“For me, it’s an incredible thing that it happened in my lifetime!”

Peter Higgs was at a loss for words. The CMS and ATLAS collaborations had just announced the discovery of a new, Higgs-boson-like particle at the Large Hadron Collider.

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4 July 2012: François Englert (left) listens as Peter Higgs speaks, after ATLAS and CMS announce their discovery (Image: Maximilien Brice/CERN)

It had been 48 years since the publication of his paper that first predicted the existence of the particle that bears his name, not long after Robert Brout and François Englert proposed a new mechanism that would give mass to elementary bosons. More than 30 years had elapsed since the LHC was first conceived and around 20 years since the ATLAS and CMS collaborations were formed. After those long years filled with anticipation, it only took the Swedish Academy of Sciences a little over one year to award Englert and Higgs the 2013 Nobel Prize in Physics.

For Peter Higgs, the discovery of the Higgs boson was the end of a remarkable journey. For particle physics, it was the beginning of a new one.

Displays of candidate VHcc events
(Image: CERN)










Higgs-like? Higgs-ish? Higgs-y?

“When you find something new, you have to understand exactly what it is that you have found,” remarks Giacinto Piacquadio, one of the conveners of the ATLAS collaboration’s Higgs group

This understanding is built up gradually over time. Back in July 2012, physicists were cautious about calling the new particle a Higgs boson, let alone the Higgs boson predicted by the Standard Model of particle physics. And with good reason: while the simplest theoretical formulations required there to be only one kind of Higgs boson, some extensions of the Standard Model proposed that there could be as many as five kinds of bosons that are involved in the mass-giving mechanism. So for the first few months after the discovery, it was referred to as Higgs-like, shorthand for “a particle that seems to behave like the Higgs boson predicted by the Standard Model but we need more data to be sure”.

The identification of two quantum-mechanical properties of the particle – quantum spin and parity – gave credence to the Standard-Model interpretation. Spin is the intrinsic spatial orientation of quantum particles, and parity refers to whether the properties of the particle remain the same when some of its spatial coordinates are flipped, like comparing the particle with a hypothetical mirror image. In the Standard Model, the Higgs boson has no spin (“0”) and “even” parity. At the time of the discovery, the fact that the Higgs boson transformed into photons meant that – unlike all other elementary bosons we know – its spin could not be 1: photons have a quantum spin of 1 themselves, so a particle transforming into two photons would have a spin of 0 (with the two spins of the photon cancelling out) or 2 (if the two spins add up).

ATLAS Higgs spin/parity plot
Differences between the positive- and negative-parity theoretical scenarios (solid and dashed lines respectively) for a particle with spin 0. The data do not show evidence for the negative-parity scenario (Image: ATLAS/CERN)

In science, you can never know something with 100% certainty, but you can rule out things that are not likely. Because spin-2 particles or parity-odd particles with spin 0 would leave subtly different signatures in the ATLAS and CMS detectors than the spin-0-parity-even particle they were looking for, the scientists were eventually able to rule out these more exotic possibilities by examining many more collision events and finding no evidence to support them. “We had to analyse two-and-a-half-times more data to drop the ‘-like’,” Piacquadio adds. By March 2013, scientists were confident calling the particle a Higgs boson.

The Goldilocks zone

The Higgs boson was the last missing piece in the Standard Model. Crucially, its mass would determine how it could be observed. At 125 gigaelectronvolts (GeV), it turned out to be just right for studying the particle at the Large Hadron Collider.

We can never directly see a Higgs boson. Like most kinds of particle in nature, it is unstable and – immediately after being produced – transforms into lighter particles through a process known as particle decay. The ATLAS and CMS detectors can therefore see only the remnants of transformations, signatures that a Higgs boson might have been produced in the LHC’s collisions. Further, the downstream remnants of a Higgs transformation hold clues for how the particle was produced in the first place.

The Higgs boson’s mass was not predicted precisely by the Standard Model, but theorists knew that the processes that produced it and the kinds of particles it transformed into would depend on how heavy the boson actually was. They had prepared elaborate plots calculating the various probabilities for a Higgs boson of a given mass to transform into particular pairs of particles. According to these so-called “branching fractions”, a light Higgs boson of around 125 GeV would have the largest variety of transformation candidates that ATLAS and CMS could detect: pairs of W bosons, Z bosons, photons, bottom quarks, tau leptons and many others. The greater the variety of observable particles the Higgs can transform into, the greater the ability of scientists to study the interplay between these particles and the Higgs boson.

Higgs branching fraction
The rates at which a Higgs boson could undergo certain transformations (vertical axis) depending on its mass (horizontal axis) (Image: CERN)

Although the Higgs field was conceived to explain the masses of the W and Z bosons, scientists realised that it could help account for the masses of the fermions, namely the particles of matter. If, due to its mass, they could only observe the interplay between the Higgs boson on one hand and the W and Z bosons on the other, the puzzle of the fermion masses would remain unsolved. Discovering the particle at a convenient mass was an unexpected kindness from nature. If it were slightly more massive, above 180 GeV or so, the options to study it at the time of its discovery would have been more limited.

The variety of available transformation products means that data from the individual channels can be combined together through sophisticated techniques to build up a greater understanding of the particle. “Doing so is not trivial,” says Giovanni Petrucciani, co-convener of the Higgs analysis group in CMS. “You have to treat the uncertainties similarly across all the individual analyses and interpret the results carefully, once you have applied complicated statistical machinery.” Combining data from the transformation of the Higgs boson to pairs of Z bosons and pairs of photons allowed ATLAS and CMS to discover the Higgs boson in 2012.

