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In Search for Higgs Boson, Physicists Report a Definite Maybe

13 December 2011 9:30 am
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MEYRIN, SWITZERLANDLike all good rumors, whispers that the long-sought Higgs boson has been spotted turn out to be half-true. This week, two teams of physicists working with the world's biggest atom smasherthe Large Hadron Collider (LHC) here at the European particle physics laboratory, CERNreported the latest results in their search for the Higgs, the fundamental particle that is key to physicists' explanation of how all particles get their mass. Just as the rumors suggested, both teams report tantalizing signs that the Higgs is there and that it has a mass about 133 times that of the proton. But one team sees additional oddities, so the results haven't bowled everyone over.

The good news is that the two groups see consistent signals at roughly the same energy, reinforcing each other's results. They also see those signs via different "decay channels," the combinations of detectable particles that result when the Higgs falls apart. However, one team's data plots also show peaks and wiggles at other masses. In effect, that team sees too many signs of possible Higgses, at various masses, and that has some physicists worrying that even the cluster of hints that coincide may be a mirage created by statistical fluctuations.

"In my heart of hearts I desperately want the Higgs to be there," says Rob Roser of Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. "And yet I'm not even willing to bet your house on [the possible sighting] after seeing the data." John Ellis, a theorist at King's College London, is also circumspect: "I'd hold off a bit" on suggesting the Higgs has been seen.

The data come from two gargantuan particle detectors, called ATLAS and CMS, fed by the $5.5 billion LHC. The collider smashes protons together in the centers of the two detectors to try to blast massive new particles into existence. ATLAS and CMS search for signs of Higgses popping into existence and decaying less than a trillionth of a second later into telltale combinations of familiar particles as predicted by physicists' prevailing theory, the standard model. ATLAS and CMS each have their own thousands-strong team of scientists; in a seminar at CERN today, both reported the latest results of their searches.  

Expectations had been running high. After 2 years of operation, ATLAS and CMS should have enough data to see some evidence of the Higgs if it is there. At first blush, both teams appear to have fulfilled that expectation, reporting possible signstoo weak to claim discoveryof the Higgs decaying into more than one combination of particles.

Physicists have been down this road before. In July, both teams also reported possible traces of the Higgs, only to see them weaken with more data. However, last summer's hints consisted mainly of supposed Higgses decaying into a combination of particles, some of which the detectors cannot catch. So experimenters couldn't nail down the candidate Higgses' mass. Now, researchers have also spotted signs of the Higgs decaying into combinations of particles that allow them to snare every particle and so get a mass estimate. So they can create crucial "mass plots" that tally the number of candidate decays versus mass. Real Higgs decays should form a peak at definite mass on a background of false ones at random masses.

That's just what the ATLAS team has delivered in its search for the Higgs decaying to two photons. In the mass plot, a single peak pokes up above the background at a mass of 126 giga-electron volts (GeV). The peak is more molehill than mountain; there's roughly a 1-in-15 chance that a random fluctuation in the background would produce a similar-sized peak somewhere in the mass range of 110 to 150 GeV, explains Andrew Lankford, an ATLAS deputy spokesperson from the University of California, Irvine.

However, ATLAS researchers also searched for the Higgs decaying into two particles called Z bosons, each of which then decayed into either an electron and an antielectron or a muon (a cousin of the electron) and an antimuon. They see three candidate Higgs decays at 125 GeV with no other events in the vicinity. Taken together, the two types of decay neatly suggest that the Higgs might be there at 126 GeV; the chances that statistical noise could produce both signs are 1%. "We have something that could be a background fluctuation or it could be the beginning of a signal," Lankford says.

At first glance, CMS appears to see something quite similar. CMS experimenters have looked for the Higgs decaying in the same two ways as their ATLAS counterparts, and into three other combinations that provide less mass resolution. CMS researchers see peaks in all five decay channels, and that's strong evidence that they're on to something, says Guido Tonelli of the University of Pisa in Italy, spokesperson for the CMS experiment. Taken altogether, the five decay combinations show a signal around 124 GeV, basically in agreement with ATLAS's signal, and the chances that background could fake such a signal are about 1 in 35.

But a closer look at CMS's mass plots for the individual channels reveals extra humps that give some physicists pause. For example, in their Higgs-to-two-photons mass plot, CMS researchers see a small peak at 124 GeV that essentially matches the peak ATLAS researchers see. But CMS researchers also see another peak at about 133 GeV, which they take to be a background fluctuation because it doesn't appear in other decay channels. Similarly, they see two Higgs-to-electrons-and-muons decays pointing to a mass of 124 GeVplus three such decays at a mass of 119.5 GeV. They take the latter cluster of events to be a fluke, too.

With the current amount of data, extraneous peaks should pop up, Tonelli says. "One cannot expect to have a completely flat background distribution and an enormous peak," he says. Fermilab's Roser says he still has reservations. "The fact that CMS has these other thingsmaybe this one is a fluctuation and this one is a signalit kind of takes away your confidence," he says.

As to whether the coincidental signs of the Higgs will pan out, opinions vary. "I would be surprised if this disappeared," Tonelli says, "but it could happen." Asked if he thought the hints will hold up, Lankford says only: "God, we need more data."

Fortunately for all, the LHC should produce the data by the end of next year. For the moment, the machine has shut down for the winter. So researchers will have plenty of time for a long, spirited debate about whether they finally have their most prized quarry in their sights.

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