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#
Heavy Flavor Physics at the sPHENIX Experiment

Zhaozhong Shi1, on behalf of the sPHENIX Collaboration
1 Physics Division, Los Alamos National Laboratory, Los Alamos, NM, 87545 USA
<EMAIL_ADDRESS>

###### Abstract

The sPHENIX experiment is a state-of-the-art jet and heavy flavor physics
detector which successfully recorded its first Au + Au collision data at 200
GeV at RHIC. sPHENIX will provide heavy flavor physics measurements covering
an unexplored kinematic region and unprecedented precision at RHIC to probe
the parton energy loss mechanism, parton transport coefficients in quark-gluon
plasma, and the hadronization process under various medium conditions. At the
center of sPHENIX, the Monolithic-Active-Pixel-Sensor (MAPS) based VerTeX
detector (MVTX) is a high-precision silicon pixel detector. The MVTX provides
excellent position resolution and the capability of operating in continuous
streaming readout mode, allowing precise vertex determination and recording a
large data sample, both of which are particularly crucial for heavy flavor
physics measurements. In this work, we will show the general performance of
heavy-flavor hadron reconstruction. In addition, we will discuss the
commissioning experience with sPHENIX. Finally, we will provide the projection
of b-hadron and jet observables and discuss the estimated constraints on
theoretical models.

##  1 Introduction

The sPHENIX experiment [1] is the first new experiment at RHIC in over 20
years. According to the 2015 [2] and 2023 [3] United States Nuclear Science
Advisory Committee (NSAC) Long Range Plan for Nuclear Science, the sPHENIX is
considered as a Department of Energy flagship experiment in heavy-ion physics.
The physics goal of sPHENIX is to probe the inner workings of quark-gluon
plasma (QGP) by resolving its properties at shorter and shorter length scales
and is complementary with experiments at the LHC.

sPHENIX is a state-of-the-art heavy flavor and jet detector at RHIC. It has a
2$\pi$ angular acceptance over the rapidity range of $|y|<1$. It consists of
tracking and calorimeter systems with excellent capabilities for studying the
strongly interacting QCD matter at RHIC. In addition, sPHENIX is equipped with
a minimum bias detector (MBD) and zero degree calorimeter (ZDC) in the far
forward region and a trigger system for global event characterization. A
schematic drawing of the sPHENIX detector is shown in Figure 1 below.

Figure 1: Schematic diagram of sPHENIX detector including all its subdetector
systems.

As the inner tracking system, the Monolithic-Active-Pixel-Sensor (MAPS) based
VerTeX detector (MVTX) and the intermediate silicon strip tracker (INTT)
leverage advanced silicon detector technologies. The outer tracking system is
made of a compact Time Projection Chamber (TPC) and the TPC Outer Tracker
(TPOT). The sPHENIX tracking system is able to operate in both triggered and
continuous streaming readout modes. The streaming readout acts as a
triggerless configuration, capable of sending all collisions to the data
acquisition system. The INTT provides us with a timing resolution of about 100
ns, which can resolve RHIC bunch crossings. Utilizing the ACTS tracking
algorithm with excellent tracking purity and efficiency [4] and thanks to the
excellent performance of the tracking detectors, sPHENIX achieves precision
momentum resolution and vertexing determination.

The sPHENIX calorimeter system includes the electromagnetic calorimeter
(EMCal) and hadronic calorimeter (HCal). Both EMCAL and HCal demonstrate
excellent energy linearities and resolutions in Fermilab test beam
experiments. Based on the hodoscope position dependent correction, the tower
averaged energy resolution of the EMCal is $\Delta E/\langle
E\rangle=3.5\%\oplus 13.3\%/\sqrt{E}$ [5]. The energy resolution of the HCal
is $\Delta E/E=11.8\%\oplus 81.8\%/\sqrt{E}$ [6]. Both of them meet the
requirements for its physics goals. Moreover, sPHENIX is the first experiment
with full $2\pi$ azimuthal barrel hadronic calorimeter in the mid-rapidity
range ($|\eta|<1$) at RHIC, enabling full jet reconstruction in heavy-ion
collisions.

