Rubbia and van der Meer Isolate the Intermediate Vector Bosons

Carlo Rubbia and Simon van der Meer isolated the atomic particles known as intermediate vector bosons, the particles that carry the weak nuclear force, which provided confirming evidence for the unified electroweak theory.

Summary of Event

One of the largest research projects in contemporary physics has been the study of the forces that act between the elementary particles of matter. The experiment that resulted in the discovery of the W⁺, W^-, and Z⁰ particles, or the “intermediate vector bosons,” provided at least partial confirmation for a theory about two of these forces. Before one can understand the experiment, however, one needs to understand the four forces in nature. Atomic particles Unified electroweak theory Weinberg-Salam electroweak unified theory[Weinberg Salam electroweak unified theory] Electroweak force [kw]Rubbia and van der Meer Isolate the Intermediate Vector Bosons (1983) [kw]van der Meer Isolate the Intermediate Vector Bosons, Rubbia and (1983) [kw]Intermediate Vector Bosons, Rubbia and van der Meer Isolate the (1983) [kw]Bosons, Rubbia and van der Meer Isolate the Intermediate Vector (1983) Intermediate vector bosons Atomic particles Unified electroweak theory Weinberg-Salam electroweak unified theory[Weinberg Salam electroweak unified theory] Electroweak force [g]Europe;1983: Rubbia and van der Meer Isolate the Intermediate Vector Bosons[05050] [g]Switzerland;1983: Rubbia and van der Meer Isolate the Intermediate Vector Bosons[05050] [c]Physics;1983: Rubbia and van der Meer Isolate the Intermediate Vector Bosons[05050] [c]Science and technology;1983: Rubbia and van der Meer Isolate the Intermediate Vector Bosons[05050] Rubbia, Carlo Van der Meer, Simon

The gravitational force is the weakest of the four forces, yet any particle with mass feels its effects. The gravitational force acts over large distances and is always attractive. It holds the universe together, keeps objects from floating off Earth’s surface, and keeps Earth in orbit around the Sun.

The electromagnetic force acts on electrically charged particles such as electrons and protons and is much stronger than the gravitational force. This force can act over an infinite distance and can be either repulsive or attractive. This latter fact can be confirmed by moving two magnets into close proximity like poles (charges) repulse each other, while opposites attract.

The other two forces are nuclear forces, a strong and a weak version, which act over very small distances. The weak force is responsible for the phenomenon of radioactive decay in radioactive materials; the strong force acts between the protons and neutrons that constitute the nucleus of the atom. The strong force thus holds the nucleus together.

Until 1967, there were four fairly well understood theories explaining each of the four forces. In that year, Abdus Salam Salam, Abdus of Imperial College, London, and Steven Weinberg Weinberg, Steven of Harvard University, and, independently, Sheldon L. Glashow Glashow, Sheldon L. of Harvard University, developed the first of the so-called unified theories. The idea is that rather than having four theories to explain four forces, one unified theory would explain them all. The theory, which came to be known as the Weinberg-Salam electroweak theory (W-S), had as an experimentally testable consequence the existence of three massive particles known as the “intermediate vector bosons.” Such theories are known as quantum field theories and describe the forces between two or more particles as the exchange of a third intermediate particle between them. In other words, rather than conceptualizing the force as simply a continuous field, the force is conceived as the exchange of discrete “quanta.” In the electromagnetic force, a massless particle known as the “photon” is exchanged; it has been known to exist since Albert Einstein first discussed the quantized nature of light in 1905. In the W-S theory, the intermediate vector bosons in addition to the photons carry the newly termed electroweak force.

If the W-S theory was correct, an experiment should have been possible that could detect the intermediate vector bosons. This was the Carlo Rubbia experiment conducted at the Conseil Européen pour la Recherche Nucléaire, or CERN (later known as the European Organization for Nuclear Research). The theory predicted the existence of three particles: the W⁺, the W^-, and the Z⁰. They carry positive, negative, and neutral electrical charges, respectively. Because of their relatively heavy masses eighty to ninety times that of the proton experimental apparatus that could generate extremely high energies would be required to isolate the particles.

Carlo Rubbia.

(The Nobel Foundation)

Two large experiments were conducted at CERN, CERN called UA-l and UA-2 for Underground Area One and Underground Area Two. The difference between the two experiments rests in the type of target used for the detection of the bosons, yet both employed the huge proton accelerator, the Super Proton Synchrotron Super Proton Synchrotron (SPS) at CERN. Rubbia’s first step in the experiment was to modify the SPS from simply a proton accelerator (a large magnetized ring that accelerates protons to very high energies) to a proton-antiproton collider (an apparatus that would accelerate protons and the particle of identical mass and opposite charge the antiproton at high energies for the purpose of smashing them together and detecting the residue of the collision, which often includes new particles). In theory, the intermediate vector bosons should be among the collision products of a proton-antiproton collision.

In 1976, Rubbia faced the problem that new proton-antiproton accelerators would not be in operation until the mid-1980’s. One such machine, Fermilab’s Tevatron Tevatron particle accelerator in the United States, would not come online until 1985. As a solution, Rubbia spearheaded the renovation of the CERN SPS, converting the proton accelerator into a collider. Simon van der Meer, formerly in charge of magnetic power at CERN, supervised the conversion, which was approved on Rubbia’s recommendation in 1978 at a cost of $100 million. Van der Meer’s most important contribution was the design of the new antiproton storage ring. In the new proton-antiproton collider, when a beam of accelerated protons is shot at a copper target, a relatively small number of antiprotons are produced. These particles are collected and stored in the Antiproton Accumulator until several billion have accumulated. At this time, they are sent back into the main ring of the accelerator, while protons are introduced simultaneously, but in the opposite direction. Eventually, at energies approaching 300 billion electronvolts, the two opposing beams are allowed to collide. Detectors are placed at the point of collision to record the products of the reaction.

