CERN. Geneva. LEP Experiments Committee

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CERN/LEPC 94-17LEPC 3916 December, 1994LEP EXPERIMENTS COMMITTEEMinutes of the 39th meeting,9 December 1994OPEN SESSION1 . LEP machine statusA. Faugier reported on the very successful LEP operations during 1994 with the 8-bunch pretzeloptics.  New records were achieved for integrated luminosities (65 pb-1 per year and 950 nb-1 perday), peak luminosity (2.4 x 1031 cm-2s-1) and overall machine efficiency (60%).  Thebackground problems in L3 were eliminated after the addition of extra beam-pipe shielding inSeptember.2 . High luminosity in LEP 1 and LEP 2S. Myers reported the results of the recent bunch-trains tests, and presented the prospects forincreased luminosities at both LEP 1 and LEP 2.  Collisions of single bunch-trains (1 x 4 bunches)in ALEPH and DELPHI gave luminosities of 5-6 x 1030 cm-2s-1, indicating expected startingperformances in 1995 (with 4 x 4 bunches) close to the best previously obtained with pretzeloptics.  The detectors operated successfully with bunch-trains, and backgrounds were acceptableusing asymmetric beam-separation bumps.  Whereas LEP 1 operates at the beam-beam limit(current per bunch), it is expected that LEP 2 will be total-current limited (RF-power limited).  Inconsequence the optimum bunch-train configuration at LEP 2 is probably 4 x 2 (or perhaps 4 x 3).The luminosity targets for 1995 are an integrated luminosity of 90 pb-1 per year and a peakluminosity of 3.5 x 1031 cm-2s-1.  For LEP 2, the target remains as 500 pb-1 over 3 years,implying a peak luminosity of 7 x 1031 cm-2s-1.  3 . Transverse polarization in collision at LEPM. Placidi presented the results of yet another recent success at LEP, performed in the context ofthe proposal to test longitudinal spin polarization (LEPC 94-8/P8).  A high transverse polarizationof the beam (40%) has been maintained during intense collisions (beam-beam tune shift ~0.04) atan energy of peak-2 GeV.  Quantitatively, the depolarization time constant was measured to be atleast a factor of 3 larger than the polarization time constant.4 . Report of the Energy Working GroupR. Jacobsen described the remarkable work of the Energy Working Group in measuring andmonitoring the absolute energy of LEP.  Effects on the beam energy are now being understood andmonitored at the sub-MeV level.  The tide of the earth’s crust (10 MeV peak shift) is one of thelargest effects; others range from the water level of Lac Leman (10 MeV) to the daily cycle in theearth’s magnetic field due to the solar wind (1 MeV).  Key requirements during a scan of the Z0peak are to record as much machine information as possible, to keep the machine stable, and toaverage over variations.  With present knowledge, the overall machine-related errors (i.e. commonto all experiments) during a scan are expected to be s  (mZ) ~ s  (G Z) = 1.0 MeV.CERN/LEPC 94-17 - 2 -5 . Status of the LEP 2 superconducting rf systemD. Boussard presented a progress report on the work of the SL-RF group in preparing the LEP 2sc rf cavities.  Niobium coating on copper is still the critical process in the fabrication of cavities.Cavity production is now proceeding smoothly, in spite of its sensitivity to small parameterchanges, following improvements in equipment and procedures by the manufacturers.  The designof the power couplers has been substantially improved, achieving a factor 6 reduction in internalpower dissipation.  The designs of the higher-order-mode couplers are also close to completion.Two sc rf modules (a module contains 4 cavities, each of 4 cells) have been successfully operatedin LEP at gradients up to the design value of 6 MV/m, for 470 and 770 hours, respectively.  Insummary, with the detailed designs almost finalized, the sc rf team is now in good shape to studythe problem of installing and operating a large sc rf system in LEP.6 . Reports on the LEP experimentsL3 S. GentileOPAL R.D. HeuerDELPHI J.E. AugustinALEPH L. RolandiAll detectors reported successful operation during 1994.  The total statistics are now around 3.5 Z0hadronic decays (130 pb-1) per experiment.  