“You are able to do it shortly, you are able to do it cheaply, or you are able to do it proper. We did it proper.” These have been among the opening remarks from David Toback, chief of the Collider Detector at Fermilab, as he introduced the outcomes of a decadelong experiment to measure the mass of a particle known as the W boson.
I’m a excessive power particle physicist, and I’m a part of the workforce of tons of of scientists that constructed and ran the Collider Detector at Fermilab in Illinois – often called CDF.
After trillions of collisions and years of information assortment and quantity crunching, the CDF workforce discovered that the W boson has barely extra mass than anticipated. Although the discrepancy is tiny, the outcomes, described in a paper printed in Science on April 7, 2022, have electrified the particle physics world. If the measurement is appropriate, it’s yet one more sturdy sign that there are lacking items to the physics puzzle of how the universe works.
The Normal Mannequin of particle physics describes the particles that make up the mass and forces of the universe.
MissMJ/WikimediaCommons
A particle that carries the weak pressure
The Normal Mannequin of particle physics is science’s present finest framework for the essential legal guidelines of the universe and describes three primary forces: the electromagnetic pressure, the weak pressure and the sturdy pressure.
The sturdy pressure holds atomic nuclei collectively. However some nuclei are unstable and endure radioactive decay, slowly releasing power by emitting particles. This course of is pushed by the weak pressure, and for the reason that early 1900s, physicists sought a proof for why and the way atoms decay.
In keeping with the Normal Mannequin, forces are transmitted by particles. Within the Nineteen Sixties, a sequence of theoretical and experimental breakthroughs proposed that the weak pressure is transmitted by particles known as W and Z bosons. It additionally postulated {that a} third particle, the Higgs boson, is what provides all different particles – together with W and Z bosons – mass.
Because the creation of the Normal Mannequin within the Nineteen Sixties, scientists have been working their means down the record of predicted but undiscovered particles and measuring their properties. In 1983, two experiments at CERN in Geneva, Switzerland, captured the primary proof of the existence of the W boson. It appeared to have the mass of roughly a medium-sized atom akin to bromine.
By the 2000s, there was only one piece lacking to finish the Normal Mannequin and tie the whole lot collectively: the Higgs boson. I helped seek for the Higgs boson on three successive experiments, and eventually we found it in 2012 on the Giant Hadron Collider at CERN.
The Normal Mannequin was full, and all of the measurements we made hung collectively superbly with the predictions.
The Collider Detector at Fermilab collected knowledge from trillions of collisions that produced hundreds of thousands of W bosons.
Bodhita/WikimediaCommons, CC BY-SA
Measuring W bosons
Testing the Normal Mannequin is enjoyable – you simply smash particles collectively at very excessive energies. These collisions briefly produce heavier particles that then decay again into lighter ones. Physicists use large and really delicate detectors at locations like Fermilab and CERN to measure the properties and interactions of the particles produced in these collisions.
In CDF, W bosons are produced about one out of each 10 million occasions when a proton and an antiproton collide. Antiprotons are the antimatter model of protons, with precisely the identical mass however reverse cost. Protons are made from smaller elementary particles known as quarks, and antiprotons are made from antiquarks. It’s the collision between quarks and antiquarks that create W bosons. W bosons decay so quick that they’re unattainable to measure instantly. So physicists monitor the power produced from their decay to measure the mass of W bosons.
Within the 40 years since scientists first detected proof of the W boson, successive experiments have attained ever extra exact measurements of its mass. However it’s only for the reason that measurement of the Higgs boson – because it provides mass to all different particles – that researchers may examine the measured mass of W bosons in opposition to the mass predicted by the Normal Mannequin. The prediction and the experiments at all times matched up – till now.
The brand new measurement of the W boson (pink circle) is way farther from the mass predicted by the Normal Mannequin (purple line) and in addition higher than the preliminary measurement from the experiment.
CDF Collaboration by way of Science Journal, CC BY
Unexpectedly heavy
The CDF detector at Fermilab is great at precisely measuring W bosons. From 2001 to 2011, the accelerator collided protons with antiprotons trillions of occasions, producing hundreds of thousands of W bosons and recording as a lot knowledge as potential from every collision.
The Fermilab workforce printed preliminary outcomes utilizing a fraction of the info in 2012. We discovered the mass to be barely off, however near the prediction. The workforce then spent a decade painstakingly analyzing the complete knowledge set. The method included quite a few inside cross-checks and required years of laptop simulations. To keep away from any bias creeping into the evaluation, no one may see any outcomes till the complete calculation was full.
When the physics world lastly noticed the consequence on April 7, 2022, we have been all shocked. Physicists measure elementary particle lots in items of hundreds of thousands of electron volts – shortened to MeV. The W boson’s mass got here out to be 80,433 MeV – 70 MeV larger than what the Normal Mannequin predicts it must be. This will appear to be a tiny extra, however the measurement is correct to inside 9 MeV. It is a deviation of almost eight occasions the margin of error. When my colleagues and I noticed the consequence, our response was a convincing “wow!”
What this implies for the Normal Mannequin
The truth that the measured mass of the W boson doesn’t match the expected mass throughout the Normal Mannequin may imply three issues. Both the maths is incorrect, the measurement is incorrect or there’s something lacking from the Normal Mannequin.
First, the maths. With a view to calculate the W boson’s mass, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to measure the Higgs boson mass to inside a quarter-percent. Moreover, theoretical physicists have been engaged on the W boson mass calculations for many years. Whereas the maths is refined, the prediction is stable and never more likely to change.
The following risk is a flaw within the experiment or evaluation. Physicists all around the world are already reviewing the consequence to attempt to poke holes in it. Moreover, future experiments at CERN could ultimately obtain a extra exact consequence that can both verify or refute the Fermilab mass. However for my part, the experiment is nearly as good a measurement as is presently potential.
That leaves the final possibility: There are unexplained particles or forces inflicting the upward shift within the W boson’s mass. Even earlier than this measurement, some theorists had proposed potential new particles or forces that might consequence within the noticed deviation. Within the coming months and years, I anticipate a raft of recent papers in search of to elucidate the puzzling mass of W bosons.
As a particle physicist, I’m assured in saying that there should be extra physics ready to be found past the Normal Mannequin. If this new consequence holds up, will probably be the most recent in a sequence of findings displaying that the Normal Mannequin and real-world measurements typically don’t fairly match. It’s these mysteries that give physicists new clues and new causes to maintain trying to find fuller understanding of matter, power, house and time.
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