In April, scientists at the European Center for Nuclear Research outside Geneva, or CERN, once again fired their cosmic weapon, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider continued to pull protons—the bare guts of hydrogen atoms—around the 17-mile electromagnetic underground racetrack. In early July, the collider will begin smashing these particles together to create sparks of primordial energy.
And so, amid new developments and the renewed hope of particle physicists, the big game of searching for the secret of the universe is about to begin again. Even before the collider was restored, it was giving clues that nature was hiding something magnificent. Mitesh Patel, a particle physicist who conducted an experiment at CERN at Imperial College London, described data from his previous studies as “the most exciting array of results I’ve ever seen in my professional life.”
A decade ago, CERN physicists made global headlines with the discovery of the Higgs boson, a long-sought particle that gives mass to every other particle in the universe. What is left to find? Optimistic physicists say almost anything.
When the CERN collider first turned on in 2010, the universe was up for grabs. The largest and most powerful machine ever built was designed to find the Higgs boson. This particle is the cornerstone of the Standard Model, a set of equations that describes everything scientists can measure about the subatomic world.
But there are deeper questions about the universe that the standard Model does not explain: Where did the universe come from? Why is it made of matter and not antimatter? What is the “dark matter” that covers the universe? What is the mass of the Higgs particle itself?
Physicists had hoped that some answers would come true in 2010, when the large collider was first powered up. Nothing but the Higgs has emerged—especially since there are no new particles that could explain the nature of dark matter. Frustratingly, the Standard Model remained unshakable.
The Collider was shut down at the end of 2018 for extensive upgrades and repairs. According to the current schedule, the collider will operate until 2025, after which it will be closed for another two years to install other extensive upgrades. These upgrades include improvements to giant detectors at four locations where proton beams collide and analyze collision debris. From July, these detectors will stop their work for them. The proton beams are compressed to make them more dense, which increases the chance of the protons colliding at their intersections – but creates confusion for detectors and computers in the form of multiple particle sprays that need to be distinguished from each other.
Dr. “The data will come in much faster than we’re used to,” Patel said. Where once only a few collisions occurred at each beam transition, now there will be more like five.
“This complicates our lives in a way because we need to be able to find things we’re interested in among all these different interactions,” he said. “But that means you’re more likely to see what you’re looking for.”
Meanwhile, various experiments have uncovered possible cracks in the Standard Model and pointed to a wider, deeper theory of the universe. These results include the rare behavior of subatomic particles in the cosmic bleaches, whose names most of us are not familiar with.
take the muon, a subatomic particle that briefly became famous last year. Muons are often called fat electrons; they have the same negative electric charge but 207 times larger. “Who ordered this?” physicist Isador Rabi said muons were discovered in 1936.
No one knows where muons fit into this grand scheme. They are created by cosmic ray collisions and colliding events, and within microseconds they radioactively decay into a burst of electrons and ghostly particles called neutrinos.
Last year, a team of nearly 200 physicists associated with the Fermi National Accelerator Laboratory in Illinois reported: muons spinning in a magnetic field were swinging significantly faster More than predicted by the Standard Model.
The inconsistency with the theoretical predictions came at the eighth decimal place of the value of a parameter called g-2, which describes how the particle responds to a magnetic field.
The scientists attributed the fractional but real difference to the quantum whisper of as yet unknown particles, which would occur for a short time around the muon and affect its properties. Confirming the existence of particles will eventually break the Standard Model.
But two groups of theorists are trying to reconcile their predictions of what g-2 should be, while awaiting more data from the Fermilab experiment.
“The g-2 anomaly is still very much alive,” said physicist Aida X. El-Khadra of the University of Illinois, who led a three-year effort to establish a consensus prediction, called the Muon g-2 Theory Initiative. “Personally, I am optimistic that cracks in the Standard Model will cause an earthquake. But the exact location of the cracks can still be a moving target.”
The muon is also in another anomaly. The main character, or perhaps the villain, in this drama is a particle called the B quark, which is one of six kinds of quarks that make up heavier particles like protons and neutrons. B stands for inferior or perhaps beauty. Such quarks occur in two-quark particles known as B mesons. But these quarks are unstable and tend to scatter in ways that seem to violate the Standard Model.
Some rare decays of a B quark involve a different, lighter type of quark and a daisy-chain reaction that ends with a pair of light particles called leptons, electrons, or their plump cousins, muons. The Standard Model assumes that electrons and muons are equally likely to appear in this reaction. (There is a third, heavier lepton called tau, but it decays too quickly to be observed.) However, Dr. Patel and colleagues found more electron pairs than muon pairs, violating a principle called lepton universality.
His team is investigating B quarks with LHCb, one of the Large Hadron Collider’s large detectors. “This could be a Standard Model killer,” Patel said. This anomaly, like the magnetic anomaly of the muon, points to an unknown “effector” – a particle or force that interferes with the reaction.
Dr. If these data hold up in the upcoming collider study, Patel says, one of the most dramatic possibilities is a subatomic speculation called the leptoquark. If a particle exists, it can bridge the gap between the two classes of particles that make up the material universe: light leptons – electrons, muons, as well as neutrinos – and heavier particles such as protons and neutrons, which are made up of quarks. Excitingly, there are six kinds of quarks and six kinds of leptons.
Dr. “We’re entering this run with more optimism that a revolution is coming,” Patel said. “Fingers crossed.”
There is another particle that behaves strangely in this zoo: the W boson, which carries the so-called weak force responsible for radioactive decay. In May, physicists with the Collider Detector, or CDF, at Fermilab, A 10-year effort has been reported to measure the mass of this particle.The Large Hadron Collider is based on approximately 4 million W bosons from collisions on Fermilab’s Tevatron, which was the world’s most powerful collider until it was built.
According to the Standard Model and previous mass measurements, the W boson should weigh about 80,357 billion electron volts, the unit of mass energy preferred by physicists. By comparison, the Higgs boson weighs 125 billion electron volts, about one atom of iodine. However, the CDF measurement of W, the most precise ever made, came in higher than estimated at 80.433 billion. The experimenters calculated that, in physics jargon, there was only one chance at 2 trillion – 7-sigma – this discrepancy was a statistical fluke.
The mass of the W boson correlates with the masses of other particles, including the notorious Higgs. So this new inconsistency, if it holds up, could be another crack in the Standard Model.
Still, the three anomalies and the theorists’ hopes of revolution may evaporate with more data. But to optimists, all three point to the same encouraging direction towards hidden particles or forces that interfere with “known” physics.
“So a new particle that could explain both the g-2 and the W mass could be available at the LHC,” said physicist Kyle Cranmer of the University of Wisconsin, who is working on other experiments at CERN.
John Ellis, a theorist at CERN and Kings College London, noted that at least 70 papers have been published suggesting explanations for the new W-mass mismatch.
“Many of these explanations also require new particles available to the LHC,” he said. “Did I mention dark matter? I mean, there’s a lot to watch out for!”
About the upcoming run, Dr. Patel said: “It’s going to be exciting. It’s going to be tough work, but we’re really excited to see what we’ve got and if there’s anything really exciting about the data.”
He added: “You can have a scientific career and not say it once. So it feels like a privilege.”