LHC sees hint of lightweight Higgs boson

 作者:亓蜥     |      日期:2019-02-28 10:11:02
By Lisa Grossman THE ultra-shy Higgs boson may finally have shown itself at the Large Hadron Collider. Both of the main detectors, ATLAS and CMS, have uncovered hints of a lightweight Higgs. If it pans out, the only remaining hole in the standard model would be filled. Even more exciting, a Higgs of this mass, around 125 gigaelectronvolts, would also blast a path to uncharted terrain. Such a featherweight would need at least one new type of particle to stabilise it. “It’s very exciting,” says CMS spokesman Guido Tonelli. “This could be the first ring in a chain of discoveries.” As the leading theory for how particles and forces interact, the standard model has been spectacularly successful since it was proposed in the 1960s. But it works only on the assumption that the Higgs boson actually exists. The particle is the calling card of an unseen entity called the Higgs field, which is thought to give all particles their mass. The trouble is the standard model cannot predict what the Higgs itself weighs. So physicists have been hunting for the simplest version of the Higgs at various particle colliders for years. Experiments have steadily ruled it out at a range of masses, except for a narrow window between 115 and 141 GeV. Now physicists at the LHC at CERN, near Geneva in Switzerland, have probed that range in more detail than ever before. On Tuesday, Tonelli and Fabiola Gianotti, head of the ATLAS detector, separately presented results from more than 300 trillion high-speed particle collisions made in the last year. “This is the first time we’re really exploring the entire [mass] region with the right sensitivity – the one that will allow you, if there is something there, to start seeing something,” says Tonelli. “This is the first time we’ve explored the region with the sensitivity to see the Higgs if it is there” The ATLAS data restricts the Higgs to within 115 and 131 GeV; CMS rules out a Higgs heavier than 127 GeV. Most excitingly, ATLAS saw a tantalising hint of the Higgs at 126 GeV; CMS saw one at 124 GeV. It is the first time both experiments have seen a signal at nearly the same mass. “We’re very competitive with each other, but once I see they’re coming with results, I’m happy,” Tonelli says. “Their results are important for us. They’re obtained in a completely independent manner.” The Higgs is expected to appear fleetingly in the wreckage of high-speed proton collisions at the LHC, but cannot be seen directly. Instead, physicists look for the shower of lighter particles and photons that result from the decay of Higgs bosons of various masses. Because garden-variety particles also produce the same decay products, Higgs hunters look out for suspicious excesses of these products in their detectors. CMS saw such an excess in five different possible decay products, including the clearest to interpret, a pair of gamma-ray photons and two Z bosons. ATLAS also saw an excess in those two products. Although both teams found an excess around the same mass, there is not yet enough data to claim a discovery. The ATLAS signal has a statistical significance of 2.3 sigma at 126 GeV, meaning that the result has around a 2 per cent chance of being down to a random fluctuation; the comparable excess at CMS has a significance of just 1.9 sigma. To claim a discovery you need a 5 sigma signal, meaning there is less than 1 in a million chance of the result being a fluke. “There’s clearly not enough to conclude anything at this stage,” Gianotti says. “It could be something interesting, or just a fluctuation.” Even a hint of a 125-GeV Higgs has some theorists sighing with relief. Although the standard model can’t predict the particle’s mass directly, it does predict how other particles interact with the Higgs – in particular, the W and Z bosons that are responsible for the weak nuclear force. Earlier experiments found that the W and Z bosons weigh 80.4 and 91.2 GeV, respectively. Because of the way those particles interact, the Higgs mass probably comes out somewhere between – about 115 and 130 GeV. A Higgs at 125 GeV or so “is just what the doctor ordered”, says Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology. That mass also paves the way for physics beyond the standard model. Thanks to subtle quantum mechanical effects, a lightweight Higgs needs a heavier companion particle “acting as a sort of bodyguard”, Tonelli says. Otherwise, the quantum vacuum from which particles appear would be unstable, and the universe would long ago have disintegrated. If the Higgs is lightweight, the fact that we are here today suggests there is at least one extra particle beyond the standard model. “A lightweight Higgs suggests there is at least one extra particle beyond the standard model” Wilczek thinks that is great news. It leaves the door open for one of the most mathematically beautiful extensions of the standard model. Supersymmetry, or SUSY for short, suggests that every known particle has an as-yet-undetected partner and promises to resolve a lot of the standard model’s shortcomings. It can unite the strong and weak nuclear forces with the electromagnetic force and offers a candidate for dark matter. The simplest version of the theory predicts that its extra partner particles should already be showing up at the LHC – and they’re not. If the Higgs really weighs about 125 GeV, it could give the ailing theory new life. “In some sense SUSY will receive some oxygen,” Tonelli says. “Maybe not the most trivial and popular models, but there will be a revival of interest in it. SUSY will still be an important area of research for LHC experiments next year.” In the meantime, ATLAS and CMS physicists will be crunching more data to find out if the hints of a lightweight Higgs hold up. Rigorously combining the two current data sets would effectively double the statistics; Tonelli suspects it would firm up the statistical significance to between 3.7 and 3.9 sigma, or a 1 in 10,000 chance of the result being a fluke. Assuming the collider keeps working well, both experiments should have enough data to confirm or deny the simplest version of the Higgs by the end of 2012. By then, physicists might look back on this moment as their first glimpse of a major discovery. “This is why there is some excitement,” says Tonneli. It would also be thrilling if the Higgs never showed up at all (see “If the Higgs vanishes”). If the current hints do disappear on further examination, physicists will wait until the LHC revs up to its full energy in 2015 to look for other particles or phenomena that could give particles mass without any need for the Higgs. “There must be something |else that plays that role,” Gianotti says. “We will be going after that something else.” Teams at both of the Large Hadron Collider’s main detectors have found signs of a Higgs boson (see main story). But the statistical significance of the detections is relatively low, and the hints could disappear with more analysis and data. If no sign of the Higgs turns up at the LHC, what could step in and fill the gap? Some physicists, such as Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology, find a world without the Higgs distasteful as it is the last undiscovered piece of the standard model of particle physics. What would such a world be like? “That’s like asking, suppose two plus two does not equal four?” Wilczek says. “I can’t give an intelligent response, it’s so outlandish.” But Nobel laureate Steven Weinberg of the University of Texas at Austin, who helped develop the standard model in the 1960s, is more open to the possibility. “I think there are alternatives to the Higgs,” Weinberg says. One is the existence of a new force, called technicolour, which would act like an extra strong version of the strong nuclear force, binding quarks together in the nuclei of atoms. The technicolour force would fill space with pairs of new particles, which would form a soup through which other particles would travel, gaining mass in the process. “That would be an outstanding alternative if the Higgs isn’t there,” Weinberg says. “In that case there would be a whole host of other particles, probably at higher energy,