Anisotropic flow measurements have demonstrated development of partonic
collectivity in $200\mathrm{GeV}$ Au+Au collisions at RHIC. To understand the
partonic EOS, thermalization must be addressed. Collective motion of
heavy-flavor (c,b) quarks can be used to indicate the degree of thermalization
of the light-flavor quarks (u,d,s). Measurement of heavy-flavor quark
collectivity requires direct reconstruction of heavy-flavor hadrons in the low
$\pt$ region. Measurement of open charm spectra to high $\pt$ can be used to
investigate heavy-quark energy loss and medium properties. The Heavy Flavor
Tracker (HFT), a proposed upgrade to the STAR experiment at midrapidity, will
measure $v_{2}$ of open-charm hadrons to very low $\pt$ by reconstructing their
displaced decay vertices. The innermost part of the HFT is the PIXEL detector
(made of two low mass monolithic active pixel sensor layers), which delivers a
high precision position measurement close to the collision vertex. The
Intermediate Silicon Tracker (IST), a 1-layer strip detector, is essential to
improve hit identification in the PIXEL detector when running at full RHIC-II
luminosity. Using a full GEANT simulation, open charm measurement capabilities
of STAR with the HFT will be shown. Its performance in a broad $\pt$ range will
be demonstrated on $v_{2}$ ($\pt > 0.5\mathrm{GeV}/c$) and $R_\mathrm{CP}$
($\pt < 10\mathrm{GeV}/c$) measurements of $\D$ meson. Results of
reconstruction of $\Lc$ baryon in heavy-ion collisions are presented.Comment: to appear in EPJ C (Hot Quarks 2008 conference volume

