10 years after its discovery, how well do we know the Higgs boson?

It is now 10 years since scientists first reported the observation of the Higgs boson in CERN’s Large Hadron Collider (LHC) on the Franco-Swiss border near Geneva, Switzerland. In the intervening decade, what have we learned about the so-called “God particle”?

The most up-to-date properties of the elementary particle are presented in two papers published in Nature, produced by the ATLAS and CMS collaborations.

On this day 10 years ago, scientists around the world gathered to declare the detection of the theorised Higgs boson by the ATLAS and CMS collaboration at the LHC.

The July 4, 2012, announcement came nearly 50 years after the particle was first theorised.

Not only did the discovery of the Higgs boson confirm the existence of the particle, but also the existence of the Higgs field which permeates all space.

The strength of any particle’s interaction with that field directly influences that particle’s mass – by extension, the authors of the CMS paper write, the Higgs field “ultimately determines the size of atoms, makes the proton stable and sets the timescale of radioactive (β) decays, which for example impact the lifetime of stars”.

How do we even know the Higgs field is there?

“[I]n everyday life, we do not notice that the Higgs field is all around us,” the CMS authors write. “The only way we have of revealing the Higgs field is to perturb it, a little like throwing a stone into water and seeing the ripples. The particle known as the Higgs boson is the manifestation of such a perturbation.”

The measurement at the LHC of a particle matching the theoretical properties of the Higgs boson was a great validation of the Standard Model of particle physics which forms the basis of our understanding for how elementary particles make up the matter and forces we see around us.

Since the initial observation, Higgs bosons have been detected more than 30 times at the LHC. So, 10 years on, how does the Higgs stack up? Does it meet expectations? Was our initial excitement justified?

The ATLAS and CMS collaborations present an analysis of the past 10 years of LHC results, including ATLAS’s “Run 2” which took place between 2015 and 2018. The key question under review is how the Higgs boson interacts with other elementary particles.

ATLAS researchers note that the errors associated with both theoretical calculations and experimental measurements are a factor of two smaller than “Run 1” (2011–2012).

“The likelihood of producing a Higgs boson in a collision becomes larger when the particles that collide interact strongly with the Higgs field, that is, when they are heavy,” the CMS paper authors write.

But, at the energy levels required, physicists only know how to collide electrons and protons – relatively light particles. So, they use the fact that occasionally heavier particles are produced in these light-particle collisions. When heavier particles are produced, their interactions with the Higgs field can be measured.

Interactions between Higgs and the heaviest known elementary particles – top and bottom quarks, Z and W bosons [responsible for the weak force] and tau lepton – were examined in the analyses. The ATLAS physicists determined that the data falls precisely in line, within reasonable error margins of between 5% and 20% according to the CMS authors, with the behaviour predicted under the Standard Model.

In particular, the researchers examined Higgs boson production and decay rates – where Higgs splits into other particles – in the collision events.

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The team suggests that, not only does the Higgs boson fall into line with predicted interactions with known particles, but the tests also “provide stringent constraints on many models of new phenomena beyond the standard model”.

The CMS perspective writers say that “interactions with very light particles, such as the electron and up and down quarks of which we are made of, are too rare for current methods to observe”.

“Although the discovery of the Standard Model Higgs boson was highly anticipated at the LHC, the ability to explore so many of its features was a surprise,” the CMS authors write.

“To have established even part of the broad picture of Higgs-boson interactions in just 10 years is a major achievement, especially when one considers that, at the time when the LHC was being commissioned, many of the production and decay channels that are central to today’s measurements were believed to be beyond the reach of the LHC.”

Their perspective concludes: “Every Higgs-related measurement so far has been consistent with the Standard Model, the simplest of all current models of particle physics: a remarkable win for Occam’s razor.

“Today, it is clear that the Higgs mechanism, first proposed in the 1960s, is responsible not only for the masses of the W and Z bosons, but also for those of the three heaviest fermions. This directly implies the existence of a fifth force, mediated by the Higgs boson.”

One key to this achievement, the researchers write, is the serendipitous mass of the Higgs boson itself, which falls nicely within experimental parameters. Our ability to piece together individual proton-proton collision results in a sea of collisions has also sharpened in the past decade, say the researchers.

Greater precision remains a goal for the collaborators.

“The path for improvement is conceptually straightforward: with 20 times more data to come in the next 15–20 years from the approved high-luminosity upgrade of the LHC, and foreseeable improvements in analysis techniques and theoretical calculations, the ATLAS and CMS experiments expect to determine the currently observed set of interactions to within a couple of per cent,” the CMS perspective authors write.

Also on the horizon are tests of Higgs interactions with lighter particles – not quite as light as “first generation” leptons like electrons and up and down quarks – such as “second generation” leptons like muons, and measurement of the Higgs potential which is the energy associated with the Higgs field.

The ATLAS team also notes that we are yet to determine the interaction of the Higgs boson with itself, and its rarer decay events. With the data on the Higgs boson expected to more than double over the coming decade, the progress made in the past 10 years to understand the “God particle” is projected to continue into the next.