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CERN in Transition

by John G. Cramer

Alternate View Column AV-72
Keywords: CERN SPS LHC relativistic heavy ions collisions quark gluon plasma
Published in the June-1995 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 11/20/94 and is copyrighted 1994 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.


    I'm writing this column from CERN, the center of European high energy physics. I and my co-workers have come here for two months to work on a new ultra-relativistic heavy ion physics experiment, NA49, which uses the new CERN beam of lead-208 nuclei that have been accelerated to energies of 160 GeV/nucleon, a total energy of 33.3 TeV (3.33 1013 electron volts) for each lead nucleus. Now, with a week of beam time remaining, we are working very hard and the experiment is beginning to collect good data. In this column, I want to describe the situation here at CERN. I'll return to the experiment after that.

    CERN spreads out over the rural countryside near Geneva, Switzerland, a lake town located in a broad valley surrounded by the Swiss and French Alps. Geneva is strategically positioned at the point where Lake Leman flows into the Rhone River. The CERN campus is on the west side of the Geneva near the village of Meyrin. The laboratory demonstrates its international character by sprawling across the border between Switzerland and France. CERN's two largest accelerators, the SPS and LEP, are both built in ring tunnels that burrow under both countries, so that their particle beams cross two international borders with each transit of the rings.

    The CERN laboratory is supported by 14 European member states that provide financial contributions based on a complex formula involving the population and gross national product of each country. This year, with Europe still struggling to recover from the worldwide recession of the early 90s, CERN is in a period of turmoil.

    The laboratory has for a decade been making plans to build what will be the world's largest and highest energy particle accelerator, the Large Hadron Collider (LHC). [See my AV column in the May'92 Analog for a description of the LHC project.] The LHC will use superconducting bending magnets with very high magnetic fields (about 10 Tesla) to bend two oppositely-directed proton beams into twin circular paths with diameters of 8.6 km. The two beams of protons will be brought into head-on collision, with a total collision energy per particle pair of about 15.4 TeV (1.54 1013 electron volts), 8,200 times the mass-energy of two protons at rest.

    The high energy physics community is very enthusiastic about the prospects for new physics with the LHC, which may hold the key to discovering the Higgs boson and understanding the "splitting" of the strong and electro-weak forces into two separate and very different fundamental forces. Two large proton experimentsand one heavy ion experiment are now being planned for the new accelerator.

    Interesting physics notwithstanding, many CERN member states that are to provide the funding for the new facility are presently having severe internal budget problems and are showing increasing reluctance to contribute their allotted share to the LHC project. In particular, Germany, which has traditionally contributed generously to the CERN budget, is staggering under the enormous cost of reunification and has asked for a long stretch-out of the CERN schedule for LHC construction. The CERN technical staff is circulating petitions and holding angry meetings to protest this probable stretch-out and the severe reduction of accelerator scientists and technicians that it would bring.

    From the perspective of the particle physics community in the USA, however, the situation at CERN looks very good despite the LHC funding problems. American physicists are still reeling from the shock of the cancellation of the Superconducting Super Collider (SSC) project, which would have been the centerpiece of experimental particle physics for the next two decades and would have produced proton-proton collisions 2.6 times more energetic than those of the LHC. In early 1994, with its great ring-tunnels carved half way around the needed circle in the Austin chalk deep under Waxahachie, Texas, and its first superconducting magnets just being tested, the SSC project was killed in its cradle by a flinty-eyed budget-cutting U. S. Congress. From the congressional debate that preceded this decision there was little understanding, from either SSC proponents or opponents, of the project or its goals.

    In the wake of the recent 1994 Congressional Elections, there have been tales here at CERN of a "Curse of the SSC" effect. The rumor is that there is a remarkable correlation between the House Members who voted against the SSC and those who were defeated for reelection this November. Even if this rumor is true, however, the turnover in Congress comes too late for the SSC, a dead project with no chance of being revived.

    The SSC cancellation leaves U. S. particle physics with years of wasted effort and facing a dead-end. The next generation of particle physics experiments at the highest energies will be conducted in the coming decades in Europe by experimental physics groups dominated by Europeans. American particle physicists have little choice but to join the CERN experiments on whatever terms they can arrange.

    These terms are now being negotiated. Earlier this week Dr. Martha Krebs, Director of Physics Research of the U. S. Department of Energy, made a short visit to CERN. I had met her in the 1970's, when she was a scientific staff member for the House Science and technology committee, and knew her to be sharp and sensible. She is now a top scientist administrator at the DOE and oversees a $3 billion budget. She had come to CERN to look the over the laboratory and to take the initial steps in studying possible US participation in the LHC project.

