The Elusive Goddamn Particle

A hundred meters under the Franco–Swiss border, two beams of a hundred trillion protons each, travelling in opposite directions in circles of 27-kilometre circumference at a pace that sees them complete over 11,000 circuits in a single second, and controlled by superconducting magnets maintained at two degrees above absolute zero, smash into each other. Over half-a-billion collisions per second. From the petabytes of data, of over 1,000 trillion such collisions spread over two years, emerged signatures of a long-sought elusive particle. This particle, the avatar of an all-pervading field, reveals its presence only through its progeny. It is the manifestation of a mechanism that gives elementary particles their mass, and was the one missing brick in the Standard Model, the organizing framework for the known elementary particles.

The Large Hadron Collider (LHC) at CERN, like other particle accelerators is, in essence, a powerful microscope. By Einstein’s mass–energy equivalence relation, new particles arise from the energy liberated by the particle collisions. The higher the energy of the beams, the heavier the new particles created. These ephemeral particles then decay to more stable ones, and an analysis of these decay products enables particle physicists to reconstruct the sequence of events and decipher its governing laws.

What makes the LHC different from other particle accelerators is the unprecedented energy with which the colliding beams smash into each other: 4 TeV each for a combined 8 TeV, with plans to ramp this up to 7 TeV each. Here, TeV stands for tera, or a trillion, electron volts. An electron volt is the energy gained by an electron accelerated through a potential difference of one volt. It is a convenient unit of energy in particle physics because, in absolute terms, the energies involved are very small. With such energies, physicists are now able to probe matter at scales of 10–18 metres.

Since J. J. Thomson discovered the first fundamental particle, the electron, in 1897 through his experiments with cathode ray tubes, this has been the modus operandi of particle physics – smash accelerated particles into stationary targets or into each other. As physicists kept increasing the energy of the colliding particles, they discovered a bewildering zoo. An organizing principle was required, a framework which would bring order to the menagerie of seemingly elementary particles. That framework is the Standard Model.

All the matter we see around us is made up of just three particles: protons, neutrons, and electrons, of which only the last is a truly elementary, indivisible particle. Protons and neutrons are made up of quarks, which come in six varieties. However, only two of them, called up and down quarks, suffice to make up protons and neutrons. The quarks, along with the electron and the ghostly neutrino, are part of a family of particles called fermions.

The infinitely rich variety of physical phenomena in the universe can be explained by just four fundamental interactions or forces. Gravity, the most familiar one, is so weak that the entire mass of the Earth pulling down on you isn’t enough to keep you grounded. It is, however, a long-range force: the Sun, for instance, exerts its force across 150 million kilometres of space to keep the Earth in its orbit. Gravity, first theorised by Isaac Newton, was also the first instance of what has become the over-arching goal of theoretical physics: unification. Newton unified the terrestrial and the celestial when he realized that the same force that pulls apples to the ground also keeps the Moon in its orbit around the Earth.

Electricity and magnetism were believed to be related but distinct phenomena, until James Clerk Maxwell unified them with his four equations. This threw up a surprising prediction: light was a wave, thus unifying optics with electromagnetism. But what was waving?

This brings us to a second great theme in theoretical physics – the concept of a field. First proposed by Michael Faraday, a field can be thought of as a set of numbers at every point in space, that represent the strength and the direction of the force a particle kept at that point would feel. There are many kinds of fields: for light, what was waving was the electromagnetic field. Gravity had to wait for Einstein for its own field theory, the general theory of relativity.

The two other fundamental forces are less familiar, confined as they are to the atomic nucleus. What enables the nucleus to exist despite the repulsion between its protons is the short-range strong nuclear force. Neutrons too, interact with each other and the protons through this force. The strong force between nucleons, however, actually comes from the interaction between their constituent quarks. This brings us to another key idea: force-carrying particles. Each fundamental interaction is mediated by particles which are called bosons. These are the quanta, a sort of unit, of their corresponding fields. The quantum of the electromagnetic field is the familiar photon, which is massless. For the strong force between quarks, the force carriers are gluons, while for gravity, the hypothesized force carrier is the graviton.

The fourth fundamental force in nature is the weak nuclear force, also a short-range force. It is responsible for negative beta decay, in which a neutron becomes a proton with the emission of an electron and an antineutrino. It is also essential for the nuclear reactions in the centres of stars like the Sun, where hydrogen is converted into helium. The force-carrying particles of the weak force are the W+, W– and Z0 bosons. In addition to carrying the force, their exchange changes the character of the particles that swap them.

