A Higgs boson without the mess
June 26, 2009 | 6:08 am
If CERN's Large Hadron Collider (LHC) can create Higgs bosons, a handful may appear in rare "exclusive" reactions that don't destroy the colliding protons--similar to a reaction now observed at Fermilab. CERN's ATLAS and CMS teams are considering adding equipment to their detectors (CMS shown here) to look for such events. Photo: CERN
Particle physicists at CERN’s Large Hadron Collider (LHC) hope to discover the Higgs boson amid the froth of particles born from proton-proton collisions. Results in the 19 June Physical Review Letters show that there may be a way to cut through some of that froth. An experiment at Fermilab’s proton-antiproton collider in Illinois has identified a rare process that produces matter from the intense field of the strong nuclear force but leaves the proton and antiproton intact. There’s a chance the same basic interaction could give LHC physicists a cleaner look at the Higgs.
A proton is always surrounded by a swarm of ghostly virtual photons and gluons associated with the fields of the electromagnetic and strong nuclear forces. Researchers have predicted that when two protons (or a proton and an antiproton) fly past one another at close range, within about a proton’s diameter, these virtual particle clouds may occasionally interact to create new, real (not virtual) particles. The original protons would merely lose some momentum and separate from the beam. Such an “exclusive” reaction–where the original particles don’t break apart–gives unusually clean data because there are so few particles to detect.
In the new experiment, researchers were looking for signs that the interaction of virtual gluons had generated short-lived particles including the Χc (”Chi-c”) and J/ψ mesons, which are charm-anticharm quark pairs that decay into muons and antimuons. The Χc reaction would be especially rare because it requires protons to donate two gluons each, a requirement that also makes detailed predictions challenging, says Fermilab’s Mike Albrow, a member of the Collider Detector at Fermilab (CDF) collaboration.
In 2007, CDF researchers observed hints of exclusive, virtual gluon reactions in the form of high-energy photons radiating from colliding protons and antiprotons. Now the team has sifted through nearly 500 muon-antimuon pairs, identifying 65 that must have come from the decay of the Χc–very close to the rate predicted in 2005 by a team at Durham University in England. Because the Χc has similar particle properties to the much heavier Higgs boson, the same basic reaction should produce the Higgs at the higher collision energies provided by the LHC, says Albrow. “It’s the strongest evidence that the Higgs boson must be produced this way, if it does exist.”
Based on the rate of Χc production, Albrow estimates LHC collisions could produce 100 to 1000 Higgs bosons per year in each of the accelerator’s two largest particle detectors, ATLAS and CMS. “Even a few dozen events per year would enable you to measure the [Higgs's] mass, spin, and other properties,” he says. That’s why ATLAS and CMS teams are reviewing proposals to add detectors to look for exclusive Higgs events.
But not everyone is so optimistic that these events would be detectable in significant numbers. “It looks hard, but one should never say never,” says Joseph Incandela of the University of California, Santa Barbara, deputy physics coordinator for CMS. Incandela points out that once the LHC is operating at full capacity, every crossing of its twin proton beams is expected to yield about 20 collisions, throwing up other particles that may obscure exclusive reactions. But he says there are scenarios such as supersymmetry, a proposed extension to the standard model (the textbook theory of particle physics) in which there could be multiple Higgs bosons. In those situations, Albrow adds, exclusive reactions might be the only ones clean enough to distinguish the different Higgs particles.
by JR Minkel
JR Minkel is a freelance science writer in Nashville, Tennessee. His first book, Instant Egghead Guide: The Universe, comes out in July.
This story was first published in Physical Review Focus and is copyright American Physical Society. Reprinted with permission.
For more information on exclusive events, see the CERN Courier.
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3 Comments »
June 27th, 2009 at 8:37 am
Speaking of PRL…someone posted this on another SB blog that highlighted the original differences in the 1964 PRL mass boson papers. Thought it was interesting as so much is written about the Fermi vs. CERN battles that the theorists just get ignored.
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Both PH and EB worked in the Lorentz (i.e., manifestly covariant) gauge. Thus the Goldstone theorem would be fully expected to apply. However, PH is also done purely classically (i.e., without quantum theory), which means that the Goldstone theorem (a result from QUANTUM field theory) really has no obvious application. What PH does is to show that with a broken symmetry condition the classical field equations can be juggled into the form of the equations of a MASSIVE vector boson. Namely, the broken symmetry condition gives mass. That is a good thing, but what of the radiation gauge avoidance of the Goldstone theorem which PH had touted in his earlier paper? Is it not strange that that avoidance mechanism is not mentioned in the PH paper?
EB do some calculations in quantum field theory in which they impose a broken symmetry condition. Given the fact that a broken symmetry condition introduces a mass parameter into the theory it is not surprising that they also find a MASSIVE vector boson. But what of the zero mass particle which they must have according to the Goldstone theorem. They need to show that there is a decoupling of that particle from the physical sectors of the theory. In other words, it needs to show the zero mass particle is purely a so-called gauge excitation. That is in fact the case, but is not shown in EB.
GHK uses the radiation gauge and shows that a massive vector particle emerges from a broken symmetry condition. Thus GHK achieves the goal of lending mass to the vector particle, but is not plagued by the encumbrance of the Goldstone theorem. Moreover, GHK shows explicitly the precise way in which the Goldstone theorem fails in the context of their model.
One can thus sum up by saying that in a sense PH and EB solved half of the problem – namely massifying the gauge particle. GHK really solved an entire problem – massifying and also showing how the deadening hand of the Goldstone theorem is avoided.
June 27th, 2009 at 9:14 am
Yes. I posted it. The differences between the PRL papers from 1964 by Brout and Englert; Higgs; and Guralnik, Hagen, Kibble have been a bit lost over time. With some in Europe trying to write the GHK team out of the history it is a good time to note the differences.
http://www.symmetrymagazine.org/breaking/2008/05/16/the-jpsi-particle-original-papers
http://prl.aps.org/50years/milestones#1964
March 6th, 2010 at 8:54 am
Most of first post is true but Higgs did not select a gauge – made some general approximations but did not select gauge or show how goldstone theorem really fails.