The cover of the current issue of Science, the 142-year-old flagship journal of the American Association for the Advancement of Science, and one of the top peer-reviewed academic journals in the world, manages to convey a blockbuster finding contained in its pages.
A chunky concrete ‘W’ sits on top of the shattered remains of a table that had neatly summarized the complex scheme of 17 particles that most physicists believe are the basic building blocks of matter.
Below the image run the words “Heavyweight: W boson mass measures higher than expected.”
The finding is getting the Muhammad Ali treatment because the theory that the shattered table represents — the Standard Model of particle physics — is “by any measure, the most successful scientific theory of all time,” according to Cambridge University theoretical physicist David Tong. “[It] gives the correct answer to hundreds of thousands of experiments, in some cases with an accuracy unprecedented in science.”
But this time, it didn’t.
Interesting Engineering sat down with particle physicist Ashutosh Kotwal, the senior author of the paper, to learn why the finding is such a big deal and how his team pulled off such a feat of engineering.
Elementary physics drove the cutting-edge research
If you want an indication of how much time is needed for the number-crunching and double-checking that modern physics requires, consider this. The data that underpins this fantastically provocative new paper was collected more than a decade ago.
Between 2002 to 2011, researchers at the Department of Energy’s Fermilab sent subatomic particles racing through a particle accelerator called Tevatron. Their job was to pay very close attention to what happened right after the particles smashed into each other. Over and over and over again.
“When the protons and antiprotons collide, a lot of particles fly out,” Kotwal says. “In any given collision, maybe 30 particles are flying out, maybe 50 particles are flying out. You want to measure the energy and the position and the direction of every one of them,” he explained.
While the experiments were running, Tevatron was the most powerful particle accelerator in the world. This was the absolute cutting edge of science, but the researchers relied on concepts from elementary physics.
“If you open an undergrad physics textbook, it says charged particles go in a circle when [they] go through a magnetic field. So what you can do is… put layer after layer of position sensors [in the particle accelerator]. So, when a charged particle comes through, it records where the particle went,” Kotwal says.
Those sensors recorded reams of data for computers to interpret, sort of like a high-tech connect-the-dots. But instead of a two-dimensional line drawing of a cool frog, these patterns produced circles whose precise dimensions contained valuable information.
“It’s textbook physics that if you can measure the radius of that circle precisely, and if you know the strength of the magnetic field precisely, you can combine these in a simple formula [to determine] the energy of the particle,” he explained.
Studying subatomic particles requires incredible precision
The researchers used roughly 30,000 sensors to measure the radii of these circles. Since measuring something as minuscule as a subatomic particle depends on getting those measurements just right, it’s absolutely essential to know exactly — with a level of precision required under hardly any other circumstance — where each sensor is located.
Kotwal and his team relied on a natural source of particles — cosmic rays —to help them align their sensors more accurately than ever before. Earth is under constant bombardment by high-energy fragments of atoms that come from supernova explosions (and maybe other places, too, though scientists aren’t entirely sure). Some of those particles are protons. When they collide with atoms in Earth’s upper atmosphere, the protons break apart to form subatomic particles called muons, which are in the same class as electrons but roughly 200 times bigger (Both muons and electrons are among the 17 constituents of the Standard Model).
These muons move at nearly the speed of light, and they’re incredibly abundant. If you hold your hand parallel to the ground, one muon will pass through it every second, on average. That means they were constantly passing through Tevatron.
“We treat them like straight lines [and] use those lines to align our sensors,” Kotwal says.
“We demonstrate that we could line them up to an accuracy of one micron each. In the past, it was three or four microns,” he says. For reference, a strand of hair is roughly 100 microns thick.
The researchers made more than a dozen such improvements over the last time they ran this type of experiment. “We describe every one of them, what impact it had and why… compared to the last time,” he says.
How does this finding fit into the bigger picture?
The laws of nature that physicists believed in would also have made sense (more or less) to non-physicists until about 1900. Then, two completely counterintuitive ideas — quantum mechanics and relativity — burst onto the scene and allowed predictions so accurate that serious physicists had no choice but to take them seriously.
