Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

High pT hadron production in Au+Au collisions at√sN N = 200 GeV

Y. Akiba,∗1 R. Akimoto,∗1 H. Asano,∗1 S. Bathe,∗1,∗2 S. Baumgart,∗1 K. Boyle,∗1 J. Bryslawskyj,∗2C-H. Chen,∗1 L. Ding,∗3 A. Enokizono,∗4 S. Esumi,∗5 T. Hachiya,∗1 S. Horiuchi,∗5 J. Koster,∗1 K. Kurita,∗4
M. Kurosawa,∗1 A. Lebedev,∗3 M. McCumber,∗6 H. Nakagomi,∗4 R. Nouicer,∗1 C. Ogilvie,∗3 Z. Rowan,∗2H. Sako,∗1 S. Sato,∗7 A. Shaver,∗3 M. Shimomura,∗1,∗3 M. Stepanov,∗8 A. Taketani,∗1 M. Wysocki,∗9and the PHENIX VTX groupluted with the detector resolution, whereas fake tracks
can have any DCA.
Figure 1 shows the raw DCA distribution in thetransverse plane. A peak is observed around zero
DCA, which is dominated by real tracks, above a background of random tracks and weak decays.
Figure 2 shows the transverse momentum distribution of tracks with and without the small-DCA requirement. At high pT , the spectrum without the DCA requirement appears unphysically ﬂat, whereas the spectrum with the requirement continues to fall. This observed behavior suggests that the DCA requirement
successfully suppresses the background.
These plots indicate the potential of this method.
Recently, signiﬁcant progress has been made in rejecting misfunctioning parts of the detector and improving
the tracking algorithm. The analysis is still in progress.

One of the most signiﬁcant discoveries at RHIC has
been the suppression of high pT hadrons in central
Au+Au collisions1) . In pQCD models, the data constrain the transport coeﬃcient, q̂ 2) .
Currently the best measurement at RHIC is achievedby neutral pions3) . For charged hadrons, the measurement is limited by a background from photon conversions and random tracks, both mimicking high transverse momentum tracks.
With the recent addition of a Silicon Vertexing
Tracker (VTX)4) to PHENIX, it is possible to significantly reject this background and to extend hadronmeasurement to a higher pT . Tracks need to be reconstructed with a small Distance of Closest Approach(DCA) of the track projection on the primary vertex.
Real tracks are reconstructed with zero DCA convo-Fig. 2. Uncorrected hadron pT spectra for diﬀerent purity
cuts. At high pT , the spectrum without the DCA requirement appears unphysically ﬂat while the spectrumwith the DCA requirement continues to fall.

Fig. 1. Raw DCA distribution in the transverse plane. The
peak around zero DCA is dominated by real tracks. The
underlying background comes from random tracks andweak decays. The fall-oﬀ at ± 0.4 cm is an artifact ofthe tracking algorithm.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9References

RIKEN Nishina Center
Department of Natural Sciences, Baruch College, CUNY
Department of Physics & Astronomy, Iowa State University
Department of Physics, Rikkyo University
Institute of Physics, University of Tsukuba
Department of Physics, University of Colorado, Boulder
Advanced Science Research Center, Japan Atomic Energy
AgencyPhysics Department, University of Massachusetts, Amherst
Physics Division, Oak Ridge National Laboratory

1) K. Adcox et al. [PHENIX Collaboration]; Phys. Rev.
Lett. 88, 022301 (2002).
2) A. Adare et al. [PHENIX Collaboration]; Phys. Rev. C
77, 064907 (2008).
3) A. Adare et al. [PHENIX Collaboration]; Phys. Rev. C
87, no. 3, 034911 (2013).
4) A. Taketani et al. [PHENIX Collaboration]; Nucl. Instrum. Meth. A 623, 374 (2010).

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