A. Adare

A. Andronic

B.I. Abelev

C. Adler

J. Adams

Jan Kapitán

O. Barannikova

P.R. Sorensen

R. Baier

S. Batsouli

Y.L. Dokshitzer

English

arXiv

Crossref

STAR inner tracking upgrade—a performance study

Springer - Publisher Connector

Eur. Phys. J. C (2009) 62: 217–221
DOI 10.1140/epjc/s10052-009-0992-4
Regular Article - Experimental Physics
STAR inner tracking upgrade—a performance study
Jan Kapitána
for the STAR Collaboration
Nuclear Physics Institute ASCR, Rez/Prague, Czech Republic
Received: 27 September 2008 / Revised: 5 March 2009 / Published online: 20 March 2009
© Springer-Verlag / Società Italiana di Fisica 2009
Abstract Anisotropic flow measurements have demon-
strated development of partonic collectivity in 200 GeV
Au + Au collisions at RHIC. To understand the partonic
EOS, thermalization must be addressed. Collective motion
of heavy-flavor (c, b) quarks can be used to indicate the de-
gree of thermalization of the light-flavor quarks (u, d, s).
Measurement of heavy-flavor quark collectivity requires di-
rect reconstruction of heavy-flavor hadrons in the low pT
region. Measurement of open charm spectra to high pT can
be used to investigate heavy-quark energy loss and medium
properties. The Heavy Flavor Tracker (HFT), a proposed
upgrade to the STAR experiment at midrapidity, will mea-
sure v2 of open-charm hadrons to very low pT by recon-
structing their displaced decay vertices. The innermost part
of the HFT is the PIXEL detector (made of two low mass
monolithic active pixel sensor layers), which delivers a high
precision position measurement close to the collision vertex.
The Intermediate Silicon Tracker (IST), a 1-layer strip de-
tector, is essential to improve hit identification in the PIXEL
detector when running at full RHIC-II luminosity. Using
a full GEANT simulation, open charm measurement ca-
pabilities of STAR with the HFT will be shown. Its per-
formance in a broad pT range will be demonstrated on v2
(pT > 0.5 GeV/c) and RCP (pT < 10 GeV/c) measure-
ments of D0 meson. Results of reconstruction of C baryon
in heavy-ion collisions are presented.
1 Introduction
Produced by the initial hard scattering, heavy quarks (c, b)
are an ideal tool to probe early stages of heavy ion colli-
sion [1]. They derive their mass from the Higgs field, and
therefore are not modified by the surrounding QCD medium
(they stay heavy even in the case of chiral symmetry restora-
tion).
a e-mail: kapitan@rcf.rhic.bnl.gov
Previous studies have identified the development of par-
tonic collectivity in heavy-ion collisions at RHIC [1], but
they have not yet demonstrated thermalization of the created
matter. The study of heavy quark collectivity may allow us
to address this issue. Measurement of elliptic flow (v2) of
open charm hadrons to low transverse momentum (pT) is of
particular interest.
Suppression of high pT hadron production at RHIC [2–4]
is commonly thought to arise from partonic energy loss in
dense matter due to induced gluon radiation [5, 6]. Radiative
energy loss of heavy quarks is expected to be suppressed
(dead cone effect) [7].
Most heavy-flavor measurements at RHIC use semilep-
tonic decay modes of heavy-flavor hadrons, but lack of pre-
cise kinematical information about the parent hadron makes
it difficult to study dynamics at low pT [8]. Measurements
of nuclear modification factor (RAA) of heavy-flavor decay
electrons at high pT [9, 10] indicate a significant energy loss
of heavy quarks. However, knowledge of the relative contri-
butions of charm and bottom decays to electron spectra is
crucial to interpret these results. Electron spectra from open
charm hadron decays are also sensitive to relative C/D me-
son yield [11], due to different branching ratios of their in-
clusive electron decay channels.
In central Au + Au collisions at RHIC, a baryon/meson
enhancement has been observed in the intermediate pT re-
gion (2 < pT < 6 GeV/c) [12–14]. These results are usu-
ally explained by a hadronization mechanism involving col-
lective multi-parton coalescence rather than independent
vacuum fragmentation. The success of the coalescence ap-
proach implies deconfinement and possibly thermalization
of the light quarks prior to hadronization.
Since C is the lightest charmed baryon, and its mass
is not far from that of the D0 meson, a similar pattern
of baryon/meson enhancement is expected in charm sec-
tor [15]. C/D0 enhancement is also believed to be a signa-
ture of a strongly coupled quark-gluon plasma [16]. There-
fore it would be very interesting to measure RCP of C
baryon and compare it to RCP of D0 mesons.
218 Eur. Phys. J. C (2009) 62: 217–221
Direct reconstruction of open charm hadrons is neces-
sary for these measurements. Given the large combinatorial
backgrounds in heavy-ion collisions, topological reconstruc-
tion is needed to achieve reasonable signal significance. The
Heavy Flavor Tracker (HFT), a proposed upgrade [17] to the
STAR experiment, will enable measurement of open charm
hadrons by reconstructing their displaced decay vertices.
2 Heavy flavor tracker design
