Happy hunting grounds

This artistic view of a Feynman diagram shows the process of proton colliding with an antiproton, producing a W’, which then decays into a top quark and an antibottom quark.

We understand nature in terms of elementary particles interacting through a set of well-known forces, which are mediated by other particles. These are the graviton (mediator of gravity), the photon (mediator of electromagnetism), the gluon (mediator of the strong force), the W and Z bosons (mediators of the weak force) and the Higgs boson. We produce and detect these particles (except the graviton) in large numbers at colliders around the world.

But is that all the universe is made of — a handful of different types of particles? We have good reasons to believe that this is not the case. New forces can exist, and the corresponding mediating particles could be seen at colliders. However, such particles have been hunted extensively at the Large Hadron Collider without success so far. If new forces are hiding so well from physicists’ determination to discover them, either they would have to be mediated by very massive bosons or these bosons would have to interact very weakly with ordinary stuff.

The W and Z boson serve as a good model for this kind of exotic stuff: In fact they are both very heavy compared to their peers and interact weakly with ordinary matter. They live very shortly before decaying into more “mundane” particles, most of the time quarks. If new forces were to exist with such properties, then the LHC would not be the best hunting ground because of its enormous production rate of quarks from ordinary forces.

A new analysis of Tevatron data performed by the CDF collaboration searches for the existence of new electrically charged, massive particles (a W’ boson) decaying into a top and a bottom quark. Top and bottom quarks leave striking signatures in the detector; W’ events would resemble ordinary production of such quarks if not for the extra energy provided by the decay of the parent particle.

The search for a W’ with data from the CDF experiment turns out to be the most sensitive for such a heavy particle with mass below 650 GeV (approximately 700 times the proton mass). Unfortunately, no surprise turned out from CDF data. The ball is now again in the hands of the LHC experiments!

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These scientists are the primary analysts for this result. Top row, from left: Fabio Anza (Oxford University), Giorgio Bellettini (University of Pisa, INFN), Ludovico Bianchi (Forschungszentrum Jülich, Germany), Daniela Bortoletto (Oxford University). Second row, from left: Matteo Cremonesi (Fermilab), Tom Junk (Fermilab), Young-Kee Kim (University of Chicago), Qiuguang Liu (Los Alamos). Third row, from left: Fabrizio Margaroli (Spenzia University of Rome, INFN) and Karolos Potamianos (Berkeley Lab).