##  2 The MVTX Detector

MVTX is a silicon pixel detector with excellent position resolution
approaching 5 $\upmu$m and offers continuous streaming readout option with a
strobe111A readout frame. Here it is set to be 89 $\upmu$s. length as short as
5 $\upmu$s [7], both of which are crucial for sPHENIX heavy flavor physics
studies. It is adapted from the inner three layers of Inner Tracking System-2
(ITS-2) detector from the ALICE experiment [8]. The sPHENIX MVTX consists of
48 staves222Rectangular flat planes that hold silicon sensors along the beam
axis. made of nine ALPIDE chips [9] and electronic signals are readout by one
front-end readout unit (RU). At the back end, the data will be transmitted to
the Front-End LInk eXchange (FELIX) board for further processing [10]. 1 FELIX
reads out eight RUs. Six FELIX systems are used to service whole MVTX system.

In addition to the detector and readout electronic systems, we employ negative
pressure cooling to maintain overall constant temperature of both the silicon
sensors and the readout electronics for their normal operation and safety.
Finally, slow control and quality control systems are used to operate and
monitor the detector status, ensuring high quality data taking.

MVTX staves were hand carried by air from CERN to LBNL. Two half barrels of
the MVTX detectors were assembled at LBNL and then shipped to BNL in October
2022. In March 2023, we installed the MVTX cooling and electronic systems and
successfully inserted the MVTX detector to the sPHENIX experiment. All 48
staves passed the electrical tests. We were able to readout the entire MVTX
and collect fake hit rate and threshold scan data before the RHIC Au ion beam
was turned on.

##  3 First Year Data Taking

sPHENIX anticipates to collect data from 2023 to 2025 [11]. In 2023, the Au +
Au collisions at $\sqrt{s_{NN}}=200$ GeV data taking had to be curtailed due
to an unfortunate valve box failure at RHIC on August 1st, 2023. Nevertheless,
from May 18th to August 1st 2023, sPHENIX has made significant progress in the
detector commissioning with beam. All sPHENIX detector subsystems were able to
take and record collision data. sPHENIX continued taking another two months of
comic data after August 1st for alignment and calibration purposes. All
sPHENIX detector subsystems were able to take and record data.

Currently, we are carrying out data production and event reconstruction.
Offline data analyses are also ongoing. As part of the tracking system, MVTX
is able to synchronize with INTT, TPC, and TPOT to record cosmic ray events. A
cosmic event display for tracking commissioning is shown in Figure 2.

Figure 2: A cosmic event display of the tracking system taken on 08/23/2023.
In this event, the Level 1 physics trigger for cosmic events, which requires
the coincident signals between two outer HCal towers, is fired. In addition,
the magnetic field is turned off (B = 0).

During the run, MVTX demonstrated a low noise level of less than 10-6 hot
pixels per chip per event after masking hot pixels [12]. The MVTX mostly
operated in continuous streaming readout mode with a few runs in triggered
mode. For this online display only, to reject the background and obtain pixels
fired by cosmic muons, we remove clusters with single pixels. We can see a
clear straight line cosmic muon track from the hits of MVTX, INTT, TPOC, and
TPOT.

In addition, we analyzed the Au + Au collision data. We correlated the total
number of pixels fired between two MVTX layers. In addition, we also correlate
clusters of INTT and TPOT. Figure 3 shows the strong correlations from the
subsystems.

Figure 3: Correlation between MVTX layer 0 and 1 within one strobe is shown on
the left and INTT and TPOT clusters is shown on the right. Strong correlations
are observed in MVTX, INTT, and TPOT with beam collisions.

The studies of both Au + Au beam collision and the cosmic events demonstrate
the functionality of sPHENIX tracking system and readiness for offline data
analysis.

##  4 Heavy Flavor Physics Program

To complete the RHIC science mission, the sPHENIX physics program consists of
jet substructure, open heavy flavor, quarkonia, bulk physics, and cold QCD
physics. sPHENIX plans to take Au + Au data at $\sqrt{s_{NN}}=200$ GeV in
2023, $p^{\uparrow}+p^{\uparrow}$ at $\sqrt{s}=200$ GeV in 2024, and high
luminosity Au + Au at $\sqrt{s_{NN}}=$ 200 GeV in 2025. With excellent
luminosity recorded, and tracking and vertexing capabilities, sPHENIX will be
able to perform precise measurements of fully reconstructed open heavy flavor
hadrons such as $D^{0}$, $B^{+}$, and $\Lambda_{c}^{+}$.