The HFT detector consists of two subsystems: The PIXEL
detector (2 layers) and Intermediate Silicon Tracker (IST,
1 layer). The midrapidity tracking system of STAR further
includes the existing Silicon Strip Detector (SSD) and the
large Time Projection Chamber (TPC). Compared to the pre-
vious version, the current design of the HFT has been opti-
mised for lower mass and better hit resolving, however it has
not been fully simulated yet. In the following, simulation re-
sults are presented for the previous design. The parameters
of these are displayed in Table 1.
The PIXEL detector is made of low-mass monolithic ac-
tive pixel sensors (MAPS, see Fig. 1) and enables high preci-
sion measurement close to the primary collision vertex, fea-
turing 18 µm pixel pitch and a thickness of only 0.28% X0
per layer. It is fabricated by CMOS technology.
The detector configuration is shown in Fig. 2. Inner layer
sensors facing the beam pipe will improve STAR’s di-lepton
capability by rejecting e+e− pairs from photon conversions
(beam pipe thickness being only ≈ 0.15% X0).
As there is a very large number of pixels, the read-
out of the PIXEL detector is ≈200 µs. Given the high lu-
minosity projected for the future RHIC-II upgrade (50 ·
1026 cm−2 s−1), the PIXEL detector will integrate over ≈10
minimum bias collisions. The IST detector is essential to
improve hit identification at PIXEL2 layer in this pile-up
environment. The IST detector consists of 1 layer of fast
single-sided strip sensors.
Table 1 Hit position resolutions of SSD + HFT layers for the two
design versions. IST2 (simulated design) has two layers (A, B) with
crossed strips
current design simulated design
layer r (cm) hit resol. r (cm) hit resol.
(r − φ × z) (r − φ × z)
(µm × µm) (µm × µm)
SSD 23 30 × 699 23 30 × 699
IST2-B − − 17 17×12000
IST2-A − − 17 12000 × 17
IST1 14 115×2900 12 17 × 6000
PIXEL2 8 5 × 5 7 9 × 9
PIXEL1 2.5 5 × 5 2.5 9 × 9
Fig. 1 MAPS principle of operation: electrons created in the epitaxial
layer thermally diffuse towards low potential n-well region. A small
contribution to the total signal also exists from electrons created in the
p++ substrate
Fig. 2 One half of the PIXEL detector with its support structure. It
will be mounted on one side, where also all the cables and cooling air
comes from
3 Simulation results
HIJING central Au + Au events at √sNN = 200 GeV with
added D0 and C were filtered through GEANT and STAR
detector response simulators. To assess the impact of pile-
up, pseudo-random hits were added to PIXEL1 and PIXEL2
layers, corresponding to a minimum bias (MB) collision rate
of 80 kHz and a primary vertex diamond size σPV _Z =
15 cm. This is upper estimate of pile-up, in fact current
RHIC-II projections show smaller luminosity (correspond-
ing to MB collision rate ≈ 50 kHz) and bigger σPV _Z .
The decay modes used for reconstruction were D0 →
K− +π+ (B.R. 3.8%) and C → K− +π+ +p (B.R. 5.0%).
A possibility of using a C decay through a resonant inter-
mediate (1520) state has been investigated, and a signifi-
cant improvement of S/B ratio could be expected. However,
a full simulation has not been performed, and results shown
hereafter are for the non-resonant decay.
Eur. Phys. J. C (2009) 62: 217–221 219
As already mentioned in Sect. 2, the track impact pa-
rameter (pointing) resolution is mainly delivered by the
PIXEL detector, which is consistent with results from full
simulation shown in Fig. 3. For particle identification (PID)
of daughter particles, K −π separation for pT < 1.6 GeV/c
and (K + π) − p separation for pT < 3.0 GeV/c is ex-
pected using the MRPC TOF detector [18], with an effi-
ciency ≈ 90%.
Reconstruction efficiencies for D0 and C are shown in
Fig. 4. The daughter tracks are required to be well recon-
structed in the TPC and have correctly associated hits in both
PIXEL layers. Efficiency for C is smaller due to the fact,
that there are three daughter tracks with, on average, lower
pT than in the case of D0.
For C reconstruction, primary track combinatorial
background is huge due to its very short decay length
(cτ = 59.9 µm) and three-body decay. To reduce this back-
ground, PID information of daughter tracks is required for
Fig. 3 Track impact parameter resolution. Symbols: full detector sim-
ulation, lines: calculation for PIXEL detector only [17]
Fig. 4 Efficiency of reconstructing daughter tracks of D0 and C in
|η| < 1. Note that no cuts to isolate signal from combinatorial back-
ground have been applied here
pT < 5 GeV/c, limiting the acceptance. It is possible to re-
construct C at higher pT, but this has not been studied in
detail yet.
Given the overall uncertainty of cc¯ cross section at RHIC,
D0 dN/dy = 0.002 per binary collision has been used for
signal estimates, which is half of the value measured by the
STAR Collaboration [19]. C/D0 ratio 0.2 was assumed for
the case of no enhancement [15]. The shape of pT spectra
for D0 and C in central Au + Au collisions was assumed
to be a power-law, with 〈pT〉 = 1.0 GeV/c and n = 11 (with
C being a normalisation constant):
1
