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Joyful birthday, Higgs boson! What we do and don’t know concerning the particle


On 4 July 2012, physicists at CERN, Europe’s particle-physics laboratory, declared victory of their lengthy seek for the Higgs boson. The elusive particle’s discovery crammed within the final hole in the usual mannequin — physicists’ finest description of particles and forces — and opened a brand new window on physics by offering a solution to study concerning the Higgs area, which includes a beforehand unstudied sort of interplay that offers particles their plenty.

Since then, researchers at CERN’s Giant Hadron Collider (LHC) close to Geneva, Switzerland, have been busy, publishing nearly 350 scientific articles concerning the Higgs boson. However, lots of the particle’s properties stay a thriller.

On the ten-year anniversary of the Higgs boson’s discovery, Nature seems to be at what it has taught us concerning the Universe, in addition to the large questions that stay.

5 issues scientists have learnt

The Higgs boson’s mass is 125 billion electronvolts

Physicists anticipated to seek out the Higgs boson finally, however they didn’t know when. Within the Nineteen Sixties, physicist Peter Higgs and others theorized that what’s now known as a Higgs area may clarify why the photon has no mass and the W and Z bosons, which carry the weak nuclear power that’s behind radioactivity, are heavy (for subatomic particles). The particular properties of the Higgs area allowed the identical arithmetic to account for the plenty of all particles, and it turned a vital a part of the usual mannequin. However the principle made no predictions concerning the boson’s mass and due to this fact when the LHC may produce it.

In the long run, the particle emerged a lot sooner than anticipated. The LHC began gathering knowledge in its seek for the Higgs in 2009, and each ATLAS and CMS, the accelerator’s general-purpose detectors, noticed it in 2012. The detectors noticed the decay of only a few dozen Higgs bosons into photons, Ws and Zs, which revealed a bump within the knowledge at 125 billion electronvolts (GeV), about 125 instances the mass of the proton.

The Higgs’ mass of 125 GeV places it in a candy spot which means the boson decays into a variety of particles at a frequency excessive sufficient for LHC experiments to watch, says Matthew Mccullough, a theoretical physicist at CERN. “It’s very weird and possibly happenstance, but it surely simply so occurs that [at this mass] you possibly can measure a great deal of various things concerning the Higgs.”

The Higgs boson is a spin-zero particle

Spin is an intrinsic quantum-mechanical property of a particle, usually pictured as an inner bar magnet. All different identified elementary particles have a spin of 1/2 or 1, however theories predicted that the Higgs must be distinctive in having a spin of zero (it was additionally accurately predicted to have zero cost).

In 2013, CERN experiments studied the angle at which photons produced in Higgs boson decays flew out into the detectors, and used this to point out with excessive likelihood that the particle had zero spin. Till this had been demonstrated, few physicists have been snug calling the particle that they had discovered the Higgs, says Ramona Gröber, a theoretical physicist on the College of Padua in Italy.

The Higgs’ properties rule out some theories that stretch the usual mannequin

Physicists know that the usual mannequin is just not full. It breaks down at excessive energies and may’t clarify key observations, such because the existence of darkish matter or why there’s so little antimatter within the Universe. So physicists have give you extensions to the mannequin that account for these. Discovering the Higgs boson’s 125-GeV mass has made a few of these theories much less engaging, says Gröber. However the mass is in a gray zone, which implies it guidelines out little or no categorically, says Freya Blekman, a particle physicist on the German Electron Synchrotron (DESY) in Hamburg. “What now we have is a particle that’s in keeping with kind of something,” she says.

The Higgs boson interacts with different particles as the usual mannequin predicts

In accordance with the usual mannequin, a particle’s mass will depend on how strongly it interacts with the Higgs area. Though the boson — which is sort of a ripple within the Higgs area — doesn’t have a job in that course of, the speed at which Higgs bosons decay into or are produced by every other given particle supplies a measure of how strongly that particle interacts with the sector. LHC experiments have confirmed that — at the least for the heaviest particles, produced most ceaselessly in Higgs decays — mass is proportional to interplay with the sector, a exceptional win for a 60-year-old principle.

The Universe is secure — however solely simply

Calculations utilizing the mass of the Higgs boson recommend that the Universe may be solely quickly secure, and there’s a vanishingly small probability that it may shift right into a decrease power state — with catastrophic penalties.

