- 3621 - HIGGS BOSON - what have we learned? Physicists are just at the beginning of their efforts to understand the Higgs field, whose unique nature makes it “behave like a portal to new physics”.
--------------------- 3621 - HIGGS BOSON - what have we learned?
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- On July 4, 2012, physicists at CERN declared victory in their long search for the “Higgs boson“. The elusive particle’s discovery filled in the last gap in the standard model that is physicists’ best description of particles and forces. It opened a new window on physics by providing a way to learn about the “Higgs field“, which involves a kind of interaction that gives particles their masses.
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- The Standard Model for particle physics divides particles into fermions and bosons. Fermions are like quarks and electrons. Bosons are force carrying particles like photons and gluons. Gluons carry the Strong Force. Photons carry the electromagnetic Force.
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- Researchers at CERN’s Large Hadron Collider (LHC) near Geneva, Switzerland, have been busy, publishing almost 350 scientific articles about the Higgs boson. Nevertheless, many of the particle’s properties remain a mystery.
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- On the ten-year, 2022, anniversary of the Higgs boson’s discovery, Nature looks at what it has taught us about the Universe, as well as the big questions that remain:
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-------------------------- The Higgs boson’s mass is 125 billion electronvolts
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- 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’s now called a Higgs field could explain why the photon has no mass and the W and Z bosons, which carry the weak nuclear force that is behind radioactivity, are heavy for subatomic particles.
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- The special properties of the Higgs field allowed the same mathematics to account for 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 therefore when the LHC collider might produce it.
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- The LHC started gathering data in its search for the Higgs in 2009, and both “ATLAS” and “CMS“, the accelerator’s general-purpose 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 bump in the data at 125 billion electronvolts (GeV), about 125 times the mass of the proton.
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- The Higgs’ mass of 125 GeV puts it in a sweet spot that means the boson decays into a wide range of particles at a frequency high enough for LHC experiments to observe.
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---------------------- The Higgs boson is a “spin-zero particle”
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- Spin is an intrinsic quantum-mechanical property of a particle, often pictured as an internal bar magnet. All other known fundamental particles have a spin of “½” or “1“, but theories predicted that the Higgs should be unique in having a spin of “zero” . It was also correctly predicted to have “zero charge“.
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- In 2013, CERN experiments studied the angle at which photons produced in Higgs boson decays flew out into the detectors, and used this to show with high probability that the particle had zero spin.
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------------------------- The Higgs’ properties rule out some theories that extend the standard model:
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- Physicists know that the standard model is not complete. It breaks down at high energies and can’t explain key observations, such as the existence of dark matter or why there is so little antimatter in the Universe. So physicists have come up with extensions to the model that account for these.
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- Discovering the Higgs boson’s 125-GeV mass has made some of these theories less attractive.
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------------------------ The Higgs boson interacts with other particles as the standard model predicts:
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- According to the standard model, a particle’s mass depends on how strongly it interacts with the Higgs field. Although the boson, which is like a ripple in the Higgs field, doesn’t have a role in that process, the rate at which Higgs bosons decay into or are produced by any other given particle provides a measure of how strongly that particle interacts with the field.
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- LHC experiments have confirmed that, at least for the heaviest particles, produced most frequently in Higgs decays, their mass is proportional to interaction with the field.
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--------------------------- The Universe is stable, but only just:
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- Calculations using the mass of the Higgs boson suggest that the Universe might be only temporarily stable, and there’s a vanishingly small chance that it could shift into a lower energy state with catastrophic consequences.
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- Unlike other known fields, the Higgs field has a lowest energy state above zero even in a vacuum, and it pervades the entire Universe. According to the standard model, this ‘ground state’ depends on how particles interact with the field.
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- Soon after physicists discovered the Higgs boson’s mass, theorists used the value alongside other measurements to predict that there also exists a lower, more preferable energy state.
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- Shifting to this other state would require it to overcome an enormous energy barrier and the probability of this happening is so small that it is unlikely to occur on the timescale of the lifetime of the Universe.
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------------------------- Can we make Higgs measurements more precise?
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- So far, the Higgs boson’s properties 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 physics theories, which are only slightly different from the standard model.
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- More data will increase the precision of these measurements and the LHC has collected just one-twentieth of the total amount of information it is expected to gather.
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-------------------------- Does the Higgs interact with lighter particles?
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- Until now, the Higgs boson’s interactions have seemed to fit with the standard model, but physicists have seen it decay into only the heaviest matter particles, such as the bottom quark.
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- Physicists now want to check whether it interacts in the same way with particles from lighter families, known as generations. In 2020, CMS and ATLAS saw one such interaction, the rare decay of a Higgs to a second-generation cousin of the electron called the “muon“.
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--------------------------- Does the Higgs interact with itself?
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- The Higgs boson has mass, so it should interact with itself. But such interactions like the decay of an energetic Higgs boson to two less energetic ones are extremely rare, because all the particles involved are so heavy. ATLAS and CMS hope to find hints of the interactions after a planned upgrade to the LHC from 2026, but conclusive evidence will probably take a more powerful collider.
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- The rate of this self-interaction is crucial to understanding the Universe. The probability of self-interaction is determined by how the Higgs field’s potential energy changes near its minimum, which describes conditions just after the Big Bang.
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- Knowing about the Higgs self-interaction could help scientists to understand the dynamics of the early Universe. Many theories that try to explain how matter somehow became more abundant than antimatter require Higgs self-interactions that diverge from the standard model’s prediction by as much as 30%.
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------------------------- What is the Higgs boson’s lifetime?
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- Physicists want to know the lifetime of the Higgs, on average how long it sticks around before decaying to other particles. Any deviation from predictions could point to interactions with unknown particles, such as those that make up “dark matter“. But, its lifetime is too small to measure directly.
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- To measure it indirectly, physicists look at the spread, or ‘width’, of the particle’s energy over multiple measurements. Quantum physics says that uncertainty in the particle’s energy should be inversely related to its lifetime. In 2021 CMS physicists produced their first rough measurement of the Higgs’ lifetime at 2.1 × 10^22 seconds. The results suggest that the lifetime is consistent with the standard model.
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--------------------------- Are any exotic predictions true?
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- Some theories that extend 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 in charge or spin. As well as checking whether the Higgs is truly a standard-model particle, LHC experiments will look for properties predicted by other theories, including decays into forbidden particle combinations.
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- Physicists are just at the beginning of their efforts to understand the Higgs field, whose unique nature makes it “behave like a portal to new physics”. There is a lot of room for excitement here.
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July 6, 2022 HIGGS BOSON - what have we learned? 3621
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