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RHIC: Big Bangs in the Lab

by John G. Cramer

Alternate View Column AV-46
Keywords: relativistic heavy ion collider RHIC Brookhaven quark gluon plasma

Published in the June-1991 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 9/30/90 and is copyrighted ©1990 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

    Imagine that we are watching the flight of a nucleus of a heavy element, say a gold nucleus with 79 protons and 118 neutrons. It has been accelerated until it is travelling with a speed of 99.9957 % of the velocity of light. A nucleus at this near-light speed will show two striking relativistic effects: (1) its mass will increase and (2) its length will shrink in the direction of travel. At 0.999957 c the mass-energy of the nucleus will increase to 108 times its rest mass (or 100 GeV/nucleon, for those who know the lingo). At the same time, its thickness in the direction of travel will shrink to 1/108 of its thickness at rest. The nucleus will resemble a pancake with a thickness about 1% of its diameter and a mass that has been increased by a factor of 108.

    Now imagine that this accelerated gold nucleus has a head-on collision with a second nuclear pancake, a gold nucleus accelerated to the same ultra-relativistic velocity but in the opposite direction. Momentarily, the two pancakes in this light-speed head-on collision will overlap. The masses of the nuclei have been increased by a factor of 108, and the mass of each is contained in a volume that has been reduced by a factor of 108. As a result, neglecting the fuzzing effects of the Heisenberg uncertainty principle, the instantaneous density of mass-energy in the overlapping collision volume is 2 x 1082 or 22,328 times the density of a normal gold nucleus at rest.

    There 197 nucleons (neutrons and protons) in each gold nucleus. Any line cut through the center of the nucleus will intersect a stack of about 7 nucleons. Therefore, the collision of the two nuclear pancakes will involve linear stacks of nucleons impacting again and again with the "wreckage" of collisions of nucleons just ahead in the stack, a seven-fold repeated "chain reaction" pileup reminiscent of a California freeway.

    The standard theory of quantum chromodynamics describes nucleons as composed of three quarks held together by gluons, the mediating particles of the color force, also called the strong interaction. A gluon that is binding two quarks can be visualized as a color flux tube or color string connecting one quark to the other. In the collision described above the color strings are repeatedly broken, snarled, and tangled, producing what has been described as color spaghetti..

    The brief phase of ultra-high mass-energy density is followed by a longer-duration phase in which the two now banged-up projectiles, still carrying very high energy and momentum, pull apart. The tangled color strings pull hard on the quarks as the projectiles separate, extracting a fraction of the huge collision energy and leaving a wake of energetic debris in the path of the separating nuclear projectiles. This wake is predicted to be almost free of identifiable nucleons and to contain 10 to 100 times the mass-energy density of a nucleus at rest. It is called a quark-gluon plasma, a completely new state of matter that has never been studied by physicists, that perhaps has not existed in our universe since the first few microseconds of the Big Bang, the time at which the cosmos cooled from a plasma of quarks and gluons to a "soup" of nucleons and mesons.

    The quark-gluon plasma thus represents a new and unexplored territory in the domain of nuclear physics. Exploring this quark-matter territory may simply provide a confirmation of our present ideas about matter under extreme conditions, but it may also reveal a new landscape dotted with strange and unanticipated wonders.

    This Alternate View column is about a new initiative in nuclear physics that will produce a new accelerator facility to permit the study of this new state of matter. The facility is the Relativistic Heavy Ion Collider (RHIC), a $397 million accelerator facility to be built over the next six years at Brookhaven National Laboratory with funds from the U. S. Department of Energy for the study of ultra-relativistic heavy ion collisions. I rarely write about my own scientific work in these columns, but this one is an exception. In June of 1990 I joined a collaboration that is proposing to build a $27 million detector to be used at RHIC in about 1997, after that facility goes into operation. I plan to do physics at RHIC.

    So I want to tell you the facility. First, I will recount the somewhat dismal events that preceded the RHIC project. In the early 1980's Brookhaven National Laboratory was in deep trouble arising from the lagging construction of a new particle accelerator called ISABELLE, a 200 GeV on 200 GeV proton-proton collider. The source of the trouble was that the project had been started too soon. In 1977 the powerful congressional delegation of New York State had overpowered the congressional funding committees and inserted the funding for ISABELLE into the Department of Energy (DOE) budget, placing it ahead of competing high energy physics projects and a full year ahead of the schedule the DOE and its advisory committees had made for funding the project.

    Pushing the ISABELLE schedule ahead by a year was, as it turns out, a disastrous blunder. The superconducting magnets that were to be the heart of the machine had worked well in prototype tests, but not enough testing had been done. When the production models of the magnets begin to come off the assembly lines, they could not be made to work at the magnetic field levels needed for the machine. The missing year of prototype testing, short circuited by political clout, had resulted in an unworkable magnet design.

    The magnet design problems were eventually fixed, but only after it was too late. In 1983 a physics advisory panel recommended that the ISABELLE project be scrapped so that more effort could be focussed on the design of the new Superconducting Supercollider project. George A. Keyworth, President Reagan's Science Advisor, announced shortly afterwards that ISABELLE had been cancelled. As someone put it, ISABELLE had become was-ABELLE.

