Resolution and radiative corrections A first order estimate for pbar p  e + e - T. H. IPN Orsay 05/10/2011 GDR PH-QCD meeting on « The nucleon structure via proton-antiproton annihilation

ep scattering vs annihilation In electron scattering (ep  ep), usually electrons are detected: – One single scattering angle  with a small   E e is fixed – Momentum resolution is good (a few in spectrometer) – Electrons travel in vacuum – Cut adjusted to minimize the error on the radiative corrections In annihilation at PANDA; pbar p  e + e - – 2 electrons – Detection at all angles (  – Momentum resolution is moderate (a few % at 1 GeV/c transverse momentum ) – Matter along the lepton path length Questions – Position of the cut ? – Dependance on the angle ? Resolution R=  p /p scales like p T X/X 0 depends on angle – Can the PANDA EMC detect the associated photon(s) – What are the effect of Kalman filtering and Kinematical Fit?

ep scattering vs annihilation In electron scattering (ep  ep), usually electrons are detected: – One single scattering angle  with a small   E e is fixed – Momentum resolution is good (a few in spectrometer) – Electrons travel in vacuum – Cut adjusted to minimize the error on the radiative corrections In annihilation at PANDA; pbar p  e + e - – 2 electrons – Detection at all angles (  – Momentum resolution is moderate (a few % at 1 GeV/c transverse momentum ) – Matter along the lepton path length Questions – Position of the cut ? – Dependance on the angle ? Resolution R=  p /p scales like p T X/X 0 depends on angle – Can the PANDA EMC detect the associated photon(s) – What are the effect of Kalman filtering and Kinematical Fit?

The method p p  e + e - Choose selected  CM (30°,40°,…,90°) Calculate corresponding laboratory angle Calculate the momentum resolution according to (see Technical Design Report of CTS, page 87) fitted on STT muons tracks with 0.2 < p T <2.5 GeV/c Calculate the e + e - invariant mass; m inv (e + e - ) Calculate the photon energy according to : (conservation of E,P) Plot E  distribution width against  CM

 CM (e + ) (deg)  Ephot (MeV) Curve is symmetric wrt 90° CM angle Weak  CM dependance   grows linearly with p inc  A cut at -2  would correspond to a 8% loss Question: which fraction of the hard/soft photons is below this -2  limit? The result

Assumption: 4% X 0 – 70% of the electrons lose 1 MeV/c or less – 80% of the electrons lose 10 MeV/c or less – 90% of the electrons lose 100 MeV/c or less At 80%, the corresponding average energy loss is always much less than the resolution, but still 20% with a higher energy loss!  a « full simulation » is necessary to « measure » the impact of external Bremsstrahlung. Even if tracking cannot resolve a 1 GeV/c electron from a 0.99 GeV/c one, can we ‘detect’ this 10 MeV  in the calorimeter and then put this condition our event selection process? Still a lot of work Effect of Bremsstrahlung in detector material Almost independent of the energy