Origins of ultra-massive jets

The mass of jets produced with transverse momentum is greater than 400 GeV/c2, and they are observed in the CDF detector.

At CDF, physicists collide protons of energy 1 TeV (trillion electron volts) with antiprotons of equal energy. They observe a phenomenon known as a jet in many of these events. A jet is a spray of particles moving in the same direction as the quark or gluon, originially from a proton-antiproton collision. The collective energy and direction of the particles making up the jet equal those. Since these particles are practically massless, the resulting jets are expected to have low mass also. However, in some cases, jets can have a mass of more than 40 percent of their total energy.

To better understand these high-mass jets, physicists want to examine the particle that caused the jet. They try to do this by studying the spread of the particles inside the jet, usually comprising of a number of pions and kaons.

This jet substructure sheds light on the physics processes in which jets are formed, and it can also suggest the existence of massive particles that have not yet been observed.

In this study, CDF physicists measure, for the first time, the attributes of jets occurring at high energies where over 40 percent of the total collision energy comes out in the form of two or more massive jets.

CDF physicists think that the masses of these jets come from the emission by the originating particle of gluons, the particle that carries the strong force and binds the three quarks of a proton together. It is the mechanism of this gluon emission that creates the ultra-massive jets, some of which can be heavier than the nucleus of a gold atom. Quarks or gluons are the only constituents of matter that can produce jets. In this experiment, scientists observed that the high-mass jets come primarily from energetic quarks.

More detailed measurements of these massive jets show that they have a two-body signature, a feature that scientists uncovered by examining two variables that help separate jets with different shapes. This confirms the model that these massive jets primarily come from a very energetic quark emitting a single energetic gluon. This knowledge will help physicists in future studies, as we continue the search for very super-massive energetic objects such as the top quark, the most massive point-like particle known to exist.

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edited by Andy Beretvas

This work was done by Raz Alon, Ehud Duchovni and Gilad Perez, all of the Weizmann Institute of Science in Israel, and Pekka Sinervo, University of Toronto in Canada.