View Column AV-52
Keywords: CERN large hadronic collider particle physics Higgs boson
Published in the May-1992 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 10/20/91 and is copyrighted ©1991 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without the explicit permission of the author.
For the past six weeks I've been living in Meyrin, Switzerland, a village sandwiched between the great city of Geneva and the French border, between the long Jura and Saléve mountain masses. I have two more weeks of near-monastic life here before I return home to Seattle, rejoin my wife and family, and take up my faculty responsibilities at the University of Washington.
In this column, however, I'm not going to talk about my own experiment here, but about CERN's fast-track plans for building a new and more powerful particle accelerator, the LargeHadronic Collider or LHC. The LHC is to come into operation around 1999. It is the principal competition for the SSC, the huge U. S. accelerator presently under construction in and around Waxahachie, Texas. [See my AV column "The Coming of the SSC", Analog March, 1988 for a description of that project.]
The European and American approaches to the construction of high energy accelerators have always been distinctly different. In the United States it is usual to turn off and walk away from aging high energy accelerators that can no longer produce forefront physics. The Brookhaven Cosmotron, the Argonne ZGS, the Princeton-Penn Accelerator, the Nevis Cyclotron at Columbia, and many smaller machines have been victims of this accelerator evolution-in-action. The Berkeley Bevalac and the LAMPF facility at Los Alamos will probably be the next to join the ranks of extinct accelerators.
It has also become standard in the USA to build any new major accelerator facility at a new place. The five original AEC (now DOE) national laboratories at Argonne, Berkeley, Brookhaven, Los Alamos, and Oak Ridge, remnants of the World War II Manhattan Project, have been systematically bypassed as sites for new large accelerator projects. Instead, major new national laboratories have been created at SLAC near Palo Alto, California, at FermiLab near Chicago, and now near Waxahachie, Texas.
There are political and sociological advantages to the rather expensive US practice of scattering new national laboratories around on the landscape. It's a mechanism for more broadly distributing federal spending, of slicing off huge slabs of federal pork for states and districts where political power is concentrated. The siting of FermiLab near Chicago and the Johnson Manned Spaceflight Center near Houston, for example, was reportedly the result of a smoke-filled-room deal cut between Senators Lyndon Johnson of Texas and Everett Dirksen of Illinois when they led their parties in the U. S. Senate.
But there are also other reasons. Beginning a new laboratory in a new place is consistent with the frontier spirit of the US. It is a chance to walk away from the entrenched time-serving bureaucracies and over-the-hill scientific and technical leadership of the arthritic older national laboratories and to start the new enterprise with a clean slate, selectively recruiting energetic new people, and developing new sense of purpose.
There is perhaps wisdom in this approach. The only recent attempt to build a new major high energy accelerator at an existing national laboratory came to disaster when the magnets designed for Brookhaven's ISABELLE proton-proton collider failed and the project had to be cancelled. The strategy of starting fresh, on the other hand, has worked very well at SLAC and FermiLab, and it seems to be working now in the construction of the SSC. It does, unfortunately, have the side effect of reducing our nation's best accelerator builders to the status of migratory workers with PhDs.
Europe decided long ago that it could not afford to do high energy physics in the spend-thrift American style. European science springs from a broader sense of history, a waste-not want-not mentality and a long tradition of high technical craftsmanship coupled with job security. Abandoning a large scientific facility, firing its employees, and starting fresh somewhere else is as alien to the European mentality as tearing down a great cathedral so that a more modern one could be erected on the site.
Most old accelerators at CERN are not turned off. Instead, they become the injectors for the new accelerators. The old 24 GeV CERN proton synchrotron or CPS, which first went into operation in 1959, serves as the injector for the 400 GeV SPS (super proton synchrotron) which first began operation in 1976. The SPS in turn serves as the injector for the large electron-positron (LEP) accelerator which came into operation at in 1989.
The CPS is a relatively small machine, located in a ring shaped tunnel about the size of a circularized football stadium. It injects the SPS ring, which is about 2 kilometers in diameter, too big to fit on the CERN site. This problem was solved by using heavy tunnel-boring equipment, the same kind used in making subway tunnels, to dig a 6.3 km tunnel beneath the French and Swiss countryside. The beam of particles in the SPS must cross international borders about 48,000 times per second during the acceleration cycle.
My CERN experiment (NA35) uses 200 GeV/nucleon sulfur ions from the SPS that come up from underground and are delivered to a large hangar-like building in the North Area, a separate part of CERN located near the village of Prevessin, France. It takes about 15 minutes to drive there from the CERN Hostel where I'm staying, on the other side of an international border.
The 6.3 km SPS tunnel, however, is dwarfed by the larger tunnel that houses the LEP accelerator. The LEP tunnel is 27 kilometers long and was bored even deeper underground. It circles under the French countryside, at its northern edge tunneling under the Jura mountains. LEP is mainly in France, and dips into Switzerland only as it approaches the CERN Meyrin site. It accelerates a 90 GeV beam of electrons in one direction and 90 GeV beam of positrons in the other, bringing them into head-on collisions at 8 points around the LEP ring. The small French villages of St. Genis, Enchevey, and Ferney-Voltaire are the sites of above-ground staging areas and deep man-made cavern that house the four major LEP experiments, ALEPH, DELPHI, L-3, and OPAL.
Charged particles give off electromagnetic radiation (light) when they are made to turn a corner. This radiation, called synchrotron radiation, is particularly apparent with electrons because they are the least massive of charged particles. Synchrotron radiation bleeds away the energy of electrons (or positrons) when they are accelerated in a circular path. The smaller the circle, the greater the loss. The circular LEP tunnel was made very large to minimize this problem.
