The prototype of a time digitizing system for the BESⅢ endcap TOF (ETOF) upgrade is introduced in this paper. The ETOF readout electronics has a distributed architecture. Hit signals from the multi-gap resistive plate chamber (MRPC) are signaled as LVDS by front-end electronics (FEE) and are then sent to the back-end time digitizing system via long shield differential twisted pair cables. The ETOF digitizing system consists of two VME crates, each of which contains modules for time digitization, clock, trigger, fast control, etc. The time digitizing module (TDIG) of this prototype can support up to 72 electrical channels for hit information measurement. The fast control (FCTL) module can operate in barrel or endcap mode. The barrel FCTL fans out fast control signals from the trigger system to the endcap FCTLs, merges data from the endcaps and then transfers to the trigger system. Without modifying the barrel TOF (BTOF) structure, this time digitizing architecture benefits from improved ETOF performance without degrading the BTOF performance. Lab experiments show that the time resolution of this digitizing system can be lower than 20 ps, and the data throughput to the DAQ can be about 92 Mbps. Beam experiments show that the total time resolution can be lower than 45 ps.
The deuteron (d) is a bound state of protons and neutrons and has long been a prominent subject of physics. In the field of nuclear physics, the coalescence model was initially proposed to explain the emission of d in reactions induced by protons with energies of 25−30 GeV [1, 2]. Over the years, the coalescence model has become an effective tool for describing the production of particles and anti-particles [3]. The study of d production is crucial to verifying theoretical models [4] and provides a basis for understanding the particle production mechanism [5]. In the fields of astrophysics and cosmology, the study of processes involving cosmic anti-nuclei, such as anti-deuterons (ˉd), serves as a sensitive probe for dark matter annihilation and allows for indirect studies of dark matter [6, 7]. This type of study is also important for understanding the properties of dense astrophysical objects, such as neutron stars [8,9]. In the field of particle physics, experimental studies of processes involving d are critical to validating the Lund string fragmentation model and predicting d production in Z boson decays. [10]. Many results have been reported by various experiments, including those on heavy ion collisions [11], proton-proton collisions [12], proton-nucleus collisions [13], and photoproduction reactions [14]. In contrast, relevant studies on positron-electron collisions are relatively limited owing to their low production cross-sections [15].
In recent years, an increasing number of hadrons, such as qˉq for mesons, qqq for baryons, qˉqqˉq for “tetraquark” states [16], and qqqqˉq for “pentaquark” states [17], have been observed in experiments. In particular, the exotic state d*(2380), with a mass around 2380 MeV/c2 and a width of approximately 70 MeV, was observed in the isoscalar double-pionic fusion process pn→dπ0π0 [18] and was subsequently confirmed in many other processes [19−22]. This state has been proposed as an excited d, a molecule with a large ΔΔ component [23], or a hexaquark state dominated by a hidden-color component [24]. Because one-third of d*(2380) decays into final states involving d, processes involving d offer potential test-beds to investigate the properties of d*(2380). The BESIII experiment, operated in the tau-charm energy region, has collected a large positron-electron collision data sample in the 4.009−4.946 GeV energy range. This provides a good opportunity to study d and ˉd production, the d*(2380) resonance, dibaryon states, and hexaquark states.
Good performance in particle identification (PID) is essential for precision measurements in quark flavor physics, τ physics, top physics, Higgs physics, and other fields [25]. The BES and BESIII Collaborations reported systematic studies of the PID efficiencies of electrons, muons, pions, kaons, and protons [26−29]. The LHCb and BaBar Collaborations have also reported extensive PID studies [30, 31]. However, little knowledge on d PID is currently available owing to the low production rate. An effective d identification method is helpful for reducing backgrounds from other particles. Previously, the ARGUS [32], BaBar [33], CLEO [34], and ALEPH [35] Collaborations reported ˉd production, whereas only the ALEPH Collaboration reported a d PID study. Usually, the specific ionization energy loss (dE/dx) and time-of-flight (TOF) measurements are used for d or ˉd identification. Details of methods and momentum ranges in d or ˉd PID studies from different experiments are shown in Table 1.
Table 1
Table 1.
Methods and momentum ranges of d or ˉd PID studies from different experiments.
In this study, we investigate d PID efficiencies using the dE/dx method with a data sample collected by the BESIII detector at center-of-mass (c.m.) energies between 4.009 and 4.946 GeV. ˉd is not considered owing to limited statistics. In addition, due to the challenges of detecting low-momentum particles using the TOF detector and the limited statistics of high-momentum d, we only aim for the PID efficiencies of d with momenta ranging from 0.52 to 0.72 GeV/c.
