Hi all,
when studying momentum resolution in lepton ID I realized that
that bremsstrahlung for electrons can introduce large biases in
p* efficiency and resolution. It can also affect the mm2 resolution
and finally it produces large biases in the inclusive Mx reconstruction.
I the past we always neglected this point.
A couple of examples. Here is the p* resolution comparison
between e and mu:
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/resoelemu.eps
that shows that the effect is large. And this is mm2 for pi0lnu:
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/mm2pi0elemu.eps
(for the inclusive calculation of mm2 the effect is smaller because the
brem photon is included in the X system and then recoil has it).
I implemented a brem recovery algorithm that is very similar to the one
in Beta. Code is in VubAnalysis (in util.icc).
It is based on two variables
 Delta(theta): it is the difference between the theta of the brem photon
and the electron one. They should be the same within resolutions.
 DD = (Phi(gamma)Phi(e production))/Phi(e EMC)Phi(e production)): here
I am requesting that the phi of the gamma is in between the phi of the
electron at the EMC and the phi at the origin. I used this ratio
(instead of a simple cut on Phi(gamma)Phi(e production) as in Beta)
because numerator and denominator are somehow proportional.
Here the distributions for electrons
Delta(theta) (for 0.3<DD<0.6)
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/Deltatheta.eps
DD (for 0.04<Delta(theta)<0.04)
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/DD.eps
I finally select brem photon with these cuts
0.04<Delta(theta)<0.04 && 0.3<DD<0.6
and I redefined the electron p4 as p4+p4(brem photon). Moreover the
brem photons are removed from the good photon list (goodPhoton[i] = 0)
The following plots are a comparison with and without the correction:
resolution in p*:
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/resoelecorr.eps
mm2 for pi0lnu:
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/mm2pi0elecorr.eps
inclusive mm2
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/mm2elecorr.eps
(small effect as expected, see above, mostly due to gain in p* cut
efficiency)
inclusive fitted X mass (mxhadfit)
http://www.slac.stanford.edu/~daniele/exclvub/bremstudy/mxhadfitelecorr.eps
Conclusions:
 brem recovery improves largely the resolution in p* and the efficiency
of p* cut (~+5%)
 it improves mm2 resolution in exclusive calculation of mm2. total mm2
cut efficiency increase by 1015% depending on the mode.
 the removal of brem photon changes and improves significantly the
reconstruction of the inclusive X mass (mxhadfit)
Daniele
