ALEPH week WW semileptonic minutes

Minutes of the leptonic session of the WW meeting during the ALEPH week

More on taunuqq selection improvements

Anne Ealet

Anne presented a status report on her work to improve the tnqq section.
Working on the new NN-based tnqq selection she realized that a limiting factor of the selection performance was the preselection step:
the efficiency after the preselection was only 75% and after the preselection the agreement data vs MC of the variables used for the NN
was poor since the preselection was not very effective against two photon events and we miss two photon MC events.
Anne tried to change the preselection introducing a missing p_t cut at 20 GeV. The efficiency after the new preselection is about 80% and
the agreement data vs MC is much better. A NN based on 14 variables (not using the hadronic mass to make this selection usable for the
mass measurement too) has been trained at different energies; requiring the same purity of the official selection (78%) the efficiency increases, on average, from 58% to 65% leading to a 9% relative improvement on the cross section statistical error.
Combing this selection with the new enqq and mnqq selections she presented in Heidelberg the overall relative improvement of the cross section statistical error is about 4%.

Fragmentation studies in semileptonic events
Jason Ward

Jason presented a status report of his fragmentation studies with enuqq and munuqq events.
Comparing event-by-event Herwig and Jetset he found that Herwig mass is higher than JETSET by about 20 MeV (mean event-by-event mass difference). The mean dijet angle is larger in Herwig than in JETSET: if the jet masses were equal between Herwig and JETSET this could explain why Herwig W mass is larger than JETSET.
Other findings are:

Looking at the particle composition at the generator level it was found that there are more baryons in JETSET than Herwig.
Moreover the reconstructed mass is the lower the larger is the number of neutrons, protons and kaons in the event.
It was found that Herwig jets are lighter than JETSET jets. Does it compensate the wider opening angle w.r.t. the mass measurement?
At the reconstruction level the charged object multiplicity distribution is broader in Herwig than in JETSET and the mean neutral multiplicity is lower in Herwig than in JETSET. The discrepancy in the charged sector are present at the generator level too.
The total jet energy is larger in Herwig than in JETSET, at the reconstruction level.

Resuming his studies on the charm sector, Jason found no striking effect on the Herwig-JETSET difference as a function of the charm content of the event.
Concerning the possibility to extract a fragmentation systematic uncertainty by comparing data and MC Jason expressed his doubts about the present attempts based on the mean comparison or the reshaping of distributions of variables sensitive to the fragmentation. Anyhow no real progress in this field.
Finally looking at the interjet angle and comparing the jet direction and the jet profile w.r.t. the quark direction he found that:

the JETSET dijet angle is smaller than in Herwig
the jet momentum profile is asymmetric about the quark direction: larger momenta between the two jets
JETSET jet profile is broader than Herwig

New fully leptonic selection

David Fayolle

David presented a status report of his studies to improve the fully leptonic event selection.
Details can be found in this site where the transparencies are collected.

Discussion was triggered by the attempt to relax the E12 veto by rejecting the events with E12 > 10 GeV. To be investigated further.

New and Old semileptonic ntuples: event by event comparison
Andrea Venturi

Andrea began to compare event by event the reconstructed lepton and jets in semileptonic events selected and reconstructed by the old code, used for the published 189 GeV mass paper and by the new code used to produce the common ntuple (version 1.0). He found many "little" differences among the two codes which explain almost all the observed discrepancy in the reconstructed lepton and jets.

SICAL cleaning is applied to the old ntuples only since it is not available in the 189 GeV MINI (used for this test) yet
The lepton track acceptance is wider in the new code (|cos(th)|<0.975 instead of 0.95)
Different prescription to extrapolate the lepton track ( the old prescription had some nasty side effects on the energy flow)
Different algorithm to lock the neutral objects close to the lepton due to missing pieces of information in the MINI
Different algorithm to evaluate the isolation angle w.r.t. the charged tracks (only good tracks were considered in the old code)
Different angular cut in the routine which correct the lepton energy for the bremstrahlung photon
Different algorithm for the bremstrahlung correction : lepton rescaling (old) vs lepton+photon sum (new)
One missing piece of information in the MINI: estimated multiple scattering between ITC and TPC.

Once this differences are removed the number of events which differs by at least 50 MeV in the lepton or in the jet sum energy and momentum is about 6 in 36k WW events. More studies will be performed in the future aiming at the release of a new version of the common ntuple code which should include all the nice features of the old code which were lost in the transition.