The Beijing Spectrometer III (BESIII) [36] is a general-purpose detector operated at the Beijing Electron-Positron Collider II [37]. The BESIII detector consists of four sub-detectors: the mult-layer drift chamber (MDC), TOF counter, electric-magnetic calorimeter, and muon chamber. The MDC sub-detector determines the momentum and vertex position for charged particles [38]. It also provides dE/dx information for PID of charged particles. It is a type of gas detector that contains multiple layers of field wires and signal wires, and its operating gas consists of a mixture of helium (He) and C3H8 in a ratio of 60:40. The energy loss of charged particles through ionization in the working gas, dE/dx, is obtained by measuring the charge deposited on the signal wires. After dE/dx calibration, a resolution of approximately 6% is obtained for minimum ionization particles. The TOF counter measures the flight time of charged particles, which is widely used for PID. The time resolution in the TOF barrel [36] region is 68 ps, whereas that in the end-cap [36] region is 110 ps. In 2015, the end-cap TOF system underwent an upgrade using multi-gap resistive plate chamber technology, resulting in an improved time resolution of 65 ps [39].
The BESIII detector is simulated by the GEANT4-based simulation software BOOST [40, 41], which includes the geometric and material description of the BESIII detector, the detector response, and digitization models.
The data samples taken at c.m. energies between 4.009 and 4.946 GeV with a total integrated luminosity of approximately 18fb−1 are used in this analysis. For the data sample at each energy point, the c.m. energy is measured using the e+e−→μ+μ− process, with an uncertainty less than 1.0 MeV [42, 43], and the integrated luminosity is measured using the Bhabha process, with an uncertainty of 1.0% [43]. The data samples are divided into seven subsamples according to different data taking periods to consider slightly different detector performances.
The e+e−→ˉpˉnd process is simulated in the phase-space model with ConExc [44, 45] at each c.m. energy. The potential backgrounds are studied using the inclusive Monte Carlo (MC) sample, corresponding to an integrated luminosity of 32 fb−1 at √s=4.178 GeV with KKMC [46, 47]. The MC samples used in this study are based on Geant4 version 9.3.p01, with the QGSP_BERT_CHIPS physics list [48].
The BESIII experiment usually combines both dE/dx and TOF information to identify charged particles. As shown in Fig. 1 and Fig. 2, both dE/dx and TOF information can effectively separate d from other particles in a certain momentum range. Because the c.m. energies are all below 5 GeV and d is heavy (approximately 1.87 GeV/c2), the momentum of d is relatively low (mostly below 0.75 GeV/c) when produced. The low momentum d is easy to circulate within the MDC, which makes it unable to hit the TOF detector. Because only a small fraction of deuterons can reach the TOF detector, identifying d with the TOF information will cause significant efficiency loss. Therefore, we use the dE/dx information to study d PID, with the help of the TOF information. See Section IV.D for more information.
Figure 1
Figure 1.
(color online) Distributions of normalized pulse height of dE/dx (NPH) versus momentum (left) and β versus momentum (right). All plots are based on the 2016 data sample, which was collected in 2016 at a center-of-mass energy of 4.178 GeV.
Figure 2.
(color online) Distributions of NPH (left) and β (right) for candidates in the momentum range 0.595−0.605 GeV/c. All plots are based on the 2016 data sample.
The dE/dx method uses the normalized residual of dE/dx, denoted as
χdE/dx=dE/dxmeas−dE/dxexpσdE/dx,
(1)
where dE/dxmeas, σdE/dx, and dE/dxexp represent the measured value, the uncertainty of dE/dxmeas, and the expected value, respectively. The χdE/dx distribution is expected to follow a normal distribution, as shown in Fig. 3.
Figure 3
Figure 3.
(color online) Fits to χdE/dx (left) and χTOF (right) of the 2016 data sample (dots with error bars). A Gaussian function is marked with a blue dashed line, and a first-order Chebychev polynomial is marked with a red dashed line.
Once the χdE/dx distribution is obtained, the probability density function of a given particle hypothesis is constructed as
prob(χ2dE/dx,1)=1−f(12,x),
(2)
with x=χ2dE/dx/2 and f(12,x)=1√π×∫x0t−12×e−tdt. Here, 'prob' represents the probability of a value greater than the observed χ2dE/dx. It must satisfy a certain value to achieve the desired identification ability. In d identification, prob(χ2dE/dx,1) is usually required to be greater than a certain value in the range of (0,1), and a larger value represents a more stringent PID requirement.
Deuterons are expected to be primarily produced through the reaction of charged particles with beam pipe materials. As a result, the flight time is divided into two segments. However, because the flight distance in the beam pipe is extremely short, the impact on the time resolution is negligible. Therefore, this effect is disregarded in this study. As shown in Fig. 1, β represents the ratio of the speed of charged particles to the speed of light, defined as
β=Lpathc×tTOF,
(3)
where Lpath, c, and tTOF represent the flight distance of the particle, the speed of light, and the flight time of the charged particle, respectively. The χTOF value is given by
χTOF=βmeas−βexpσTOF,
(4)
where βmeas, σTOF, and βexp represent the measured value, the uncertainty of βmeas, and the expected value, respectively. Similarly, the χTOF distribution is expected to follow a normal distribution, as shown in Fig. 3.
