10 Years of the Higgs Boson: How Far Have We Come, and What Next?

A side-on view of the CMS detector at the Large Hadron Collider. Photo: CERN

Ten years ago on this day, scientists at the world’s largest physics experiment announced that they had discovered the Higgs boson. The discovery of this subatomic particle validated the idea that the Higgs field – a field of energy that pervades the universe, and which gives subatomic particles their mass – is real.

Proving that the Higgs boson exists is also proof that the Higgs mechanism, first predicted in the 1960s, exists. So the discovery of the particle closed an important avenue of physics research after many decades to much celebration and fanfare.

However, it added new questions to the pile of unresolved mysteries.

Based on which questions they would like to have answered and how, physicists today have to make crucial decisions about what their next experiments will do and at what cost. The colossal apparatus that helped physicists find the Higgs boson was the Large Hadron Collider (LHC), a 27-km long tube in which protons are accelerated with the help of very strong magnets to nearly the speed of light, and smashed head on. Detectors on the collider, including two called ATLAS and CMS, study the detritus of these collisions for signs of the Higgs boson.

The Higgs field gives mass to subatomic particles through the Higgs mechanism. The existence of the mechanism was predicted by three independent groups in 1964. One of these groups was just Peter Higgs, a British theoretical physicist. The other groups were 1) Robert Brout and François Englert and 2) Gerald Guralnik, C.R. Hagen and Tom Kibble.

Most people refer to the mechanism as the Higgs mechanism but Peter Higgs has said that he calls it the “Anderson-Brout-Englert-Guralnik-Hagen-Higgs-Kibble-‘t Hooft mechanism”. ‘Anderson’ stands for Philip Warren Anderson and ”t Hooft’ for Gerardus ‘t Hooft.

After the particle’s discovery, Peter Higgs and François Englert were awarded the Nobel Prize for physics in 2013.

ATLAS collaboration spokesperson Fabiola Gianotti presenting evidence of the Higgs boson’s discovery at CERN, July 4, 2012. Photo: CERN

A photon is a particle that physicists understand to be an excitation of the electromagnetic energy field. Similarly, the Higgs boson is an excitation in the Higgs field. The strength with which the Higgs boson interacts with a particle is proportional to the particle’s mass. So for example, the top quark is the heaviest known elementary particle because it interacts most strongly with the Higgs boson. The less strong the interaction with the Higgs boson, the lighter a particle will be.

(Note that only elementary particles get their mass through the Higgs mechanism. Composite particles, like protons and neutrons, can get their mass from many sources, only one of which is the masses of their constituent particles.)

This said, based on 10 years’ worth of data from the LHC, physicists have thus far studied the Higgs boson’s interactions with the heavier particles more than with the lighter ones, like electrons and positrons. Physicists also need to study further how the Higgs boson couples with itself, to explain how the particle gets its own mass.

The Higgs boson is often called the “god particle”, but the particle has no theological connotations, contrary to several pseudoscientific articles that have appeared in the popular press since its discovery. This name is a modification of “goddamn particle”, which is what physicist Leon Lederman called the Higgs boson in 1993 because it was proving so hard to find.

Indeed, the LHC is a commensurately awe-inspiring machine – the largest science experiment even today, 12 years after it started operating. But finding the Higgs boson has remained the last of the LHC’s sensational discoveries. To be fair, the LHC has supported numerous incremental findings and helped improve the precision of some discoveries and invalidate or modify others. The machine has also thrown up negative results that have constrained several theoretical predictions.

For example, we don’t know what dark matter is. Some theories predict dark-matter particles with some properties. Physicists at the LHC then look for evidence of these particles in their detectors, at different energies. (The energy at which a particle is found is important because it is related to the particle’s mass and indicates which other particles it may decay to.)

Thus far, the LHC hasn’t found evidence for any such particles. This has meant not that dark matter particles don’t exist but that these particles, if they exist, don’t at the energies and other conditions in which the LHC looked for them.

Another equally important problem is the mass of the Higgs boson: it is much heavier than Higgs et al. predicted it would be. Why? There is another related problem. The existing framework of rules physicists use to understand the properties of subatomic particles is called the Standard Model. And it predicts that the Higgs boson’s mass is unstable and could drastically change one day, with catastrophic consequences for the universe (and mystery also related to the mass of an unusual particle called the top quark). Could it, really?

Both these questions have major implications for our understanding of the universe – and also means that the Standard Model could be wrong or incomplete in some way. But we don’t know in what way.

Also read: We Discovered the Top Quark 25 Years Ago. Its Mass Is Still Fascinating. (2020)

A popular resolution to these problems is called supersymmetry. It’s a theory that predicts that every matter particle has a complementary force particle, and vice versa. These complementary particles are called supersymmetric partners. If we found them, the math of the existing rules of particles would shift in a way that could explain why the Higgs boson’s mass is what it is.

Many physicists expected the LHC to find supersymmetric partners, but so far the data has come up blank. One such negative result prompted a UK spokesperson for one of the collaborations on the LHC to tell the BBC in 2012, “Supersymmetry may not be dead but these latest results have certainly put it into hospital.”

This wasn’t a positive result but that doesn’t mean it wasn’t informative.

The LHC and its detectors receive periodic upgrades that improve their sensitivity, resolution, timing, collision output, etc. After the last round of upgrades, beginning in December 2018, the LHC reopened on April 22. The next round of upgrades will happen in 2026.

