The Indefinite Particle

The 2012 Nobel Prize in Physics will be awarded next month, with the recent discovery of the Higgs boson sparking mass conjecture about the likely winners. However, apportioning credit for this historic scientific find is no simple matter.

On the morning of Tuesday, October 9, a brief telephone call from Stockholm will inform one, two or maybe even three people, that they are the Nobel laureates in physics for 2012. Shortly afterward, a press conference will be held to announce the winners to the rest of the world. Officially, the prize committee was supposed to select the winner this month, following a series of lengthy meetings over many months. But from conversations with physicists that are familiar with the process, it seems that a decision was already made in mid-August.

The decision-making process may have taken a sharp turn about a month earlier, on July 4, when scientists from the large particle accelerator in Switzerland announced their discovery of a new particle whose properties are consistent with the Higgs boson, more than 40 years after theoreticians predicted its existence.

This historic discovery may complete one of the last missing pieces in the so-called Standard Model of particle physics - the set of theories that describe the forces in nature and elementary particles from which matter is composed in our universe.

Alternatively, the discovery may lead scientists beyond the Standard Model which, despite being the most well-founded and solid theory in scientific history, has spawned many essential problems that occupy scientists. The nature of the theoretical implications depends on the nature of the particle, and it is a bit too early to discuss that. Most researchers believe that this is indeed the Higgs boson that was predicted in 1964, or a particle that serves a similar function in the physical fabric of the universe, in that it is connected to a mechanism that gives mass to the rest of the particles.

However, before a final answer to this issue can be provided, the physicists at CERN - the European Organization for Nuclear Research, which houses the accelerator known as the Large Hadron Collider (LHC ) - have a lot of work left to determine the detailed properties of the new particle.

A thorny problem

Despite its heavy media coverage, it is not certain that this year's prize will go to the discoverers of the Higgs. There are scientists - among them physicist Prof. Yaron Oz, who is dean of the Faculty of Exact Sciences at Tel Aviv University - who argue that the uncertainty surrounding the identity of the particle will prevent the theoreticians who predicted the existence of Higgs or the researchers who discovered it from being granted the Nobel Prize this year.

"It is difficult to believe it will happen," Oz says. "It is almost verging on the impossible. We know there is a particle there and it appears to have properties similar to Higgs, but there are questions that must be answered first. Right now it is still too early."

Fellow physicist Prof. Eilam Gross, of the Weizmann Institute of Science, disagrees. Gross, the Higgs search convener (who coordinates the search for the Higgs at the ATLAS detector) at the ATLAS group - one of the two groups that discovered the particle in the particle accelerator - believes the Higgs will be at the center of the Nobel Prize this year: "I have no doubt about it," he says.

Whether the prize is awarded to them this year or in coming years, there would seem to be two groups of Nobel Prize candidates for the Higgs: the theoreticians who predicted, and the experimenters who discovered.

When it comes to the experimenters, the prize committee is faced with a thorny problem. More than 6,000 experimenters took part in the research groups of the two particle detectors, ATLAS and CMS, which were responsible for the discovery. But Nobel rules dictate that no more than three people can be awarded a prize. Alfred Nobel did not live in an era of research groups comprised of hundreds and thousands of scientists, so his will did not address such a possibility. Some are hopeful that, contrary to the customary practice, the prize committee members considered - or will consider - the possibility of granting the Nobel to the two particle detector research groups, or to the CERN organization itself.

"I'm not betting on it, because the Prize committee is very conservative," says physicist Prof. Eliezer Rabinovici, director of the Institute for Advanced Studies at the Hebrew University of Jerusalem, and who serves as Israel's liaison with CERN. "But who knows, the committee can also surprise us. It is one of the options at its disposal."

"I've calculated that if CERN gets a Nobel Prize, my share in the business will be $100," jokes physicist Prof. Ehud Duchovni of the Weizmann Institute of Science, who was part of the ATLAS detector group. He thinks the discovery warrants two prizes, one for the theoreticians and the other for CERN.

Ultimately, it is likely that the theoreticians will be the ones to win the Nobel Prize in connection with the Higgs, if and when one is awarded. But a little political debacle awaits the prize committee on the theoretical front as well. Back in 1964, six scientists working in three separate teams published three separate articles, one after the other, laying down the theoretical groundwork for the mechanism that grants elementary particles their mass. Even those who were not the first to publish their work claim that they were working independently from others in the field. But as we know, only three can win the prize.

The first article was by the Belgian Francois Englert and the Belgian-American Robert Brout. Immediately afterward came the article by Britain's Peter Higgs. These three were the first to propose a mechanism explaining how particles acquire their mass within the framework of the Standard Model of particle physics. Brout died last year, leaving Englert and Higgs the leading candidates for a Nobel.

There are several researchers who are hoping to become Nobel laureates alongside Englert and Higgs, headed by the Americans Carl Richard Hagen and Gerald Guralnik and Briton Tom Kibble. They say they independently arrived at similar findings to those of Higgs, Brout and Englert, but were a few days late sending in their article. A short while before they mailed their own - Gross recounted last year at a Hebrew University lecture - they received Higgs' article.