Photograph corresponding to CERN courier article: Inspired by software (2019MarApr)
Photograph featured in the CERN courier article for issue 2019MarApr. Contains an image of ATLAS Higgs event, accompanied with a piece of event selection code of an CMS analysis reimplemented by theorists in open code CheckMATE. (Image: CERN)










Generation gaps

The LHC started operations at a collision energy of 7 teraelectronvolts (TeV) before ramping up to 8 TeV over the course of its first run (2010–2013). The data collected over this period not only led to the discovery of the Higgs boson but showed the relationship (“coupling”) between the Higgs boson and elementary bosons: it was observed transforming into pairs of Ws, Zs and photons. And, while transformations to gluons are impossible to observe, the scientists could probe this coupling through the Higgs production itself: the most abundant way for a Higgs to be created in proton–proton interactions is for two gluons – one from each proton – to fuse together, accounting for nearly 90% of Higgs bosons produced at the LHC.

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A candidate for a Higgs boson transforming into two photons (Image: CMS/CERN)

The next challenge was to observe the coupling to fermions, to cement the role of the Higgs field as the origin of mass of all elementary massive particles. These couplings had been probed indirectly: the Standard Model tells us that the gluon-fusion production mechanism and the Higgs transformation to photon pairs require the creation and annihilation of “virtual” top–antitop pairs. However, a direct observation of Higgs couplings to fermions was lacking.

Curiously, both kinds of fermions – quarks, which make up compound particles like protons, and leptons, like the familiar electron – come in three generations of particles, each heavier than the previous. And unlike bosons, whose coupling strengths to the Higgs are proportional to their masses, the Higgs-coupling strengths of fermions is proportional to the square of their masses.

The third generation of fermions – the heaviest – are therefore the most likely particles to manifest in processes involving the Higgs boson. “The connection between the Higgs and the top quark in particular is very exciting to look into,” remarks María Cepeda, Petrucciani’s fellow convener on CMS. Despite their relative abundance in such processes, these particles are challenging to identify. Since quarks cannot exist freely, two bottom quarks (a quark and an antiquark) emerging from a Higgs transformation rapidly combine with other quarks pulled out of the vacuum and form jets of particles. The experimentalists have to then tag jets of particles that carry the signature of a bottom quark, in order to isolate the signal. The top quark on the other hand is heavier than the Higgs and so a Higgs can never be observed transforming into two top quarks. Scientists have to therefore measure its coupling with the Higgs by looking for collision events in which a Higgs boson is produced in association with two top quarks. The second run of the LHC (2015–2018) was at an energy of 13 TeV and the large data volume collected allowed ATLAS and CMS to observe the interplay between the Higgs boson and the bottom quark, the top quark and the tau lepton.

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A candidate for a Higgs boson transforming into a b-quark and a b-antiquark (Image: ATLAS/CERN)

Couplings to the second generation of fermions are much weaker and neither ATLAS nor CMS have so far observed Higgs transformations into charm quarks, strange quarks or muons. The next run of the LHC (2021 onwards) is expected to provide enough data to begin to shed light on some of these interactions. “The LHC’s instantaneous luminosity – the rate at which it collides protons – has increased dramatically over its first two runs,” notes Piacquadio with excitement. “This means that the number of Higgs bosons produced by the LHC continues to rise, as do the odds that we observe them undergoing rarer transformations.”

But for the second generation of fermions, the LHC’s data volume over its whole operational life may not be enough to breach the 5σ statistical threshold to claim a Higgs transformation to all these particles. Although the High-Luminosity LHC, which will be the collider’s incarnation from 2026, is expected to allow ATLAS and CMS to see the Higgs transforming into pairs of muons, transformations to second-generation quarks will probably remain out of reach.

More data, more precision

The Higgs boson holds the key to our understanding of nature beyond what is shown by the Standard Model.

ATLAS and CMS are, for example, looking for so-called “invisible decays” of the Higgs boson, in which it transforms into particles that the detectors cannot observe. These invisible particles might be manifestations of dark matter. And measurements of couplings that deviate from the theoretical predictions could provide an alternative explanation for the masses of the different generations of fermions, explaining why they exist in distinct generations to start with and possibly hinting at the existence of other Higgs bosons.

Yet, the Brout-Englert-Higgs mechanism remains among the least-understood phenomena in the Standard Model. Indeed, while scientists have dropped the “-like” suffix and have understood the Higgs boson remarkably since its discovery, they still do not know if what was observed is the Higgs boson predicted by the Standard Model. Couplings to the second-generation fermions remain elusive and the couplings that have been observed are known with an uncertainty of 10 to 20%, expected to reduce to the 2–4% range with the High-Luminosity LHC. Observation of as-yet-unseen phenomena and precision measurements of those that have been seen may require data volumes far greater than the LHC can provide over its lifetime.

The global particle-physics community is therefore keen on building a “Higgs factory”, a dedicated accelerator with a focus on producing Higgs bosons in unimaginably large quantities, to allow the continued exploration of this strange particle. A high-energy Higgs factory would also enable scientists to produce two Higgs bosons at a time, to address the question of the so-called “Higgs self-interaction”, the process through which the Higgs boson itself gains mass.

Since its discovery nearly eight years ago, ATLAS and CMS have published hundreds of papers on the Higgs boson and our understanding of the particle has grown incrementally but greatly. Today, we know with great precision what its mass is, what its most abundant transformation channels are and how it is produced in the first place. But a lot remains unknown, about both the Higgs boson and the quantum world in general.

The Higgs may be the most important discovery of the LHC so far, but there is much still to learn from this remarkable machine. Our next story in this series will take a look at searches for dark matter at the Large Hadron Collider.