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CERN celebrates as Higgs signal reaches significance
CERN celebrates as Higgs signal reaches significance
A strong signal emerges as the number of collisions pile up.
A four-lepton decay, a possible sign of the Higgs, seen by the ATLAS detector.
Today, in two seminars held at CERN, the European center for physics, announced evidence that the elusive Higgs particle has finally been discovered.
Physics' Standard Model describes the fundamental particles that make up all matter, like quarks and electrons, as well as the particles that mediate their interactions through forces like electromagnetism and the weak force. Back in the 1960s, theorists extended the model to incorporate what has become known as the Higgs mechanism, which provides many of the particles with mass. One consequence of the Standard Model's version of the Higgs is that there should be a force-carrying particle, called a boson, associated with the Higgs field.
For decades, physicists have been sifting through the output of colliders like the Tevatron and LEP, looking for an indication that the Higgs was present in the spray of exotic particles they detected. The closest they got was a hint of a signal that didn't rise far enough above the background. Now, in less than two years of operation, the Large Hadron Collider's detectors have found clear evidence of a particle that looks a lot like the Higgs.
Finding the Higgs was always a matter of probability. We can't detect the particle directly, but the Standard Model tells us what its decay pathways will look like, provided we feed the equations a specific mass. So, for example, we can calculate that a Higgs boson weighing in between 115 and 135GeV (the range suggested by the Tevatron data) should decay into two photons with some frequency; two Z bosons with a different frequency, and other combinations of particles with additional probabilities.
The challenge comes from the fact that the Standard Model also predicts that processes that don't involve a Higgs will also produce similar looking patterns of particles. So, we're left with probabilities. Do we see an excess of these events that can't be accounted for by non-Higgs decays? How statistically significance is that excess?
Particle physicists have settled on a specific measure of significance called five sigma (or five standard deviations) before they're willing to accept that we've spotted a new particle. When the LHC wrapped up last year, its detectors both saw a signal near 125GeV that reached nearly three sigma—tantalizing, but not enough to claim discovery. At the time, CERN's director basically said "wait until next year," when the hardware would gather far more collisions, enough to provide a greater degree of statistical certainty. To make sure that next year was worth waiting for, the LHC operators planned on running the machine both with a high number of proton bunches (which increases the total number of collisions) and at a slightly higher energy (which increases the probability that a collision will produce a heavy particle).
The hardware performed brilliantly, as the LHC reached its planned luminosity quickly and started pumping out the data. By somewhere in June, it had already produced as many collisions as it had in all of last year, and should double the available data again before this year's run is over.
But the huge number of collisions created its own problems. At times, up to 30 collisions were taking place nearly simultaneously, and the computer systems had to reconstruct which signals came from what collisions and trigger the system to save the data if something looked interesting—all within a fraction of a second. According to the presentations at CERN, the software triggers were improved, the code reconstructed events faster, and the computing grid was given more sophisticated analysis tools to identify events that could come from a Higgs decay. The net result was today's announcement (and yesterday's accidental pre-announcement).
Where do we now stand? There are a lot of ways to look at it. One is basically the probability of finding the Higgs at a specific mass. If we assume the Higgs is 125GeV, we see a signal that's a specific sigma above background. But there's no particular reason to assume 125GeV and not, say, 135GeV, and the statistics need to compensate for this (called the "look elsewhere effect"). Then there are multiple channels thanks to the different decay pathways, and two different detectors. So, for the CMS detector, the two-photon channel produces a local Higgs signal that's 4.5 sigma, but that drops to 2.5 sigma when the look elsewhere effect is considered. It's only by combining all its channels that CMS reaches a 4.9 sigma, and the data from both detectors had to be combined to be able to push things over five sigma and declare discovery.
Using the standard way of displaying the data where green indicates one sigma and yellow two (hence the nickname "Brazil plots"), the peak looks both clean and enormous.
That sure looks like a significant signal to me.
There are a number of reasons to be confident in this result. As we mentioned above, the Higgs at this mass has several different pathways that it might use to decay (two photons, two Z particles, etc.). A signal was seen in several of these channels, indicating it's not just an artifact of a specific analysis. In addition, this mass is consistent with a weaker signal seen in the Tevatron data, which not only has distinct detectors, but also collides different particles (protons and their antimatter equivalent instead of the LHC's two protons).
The other nice thing about the expanded data is that they got rid of something that was a bit awkward in last year's data. The two detectors, ATLAS and CMS, both saw signals near 125GeV, but the peaks were on opposite sides: CMS at 124GeV, ATLAS at 127GeV. With more data, that apparent discrepancy seems to have gone away, and everyone is now saying 126GeV. (Someone noted that's roughly comparable in mass to an iodine atom.)
So, what's next? We know we have a boson thanks to its decay pathways, and it's behaving largely as the standard model would predict if it were the Higgs. But the LHC should be able to produce many more of these, which will push the individual decay channels up to five sigma territory. At that point, the numbers should tell us if there's something odd about individual decay pathways—do we see an excess of two photon decays? Fewer four lepton results than predicted? This will provide fine-scale tests of the Standard Model.
In addition, we'll get a better grip on the particle's mass. Some of the decay channels we're using involve the production of neutrinos and, since we don't know how much they weigh, we can't tell how much mass and energy they carry away when a Higgs decays. That helps broaden out the mass peak. More data, particularly from those channels that don't involve neutrinos, will narrow that down.
Further into the future, the LHC will go into a long shutdown at the end of this year, so that its hardware can be upgraded to operate at its full potential, reaching energies of 14TeV. When it comes back on line in a few years, the focus will shift to seeing if there's anything out there that the Standard Model doesn't predict.
UPDATE: CERN has indicated it will extend this year's LHC run by several months in order to get enough data to know more things about the newly discovered boson. This is the last chance they'll get before the extended shutdown for upgrades, and they probably have some sense of what it will take to push key measurements into statistical significance now.
Source: Ars Technica
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