The Higgs boson: What makes it special?
By: Achintya Rao
4 MAY, 2020 · Voir en français
Our second story in the LHC Physics at Ten series visits the LHC’s most important discovery so far
As a layman I would now say… I think we have it.
“It” was the Higgs boson, the almost-mythical entity that had put particle physics in the global spotlight, and the man proclaiming to be a mere layman was none other than CERN’s Director-General, Rolf Heuer. Heuer spoke in the Laboratory’s main auditorium on 4 July 2012, moments after the CMS and ATLAS collaborations at the Large Hadron Collider announced the discovery of a new elementary particle, which we now know is a Higgs boson. Applause reverberated in Geneva from as far away as Melbourne, Australia, where delegates of the International Conference on High Energy Physics were connected via video-conference.
So what exactly is so special about this particle?
“Easy! It is the first and only elementary scalar particle we have observed,” grins Rebeca Gonzalez Suarez, who, as a doctoral student, was involved in the CMS search for the Higgs boson. Easy for a physicist, perhaps…
Elegance and symmetries
At the subatomic scale, the universe is a complex choreography of elementary particles interacting with one another through fundamental forces, which can be explained using a term that physicists of all persuasions turn to: elegance.
“In the 1960s, theoretical physicists were working on an elegant way of describing the fundamental laws of nature in terms of quantum field theory,” says Pier Monni, of CERN’s Theory department. In quantum field theory, both matter particles (fermions such as electrons, or the quarks inside protons) and the force carriers (bosons such as the photon, or the gluons that bind quarks) are manifestations of underlying, fundamental quantum fields. Today we call this elegant description the Standard Model of particle physics.
The Standard Model is based on the notion of symmetries in nature, that the physical properties they describe remain unchanged under some transformation, such as a rotation in space. Using this notion, physicists can provide a unified set of equations for both electromagnetism (electricity, magnetism, light) and the weak nuclear force (radioactivity). The force which is thus unified is dubbed the electroweak force.
But these very symmetries presented a glaring problem: “The symmetries explained the electroweak force but in order to keep the symmetries valid, they forbid its force-carrying particles from having mass,” explains Fabio Cerutti, who co-led Higgs groups at ATLAS on two separate occasions. “The photon, which carries electromagnetism, we knew was massless; the W and Z bosons, carriers of the weak force, could not be.” Although the W and Z had not been directly observed at the time, physicists knew that if they were to have no mass, processes such as beta decay would have occurred at infinite rates – a physical impossibility – while other processes would have probabilities greater than one at high energies.
In 1964, two papers – one by Robert Brout and François Englert, the other by Peter Higgs – purported to have a solution: a new mechanism that would break the electroweak symmetry. The Brout-Englert-Higgs mechanism introduced a new quantum field that today we call the Higgs field, whose quantum manifestation is the Higgs boson. Only particles that interact with the Higgs field acquire mass. “It is exactly this mechanism,” Cerutti adds, “that creates all the complexity of the Standard Model.”
Originally conceived to explain the masses of the W and Z bosons only, scientists soon found they could extend the Brout-Englert-Higgs mechanism to account for the mass of all massive elementary particles. “To accommodate the mass of the W and Z bosons, we don’t need the same Higgs field to give mass to any other particles such as electrons or quarks,” remarks Kerstin Tackmann, a co-convener of the Higgs group on ATLAS. “But it is a convenient way to do so!”
The mathematical puzzle had been solved decades ago but whether the maths described physical reality remained to be tested.
Something in nothing
The Higgs field is peculiar in two particular ways.
Imagine an empty region of space, a perfect vacuum, without any matter present in it. Quantum field theory tells us that this hypothetical region is not really empty: particle–antiparticle pairs associated with different quantum fields pop into existence briefly before annihilating, transforming into energy. However, the “expectation value” of these fields in a vacuum is zero, implying that on average we can expect there to be no particles within the perfect vacuum. The Higgs field on the other hand has a really high vacuum expectation value. “This non-zero vacuum expectation value,” Tackmann elaborates, “means that the Higgs field is everywhere.” Its omnipresence is what allows the Higgs field to affect all known massive elementary particles in the entire universe.