pT
dN
dpT
= C ·
(
1 + pT
p0
)−n
(1)
p0 = 〈pT〉 · n − 32 (2)
To reduce the large combinatorial background, topo-
logical cuts have been applied: only tracks with Distance
of Closest Approach to event Primary Vertex DCAPV >
DCAPV,cut have been used, where the cut is in the range
40–80 µm depending on transverse momentum of recon-
structed D0 (C). The DCA of daughter tracks to the decay
vertex was required to be less than 2σ and D0 (C) momen-
tum was required to point back to the event primary vertex.
Finally, a 2 (3) particle invariant mass cut has been applied.
Signal significance of D0 + D0 from 100 × 106 cen-
tral Au + Au collisions is shown in Fig. 5. To estimate
background for D0 (C) at different centralities, (Npart )2
((Npart )3) scaling was used. For the signal, the same RCP
was assumed as measured by STAR for charged hadrons [3].
This gives a factor 4 higher yield at high pT in peripheral
(60–80%) collisions than given by Nbin scaling. Ratio of
Nbin between central (0–10%) and peripheral (60–80%) col-
lision is ≈44.
Fig. 5 D0 signal significance in central Au + Au events. The effect of
pile-up is visible in low and medium pT region
220 Eur. Phys. J. C (2009) 62: 217–221
For measurements of D0 v2 and RCP, statistics of 500 ×
106 minimum bias Au+Au events was assumed. This is the
amount of data the STAR detector can take in about 1 month
of running (with DAQ rate 500 Hz and 40% beam up time).
Estimated statistical errors on v2 (for D0 + D0, |η| < 1)
are shown in Fig. 6. For pT > 1.0 GeV/c, the HFT will be
able to distinguish between extreme elliptic flow scenarios
in the coalescence picture in the first year of operation.
500 × 106 minimum bias events include 62.5 × 106 cen-
tral and 125 × 106 peripheral events for RCP measurement.
D0 signal significance (100 × 106 central events) in 9 <
pT < 10 GeV/c bin is about 15σ (consistent with Fig. 5).
The background is negligible here, so signal significance in
100 × 106 peripheral events will be √(44/4) ≈ 3.3 times
smaller. In a similar way, the calculation was done for lower
pT (taking into account the background here), and the esti-
Fig. 6 Two scenarios for D0 elliptic flow (charm quark flow zero or
equal to the flow of light quarks) and estimated statistical errors
Fig. 7 Estimated statistical errors for RCP measurement of D0 meson,
in 500 × 106 minimum bias Au + Au events
mated statistical errors on D0 RCP measurement are shown
in Fig. 7. At highest pT, the relative error is 20%, which is a
good precision for this region.
For measurements of C, statistics of 2 × 109 minimum
bias and 250 × 106 central triggered events was assumed,
giving 500 × 106 central and 500 × 106 peripheral events
used for RCP. C reconstruction cuts were optimized in pT
range 2–5 GeV/c, estimated mass peak for the middle pT
bin is shown in Fig. 8. Estimated signal significance for this
pT bin in 500 × 106 central Au + Au collisions is 8σ .
To estimate errors of C/D0 measurement, two scenarios
have been used: 1. no enhancement, ratio equal to 0.2 and
flat in pT, 2. the same enhancement as /K0S [14]. D0 has
much larger yield and cτ than C, statistical errors coming
from its measurement are therefore negligible. C/D0 ratio
in peripheral collisions is assumed flat (in rough agreement
Fig. 8 Expected invariant mass distribution of C,
3 < pT < 4 GeV/c, for 500 × 106 central events
Fig. 9 Expected errors on C/D0 measurement, assuming 500 × 106
central and 500 × 106 peripheral collisions. Except for the lowest pT
bin, the errors are dominated by measurement of C in peripheral col-
lisions
Eur. Phys. J. C (2009) 62: 217–221 221
with /K0S [14]), with statistical errors given by rescaling
signal and background from central to peripheral collisions,
as described above.
Statistical errors on RCP(C)/RCP(D0) are shown in
Fig. 9, where the two extreme cases can be well distin-
guished. Thus, baryon/meson ratio in charm sector will be
measured with good precision, for the first time in heavy ion
collisions.
4 Conclusions
The HFT detector will measure open charm hadrons over a
broad pT range, enabling precision study of charm collectiv-
ity, energy loss and baryon/meson ratios. These are impor-
tant ingredients for a systematic study of the dense medium
created in heavy-ion collisions at RHIC.
This will be achieved by using low mass MAPS sensors
(PIXEL) together with a fast strip detector (IST), deliver-
ing high efficiency and ultimate pointing resolution at low
pT, even in the high luminosity environment of RHIC-II.
Improved reconstruction efficiency is expected due to im-
proved detector design and to possibly lower pile-up hit den-
sities in RHIC-II luminosity, to be fully simulated in the fu-
ture.
Acknowledgements This work was supported in part by the IRP
AV0Z10480505, by GACR grant 202/07/0079 and by grant LC07048
of the Ministry of Education of the Czech Republic.
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STAR inner tracking upgrade - A performance study

STAR inner tracking upgrade - A performance study

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