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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 10-15% depending on the mode. - the removal of brem photon changes and improves significantly the reconstruction of the inclusive X mass (mxhadfit) Daniele