Thanks to their heavy masses, which are much greater than the QCD scale,
$\Lambda_{QCD}$, and QGP temperature, $T_{QGP}$, ($m_{Q}\gg\Lambda_{QCD}$ and
$m_{Q}\gg T_{QGP}$), heavy quarks, such as charm and beauty quarks, are
produced in hard scattering processes in the early stage of heavy-ion
collisions and have long thermal relaxation times. Their initial production
spectra can be calculated by perturbative QCD. Heavy quarks retain their
flavor and mass identities as they traverse through the QGP, making them
excellent probes to study the transport properties of QGP. A heavy flavor
particle widely used to study QGP is the $D^{0}$ meson because of its large
charm-quark fragmentation fraction and simple hadronic final states in the
decay channel of $D^{0}\to K^{-}\pi^{+}$. Figure 4 shows fully reconstructed
$D^{0}$ mesons in the decay channel of $D^{0}\to K^{-}\pi^{+}$ with sPHENIX
full detector simulations of Au + Au collisions.

Figure 4: Invariant mass distribution of both prompt and non-prompt $D^{0}$ in
the decay channel of $D^{0}\to K^{-}\pi^{+}$ from with HIJING Au + Au event
generator with pile up with the sPHENIX detector in GEANT 4 simulations [13].
In the simulations, KFParticle package is applied for secondary vertex
reconstruction. It should also be noted that this is 25 minutes of full
luminosity data.

According to the simulation studies shown in Figure 4, we expect to obtain the
statistics that can observe clear $D^{0}$ resonance with about 25 minutes of
data taking at 15 kHz. Thanks to the large minimum bias p + p and Au + Au
datasets, excellent statistics can be achieved for precise and differential
$D^{0}$ measurements. The projected performance of classic observables such as
$D^{0}$ nuclear modification factor $R_{AA}$ and elliptic flow $v_{2}$, which
characterize the microscopic properties of the QGP such as energy loss and
transport coefficients, are shown in Figure 5

Figure 5: The projected performance of nuclear modification factor $R_{AA}$
(left) and elliptic flow $v_{2}$ (right) of prompt (black) and non-prompt
(blue) $D^{0}$ as a function of $D^{0}$ $p_{T}$ with the sPHENIX experiment
from simulations [11] are shown above.

With the precision vertex resolution, we can determine prompt and non-prompt
$D^{0}$ meson using the distance of closest approach (DCA) in a data-driven
manner. The studies of $R_{AA}$ and $v_{2}$ of prompt $D^{0}$ mesons will help
us investigate charm-quark thermalization in the QGP and interaction with the
medium constituents. From the $R_{AA}$ and $v_{2}$ measurements of non-prompt
$D^{0}$ feed down from b hadrons, we can study flavor dependence of energy
loss and constrain beauty-quark transport coefficients in the QGP.

The charm baryon $\Lambda_{c}^{+}$ has a more complex 3-prong hadronic decay
topology: $\Lambda_{c}^{+}\to pK^{-}\pi^{+}$. sPHENIX is also able to fully
reconstruct prompt $\Lambda_{c}^{+}$. Preliminary studies to tag protons using
$dE/dx$ is underway to improve $\Lambda_{c}^{+}$ reconstruction. Figure 6
shows the projected $\Lambda_{c}^{+}/D^{0}$ ratio performance with sPHENIX in
p + p and Au + Au collisions

Figure 6: Projected performance $\Lambda_{c}^{+}/D^{0}$ as a function of
$\Lambda_{c}^{+}$ $p_{T}$ with full sPHENIX detector simulations in p + p
(black) and Au + Au (blue) [11]. The data point of $\Lambda_{c}^{+}/D^{0}$
with STAR Au + Au data at 200 GeV (red) is presented [14]. Theoretical model
predictions including Tsinghua, Catania, TAMU, and PYTHIA 8 are also overlaid.

The precise and differential measurement of $\Lambda_{c}^{+}/D^{0}$ with
sPHENIX over a wide range of $p_{T}$ and event multiplicity or centrality from
p + p to Au + Au will pinpoint the fragmentation and recombination mechanisms
of charm quark hadronization from vacuum to QGP at RHIC.

In addition to fully reconstructed open heavy flavor hadrons, sPHENIX is
capable of studying b-jets substructure and heavy flavor jet physics at RHIC.
Figure 7 shows the $R_{AA}$ as a function $p_{T}$ and jet $z_{g}$
distributions of b-jet in p + p and Au + Au collisions.

Figure 7: Projected precision of b-jet $R_{AA}$ as a function of $p_{T}$
(left) and substructure observable $\frac{1}{N_{jet}}\frac{dN}{dz_{g}}$ as a
function jet $z_{g}$ in p + p and Au + Au with full sPHENIX detector
simulations [11].

