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Hints of Higgs Boson Appear Weaker
22 August 2011 1:12 pm
Now for a bit of a reality check. Last month, physicists working with the world's highest-energy atom-smasher reported possible evidence of the long-sought Higgs boson, the last missing piece of scientists' standard model of fundamental particles and the key to physicists' explanation of how all particles get their mass. Today, however, the same two teams reported that, with more data, those signs appear slightly weaker, suggesting that they could be a statistical fluctuation in the "background" produced by decays of familiar particles. Still, the curious excess of possible Higgs bosons remains.
Physicists are hoping that the 27-kilometer-long Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, near Geneva, Switzerland, will produce the Higgs boson as the LHC smashes protons at unprecedented energies. Researchers then hope to observe the fleeting Higgs bosons as they decay into various combinations of known particles. Last month, the teams working with ATLAS and CMS, the two largest particle detectors fed by the LHC, both reported slight excesses of events that appear to be Higgs bosons decaying primarily into two particles known as W bosons. Intriguingly, those excesses appeared to be in the same mass region, between about 110 and 170 giga-electron volts (GeV)—about 117 and 181 times the mass of a proton.
But both teams have now analyzed substantially more data than they had in July, and the signals have not grown stronger, as they reported today at the biannual Lepton Photon conference in Mumbai, India. "If you add 50% more data, you expect the signal to grow, and it does not," says Vivek Sharma, a physicist at the University of California, San Diego, who reported the CMS results. Both CMS and ATLAS have now combed through roughly 50% more data for some decay "channels," although ATLAS has searched through more than twice as much data looking for the Higgs to decay in one particular way.
Still, neither group can prove that their supposed signal is just background fluctuation, says Bill Murray, a physicist at Rutherford Appleton Laboratory near Didcot, United Kingdom, and senior convener of the ATLAS Higgs working group. "It's clear that the old data and the new data are telling different stories," Murray says. "Which one is telling us the right story, we can't yet say."
The real news from the conference is that both ATLAS and CMS have now analyzed enough data and looked at enough ways in which the Higgs might decay to say that with high confidence where does not exist: essentially, across the entire mass range from 145 GeV to 400 GeV. Physicists generally don't expect it to be more massive than that. That means that between them, ATLAS and CMS have now backed the particle into a corner between 115 GeV (the limit set by a previous collider) and 145 GeV. And at the rate the two experiments are collecting data, working together they should be able to spot the particle or rule it out by year's end. "If the Higgs exists, is has to be there," Sharma says. "And if it's not there, it will be known to be science-fiction by December."
Murray is a bit more cautious. He notes that statistically, the limits prove that there is less than a 5% chance that the Higgs is there and physicists didn't spot it. "It's enough for something you don't really think is there," he says. "It's not enough to exclude something like the Higgs." Still, it's intriguing that physicists are now talking about when they can say they've proved that the Higgs doesn't exist. If that happens, theorists will have some serious explaining to do.