Speaker
Description
In heavy-ion collisions, the study of recoil jets tagged by high transverse-momentum "direct photons" ($\gamma_{dir}$) should provide a measurement of the partonic energy loss in the hot, dense medium produced in such collisions\footnote{X.-N. Wang, Z. Huang, and I. Sarcevic, Phys. Rev. Lett. 77, 231 (1996)}. Since a $\gamma_{dir}$ does not interact strongly with the medium, it closely approximates the initial energy of the recoiling parton. It is also interesting to compare the recoil jets tagged by $\gamma_{dir}$ to those tagged by high transverse-momentum $\pi^{0}$. In contrast to the $\gamma_{dir}$, high transverse-momentum $\pi^{0}$ are assumed to be biased towards being produced near the surface of the medium. Moreover, the production mechanisms of $\gamma_{dir}$ favor recoiling quarks over gluons, but the production mechanisms of $\pi^{0}$ show no such preference. Thus the comparison of $\gamma_{dir}$-tagged recoil jets to $\pi^{0}$-tagged recoil jets may shed light on the path-length and color-factor dependence of in-medium partonic energy loss.
To establish a vacuum fragmentation reference, we present the measurement of the yields of full recoil jets (recoil jets consisting of both charged and neutral particles) in p+p collisions. The yields are measured using the STAR Time Projection Chamber and Barrel Electromagnetic Calorimeter in p+p collisions at $\sqrt{s_{NN}}=200$ GeV tagged by neutral-particle triggers recorded during the running year 2009. The neutral-particle triggers satisfy $9 < E_{T}^{trig} < 20$ GeV and $|\eta^{trig}| < 1$, and are separated into a sample of identified $\pi^{0}$ triggers and a sample of triggers with an enhanced fraction of $\gamma_{dir}$. Jets are reconstructed from charged tracks and neutral towers with $p_{T} > 0.2$ GeV/c and $E_{T} > 0.2$ GeV respectively and $|\eta| < 1$ using the anti-$k_{T}$ algorithm for resolution parameter $0.3 \leq R \leq 0.6$. To assay the effect of reconstructing full jets versus charged-only jets in such studies, the yields of charged recoil-jets are compared to the yields of full recoil-jets. The data are corrected for instrumental effects and compared to Pythia simulations\footnote{T. Sj\"ostrand, S. Mrenna and P. Z. Skands, Comput. Phys. Commun. 178 (2008) 852 [arXiv:0710.3820 [hep-ph]]}.
| Preferred Track | Jets and High pT Hadrons |
|---|---|
| Collaboration | STAR |
