In April, scientists at the European Center for Nuclear Research, or CERN, outside Geneva once again fired their cosmic weapon, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider has resumed launching protons, the bare guts of hydrogen atoms, around its 17-mile underground electromagnetic racetrack. In early July, the collider will begin colliding these particles to create sparks of primordial energy.
And so the great game of the search for the secret of the universe is about to begin again, amid new developments and renewed hopes from particle physicists. Even before its renovation, the collider had been hinting that nature might be hiding something spectacular. Mitesh Patel, a particle physicist at Imperial College London conducting an experiment at CERN, described the data from his previous runs as “the most exciting set of results I’ve ever seen in my professional life.”
A decade ago, physicists at CERN made global headlines with the discovery of the Higgs boson, a long-sought particle that imparts mass to all other particles in the universe. What is left to find? Almost everything, say the optimistic physicists.
When the CERN collider first ignited in 2010, the universe was at stake. The machine, the largest and most powerful ever built, was designed to find the Higgs boson. That particle is the cornerstone of the Standard Model, a set of equations that explains everything scientists have been able to measure about the subatomic world.
But there are deeper questions about the universe that the Standard Model doesn’t explain: Where did the universe come from? Why is it made of matter instead of antimatter? What is the “dark matter” that pervades the cosmos? How does the Higgs particle itself have mass?
Physicists hoped that some answers would materialize in 2010 when the large collider first ignited. Nothing turned up except the Higgs, in particular, no new particles that could explain the nature of dark matter. Frustratingly, the Standard Model remained steadfast.
The collider was closed in late 2018 for extensive upgrades and repairs. Based on the current schedule, the collider will run until 2025 and then shut down for two more years for other extensive upgrades to be installed. Among this set of updates are improvements to the giant detectors that sit at the four points where proton beams collide and analyze debris from the collision. Starting in July, those detectors will have their work cut out for them. The proton beams were compressed to make them more intense, increasing the chances of protons colliding at crossing points, but creating confusion for detectors and computers in the form of multiple aerosols of particles that must be distinguished from each other.
“The data will come in at a much faster rate than we are used to,” said Dr. Patel. Where before only a couple of collisions occurred at each lightning crossing, there would now be more than five.
“That makes our lives more difficult in a way because we have to be able to find the things that interest us among all these different interactions,” he said. “But it means there’s a higher chance of seeing what you’re looking for.”
Meanwhile, a variety of experiments have revealed possible cracks in the standard model and hinted at a broader and deeper theory of the universe. These results involve unusual behaviors of subatomic particles whose names are unknown to most of us on the cosmic bleachers.
Take the muon, a subatomic particle that briefly became famous last year. Muons are often called fat electrons; they have the same negative electrical charge but are 207 times more massive. “Who ordered that?” said physicist Isador Rabi when muons were discovered in 1936.
No one knows where muons fit into the grand scheme of things. They are created by cosmic ray collisions, and in collider events, and radioactively decay within microseconds into a fizz of electrons and the ghostly particles called neutrinos.
Last year, a team of about 200 physicists associated with the Fermi National Accelerator Laboratory in Illinois reported that muons spinning in a magnetic field had wobbled significantly faster than predicted by the Standard Model.
The discrepancy with theoretical predictions occurred to the eighth decimal place of the value of a parameter called g-2, which described 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 that would briefly materialize around the muon and affect its properties. Confirming the existence of particles would finally break the Standard Model.
But two groups of theorists are still working to reconcile their predictions of what g-2 should be, while they await more data from the Fermilab experiment.
“The g-2 anomaly is still very much alive,” said Aida X. El-Khadra, a physicist at the University of Illinois who helped lead a three-year effort called the Muon g-2 Theory Initiative to establish a consensus prediction. “Personally, I am optimistic that the cracks in the Standard Model will add up to an earthquake. However, the exact position of the cracks may still be a moving target.”
The muon also figures in another anomaly. The main character, or perhaps the villain, in this drama is a particle called a B quark, one of the six varieties of quark that make up heavier particles like protons and neutrons. B stands for background or, perhaps, beauty. Such quarks occur in two-quark particles known as B mesons. But these quarks are unstable and tend to fall apart in ways that appear to violate the Standard Model.
Some rare B-quark decays involve a chain of reactions, ending in a different type of lighter quark and a pair of light particles called leptons, either electrons or their fat cousins, muons. The standard model holds 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.) But Dr. Patel and his colleagues have found more electron pairs than muon pairs, violating a principle called lepton universality.
“This could be a Standard Model killer,” said Dr Patel, whose team has been investigating B quarks with one of the large detectors at the Large Hadron Collider, LHCb. This anomaly, like the muon magnetic anomaly, points to an unknown “influencer”: a particle or force that interferes with the reaction.
One of the most dramatic possibilities, if this data holds up in the collider’s next run, says Dr. Patel, is a subatomic speculation called a leptoquark. If the particle exists, it could bridge the gap between two classes of particles that make up the material universe: the light leptons (electrons, muons and also neutrinos) and heavier particles like protons and neutrons, which are made of quarks. Interestingly, there are six types of quarks and six types of leptons.
“We’re going into this race with more optimism that a revolution could be on the way,” said Dr. Patel. “Fingers crossed.”
There is another particle in this zoo that is behaving strangely: the W boson, which transmits the so-called weak force responsible for radioactive decay. In May, physicists with the Collider Detector at Fermilab, or CDF, reported a 10-year effort to measure the mass of this particle, based on some 4 million W bosons collected from collisions at Fermilab’s Tevatron, which was the world’s most powerful collider. until the Large Hadron Collider was built.
Based on the Standard Model and previous mass measurements, the W boson should weigh about 80.357 billion electron volts, the preferred unit of mass-energy for physicists. By comparison, the Higgs boson weighs 125 billion electron volts, about as much as an atom of iodine. But the CDF measurement of W, the most accurate ever, came out higher than expected at 80.433 billion. The experimenters calculated that there was only one chance in 2 trillion (7 sigma, in physics jargon) that this discrepancy was a statistical fluke.
The mass of the W boson is connected to the masses of other particles, including the infamous Higgs. So this new discrepancy, if it holds up, could be another crack in the standard model.
Still, all three anomalies and theorists’ hopes for a revolution could evaporate with more data. But for optimists, all three point in the same encouraging direction toward hidden particles or forces that interfere with “known” physics.
“So a new particle that could explain both g-2 and W mass could be within reach of the LHC,” said Kyle Cranmer, a physicist at the University of Wisconsin who works on other experiments at CERN.
John Ellis, a theorist at CERN and King’s College London, noted that at least 70 papers have been published suggesting explanations for the new mass discrepancy W.
“Many of these explanations also require new particles that can be accessible to the LHC,” he said. “Did I mention dark matter? So, there are many things to consider!”
Of the upcoming race, Dr. Patel said: “It’s going to be exciting. It will be hard work, but we are very interested to see what we have and if there is anything really exciting in the data.”
He added: “You could go through a scientific career and not be able to say that once. So it feels like a privilege.”