The period for which the LHC collects data between upgrades is called a ‘run’. Every run produces copious amounts of data that physicists (using computers) can’t finish processing in that run itself. Many of them are still sifting through data produced in previous runs, looking for interesting results.

A view of the LHC operations centre, May 21, 2021, when the machine achieved a proton collision energy of 13 TeV for the first time. Photo: CERN

With regards to the Higgs boson itself, there are quite a few questions waiting to be answered. They can be divided into three types, sensu lato:

* What we already know but need to know better – For example, we have studied the Higgs boson’s interactions with particles called leptons and quarks only to a precision of up to 5%. This isn’t good enough: we need the data to be much more precise because the possibility exists that at higher precision, the observed numbers deviate from that predicted by theory. This in turn could mean, according to one hypothesis, for example, that the Higgs boson is not a fundamental particle but is made up of smaller particles.

* What we still don’t know but expect to know – Is there only one ‘type’ of Higgs boson? How do two Higgs bosons interact with each other? How does the Higgs boson interact with lighter particles? Does the decay of a Higgs boson follow or violate existing laws of physics? Are there decay pathways we haven’t found yet? Why is the Higgs boson’s mass so much lower than what physicists’ calculations say it should be?

* What the Higgs boson can tell us about other mysteries – Why does our universe have more matter than antimatter? What is dark matter? Why did cosmic inflation happen?

The pursuit of the answers to any of these questions itself doesn’t mean they enjoy immunity from doubt. New information that we discover could cast old questions in new light and – as with the Higgs boson’s discovery – throw up more, and probably more important, questions.

The next logical step in this direction is to improve the LHC – which is already planned – and, further down the line, to find ways to study the Higgs boson’s interactions with particles that cannot be produced in sufficient quantities at LHC collisions.

This is why, while both the LHC and data from the LHC will be around for at least two more decades, producing an abundance of data that will help physicists improve existing measurements, physicists – and others – have also started thinking about what kind of machine they should build next, a machine that can elucidate the Higgs boson’s dance with the other particles. They start to think now because planning and building machines of this size and sophistication can take up to a decade.

There are already three major contenders, at least after a round of (very) public debates just before the pandemic. Two of them are similar in design and purpose but differ in their power and location.

Both CERN, the European research facility that currently hosts the LHC, and China have proposed circular colliders like the LHC – but larger and more powerful.

CERN’s pitch is called the Future Circular Collider (FCC). Its beam pipe will be 100 km long, compared to the LHC’s 27 km, and will cost around $15 billion to build. CERN had said before the pandemic that the FCC could come ‘online’ by 2050.

China’s pitch is for the Circular Electron-Positron Collider (CEPC), to be built at a cost of $5 billion. It will be able to achieve lower collision energies than the FCC. Before 2020, China had said it could build this machine by 2022.

Also read: CERN’s Concept Design for Next-Gen ‘Supercollider’ Mirrors China’s Plans (2019)

The third major contender is the International Linear Collider (ILC), put together by an international collaboration. It will consist of an approximately 30-km-long tube that will accelerate two kinds of particles from the two ends in a straight line, and smash them into each other. It was expected to be located in Japan before scientists there raised concerns about how the government would split the cost of building and running the machine with other countries.

Physicists are also considering another design for a machine called the Compact Linear Collider.

The FCC, the CEPC and the ILC all plan to smash electrons with positrons1 – unlike the LHC, which smashes protons with protons. Protons are composite particles (they are made of quarks and gluons), so their collision data is very noisy and requires lots of modelling and filtering to make sense of. Positrons and electrons are both elementary particles, on the other hand, so their collisions are said to be ‘cleaner’ and more conducive for analysis.

Both types of collisions can produce Higgs bosons for detectors to study. However, in a circular device, accelerating electrons and positrons to some velocity has a higher energy cost than accelerating protons to the same velocity.

An infographic explaining the constituents of matter. Gravitons (bottom right) are still hypothetical. Image: CERN

Then again, not all physicists agree that the world needs an even bigger machine to make sense of the Higgs boson, the existence of supersymmetric partners and to explore solutions to other problems. Some have contended that unless physicists can be more sure of what they can or can’t find, we shouldn’t spend so much money on a single, big project. Even others have identified arguments both for and against this position.

Recently, theoretical physicist Matthew Strassler spelled out two arguments on his blog in favour of building a bigger collider than the LHC. Both of them are related to understanding the “structure” of the Standard Model. Strassler also acknowledged a few arguments against the idea – including the potential carbon footprint of the power demand and the capex/opex of running a bigger machine.

Another argument is that there is no guarantee a particle accelerator capable of reaching even 10x the collision energy at the LHC will find new particles.

Some have also argued that the money ‘saved’ by not building a bigger particle accelerator should be spent on multiple smaller projects – but this is not really possible. The money on building colliders often comes from multiple governments, not one, and over several years. A lot of the value is also ‘locked’ in manufacturing and research contracts and can’t be converted directly to spendable funds.

Second, at least one example from history has told us that when a government cancels a big physics project, it doesn’t earmark the money ‘saved’ for other physics projects, but will spend it on anything else the government deems important.

But for further Higgs boson research, physicists need more and more data. A lot is still pouring in from the LHC’s and its predecessors’ archives. They may also decide that they need a different machine from what the LHC currently is, even probably a new machine – one that can tell us, by unravelling more about the goddamned particle, why our universe is the way it is.

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