CERN. Geneva. LEP Experiments Committee

CERN Document Server

CERN/LEPC 96-9LEPC 444 December 1996LEP EXPERIMENTS COMMITTEEMinutes of the 44th meeting,19 November 1996OPEN SESSION1 . LEP2 statusM. Lamont reported on the status of LEP.  Prior to the technical stop in August, a total luminosityof 12.1 pb-1 at 161 GeV and 0.5 pb-1 at the Z0 (for detector calibration) had been delivered thisyear.  After installing extra super-conducting (sc) rf cavities (bringing the total to 176) the LEPenergy was raised to Ö s = 172 GeV, marking the completion of LEP2 - Phase II.  A total of8.5 pb-1 at 172 GeV and 0.8 pb-1 at the Z0 was delivered with 90/60 optics prior to a 3-day stop tore-cable the sextupoles for tests of the 108/90 optics.  Over a period of two weeks, about 2 pb-1had been delivered at 172 GeV with 108/90 optics.With the 90/60 optics, LEP performed well at Ö s = 172 GeV with 4x1 bunches per beam: amaximum current of 4.2 mA, a peak luminosity of 3.4x1031 cm-2s-1, a maximum integratedluminosity of 1.1 pb-1 per day, and low detector backgrounds.  A small betatron couplingparameter (emittance ratio), k =0.005, and large beam-beam tune shift of 0.04 have again beenachieved.  This performance is better than expected and indicates that the 90/60 optics may besuitable at even higher energies.  Furthermore a beam polarization of 12% was achieved at Eb=50GeV - the highest energy so far - which is an encouraging sign for LEP2 energy calibration.  In contrast, however, the performance of the 108/90 optics has not yet met expectations - but therehas been little time so far for optimisation.  Although a maximum current of 4.2 mA has beenachieved, the peak luminosity has been limited to 2.2x1031 cm-2s-1 and a large coupling, k , hasbeen observed.  Furthermore, high detector backgrounds have been experienced in two interactionregions (L3 and OPAL).  The limited performance results from a relatively poor dynamic apertureat present, due to the higher sensitivity of this optics to machine imperfections.  During the 96-97 shutdown another 16 sc rf modules (64 cavities) will be installed and about onequarter of the warm copper cavities in IP2 will be removed.  By June 1997 the installed rf cavitiesare expected to total 240 sc and 86 copper, providing a maximum operating accelerating voltage of2.5 GV and a maximum operating energy Ö s = 186-188 GeV.  In view of the uncertainties withthe optics, it is hard to reliably predict the LEP2 performance in 1997.  However, with 4x2bunches per beam, a total current of 8 mA at the start of a fill appears realistic for both the 90/60and the 108/90 optics.  This would give a peak luminosity of about 6x1031 cm-2s-1 and an averageluminosity of about 1 pb-1 per day.  With 108 days for physics, and after allowing for Z0calibration data, the total luminosity at high energy in 1997 would be about 100 pb-1.2 . LEP energy calibrationM. Hildreth reported on the progress of the LEP Energy Working Group, both for Z0 massmeasurement with 93-95 data and for the high energy running with LEP2.  Numerous sources ofsystematic shifts in the beam energy have been identified, each causing variations of betweenabout 1 and 10 MeV.  These have been carefully monitored and successfully modeled, providingoverall uncertainties at the Z0 from the LEP beam energy of s (MZ) = 1.5 MeV and s (G Z) = 1.7MeV.  The central value of the beam energy has recently been revised downwards (resulting in ashift in MZ of about -4 MeV).  This is due to a gradual rise of the beam energy during each fill (5-CERN/LEPC 96-9 - 2 -10 MeV), which had been observed previously in the reference dipole, but as a much smallereffect  (~1 MeV).  The final uncertainties from the LEP beam energy compare favourably with theoverall errors from the combined LEP detectors of s (MZ) = 2.0 MeV and s (G Z) = 2.7 MeV.At LEP2, since there is no beam polarization near the physics energy, an alternative techniquemust be developed.  The aim is to make precise beam energy measurements by resonantdepolarization at two - and preferably more - points in the region of the Z0 and as high above aspossible (perhaps up to 55-60 GeV).  