Hard scatterings in p+p collisions produce back-to-back "jets" of particles, but in nuclear collisions at RHIC, the presence of dense nuclear matter modifies the jets' properties.
star focus: jets in nuclear collisions
Part 1 of a series on STAR analysis topics
Posted: Aug 3, 2006

In high energy proton-proton collisions, the hard scattering of quarks and gluons early in the collision leads to the production of jets, narrow streams of fast-moving particles that allow physicists to detect and understand the scattering. In nuclear collisions at RHIC, jets instead serve as a penetrating probe of the extremely dense nuclear matter formed in the collision. Comparing characteristics of jets in nuclear collisions to jets in p+p collisions has uncovered special properties of dense nuclear matter at RHIC.

Jets and azimuthal correlations
High-energy jets occur very infrequently, and millions of events are recorded to acquire enough statistics to study them. In addition, a head-on nuclear collision at RHIC generates thousands of particles, only a small fraction of which might be from a jet. To extract these rare signals from the data, the intrinsically correlated directions of particles from back-to-back jets are used. The technique, called azimuthal correlations, can be applied to a wide range of particle momenta, particle type, collision energy, and other variables to study the effect of the nuclear medium in detail.

Jet suppression and enhancement in STAR
In 2002-03, STAR announced the surprising observations that back-to-back jets measured with azimuthal correlations of high-energy particles showed a strong suppression (paper) of one of the jets in head-on Au+Au collisions, while the same studies in p+p and deuteron+Au collisions showed no such suppression (paper). The suppression of the "away-side jet" in Au+Au led to the idea that hard scatterings which take place near the surface of the collisions allow one jet to escape but cause the other jet to lose significant energy in the dense matter. In contrast, a subsequent study of azimuthal correlations of lower-energy particles using the same data showed an increase of away-side particle production (paper). The two observations, shown in the following figure, are consistent with the notion of parton energy loss ("jet quenching") in which a quark's or gluon's energy is transferred to slower quarks and gluons, either through radiation or collisions.

More statistics, higher-energy jets
The large increase in Au+Au statistics in the 2004 RHIC run allowed studies of azimuthal correlations in STAR to extend to more energetic jets.  Recently STAR has shown that, as the jet energy is increased (by increasing the momentum of particles in the azimuthal correlations analysis), a narrow away-side peak emerges above the background that looks very similar to the away-side peak in d+Au collisions, with a reduced peak height (paper). The direct observation of "dijets" potentially provides an additional probe of the nuclear medium, in particular isolating the conditions for which quarks and gluons do not interact and lose energy. Ongoing theoretical work should confront these questions.

Left: STAR data from the 2004 run, showing the emergence of narrow, back-to-back peaks in head-on Au+Au collisions as momentum of trigger particles is increased (left to right in figure). Right: Comparison of deuteron+Au and two samples of Au+Au collisions showing the increased supression of back-to-back peaks as the system size is increased (left to right in figure). In both figures, the rows show different momentum ranges for associated particles.

Current, future measurements
Further uses of azimuthal correlations to study jets and the nuclear medium in detail are underway:

  • Photon-jet azimuthal correlations, even rarer than normal dijets, allow the jet's energy to be determined exactly, because the outgoing photon doesn't interact with nuclear matter.
  • Azimuthal correlations involving electrons can be used to detect heavier quarks (charm and bottom), which are predicted by theorists to lose less energy than light quarks when propagating through the medium.

Future upgrades to the STAR detector, as well as larger datasets, will give collaborators more flexibility in exploring the details of parton energy loss and the underlying properties of the dense nuclear matter formed in the collision.

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