CERN. Geneva. LEP Experiments Committee

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CERN/LEPC 2000-006LEPC 5521 July 2000LEP EXPERIMENTS COMMITTEEMinutes of the 55th meeting,Thursday, 20 July 2000OPEN SESSION1 . Status of LEPR. Assmann reported on the status and performance of LEP during the current run.  In order tomaximise the discovery potential, the operation strategy for LEP in its final year has aimed for thehighest possible energy at the expense of reduced luminosity.  So far a total luminosity of114 pb-1 per experiment has been delivered (1.07 pb-1 per day), mainly centred around three beamenergies: 102.7, 103.4 and 104.1 GeV.  These correspond, respectively, to a margin of 2, 1 and0 klystron-trips before the beam is lost.  Operation with 0 klystron margin at the highest energyresults in a typical beam lifetime of 14 min (compared with 1.5 h at 1 klystron margin).To improve luminosity efficiency, LEP has therefore been operated in a 'mini-ramp' mode, wherethe energy of a fill is progressively increased by reducing the klystron margin in steps from 2 to0 as the current decreases.  Despite pushing LEP to its energy limit, the overall luminosity hasbeen very good, slightly exceeding 1998 performance and only 20% below the record operation in1999.  In addition to high reliability of all the hardware systems, an important factor in maintaininga high integrated luminosity at LEP has been the improvement in turn-around time between fills,which now averages 69 min compared with 92 min in 1999.  Thankfully, movements of the LEPring due to LHC civil engineering have so far been modest and have not appreciably affectedoperation.  The energy increase since last year of 3.4 GeV (from 101 to 104.4 GeV) is due to improvementsin the RF system (1.1 GeV), changes in the RF operational procedures (2.2 GeV) and an increasein the bending length of the ring (0.2 GeV).  The sc RF cavities are being operated at an averagegradient of 7.4 MV/m, well above their design performance of 6 MV/m.  Despite this demandingoperation, only 4 cavities have been lost (1 in 1999 and 3 this year).For the remaining high-energy operation of LEP, a recent short MD has indicated that operationwith 2+2 bunches, rather than the present 4+4, results in only a small decrease in luminosity for alarge reduction of current.  This seems to significantly improve the RF stability and, inconsequence, to improve the operation of the LEP at its highest energy near 104 GeV.  If this isconfirmed with further tests, LEP will be operated in future with 2+2 bunches and ramped to104 GeV as early as possible in each fill.  Depending on the outcome of these tests, the luminosityper day at 104 GeV may be between 0.2 and 0.64 pb-1, representing at least a factor5 improvement over present operation.  2 . LEP detector status reportsOPAL S. YamashitaALEPH G. TaylorDELPHI W. MurrayL3 S. GentileAll detectors reported successful operation during the present run, collecting about 95 pb-1 at highenergy so far.  Despite a larger horizontal beam size the machine backgrounds have mainly beengood, thanks to optimisation of the optics and collimators by the accelerator team.  CERN/LEPC 2000-006 - 2 -The physics results are in agreement with the expectations of the Standard Model, although severalinteresting channels and events were highlighted.  The latter included some excess in the lightsbottom channel from ALEPH.  In addition, a few high mass SM Higgs candidates were reportedaround 115 GeV, but these are not statistically distinct from a background fluctuation.  Lowermass limits of 108-111 GeV (95% CL) for the SM Higgs were reported—an improvement in massreach by about 5 GeV since last year.For the final operation of LEP, all detectors agree that the next priorities are to obtain about 10 pb-1near 104 GeV for chargino searches, followed by optimisation of LEP operation for the SM Higgssearch.  Concerning an extension beyond the presently-scheduled shutdown on 15 September, alldetectors requested running in the two week reserve until the end of the month.  A final 0.5 pb-1sample of Z calibration data is also requested near the end of the run.  In addition, assuming nosign of new physics emerges, three of the four detectors requested an extension of the run intoDecember with LEP operated at 161 GeV total energy, in order to obtain a 200 pb-1 sample for athreshold measurement of the W mass.3 . LEP working group reportsHiggs A. ReadSUSY A. De MinElectroweak P. RentonThe search and electroweak working groups reported on the combined results from the run inprogress.  No evidence of a new physics signal was reported.  The combined  lower mass limit onthe SM Higgs is 113.3 GeV (95% CL).  The stau excess observed earlier is not confirmed in thisyear's data and is now interpreted as a statistical fluctuation.The combined direct reconstruction measurement of the W mass from LEP is80.428±0.046 GeV, which is dominated by the systematic uncertainty of 36 MeV.  Based onreasonable assumptions of reductions in the systematic errors, the estimated final W mass error is35 MeV.  