August 23, 2011 | 41
A few years ago, celebrated British physicist Stephen Hawking was widely reported in the press to have placed a provocative public bet that the LHC (along with all particle accelerators that preceded it) would never find the Higgs boson, the so-called “God particle” believed responsible for having imbued massive particles with their mass when the universe was very young.
His pronouncement caused a stir in the global physics community, and the Scottish physicist Peter Higgs, whose name had gotten attached to the hypothetical particle (Higgs had done some work in the 1960s, as had several other physicists, paving the way for the theoretical existence of the mass-imparting boson) took the challenge personally, complained about Hawking, and later lamented that to answer Hawking’s challenge would have been “like criticizing the late Princess Diana.”
In fact, informal polls of physicists over the last decade have shown that an overwhelming majority believed that the existence of the Higgs was a foregone conclusion and that all that was needed was simply to run the LHC long enough: the Higgs would eventually show up. Hawking—known for controversial and contrarian pronouncements—was seen as simply throwing around his weight.
But the Higgs boson never appeared. Running continually at an unprecedented energy level of seven trillion electron volts since March 31, 2010, the LHC has been amassing petabytes of data that are being analyzed by a grid of interlinked computers worldwide in search of the missing boson. And yesterday, August 22, at the Biennial International Symposium on Lepton-Photon Interactions at the Tata Institute of Fundamental Research in Mumbai, India, the bombshell was dropped: CERN scientists declared that over the entire range of energy the Collider had explored—from 145 to 466 billion electron volts—the Higgs boson is excluded as a possibility with a 95% probability.
The search for the Higgs is a statistical hunt that involves looking at the particles that emanate from the high-energy collisions of protons inside the LHC, measuring their energies and directions of flight, as well as other parameters, and trying to assess whether it is likely that some of these particles result from the decay of a Higgs boson created by the collision. These assessments carry a probability measure, such as 95%, 99%, or—as traditionally required in particle physics for a “definitive” conclusion about the existence of a new particle: 99.99997% (this is the infamous “five-sigma” requirement).
To be sure, the new, negative results presented in Mumbai yesterday are of a different nature. They state that, with a 95% probability, the Higgs does not exist within the range of energies the LHC has so far explored, between 145 and 466 billion electron volts. The probability of nonexistence is not overwhelming—there is still a 5% chance that the Higgs is hiding somewhere within this energy range. And, more importantly, the lower energy range from 114 to just under 145 billion electron volts, a region of energy that Fermilab has determined, through earlier experiments, may harbor the Higgs, has not been ruled out. But the Higgs is quickly running out of places to hide. Lower energy levels have been accessible to smaller accelerators, such as the Tevatron at Fermilab and the LEP—the LHC’s predecessor at CERN—and neither collider had found it. Perhaps the Higgs does not exist at all.
So while CERN will continue its search for the Higgs at least until the end of this year, if no positive results about the Higgs should come out, Stephen Hawking—betting against the entire world of physics, as it were—would be able to cash in on his wager. And in that case, Congress may feel that even though its 1993 decision to cancel the American alternative to CERN—the Superconducting Super Collider—was generally met with chagrin by the American physics community, it may have been the right move one after all: to spend billions of taxpayer dollars in search of a particle that likely does not exist would have been wasteful.
But if the Higgs doesn’t exist, where does mass in the universe come from? Theories that go beyond the “standard model” of particle physics (of which the Higgs is the keystone—the one missing piece needed to explain how the universe we know came to be) may be necessary. Steven Weinberg, who in his landmark 1967 paper on the unification of the electromagnetic and the weak interactions had made key use of the Higgs for “breaking the symmetry” and separating the electromagnetic from the weak forces, has since gone beyond the standard model in his research. Weinberg has proposed a theory called Technicolor, within which the primeval symmetry of our universe can be broken through a different mechanism than the action of the elusive Higgs. But to prove the validity of the Technicolor theory may require an energy level that would dwarf that available to the LHC—at an equally astronomical cost.