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

On July 4, 2012, physicists at CERN, Europe’s particle physics laboratory, declared victory in their long search for the Higgs boson. The discovery of the elusive particle filled the last gap in the Standard Model — physicists’ best description of particles and forces — and opened a new window on physics, offering a way to learn about the Higgs field that one hadn’t before The type of interaction studied gives the 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. Nevertheless, many properties of the particle remain a mystery.

10th anniversary of the discovery of the Higgs boson Nature looks at what it has taught us about the universe and the big questions that still remain.

5 things scientists have learned

The mass of the Higgs boson is 125 billion electron volts

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

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

Higgs’ mass of 125 GeV puts it in an optimal range, meaning the boson decays into a broad range of particles with a frequency high enough to be observed by LHC experiments, says Matthew McCullough, a theoretical physicist at CERN. “It’s very bizarre and probably random, but it just so happens [at this mass] You can measure many different things about the Higgs.”

The Higgs boson is a spin-zero particle

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

In 2013, CERN experiments studied the angle at which photons produced in the Higgs boson decay flew into the detectors and used this to show with a high probability that the particle had zero spin. Until this was demonstrated, few physicists could name the particle they found Higgs, says Ramona Gröber, a theoretical physicist at the University of Padua in Italy.

Higgs’ property rule out 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 like the existence of dark matter or why there is so little antimatter in the universe. Therefore, physicists have developed extensions of the model that take them into account. The discovery of the 125 GeV mass of the Higgs boson has made some of these theories less attractive, says Gröber. But the mass is in a gray area, so exclude very little categorically, says Freya Blekman, a particle physicist at the German Electron Synchrotron (DESY) in Hamburg. “What we have is a particle that matches pretty much everything,” she says.

The Higgs boson interacts with other particles as predicted by the Standard Model

According to the Standard Model, the mass of a particle depends on how strongly it interacts with the Higgs field. Although the boson – which is like a wave in the Higgs field – plays no role in this process, the rate at which Higgs bosons decay into or are produced by another given particle provides a measure of how strong that particle is the field interacts with it. LHC experiments have confirmed that, at least for the heaviest particles most commonly produced in Higgs decays, mass is proportional to interaction with the field, a remarkable win 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 may be only temporarily stable, with an infinitesimally small chance of transitioning 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 permeates the entire universe. According to the Standard Model, this “ground state” depends on how particles interact 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 favorable energy state also existed.

Shifting to this other state would require crossing an enormous energy barrier, McCullough says, and the probability of doing so is so small that it’s improbable on the universe’s lifetime timescale. “Our end of the world will come much earlier for other reasons,” says McCullough.

Graphical representation of events recorded by the CMS detector in 2012 consistent with the decay of the Higgs boson

A computer image of events recorded by CERN’s Compact Muon Solenoid Detector in 2012 shows features expected from the decay of a Higgs boson into a pair of photons (dashed yellow lines and green towers).recognition: Thomas 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 its interaction strength – agree with the predictions of the Standard Model, but with an uncertainty of about 10%. This isn’t good enough to show the subtle differences predicted by new physical theories, 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 the total amount of information expected. Seeing evidence of new phenomena in precision studies is more likely than observing a new particle directly, says Daniel de Florian, a theoretical physicist at the National University of San Martín in Argentina. “For the next decade or more, the game will be precision.”

Does the Higgs interact with lighter particles?

So far, the Higgs boson’s interactions seemed to fit the Standard Model, but physicists have seen it decay into only the heaviest particles of matter, like the bottom quark. Physicists now want to check whether it interacts in the same way with particles from lighter families, so-called generations. In 2020, CMS and ATLAS saw one such interaction – the rare decay of a Higgs into a second-generation cousin of the electron called the muon1. Although 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 a high-energy Higgs boson into two lower-energy ones – are extremely rare because all the particles involved are so heavy. ATLAS and CMS hope to find clues to the interactions after a planned upgrade of the LHC starting in 2026, but conclusive evidence will likely require a more powerful collider.

The rate 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 conditions just after the Big Bang. Knowing how Higgs self-interacts 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 common than antimatter require Higgs self-interactions that differ by up to 30% from the Standard Model’s prediction. “I can’t stress enough how important this measurement is,” says McCullough.

How long does the Higgs boson last?

Physicists want to know the lifetime of the Higgs — how long it lasts, on average, before decaying into other particles — because any deviation from predictions could indicate interactions with unknown particles, such as B. those that make up dark matter. But its lifespan is too short to be measured directly.

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

Are any exotic predictions 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 that behave similarly but differ in charge or spin, for example. In addition to verifying that Higgs is truly a Standard Model particle, LHC experiments will look for properties predicted by other theories, including decay into forbidden combinations of particles.

Physicists are just beginning their efforts to understand the Higgs field, which due to its unique nature “acts like a portal to new physics,” says de Florian. “There is a lot of room for enthusiasm here”

Leave a Comment

%d bloggers like this: