Mx<1.55GeV.
http://www.slac.stanford.edu/~daniele/vub/mx155.ps
top left: S/sqrt(S+B) vs cut on Q2 (S=vub events, B=vcb events both
for Mx < Mxcut)
top right: S vs cut on Q2
bottom left: S vs cut on Q2
Similar plots for different Mx cuts.
As you see, no gain comes from an additional cut on Q2 (as far as the
statistical error is concerned).
BTW a study of the result as a function of this cut seems to be feasible.
The MC statistics used corresponds to ~200 fb-1 (with BR(b->ulnu) = 1.3
* 10-3). With the number of signal events we will have in 80fb-1 (>200) we
could perform plots similar to the ones I showed above. It should be very
interesting to check the stability of the result as a function of the Q2
cut (since theorists think that the best theoretical error should be
obtained with a combined cut).
Daniele