In contrast to different identified fields, the Higgs area has a lowest power state above zero even in a vacuum, and it pervades all the Universe. In accordance with the usual mannequin, this ‘floor state’ will depend on how particles work together with the sector. Quickly after physicists found the Higgs boson’s mass, theorists used the worth (alongside different measurements) to foretell that there additionally exists a decrease, extra preferable power state.

Shifting to this different state would require it to beat an infinite power barrier, says Mccullough, and the likelihood of this occurring is so small that it’s unlikely to happen on the timescale of the lifetime of the Universe. “Our doomsday shall be a lot sooner, for different causes,” says Mccullough.

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

A pc picture of occasions recorded with CERN’s Compact Muon Solenoid detector in 2012 reveals traits anticipated from the decay of a Higgs boson to a pair of photons (dashed yellow strains and inexperienced towers).Credit score: Thomas Mc Cauley, CMS Collaboration/CERN

5 issues scientists nonetheless wish to know

Can we make Higgs measurements extra exact?

To date, the Higgs boson’s properties — similar to its interplay energy — match these predicted by the usual mannequin, however with an uncertainty of round 10%. This isn’t ok to point out the refined variations predicted by new physics theories, that are solely barely completely different from the usual mannequin, says Blekman.

Extra knowledge will enhance the precision of those measurements and the LHC has collected simply one-twentieth of the full quantity of knowledge it’s anticipated to assemble. Seeing hints of recent phenomena in precision research is extra possible than immediately observing a brand new particle, says Daniel de Florian, a theoretical physicist on the Nationwide College of San Martín in Argentina. “For the following decade or extra, the secret is precision.”

Does the Higgs work together with lighter particles?

Till now, the Higgs boson’s interactions have appeared to suit with the usual mannequin, however physicists have seen it decay into solely the heaviest matter particles, similar to the underside quark. Physicists now wish to test whether or not it interacts in the identical manner with particles from lighter households, often known as generations. In 2020, CMS and ATLAS noticed one such interplay — the uncommon decay of a Higgs to a second-generation cousin of the electron known as the muon1. Though that is proof that the connection between mass and interplay energy holds for lighter particles, physicists want extra knowledge to verify it.

Does the Higgs work together with itself?

The Higgs boson has mass, so it ought to work together with itself. However such interactions — for instance, the decay of an brisk Higgs boson to 2 much less energetic ones — are extraordinarily uncommon, as a result of all of the particles concerned are so heavy. ATLAS and CMS hope to seek out hints of the interactions after a deliberate improve to the LHC from 2026, however conclusive proof will in all probability take a extra highly effective collider.

The speed of this self-interaction is essential to understanding the Universe, says Mccullough. The likelihood of self-interaction is set by how the Higgs area’s potential power modifications close to its minimal, which describes situations simply after the Large Bang. So realizing concerning the Higgs self-interaction may assist scientists to grasp the dynamics of the early Universe, says Mccullough. Gröber notes that many theories that attempt to clarify how matter by some means turned extra considerable than antimatter require Higgs self-interactions that diverge from the usual mannequin’s prediction by as a lot as 30%. “I can’t emphasize sufficient how essential” this measurement is, says Mccullough.

What’s the Higgs boson’s lifetime?

Physicists wish to know the lifetime of the Higgs — how lengthy, on common, it sticks round earlier than decaying to different particles — as a result of any deviation from predictions may level to interactions with unknown particles, similar to people who make up darkish matter. However its lifetime is simply too small to measure immediately.

To measure it not directly, physicists have a look at the unfold, or ‘width’, of the particle’s power over a number of measurements (quantum physics says that uncertainty within the particle’s power must be inversely associated to its lifetime). Final yr, CMS physicists produced their first tough measurement of the Higgs’ lifetime: 2.1 × 10−22 seconds2. The outcomes recommend that the lifetime is in keeping with the usual mannequin.

Are any unique predictions true?

Some theories that stretch the usual mannequin predict that the Higgs boson is just not elementary, however — just like the proton — is made up of different particles. Others predict that there are a number of Higgs bosons, which behave equally however differ, for instance, in cost or spin. In addition to checking whether or not the Higgs is really a standard-model particle, LHC experiments will search for properties predicted by different theories, together with decays into forbidden particle combos.

Physicists are simply at first of their efforts to grasp the Higgs area, whose distinctive nature makes it “behave like a portal to new physics”, says de Florian. “There’s a number of room for pleasure right here.”

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