Science! It Marches On | Cosmic Variance | Discover Magazine
noting Physicists Will Have to Wait a Little Longer for Higgs Boson - NYTimes.comThe news from Geneva this morning is in. Essentials: what we’re seeing is pretty consistent with the existence of a Higgs boson around 123-126 GeV. The data aren’t nearly conclusive enough to say that it’s definitely there. But the LHC is purring along, and a year from now we’ll know a lot more.
The Large Hadron Collider recently finished its first full year of operation, and a successful year at that. It has performed better than expected, and it has successfully tested much of the Standard Model. But there is a part of the Standard Model that has eluded discovery: the Higgs particle.
What is it and what does it do? This particle is expected to have a very odd property: its lowest-energy state has a nonzero field value. Remember wave-particle duality. It is also expected to interact with most other Standard Model particles, an interaction much stronger for some particles than others. Since the Higgs particle is, in a way, always there, its interactions sort-of-drag the other particles, giving them masses. The stronger the interaction with the Higgs, the greater the mass.
That nonzero field value can be explained with a simple analogy. Most elementary-particle fields have a potential-energy function analogous to a bowl. Drop a marble into it and it rolls around, slowing down and coming to rest in the center. The Higgs particle, however, has a hump in the middle of the bowl, and the marble comes to rest in the trough around the hump.
How was it observed? The LHC has two rings that store protons going in opposite directions at very close to c, being about 10^(-7) c less than c itself. They have a kinetic energy of about 3.5 TeV, nearly 4000 times a proton's rest mass. Where the rings cross each other, the protons can collide, and detector complexes pick up the particles that are produced. Why protons and not electrons? A charged particle going in a circle acts like a radio antenna, making "synchrotron radiation". For the same energy, this loss is worse for less massive particles, like the accelerated electrons and positrons that had previously occupied the LHC's tunnels. So that's why the LHC uses protons instead. It also collides protons with other protons, not the Fermilab Tevatron's antiprotons, because that saves the step of making antiprotons. Ram some protons into a target at a few GeV, and about 10^(-7) of the collisions make antiprotons -- very inefficient.
The big downside of protons (and antiprotons) is that they are composite, being composed of quarks and gluons, and a collision typically makes a *lot* of particles. The interesting events are usually VERY rare, and one has to look through a LOT of collisions to find them. They also have some "background", similar-looking events made by different processes. The Higgs searchers have been looking for events like pairs of energetic photons coming from the collision site, but two quarks colliding can also make pairs of energetic photons. Why look for such events? Because like other very massive particles, the Higgs particle will decay long before it can reach a detector. Long in elementary-particle terms, of course!
There are two detectors being used in the Higgs search, ATLAS and CMS, each run by a separate team of physicists. That provides a valuable check of putative discoveries.
Now the big story.
The ATLAS team finds a bump in the events at a putative Higgs mass of 126 GeV, about 3.6 standard deviations (sigma) high. Since it could be elsewhere (the look-elsewhere effect), taking into account that possibility yields a significance of 2.3 sigma.
The CMS team finds one at 124 GeV, with a height of 2.6 sigma, and a look-elsewhere significance of 1.9 sigma.
Each bump may still be an accident, just like a gambler getting a lucky streak. But they are close to each other, and a crude combination of them gives a height of 4.4 sigma and a look-elsewhere significance of 3.2 sigma.
Almost but quite enough to declare a discovery. Physicists have a convention of 5 sigma, which means that only a one-in-a-million lucky streak can duplicate it.
The LHC is now shut down for maintenance, but next year, it should be able to get past 5 sigma, if the Higgs particle exists.
Exploration of the implications of this putative discovery: [1112.3017] Implications of a 125 GeV Higgs scalar for LHC SUSY and neutralino dark matter searches
Howard Baer, Vernon Barger, Azar Mustafayev
(Submitted on 13 Dec 2011)
Short-short summary: according to the simpler supersymmetric models, it may be awfully hard for the LHC to make supersymmetry partners -- their masses go into the TeV range.