LEP EXPERIMENTS COMMITTEE
Minutes of the 51st meeting,
24 March 1999

OPEN SESSION

1. Summary of the 1999 Chamonix Workshop

R. Bailey reported on the conclusions of the Chamonix IX workshop, January 1999. Although 1998 was a very successful year for LEP, its performance was current-limited by the antenna cable problem and by the available cryogenic power. Both these limitations are now removed; the cryogenic system has been upgraded and can handle up to 12mA total beam current at 7 MV/m, and all (about 500) antenna cables in the Nb/Cu sc cavities have been replaced. With the installation of the final four Nb/Cu sc rf modules, the full complement of rf voltage is available at LEP. The sc rf system comprises 72 modules x 4 cavities = 288 sc cavities (with 1728 m2 of sc surface). Together with the 48 warm Cu cavities, the available accelerating voltage is 2750 MV under the 1998 conditions of 6 MV/m, and up to about 3200 MV if a mean gradient of 7.0 MV/m can be reached. The latter would permit beam operation at 100 GeV. The detailed running strategy in 1999 is to establish stable running conditions in increasing steps throughout the year, as follows: 1) start with 1998 conditions, namely 6 MV/m and 6 mA total beam current, giving a beam energy of 96 GeV, then 2) raise the energy to 98 GeV at 6 mA, then 3) increase the beam current to 8 mA at 98 GeV, and finally 4) increase the accelerating voltage to approach 100 GeV beam energy.

The successful 102/90 beam optics will be retained for 1999 - both for high energy running and for the Z calibration data. Some improvements are anticipated in the filling times, which may become significant as the coast time becomes rather short (around 3 h). The photon background storms observed in ALEPH and DELPHI in 1998 are now understood and can be controlled by careful adjustment of the machine tune. The luminosity imbalance between the interaction points will be minimised in 1999 within the constraint of maintaining a high overall luminosity at LEP.

A new idea was discussed at Chamonix that could lead to a further increase of about 1 GeV in the beam energy. The idea is to decrease the energy loss per turn by making the quadrupoles participate in the bending of the beam. This could be done by installing dipole coils in the quadrupoles. The feasibility of this scheme, including its cost and possible side effects, are under study.

2. LEP2 physics jamboree

Each experiment reported integrated samples of about 3500 WW events, providing statistical precisions on the W mass of 65-80 MeV. The combined LEP value for the W mass now has an error of under 60 MeV, including systematic errors, which is comparable to the Tevatron value. The lower mass limits on the SM Higgs are 90-95 GeV per experiment. The lower mass limits on the lightest Higgs of minimal SUSY (h,A) are 75-82 GeV per experiment for large values of tan b; the low tan b range (0.6-1.5) starts to be excluded by some experiments.

3. Summary of the LEP2 Monte Carlo Workshop

R. Pittau reported on the LEP2 Monte Carlo workshop, 12-13 March. This brought together theoretical and experimental experts who discussed the open problems facing the LEP2 physics simulations. For several measurements at LEP the theoretical uncertainties are now comparable to the statistical precisions. The workshop has established a platform for close collaboration between the theorists and experimentalists and has identified the pressing tasks for further work; first priority will be given to improving generators for backgrounds to new particle searches.

CLOSED SESSION

Present: R. Cashmore, J. Colas, M. Delfino, M. Doser, F. Gasparini, N. Harnew, K. Hübner,
P.O. Hulth, P. Janot, L. Jonsson, J. Kirkby (Secretary), K.-H. Kissler, W. Lohmann,
M. Mangano, C. Matteuzzi, J. May, R. Miquel, T Müller, S. Myers, L. Pape (representing
V.G. Goggi), D. Schaile, Y. Sirois, A. Smith, M. Spiro (Chairman), R. Tenchini.

1. Approval of the minutes of the 50th meeting

The minutes of the 50th meeting (LEPC 98-9/LEPC 50) were approved without modification.

2. Chairman's report

The Chairman announced that the four experiments have agreed to the presentation of a fast "ADLO" combination of the LEP200 data by the working groups at a special meeting of the LEPC in July 2000. This would allow sufficient time for a decision to be made to extend LEP if there were strong evidence for a major discovery. It was considered important to test the fast combination procedures in advance. To this end, the search working groups are encouraged to reach a consensus on their procedures over the coming months, and they will be invited to present some combined results of the current (1999) data at the next LEPC meeting in September.

