Minutes of the 39th meeting,
9 December 1994
1. LEP machine status
A. Faugier reported on the very successful LEP operations
during 1994 with the 8-bunch pretzel optics. New records were achieved
for integrated luminosities
(65 pb-1 per year and 950 nb-1 per day), peak luminosity (2.4 x 1031 cm-2s-1) and overall machine efficiency (60%). The background problems in L3 were eliminated after the addition of extra beam-pipe shielding in September.
2. High luminosity in LEP 1 and LEP 2
S. Myers reported the results of the recent bunch-trains
tests, and presented the prospects for increased 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 starting performances in 1995
(with 4 x 4 bunches) close to the best previously obtained with pretzel optics. The detectors operated successfully with bunch-trains, and backgrounds were acceptable using 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). In consequence 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 peak luminosity 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 LEP
M. Placidi presented the results of yet another recent
success at LEP, performed in the context of the proposal to test longitudinal
(LEPC 94-8/P8). A high transverse polarization of the beam (40%) has been maintained during intense collisions (beam-beam tune shift ~0.04) at an energy of peak-2 GeV. Quantitatively, the depolarization time constant was measured to be at least a factor of 3 larger than the polarization time constant.
4. Report of the Energy Working Group
R. Jacobsen described the remarkable work of the
Energy Working Group in measuring and monitoring the absolute energy of
LEP. Effects on the beam energy are now being understood and monitored
at the sub-MeV level. The tide of the earthÝs crust (10 MeV peak shift)
is one of the largest effects; others range from the water level of Lac
Leman (10 MeV) to the daily cycle in the earthÝs magnetic field due to
the solar wind (1 MeV). Key requirements during a scan of the Z0 peak are
to record as much machine information as possible, to keep the machine
stable, and to average over variations. With present knowledge, the overall
machine-related errors (i.e. common to all experiments) during a scan are
expected to be s (mZ)
~ s (GZ)
= 1.0 MeV.
5. Status of the LEP 2 superconducting rf system
D. Boussard presented a progress report on the work
of the SL-RF group in preparing the LEP 2 sc 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 parameter changes, following improvements in equipment and procedures
by the manufacturers. The design of the power couplers has been substantially
improved, achieving a factor 6 reduction in internal power 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 operated in LEP at gradients up to the design value of
6 MV/m, for 470 and 770 hours, respectively. In summary, with the detailed
designs almost finalized, the sc rf team is now in good shape to study
the problem of installing and operating a large sc rf system in LEP.
6. Reports on the LEP experiments
Present: 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 meeting
The minutes of the 38th meeting (LEPC 94-14, LEPC
38) were approved without modification.
2. Chairman's report
In the open session, the Chairman had announced that the first General Meeting of the workshop on LEP 2 physics is scheduled for 2-3 February 1995, in the Main Auditorium at CERN. He had also announced that an audio-visual broadcast of the open session was taking place in real-time on Internet, on an experimental basis, for the benefit of LEP physicists and others who could not attend 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 a remarkably successful year: record luminosities, successful bunch-trains tests, observation of transverse polarization in collision, precise calibration and monitoring of the beam energy and, of particular 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 to construct a very small angle tagger (VSAT)
for two-photon physics. The proponents have indicated that a detailed document
will be submitted shortly to the LEPC and have requested an early 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 recommendations made at the last LEPC meeting.
3. Discussion on bunch-trains
With the positive results of the bunch-trains tests,
and with the readiness of the detectors, the committee recommended approval
for LEP operation with bunch-trains, commencing at the start of the
1995 cycle. It was noted that the pretzel separators will now remain in
place until June, 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 operation of LEP at
its June 1995 meeting.
4. Discussion of the energy calibration and run plan for 1995
The committee was impressed by the fine work of the Energy Calibration Group. The group is making 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 from each 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 take place after the
bunch-trains are commissioned. The LEP co-ordinator was asked to co-ordinate
the requests of the four detectors and to submit a detailed plan at the
June 1995 LEPC meeting. The plan should ensure compatibility between the
developments foreseen for the machine and the scanning requirements of
a stable, well-monitored machine. Although one detector (L3) requested
that high-energy running for physics be postponed until 1996, the committee
a high-energy run for the period following the 2nd technical stop in
October, assuming the presence of sufficient sc rf cavities. It is hoped
that up to 5 pb-1 per experiment could
be generated at an energy of about 140 GeV, which would allow a sensitive
search for charginos in an unexplored mass range.
5. Discussion of the LEP 2 superconducting rf system
The committee congratulated the sc rf team for their substantial progress in 1994. The designs of the main couplers and higher-order mode couplers are almost finalized, cavity manufacture is now reliable, and operational experience of sc cavities in LEP has begun. A first planning for the installation 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 2 Working Group. At LEP 2 (Phase II), with 192 sc cavities and 56 (i.e. half of the total) copper cavities , a luminosity of about 7 x 1031 cm-2s-1 can be reached at a beam energy of 90 GeV. At LEP 2 (Phase III) - using all 224 sc rf cavities that have been ordered from industry, together with half the copper cavities - this luminosity can be reached at the maximum energy of about 93 GeV.
Since industrial manufacture of cavities for LEP
2 will end in the 2nd half of 1995, the committee asked for a discussion
to take place at its June 1995 meeting on the physics and machine aspects
related to the peak energy of LEP 2. The primary physics concern is the
overlap of the accessible mass ranges for a Higgs between LEP 2 and the
LHC. F. Zwirner was appointed to co-ordinate the 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 Spin Polarization at LEP
The committee congratulated the proponents on their
successful demonstration of high (40%) transverse polarization while beams
are colliding, which indicates the potential exists for a physics programme
at LEP based on longitudinal beam polarization. The referees (W. Bartel,
W. Hollik and G. Matthiae) presented further reports, and the proponents
(A. Blondel and M. Placidi) were invited to part of the discussion. The
importance of the ultimate physics goal - namely the measurement of sin2qW
to a precision of 1 x 10-4 from the left-right
polarization asymmetry ALR - was underscored
once more by the committee. After a long discussion, however, and in view
of the conflict of the proposed schedule with the higher priority LEP 2
programme, the committee recommended not to perform the spin rotator
test in 1996, as presented in this proposal. Nevertheless the committee
encouraged the continuation of tests involving transverse polarization,
which are of interest both to this proposal and to the energy calibration
of LEP. It was noted that results would be obtained from the spin rotators
that are planned for installation in H1 and ZEUS at HERA in 1997. The committee
expressed its interest that longitudinal beam polarization remain open
as a possible future option beyond LEP 2 and recommended that the matter
be reviewed again in 1997 taking into account the planning for the LHC.
7. Discussion of the 1995 LEPC meeting at Cogne
The committee discussed the draft agenda for the
LEPC meeting at Cogne, 17 - 23 September 1995, to review LEP physics and
8. Report from the LEP Co-ordinator
T. Camporesi reported that the 1995 LEP schedule
(attached) had been approved by the Research Board.
In closing the meeting, the Chairman warmly thanked
Jim Allaby and Pier Giorgio Innocenti for their 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,
Giorgio Goggi and Michal Turala.
10. Next LEPC meeting
The LEPC meeting on 31 January - 1 February has been
cancelled. The dates of the next meeting of the LEPC are therefore
Tuesday and Wednesday, 6 - 7 June