…and measures suppression of single jets in heavy-ion collisions

While primarily designed for proton–proton collisions, the ATLAS detector is also an excellent tool to perform measurements in the hot, dense environment of heavy-ion collisions, where temperatures reach tera-kelvin scales. So far, results include detailed measurements of collective properties of the system, such as “elliptic flow”, as well as of “hard probes”, such as jets, quarkonia and vector bosons.

Using the initial 2010 heavy-ion collision data from the LHC, the ATLAS collaboration published the first direct evidence that jets lose energy as they pass through the hot, dense medium, a process called jet “quenching”, leading to event-by-event asymmetries in the energies of the two jets. To characterize the effects of quenching from a different perspective, the next major jet measurement in lead–lead collisions undertaken by ATLAS was to establish the overall reduction in the rate of jets in more “central” collisions, where the two nuclei overlap more completely.

For the Quark Matter 2011 conference, ATLAS compared the rates for central events with those in more peripheral events that consist primarily of a few simultaneous nucleon–nucleon collisions. One surprising result is that, for jets above 100 GeV, the measured jet-suppression factor is independent of the measured jet energy. An even more surprising finding is that this result is the same for jets reconstructed with different “cone” radii, implying that the suppression is not accompanied by a substantial modification of the distribution of energy within a jet. By contrast, an ATLAS measurement of W boson yields using single muons showed no suppression at all.

This comparison, shown in the figure, was quantified using the variable R_CP, the ratio of yields measured in central and peripheral collisions, each yield normalized by the relevant number of binary collisions. This quantity is unity if jets are produced in proportion to the number of binary collisions, but falls below one if the yields are suppressed in more central collisions.

The higher luminosities expected in 2011 will provide increased jet statistics, allowing the measurement of jets with even higher energies. At the same time, a more precise understanding of the fluctuations of soft particles, mainly from a rich spectrum of collective modes, will allow the measurement of lower-energy jets, which in preliminary results from the Relativistic Heavy Ion Collider show stronger modification from passage through the medium.