LHC results negate existence of new particle at higher energies

August 06, 2016 05:24 pm | Updated 05:24 pm IST

Scientists at the Large Hadron Collider show that the 750 GeV bump is not due to a particle state.

Scientists at the Large Hadron Collider show that the 750 GeV bump is not due to a particle state.

A great “might have been” for the universe, or at least for the people who study it, disappeared on August 5.

In December, two teams of physicists working at CERN’s Large Hadron Collider reported that they might have seen traces of what could be a new fundamental constituent of nature, an elementary particle that is not part of the Standard Model that has ruled particle physics for the last half-century.

A bump on a graph signalling excess pairs of gamma rays was most likely a statistical fluke, they said. But physicists have been holding their breath ever since.

If real, the new particle would have opened a crack between the known and the unknown, affording a glimpse of quantum secrets undreamed of even by Einstein. Answers to questions like why there is matter but not antimatter in the universe, or the identity of the mysterious dark matter that provides the gravitational glue in the cosmos. In the few months after the announcement, 500 papers were written trying to interpret the meaning of the putative particle.

On August 5, physicists from the same two CERN teams reported that under the onslaught of more data, the possibility of a particle had melted away.

“We don’t see anything,” said Tiziano Camporesi of CERN, the European Organization for Nuclear Research and a spokesman for one of the detector teams known as CMS, on the eve of the announcement. “In fact, there is even a small deficit exactly at that point.”

His statement was echoed by a member of the competing team, known as ATLAS. James Beacham, of Ohio State University, said, “As it stands now, the bumplet has gone into a flatline.”

The new results were presented in Chicago at the International Conference of High Energy Physics, ICHEP for short, by Bruno Lenzi of CERN for the ATLAS team, and Chiara Rovelli for their competitors named for their own detector called CMS, short for Compact Muon Solenoid.

The presentations were part of an outpouring of dozens of papers from the two teams on the results so far this year from the collider, all of them in general agreement with the Standard Model.

The non-result has further deepened an already deep mystery about the famous Higgs boson, which explains why other particles have mass, and whose discovery resulted in showers of Champagne and Nobel Prizes four years ago.

The Higgs, one of the heaviest elementary particles known, weighs about 125 billion electron volts, in the units of mass and energy favoured by particle physicists — about as much as an entire iodine atom. That, however, is way too light by a factor of trillions according to standard quantum calculations, physicists say, unless there is some new phenomenon, some new physics, exerting its influence on the universe and keeping the Higgs mass from zooming to cataclysmic scales. That would mean new particles.

“We have seen the Higgs, we expect to see something else,” said Lisa Randall, a Harvard particle theorist who was not part of the CERN experiments. Hence the excitement over the December bump. Its mass, about 750 billion electron volts, was in the range where something should happen.

“It would have been great if it was there,” Randall said. “It is the sort of thing they should be looking for if we want to understand the Higgs.”

For a long time, the phenomenon physicists have thought would appear to save the day is a conjecture known as supersymmetry, which comes with the prediction of a whole new set of elementary particles, known as WIMPs, for weakly interacting massive particles, one of which could comprise the dark matter that is at the heart of cosmologists’ dreams.

But so far, WIMPs have not shown up either in the collider or in underground experiments designed to detect wimps floating through space. Neither has evidence for an alternative idea that the universe has more than three dimensions of space.

The Large Hadron Collider is expected to run for another 20 years. So, these could still be exciting times.

The CERN collider was built at a cost of some $10 billion, to speed protons around an 18-mile underground track at more than 99 percent of the speed of light, and smash them together with a combined energy of 14 trillion electron volts, in search of new particles and forces of nature. The more energy they can pour into these collisions, microscopic samples of primordial fire, by virtue of Einstein’s equivalence of mass and energy, the more massive particles can come out of them. During its first two years of running the collider, hampered by electrical problems, ran at only half power but still managed to find the Higgs boson.

— New York Times News Service

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