Happy Birthday, Higgs boson! What we do and don’t know about the particle

On July 4, 2012, physicists at CERN, the European Laboratory of Particle Physics, declared victory in their long search for the Higgs boson. The discovery of the elusive particle filled the last gap in the Standard Model – the best description of particles and forces by physicists – and opened a new window on physics by providing a way to learn more about the Higgs field, that involves a previously unstudied type of interaction that gives particles their mass.

Since then, researchers at CERN’s Large Hadron Collider (LHC) near Geneva, Switzerland, have been busy publishing nearly 350 scientific papers on the Higgs boson. Yet many of the particle’s properties remain a mystery.

On the tenth anniversary of the discovery of the Higgs boson, Nature looks at what it has taught us about the universe, as well as the big questions that remain.

5 things scientists have learned

The mass of the Higgs boson is 125 billion electron volts

Physicists expected to find the Higgs boson eventually, but they didn’t know when. In the 1960s, physicist Peter Higgs and others theorized that what is now called a Higgs field might explain why the photon has no mass and that the W and Z bosons, which carry the weak nuclear force behind radioactivity, are heavy. are (for subatomic particles) . The special properties of the Higgs field allowed the same math to explain the masses of all particles, and it became an essential part of the Standard Model. But the theory made no predictions about the boson’s mass and thus when the LHC might produce it.

In the end, the particle appeared much earlier than expected. The LHC began collecting data in 2009 during its search for the Higgs, and both ATLAS and CMS, the accelerator’s general-purpose detectors, spotted it in 2012. The detectors observed the decay of just a few dozen Higgs bosons in photons, Ws and Zs. , which revealed a bump in the data at 125 billion electron volts (GeV), about 125 times the mass of the proton.

The Higgs mass of 125 GeV puts it in a sweet spot, meaning the boson decays into a wide range of particles at a frequency high enough to be observed by LHC experiments, said Matthew McCullough, a theoretical physicist at CERN. “It’s very bizarre and probably a coincidence, but it just so happens that” [at this mass] you can measure a lot of different things about the Higgs.”

The Higgs boson is a spin-zero particle

Spin is an intrinsic quantum mechanical property of a particle, often depicted as an internal bar magnet. All other known fundamental particles have a spin of 1/2 or 1, but theories predicted that the Higgs would be unique in having a spin of zero (it was also correctly predicted to have zero charge).

In 2013, CERN experiments studied the angle at which photons produced in the decay of the Higgs boson flew into the detectors, and used this to show with high probability that the particle had no spin. Until this was demonstrated, few physicists could call the particle they found Higgs, says Ramona Gröber, a theoretical physicist at the University of Padua in Italy.

Higgs’ property rule from some theories that extend the standard model

Physicists know that the Standard Model is not complete. It breaks down at high energies and cannot explain important observations such as the existence of dark matter or why there is so little antimatter in the Universe. So physicists have devised extensions to the model that are responsible for this. The discovery of the 125-GeV mass of the Higgs boson has made some of these theories less appealing, Gröber says. But the mass is in a gray zone, meaning it categorically excludes very little, says Freya Blekman, a particle physicist at Germany’s Electron Synchrotron (DESY) in Hamburg. “What we have is a particle that is consistent with more or less everything,” she says.

The Higgs boson interacts with other particles as the Standard Model predicts

According to the Standard Model, the mass of a particle depends on how strongly it interacts with the Higgs field. Although the boson — which resembles a ripple in the Higgs field — plays no part in that process, the rate at which Higgs bosons decay or are produced by another particular particle provides a measure of how strongly that particle interacts with the field. . LHC experiments have confirmed that — at least for the heaviest particles, which are most often produced in Higgs decay — mass is proportional to the interaction with the field, a remarkable victory for a 60-year-old theory.

The universe is stable – but only just

Calculations using the mass of the Higgs boson suggest that the universe could only be temporarily stable, and there is a negligible chance that it could shift to a lower energy state – with catastrophic consequences.

