NEW CHECKS ON M(ee) IN hee EVENTS BY HZHA AND WPHACT

April 29, 1998

The following figure is an update of the one presented in the previous note .

Now the red histogram displays the (e+e- gamma) mass in hee events at 184 GeV with Mh=60 GeV, where gamma is a final state radiation photon emitted up to 90 degrees wrt an electron. Remember that wphact has no fsr photons (JETSET was assumed to provide them, but this features works only within parton shower, that is for quarks/gluons but not leptons). The agreement below the peak is nearly perfect (the difference for low masses is probably due to generation cuts in wphact, namely min f fbar mass 2 GeV and min ferm en. 1 GeV, and is irrelevant). There is still a significant disagreement above the peak. This may be due to a different treatment of the initial state radiation, that is implemented in both generator (only collinear for WPHACT). In order to check this hypotesis, new histograms have been produced, including in the e+e-mass also the isr photons. The results are presented here.

The agreement is now very good over all the spectrum, except the low mass region (quantitavely very marginal, anyway). The next problem is whether this different treatment of isr/fsr may have a significant impact on the analysis, and what should we believe (wphact is missing of pt in isr and all fsr, hzha may be suspected to overestimate the radiative effects). Since part of fsr should be readsorbed in the electron e.m. shower and the experimental resolution smears the mass spectra, the mass distributions have been produced after full simulation, reconstruction and standard analysis, including all cuts but the btag one. The mass is taken from the 4C fit. The comparison (see next figure) has been done for Mh=80 GeV since only that sample was available for wphact. For comparison hzha wit Mh=60 GeV is presented too.

CONCLUSION: it appears that at this level we cannot observe worrying differences, and we can go on generating MC samples with both generators, but a decision should be taken in the teams.