- Systematic study on e+e- -> qqbar at sqrt(s)=183 GeV ---------------------------------------------------- Marie-Noelle has studied the dependence of the statistical and systematic errors of the measurement of the hadronic x-section as a function of the cut on s'. The LEP working group has decided to use the cut sqrt(s'/s)>0.85, but it is not a priori clear if this is the best choice for ALEPH. The sources of systematic errors are: In the efficiency (96.2%) we have an (absolute) error of 0.6% from the understanding of the radiative return and of 0.7% from the energy scale. The latter is estimated by scaling coherently all ECAL (HCAL) energy deposits by 1.5 (4) % and then reapplying the E-Flow. The WW background amounts to 28.6% at sqrt(s'/s)>0.85, with an error of 1.2%, again from the energy scale. Here the biggest contribution comes from semileptonic WW events, since the rescaling of the energy of the hard electron has a big impact. It has been discussed if one should cut harder to remove this kind of events, for example via a cut on Mvis/Elep < 0.7, which removes most of them. The efficiency would go down, however. The double radiative events have been studied via a comparison of the scatter plot in Mvis vs. sqrt(s'/s) between data and MC. Double radiative events lie mostly in a slice with 70GeV < Mvis < 107 GeV and 0.6 < sqrt(s'/s) < 0.8. There a difference of about 5.7% is observed. A similar check on Z returns (medium s') shows agreement within 3%. In summary, when plotting the efficiency and backgrounds vs. the s'-cut, one observes a decrease of the WW background from 30% (at 0.8) to about 16% (at 0.95), and a similar decrease of the double radiative events from 10% to 5%, with the efficiency going from 97% to 85%. When looking at the resulting total systematic error, an optimal working point appears to lie at sqrt(s'/s) = 0.92. The conclusion is that in ALEPH we should rather use a cut on sqrt(s'/s) of 0.9. However, the analysis will also be done for 0.85 in order to facilitate the combination with the other experiments. Marie-Noelle has not yet redone the 183 analysis, since she is waiting for the final reprocessing. She also has to check and monitor the gamma-gamma trigger, which apparently has changed during the run, as the rate is not stable.
- Update on Rb measurement at LEP2 (Mario Antonelli) -------------------------------------------------- b-tag efficiency ------------- Mario has found the explanation for the problem he reported on at the meeting in October, namely that the b-tag efficiency was found to be different at the different centre-of-mass energies. Whereas the impact parameter is quasi lorentz invariant, the distance of the secondary vertex from the primary one scales with the boost. He found out that in QIPBTAG there is a fixed cut on this distance of D<1.5cm, in order to reject conversions, nuclear interactions and long lived particles. If he scales this cut with the boost, he gets pratically the same efficiency at every energy point, i.e. at 91 GeV and at the higher energies. This discrepancy was important to understand since he wants to use the 91 GeV data and the world average to estimate the systematic errors of the high energy measurements. 1997 results: ----------- He used the old reprocessing up to now, the new reprocessing will be used soon. The s'-cut he applies is 0.85. From the 183 MC he gets a b-tag efficiency of 41+-2%, with a purity of 96%. The backgrounds are 2% from WW and 4% from ZZ. From the 183 data he has analyzed a sample of L=55.92 pb-1. This amounts in 114 tagged events, giving a bbar x-section of sigma_bb(183) = 4.45 +- 0.42 (stat) pb whereas Zfitter predicts 4.00 pb. From the 130 GeV MC (1997) he gets an efficiency of 43% and a purity of 97%. In a data sample of L=6.18 pb-1 he finds 37 b-tagged events, which translates into a cross section of sigma_bb(130) = 13.5 +- 2.2 (stat) pb to be compared to the prediction of 15.57 pb. Regarding the 136 GeV data, he combined 95 and 97 data. He finds 39 events, giving sigma_bb(136) = 12.3 +- 2.0 (stat) pb to be compared to 12.23 pb predicted. If taking the 95 data, he has to check if the missing active layers of the VDET are taken into account. If not, one could just suppress the MC hits found in layers which actually were inactive. Remarks and Conclusions : ---------------------- He points out that for the Z data in 1997 the old reprocessing was "bad", in the sense that about 1pb-1 are not usable due to bad calibrations. The new reprocessing gives a b-tag efficiency of 51 % (note that he changed the cuts now) and a purity of 95%, resulting in Rb = 0.213 +- 0.002 (stat), the world average being Rb = 0.2170 +- 0.0009 . He concludes reminding that the "efficiency problem" has been understood, that preliminary measurements of sigma_bb at 130,136,161,172 and 183 GeV have been obtained, that he is going to reanalyse all the data after the new reprocessing is finished, that he wants to use the Z data to estimate the systematic errors and finally that an ALEPH note is in preparation. Marie-Noelle points out that one doesn't take into account the VDET problems we experienced this year (e.g. rphi short etc) if one takes the Z data to estimate the systematic error. Ian proposes to look at the negative part of the impact parameter distribution to compare data and MC at different energies and to study the stability over the running period. He thinks that the resolution at the highest energies could be better because of reduced multiple scattering and wonders if this could have an impact on the systematics. Mario will perform the proposed checks.
- Discussion ---------- Dieter: ------- We should have results ready for the end of January, however, we have to wait for the reprocessing. The contents of the paper would be the x-sections, asymmetries, Rb and limits (contact terms, Z', leptoquarks). Marie-Noelle: ------------ She points out that there is a problem for the agreement data-MC, in the sense that the Julia version used for the MC is not the same as for the data. She would like the MC to be reprocessed. We also have to wait for an understanding of the ECAL problems. Frederic: --------- Regarding the ISR-FSR interference effect, we will have an estimation of its impact on the efficiency in time, but won't have a full MC very soon. That's what he learned from Was and Jadach. Marc and Elsa: ------------- The tau MC was found to be wrong (the Branching ratio was wrong). Marc has announced it to the MC Web page maintainer, but there seems not to be any warning on the Web. Dieter asks for an ALNEWS to be posted in order to warn people. Elsa has produced a private MC, which she could move to official ALEPH tapes.
- Next meeting ---> early or mid January
Guenther Dissertori
Tue December 9, 11:59:38 MET DST 1997