LEP EXPERIMENTS COMMITTEE
Minutes of the 45th meeting,
29 May 1997

OPEN SESSION

1. Conclusions of 1997 Chamonix Workshop

S. Myers reported on the conclusions of the Chamonix VII workshop, January 1997. The machine will be operated with 90/60 optics in 1997 at a maximum beam energy near 92 GeV. Although 90/60 optics requires a small reduction (0.4 GeV) in beam energy relative to the alternative 108/90 optics, it is expected to result in a higher integrated luminosity. The integrated luminosity in 1997 cannot be reliably estimated in view of the SPS building fire and subsequent cleaning operation, but a delay in LEP startup of up to about 7 weeks is expected. In 1998, a peak luminosity of 1032 cm-2s-1 could be reached at ÷s = 192 GeV, which would result in an integrated luminosity of about 200 pb-1, assuming 120 days of physics.

During 1999 and 2000, vertical movements of the LEP tunnel by up to 30 mm are expected near the experimental pits, due to excavations for the LHC. These movements will result in the need for frequent vertical realignment of the affected machine elements during the course of the year.

2. LEP2 physics prospects

Supersymmetry John Ellis
High precision measurements of the Standard Model David Charlton

Two talks were presented on the physics motivation for LEP2 and the interest in reaching a total energy close to 200 GeV. The arguments are based primarily on the Higgs sector and supersymmetry.

Precision electroweak data favours a light Higgs mass, centred near 150 GeV within a factor of about 2. With a total energy of 200 GeV, LEP2 can cross the difficult mass range between the Z mass and 100 GeV. With 200 pb-1 per experiment at ÷s = 192 GeV, the combined LEP discovery limit for the SM Higgs extends to a mass of 96 GeV and the exclusion limit to 99 GeV. At ÷s = 200 GeV and with 150 pb-1 per experiment, the SM Higgs discovery mass extends to
100 GeV and the exclusion mass to 106 GeV, providing safe overlap with the Higgs search at the LHC.

Moreover if the fine tuning of parameters is restricted, charginos and neutralinos are expected to have masses below 100 GeV and 70 GeV, respectively. At ÷s = 200 GeV, LEP2 will allow a thorough exploration of this range of chargino masses, while the lightest neutralino will be completed explored below a mass of 50 GeV.

These considerations underscore the importance of the energy region at the limit of LEP2 and the significant potential for discovery.

3. Reports from the LEP experiments

           DELPHI  Tiziano Camporesi
           L3            Martin Pohl
           OPAL      Rolf-Dieter Heuer
           ALEPH   Gigi Rolandi

All experiments reported W mass measurements from their total data at 161 and 172 GeV, with statistical precisions in the range 0.25-0.29 GeV per experiment. The combined LEP measurement is now m(W) = 80.38±0.14 GeV, to be compared with 80.366±0.031 GeV from the combined Standard Model fit to all data [LEP EWWG]. The new data have revealed no clear evidence for new physics, with, for instance, cross section upper limits of about 0.5 pb for the production of SUSY particles. The most significant anomaly is the ALEPH 4-jet signal - a peak containing 19 events, with only 4.3 events expected from standard processes. None of the other three detectors sees this signal even though they have now directly confirmed that their sensitivities to the 4-jet events are comparable to ALEPH's.

Regarding preferences for high-energy data in 1997, DELPHI, L3 and OPAL requested running at 184 GeV for the entire year. ALEPH also requested a sufficient integrated luminosity be collected at 184 GeV, but in addition requested the possibility of collecting 12 pb-1 at ÷s = 130-136 GeV at the end of the run in order to resolve the 4-jet puzzle.

Concerning the future operation of LEP, the four collaborations unanimously supported the operation of LEP in 2000 to fully exploit its physics potential. The highest emphasis for the final stage of operation of LEP was placed on reaching the maximum possible energy near
÷s = 200 GeV and collecting sufficient luminosity to effectively cover the Higgs sector in the experimentally-difficult mass range up to 100 GeV.

CLOSED SESSION

Present: S. Bethke, J. Drees, L. Foà, G. Goggi, P. Hansen, K. Hübner, J. Kirkby (Secretary),
                 W. Lohmann, T. Lohse, M. Mangano, B. Mansoulié, R. Marshall, J. May, S. Myers,
                 J. Panman,L. Pape, K. Peach, I. Videau, P. Wells and P. Zerwas (Chairman).

Apologies: M. Calvetti, J. Dainton, G. Kantardjian, K.-H. Kissler, M. Klein and S. Pokorski.

1. Approval of the minutes of the 44th meeting

The minutes of the 44th meeting (LEPC 96-9/LEPC 44) were approved without modification.

2. Chairman's report

The Chairman welcomed the new members of the committee: Wolfgang Lohmann and Jürgen May. He also expressed his appreciation to Philippa Wells for having accepted to continue as LEP Coordinator for an additional year.

