In April, scientists from the European Center for Nuclear Research, or CERN, outside Geneva, once again launched their space pistol, the Large Hadron Collider. After a three-year halt to repairs and upgrades, the collider resumed the launch of protons – the bare entrails of hydrogen atoms – around its 17-kilometer electromagnetic underground runway. In early July, the collider will begin to break these particles together to create sparks of primary energy.
And so the great game of hunting for the secret of the universe is about to start again, against the background of new developments and refreshed hopes of particle physicists. Even before its renovation, the collider hinted that nature may be hiding something spectacular. Mitesh Patel, a particle physicist at Imperial College London who is conducting an experiment at CERN, describes data from his previous research as “the most exciting set of results I’ve seen in my professional life.”
A decade ago, CERN physicists made global headlines with the discovery of the Higgs boson, a long-sought-after particle that adds mass to all other particles in the universe. What remains to be found? Almost everything, say optimistic physicists.
When the CERN collider was first launched in 2010, the universe was ready to be captured. The machine, the largest and most powerful ever created, was designed to find the Higgs boson. This particle is the keystone of the Standard Model, a set of equations that explains everything scientists have been able to measure for 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 and not antimatter? What is the “dark matter” that fills space? How does the Higgs particle itself have mass?
Physicists hoped that some answers would materialize in 2010, when the big collider was first turned on. Nothing came up except Higgs – in particular, no new particles that could explain the nature of dark matter. It is disappointing that the Standard Model remained unshakable.
The collider was stopped at the end of 2018 for extensive upgrades and repairs. According to the current schedule, the collider will operate until 2025 and then will be closed for another two years to install other extensive upgrades. Among this set of improvements are improvements to the giant detectors, which are located at the four points where the proton beams collide and analyze the remnants of the collision. From July, these detectors will have their work. Proton beams are compressed to make them more intense, increasing the chances of protons colliding at points of intersection – but creating confusion for detectors and computers in the form of multiple sprays of particles that need to be distinguished from each other.
“The data will come at a much faster pace than we’re used to,” said Dr. Patel. Where once there were only a few collisions at each beam intersection, there will now be more than five.
“It makes our lives more difficult in a way, because we need to be able to find the things that interest us in 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 hinted at a broader, more in-depth theory of the universe. These results include the rare behavior of subatomic particles whose names are unknown to most of us in space tribunes.
Take the muon, a subatomic particle that became known briefly last year. Muons are often called fat electrons; they have the same negative electric charge, but are 207 times more massive. “Who ordered this?” physicist Isador Rabi said when the muons were discovered in 1936.
No one knows where the muons fit into the big picture. They are created by collisions of cosmic rays – and in collider events – and decay radioactively in microseconds in the hiss of electrons and ghostly particles called neutrinos.
Last year, a team of about 200 physicists linked to the National Fermi Accelerator Laboratory in Illinois reported that muons rotating in a magnetic field swung significantly faster than predicted by the Standard Model.
The discrepancy with theoretical predictions came on the eighth decimal place of the value of a parameter called g-2, which describes how a particle reacts to a magnetic field.
Scientists attribute the partial but real difference of the quantum whisper to as yet unknown particles, which will materialize briefly around the muon and affect its properties. Confirming the existence of the particles would ultimately violate 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 Fermilab’s experiment.
“The g-2 anomaly is still very much alive,” said Aida H. El-Qadra, a physicist at the University of Illinois who helped run a three-year effort called the Muon g-2 Theory Initiative to establish a consensus forecast. . “Personally, I am optimistic that the cracks in the Standard Model will lead to an earthquake. However, the exact position of the cracks may still be a moving target. “
The muon also appears in another anomaly. The protagonist or perhaps villain in this drama is a particle called the B quark, one of the six types of quark that make up heavier particles such as protons and neutrons. B means bottom or, perhaps, beauty. Such quarks are found in two-quark particles known as B mesons. But these quarks are unstable and tend to break down in ways that seem to violate the Standard Model.
Some rare decays of the B quark include a series of reactions ending in a different, lighter type of quark and a pair of light particles called leptons, or electrons, or their complete cousins, muons. The standard model states that electrons and muons are equally likely to occur 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 discovered more electron pairs than muon pairs, violating a principle called lepton universality.
“This could be a standard model killer,” said Dr. Patel, whose team studied the B quarks with one of the Large Hadron Collider’s major detectors, the LHCb. This anomaly, like the muon’s magnetic anomaly, hints at an unknown “influencer” – a particle or force that interferes with the reaction.
One of the most dramatic possibilities if this data stays in the upcoming collider, says Dr. Patel, is a subatomic speculation called leptoquark. If the particle exists, it could 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 of quarks. Interestingly, there are six types of quarks and six types of leptons.
“We are entering this series with more optimism that there may be a revolution,” said Dr. Patel. “Fingers crossed.”
There is another particle in this zoo that behaves strangely: the W boson, which transmits the so-called weak force responsible for radioactive decay. In May, physicists with Collider Detector at Fermilab, or CDF, reported a 10-year effort to measure the mass of this particle based on about 4 million W bosons collected from collisions at Fermilab’s Tevatron, the world’s most powerful collider. . until the construction of the Large Hadron Collider.
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 the size of an iodine atom. But the CDF measurement of W, the most accurate ever made, was higher than predicted at 80.433 billion. The experimenters estimated that there is only one chance out of 2 trillion – 7-sigma, in the jargon of physics – this discrepancy is a statistical coincidence.
The mass of the W boson is related to the masses of other particles, including the famous Higgs. So this new discrepancy, if persisted, could be another crack in the Standard Model.
Yet all three anomalies and the hopes of revolution theorists can evaporate with more data. But for optimists, all three point in the same encouraging direction to hidden particles or forces that interfere with “famous” physics.
“So a new particle that can explain both g-2 and W mass could be in the LHC’s range,” 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 explaining the new W-table mismatch.
“Many of these explanations also require new particles that may be available to the LHC,” he said. “Did I mention dark matter?” So, there are many things to watch out for! ”
Regarding the upcoming run, Dr. Patel said: “It will be exciting. It’s going to be a tough job, but we really want to see what we have and if there’s anything really exciting about the data. “
He added: “You can go through a scientific career and not be able to say it once. So it feels like a privilege. “
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