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The purpose of this White Paper is to examine the BESIII program [1], to consider further physics opportunities, and to plan for possible upgrades of the BEPCII accelerator and the BESIII detector [2], in order to fulfill the physics potentials of the BESIII experiment. The BESIII Yellow Book [1] documented the original plan for the BESIII physics program before its commissioning. The discovery of
$ Z_c $ (3900) [3], followed by many experimental results for XYZ hadrons by BESIII [4-6], were pleasant surprises, which were not foreseen in the Yellow Book. Another surprise came from the first systematic absolute measurements of the$ \Lambda_c^+ $ decay properties based on the thereshold$ \Lambda_c^+\bar{\Lambda}{}_c^- $ pair production [7, 8]. The physics of XYZ hadrons and (heavier) charmed baryons has also become the focal point of the Belle II and LHCb experiments, and is an exciting area for the BESIII experiment in the future. In addition, a full spectrum of other important experimental opportunities, as discussed in this White Paper, will be continually pursued by BESIII, such as light hadron spectroscopy and charmed meson physics.The integration of quantum theory and Maxwell’s electrodynamics has led to a new, powerful theoretical scenario, quantum electrodynamics (QED), which was the first building block of what is called today the Standard Model (SM) of particle physics. Experimental progress led to discoveries of new particles and characterization of their properties, which helped to develop the theoretical framework towards a common understanding of the weak and electromagnetic interactions, called the electroweak theory. The modern theory of the strong interaction, called quantum chromodynamics (QCD), was modeled in a similar way and based on the exact color SU(3) symmetry of quarks and gluons.
Despite being very successful, several issues remain un-answered in SM. The strong interaction only allows the existence of composite objects; free quarks and gluons have never been observed. This is called confinement, but it is far from being theoretically understood due to its non-perturbative nature. A detailed study of composite objects and their properties will shed light on this part of QCD. Furthermore, it is suspected that additional features or underlying symmetries beyond SM might not have been discovered yet, which is usually summarized by the term ‘new physics’.
The hadron physics experiments in the 1970’s and 1980’s concentrated on studying the spectroscopy of the newly discovered hadrons containing relatively heavy charm and bottom quarks, or tried to understand specific questions in the light-hadron sector with dedicated experiments. For the heavy-quark mesons, no clearly superfluous or ambiguous hadron states have been reported. The recent discoveries of ‘exotic’ charmonium-like states have made the picture more complicated [38]. Furthermore, the situation has always been less straightforward for light mesons and baryons containing only light quarks [10]. Here, the high density of states and their broad widths often make the identification and interpretation of observed signals rather ambiguous. So far the unambiguous identification and understanding of gluonic hadrons is clearly missing. However, the self-interaction of gluons is central to QCD and leads to a flux tube of gluons binding the quarks together inside a hadron. Due to the self-interaction, bound states of pure gluons (named glueballs), or their mixing with conventional mesonic state, should exist as well as the so-called hybrids, where quarks and gluonic excitations contribute explicitly to the quantum numbers.
The energy regime in which BESIII is operating and the detector design allow a detailed study of charmonium and the light-quark region. Charmonium physics received a major renewal of interest in the 2000’s when many new, unexpected resonances, called X, Y and Z states [38], were discovered but could not be accommodated by the quark model. Many of those were found by Belle, BaBar, CDF, D0 and later the LHC experiments, but only BESIII is dedicated to the energy region where most of these states appear. It is therefore not surprising that detailed studies with a much higher statistics can only be performed at BESIII. Nowadays, BESIII is one of the main contributors to the understanding of the XYZ states. At the same time, the high production cross-section of charmonia at BEPCII together with a modern, almost hermetic detector for charged and neutral particles, allows also high-precision studies of light-quark hadrons in the decay of charmonia. Since this decay into light quarks proceeds via gluons, it is likely that the desired studies of gluonic excitations may be performed at BESIII, as is shown in this White Paper.
Despite the discovery of the charm quark more than 40 years ago, many questions about charmed particles still remain unsolved [38]. An upgraded BEPCII and BESIII can make key contributions to the lepton flavor universality, unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, validity of lattice QCD (LQCD), as well as theories of decay constants and form factors, by studying the leptonic and semileptonic decays of charmed particles. These studies can give insight into the applicability of QCD in low-energy nonperturbative context and can greatly expand our knowledge of charmed baryon properties. Open questions include the missing
$ \Lambda_c $ decay modes (e.g. those with as yet undetected neutral or excited final-state baryons) and baryon electromagnetic structure. -
The BESIII detector and BEPCII accelerator represent major upgrades over the previous version of BES [11, 12] and BEPC [13]; the facility is used for studies of hadron physics and
$ \tau $ -charm physics. The BEPCII collider, installed in the same tunnel as BEPC, is a double-ring multi-bunch collider with a design luminosity of$ 1\times 10^{33}\; \text{cm}^{-2} \text{s}^{-1} $ optimized at a center-of-mass (cms) energy of$ 2 \times 1.89 $ GeV, an increase of a factor of 100 over its predecessor. The design luminosity was reached in 2016, setting a new world record for the accelerator in this energy regime [14].The BESIII detector is designed to fulfill the physics requirements and the technical requirements for a high luminosity multi-bunch collider. Detailed description of the BESIII detector can be found in Ref. [2]. Figure 1.1 shows a schematic view of the BESIII detector, which covers 93% of the 4
$ \pi $ solid angle. It consists of the following components:
Figure 1.1. (color online) An overview of the BESIII Detector.
