The First Full Jet Measurements in Heavy Ion Collisions
By Sevil Salur
Jets are collimated sprays of particles that are remnants of hard-scattered quarks and gluons. They are fundamental to QCD and are observed in high energy collisions of all kinds. At RHIC, jets are put to a new use to study the hot QCD matter through their interaction and energy loss in the medium ("jet quenching"). Until now, only indirect measurements of jet quenching have been made via inclusive hadron distributions and di-hadron correlations at high transverse momentum, to avoid the complex backgrounds of heavy ion events. However, these jet fragmentation particles are biased towards the population of jets that has the least interaction with the medium. A more complete picture of jet quenching would be obtained using full jet reconstruction. This article reports results from a recent new approach of jet measurement in heavy ion collisions, utilizing the high luminosity Au+Au data set from 2007 RHIC run. Comparison of the inclusive jet cross section obtained in central Au+Au events with that in p+p collisions, published previously by STAR, suggests that unbiased jet reconstruction in the complex heavy ion environment indeed may be possible. The experimental details can be found in  for the direct measurement of jets and  for the accompanying jet fragmentation studies in heavy ion collisions utilizing the STAR experiment.
Direct jet measurements in p+p collisions at RHIC have been carried out since the third year of RHIC operations. See Figure 1 for the published jet spectrum in p+p collisions by the STAR experiment . Since 2006, the STAR barrel electromagnetic calorimeter (BEMC) has been operated with full azimuthal coverage (φ) and large pseudorapidity (η) acceptance. This detector upgrade together with the increased beam luminosities of RHIC and data recording capabilities of STAR, enables the study of full jet reconstruction in heavy ion collisions for the first time at RHIC. Based on scaling of the p+p spectrum shown in Figure 1, the Au+Au dataset from the 2007 RHIC run is expected to contain statistically significant jet yield beyond 50 GeV. However, measuring jets above the complex heavy ion background is a challenging task.
During the last 20 years, various jet reconstruction algorithms have been developed for both leptonic and hadronic colliders. For a detailed overview of jet algorithms in high energy collisions, see the references in . Most recently a BNL research associate, Gregory Soyez, together with his collaborators Matteo Cacciari and Gavin Salam, developed a new approach to jet reconstruction, motivated by the need of precision jet measurements in the search for new physics in high luminosity p+p collisions at the LHC . A key feature of their approach is a new QCD inspired algorithm for separating jets from the large backgrounds due to pile up. As it turns out from simulations, these improved techniques can also be used in heavy ion environments where the background subtraction is essential for jet measurements. Sequential recombination algorithms (kT and Cambridge/Aachen (CAMB)) encoded in the FastJet suite of programs , along with an alternative seeded cone algorithm (labeled LOHSC) are utilized to search for jets in the Au+Au collisions. Figure 2 shows an example of an identified di-jet event for central Au+Au collisions, using both the neutral energy from the BEMC and charged particles from the Time Projection Chamber of the STAR experiment.
In order to assess the bias of the heavy ion jet measurements, the inclusive jet cross section is compared to that from p+p collisions presented in the Figure 1. Figure 3 shows the comparison of the inclusive jet spectrum for central Au+Au collisions (taken with a Minimum Bias online trigger "MB-Trig" ) to the NBin scaled p+p spectrum, for the kT, CAMB and LOHSC algorithms. To account for nuclear geometric effects, the p+p spectrum is scaled by NBin, the number of binary nucleon+nucleon collisions equivalent to a central Au+Au collisions, as calculated by a Glauber model. In the case of jet reconstruction, NBin scaling is expected if the reconstruction is unbiased, i.e. the jet energy is recovered independent of the fragmentation, even in the presence of strong jet quenching. This scaling is analogous to the cross section scaling of high pT direct photon production in heavy ion collisions, observed by the PHENIX experiment . At present, the total systematic systematic uncertainty on the normalisation of the inclusive p+p jet spectrum is around 50%. Figure 3 shows that the heavy ion jet spectrum agrees well with the scaled p+p measurement within the systematic uncertainty. In , the jet spectra are measured using different threshold cuts on the track momenta and calorimeter tower energies (pTcut). It is found that the agreement between the binary scaled p+p spectra and the Au+Au measurement is worse for larger pTcut. This suggests that the threshold cuts introduce biases which are not fully corrected with the current procedure that uses fragmentation models that are developed for e++e− and p+p collisions. It could also be an indication of modified fragmentation due to jet quenching.
Unbiased reconstruction of jets in central heavy ion collisions at RHIC energies would be a breakthrough to investigate the properties of the matter produced at RHIC. The study shown here indicates that unbiased reconstruction of jets may be possible in heavy ion events. However, spectrum corrections are currently based on model calculations using PYTHIA fragmentation. This aspect, together with the spectrum variations due to cuts and reconstruction algorithms, must be investigated further in order to assess the systematic uncertainties of this measurement.
A copious production of very energetic jets, well above the heavy ion background is predicted to occur at the LHC. The large kinematic reach of high luminosity running at RHIC and at the LHC may provide sufficient lever-arm to map out the QCD evolution of jet quenching. The comparison of full jet measurements in the different physical systems generated at RHIC and the LHC will provide unique and crucial insights into our understanding of jet quenching and the nature of hot QCD matter.
 STAR Collaboration, B. I. Abelev, et. al. Phys. Rev. Lett. 97 252001 (2006).