When charged particles pass through a detector, they interact with detector materials, thereby causing momentum loss. To account for this effect, BESIII has developed a track fitting algorithm based on the Kalman filter method [38]. This algorithm carefully handles the effects of multiple scattering, energy loss, non-uniformity of the magnetic field, and wire sag. To improve the momentum resolution and reduce the mean value of the difference between truth and reconstructed momenta in candidate events, we apply the track fitting algorithm for d reconstruction. The input/output check shows that this algorithm works well in d momentum correction. As shown in Fig. 4, the difference between the corrected and true momenta is significantly improved in the simulated sample.
Figure 4
Figure 4.
(color online) Momentum difference between reconstruction and truth before (left) and after (right) correction. Two Gaussian functions are marked by blue and black dashed lines, and a first-order Chebychev polynomial is marked by a red dashed line.
To select d candidates, the number of charged tracks must be larger than two for data, whereas it must be two and the net charge must be zero for MC simulation. Charged tracks are reconstructed with the MDC hits within the range |cosθ| < 0.93, where θ is the polar angle with respect to the z-axis. They are required to originate from the interaction region, defined as Rxy < 1.0 cm and |Vz| < 10.0 cm, where Rxy and |Vz| are the projections of the distances from the closest approach of the tracks to the interaction point in the x-y plane and the z direction, respectively. The combined dE/dx and TOF information is used to identify the observed particles to improve the purity of the sample. The probabilities of identifying the track as electrons, pions, kaons, and protons must be less than 0.001, which ensures high purity of the selected d samples.
After applying all the above selection criteria, the remaining backgrounds are studied with the large inclusive MC sample generated at √s=4.178 GeV. In the d signal region, the distribution of NPH versus momentum of the inclusive MC sample is a distinct banded distribution. With the topology and event visualization tool [49], we find that these tracks predominantly originate in the beam pipe. This is further supported by the two-dimensional distribution of Vy versus Vx in the data sample, and the d sample is significantly larger than the ˉd sample. Therefore, the deuterons used are mainly produced by the reaction of secondary charged particles with the beam pipe materials.
To improve the purity of the d sample, some further selection criteria of pd∈(0.52,0.72) GeV/c, χdE/dx∈(−3,3), and χTOF∈(−3,3) are imposed on the selected candidates. The transverse momentum and cosθ distributions of the accepted candidates in data and MC simulation are shown in Fig. 5. The data-MC consistency in the cosθ distribution is good, while the transverse momentum is distributed in the same interval with a small difference in shape. The difference in the transverse momentum is mainly caused by the different sources of d production in data and MC simulation. In data, d is not subject to channel restrictions and is mostly produced through secondary particle reactions with the beam pipe. In MC simulation, d is produced through an exclusive process at the collision point. This discrepancy leads to a small difference in the transverse momentum distributions between data and MC simulation.
Figure 5
Figure 5.
(color online) Transverse momentum (left) and cosθ (right) distributions of candidates in the 2016 data sample and signal MC sample.
The purity of d samples, as shown in Table 2, is obtained by integrating the corresponding χ distribution within the ±3σ range. The purity of the obtained d samples is higher than 97%.
Table 2
Table 2.
Purity of d samples in different χdE/dx and χTOF signal intervals.
To calculate the d PID efficiency using the dE/dx information, we must first obtain the total number of deuterons. Because no exclusive control sample is available, the total number of deuterons can only be obtained from data. Considering that the d PID method with only TOF information has been well established, the d sample selected by the TOF information is used as the control sample. The d PID efficiency is determined via
ε=NobsNtot,
(5)
where Ntot is the number of signals obtained from the fit to the χTOF distribution, and Nobs is the number of signals obtained from the fit to the χdE/dx distribution of the accepted candidates. Ntot is obtained by integrating the fitted signal shape within (mean−3σ,mean+3σ), whereas Nobs is obtained from the fit to the obtained χdE/dx distribution. The "mean" value corresponds to the expected value obtained from the Gaussian fit. As an example, Fig. 6 shows the fits to the χdE/dx and χTOF distributions of the 2016 data sample.
Figure 6
Figure 6.
(color online) Fits to χdE/dx (left) and χTOF (right) of candidates in the momentum interval 0.52−0.57 GeV/c in the 2016 data sample. A Gaussian function is marked with a blue dashed line, and a first-order Chebychev polynomial is marked with a red dashed line.