OPAL has almost completed the repair of its Si micro-vertex detector, which had been damaged in an accident in October.  All detectors indicated theirreadiness to operate with bunch-trains at the beginning of 1995 and, in particular, ALEPHindicated progress towards operating with a train length of 740 ns (i.e. operating efficiently with4 x 4 bunches).  Regarding the requests for LEP running in 1995, all detectors are interested inincreased Z0 statistics with bunch-trains operation and in a scan of the Z0 peak (with an off-peakluminosity of about 20 pb-1).  Three detectors requested high-energy running for physics (a fewpb-1 at ~140 GeV) after the October technical stop; L3 places a higher priority in 1995 on increasedZ0 statistics, and proposed that high-energy running for physics be postponed until the start of the1996 cycle.CLOSED SESSIONPresent: J. Allaby, W. Bartel, M. Calvetti, T. Camporesi, G. Coignet, J.D. Dowell (Chairman),L. Foà, B. Foster, P. Hansen, W. Hollik, K. Hübner, P.G. Innocenti, G. Kantardjian, J. Kirkby(Secretary), K.H. Kissler, M. Klein, B. Mansoulié, G. Matthiae, D.J. Miller, S. Myers,J. Panman, D. Schlatter, D.O. Williams and F. Zwirner.Apologies: A. Weidberg.Part Time: A. Blondel, M. Placidi.1 . Approval of the minutes of the 38th meetingThe minutes of the 38th meeting (LEPC 94-14, LEPC 38) were approved without modification.2 . Chairman's reportIn the open session, the Chairman had announced that the first General Meeting of the workshopon LEP 2 physics is scheduled for 2-3 February 1995, in the Main Auditorium at CERN.  He hadalso announced that an audio-visual broadcast of the open session was taking place in real-time onInternet, on an experimental basis, for the benefit of LEP physicists and others who could not- 3 - CERN/LEPC 94-17attend the session in person.  This was the first such broadcast of a CERN committee session(further information can be obtained from Christian Isnard/CN).In the closed session, the Chairman joined with the committee in congratulating the LEP team for aremarkably successful year: record luminosities, successful bunch-trains tests, observation oftransverse polarization in collision, precise calibration and monitoring of the beam energy and, ofparticular note, great progress on the sc rf system for LEP 2.  The Chairman has received a memorandum from the spokesman of L3 concerning a proposal toconstruct a very small angle tagger (VSAT) for two-photon physics.  The proponents haveindicated that a detailed document will be submitted shortly to the LEPC and have requested anearly decision in order to allow installation to take place in the 95-96 shutdown.  A referee(M. Klein) was appointed.Finally, the Chairman  reported that the Research Board had approved all the recommendationsmade at the last LEPC meeting.3 . Discussion on bunch-trainsWith the positive results of the bunch-trains tests, and with the readiness of the detectors, thecommittee recommended approval for LEP operation with bunch-trains, commencingat the start of the 1995 cycle.  It was noted that the pretzel separators will now remain in place untilJune, providing an effective safety net in case unforeseen difficulties are encountered with bunch-trains.  The committee looks forward with interest to hearing the results of bunch-trains operationof LEP at its June 1995 meeting.4 . Discussion of the energy calibration and run plan for 1995The committee was impressed by the fine work of the Energy Calibration Group.  The group ismaking a vital contribution to LEP 1 physics, and it will have an equally important rôle at LEP 2.The committee encourages the continued active participation of experts from the machine and fromeach of the four LEP detectors in the Energy Calibration Group.Regarding the run plan for 1995, the committee recommended a scan of the Z0 peak to takeplace after the bunch-trains are commissioned.  The LEP co-ordinator was asked to co-ordinate therequests of the four detectors and to submit a detailed plan at the June 1995 LEPC meeting.  Theplan should ensure compatibility between the developments foreseen for the machine and thescanning requirements of a stable, well-monitored machine.  