Saturday, July 23, 2011

Weak Hints Of The Higgs

Finally, after years of results where large energy range are excluded for the Higgs, we are starting to get hints of the Higgs signature, and it came out of ATLAS and CMS at the LHC. Both were reported at this week's EPS meeting.

Both experiments found excesses in the 130-150 GeV mass region. But the excesses did not have enough statistical significance to count as evidence of the Higgs.

Scientists measure statistical significance in units called sigma, written as the Greek letter σ. These high-energy experiments usually require 3σ  level of confidence, about 99.7 percent certainty, to claim they’ve seen evidence of something. They need 5σ to claim a discovery. The ATLAS experiment reported excesses at confidence levels between 2 and 2.8σ, and the CMS experiment found similar excesses at close to 3σ.

Still, there's a very long way to go (and more data collection and analysis) before one can actually claim discovery. Unlike pseudosciences where even a weak correlation seems to be sufficient to claim that a phenomenon exists, in high energy physics, not only do you need a high confidence level that your result isn't simply due to chance, but you also need another independent detector, measuring things differently, to agree with your result! The fact that both ATLAS and CMS are getting almost the identical result is a very good start. And it is only a start.

Interestingly enough, the energy range where this is detected is also accessible at the Tevatron. I wonder if CDF and DZero might zero in (no pun intended) in this range and see what their data looks like with various background subtraction schemes.



Logan Wright said...

The cost of that extra confidence seems a tad gratuitous, but I will admit I'm not a particle physicist. Bell's inequality, for sure, has been violated (albeit with your choice of loophole) to well beyond 5 standard deviations, but the cost, in expertise, time and dollars, "per sigma" is significantly less. While I'm excited by the preliminary results, I can't help but wonder if maybe those extra decimal points of certainty really add as much to the result as they should given the tradeoffs. It would seem silly to just have everyone pack up and go home at 3 sigma, but it raises the question of why 5 sigma is the bare minimum. Has anything ever been honestly "discovered" at <5 std. deviations and been subsequently dismissed as an extremely unlucky error?

Pi-Guy said...

Logan, I can see what you mean. But if nothing else the 5-sigma represents accountability. The LHC is a several-billion-Euro project with or without the extra standard deviations. With those extra a discovery becomes that much less a questionable claim; that much more a quantified fact that the data speak for themselves. This is especially important in experiments with quantum field theory, which is inherently statistical in what it yields.