Equipped with high performance HCal, sPHENIX is expected to deliver the first
full b-jet measurements at RHIC. Moreover, sPHENIX will have excellent b-jet
reconstruction and tagging capabilities thanks to the calorimeters and MVTX.
sPHENIX will be able to perform differential subjet splitting function
measurements with good precision at low $p_{T}$. The b-jet substructure
measurements will test pQCD model calculations in p + p collisions and
quantify the medium modification in the unique sPHENIX kinematic region in Au
+ Au collisions, complementary to LHC jet substructure measurements

Finally, quarkonium spectroscopy stands as one of the flagship measurements of
sPHENIX hidden heavy flavor physics. Because of the color screening effect,
the binding energy of $\Upsilon$ will decrease as the temperature of QGP
increases [15]. In experiments, the sequential suppression of $\Upsilon(1S)$,
$\Upsilon(2S)$, and $\Upsilon(3S)$ production will be quantified by nuclear
modification factor $R_{AA}$ [16]. Hence, Upsilon can be used as a thermometer
to measure the temperature of QGP. The excellent performance of the sPHENIX
EMCAL and tracking detectors enables outstanding electron identification
capabilities to perform precise $\Upsilon\to e^{+}e^{-}$ measurements with
sufficient invariant mass resolutions to distinguish all three Upsilon states.
Figure 8 shows the dielectron invariant mass distribution near the Upsilon
resonances in central Au + Au simulations and $\Upsilon(1S)$, $\Upsilon(2S)$,
and $\Upsilon(3S)$ $R_{AA}$ as a function of $p_{T}$.

Figure 8: Projected performance $\Upsilon(1S)$, $\Upsilon(2S)$, and
$\Upsilon(3S)$ invariant mass distribution in 0-10% Au + Au collisions at
$\sqrt{s_{NN}}=200$ GeV from dielectron decay channel using sPHENIX simulation
scaled up to 24 billion events is shown on the left. The $\Upsilon(1S)$,
$\Upsilon(2S)$, and $\Upsilon(3S)$ $R_{AA}$ as a function of $p_{T}$ of
sPHENIX projection (solid circle) as well as the STAR data (open square) [17]
are shown on the right.

We can see that sPHENIX will be able to provide very precise and broad $p_{T}$
coverage (up to about 9 GeV/c) of $\Upsilon$ $R_{AA}$ measurements for
quarkonia physics studies. In addition, the $\Upsilon(3S)$ resonance may
potentially be separated from the $\Upsilon(2S)$ and observed in Au + Au
collisions for the first time by sPHENIX at RHIC, which is complementary to
the recent observation and $R_{AA}$ measurements of $\Upsilon(3S)$ with the
CMS 2018 LHC PbPb data. [18].

##  5 Summary

We have introduced the sPHENIX experiment, which is the first new experiment
at RHIC in over 20 years to study the microscopic properties of QGP and is
currently taking data. The sPHENIX physics program consists of jets, open
heavy flavor, quarkonia, bulk physics, and cold QCD. In the 2023 Au + Au run,
sPHENIX has demonstrated overall detector functionality and readiness for
offline data analysis. The correlation studies of tracking and colorimeter
systems validate the synchronization among subdetectors. Aside from the 200
GeV Au + Au collision data, sPHENIX have taken large cosmic datasets for
detector alignment and calibration studies. In several cosmic ray events, a
clear straight line muon tracks are observed without the magnetic field,
showing overall good quality of the sPHENIX data.

We have also reported the projected physics measurements with full sPHENIX
detector simulations assuming the luminosity documented in the 2022 sPHENIX
Beam Use Proposal [11]. We expect to achieve high statistics, fully
reconstructed charm and beauty hadron measurements to study heavy quark
diffusion, energy loss, and hadronization in QGP. Moreover, thanks to the
excellent calorimeter, tracking, and vertexing performance, sPHENIX will carry
out the first inclusive b-jet measurements with high precision at low $p_{T}$,
complementary to LHC experiments. Finally, Upsilon spectroscopy, one of the
major physics topics of the sPHENIX experiment, provides us with precision
measurement of QGP temperature. Potential observation and $R_{AA}$
measurements of $\Upsilon(3S)$ for the first time may be accomplished by
sPHENIX at RHIC.

##  6 Acknowledgement

We would like to express our gratitude to the ISMD 2023 conference organizers
for inviting us to present this work and providing useful information for the
proceedings submission. We particularly want to thank Professor Máté Csanád
for his friendly correspondence and cordial reception. This work is supported
by the United States Department of Energy Office of Science and Los Alamos
National Laboratory Laboratory Directed Research & Development (LDRD).

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