When the universe had just come into being and was extremely hot, its energy density was higher than the energy associated with the vacuum expectation value of the Higgs field. As a result, the symmetries of the Standard Model could hold, allowing particles such as the W and Z to be massless. As the universe started to cool down, the energy density dropped, until – fractions of a second after the Big Bang – it fell below that of the Higgs field. This resulted in the symmetries being broken and certain particles gained mass.
The other property of the Higgs field is what makes it impossible to observe directly. Quantum fields, both observed and hypothesised, come in different varieties. Vector fields are like the wind: they have both magnitude and direction. Consequently, vector bosons have an intrinsic angular momentum that physicists call quantum spin. Scalar fields have only magnitude and no direction, like temperature, and scalar bosons have no quantum spin. Before 2012 we had only ever observed vector fields at the quantum level, such as the electromagnetic field.
“You can observe a field by observing a particle interacting with it, like electrons bending in a magnetic field,” Monni explains. “Or you can observe it by producing the quantum particle associated with the field, such as a photon.” But the Higgs field, with its constant non-zero value, cannot be switched on or off like the electromagnetic field. Scientists had only one option to prove it exists: create – and observe – the Higgs boson.
Bump-hunting at the Large Hadron Collider
Particle collisions at sufficiently high energies are necessary to produce a Higgs boson, but for a long time physicists were hunting in the dark: they did not know what this energy range was.
They had searched for signs of the Higgs boson in particle-collision debris at the Large Electron–Positron collider (LEP), which was the Large Hadron Collider’s direct predecessor, and at Fermilab’s Tevatron in the US. The Large Hadron Collider had the capacity to explore the entire predicted energy range where the Higgs boson could appear, and the two general-purpose particle detectors at the LHC – ATLAS and CMS – were meant to provide a definitive answer on its existence. For some, like Monni, the LHC’s calling was irresistible, leading him to switch careers from aerospace engineering to theoretical physics.
Gonzalez Suarez’s colleagues and friends were in the CMS and ATLAS control rooms when the LHC embarked on its high-energy journey on 30 March 2010. She herself was in her office at CERN’s main site in Geneva. “I was writing my doctoral thesis on one screen and looking at the live stream of the collisions on a second. I wanted to know if the code I had written to identify particles produced in the collisions worked!”
When two protons collide within the LHC, it is their constituent quarks and gluons that interact with one another. These high-energy interactions can, through well-predicted quantum effects, produce a Higgs boson, which would immediately transform – or “decay” – into lighter particles that ATLAS and CMS could observe. The scientists therefore needed to build up enough evidence to suggest that particles that could have appeared from a Higgs production and transformation were indeed the result of such a process.
“When the LHC programme started, popular belief was that we would only see a Higgs boson after several years of data collection,” recounts Vivek Sharma, who co-led the CMS search when the LHC began operations. Sharma and his colleagues presented a plan to CMS in September 2010 of how to tackle the problem with half that data. It required not only a thorough understanding of one’s own detector hardware, its reach and its limitations, but also a team with a variety of technical expertise. “By the time ATLAS and CMS gave a joint talk to CERN’s Scientific Policy Committee in March 2011,” Sharma continues, “there was a force building up that the Higgs boson could be hunted with even smaller datasets.”
A routine end-of-year seminar by ATLAS and CMS in December 2011 overloaded CERN’s webcast servers, as thousands tuned in to hear the latest updates from the collaborations. Early signs of the Higgs boson were there: both detectors had seen bumps in their data that were starting to look distinct from any statistical fluctuations or noise. But the results lacked the necessary statistical certainty to claim discovery. The world had to wait nearly seven months before Joe Incandela of CMS and Fabiola Gianotti of ATLAS could do so in July 2012. The collaborations had performed better than expected to discover the Higgs boson with just two years of data from the LHC.
In CERN’s auditorium, Peter Higgs wiped away tears of joy, and François Englert paid tribute to his late colleague and collaborator, Robert Brout, who did not live to see proof of the mechanism that bears his name.
Gonzalez Suarez celebrated with mixed emotions. Her post-doctoral research took her away from Higgs research before the discovery, and eventually from CMS, to the ATLAS collaboration. “The discovery of the Higgs boson was a historic event, but we are still only at the beginning in our understanding of this new particle.”
The road from data to discovery was challenging. But what have we learnt about the Higgs boson since then? Find out more in part two of the Higgs saga (coming soon).