There is a crucial difference, however, between the force carriers of the three other forces and that of the weak force. The photon, the gluon, and the graviton are all massless, while the W and Z bosons are heavy, with masses about 100 times that of a proton.

During the 1960s, Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg independently found away to unify the weak and electromagnetic forces in one mathematical formalism, the electroweak interaction.

The two forces were now on an equal footing – a symmetry. The difference in masses of the messenger particles, and hence the strength and range of the two forces, however, meant that this symmetry was “broken” by some mechanism that gives mass to the particles exchanged in weak interactions but not to the photons exchanged in electromagnetic interactions. This mechanism, the Higgs field which permeates all space, and whose manifestation is the Higgs boson, was proposed almost 50 years ago by three groups of physicists: Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, Carl Hagen, and Tom Kibble. By interacting with this field, the elementary particles acquire their masses. Later, the strong force was incorporated into the electroweak theory, resulting in what physicists call the Standard Model.

We spoke to Prof. James Libby of the Physics department of IIT Madras, who is an experimental particle physicist. Prof. Libby has been involved with various experiments at CERN. “It goes back to when I was a Ph.D. student. I worked for the predecessor of LHC, the Large Electron-Positron (LEP) Collider, and I was there in the late 90’s. That was all coming to an end because the LHC was due to start running in 2005. The main thing the LEP was designed to do, was to measure the parameters of the Z boson very carefully, one of which is its mass,” he recalls. “Electroweak unification was, by this time, very well established because the previous experiments at CERN had obtained signatures of the W and the Z bosons.”

Symmetry has been one of the great themes in theoretical physics in the last century. If the symmetry inherent in the Standard Model is not broken, all particles in the world would have been massless, just like the photon. But all elementary particles, except photons and gluons, are known to carry a rest mass.

“That is what the Higgs boson does. It breaks the symmetry and gives mass to these particles, through a process called spontaneous symmetry breaking,” explains Prof. Libby. Seeing the puzzled look on our faces, he elaborates:

“The best example I’ve heard when someone is trying to explain it in layman’s terms, is to imagine a circular table. Everyone has got their places laid around this. When it is all laid and neat, I can rotate this table and it will look the same. But as soon as someone sits down, lifts something up, say a napkin, it becomes different. I cannot rotate it anymore and the symmetry is broken. But this can happen anywhere around the table, there is no fixed place where it can happen and that’s spontaneous symmetry breaking.”

The nucleons – protons and neutrons – are made up of three quarks each. However, their mass is much greater than what would be expected from adding up the masses of the constituent quarks. “All that additional energy of a nucleon is coming from the binding, from the gluons,” explains Prof. Libby. Thus, real fermions only carry a tiny amount of the mass of what we see as regular matter. The rest has to do with the gluons and the energy they have due to strong binding. “So it is a bit of a misnomer when people say that the Higgs mechanism explains the mass of everything. Gluons have energy as an equivalent of mass,” says Prof. Libby, thus busting the myth of the “god particle” being responsible for all mass.

“Another reason the Higgs mechanism is not responsible for all the mass in the universe, is that regular mass accounts for only 5% of the energy of the universe,” he points out. To explain the observed gravitational effects on the visible, regular matter, cosmologists have hypothesized the existence of dark matter, which is not explained by the Higgs mechanism. “The Standard Model is incomplete, or an approximation to a higher theory, because there is no candidate for the dark matter in the Standard Model,” he adds animatedly.

Although six theorists were, independently, involved in the formulation of the Higgs mechanism, the Nobel Prize Committee chose to recognise only Peter Higgs and Francois Englert.

Prof. Libby feels this was slightly unfair: “The award seems fair given the no-more-than-three-living rule, but Tom Kibble – born in Madras – and collaborators did make a significant contribution, which has been recognised by other prizes for this work, such as the 2010 Sakurai prize. They should have been recognized with a Nobel.”

We also spoke to Prof. Balakrishnan, who concurs. “Hagen, Kibble, and Guralnik were really unfortunate, as the rules do not permit more than three recipients in a given year. It is reminiscent of the 1965 Prize, when Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga were honoured for quantum electrodynamics, while Freeman Dyson ‘missed the bus’. Englert’s collaborator R. Brout, too, missed the award, as he passed away just a few years ago,” he observes.