This calibrates, at these energies, the flux loop and NMRmeasurements of the magnetic field in LEP dipoles.  The beam energy at a higher point is thendetermined from the flux loop and NMR measurements, which are corrected by the (extrapolated)lower-energy calibrations.  A large apparent discrepancy (30 MeV) that had been observedbetween measurements made at 45 GeV and 50 GeV was found to be due to sideband resonances,and there is now good agreement.  The near-term goal is to achieve a precision for Moriond ‘97 ofs (Ebeam) = 50 MeV at Ö s = 161 GeV, with a final goal of s (Ebeam) ~15 MeV for LEP2 (to becompared with the expected combined LEP experimental error s (MW) = 30-40 MeV for 500 pb-1).  3 . Reports from the LEP experimentsOPAL A. BallALEPH F. RagusaDELPHI J. TimmermansL3 D. SticklandAll experiments reported successful data taking at 172 GeV, with high efficiency.  The mainproblem had been large backgrounds for L3 and OPAL with the 108/90 optics; if extrapolated to186 GeV they would prevent at least one of the detectors (L3) from operating.Each experiment observes a clear W signal at 172 GeV in the reconstructed mass fromW+W- fi  qq qq  and qq ln  decays, but only preliminary cross section and mass measurements areavailable at present.  The new data have revealed no clear evidence for new physics, with crosssection upper limits of about 0.5 pb.  The main puzzle is the ALEPH 4-jet peak in the total-massregion of 105 GeV, which has been further strengthened by the new data.  The ALEPH peak nowcontains 18 events, with only 3.1 events expected from standard processes.  None of the otherthree detectors confirms this signal.Concerning energy preferences for 1997, DELPHI and L3 requested running the whole year at thehighest available energy, whereas ALEPH and OPAL wish to keep open the option to run thesecond part of the year at 161 GeV, assuming sufficient luminosity could be provided, in order toimprove the threshold measurement of m(W).4 . Report on the Electroweak Working GroupBob Clare reported on the recent progress of the LEP Electroweak Working Group.  The mostsignificant changes have occurred in the measurements of the heavy flavour branching fractions ofthe Z0.  The preliminary new values are Rb = 0.2178 – 0.0011 and Rc = 0.1715 – 0.0056, in goodagreement with the Standard Model values of 0.216 and 0.172, respectively.  A large number ofelectroweak parameters are now very precisely determined; in particular, the combinedmeasurement  of the weak mixing angle is sin2q lept (eff) = 0.23165– 0.00024.- 3 - CERN/LEPC 96-9CLOSED SESSIONPresent: S. Bethke, J. Drees, P. Hansen, H.-J. Hilke, K. Hübner, G. Kantardjian, J. Kirkby(Secretary), K.-H. Kissler, M. Klein, T. Lohse, M. Mangano, B. Mansoulié, R. Mount,S. Myers, J. Panman, L. Pape, S. Pokorski, I. Videau, P. Wells, D. Williams and P. Zerwas(Chairman).Apologies: M. Calvetti, J. Dainton, L. Foà, G. Goggi and R. Marshall.1 . Approval of the minutes of the 43rd meetingThe minutes of the 43rd meeting (LEPC 96-7/LEPC 43) were approved without modification.2 . Chairman's reportOn behalf of the committee, the chairman congratulated the machine team for raising LEP’s energyto a new record of 172 GeV and successfully achieving the luminosity goal.  He announced the formation of four new working groups for preparing combined results from theLEP detectors.  The full list of LEP working groups and their chairpersons is as follows (the newgroups are starred):1. B lifetime Lucia Di Ciaccio / DELPHI2. B mixing Olivier Schneider‡ / ALEPH3. Electroweak Robert Clare / L34. Four-jet events * Dieter Schlatter / ALEPH5. Higgs * Peter Igo-Kemenes / OPAL6. LEP energy Pippa Wells / OPAL7. QCD * Siegfried Bethke / OPAL8. SUSY * Luc Pape / DELPHI‡ contact person3 . Discussion on physics aspects of the open sessionThe committee noted that the recent measurements of Rb and Rc in Z0 decays are bringing theworld average into agreement with the Standard Model, in contrast with the situation a year ago.While the new values of Rb and Rc leave room for new physics, they no longer require it.Regarding LEP2 data, the committee was once again impressed by the rapid production of physicsresults by the LEP detectors at a new energy.  