    Physicists at CERN are optimistic that a deal will be arranged. CERN needs extra funds for the project and the USA needs an alternative to the SSC. In the coming year an elaborately choreographed dance will take place between CERN management, DOE officials, and Congressional representatives on the details of the arrangements.

    As a part of her visit, Krebs spent an hour with the American physicists at CERN. There were about 80 of us in the CERN Council Hall, many from my own experiment since this is the month for heavy ion running at CERN and many of the Americans from the proton and electron experiments have already gone home. In broad strokes Krebs described the physics program of the Department of Energy and our place in it. She clearly delineated the rivalry and inter-dependence of fundamental science and applications. She also touched on the difficulty of communicating the value of scientific research to administration officials and congressional staff, many of whom have law degrees.

    After her talk, Martha Krebs asked for input from us. She wanted to know what it is like for Americans to do physics at CERN, to have to come here for months or years to do our work. She was concerned that US participation at CERN might produce a "reverse brain drain". She wanted to understand what kind of training the American graduate students at CERN receive.

    When she opened the floor for questions, there were many. Most of them centering around the SSC cancellation and it implications. The international speech patterns of the questioners was interesting. While all were from American institutions, few had accents that would have branded them as Americans. The formal question period ended with a coffee break, and a circle of us continued the discussion with Krebs. She was optimistic about the prospects for basic science in the new US Congress. She was interested to hear that while staff positions at US high energy laboratories were open to all, those at CERN were reserved for citizens of member states, with US citizens largely excluded.

    I left the meeting with some optimism. In the present situation, CERN needs US participation in the LHC, and the US particle physics community needs large-scale participation at CERN. It is a marriage made in heaven. Martha Krebs seems an ideal matchmaker.

    Our NA49 experiment is located in the North Area of CERN, a complex of offices, workshops, and experiment halls in Prevessin, France, about 2 kilometers from the main CERN complex in Meyrin, Switzerland.

    About a decade ago, the CERN leaders decided to devote some of their resources to a new kind of physics, the physics of ultra- relativistic heavy ions. Instead of accelerating electrons or protons (hydrogen nuclei), they concluded that it was feasible to use the CERN synchrotron complex to accelerate the nuclei of heavier atoms to near light-speed. The collision of such a nucleus with another nucleus at rest was expected to produce a nucleus-size fireball of extremely high energy density, a mini-Big-Bang in the laboratory . In theory, if the energy density in the fireball is high enough, we might produce a quark-gluon plasma in which quarks and gluons become free particles in a continuous plasma-like state that may have existed in the first microsecond of the Big Bang.

    The initial CERN heavy ion program, beginning about 1986, used the SPS to accelerated nuclei of 16O (oxygen-16) to 200 GeV per nucleon. Since that time, CERN has devoted about 1 month per year of its running schedule to producing heavy ion beams. In 1990, beams of 32S (sulfur-32) at 200 GeV per nucleon became available. Now, in late 1994, CERN is beginning to accelerating 208Pb (lead-208) to 160 GeV per nucleon. Lead ions in a very high charge state are produced in an electron cyclotron-resonance ion source built in Grenoble, France, accelerated by a new linear accelerator build at the GSI Laboratory in Darmstadt, Germany, further accelerated in the recently refurbished CERN Booster Synchrotron, then accelerated to about 1 GeV per nucleon in the Proton Synchrotron (PS), and finally accelerated to 160 GeV per nucleon in the Super Proton Synchrotron (SPS). In early November the first 160 GeV per nucleon lead beams from the SPS were delivered to our experimental area, and we started to make measurements and collect data.

    The collisions make a few thousand particles, which are tracked in our large time projection chambers (TPCs). The TPCs use about 80,000 channels of electronics, each channel with its own amplifier, charge storage array, and analog-to-digital converter. We have been collecting data for about a week, gulping down data at the rate of 6 megabytes per collision event, with several thousand events per day. We have already produced some spectacular pictures from the TPCs, starbursts of pions streaming from the collision like a surreal Star of Bethlehem.

    The physics understanding of the mini-Big-Bangs we are producing will require some time for analysis and calculation. However, we already know that with the lead beam we are producing about 6 times more particles per collision than in previous experiments with sulfur beams, and that we have reached a new high in energy density, about 3.5 GeV per cubic femtometer as compared to the old "record" of 2.5.

    Theory suggests that this density should be enough for quark-gluon plasma formation. Is a clear signature of a quark-liberating plasma or some other "new physics" buried somewhere in the terabytes of data we will collect? We don't know yet. But we are carefully looking where no one has looked before. We stand a good chance, we think, of seeing what no one has seen before.

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