However, the nature of research in modern-day science, especially in a field such as particle physics, is collaborative, and progress is incremental. Two armies of about 3,000 physicists each worked on the two detectors at LHC: the ATLAS and the CMS. While the Nobel Prize citation mentions this experimental work, there is a feeling that the prizes could have acknowledged it too.

Prof. Balakrishnan agrees that this is, on the whole, unfortunate. “The award, unfortunately, did not extend to the massive experimental effort by the groups at CERN that worked so hard and with such single-minded devotion and organization to hunt the elusive Higgs boson down.” He expresses the hope that sometime in the near future, the on-going CERN effort will be honoured with a Nobel Prize when the present discovery is placed in perspective and buttressed by other related discoveries. For example, the theory admits more than one Higgs boson, and it is possible that the others too, would be found.

If and when this happens, it will be a paradigm shift as far as the Physics Prize is concerned. So far, it is only the Peace Prize that has ever been given to organizations such as the International Red Cross. “But now the situation may actually call for organizations and labs themselves to be honoured,” points out Prof. Balakrishnan.

Despite this, he has slightly ambivalent views when it comes to this year’s award. “The Prize, this time, has honoured a deep and pristine idea that was purely an idea in field theory at the time it was mooted, long before the Standard Model fell into place, pre-dating even the electroweak unification. As such, it is of a very different nature and character than the technical tour-de-force represented by the experimental detection of the Higgs particle almost 50 years later,” he says.

Further, he points out that finding the Higgs boson was not a de novo discovery. He compares its detection, from the trillions of collisions and the petabytes of data, to finding a needle in a haystack. However, he says, “the theory is what asserted, in the first place, that the haystack would contain a needle at all. Moreover, the bounds on the mass placed by theory and phenomenology very helpfully pointed out the specific corner of the haystack in which the needle was likely to be hidden.”

Giving examples of several other experimental discoveries which he feels were truly unexpected and revolutionary, he says, “This was not at all like the experimental discoveries of parity-violation, time-reversal symmetry violation, the quantization of the Hall effect, and many others.To give a geographical analogy, this was more like the conquest of a very difficult mountain peak by a well-equipped and well-planned expedition, rather than the unexpected discovery of a whole new continent.”

Prof. Libby counters: “Knowing where to look for the Higgs comes largely from precision electroweak data, principally the masses and widths of the Z and W bosons and the top quark, where the level of accuracy is sufficient to probe the loop corrections – including those with a Higgs – to these observables. This was mainly done by experiments at LEP/CERN and at the Tevatron/Fermilab. Neither has received any recognition from Stockholm,” he points out.

“Recognition of the theory is just, but I think you may sense where some of the experimentalist’s bunker mentality on this topic comes from. Particularly given the last accelerator experiment to win is UA1 in the early 1980s. For example, at LEP, the painstaking work reducing the uncertainties in the mass of the Z due to those in the beam energy involved studying train tables and understanding the influence of the tides in Lac Leman, which is a very different type of accomplishment but ingenious all the same,” he says, giving examples of the kind of painstaking work experimentalists do.

“There should have been some recognition of these feats,” agrees Prof. Balakrishnan. “But history is full of these ‘errors of omission.’ Madame Wu should perhaps have got the Nobel Prize because, after all, the direct demonstration of the violation of such an ‘obvious’ symmetry (parity) doesn’t happen every day.”

This discovery of parity violation led Robert Marshak and George Sudarshan and later, Richard Feynman and Murray Gell-Mann, to propose a new mechanism called V−A for weak interactions. In this theory, the weak interaction acts only on left-handed particles – those with the direction of their spin vector opposite to their direction of motion. Since the mirror reflection of a left-handed particle is right-handed, this explains the violation of parity.

However, as Prof. Balakrishnan points out, “It’s ridiculous that all the other weak interaction breakthroughs, starting from Becquerel right up to Rubbia and Van der Meer have been recognised with a Nobel, but not V–A. Sudarshan and Marshak should certainly have got it for V–A.”

Thus, the Nobel Prizes, though unmatched in their prestige, are certainly not the sole measure of, or the sole reward for, scientific accomplishment. Subrahmanyan Chandrasekhar, a Nobel Laureate himself, often quoted from a letter of his friend Edward Milne: “Posterity, in time, will give us our true measure and assign to each of us our due measure and humble place; and in the end, it is the judgement of posterity that really matters. He really succeeds who preserves accordingly to his lights, unaffected by fortune, good or bad. And it is well to remember that there is, in general, no correlation between the judgement of posterity and the judgement of contemporaries.”