    The old ISABELLE accelerator vault has been preserved at Brookhaven, along with all the cryogenics, staging areas, and the support facilities that would have been used by the cancelled superconducting accelerator. RHIC, the new accelerator, will be built in the tunnel and will use all the liquid helium refrigeration and other equipment installed almost a decade earlier. RHIC will rise from the ashes of ISABELLE, and as a result will be a factor of 2 cheaper than if it were constructed elsewhere. The startup funding for RHIC is in the new Fiscal Year 1991 DOE budget that finally passed the U.S. Congress last September after the big budget logjam was broken. Accelerator physicists and engineers at Brookhaven are swinging into action to produce the machine, which should begin initial beam tests in about five years.

    RHIC will use not only the ISABELLE equipment, but also will take advantage of the accelerator complex that already exists at Brookhaven. The RHIC beam of particles will start one of two existing MP Tandem Van de Graaff accelerators, then be routed some distance uphill to an accumulator/booster ring accelerator and into the Alternating Gradient Synchrotron, a 1960's accelerator that was once the premier facility of high energy physics and is still in operation. Heavy ions from the AGS will be sent in both clockwise and counterclockwise paths along the RHIC ring, and both beams will be accumulated to the desired intensity and then ramped up in energy to the full design energy of the machine.

    At six regions around the RHIC ring the two beams travelling in opposite directions will be squeezed together and caused to intersect and collide. At four of these these intersection points, experimental physicists (including my collaboration, we hope) will construct detectors to study the collisions and try to understand the new state of quark matter produced. The RHIC beams recirculate through the ring, passing through the intersection regions over and over until, after some hours or days the two beams have dropped to an unacceptably low level. They are then dumped, and the RHIC accelerator ring is reloaded with fresh beams of heavy ions.

    Here are some frequently asked questions and answers about this new accelerator facility.

Q: Why build two expensive accelerators, RHIC and the Superconducting Supercollider?

A: RHIC and the SSC are both colliders, but they are very different machines both in cost as well as purpose. RHIC is much cheaper. The entire RHIC accelerator facility costs about as much as a single SSC detector. Moving upward, the SSC total cost is about equal to the increase last year in the estimated cost of NASA's Space Station Freedom. And S.S. Freedom, at worst, will cost as much as a month or so of Operation Desert Shield. And so on.

Q: But why can't the RHIC physics be done at the SSC?

A: The SSC is designed primarily to discover the Higgs boson and the top quark, if the latter has not been found by the time the SSC comes into operation around the year 2000. It will operate at energies a factor of 100 higher than RHIC, but in its initial configuration will accelerate proton beams exclusively. No heavy ion injection system for beams of heavy nuclei are planned for the SSC. Its beam stations will be used exclusively for experiments in its higher energy domain. The SSC has much higher collision energies, but only a few quarks localized in a small volume are involved in its collisions. No significant formation of a quark-gluon plasma from SSC proton collisions is expected.

Q: What happens if a quark-qluon plasma is formed?

A: The quark-gluon plasma has only a brief lifetime, at the end of which its energy goes into particle production. It has been calculated that a single RHIC collision event might produce 4,900 charged pi-mesons, 500 charged K-mesons, 300 neutrons, 300 protons, 130 antiprotons, and few thousand gamma rays, and a few hundred other miscellaneous particles. The task of investigating the collision event therefore becomes one of measuring some fraction of these particles and drawing conclusions from these measurements.

Q: What will the RHIC detectors do?

A: There are two basic detector strategies. One is to select some specific signature of the quark-gluon plasma and to design a detector that focuses almost exclusively on that signature. Most of the detectors proposed for RHIC are based on that strategy. The alternative strategy is to track as many of the particles from the collision as possible. The detector design in which I am participating is one of two designs proposed using the second strategy and involves construction of a time-projection chamber, a modern electronic equivalent of the cloud chambers that were used in nuclear physics in the 1930's.

    Let me finish by saying why I decided, a few months ago, to get involved with this kind of physics, which is on a far larger scale than the experiments I have done in the past. The Hanbury-Brown-Twiss (HBT) effect is a quantum optics trick used by astronomers to measure the sizes of nearby stars. If light from a nearby star is detected simultaneously in two telescopes and the detector signals are multiplied, the combined signal shows an interference pattern characteristic of the diameter of the star.

    It happens that the HBT technique also can be used with the pi mesons produced in a RHIC collision, a brilliant "flash bulb" of particles casting bright illumination on the event. These particles, through the HBT effect, can be used to reconstruct the size, shape, and eccentricity of the source that emitted them, which might be a cooling quark-gluon plasma.

    Thus in this new domain of energy density, where new physical phenomena may be lurking, there is the opportunity to study individual collisions, not in some statistical average but as single collisions. If there is a hint of new physics, the events, without any pre-judgment about signatures, can be studied in in excruciating detail.

    It is said that if you would see what none has seen before, you must look where none has looked before. It is my hope that RHIC will allow me and my colleagues to do just that.


Quark-Gluon Plasma:
Larry McLerran, Reviews of Modern Physics 58, 1021 (1986).

Hanbury-Brown-Twiss Effect:
D. H. Boal, C.-K. Gelbke, and B. K. Jennings, Reviews of Modern Physics 62, 553 (1990).

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