Protons have 1836 times more mass than electrons and produce far less synchrotron radiation when turning a corner. The limiting factor for the acceleration of protons is not radiative energy loss but the size of the magnetic field needed to bend the energetic beam of particles into a circular path.
The maximum energy of the accelerator depends on the product of the size of the magnetic field and the radius of the circular path. CERN, in frugal European style, plans to put their new accelerator into the LEP tunnel. The large LEP ring is well suited for a proton accelerator, but a much higher magnetic field than that used for LEP will be needed.
The LHC design depends on a major engineering breakthrough, the development of a 10 Tesla dual-field superconducting magnets. To appreciate the magnitude of this engineering extrapolation, you must realize that the superconducting magnets used at FermiLab develop fields of about 3 Tesla. The SSC, which is under design at about the same time as the LHC, is using more conservative 6.6 Tesla single-field magnets. The stored energy in the magnetic field goes up as the square of the magnetic field strength, and engineering difficulty of producing the magnet probably increases as the cube of the field strength. Thus the LHC engineers are taking on a problem that is perhaps 3.5 times harder than the design of the SSC magnets.
There is also another major design difference between the SSC magnets and the LHC magnets. In the SSC, the magnets have a single cylindrical hole which contains the magnetic field and through which a beam passes. Because there are two beams (clockwise and counter-clockwise) in the SSC, separate magnets are needed and the cost of magnets is doubled. There are, however, operational advantages in using separate magnets. For example, the two proton beams can have different energies, an operating mode that offers advantages for some physics applications.
The LHC magnets are more frugal with their magnetic flux, using it twice. The field lines are bent in a circle and pass through one beam cavity on the way up and another on the way down, providing magnetic deflection for the clockwise proton beam and the counterclockwise proton beam within the same structure. Because the same field bends both beams, the magnets are both more compact and cheaper.
The LEP tunnel was constructed with expansion in mind and is roomy enough to house both LEP and LHC magnets. The LHC's cryogenic superconducting magnets will be placed just above the "warm" normal-conductor LEP magnets in the LEP tunnel. With 10 Tesla magnets bending the two particle beams in a 8.6 km diameter circle, the LHC will be able to bring pairs of protons into head-on collisions with a total collision energy of about 15.4 TeV (15.4 x 1012 electron volts), 8,200 times the mass-energy of the two protons at rest.
In contrast, the SSC, which will probably come into operation at Waxahachie a few years after the LHC, will produce collisions with energies 2.6 times larger than this. The SSC is designed to accelerate proton beams at 20 TeV per beam, so that the collision has 21,000 times the mass-energy of the colliding protons at rest.
These accelerator projects have common goals. Finding the top quark and the Higgs boson(s) are chief among them. The top quark, the heaviest and most elusive of the spin 1/2 particles, has not yet been found, although groups at both CERN and FermiLab are now searching for it. It is known that the mass-energy of the top quark is not smaller than 90 GeV. This means that it is at least 19 times heavier than the heaviest of the known quarks, the bottom, which has a mass-energy of 4.7 GeV. It is possible that the top quark will have already been discovered when the LHC and SSC go into operation. However, as the time passes and experiments with higher precision and energy fail to find the top, it seems increasingly likely that its discovery must wait for the next generation of accelerators.
The Higgs boson is the other particle-in-waiting. Like the photon, the gluon, and the Z and W bosons, it a mediating particle that is exchanged in an interaction. However, the interaction that characterizes the Higgs is not a force, like the electromagnetic, strong, and weak forces that the other vector bosons mediate. It is the breaking of a symmetry between the forces. The standard model tells us that in regions of space where the energy density is sufficiently high, forces and particles become indistinguishable. Quarks and electrons, photons and neutrinos become identical in mass and interactions. As the region of space cools and the energy density drops, this symmetry breaks and the particles assume their familiar and distinctive characteristics. The mediating particle for this breaking of symmetry is the Higgs particle, and its mass sets the scale at which the symmetry-breaking transition occurs.
It is possible that there is only one Higgs particle (or that its role is assumed by a very heavy top-anti-top quark pair), but there may also be several Higgs particles with different masses. The Higgs must have a mass-energy of at least 90 GeV, or it would already have been observed experimentally. There are theoretical reasons for believing that the mass-energy of lightest Higgs is less than 1 TeV. The 15.5 TeV collisions at the LHC and the 40 TeV collisions at the SSC therefore provide an excellent chance of producing these particles.
There is also a third reason for building these accelerators, one that you will not find in the explicit justifications for the machine. It is a maxim in physics that if you look where no one has looked before, you may find what none has found before. The LHC and SSC will open up a new energy domain for exploration. There may be surprises and hidden aspect of nature there for the discovering, as there have been when other high energy accelerators made access to new domains of energy.
For most experimental physicists, working to test theoretical predictions, e.g., searching for the top quark and the Higgs, is what is done to keep busy while you are waiting for the real discovery, the breakthrough discovery that no one had even suspected. The standard model of particle physics works very well, and lately it has an unbroken string of successes in making testable predictions.
But standard model is clearly incomplete. It has some 39 "adjustable" parameters - particle masses, force constants, and so on, which must have their basis in some deeper and more fundamental underlying aspects of nature. Most of the physicists who will be working at CERN and Waxahachie on the new accelerators will be there because the machines represent their best chance for digging deeper and perhaps discovering some of these.
Physics at CERN:
Nobel Dreams: Power, Deceit, and the Ultimate Experiment, Gary Taubes, Random House, New York (1986).
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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This page was created by John G. Cramer on 7/12/96.