The d PID efficiencies of data and MC simulation as well as their differences in different momentum ranges and data taking periods are shown in Table 3. Figure 7 shows the variation in the d PID efficiency of data with a momentum interval. We find that the d PID efficiencies of the data samples from 2011 to 2017 are higher than 97%, whereas those of the data samples from 2020 to 2021 are slightly lower but not less than 95%. This indicates that the dE/dx method is an effective approach for d identification.
Table 3
Table 3.
PID efficiencies and the differences in PID efficiencies between data and MC simulation of d in different momentum ranges and data taking periods.
The difference in the d PID efficiencies between data and MC simulation is determined via
Δdiff=|εdata−εMCεMC|,
(6)
where εdata and εMC represent the efficiencies of data and MC simulation, respectively. The uncertainty of Δdiff is assigned as the difference between the fit and sideband methods and is calculated using
Δfit=√(ΔεdataεMC)2+(εdataΔεMCε2MC)2,
(7)
Δεdata=|εfitdata−εsidedataεfitdata|,
(8)
ΔεMC=|εfitMC−εsideMCεfitMC|,
(9)
where εfitdata, εsidedata, εfitMC, and εsideMC represent the efficiencies of data and MC simulation with the fit and sideband methods, respectively.
The obtained results are summarized in Table 3. For the majority of the samples, there is good consistency between the data and MC simulation in the d PID efficiencies.
In this study, we identify d in the momentum range 0.52−0.72 GeV/c using e+e− collision data taken in the c.m. energy range 4.009−4.946 GeV at BESIII. Based on the dE/dx method, the d PID efficiencies of data are higher than 95%, with a maximum difference of (4.9±1.0)% between data and MC simulation. For the data samples collected from 2011 to 2017, the d PID efficiencies are higher than 97%. This indicates good performance of d identification. Additional methods are expected for investigating d identification in higher or lower momentum ranges. In addition, the data samples taken at higher c.m. energies by BESIII in the near future will offer new opportunities to further explore d identification [50, 51].
The authors are grateful to the BESIII software group for useful discussions. We express our gratitude to the BESIII Collaboration and BEPCII team for their strong support.
CAO Ping, SUN Wei-Jia, JI Xiao-Lu, FAN Huan-Huan, WANG Si-Yu, LIU Shu-Bin and AN Qi. Prototype of time digitizing system for BESⅢ endcap TOF upgrade[J]. Chinese Physics C, 2014, 38(4): 046101. doi: 10.1088/1674-1137/38/4/046101
CAO Ping, SUN Wei-Jia, JI Xiao-Lu, FAN Huan-Huan, WANG Si-Yu, LIU Shu-Bin and AN Qi. Prototype of time digitizing system for BESⅢ endcap TOF upgrade[J]. Chinese Physics C, 2014, 38(4): 046101.
doi: 10.1088/1674-1137/38/4/046101
Share Article
Milestone
Received: 2013-06-03
Revised: 2013-07-11
Article Metric
Article Views(2057) PDF Downloads(261) Cited by(0)
Policy on re-use
To reuse of subscription content published by CPC, the users need to request permission from CPC, unless the content was published under an Open Access license which automatically permits that type of reuse.
Abstract: The prototype of a time digitizing system for the BESⅢ endcap TOF (ETOF) upgrade is introduced in this paper. The ETOF readout electronics has a distributed architecture. Hit signals from the multi-gap resistive plate chamber (MRPC) are signaled as LVDS by front-end electronics (FEE) and are then sent to the back-end time digitizing system via long shield differential twisted pair cables. The ETOF digitizing system consists of two VME crates, each of which contains modules for time digitization, clock, trigger, fast control, etc. The time digitizing module (TDIG) of this prototype can support up to 72 electrical channels for hit information measurement. The fast control (FCTL) module can operate in barrel or endcap mode. The barrel FCTL fans out fast control signals from the trigger system to the endcap FCTLs, merges data from the endcaps and then transfers to the trigger system. Without modifying the barrel TOF (BTOF) structure, this time digitizing architecture benefits from improved ETOF performance without degrading the BTOF performance. Lab experiments show that the time resolution of this digitizing system can be lower than 20 ps, and the data throughput to the DAQ can be about 92 Mbps. Beam experiments show that the total time resolution can be lower than 45 ps.
HTML
目录
Export File
Citation
CAO Ping, SUN Wei-Jia, JI Xiao-Lu, FAN Huan-Huan, WANG Si-Yu, LIU Shu-Bin and AN Qi. Prototype of time digitizing system for BESⅢ endcap TOF upgrade[J]. Chinese Physics C, 2014, 38(4): 046101.
doi: 10.1088/1674-1137/38/4/046101
Table 3.
PID efficiencies and the differences in PID efficiencies between data and MC simulation of d in different momentum ranges and data taking periods.