By late 1981, the new collider was in operation and produced energies nearly twenty times greater than the energies that the unmodified SPS could generate. Rubbia enlisted almost two hundred experimental and theoretical physicists to take part in the project. The team at UA-1 employed a target to observe collision products that weighed almost 2,000 tons and cost $20 million. The target’s size could be attributed to some future planning on the part of the Rubbia team. Not only could this target detect the intermediate vector bosons but also it could, under the proper conditions, detect the more elusive Higgs particle, another prediction of W-S theory. This detector can measure the energy of particles by measuring the curvature of their paths in the presence of a large magnetic field. UA-2 employed a target one-tenth the size of UA-1’s, which was designed exclusively for the intermediate vector boson search. Experimental runs on both targets began in early 1982 after some crucial trial-and-error adjustments on the newly constructed collider.

In order to detect any subatomic particle, experimenters do not look for them directly they are far too small for even the most powerful microscopes. Instead, the targets are embedded with sophisticated electrical equipment that can detect the properties attributed to the particles. The search for the intermediate bosons is one such case. Theoretically, after a highly energetic proton-antiproton collision, if any bosons are produced, their high mass and equivalently (according to Einstein’s theory of relativity) high energies would make them very short-lived particles; they would decay quickly into other, less interesting particles. As a result, Rubbia and his team were forced to look for specific patterns on their targets that would indicate the presence of the bosons. Each of the three bosons has a unique decay pattern, according to the W-S theory.

The Z⁰ particle, being electrically neutral, would decay into two less massive particles with opposite charge, such as an electron and a positron. Physicists can recognize easily the tracks of positrons and electrons on a detector, and a highly energetic pair of them traveling in opposite directions would provide evidence for the Z⁰ boson. Each of the W particles carries a charge; therefore, their decay products must have an equivalent sum charge. For example, a W⁺ carries an electrical charge of +1. Its decay products might consist of a positron, which carries a +1 charge, and an electron neutrino, which has no charge. Because neutral particles such as electron neutrinos cannot be imaged in electrical detectors, evidence of the W⁺ would consist of a single, highly energetic positron. By January of 1983, Rubbia and his team were reporting success.

By the end of the summer of 1983, Rubbia and 134 other physicists had collaborated in an experiment that isolated all three of the intermediate vector bosons in very close agreement with the predictions of the W-S theory. UA-1 reported fifty-five events that they maintained produced both W⁺ and W^- particles, while UA-2 reported thirty-five events. Both reported a mass of approximately 81 billion electronvolts, plus or minus 2 billion electronvolts; the W-S theory predicted 83 billion electronvolts. UA-l reported six Z⁰ events, while UA-2 reported three, at masses that were also in very close agreement with the experiment. This confirmation of the W-S theory further justified the Nobel Committee’s awarding of the 1979 Nobel Prize in Physics to Salam, Weinberg, and Glashow for the theory. In 1984, Rubbia and van der Meer shared the Nobel Prize in Physics Nobel Prize in Physics;Carlo Rubbia[Rubbia] Nobel Prize in Physics;Simon van der Meer[Vandermeer] for their experimental achievement.

Significance

The impact of the isolation of the intermediate vector bosons has taken on significant scientific and technological dimensions. Scientifically, the discovery had the immediate effect of confirming the predictive success of the Weinberg-Salam electroweak unified theory. In addition, it has provided the impetus for further work in unifying the other forces, the gravitational and the strong nuclear force. Several attempts to unify the electroweak theory with a theory of the strong nuclear force have met with some limited success, although a unification of all four forces has yet to be accomplished. Intermediate vector bosons

One particularly striking effect of the experiment was the discovery of so-called weak neutral currents. What is so striking is that this discovery occurred before the discovery of the bosons. The experiments that led to the discovery of weak neutral currents were actually a fortunate consequence of some of the early studies of the intermediate vector bosons. A weak neutral current is one way in which the presence of the Z⁰ particle can be detected. In 1973, experiments conducted on weak interactions revealed that charge was preserved before and after weak interaction experiments, indicating that a neutral particle must be exchanged in the interaction. That particle, according to W-S theory, was the Z⁰. Rubbia, together with David B. Cline Cline, David B. and Alfred Mann, Mann, Alfred made this discovery in experiments conducted at Fermilab. The result not only gave an early vote of confidence for the new W-S theory but also spurred Rubbia to continue the search for the intermediate vector bosons at CERN.

Technologically, the success of the boson search was a victory to those physicists who favored the use of proton accelerators in high-energy physics experiments. The Rubbia results were instrumental in the U.S. Department of Energy’s push to develop the massive Superconducting Super Collider Superconducting Super Collider (SSC), a proton-antiproton collider that would attain energies at least forty times those of CERN’s collider. Construction of the SSC was canceled in 1993, however, because of rising cost estimates and other factors.

The results also struck a blow to proponents of electron-positron colliders, who claimed that proton-antiproton machines create too much interference to detect such short-lived particles as the intermediate vector bosons. Rubbia’s results indicated that this was clearly not the case, although Rubbia has favored using both types of machines. CERN’s electron-positron collider was in operation from 1989 to 2000. A similar device was scrapped in mid-construction at the United States’ Brookhaven Laboratory in favor of the much more powerful Superconducting Super Collider. Intermediate vector bosons Atomic particles Unified electroweak theory Weinberg-Salam electroweak unified theory[Weinberg Salam electroweak unified theory] Electroweak force

Rubbia and van der Meer Isolate the Intermediate Vector Bosons Summary