In April, scientists at the European Center for Nuclear Research, or CERN, outside Geneva, fired back their cosmic weapon, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider has restarted the launch of protons, the bare entrails of hydrogen atoms, around its 17-mile underground electromagnetic track. In early July, the collider will begin to collide with these particles to create sparks of primordial energy.
And so the great game of hunting the secret of the universe is about to begin again, amidst new developments and the renewed hopes of particle physicists. Even before its renewal, the collider had been giving clues that nature could be hiding something spectacular. Mitesh Patel, a particle physicist at Imperial College London conducting an experiment at CERN, described the data from his previous tests as “the most exciting set of results I’ve ever seen in my professional life”.
A decade ago, CERN physicists appeared in the headlines with the discovery of the Higgs boson, a highly sought-after particle that imparts too much to every other particle in the universe. What’s left to find? Almost everything, say 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. This particle is the key to 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 does not explain: where does the universe come from? Why is it made of matter rather than antimatter? What is the “dark matter” that floods the cosmos? How much of the same Higgs particle is too much?
Physicists expected some answers to materialize in 2010, when the large collider was first turned on. Nothing appeared except the Higgs, in particular, any new particles that could explain the nature of dark matter. Frustratingly, the standard model remained intact.
The collider closed in late 2018 for major upgrades and repairs. Under the current schedule, the collider will run until 2025 and then stop for two more years for other major updates to be installed. Among this set of upgrades are improvements to the giant detectors that sit at the four points where the proton beams collide and analyze the debris from the collision. As of July, these detectors will be out of work. Proton beams have been compressed to make them more intense, increasing the chances of protons colliding at intersection points, but creating confusion for detectors and computers in the form of multiple particle aerosols that must be distinguished between Yes.
“The data will come at a much faster rate than we are used to,” Dr. Patel said. Where before there were only a couple of collisions at each beam crossing, now there would be more than five.
“This makes life more difficult for us in some way because we need to be able to find the things that interest us between all these different interactions,” he said. “But that means you’re more likely to see what you’re looking for.”
Meanwhile, a variety of experiments have revealed possible cracks in the standard model and have hinted at a broader and deeper theory of the universe. These results involve rare behaviors of subatomic particles whose names are unknown to most of us in the cosmic ranks.
Take the muon, a subatomic particle that briefly became famous last year. Muons are often called fatty electrons; they have the same negative electric charge but are 207 times more massive. “Who commissioned it?” said physicist Isador Rabi when muons were discovered in 1936.
No one knows where the muons fit into the grand scheme of things. They are created by cosmic ray collisions, and in collision events, and disintegrate radioactively in microseconds in an envelope of electrons and phantom particles called neutrinos.
Last year, a team of about 200 physicists associated with the Fermi National Accelerator Laboratory in Illinois reported that the rotating muons in a magnetic field had staggered significantly faster than expected by the standard model.
The discrepancy with the theoretical predictions occurred in the eighth decimal place of the value of a parameter called g-2, which described how the particle responds to a magnetic field.
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 the 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 waiting for 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 prediction. of consensus. “Personally, I’m optimistic that the cracks in the standard model will add to an earthquake. However, the exact position of the cracks may still be a moving target.”
The muon also appears in another anomaly. The main character, or perhaps the villain, of this drama is a particle called B quark, one of the six varieties of quarks that make up heavier particles such as protons and neutrons. B means background or, perhaps, beauty. These quarks occur in particles of two quarks known as mesons B. But these quarks are unstable and prone to break down in ways that appear to violate the standard model.
Some rare decays of a B quark involve a chain of reactions that end in a different, lighter type of quark and a pair of light particles called leptons, either electrons or their first thick, muons. The standard model holds that electrons and muons have the same probability of appearing in this reaction. (There is a third heavier lepton called tau, but it decays too fast to be observed.) But Dr. Patel and his colleagues have found more electron pairs than muon pairs, violating a principle called the universality of lepton.
“This could be a killer of the standard model,” said Dr. Patel, whose team has been investigating B-quarks with one of the large detectors of the Large Hadron Collider, LHCb. This anomaly, like the muon’s magnetic anomaly, indicates an unknown “influencer”: a particle or force that interferes with the reaction.
One of the most dramatic possibilities, if this data is kept in the next collider, says Dr. Patel, is a subatomic speculation called leptoquark. If the particle exists, it could bridge the gap between two classes of particles that make up the material universe: light leptons (electrons, muons, and also neutrinos) and heavier particles such as protons and neutrons, which are made of quarks. Temptingly, there are six types of quarks and six types of leptons.
“We’re going into this race with more optimism that there could be a revolution,” Dr. Patel said. “Fingers crossed.”
In this zoo there is another particle that behaves strangely: the W boson, which transmits the so-called weak force responsible for radioactive decay. In May, physicists with the Fermilab Collider Detector, or CDF, reported a 10-year effort to measure the mass of this particle, based on about 4 million W bosons harvested from collisions in Fermilab’s Tevatron, which was the world’s most powerful collider. until the Great Hadron Collider was built.
According to the standard model and previous mass measurements, the W boson should weigh about 80,357 million volts, the unit of energy too favored by physicists. In comparison, the Higgs boson weighs 125 billion volts, about the size of an iodine atom. But the W’s CDF measurement, the most accurate ever made, was higher than expected at 80.433 billion. Experimenters calculated that there was only a 2 billion (7-sigma, in physics slang) possibility that this discrepancy was a statistical coincidence.
The mass of the W boson is connected to the masses of other particles, including the famous Higgs. Therefore, this new discrepancy, if maintained, could be another loophole in the standard model.
However, the three anomalies and the hopes of the theorists of 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 the g-2 and W masses could be available to the LHC,” said Kyle Cranmer, a physicist at 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 articles have been published suggesting explanations for the new discrepancy in the W mass.
“Many of these explanations also require new particles that may be accessible to the LHC,” he said. “Did I mention dark matter? So there’s a lot to keep in mind!”
On the next race, Dr. Patel said: “It’s going to be exciting. It’s going to be hard work, but we’re really looking forward to seeing what we have and if there’s anything really exciting about the data.”
He added: “You could go through a scientific career and not be able to say it once. So it feels like a privilege.”