Exciting the vacuum

This plot shows mass spectra of π+π pairs at center-of-mass energies (√s) of 0.9 and 1.96 TeV and their ratio. The main peak is a spin-2 f meson; there is evidence for spin-0 f mesons at 980 MeV/c2 (not shown here) and 1,370 MeV/c2 and some structures at higher mass.

Among all the elementary and composite particles we know, there are a few that have the odd property of being much like the vacuum: They have no electric charge and no spin, and they are identical to their mirror image. They must be present in the vacuum as virtual states; they cannot become real without violating energy conservation (except for appearances of extremely short duration).

One of these vacuum-like particles is the Higgs boson, an excitation of the all-pervading Higgs field. It decays exceedingly quickly, after only a few proton diameters. Most believe it to be truly elementary — that is, without component parts. The other known vacuum-like particles are called f mesons and χ (chi) mesons. They are understood to be quark-antiquark bound states. Very unstable, they decay quickly, even faster than the Higgs boson, sometimes into a pair of particles called pions.

There is yet another type of vacuum-like particle expected in our theory of strong interactions: a pair of gluons bound together. Gluons bind the quarks together through the strong force. The lightest “glueball” should be vacuum-like and with a mass in the range 1,000 to 1,700 MeV/c2; scientists have proposed candidates, but none is well-established.

A promising search technique is to select collisions in the Tevatron in which outgoing protons and antiprotons go down the beam pipes and a glueball pops out of the vacuum. The glueball can immediately decay into a pion pair.

As detailed in a paper submitted to Physical Review Letters, CDF scientists detected pion pairs, with no other particles seen in the detector (apart from the protons going into the beam pipes), and reconstructed the combined mass of the two pions. A glueball would show up as a bump in the distribution of those measured masses (see upper figure).

The figure also shows data taken with about half the normal Tevatron energy as well as the ratio of the distributions at the two energies. The spectrum shows several structures consistent with known f mesons and some other unexpected structures, which remain to be explained. Unfortunately, the glueball remains elusive. Does it not exist, or is it just lost among the zoo of meson states? This is an unfinished chapter of strong interaction physics.

Using similar selections and more data taken at low luminosity and by looking for different decay modes (for example π+π, K+K, K0K0 and φφ), maybe glueballs can be found at the LHC.

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These scientists are the primary analysts for this result. Top row from left: Mike Albrow (Fermilab) and Maria Zurek (University of Cologne, Germany). Second row, from left: Inna Makarenko (National University of Kyiv, Ukraine), Jon Wilson (Texas A&M) and Denys Lontovskyi (National University of Kyiv, Ukraine).