3. Discussion on the LEP machine report

The committee was impressed by the thoroughness and creativity of the machine team in reviewing LEP operation and preparing plans for the final two years of LEP operation. Great interest and encouragement was expressed for the new idea to raise the beam energy by an extra 1 GeV by installing extra dipole coils in the quadrupoles, which would reduce the synchrotron radiation losses. The committee looks forward to hearing the outcome of the feasibility study for this programme (which should be completed by June) at its next meeting in September.

The committee fully supports the plans of the LEP machine team for systematically raising the beam energy and luminosity in steps during 1999, as described under point 1 of the open session. In addition are foreseen the standard Z calibration runs of 2.5 pb-1 at the start of the cycle, and
0.5 pb-1 near the end. The highest energy may be approached by the summer. In any event, LEP cannot operate above 100 GeV per beam until the necessary authorisation in the framework of the LEP INB Convention is received from the French Authorities. This is expected before the end of September 1999.

4. Discussion on the LEP experiments

The committee congratulated the LEP experimental community for the impressive physics results presented in the jamboree, reflecting the excellent operation of LEP in 1998. It was noted that the combined LEP value for the WW cross section at 189 GeV is about 2s below the theoretical expectation, which underscores the importance of further work to reduce the present theoretical uncertainty in the higher order corrections. The importance was also stressed of achieving a deeper understanding of the influence on the W mass measurement from Bose Einstein effects and colour recombination. This will require a coordinated approach across the combined LEP data and, as such, is an important part of the work of the W physics working group.

Concerning the LEP data archive, the committee strongly supports the maintenance of the necessary hardware and software tools to allow analysis of LEP data in the period 2001-2006. CERN undertakes to provide the necessary support in terms of storage and software - although it was pointed out that CERNLIB will end in 2003. Both CERN management and the committee emphasized the need for the LEP experiments to continue to upgrade their desktop computing equipment during this period. The continued analysis of LEP data will require a significant financial investment into desktop computing by the LEP collaborations over the six-year period after LEP has shut down. The committee looks forward to hearing definite plans and requests to CERN from each of the experiments for this extended data analysis. These could be included in the experimental status reports in open session of the next LEPC.

5. Discussion on the LEP2 Monte Carlo workshop

The committee expressed its appreciation of the progress made in the LEP200 Monte Carlo workshop, and strongly encourages the continuation of this activity within the framework of a new LEP working group comprising theoreticians and representatives of the four experiments. Following an outline by M. Mangano of the draft proposal from the workshop, the committee supports the plan to prepare a detailed document by the end of the year and to continue this activity through 2000. The Chairman will discuss these ideas with the spokesmen of the four experiments and, after consultation, nominate a chairperson for the new working group.

6. Report from the LEP Coordinator

The LEP coordinator presented the LEP schedule for 1999, which has been approved by the Research Board. Physics will start on 10 May and stop on 25 October, with a total of about 160 days for LEP operation. The final date is limited by the budget for electrical energy; the two weeks (43 and 44) immediately following the scheduled stop are kept in reserve in case of an unforeseen interruption of LEP operation earlier in the year.

The LEP coordinator also presented the results of detailed studies of how the luminosity and energy of LEP can be traded to optimise the search potential for new particles. The hardest challenge is the search for the standard model Higgs, which requires both luminosity and energy; improving the Higgs mass sensitivity by 1 GeV requires either a 0.5 GeV increase in beam energy or a factor of two increase in luminosity at a fixed energy. In contrast, however, observation of charginos requires only a small luminosity above threshold, which may suggest operating LEP at the absolute maximum energy in its final days.

7. Any other business

The Chairman announced that last meeting of the LEPC is expected to take place in November 2000 (barring a major discovery of course!). The present membership of the LEPC would be invited to continue through until that time.

8. Next LEPC meeting

The dates of the next meeting of the LEPC are Tuesday-Wednesday, 7-8 September. As well as inviting presentations from the search working groups, the LEP experiments will be invited to make brief technical status reports at this meeting. The date of the final meeting in 1999 is 9-10 November.

J. Kirkby