However, there are precedents in their favor. Gross points out that six researchers won the J.J. Sakurai Prize for Theoretical Particle Physics in 2010: Brout, Englert, Higgs, Hagen, Guralnik and Kibble. The Wolf Prize, by contrast - which is considered a barometer for the Nobel Prize - was awarded only to the first trio. "If you have to give it to only three researchers, then it goes to Englert and Higgs [and Brout], because Guralnik and Hagen sent it in late," Gross concludes.

Another contender for the crown is the American physicist and Nobel Prize laureate Philip W. Anderson, who was among the first to come up with the possibility of a mechanism for generating, but did not develop this possibility into a full-fledged theory.

Nevertheless, Englert and Higgs remain the leading candidates for a Nobel when it comes to the mechanism for generating mass in the Higgs boson. To understand why this is so, we have to go back to the 1950s and '60s and to the problems physicists grappled with at the time.

Forces of nature

First though, a little scientific background: The elementary forces are carried by particles, which serve as emissaries of the forces. The electromagnetic force, for example, is carried by the photon (the light particle ). The weak nuclear force is carried by three different particles.

The weak force is a short-range force; in contrast to the long-range forces, which are the electromagnetic force and gravity, a short-range force cannot affect distant objects. The important point in terms of the Higgs is that long-range forces are carried by zero mass particles, such as the photon, whereas the carriers of the short-range forces are particles with mass.

In the late 1950s, scientists attempted to draft equations that describe the weak force, but they reached a dead-end. On the one hand, the weak nuclear force is a short-range force, and therefore the particle carrying it must have mass. On the other, the researchers discovered that the equations do not allow for such a particle to have mass.

"One possibility that physicists raised at that time was that their equations for describing the weak nuclear forces were simply incorrect," Prof. Rabinovici says now, "but that was hard to accept because these equations had had no little success."

The first to publish an article in 1964 related to this problem were Englert and Brout. According to Rabinovici, the researchers did not address the problem of the weak force directly; instead they discussed another hypothetical problem that was remarkably similar to the actual problem. Either way, he says, "they managed to solve the problem."

In order that a particle of the kind the researchers were trying to describe could acquire mass, Brout and Englert (and later Higgs ) had to assume the existence of three new particles. When the researchers' hypothetical solution to the actual problem was implemented, physicists found that each of these particles can similarly "serve" also the three carriers of the weak nuclear force, and resolve the stubborn theoretical problem: when these three new particles, called Goldstone bosons, interact with the carriers of the weak nuclear force, these force carriers acquire mass.

None of these three particles is the particle for which the scientists at the LHC in Switzerland were searching until recently. The innovation in Higgs' work was that, if we presume the existence of the three Goldstone bosons, we must presume the existence of another particle. This is the Higgs boson.

And what is the function of the fourth boson? To their great joy, physicists discovered that in addition to the first three bosons, the Higgs boson plays a key role in the cosmic fabric by granting mass to the electron and to the particles that make up the proton and neutron (the quarks ). In this manner, the Higgs gives us life as we know it, Rabinovici says. For physicists, this discovery provided strong theoretical reinforcement that the mechanism proposed by Englert, Brout and Higgs, is consistent with a description of the weak force.

Up until July 4 this year, we knew about the existence of the first three bosons out of the four. On that particular day, the world learned that a particle consistent in its properties with that fourth Higgs boson had been discovered.

"It's incredible that a single particle like the Higgs can give mass to so many different particles. It is admirably economical," Rabinovici says. He adds, however, that it may turn out that additional particles exist to serve the same function as the Higgs. "Such a discovery will only delight theoreticians, who are hoping with all their might for there to be something beyond the Standard Model," he adds.

Rabinovici believes the three researchers have deserved a Nobel Prize for years, regardless of the latest discovery in the accelerator. In many respects, he says, the existence of the Higgs Mechanism - even in a different version than the one they proposed - has already been bolstered by consistencies with other theories, and by the discovery of the first three bosons.

They have not previously been awarded the prize because, Rabinovici says, "judging by past experience, the committee does not like to award repeat prizes for observations regarding the same natural phenomenon" (the phenomenon in this case being the weak nuclear force ). Nobel Prizes relating to the weak force were already awarded in 1979, 1984 and 1999.

Rabinovici adds that the discovery of the new particle, even if it is not clear whether it is exactly the same particle predicted by Peter Higgs, might make it easier for the committee to award the prize to the 79-year-old Englert and the 83-year-old Higgs, who have waited so long for a Nobel and would have the privilege of receiving it in their lifetimes.

Ehud Duchovni aptly describes the prevalent sentiment among physicists today: "It's pretty amazing," he says, "that 40-something years ago, a man sat at a table, calculated and said, 'There must be a particle.' And then, 40-something years, $10 billion and 10,000 people later, we prove that it exists."