Short Biased Minutes of

W meeting 26/1/2000 in 40/SS-C01 from 9 am to 1 pm

1) Status of CC03 cross sections with DPA (Reisaburo)

Double Pole Approximation (DPA) is a double expansion in alpha_qed (first order) and GammaW/MassW (order zero), valid for centre of mass energies above 170 GeV and with expected uncertainty of 0.5%. The two very recent programs using this techinque are RacoonWW and YFSWW. For the latter see the presentation of Placzek (meeting of 01/12/99) , as far as RacoonWW is concerned we will have a presentation of Dittmaier next meeting (16/2/2000). The first numerical results using RacoonWW give a cross-section at 189 GeV lower by 2.8% with respect to Gentle. This result should be lowered by 0.5% because the soft photon exponentation is not yet available with RacoonWW. In conclusion we expect a -2.3% difference with respect to the presently used WW cross-section. There are also calculations available for the Higgs mass depencence, which is very small for m(higgs).gt.100 GeV. We expect a comparison between the various programs to be ready for the winter conferences.

2) Effects of improved theoretical calculations (YFSWW) (Andrea)

Andrea discussed how in practice we can insert the new results in the W mass measurement, in the WW cross section measurement and in the TGC's. One important point already stressed by Placzek is that the 4f corrections should be calculated with the old tools (for instance Koralw) as the new programs are CC03 only. The new calculations induce both a lowering of the theo cross section and of the expected angular distributions, especially on the W production angle. No distortion of the presently used invariant mass spectrum due to ISR/FSR is expected (we use Photos). Andrea proposed to start our studies by first running YFSWW as a standalone program to compare the distributions, then use kingal-level YFSWW to rreweight koralw cc03 samples, then maybe go to full simulation. A different issue concerns the non-factorizable O(alpha) corrections, available in RacoonWW and not in YFSWW. They are responsible for a 5 MeV mass shift in the mass peak with respect to Koralw, while they have no effect on the cross section. The actual effect on our mass measurement has to be evaluated, it is expectd to be small, anyway.

3) Status of Colour Reconnection in Herwig (Gerald)

Here it is a brief history since Siena with the words of Gerald himself.

on 9 Nov 99: discovered that NO color reconnections happen between W's in HERWIG WW->4q events. This was due to a program bug (known to the authors at that time!) such that the Lorentz-invariant space-time distances of final partons were only |Delta x| = 10^-5 fm, tiny compared to the W decay distance = 0.1 fm. (This also means that the W mass shifts quoted for HERWIG in our 183 GeV W mass paper had this bug and were actually zero) Correction of the bug, and retuning the fragmentation parameters at the Z -> rather strong effects are seen even for a standard value of Preco=1/9: - excess charged multiplicity(for p_T>0.2 GeV) Delta N_ch (4q-2*2q) = 1 unit, mainly in the soft region - reconnections between W's, per event, Delta N_rec = 0.15 (compare: 0.10 reconnections inside each W) - shift of reconstructed W mass, relative to the no-reconnection case, Delta M_W = 100 MeV (at generator level) With this version, MC production was done.

on 4 Dec 99, B.Webber noted that VMIN2 = minimum squared virtuality of quarks and gluons is a sensitive parameter, and its default value 10^-4 GeV^2 is too small leading to unphysical distances |Delta x| approx 1000 fm. A more reasonable value is 0.1 GeV^2, i.e of the order of the hadronic mass scale, giving |Delta x| approx 1 fm. (The formula for partons with on-shell mass m_0 is |Delta x|=(hbar c) m_0/VMIN2; m_0(gluon)=0.8 GeV, hbar*c=0.2 GeV*fm ) Again, fragmentation parameters were retuned with this setting (little change). Z data are not very sensitive to VMIN2, but 10^-4 seems disfavored; the global fit gives values around (0.6 GeV)^2. On the other hand, Delta M_W is very sensitive to this parameter. Effects between W's are now much reduced (with VMIN2=0.1 GeV^2): Delta N_ch = 0.11, Delta N_rec = .06, Delta M_W = 40 MeV. This is the version to be used for the next MC production.

Everybody thanked Gerald for his efforts to obtain reasonable modelling of CR from Herwig !

4) 1999 WW cross section after reprocessing (Julian)

The cross section after reprocessing is available, still using LEPC Monte Carlo. Very small changes with respect to LEPC results. We are in good shape to provide preliminary results to La Thuile (Maria's talk !)

5) LEP WW working groups : the Crete project (Ann)

The effort to understand in details and compare the various LEP WW analysis is continuing after Crete. An informal workshop is planned just before Easter at cern, a longer workshop in the style of last year Crete will take place in the autumn. Participation to these activities and to preparation meetings is warmly welcomed !

6) Status of Single W (Vincent)

The result to be used for the 189 GeV TGC paper is close to the one given to Tampere, with re-evaluation of the systematic errors. A problem has been encountered in computing the calorimeter systematic error, after the meeting the problem has been understood. It was due to a clash between the routine which rescales PECO (and therefore SICAL objects, too ! It is now protected from this !) and the sical cleaning. The E12 veto which is important for single W made this analysis particularly sensitive to this "feature". Also the fact that the syst error was computed on data and not on a large MC sample was not helping. On the medium term we hope to have an improved single W selection (NN based), the use of TGCFITTER to extract the couplings and the use of the angular information to extract the couplings. It has been recommended to keep the present analysis as a backup and for comparisons.

7) The TGC's (Tim + Gonzalo)

Please have a look at the minutes of the W jamboree (24/1/2000) to see the status. We decided to present the results and propose a paper to the collaboration during the thursday meeting of 27th of january. This has actually happened and the paper is being written. Still some work to do on the comparison of the results from the 3 techniques.