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Large Hadron Collider Sees Tantalizing Hint of Higgs Particle

Excess events suggest LHC is homing in on elusive particle.

By Geoff Brumfiel of Nature magazine

For now, physicists are only willing to call them "excess events," but fresh data from two experiments at the Large Hadron Collider (LHC) are hinting at something unusual--and it could be the most sought-after particle in all of physics.

Both ATLAS and the Compact Muon Solenoid (CMS) experiments are seeing an unusual surplus of events in a rough mass range of 130-150 gigaelectronvolts (energy and mass are used interchangeably in particle physics). The data are far from conclusive, but physicists believe this could be the first indication of the Higgs particle, believed to be responsible for the masses of other particles. The results were presented this afternoon at the Europhysics Conference on High Energy Physics in Grenoble, France.

Physicists familiar with the experiments urge caution. The new data are a long way from a discovery, says Matthew Strassler, a theoretical physicist at Rutgers University in New Jersey. "I would call it tantalizing."

The hung for Higgs

Since its prediction in 1964, the Higgs has been the most wanted particle in all of physics. The particle is part of the mechanism that endows all other particles with mass. The standard model of particle physics, which has been verified to astonishing accuracy, also requires a Higgs (or something like it) to unify the weak nuclear force, which regulates nuclear decays, and the electromagnetic force, which governs electricity, magnetism and light.

To track down the Higgs, high-energy physicists have built the LHC--a 27-kilometre ring housed at CERN, the European particle-physics laboratory near Geneva, that accelerates protons to near the speed of light and then smashes them together. The collisions can briefly create heavier particles. Those heavier particles then decay into a cascade of lighter particles that are picked up by building-sized detectors.

The two largest detectors, ATLAS and CMS, are now reporting excess numbers of lighter particles in the range of 130-150 GeV. That is smack in the middle of the range in which some physicists believe the Higgs particle might exist.

The statistics are far from conclusive, and the signal could yet disappear. "We really need to be very prudent; we really need to be systematic," says Guido Tonelli, the spokesperson for the CMS experiment. In the coming months, Tonelli says that CMS will focus its efforts on probing the unusual bump.

"I think that we have to be extremely careful," agrees Fabiola Gianotti, spokesperson for the ATLAS detector. Gianotti says that these low-energy excesses, and a few others seen at higher energies, could yet disappear when more data are collected.

Much of the increase seems to be in the decay of particles called W bosons. W bosons help to moderate the weak nuclear force, and theory predicts that the Higgs can sometimes decay into a pair of Ws. Both experiments have seen an unusually large number of W pairs. But they do not have enough events to confirm the excess as a signal. What's more, the way W particles themselves decay means that they cannot give a precise value of the Higgs' mass.

Even if the excess is a real signal, it could be down to a theoretical miscalculation, says Strassler. Predicting the precise quantities in which W bosons decay can be tricky, and the standard model may simply need an adjustment to explain the excess.

The picture will change rapidly in the coming months. At present, the LHC is "going gangbusters", says Vivek Sharma, a researcher at the University of California, San Diego, who is heading the Higgs search at the CMS. ATLAS and the CMS will combine their results at next month's Lepton Photon meeting in Mumbai. Additional data this autumn and winter should either confirm the excess as a real signal or show once and for all that a Higgs, at reasonably low masses, doesn't exist.

"I'm excited," Tonelli says. "We have been working in this field for 20 years and now in a matter of months we'll know the answer."

This article is reproduced with permission from the magazine Nature. The article was first published on July 22, 2011.

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