Heavy Flavor Physics at the sPHENIX Experiment

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.


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 2π 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.

The MVTX Detector
MVTX is a silicon pixel detector with excellent position resolution approaching 5 µm and offers continuous streaming readout option with a strobe1 length as short as 5 µs [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 staves2 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 √ s N N = 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.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.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.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 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.

Heavy Flavor Physics Program
The charm baryon Λ + c has a more complex 3-prong hadronic decay topology: Λ + c → pK − π + .sPHENIX is also able to fully reconstruct prompt Λ + c .Preliminary studies to tag protons using dE/dx is underway to improve Λ + c reconstruction.Figure 6 shows the projected Λ + c /D 0 ratio performance with sPHENIX in p + p and Au + Au collisions The precise and differential measurement of Λ + 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.
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 Υ will decrease as the temperature of QGP increases [15].In experiments, the sequential suppression of Υ(1S), Υ(2S), and Υ(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 Tsinghua: simul.coal, 10-80% Tsinghua: seq.coal, 10-80% Catania: coal only, 0-20% Catania: coal+frag, 0-20% TAMU: di-quark 0-5% TAMU: 3-quark 0-5% PYTHIA8 (CR) PYTHIA8 (Monash) Figure 6: Projected performance Λ + c /D 0 as a function of Λ + c p T with full sPHENIX detector simulations in p + p (black) and Au + Au (blue) [11].The data point of Λ + 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.and tracking detectors enables outstanding electron identification capabilities to perform precise Υ → e + e − measurements with sufficient invariant mass resolutions to distinguish all three Upsilon states.Figure 8  We can see that sPHENIX will be able to provide very precise and broad p T coverage (up to about 9 GeV/c) of Υ R AA measurements for quarkonia physics studies.In addition, the Υ(3S) resonance may potentially be separated from the Υ(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 Υ(3S) with the CMS 2018 LHC PbPb data.[18].

Summary
We have introduced the sPHENIX experiment, which is the first new experiment at   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 Υ(3S) for the first time may be accomplished by sPHENIX at RHIC.

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

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

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).
Number of pixels over threshold per strobe (L0)

Figure 3 :
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.

Figure 4 :
Figure 4: Invariant mass distribution of both prompt and non-prompt D 0 in the decay channel of D 0 → K − π + 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.

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.
shows the dielectron invariant mass distribution near the Upsilon resonances in central Au + Au simulations and Υ(1S), Υ(2S), and Υ(3S) R AA as a function of p T .
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

Figure 7 :
Figure 7: Projected precision of b-jet R AA as a function of p T (left) and substructure observable 1 N jet

Figure 8 :
Figure 8: Projected performance Υ(1S), Υ(2S), and Υ(3S) invariant mass distribution in 0-10% Au + Au collisions at √ s N N = 200 GeV from dielectron decay channel using sPHENIX simulation scaled up to 24 billion events is shown on the left.The Υ(1S), Υ(2S), and Υ(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.