ACTIONS 1999INTERACTIONS 1999
Ten years at CERN - and the Energy Keeps Going Up
by Helmut Vogel
While you are reading this newsletter, electrons and positrons of nearly 100 GeV energy each, are colliding head-on several hundred feet underground between Lake Geneva, Switzerland and the French Jura mountains. The particle accelerator that makes this happen is the Large Electron-Positron collider (LEP) at the high energy physics laboratory CERN.
LEP is a circular machine of 17 miles circumference. The electron-positron collisions occur at four symmetrically located points around the ring. Surrounding each collision point are large experiments composed of detectors recording the passage of the subatomic particles generated in the collisions. Each experiment is run by an international collaboration of typically 300 physicists, and carries a price tag of about $100 million.
| Ten years ago, on Bastille Day, 1989, the LEP experiments began operations, and they have taken data ever since. A group of Carnegie Mellon physics faculty: Arnold Engler, Tom Ferguson, Bob Kraemer, Roger Sutton (now emeritus) and Helmut Vogel, were founding members of one of the experiments, named "L3" (see Figure 1) and led by Nobel laureate Samuel Ting. The Carnegie Mellon group has worked on L3 during the past 10 years, and has always maintained a staff of three postdocs permanently based at CERN. Graduate students joining the group are sent off to CERN after passing their qualifying exams at Carnegie Mellon. They spend typically two to three years there before returning to Carnegie Mellon to finalize their theses. Over the years our postdocs have included individuals from England, Germany, Greece, The Netherlands, Pakistan and the U.S. Our graduate students have come from China, Ecuador, India, Korea and the U.S. This mix of nationalities within the Carnegie Mellon group — unusual even among the already multinational collaborating institutions — was part of a feature article in the Chronicle of Higher Education a few years ago. | 
Figure 1. View of the L3 experiment. |
Figure 2. Bob Kraemer inspecting the L3 luminosity monitor during assembly. |
The main hardware contribution of the Carnegie Mellon group
to L3 has been the "luminosity monitor," a detector system
consisting of more than 600 large single crystals of bismuth germanate (BGO,
see Figure 2) and finely segmented silicon strip particle detectors. BGO
has a density equal to that of steel but is optically transparent and
gives off scintillation light pulses when traversed by electrically
charged particles. This light is read out via photodiodes attached to the
ends of the crystals. The luminosity monitor detects electrons and
positrons scattered at small angles off their initial flight directions.
The information it provides is essential for all precision
cross-section measurements. The purpose of the experiments at LEP has been twofold. One, as usual when entering a new energy regime, was to keep eyes — and detectors — open for unexpected discoveries. The other was a systematic program to test the "Standard Model" (SM) of electro-weak unification to unprecedented precision. |
According to this theory, the electromagnetic and the weak nuclear force are just two manifestations of a unified "electro-weak" interaction — much as the electric and magnetic forces are different manifestations of the unifying "electromagnetic" interaction. Crucial ingredients of the SM are the particles Z0, W+, and W-), which are the heavy partners of the photon, have masses of 91 GeV/c2 and 80 GeV/c2, respectively, and are carriers of both the electromagnetic and the weak nuclear force. The Z 0 and W’s had been postulated in 1967 (Nobel prize 1979 for Glashow, Salam and Weinberg) and discovered in 1983 (Nobel prize 1984 for Rubbia and Van der Meer), but their properties were known only rather crudely prior to the startup of LEP. Another crucial ingredient is a particle called the "Higgs boson," responsible for giving mass to all massive particles. Although the Higgs remains elusive to this day, the results from LEP (and from other accelerators like the Stanford SLC and the Fermilab proton-antiproton collider) have by now placed stringent limits on its mass.
| The first years of LEP, until 1995, were devoted to
delivering to each experiment a total of more than five million Z0’s.
These are produced copiously when the center-of-mass (c.o.m.) energy of
the e + e- collisions is tuned to 91 GeV, the peak of the Z0
resonance. Since then, the c.o.m. energy of LEP has been steadily
increased to nearly 200 GeV, the highest ever achieved in e + e-
collisions. This energy is above "W-pair threshold," i.e., the
collision can produce pairs of W bosons. Figure 3 shows a compilation of
cross section measurements from L3 over the entire c.o.m. range covered.
LEP and its experiments have produced a wealth of physics results, too numerous to recount here. Our group has had strong involvement in several of the most notable among these, including: the mass of the Z0 has been measured to two parts in 105. (Achieving this precision required correcting for effects of the Moon’s(!) tidal force on the shape of the accelerator ring, the water level of nearby Lake Geneva and the schedule of the high-speed electric TGV trains leaving Geneva railway station.) The number of light neutrino species in the universe has been determined to be "three" (see Inter-Actions 1993). The precision in the "mixing angle," a universal parameter applying to all electro-weak processes, has been improved dramatically. Through all this, the SM has withstood all precision tests. The past 10 LEP years have been a triumphant decade for the electro-weak theory. |
Figure 3. L3 measurements of cross sections for the production of the Z 0 and W particles, spanning the past 10 years. The abscissa denotes the LEP center-of-mass energy. The curves show the excellent agreement with the SM theory. (The most recent data points at 189 GeV are still preliminary.) |
The LEP experiments are scheduled to take data until the end of the year 2000. The search for the Higgs particle will by then extend to a mass range near 100 GeV/c2 , close to the range predicted by the SM when including all other electro-weak experimental results. The measurement of the W mass will reach a precision of better than 0.04%, permitting one remaining crucial comparison of the experimental value with that predicted by the theory. Any significant discrepancy would indicate "new physics" beyond the SM. This very much resembles the situation in atomic physics 50 years ago when precision measurements of hydrogen spectra ("Lamb shift") and the magnetic moment of the electron ("g minus two") revealed the limitations of "standard" quantum mechanics and the need for quantum electrodynamics.
At the end of 2000, LEP will shut down to make room for the Large Hadron Collider (LHC). The LHC will occupy the present LEP tunnel — at great savings in civil construction costs — and collide two counter-rotating beams of protons at a c.o.m. energy of 14,000 GeV starting in 2005. At this new high energy frontier, new fundamental questions of elementary-particle physics become accessible that go beyond the realm of the SM (see Ira Rothstein’s article in last year’s Inter-Actions). The Carnegie Mellon group is again part of this effort — weakened by the retirements of Roger, Arnold and Bob — but restrengthened by the inclusion of Jim Russ (who still leads a strong experimental program at Fermilab) and Roy Briere, our newest faculty member, who has joined us from Harvard.
