II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Improvement of the DCA resolution for PHENIX
G. Nukazuka,∗1 Y. Akiba,∗1 T. Hachiya,∗1,∗2 and T. Todoroki∗3

DCA vs residual of clusters in s direction
350

300

300

DCA (µm)

200

250

100

200

0−100

150−200

100−300

50−400−0.1 −0.08 −0.06 −0.04 −0.02 0 0.02 0.04 0.06 0.08
Residual in s direction (cm)0.1

Entries

4000Fig. 1. Correlation between DCA and the cluster residual inthe s direction in the innermost layer of the west arm withthe selection 1.5 < pT < 2.0 GeV/c.
∗1∗2∗3RIKEN Nishina Center
Department of Mathematical and Physical Science, Nara
Women’s University
Tomonaga Center for the History of the Universe, Universityof Tsukuba

DCA distributions
6000

DCA
Entries
104869Mean
2.233
Std Dev
83.08
χ2 / ndf
29.88 / 24
Constant
4256 ± 20.2
Mean
3.666 ± 0.366
Sigma
76.26 ± 0.49
Corrected DCA
Entries
104869Mean0.1347
Std Dev
59.66
χ2 / ndf
23.21 / 17
Constant
5853 ± 27.5
Mean
1.18 ± 0.26
Sigma
55.62 ± 0.35DCA

5000

Fitting to DCA with gaussianCorrected DCAEntries

4000

Fitting to corrected DCA
with gaussian

3000
2000
1000
0−400−300−200−1000100
DCA (μm)

200

300

400

Fig. 2. DCA (dark black) and corrected DCA (light blue)
distributions taken by the west arm with the selection
1.5 < pT < 2.0 GeV/c.
100

DCA resolution as a function of pT
Nominal in 2011
West in 2016 without correction

90
DCA resolution (µm)

The PHENIX experiment accumulated data from theRelativistic Heavy Ion Collider at Brookhaven NationalLaboratory from 2001 to 2016 to study quark-gluonplasma and the spin structure of the nucleon.
The measurement of the distance of closest approach(DCA), which is the minimum distance from a beamcollision point to the trajectory of a reconstructed particle, significantly suppress the background to heavy-flavor
production measurements in a single electron channel.
The vertex tracker (VTX)1,2) consists of two layers withsilicon pixel sensors and two layers with silicon strip sensors. The VTX in each of the west and east arms measures the trajectory, and the beam-beam counter determines the Z-coordinate of the beam collision point. DCA
is calculated event by event using this information.
In 2016, PHENIX measured Au-Au collisions at the√collision energy sN N = 200 GeV. One of the tasksneeded to start the data summary tape production of
the data is the alignment of the VTX detector. In thealignment process, we found strong linear correlations
between DCA in the transverse direction with respect tothe beam-axis and the residual in track fitting in the direction s = r∆ϕ (Fig. 1), where r is the distance from thebeam axis and ∆ϕ is the relative azimuth angle between
hits and track projections on the VTX plane. The abscissa and ordinate axes represent the residual in the s direction and the transverse DCA, respectively. Only clusters with a transverse momentum pT between 1.5 GeV/cand 2.0 GeV/c are shown in Fig. 1. Green points show
average values over the DCA of a residual bin. A linear
fitting shown with the red line estimates the correlation.
The correlation strength depends on pT , the arm, andthe VTX layer. The correlations can be used to correct
the DCA value and to improve the DCA resolution. The
corrected DCA distribution is narrower than the raw dis-West in 2016 with correction
East in 2016 without correction

80

East in 2016 with correction

7060
5040300123456p (GeV/c2)

78910TFig. 3. DCA resolutions as a function of pT . Points except
black ones are slightly shifted for visibility.

tribution (Fig. 2). The dark and light blue histograms
represent the raw and corrected DCA distributions, respectively. Fittings with a Gaussian function to the raw
DCA (red) and the corrected DCA (orange) yield DCAresolutions.
Figure 3 shows DCA resolution as a function of pT .
The dark and light blue graphs are the raw and corrected
DCA resolutions in the west arm, respectively, while thegraphs in dark and light red are those in the east arm.
The black points indicate the nominal DCA resolution
for a run in 2011.2) By applying all corrections, the resolution is improved by 15% to 35%.
In 2016, VTX was operated without one strip layer inthe west arm owing to a beam accident in 2015. Under those severe conditions, a DCA resolution less than
50 µm was achieved in pT > 2.0 GeV/c, while it wasapproximately 60 µm in 2011.2)
This method may reduce background in the heavy flavor measurements because it is valid for data from other
PHENIX runs, in principle.
References

1) K .Adcox et al., Nucl. Instrum. Methods Phys. Res. A
499, 469 (2003).
2) A. Adare et al., Phys. Rev. C 93, 034904 (2016).

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