Subatomic particles of the same mass as a proton but having a negative electric charge and oppositely directed magnetic moment, antiprotons are the antiparticles of protons. Even though they are stable, they are typically short-lived as collision with any proton causes both particles to annihilate in a burst of energy.
Even though antiprotons were discovered only in 1955, we will have to go back a few decades further to get our foundations right. For it was in 1928 that Paul Dirac, a brilliant and eccentric English theoretical physicist, formulated an equation to describe the behaviour of relativistic electrons in electric and magnetic fields.
Strong implications
Dirac’s theory, which helped him win the 1933 Nobel Prize in Physics, considered both the special theory of relativity formulated by theoretical physicist Albert Einstein and the effects of quantum physics proposed by physicists Erwin Schrodinger and Werner Heisenberg. Even though the majority of the scientists of the time assumed that a particle’s energy must always be positive, Dirac’s equation permitted negative and positive values for energy.
While the idea went against both common sense and what was observed in physics, the implications were obvious. And when a young physicist, Carl David Anderson, recorded a now historic photograph in a project at the California Institute of Technology in 1932, things changed dramatically. Anderson had discovered an antiparticle that was later named positron, a discovery that made him one of the youngest to win the Nobel Prize for Physics when he received the honour in 1936.
The existence of the positron, the antimatter counterpart of the electron, suggested that the proton too should have an antimatter counterpart. This meant that antiprotons had to exist and the search for them soon got under way.
Bevatron is born
The accelerators available at the time were not powerful enough to produce these particles as two billion electron volts (eV) were required to create proton-antiproton pairs. The best way to do that would be by striking a beam of protons accelerated to an energy of about six billion eV on a stationary target of neutrons.
With these numbers in mind, American nuclear scientist Ernest Lawrence – winner of the 1939 Nobel Prize for Physics for the invention of the cyclotron – commissioned the Bevatron accelerator in 1954 to reach the required energy levels (while billion electron volts is now known as GeV, it was then denoted as BeV, giving the accelerator its name). Even though this wasn’t Bevatron’s officially declared purpose, its energy range – designed to accelerate protons up to 6.5 GeV – wasn’t chosen arbitrarily and it was built to go after the antiproton.
Half the challenge
With a machine at their disposal, two teams were put together to hunt for the antiproton. Edward Lofgren, who managed operations at the Bevatron, headed one team, and the other team was led by physicists Emilio Segre and Owen Chamberlain.
While creating the necessary conditions to produce antiprotons was one huge challenge, devising the means to identify them once they were created was an equally difficult task. Not only would 40,000 other particles be created for every antiproton created, but these antiprotons then annihilated in just microseconds.
It was understood that at least two independent quantities had to be measured for the same particle to identify it as an antiproton. Several possibilities were considered, but they concluded that it had to be momentum and velocity in the end. Apart from putting in place elaborate systems to measure momentum and velocity, an additional experiment to see the signature star image of an annihilation event was also readied to further check if a suspect particle was truly an antiproton.
Segre, Chamberlain, and their group went first in the first week of August in 1955. After their run lasted for five days, Lofgren and his collaborators ran their experiments for the next two weeks. Segre and Chamberlain returned for their next go on August 29, only to see the Bevatron break down on September 5.
60 antiprotons
Even though Lofgren’s crew was scheduled to begin on September 21, a week after repairs had been completed, he generously ceded his time so that Segre and Chamberlain could complete their experiments. It was during that run that they got the first evidence of the antiproton based on momentum and velocity, later confirmed by analysis of images that revealed the signature annihilation star.
The experiments detected 60 antiprotons in all during a run that lasted nearly seven hours. The official announcement of the discovery was made on October 19, and within a couple of weeks, on November 1, a paper titled “Observation of antiprotons” was published in Physical Review Letters.
Segre and Chamberlain won the 1959 Nobel Prize for Physics for their discovery. The Bevatron remained operational for nearly 40 years and the structure was finally demolished in 2011. As for antiprotons, they are now produced in the trillions and are central to high-energy physics experiments.