The Chairman reported on the outcome of a special LEPC meeting that had taken place on
25 February to discuss the L3 request for 2.5 pb-1 Z calibration data be taken at the start of LEP running each year. The participants had included the four LEP spokesmen, P. Dornan, S. Myers, M. Pohl, the Directors of Research and Accelerators, the LEP Coordinator, and the Chairman and Secretary of the LEPC. The conclusion of the meeting was that a clear case exists for the requested calibration data from L3. Therefore a total of 2.5 pb-1 Z calibration data will be taken at the beginning of the LEP cycle this year. Under normal running conditions with 90/60 optics, this is estimated to take about 6 days; if, for unforeseen reasons, it is found to require a significantly longer time then the decision to switch to high-energy running will be taken by the Director of Research , in consultation with the LEP spokesmen.

3. Report from the LEP Coordinator

The LEP Coordinator presented the new draft schedule for 1997. As a result of the fire in the SPS surface building and the consequent cleaning operation, the original estimates for 1997 of 108 days for physics and 27 days for machine development have been severely reduced. With an estimated startup around the beginning of August, there would be 60-70 days for physics, complemented by 20 days for machine development and energy calibration. After accounting for the Z calibration running, about 50 pb-1 of high energy data could be delivered per experiment.

The loss of beam time could be partially compensated by extra running at the end of 1997 and an early startup in 1998. Also, one extra week (no. 40) is expected to be available in 1997 due to the cancellation of the Pb ion run. Early running in 1998 benefits from higher energy (192 GeV), higher luminosity and lower power costs. After reviewing the schedule of work on the LEP machine during the 97/98 shutdown, it appears possible to start up one month earlier in 1998.

Regarding the beam energy calibration at LEP2, the expected near-term precisions are s(Ebeam) = 30 (40) MeV at ÷s = 172 (184) GeV. The final goal for LEP2 remains at s(Ebeam) ~15 MeV (to be compared with the expected combined LEP experimental error s(MW) = 30-40 MeV for
500 pb-1).

4. Near-term LEP schedule

After discussion, the committee made the following recommendations on the near-term LEP schedule.

Firstly, following the 2.5 pb-1 of Z calibration data at the start of the cycle, the priority in 1997 is to accumulate 40-50 pb-1 at 184 GeV.

Secondly, concerning the ALEPH request for 12 pb-1 data at 130-136 GeV, the committee recognises the importance of resolving the 4-jet puzzle experimentally. ALEPH is invited to prepare a written report for the next meeting in September summarising their request for dedicated running, together with the supporting physics arguments. The committee indicated that the earliest times for any running at 130-136 GeV would be at the end of this year, assuming sufficient high energy data had been collected, or else during 1998.

Finally, with regard to compensation for the loss of LEP running at the start of this year, the committee notes the extra week foreseen in 1997 (no. 40; due to the cancellation of the Pb ion run) and, in addition, recommends starting 4 weeks early in 1998, around 11 May. Furthermore, if funds are available and provided it does not compromise the early startup in 1998, the committee recommends adding a further week in November 1997 (no. 45), at the end of the cycle.

5. Future operation of LEP in 2000

S. Myers presented the prospects for the maximum LEP energy, which depends on the maximum rf accelerating voltage that can be achieved. Tests will be carried out this year to confirm that the sc rf cavities can be reliably operated at 7 MV/m. With the extra cooling capacity of the LHC installation and additional compressors, the average accelerating gradient of the sc rf system could be raised from the present 6 MV/m up to 7 MV/m. After installing the four spare sc rf modules, this would allow LEP to reach a total energy of about 200 GeV. This installation could be ready for March 1999.

The committee considers there is a clear and compelling physics motivation for reaching
200 GeV. The SM Higgs discovery region will cross the experimentally difficult range through the Z resonance up to 100 GeV, providing safe overlap with the LHC. The SUSY Higgs discovery region will provide complete coverage for small tan b. The parameter space for many SUSY particles - charginos, neutralinos, etc. - will be significantly extended in a very interesting mass range. New forces (contact interactions) between leptons and quarks will be probed up to mass scales near 8 TeV.

The committee unanimously and enthusiastically supports the request from the LEP collaborations to fully exploit the physics potential of LEP by raising the energy to the highest possible value, near 200 GeV. To exploit the physics potential, in particular the Higgs sector, requires a luminosity of about 200 pb-1 per experiment at an energy near 200 GeV. In order to meet this goal, the committee recommends very strongly that LEP be operated in 2000.

The committee welcomed the decision by CERN management to advance the schedule of the LHC cryogenics installation, which brings 200 GeV operation of LEP within reach.

6. Next LEPC meeting

The dates of the next meeting of the LEPC are Tuesday-Wednesday, 9-10 September 1997.