Physicists close in on the exceedingly short life of the Higgs boson
Just because scientists have discovered a particle doesn't mean they know all its properties.
1.6 x 10-22 seconds: That, according to theory, is the lifetime of the Higgs boson, one of the most sought-after particles in the subatomic world. This time is so short that tens of trillions of Higgs bosons might live and die before the light from the device you’re using to read this reaches your eyes.
Physicists are zeroing in on this lifetime in the real world. Poring over data from CERN’s Large Hadron Collider (LHC), scientists have narrowed down the Higgs’ lifespan to something around that 1.6 x 10-22 figure. The scientists were able to do so thanks to data from the CMS, one of the LHC’s detectors. Their work is a major advance–and it’s a sign that, nearly a decade after the Higgs boson’s discovery, there is still quite a bit to learn about the particle.
“This is a good achievement, a great milestone, but it’s just the first step,” says Caterina Vernieri, a particle physicist at the SLAC National Accelerator Laboratory in California, who has worked with the CMS group in the past but was not involved with this current research.
The Higgs boson is the reason that many particles have mass, to make a long story involving complex concepts called “quantum fields” and “symmetry breaking” short. It was first theorised in the 1960s—its namesake is Peter Higgs, a Nobel-winning British physicist—but it eluded scientists for decades.
Smashing particles together at higher and higher energy was the key to its discovery, made possible by the LHC, where particles circle through a 17-mile-long ring on the French-Swiss border. The LHC went online in 2008. In 2012, physicists working there found the fingerprints of something that could have been the Higgs boson; by the end of 2013, they’d determined that their results weren’t just random statistical noise.
The search for the Higgs boson was over. But just because scientists have discovered a particle–or anything else–doesn’t mean that they understand all of its properties.
Theoretical physicists predicted many of the Higgs boson’s properties in the decades before its discovery. If those theoretical predictions matched well with what scientists ultimately found, then it would be additional evidence that the Higgs boson fits into the theory behind modern particle physics–the so-called Standard Model. It would help scientists learn more about how the universe ticks on the tiniest scales
But scientists are trying to study things that don’t exactly reveal themselves to the world. Particles like the Higgs, on top of their puny size, might only show themselves for vanishingly short timespans before decaying into a charcuterie board of other particles.
“The lifetime of the Higgs boson is extremely small,” says Vernieri. “So when it’s produced in our experiment, we don’t really actually measure the Higgs boson or see a Higgs boson, but what we see is the debris…of the particles it decays into.”
So the CMS scientists pored over data from LHC experiments undertaken between 2015 and 2018. By looking at the particles that the Higgs boson decayed into, they could backtrack and find a range of masses that the Higgs boson could have. Thanks to a quantum property called the uncertainty principle, that range is inversely proportional to the particle’s lifetime–allowing the physicists to calculate the latter from the former.
According to their calculations, the Higgs boson’s lifetime lies somewhere between 1.2 x 10-22 seconds and 4.4 x 10-22 seconds. That’s the most precise estimate of the Higgs boson’s lifetime yet, aligning well with the 1.6 x 10-22 number that theorists predicted.
And, yet, it’s not precise enough for some physics.
There’s a possibility, for instance, that there’s a strange, currently unknown exotic particle that the Higgs boson decays into, which the Standard Model doesn’t account for. That would influence the Higgs boson’s lifetime–but so subtly that even this calculation couldn’t detect it.
“This would be a tiny, tiny change in the lifetime value,” says Vernieri. “So we need, really, to measure the lifetime with very good precision.”
Fortunately, particle physicists think they can get better in that regard. “The precision of the measurement is expected to improve in the coming years with data from the next LHC runs and new analysis ideas,” says Pascal Vanlaer, a physicist at CMS and one of the physicists behind the project, in a statement.
The first of those next runs is, according to plan, not too far in the future. Since 2018, the LHC has been shut down for a lengthy period called, fittingly, Long Shutdown 2. During that time, the collider and CERN’s surrounding facilities have undergone a raft of upgrades. Following a disruption to that timetable caused by COVID-19, the collider is currently set to turn on again in February 2022.
And there are many other things about the Higgs boson that we still don’t know for sure — from how it’s produced to how it reacts to other particles to how it interacts with itself. To determine those features, not even the LHC may be sensitive enough.
“We produce a Higgs boson every billion collisions at LHC,” says Vernieri, and often, trying to see Higgs bosons means having to look through a whole sea of other particles. “It’s a very challenging environment to study, very precisely, particle production.”
The key will be a cleaner environment to study the Higgs boson with higher precision, Vernieri says. Perhaps, then, that’s a job for one of the LHC’s proposed successors.