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Revolutionary neutral-current discovery honored 36 years later

In a room at a German university a physicist went through his daily routine. He walked down a line of bubble-chamber photo scanners.  He leafed through the piles of particle-interaction photos the scanners had recorded.

Bubble chamber photo of first leptonic neutral current event, found in Aachen. Courtesy Jorge Morfin.

Bubble-chamber photo of first leptonic neutral current event, found in Aachen. Courtesy Jorge Morfin.

One image caught his eye.
Soon another physicist was called over to look, and another. The excitement was palpable. By evening, everyone in the Aachen group -- and, soon thereafter, everyone in the rest of the Gargamelle collaboration -- knew particle physics would never be the same.

"It actually provided the experimental foundation for what we now call the Standard Model," says Harry Weerts, recounting the observation of the first neutral current event in 1972 - a single electron scattering.

 Jorge Morfin, also on the team searching for this leptonic event, agrees. "Seldom has a single interaction had such a dramatic impact on particle physics" he says. 

 Weak neutral current interactions are similar to electromagnetism. They occur through the exchange of neutral particles.  The Gargamelle bubble chamber had been set up to observe neutrinos, which leave no tracks in bubble-chamber fluid but can be seen indirectly through their interactions with other particles as they pass through. It was one of those interactions, in which a neutrino scatters a single electron, that directly revealed the presence of the weak neutral current.   Ten years later, experiments at CERN discovered the Z, the particle associated with weak neutral currents.

The single picture of the single electron found by the Aachen group would drive the collaboration to confirm the observation with a larger, more complicated hadronic study that involved events with and without muons and more than one million bubble-chamber images.

In 1973 the collaboration published a paper that would become one of two discovery cornerstones for the electroweak force theory. This theory postulates that at very high energies, two of the four fundamental forces of nature - the weak force, which is responsible for radioactive decay, and the electromagnetic force, which is responsible for electric and magnetic interactions - become one.

More than three decades later, at a July 20 conference in Krakow, Poland, the Gargamelle collaboration will receive the European Physical Society High Energy and Particle Physics Prize for discovering the weak neutral current. The men who first predicted weak neutral currents won the Nobel Prize in 1979

The Gargamelle discovery was the first time that more than one or two physicists stood out as key players in a discovery.  The 55 physicists who authored the paper, which some have called the most important discovery made by CERN scientists so far,  came from working groups in Germany, Belgium, Italy, France and England, all using a neutrino beam based at CERN. 

Professor Fabio Zwirner, a member of the EPS-HEPP Board, called the award "long overdue."

From observation to discovery:

The road to discovery was nearly as unusual as the road to recognition.

Former Gargamelle collaboration members and the first bubble chamber photos of neutral current interactions. Jorge Morfin holds the leptonic event and Harry Weerts holds one of the hadronic events.

Former Gargamelle collaboration members and the first bubble chamber photos of neutral current interactions. Jorge Morfin holds the leptonic event and Harry Weerts holds one of the hadronic events.

In the winter of 1972, when the neutral current search team in Aachen, Germany, which included Morfin and Weerts, found the first hint the neutral current existed -- the single electron event --  the theory behind it was only a few years old. A strong bias that this type of interaction shouldn't exist was just starting to fade.

The research team had received the fateful photo by chance, after the event photos were divided among universities. 

"We called it our Christmas present," says Morfin. The team worked for months trying to rule out the possibility that the event was caused by background and not a new discovery.

Subsequently, Gargamelle was racing the HPWF (Harvard, Pennsylvania, Wisconsin, Fermilab) experiment at Fermilab to discover hadronic neutral currents. The race was dubbed "alternating neutral currents" for the announcements of "we found it, no we didn't, etc., " Morfin says.

The entire Gargamelle collaboration kicked into high gear, compiling event data and analysis from all members, until the discovery was doubt proof, culminating in a 1973 publication.

"CERN and Fermilab both had the evidence, but we were trying to convince ourselves that what we were seeing wasn't background neutrons," Morfin says. "We finally were convinced there were no other explanations that would fit the data. The energy and spatial distribution of the muon-less events were not consistent with neutrons."

Forging a legacy and a bond:

Four of the living collaborators currently work in the United States: Morfin at Fermi National Laboratory, Weerts at Argonne National Laboratory, Robert Palmer at Brookhaven National Laboratory and William "Jack" Fry at the University of Wisconsin-Madison, where he is a professor emeritus.

"Our committee rewards a milestone experimental discovery in the physics of elementary interactions," said Professor Stefan Pokorski, from the University of Warsaw, in a press release. He called the discovery "a benchmark to full understanding of the phenomenon of radioactivity, observed more than a hundred years ago by Henri Becquerel, Marie and Pierre Curie and Ernest Rutherford."

The discovery, made in a 12-cubic-meter liquid Freon bubble chamber, created a foundation for future experiments, including the NuTev experiment at Fermilab that looked at the neutrino ratio of neutral currents to charged currents and the searches for the Higgs boson at Fermilab's Tevatron and CERN's Large Hadron Collider, which could round out the understanding of the electroweak interaction.

"This was a very exciting time," Weerts says. "What this does is it binds people together for a lifetime. As a group, you feel that you accomplished something great."

Read more about the historic discovery in this AAPPS Bulletin article. And in this review of the discovery

View a CERN film about the construction and operation of this giant bubble chamber here.

View past  EPS HEPP prize winners here.