Unlike other known fields, the Higgs field has a lowest energy state above zero, even in a vacuum, and it permeates the entire universe. According to the Standard Model, this ‘ground state’ depends on the interaction of particles with the field. Shortly after physicists discovered the mass of the Higgs boson, theorists used the value (among other measurements) to predict that a lower, more preferred energy state also exists.

To switch to this other state would have to overcome a huge energy barrier, McCullough says, and the chances of this happening are so slim that it’s unlikely to happen on the timescale of the universe’s lifetime. “Our doomsday will be much earlier, for other reasons,” says McCullough.

Graphical representation of events recorded in 2012 with the CMS detector that correspond to the decay of the Higgs boson

A computer image of events recorded in 2012 with CERN’s Compact Muon Solenoid detector shows features expected from the decay of a Higgs boson down to a few photons (dashed yellow lines and green towers).CreditThomas McCauley, CMS Collaboration/CERN

5 things scientists still want to know

Can we make Higgs measurements more accurate?

So far, the properties of the Higgs boson – such as the interaction strength – are in line with those predicted by the Standard Model, but with an uncertainty of about 10%. That’s not good enough to show the subtle differences predicted by new theories of physics, which differ only slightly from the Standard Model, Blekman says.

More data will increase the accuracy of these measurements, and the LHC has collected only one-twentieth of the total amount of information it is expected to collect. Seeing hints of new phenomena in precision studies is more likely than directly observing a new particle, says Daniel de Florian, a theoretical physicist at the National University of San Martín in Argentina. “For the next decade or more, the name of the game is precision.”

Does the Higgs interact with lighter particles?

Until now, the Higgs boson’s interactions seemed to fit the Standard Model, but physicists have seen it decay to only the heaviest matter particles, such as the bottom quark. Physicists now want to find out if it interacts in the same way with particles from lighter families, known as generations. In 2020, CMS and ATLAS saw such an interaction – the rare decay of a Higgs to a second-generation cousin of the electron called the muon1† While this is evidence that the relationship between mass and interaction strength holds for lighter particles, physicists need more data to confirm this.

Does the Higgs interact with itself?

The Higgs boson has mass, so it should interact with itself. But such interactions — for example, the decay of one energetic Higgs boson to two less energetic bosons — are extremely rare, because all the particles involved are so heavy. ATLAS and CMS hope to find hints about the interactions after a planned upgrade to the LHC from 2026, but conclusive evidence will likely require a more powerful collider.

The speed of this self-interaction is critical to understanding the universe, McCullough says. The probability of self-interaction is determined by how the potential energy of the Higgs field changes near its minimum, which describes the conditions just after the Big Bang. So knowledge of Higgs’ self-interaction could help scientists understand the dynamics of the early Universe, McCullough says. Gröber notes that many theories that try to explain how matter somehow became more abundant than antimatter require Higgs self-interactions that differ by up to 30% from the Standard Model prediction. “I cannot emphasize enough how important” this measurement is, says McCullough.

What is the lifetime of the Higgs boson?

Physicists want to know the Higgs’ lifetime — how long it lasts on average before decaying to other particles — because any deviation from predictions could indicate interactions with unknown particles, such as those that make up dark matter. But the service life is too small to measure directly.

To measure it indirectly, physicists look at the spread, or “width,” of the particle’s energy across multiple measurements (quantum physics says uncertainty in the particle’s energy must be inversely proportional to its lifetime). Last year, CMS physicists produced their first rough measurement of the Higgs’ longevity: 2.1 × 10−22 seconds2† The results suggest that the longevity is consistent with the Standard Model.

Are all the exotic prophecies true?

Some theories that extend the Standard Model predict that the Higgs boson is not fundamental, but – like the proton – consists of other particles. Others predict that there are several Higgs bosons, which behave the same, but differ, for example in charge or spin. In addition to checking whether the Higgs really is a Standard Model particle, LHC experiments will look for properties predicted by other theories, including decay in forbidden particle combinations.

Physicists are only at the beginning of their efforts to understand the Higgs field, whose uniqueness makes it “act like a portal to new physics,” de Florian says. “There’s a lot of room for excitement here.”

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