An additional 200 pb-1 run at 161 GeV would provide an independent measurement with48 MeV precision which, when combined with the direct reconstruction measurement, wouldresult in a reduction of the final error on the W mass to 30 MeV.CLOSED SESSIONPresent: R. Cashmore, J. Colas, M. Delfino, F. Gasparini, V.G. Goggi, N. Harnew,K. Hübner, P.O. Hulth, P. Janot, L. Jonsson, J. Kirkby (secretary), W. Lohmann,L. Maiani, M. Mangano, J. May, C. Matteuzzi, T Müller, S. Myers, D. Schaile,A. Smith, M. Spiro (chairman), R. Tenchini.Apologies: M. Doser, R. Miquel, Y. Sirois.1 . Approval of the minutes of the 54th meetingThe minutes of the 54th meeting (LEPC 2000-2/LEPC 54) were approved without modification.2 . Chairman's reportThe chairman announced there will be a final round of status reports from LEP on5 September, which represents the latest date for the input of new data regarding any possibleextension of LEP (to be based on the grounds of a potential discovery).  Short presentations fromthe machine, the four detectors and the Higgs and SUSY working groups will be included.  Afterthese presentations the LEP detector spokesmen will meet together with CERN and LEP- 3 - CERN/LEPC 2000-006management, the LEP coordinator and the LEPC chairman and secretary to decide how to run LEPfor the final period.  The chairman invited the LEPC to the LEP Fest, scheduled for 10-11 October, where the latestresults from LEP will be presented as well as perspectives on the history of LEP and itscontributions to physics and to accelerator and detector technologies.  A LEPC working lunch hasbeen arranged during the meeting, on 11 October.  Two further meetings of the LEPC will be scheduled in 2001 (March and November) in order todiscuss the physics analysis, detector dismantling and data archiving.3 . Discussion on the LEP machine reportThe chairman joined the LEPC in congratulating the accelerator team for increasing the energy ofLEP still further in 2000 while maintaining a high integrated luminosity, despite operating thehardware at its limit.  The committee deeply appreciates the high standard of the entire CERNaccelerator complex in achieving this performance.  The committee supports the new strategy to operate LEP at a lower beam current with 2+2 bunchesif further tests prove successful.  Although no further energy increase can be obtained, thisstrategy may provide a higher daily luminosity at 104 GeV if it indeed leads to improved stabilityof the sc RF system.4 . Discussion on the reports from the LEP experiments and LEP working groupsThe committee was once again impressed by the quasi real time physics analyses of the LEPdetectors and in the fast combined analyses of the working groups.  The spirit of opennessbetween the LEP collaborations has been instrumental in the unqualified success of the physicsoutput from the working groups.  Although no clear signal of new physics has emerged, someinteresting events have been observed and the new data are eagerly anticipated.The committee strongly supports operation of LEP in the two week reserve until the end ofSeptember.  This will significantly help to explore the SM Higgs mass window between 113 and115 GeV.  (LEP operation during the two week reserve has now been approved by the DirectorGeneral; so LEP will run until 1 October.)  Concerning the request for an extension of the run tomake a threshold measurement of the W mass at 161 GeV, while recognising that the measurementhas some aspects of basic interest, the committee considers the resulting improvement of the Wmass to be too modest and therefore does not support an extension of LEP for this measurement.Following the original understanding, any extension of LEP beyond the end of September willrequire a good chance of transforming a significant signal of new physics into a discovery.  5 . Report from the LEP coordinator and planning for the final operation of LEPThe LEP coordinator reported on the progress of the beam energy calibration.  Both the NMRextrapolation method and the Qs vs. VRF method seem likely to reach their precision goals of20 MeV each.  The precision spectrometer (15 MeV goal) has encountered some unexpectedsystematic errors, whose origin is under study.  So far this year the delivered luminosity has been divided as follows: near to 102.7 GeV(72 pb-1), 103.4 GeV (42 pb-1) and 104.1 (2.3 pb-1), corresponding to 2, 1 and 0 klystronsmargin, respectively.  The most recent operation suggests that the time spent at 102.7 GeV can besomewhat reduced, so the performance of the present mini-ramp scheme—which is close to idealfor the Higgs search— may be improved.  Following the proposals of the experimental teams and the LEP coordinator, in consultation withthe machine team, the committee recommends the following ordered sequence of LEP operations:CERN/LEPC 2000-006 - 4 -a) Operation at the highest energy near 104 GeV for 10 pb-1 to optimise the chargino search.This should be done now since the available RF voltage tends to decrease with time.b) If a signal is observed then continue operation at this energy.c) If no signal is observed then optimise the operation for the SM Higgs search and continuethis way until the end of September (while allowing time for beam energy MDs and 0.5pb-1 Z calibration data).  d) This operation strategy of LEP will be reviewed on 5 September in the light of the newdata.6 . Next LEPC meetingThe date of the next LEPC meeting is the LEP Fest, Tuesday-Wednesday 10-11 October.Although there will not be a formal closed session, a LEPC working lunch is scheduled for11 October 2000.J. Kirkby

Minutes of the 55th meeting, Thursday, 20 July 2000

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