Yearning for new physics at CERN

The world’s biggest and most expensive time machine is running again. Underneath the fields and shopping centres on the French-Swiss border outside Geneva, Switzerland, in the Large Hadron Collider, the subatomic particles known as protons are zooming around a 17-mile electromagnetic racetrack and banging into one another at the speed of light, recreating conditions of the universe when it was only a trillionth of a second old. Some 5,000 physicists are back at work here at CERN, the European Organisation for Nuclear Research, watching their computers sift the debris from primordial collisions in search of new particles and forces of nature.

Science is knocking on heaven’s door, as the Harvard physicist Lisa Randall put it in the title of her book about particle physics. But what if nobody answers? What if there is nothing new to discover? That prospect is now a cloud hanging over the physics community. It has been five years and more than seven quadrillion collisions of protons since 2012, when the collider discovered the Higgs boson, the particle that explains why some other elementary particles have mass. That achievement completed an edifice of equations called the Standard Model, ending one significant chapter in physics.

A 2015 bump in the collider data hinted at a new particle, inspiring a flood of theoretical papers before it disappeared into the background noise as just another fluke of nature. But since then, the silence from the frontier has been ominous. “These are difficult times for the theorists,” Gian Giudice, the head of CERN’s theory department, said. “Our hopes seem to have been shattered. We have not found what we wanted.” What the world’s physicists have wanted for almost 30 years is any sign of a phenomenon called supersymmetry, which has hovered just out of reach like a golden apple, a promise of a hidden mathematical beauty at the core of reality.

Theorists in the 1970s posited a relationship between the particles that carry forces, like the photon that conveys electromagnetism or light, and the basic constituents of matter, electrons and quarks. If the theory of supersymmetry is correct, there should be a whole new set of elementary particles to be discovered, so-called super-partners of the quarks, electrons and other particles we already know and love.

Mystery particles

Colliders get their mojo from Einstein’s equivalence of mass and energy. When a pair of protons collide in the Large Hadron Collider, they recreate a smidgen of the original Big Bang that jump-started the cosmos. Whatever forms of matter can be made from that bank of energy can reappear and briefly strut their stuff through labyrinths of electronic detectors and computers. Every time colliders get a little more energy to spend, scientists get access to realms of time, nature and possibility we have never experienced, and we get a little closer to the mathematical bones of reality. The Large Hadron Collider was designed to collide protons with energies of seven trillion electron volts a piece. That was enough, physicists knew, to discover the Higgs or to prove that it was wrong.

Many theorists had also hoped that supersymmetrical particles would show up when the Large Hadron Collider was finally turned on in 2010. Indeed the mystery particles could have shown up even earlier, in the collider’s predecessors, according to some versions of the theory. In May, a new analysis by the 3,000 physicists monitoring the big Atlas detector reported no hints of superparticles up to a mass of almost two trillion electron volts.

The idea that the Large Hadron Collider would discover the Higgs boson but nothing else has long been physicists’ worst nightmare. Among other things, it would leave them with no explanation for their greatest achievement: the Higgs itself. According to CERN, the long-sought boson weighs 125 billion electron volts or as much as a whole iodine atom. But that is too light, according to theoretical calculations. The mass of the Higgs should be some thousands of quadrillion times as high. The cause is quantum weirdness, one principle of which is that anything that is not forbidden will happen. That means the Higgs calculation must include the effects of its interactions with all other known particles, including so-called virtual particles that can wink in and out of existence.

Theorists have to doctor their equations for the Higgs and other numbers to come out right under the Standard Model. But when the alleged supersymmetric particles are inserted in the mix, a miracle occurs. They cancel out the effects of the other particles, leaving the Higgs with a perfectly finite, normal mass. This is the way nature should be, they say. Supersymmetry is such a general idea that there is always another version that can be proposed. Guido Tonelli, a professor at the University of Pisa in Italy who was one of the leaders of the Higgs hunt, said, “For a while we thought we could discover the Higgs and new physics at the same time — that was very exciting.” But he said he did not share his colleagues’ depression that it did not happen: “The fact that the Higgs fits the Standard Model means new physics is farther up the energy scale. We know it is there, we just don’t know if it is tomorrow or the next decade.”

Opportunity for new ideas

By the end of 2018, the collider would have logged some 15,000 trillion collisions. If something does not show up by then, Gian said, it will be time to go back to the drawing board. “It’s a high point of research when we have confusion,” he said. “Confusion,” he explained, “means an opportunity for new ideas.” As the universe expands and evolves during the Big Bang, the Higgs field, of which the boson is an expression, undergoes phase transitions, like water turning to ice.

At some point, it gets stuck. “What fixes the value of the Higgs is the history of the universe,” Gian said. But that would make the Higgs field unstable over very long time frames and could eventually collapse, dissolving what we think of as reality.

Another possibility, which is anathema to many card-carrying Einsteinians, is that these problematic numbers are due to random chance. There are an infinite number of possible universes with different Higgs masses, but only one that has the capability of evolving into stars, planets, us.

One encouraging hint has come from recent CERN studies of a weird short-lived little particle called a B-meson, which among other things flips back and forth from being itself and its antimatter opposite trillions of times a second. According to the Standard Model, these particles should have an equal chance of producing electrons as their fat cousins the muons, when they decay in certain ways.

However, measurements at the CERN collider have shown a definite propensity for the mesons to underproduce muons, as reported at CERN in April. The same quantum weirdness that blows up the theoretical mass of the Higgs might also be at work here, physicists say, hinting at a new very massive particle called a leptoquark. Or it could just be a fluke. “Needless to say, if these signals hold up then it would be an extremely big deal, but it is too soon to say,” said Guy Wilkinson, an Oxford
professor who is the spokesman for the LHCb collaboration.