The results of two recent experiments at the Large Hadron Collider (LHC) near Geneva suggest physicists are close to discovering the Higgs boson, the so-called God particle. Combined, the two experiments have narrowed the possible band of possible Higgs boson masses to between 115 and 130 GeV (gigaelectron volt). Rather than look directly for this fleeting would-be particle itself, physicists look for the various combinations of particles into which Higgs bosons are thought to decay. Independent analyses have verified excesses of these particles from the low mass region 124 to 126 GeV. Though not sufficient to claim a discovery, the latest experiments restrict the region in which the Higgs boson might be hiding.
Remind me. The Higgs whoson?
The existence of the Higgs boson was first proposed in the 1960s by, among others, Peter Higgs, as an explanation for why particles have mass. The idea is that particles gain mass by interacting with a universal, omnipresent, but for now theoretical, Higgs field. Higgs bosons, if they exist, are the particles that comprise that field, and its discovery would fill the last hole in what physics calls the Standard Model.
Why the God particle?
Interesting story. The nickname, loved by journalists, disliked by some scientists, comes from the title of Fermilab Director Leon Lederman's book The God Particle: If the Universe is the Answer, What is the Question? According to Peter Higgs, Lederman wanted to call it the "goddamn particle", presumably a reference to its elusiveness, but his editor preferred God particle - perhaps on the grounds that it would sell more copies. It's a nickname the media has latched on to, but don't read too much into it.
And now they've found it?
No, but, they're getting there. A seminar held at CERN yesterday announced results from two of the LHC's detectors, ATLAS and CMS. ATLAS identified that the mass of the Higgs boson is most likely in the range of 116-130 GeV, and CMS 115-127 GeV, though it sounds as if the range 124-126 GeV is looking particularly interesting. Ongoing experiments are narrowing the remaining possible range as we speak. It's a case of if your quarry goes to ground ... That said, its precise form is by no means certain, nor is its discovery assured.
Hang on.1 eV is an electron volt, right? How is that a unit of mass?
The electron volt is a unit of energy. 1 GeV is one gigaelectron volt, or 1 billion electron volts. But thanks to mass-energy equivalence it's also a unit of mass. In Einstein's famous equation, energy equals mass times the speed of light squared. As the speed of light is a constant, it can be assigned a value of 1, making energy and mass equivalent. Incidentally, a proton has a mass of 0.938 GeV, so in relative terms, Higgs bosons, if they exist, are huge.
And how do they look for Higgs bosons, exactly?
This is where a particle accelerator like CERN's LHC or Fermilab's Tevatron (until it was shut down) comes in handy. By colliding protons traveling at close to the speed of light, physicists can create collisions which replicate the conditions of the first billionth of a second after the universe began: the only conditions in which certain particles are formed. The Higgs boson itself may exist too fleetingly to observe directly, but through the particles into which it decays, it should prove possible to trace.
When will we know for sure?
It's impossible to say with certainty but some physicists are saying that a 2012 discovery is possible. Because the experiments rely on looking for peaks in reams of data rather than identifying the particle itself, there is a question of statistical certainty. According to the Guardian's Ian Sample, its correspondent at CERN, four times as much data as has been analyzed to date is required to be sure. There will be a race to gather that data before the end of 2012 when LHC closes for at least 12 months for an upgrade.
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