Although one detector (L3) requestedthat high-energy running for physics be postponed until 1996, the committee recommended ahigh-energy run for the period following the 2nd technical stop in October, assuming thepresence of sufficient sc rf cavities.  It is hoped that up to 5 pb-1 per experiment could be generatedat an energy of about 140 GeV, which would allow a sensitive search for charginos in anunexplored mass range.5 . Discussion of the LEP 2 superconducting rf systemThe committee congratulated the sc rf team for their substantial progress in 1994.  The designs ofthe main couplers and higher-order mode couplers are almost finalized, cavity manufacture is nowreliable, and operational experience of sc cavities in LEP has begun.  A first planning for theinstallation of the Phase II sc cavities will be discussed at the LEP Performance Workshop,Chamonix, January 1995.S. Myers said that the peak beam energies and luminosities at LEP are under study by the LEP 2Working Group.  At LEP 2 (Phase II), with 192 sc cavities and 56 (i.e. half of the total) coppercavities , a luminosity of about 7 x 1031 cm-2s-1 can be reached at a beam energy of 90 GeV.  AtLEP 2 (Phase III)—using all 224 sc rf cavities that have been ordered from industry, together withhalf the copper cavities—this luminosity can be reached at the maximum energy of about 93 GeV.CERN/LEPC 94-17 - 4 -Since industrial manufacture of cavities for LEP 2 will end in the 2nd half of 1995, the committeeasked for a discussion to take place at its June 1995 meeting on the physics and machine aspectsrelated to the peak energy of LEP 2.  The primary physics concern is the overlap of the accessiblemass ranges for a Higgs between LEP 2 and the LHC.  F. Zwirner was appointed to co-ordinatethe physics aspects and S. Myers to co-ordinate the machine aspects for this discussion.6 . Discussion of the Proposal LEPC 94-8/P8: A Test of Longitudinal SpinPolarization at LEPThe committee congratulated the proponents on their successful demonstration of high (40%)transverse polarization while beams are colliding, which indicates the potential exists for a physicsprogramme at LEP based on longitudinal beam polarization.  The referees (W. Bartel, W. Hollikand G. Matthiae) presented further reports, and the proponents (A. Blondel and M. Placidi) wereinvited to part of the discussion.  The importance of the ultimate physics goal—namely themeasurement of sin2 q W to a precision of 1 x 10-4 from the left-right polarization asymmetryALR—was underscored once more by the committee.  After a long discussion, however, and inview of the conflict of the proposed schedule with the higher priority LEP 2 programme, thecommittee recommended not to perform the spin rotator test in 1996 , as presented inthis proposal.  Nevertheless the committee encouraged the continuation of tests involvingtransverse polarization, which are of interest both to this proposal and to the energy calibration ofLEP.  It was noted that results would be obtained from the spin rotators that are planned forinstallation in H1 and ZEUS at HERA in 1997.  The committee expressed its interest thatlongitudinal beam polarization remain open as a possible future option beyond LEP 2 andrecommended that the matter be reviewed again in 1997 taking into account the planning for theLHC.7 . Discussion of the 1995 LEPC meeting at CogneThe committee discussed the draft agenda for the LEPC meeting at Cogne, 17 - 23 September1995, to review LEP physics and plans.  8 . Report from the LEP Co-ordinatorT. Camporesi reported that the 1995 LEP schedule (attached) had been approved by the ResearchBoard.  9 . A.O.B.In closing the meeting, the Chairman warmly thanked Jim Allaby and Pier Giorgio Innocenti fortheir important contributions to the LEPC during their terms as Division Leaders of PPE and ECP,respectively.  They will be replaced from the next meeting by the new Division Leaders, GiorgioGoggi and Michal Turala.1 0 . Next LEPC meetingThe LEPC meeting on 31 January - 1 February has been cancelled.  The dates of the next meetingof the LEPC are therefore Tuesday and Wednesday, 6 - 7 June.  J. Kirkby

LEP Experiments Committee: Minutes of the 39th meeting 9 Dec. 1994

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