On July 4, 2012, physicists at CERN, the European laboratory for 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 in physicists) and opened a new window on physics providing a way to learn about the Higgs field, which involves a type of interaction not previously studied 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. However, many of the properties of the particle remain a mystery.
In the ten years since the discovery of the Higgs boson, Nature look at what he has taught us about the Universe, as well as the great questions that remain.
5 things scientists have learned
The mass of the Higgs boson is 125 billion electron volts
Physicists hoped to finally find the Higgs boson, but did not know when. In the 1960s, physicist Peter Higgs and others theorized that what is now called the Higgs field could explain why the photon does not have mass and the W and Z bosons, which carry the weak nuclear force behind them. of radioactivity, are heavy (for subatomic particles). . The special properties of the Higgs field allowed mathematics itself 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 mass of the boson and therefore when it could be produced by the LHC.
In the end, the particle emerged much earlier than expected. The LHC began collecting data in its Higgs search in 2009, and both ATLAS and CMS, the general-purpose accelerator detectors, saw it in 2012. Detectors observed the disintegration of a few dozen of Higgs bosons in photons, Ws and Zs. , which revealed an increase in data to 125 billion electron volts (GeV), about 125 times the mass of the proton.
Higgs ’125 GeV mass places it at a sweet spot which means the boson disintegrates into a wide range of particles at a frequency high enough for LHC experiments to observe, says Matthew Mccullough, a theoretical physicist at CERN. “It’s very strange and probably accidental, but it happens that way [at this mass] you can measure a lot of different things about the Higgs. “
The Higgs boson is a zero spin particle
Spin is an intrinsic quantum mechanical property of a particle, often represented as an internal bar magnet. All other known fundamental particles have a 1/2 or 1 turn, but theories predict that Higgs should be unique in having a zero turn (it was also correctly predicted that it would have zero charge).
In 2013, CERN experiments studied the angle at which photons produced in Higgs boson decays protruded toward detectors and used it to prove with high probability that the particle had zero rotation. Until this was proven, few physicists were comfortable calling Higgs the particle they had found, 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 at high energies and cannot explain key observations, such as the existence of dark matter or why there is so little antimatter in the Universe. Thus, physicists have devised extensions of the model that take this into account. Discovering the 125 GeV mass of the Higgs boson has made some of these theories less attractive, Gröber says. But the mass is in a gray area, which means it rules out very little categorically, says Freya Blekman, particle physicist at the German Electronic Synchrotron (DESY) in Hamburg. “What we have is a particle that is consistent with more or less anything,” he 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 the force that interacts with the Higgs field. Although the boson, which is like a ripple in the Higgs field, plays no role in this process, the rate at which Higgs bosons disintegrate or are produced by any other particle provides a measure of the force that this. particle interacts with the field. LHC experiments have confirmed that, at least for the heaviest particles, most frequently produced in Higgs decays, the mass is proportional to the interaction with the field, a remarkable victory for a 60-year-old theory.
The Universe is stable, but only fair
Calculations using the mass of the Higgs boson suggest that the Universe could only be temporarily stable, and there is a very small chance that it could move to a lower energy state, with catastrophic consequences.
Unlike other known fields, the Higgs field has a lower energy state above zero even in a vacuum, and permeates the entire Universe. According to the standard model, this “ground state” depends on how the particles interact with the field. Shortly after physicists discovered the mass of the Higgs boson, theorists used value (along with other measurements) to predict that a lower and more preferable energy state also exists.
Moving to this other state would require it to overcome a huge energy barrier, Mccullough says, and the likelihood of this happening is so small that it is unlikely to occur on the time scale of the life of the Universe. “Our day of final judgment will be much earlier, for other reasons,” Mccullough says.
A computer image of events recorded with CERN’s compact muon solenoid detector in 2012 shows the expected characteristics of the disintegration of a Higgs boson to a pair of photons (dashed yellow lines and green towers).: Thomas Mc Cauley, CMS / CERN Collaboration
5 things scientists still want to know
Can we make more accurate Higgs measurements?
To date, the properties of the Higgs boson, such as its interaction strength, match those predicted by the standard model, but with an uncertainty of around 10%. This is not good enough to show the subtle differences predicted by new theories of physics, which are only slightly different from the standard model, Blekman says.
More data will increase the accuracy of these measures and the LHC has only collected one-twentieth of the total amount of information it is expected to collect. Seeing evidence of new phenomena in precision studies is more likely than directly observing a new particle, says Daniel de Florian, a theoretical physicist at the Universidad Nacional de San Martín in Argentina. “For the next decade or so, the name of the game is precision.”
Does the Higgs interact with lighter particles?
Until now, the interactions of the Higgs boson seemed to fit the standard model, but physicists have seen it disintegrate only in heavier particles of matter, such as the lower quark. Physicists now want to test whether it interacts in the same way with particles from lighter families, known as generations. In 2020, CMS and ATLAS saw one of these interactions: the rare decomposition of a Higgs into a second-generation cousin of the electron called muon1. While this is evidence that the relationship between mass and interaction force is maintained for lighter particles, physicists need more data to confirm this.
Does the Higgs interact with itself?
The Higgs boson has too much, so it should interact with itself. But these interactions, for example, the disintegration of one energetic Higgs boson to two less energetic ones, are extremely rare, because all the particles involved are very heavy. ATLAS and CMS hope to find clues to the interactions after a planned LHC upgrade starting in 2026, but conclusive testing is likely to take a more powerful collider.
The speed of this self-interaction is crucial 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. Therefore, knowing Higgs ’self-interaction could help scientists understand the dynamics of the early Universe, Mccullough says. Gröber points out that many theories that attempt to explain how matter was made in some way more abundant than antimatter require Higgs self-interactions that diverge from the prediction of the standard model by up to 30%. “I can’t stress enough the importance” of this measure, Mccullough says.
What is the life of the Higgs boson?
Physicists want to know the useful life of the Higgs, how long, on average, it remains before it decomposes into other particles, because any deviation from predictions could point to interactions with unknown particles, such as those that form dark matter. But its useful life is too small to be measured directly.
To measure this indirectly, physicists look at the propagation, or “width,” of the energy of the particle in multiple measurements (quantum physics says that uncertainty in the energy of the particle should be inversely related to the its useful life). Last year, CMS physicists produced their first approximate measure of Higgs lifespan: 2.1 × 10−22 seconds2. The results suggest that the service life is consistent with the standard model.
Are any exotic predictions true?
Some theories extending the standard model predict that the Higgs boson is not fundamental, but, like the proton, is made up of other particles. Others predict that there are multiple Higgs bosons, which behave similarly but differ, for example, in charge or rotation. In addition to checking whether Higgs is really a standard model particle, the LHC experiments will look for properties predicted by other theories, including decays in forbidden particle combinations.
Physicists are at the beginning of their efforts to understand the Higgs field, the unique nature of which makes it “behave like a portal to new physics,” de Florian says. “There’s a lot of room for excitement here.”