Several interesting events have been observedwhich are candidates for new physics but which could be also be explained by standard processes.The most significant anomaly is the ALEPH four-jet signal, which is now well beyond a possiblestatistical fluctuation and in clear contradiction with the other three detectors.  Not surprisingly, theevents may be interpreted within extended SUSY models, for example by pair production ofcharginos of mass 53 GeV.  The committee looks forward to further high energy data and tohearing at a future meeting of the progress of the joint LEP working groups on this and othersubjects.4 . Discussion on LEP2 energy calibrationThe committee was pleased to hear of the resolution of the earlier inconsistency between the fluxloop and NMR measurements on the one hand and the resonant  depolarization measurement onthe other.  This should allow the beam energy to be known to s  ~50 MeV in the near future, whichis sufficiently precise as not to limit present measurements of the W mass.  The LEP coordinatorCERN/LEPC 96-9 - 4 -indicated that resonant depolarization measurements will be attempted in 1997 at beam energies of55 GeV and 60 GeV in order to improve the precision of the extrapolation to higher energy.5 . Report from the LEP Coordinator on planning for the 1997 LEP scheduleThe LEP Coordinator presented the draft schedule for 1997 (attached).  The totals for 1997 are108 days for physics and 27 days for machine development.  The choice of optics for 1997 will be made at the Chamonix Workshop, 14-17 January 1997; theprimary choice is between 90/60 and 108/90, although 90/90 is a possible third option.  Based onpresent knowledge, both 90/60 and 108/90 could probably operate up to the highest LEP2 energy.Furthermore they are estimated to have similar potential for luminosity at higher energies, since the108/90 optics has so far not provided any improvement in the dynamic aperture.  The committee discussed the large detector backgrounds for L3 (Point 2) and OPAL (Point 6) withthe 108/90 optics.  Due to the presence of copper cavities near these experiments (which take upmore space), there is an optics asymmetry between points 2/6 and points 4/8.  The highbackgrounds may also be connected with the poor dynamic aperture (resonances).  The committeeencouraged the work in progress to understand these backgrounds and find solutions.6 . Discussion on the LEP schedule for 1997An initial discussion took place on the LEP energies for physics in 1997 and on the detectorcalibration data at the Z0.  In the latter case, a total of 1 pb-1 of Z0 data at the start of the runsatisfies the requirements of three detectors (ALEPH, DELPHI and OPAL), whereas the fourthdetector (L3) requires 2.5 pb-1 of Z0 data at the start of the run.  The committee was pleased to notethat improvements in the L3 TEC calibration procedure, notably after including the SMD, havereduced the requirements from the previous figure of 4 pb-1 of Z0 data.  The extra requirement ofL3 beyond the other detectors is now reduced to 1.5 pb-1 (which is estimated to require anadditional period of about 3 days for 90/60 optics, or 4.5 days for 108/90 optics).  After a lengthydiscussion, it was decided to formulate the recommendation on the Z0 calibration data at the Maymeeting, when the planning for the initial high-energy running would also be discussed.  At thattime a better estimate can be made of the time required to collect Z0 data, since a decision on theoptics should be known from the ‘97 Chamonix workshop.7 . A.O.B.The chairman announced that a LEP2 physics jamboree is scheduled at CERN for the afternoon of25 February 1997, to be organized within the framework of the Particle Physics Seminars.8 . Next LEPC meetingThe dates of the next meeting of the LEPC are Thursday-Friday, 29-30 May 1997.J. Kirkby

LEP Experiments Committee: Minutes of the 44th meeting on 19 November 1996

Abstract

Similar works

Full text

Available Versions

CERN Document Server