More than a century later, researchers are still hunting for a way to stitch these theories together into one perfect “theory of everything.” But a century ago, physicist Paul Dirac came close. He “put a joint theory together” that combined principles of the two approaches, Kotwal says.
Early on, there was evidence that his approach of using math to find deep truths about the nature of matter was paying off.
“One of the fantastic things that came out of Dirac’s work was the prediction that something like antimatter should exist,” he says. This prediction came from equations that implied that a particle must have a corresponding particle that’s its mirror opposite.
“And soon enough the anti-electron — the positron — was discovered,” he says.
As the decades passed, Dirac’s basic theory grew as physicists made more advancements. They were aided by the fact that a certain branch of mathematics — group theory — seemed to underlie many of the disparate threads they were tugging on.
The theory grew into a set of self-consistent “principles collectively describing all the matter we know, all the forces… and all the interactions between the matter and the forces,” Kotwal says. “This is how it steadily became more and more encompassing.”
The Standard Model is born
But there was a problem.
“One thing that prevents [this theory] from working — I will make a strong statement — that prevents it from working, is the fact that particles have masses,” Kotwal says. Adding masses to the equation caused the theory to “fall apart.”
But that wasn’t the end of the equation. “Some people figured out that you don’t have to discard the whole theory that was already working well. You have to just modify it in a small way,” Kotal says. That modification came in the form of a new particle: the then-unseen Higgs boson.
“It became at that point, what we now call the Standard Model, because now it explained one more conundrum, which is, how do masses arise in this whole picture,” he says.
This view was confirmed later, in 2012, when the Higgs boson was observed for the first time. That happened in the Large Hadron Collider, just two years after that accelerator usurped Tevatron as the world’s most powerful.
The Standard Model doesn’t explain absolutely everything. It can’t account for dark matter, the ratio of ordinary matter to antimatter, certain aspects of the Higgs boson, or — most notably — gravity. But it does explain just about everything else.
So, what is the W boson?
Protons and neutrons — the particles bunched together like grapes in the nucleus of an atom — aren’t among the 17 particles in the Standard Model. That’s because they’re made of even smaller particles, called quarks.
Protons and neutrons are made of three quarks each (that is, the total number of quarks minus the total number of antiquarks is always three). However, it’s the all-important third quark that determines if a particle is a proton or a neutron. That difference is huge because protons need neutrons in order to stick together and make anything beyond subatomic soup.
“All the elements that we know contain neutrons as well as protons,” Kotwal says. “Without [neutrons], the nucleus of the atom cannot form.”
The W boson is so important because it transforms that third quark in a proton and converts the whole thing into a neutron. It’s not something that happens in everyday life, but it’s absolutely essential. Without the W boson, nothing would exist as we know it.
The universe “would have been protons and electrons. It would have been just hydrogen, hydrogen all over. Nothing about the universe that we see around us — all the richness, all the complexity, us — could have happened… without the exchange of the W boson,” he says.
Does the new finding spell doom for the Standard Model?
It’s impossible to say what the new findings will ultimately mean for physics. For one thing, they have to be confirmed. “While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully,” says Fermilab Deputy Director Joe Lykken.
Then, it will be up to theoretical physicists to make sense of the new, slightly larger mass. It’s possible that the new findings actually do fit into the equations. They’ll “look at the pure Standard Model calculation carefully to if there is any wiggle room there,” Kotwal says. While that sounds unlikely, the Standard Model is incredibly complex.
Other theorists will probably look at “extensions” to the theory that would update the equations to reflect the new findings. It would hardly be the first time that new information led physicists to reimagine this equation in light of new evidence.
And eventually, there will be more experiments. The Large Hadron Collider, for example, is in hot pursuit of these very questions.
“This is a trigger for all of us to think broadly,” Kotwal says. “I like to say, leave no stone unturned. That’s what we’re in it for. So, let’s go do everything we can do. Once in a while, nature will show us the next mystery. Perhaps that’s around the corner… that’s been the history of the Standard Model. New mysteries popped up [and] people figured out what they meant.”