Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 00 (0)√Measurement of D 0 in p + p collisions at s = 200 GeV using theSilicon Vertex Tracker at RHIC-PHENIX
T. Moon,∗1,∗2 A. Adare,∗3 Y. Akiba,∗1 R. Akimoto,∗1,∗4 H. Asano,∗1,∗5 S. Bathe,∗1 J. Bryslawskyj,∗6 A. Dion,∗7T. Koblesky,∗3 T. Hachiya,∗1 M. Kurosawa,∗1 M. Mccumber,∗8 D. Mcglinchey,∗3 H. Nakagomi,∗1,∗9 R. Nouicer,∗1H. Sako,∗1 S. Sato,∗1 M. Shimomura,∗10 T. Sumita,∗1 A. Taketani,∗1 H. Torii,∗1 and the PHENIX VTX group
Measurements of the D0 meson (and its charge conjugate) via the hadronic channel D0 → K − + π +(D̄0 → K + + π − ) in p + p collisions provide important
information. First, the total cross section of charm
production can be obtained and compared with perturbative Qantum Chromodynamics (pQCD) calculations. Second, it provides the baseline measurementof charm production in order to understand the properties of the hot and dense QCD medium, created byheavy ion collisions, by comparing D0 production inp + p and Au+Au collisions.
Direct reconstruction of the D 0 meson is challenging
because of its short decay lengths, cτ = 122.9µm, anda very large combinatorial background. However, theSilicon Vertex Tracker (VTX),1),2) installed into thePHENIX experiment during the 2011 run, can separate the primary vertex and the D0 → K − + π + decay vertex with high resolution. This can substantiallyeliminate the combinatorial background.
The strategy to extract D0 production yield is basedon an invariant mass analysis of all the possible pairsof two oppositely charged tracks. The invariant mass,MKπ , is defined as follows.
→→p − )2(1)p++−MKπ = (E+ + E− )2 − (−E± =

→m2± + −p 2±-

The invariant mass distribution of K π+ before applying a DCA cut
CutMass48

Entries
Mean
RMS

22000

1.02221e+07
1.83
0.1697

Counts/5MeV/c2

20000

18000

16000

14000

12000

MD0 = 1.86486 GeV/c2
1.6

1.7

1.8

1.9

2

2.1

MK-π+ (GeV/c 2)-The invariant mass distribution of K π+ after applying a DCA cut
CutMass52
Entries
683224Mean
1.823
RMS
0.16851300Counts/5MeV/c2

1200
1100
1000
900
800700600

MD0 = 1.86486 GeV/c2

500
1.6

1.7

1.8

1.92

2.1

MK-π+ (GeV/c 2)

Fig. 1. The invariant mass distribution of K − π + before
and after applying a DCA cut

track, K − or π + , of the D0 meson is larger than that ofprimary track, which comes from the primary vertex.
Therefore, we can reject those primary tracks used in
the invariant mass analysis by applying a DCA cut.
The intermediate result is shown in Fig. 1, whichindicates the invariant mass distribution of K − π + before and after applying a DCA cut. There is a hint ofa small peak at the D0 mass region in the mass spectrum with the DCA cut (the lower panel). However,there still remains a substantial combinatorial background. Therefore, it is necessary to further reducethe combinatorial background without sacrificing too
many D0 signals, and it is expected to be solved by
selecting tracks in accordance with the D0 decay kinematics using the VTX. We have been developing ananalysis code that selects the decay kinematics of D0
appropriately, leaving room for improving of the signal
to background ratio.

(2)

In the above equations, the subscript + or − indicates the charge of the track. We assigned the mass,(m+ , m− ) = (mK , mπ ) or (mπ , mK ).
In the absence of any ion pair selection, the signal
to combinatorial background ratio would be too small
to extract the D0 signal in the invariant mass distribution. It is, therefore, mandatory to select tracks on thebasis of kinematical and geometrical considerations.
One of the quantities that is useful for track selectionis the distance of the closest approach (DCA) of thereconstructed track to the primary vertex in the planetransverse to the beam direction. DCA of the daughter
RIKEN Nishina CenterDepartment of Physics, Yonsei University∗3 Department of Physics, University of Colorado∗4 CNS, Univ. of Tokyo∗5 Department of Physics, Kyoto University∗6 Department of Physics, Baruch Colleage, CUNY
∗7 Department of Physics, Stony Brook University∗8 Los Alamos National Laboratory∗9 Department of Physics, Univ. of Tsukuba∗10 Department of Physics, Iowa State University∗1∗2References1) A. Taketani et al. [PHENIX Collaboration]; Nucl. Instrum. Meth. A. 623, 374, 2010.
2) T. Hachiya et al.: RIKEN Accel. Prog. Rep. 44, 179(2011).

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