As the Large Hadron Collider revives, physicists’ hopes soar

In April, scientists at the European Center for Nuclear Research, or CERN, outside Geneva, once again fired their cosmic cannon, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider has resumed firing protons — the bare guts of hydrogen atoms — around its 27-mile electromagnetic underground racetrack. In early July, the accelerator will collide these particles together to create sparks of primordial energy.

And so the great game of the hunt for the secret of the universe is about to begin again, amid new developments and the renewed hopes of particle physicists. Even before the renovation, the collider had produced hints 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 runs 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 particle that imparts mass to all other particles in the universe. What else is there to find? Almost anything, optimistic physicists say.

When the CERN collider first turned on in 2010, the universe was up for grabs. 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” engulfing the cosmos? How does the Higgs boson itself have mass?

Physicists hoped some answers would come out in 2010, when the big accelerator was first turned on. Nothing showed up except the Higgs — in particular, no new particle that could explain the nature of dark matter. Frustratingly, the standard model remained unwavering.

The accelerator was shut down in late 2018 for extensive upgrades and repairs. Under the current schedule, the accelerator will run until 2025 and then shut down for another two years to install other extensive upgrades. This suite of upgrades includes improvements to the giant detectors located at the four points where the proton beams collide and analyze the collision debris. From July, those detectors will have to do their job. The proton beams have been squeezed to make them more intense, increasing the likelihood of protons colliding at the intersections — but confusion arises for the detectors and computers in the form of multiple sprays of particles that must be distinguished from each other.

“The data is coming in much faster than we were used to,” said Dr. patella. Where once there were only a few collisions at each jet crossing, there are now more than five.

“That makes our lives more difficult in a way, because in all those different interactions we have to be able to find the things that we are interested in,” he said. “But it means there’s a higher chance of seeing the thing you’re looking for.”

Meanwhile, several experiments have revealed possible cracks in the Standard Model — and have suggested a broader, deeper theory of the universe. These results relate to rare behavior of subatomic particles whose names are unknown to most of us in the cosmic stands.

Take the muon, a subatomic particle that briefly gained fame last year. Muons are often called fat electrons; they have the same negative electrical charge, but are 207 times as heavy. “Who ordered that?” said the physicist Isador Rabi when muons were discovered in 1936.

No one knows where muons fit into the grand scheme of things. Created by cosmic ray collisions — and upon collisions — they radioactively decay in microseconds into a vortex of electrons and the ghostly particles called neutrinos.

Last year, a team of some 200 physicists at the Fermi National Accelerator Laboratory in Illinois reported that muons spinning in a magnetic field fluctuated significantly faster than predicted by the Standard Model.

The discrepancy with theoretical predictions came 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 whispers of as-yet-unknown particles that would briefly materialize around the muon, affecting 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, as 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 develop a to establish a consensus forecast. “Personally, I’m optimistic that the cracks in the Standard Model will lead to an earthquake. However, the exact position of the cracks could still be a moving target.”

The muon also occurs in another anomaly. The main character, or perhaps the villain, in this drama is a particle called a B quark, one of six types of quarks that make up heavier particles, such as protons and neutrons. B stands for soil or maybe beauty. Such quarks occur in two quark particles known as B mesons. But these quarks are unstable and tend to break apart in ways that seem to violate the Standard Model.

Some rare fakes of a B quark involve a daisy chain reaction, ending in another, lighter kind of quark and a few lightweight particles called leptons, either electrons or their fat cousins, muons. The Standard Model states that electrons and muons are equally likely to appear in this reaction. (There is a third, heavier lepton, the tau, but it decays too quickly to be observed.) But Dr. Patel and his colleagues have found more electron pairs than muon pairs, which violates a principle called lepton universality.

“This could be a Standard Model killer,” said Dr. Patel, whose team examined the B quarks with one of the large detectors at the Large Hadron Collider, LHCb. This anomaly, like the magnetic anomaly of the muon, indicates an unknown “influencer” – a particle or force that disrupts the reaction.

One of the most dramatic possibilities, if this data holds up in the coming collider run, says Dr. Patel, is a subatomic speculation called a leptoquark. If the particle exists, it can bridge the gap between two classes of particles that make up the material universe: lightweight leptons — electrons, muons, and also neutrinos — and heavier particles like protons and neutrons, which are made of quarks. Temptingly enough, there are six types of quarks and six types of leptons.

“We’re going into this run with more optimism that a revolution could be coming,” said Dr. patella. “Fingers crossed.”

There is another particle in this zoo that behaves strangely: the W particle, which transmits the so-called weak force responsible for radioactive decay. In May, physicists with the Collider Detector in Fermilab, or CDF, reported on a 10-year effort to measure the mass of this particle, based on about 4 million W bosons harvested by collisions in Fermilab’s Tevatron, the world’s most powerful collider until the Large Hadron Collider 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. In comparison, the Higgs boson weighs 125 billion electron volts, about as much as an iodine atom. But the CDF measurement of the W, the most accurate ever done, came in higher than predicted at 80.433 billion. The researchers 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 particle is linked to the masses of other particles, including the infamous Higgs. So this new discrepancy, if it holds up, could be another crack in the stock 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 to hidden particles or forces that disrupt “familiar” physics.

“So a new particle that could account for both g-2 and W masses could be within reach at 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 Kings College London, noted that at least 70 papers have been published suggesting explanations for the new W-mass discrepancy.

“Many of these explanations also require new particles that may be accessible to the LHC,” he said. “Did I mention dark matter? So plenty to watch out for!”

Speaking of the upcoming run, Dr. Patel: “It will be exciting. It will be hard work, but we are very curious to see what we have and if there is something really exciting in the data.”

He added: “You could go through a scientific career and not say that once. So it feels like a privilege.”

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