LEPC SPECIAL SEMINAR

Minutes of the meeting,
5 September 2000

OPEN SESSION
 

       LEP machine report
                LEP status: S. Myers

       LEP detector reports
                ALEPH:   D. Schlatter
                DELPHI:  T. Camporesi
                L3:           J.J. Blaising
                OPAL:      C. Rembser

       LEP search working group reports
                Higgs:      C. Tully
                SUSY:     S. Rosier-Lees
 

CLOSED SESSION

Present: G. Altarelli, J.J. Blaising, T. Camporesi, R. Cashmore, J. Ellis, K. Hübner,
P. Igo-Kemenes, P. Janot, J. Kirkby (secretary), L. Maiani, S. Myers, L. Pape, D. Plane,
D. Schlatter, M. Spiro (Chairman).

A special closed session took place after the open presentations to evaluate the options for the
final operation of LEP in the light of the latest data.

The Chairman joined with CERN management in congratulating the LEP machine team for the
excellent performance of LEP and in congratulating the LEP experiments and working groups
for their fast and impressive physics analyses.

The discussion focused on the single indication of new physics in the open presentations,
namely the observation by ALEPH of candidate SM Higgs events at a mass of around 114 GeV.

The experimental situation is as follows. With tight selection criteria (signal/background = 2.0),
ALEPH has observed 3 Higgs candidates near 114 GeV in the 4-jet channel, with an expected
background of 0.3 events. The expected number of signal events in this channel for a
114 GeV Higgs is 0.6 events for ALEPH alone. No events of that quality are observed by
ALEPH in any other channel (jj+ll, jj+Emiss).

Using equivalent tight selection criteria, no events of that high quality are observed in any of
the other 3 experiments, although DELPHI has observed with similar cuts and in the same
4-jet channel 2 candidates with mass above 109 GeV on a background of 0.3. Due to the high
quality (weights) of the observed events, the significance is 3.9s for the ALEPH observation
alone, and 2.7s for all 4 experiments combined. The possibility of a real signal in ALEPH is not
ruled out by the combined data from the other 3 experiments. In short, an excess of Higgs candidate
events has appeared in mostly one experiment and mostly in one channel but, given the small
number of events, there is no statistical discrepancy. These data correspond, in average per
experiment, to a recorded luminosity of 65 pb-1 above 103 GeV beam energy.

Based on the ALEPH observation and the combination of data, the four experiments are
unanimously requesting a continuation of LEP operation until 1 December, i.e. a 2-month extension
beyond the presently-scheduled final shutdown of LEP on 1 October. With the current machine
performance of 1.1 pb-1 /day, the expected delivered luminosities per experiment above 103 GeV
beam energy are approximately as follows:

                    1 October shutdown 100 pb-1
                    2 November " 136 pb-1
                    4 December " 173 pb-1
These are to be compared with a delivered luminosity of 70 pb-1 for the current results. Assuming a
doubling of the luminosity, the 2.7s signal, if due to a standard model Higgs, might be statistically
expected to become a 3.3s signal. This last figure is preliminary and could get slightly higher since
the analysis is not yet optimized. If the effect is purely statistical, it would be reduced to about
2s significance.

All four experiments recognize that the requested extension would not transform this hint of a signal
into a clear discovery. Nevertheless, the experiments consider a doubling of the luminosity above
103 GeV to be a substantial step.

Notwithstanding the physics interest, it is recognized that the LHC schedule should not be
significantly disturbed and that the cost of any extension should not be excessive. After an extensive
discussion,the two options for LEP operation beyond the presently-scheduled shutdown on
1 October are seen as follows:

            1.   Stop on 2 November, for an estimated doubling of the present luminosity and a 3.3 sigma
                   potential for the discovery of a 114 GeV Higgs with the present analysis. The extra operation
                   costs of LEP are 4 MCHF for electricity only. At present it is expected that the impact on the
                   LHC schedule is mild

            2.    Stop on 4 December for an estimated 3.7 sigma potential for the discovery of a 114 GeV Higgs
                   with the present analysis. The extra operation costs of LEP are 9 MCHF. This would delay
                   LHC civil engineering work until after the start of the new year, affecting strongly the planning
                   of LHC.  LHC additional cost is being evaluated in view of the Research Board meeting of
                   14   September 2000.

The DG requested the Chairman to ask for the scientific advice of the LEPC. The original understanding
was that any extension of LEP would require good prospects of transforming a tentative observation
into a significant discovery. The main issue for the LEPC is whether, in its view, options 1)
or 2) fulfil this condition and whether an extension is justified.
 
 
 

Jasper Kirkby