● Helium-gas based drift chamber (MDC) with a single wire resolution that is better than 120
$ \mu $ m and a dE/dx resolution better than 6%. The momentum resolution in the 1.0 T magnetic field is better than 0.5% for charged tracks with a momentum of 1 GeV/c.● CsI(Tl) crystal calorimeter with an energy resolution that is better than 2.5% and position resolution better than 6 mm for 1 GeV electrons and gammas.
● Time-of-Flight (TOF) system with an intrinsic timing resolution of 68 ps in the barrel part and 110 ps in the end-cap part.
● Super-conducting solenoid magnet with a central field of 1.0 Tesla.
● 9-layer RPC-based muon chamber system with a spatial resolution that is better than 2 cm.
Details of each sub-detector and their performance, together with the trigger system, are discussed in Ref. [2].
The BESIII detector has been operating since 2009, and BEPCII has delivered around 30 fb
$ ^{-1} $ of integrated luminosity at different cms energies. The experiment has received several upgrades, and new upgrades of both the detector and accelerator are being considered. -
In order to improve the capability for particle identification of the BESIII experiment, the end-cap time-of-flight (ETOF) detector was upgraded with the multi-gap resistive plate chamber (MRPC) technology in 2015 [15]. MRPC is a new type of gaseous detector that has been successfully used as TOF detector in several experiments. The new ETOF system of BESIII consists of two end-caps; each end-cap station has 36 trapezoidal shaped MRPC modules arranged in circular double layers as shown in Figs. 1.2 and 1.3. Each MRPC is divided into 12 readout strips which are read out from both ends in order to improve the timing resolution. The readout electronics system of MRPC detectors consists of FEE boards, time-to-digital conversion modules, calibration-threshold-test-power board, fast control module and a clock module in NIM crates that communicates with and is controlled by the data acquisition system. A multi-peak phenomenon in the time-over-threshold distribution was observed, and the reflection of the inductive signal at the ends of the strip is the main contribution. An empirical calibration function based on the analysis of the correlation of raw measured time, time-over-threshold and extrapolated hit position of the charged particle, is implemented using the real data for Bhabha events. Performance checks show that the overall time resolution for pions with a momentum around 0.8 GeV/c is about 65 ps, which is better than the original design goal.
Figure 1.2. (color online) Schematic drawing of MRPC ETOF at BESIII.
Figure 1.3. (color online) The cross-sectional view of the MRPC module along its length.
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MDC is the main tracker of BESIII with the capability of accurate measurements of the position and momentum of charged particles produced in
$ e^+ e^- $ collisions, as well as charged particles identification by measuring dE/dx. MDC is a low-mass cylindrical wire chamber with small-cell geometry, using helium-based gas and operating in a 1 T magnetic field. It consists of an inner chamber (8 layers) and an outer chamber (35 layers), which are joined together at the endplates and share a common gas volume. After running since 2009, MDC is suffering from ageing due to beam-induced background with a hit rate up to 2$ {\rm kHz/cm}^2 $ [16], which has caused the cell gains of the inner chamber to drop dramatically (about 39% drop for the first layer cells in 2017 as shown in Fig. 1.4), and has furthermore led to a degradation of the spatial resolution and reconstruction efficiency. Because of the radiation damage of the inner chamber, a cylindrical gas electron multiplier (CGEM) has been selected as one of the options for the upgrade, due to its attractive features such as high counting rate capability and low sensitivity to ageing. The CGEM inner tracker (CGEM-IT) project deploys a series of innovations and special attributes in order to cope with the requirements of BESIII, as listed in Table 1.1.Value Requirements σxy $\leqslant$ 130 µm
$\sigma_{z}$ 
$\leqslant$ 1 mm
dp/pfor 1 GeV/c 0.5% Material budget $\leqslant$ 1.5% X0
Angular Coverage 93%×4π Hit Rate 104 Hz/cm2 Minimum Radius 65.5 mm Maximum Radius 180.7 mm Table 1.1. List of requirements for the new inner tracker
$ . $ CGEM-IT consists of three layers of triple cylindrical GEM [17], shown in Fig. 1.5. Each layer is assembled with five cylindrical structures: one cathode, three GEMs and the anode readout (see Fig. 1.5) [18]. The GEMs and electrode foils are produced in planes and then shaped as cylinders. The assembly is performed inside a vertical inserting machine. To minimize the material budget, there are no support frames inside the active area, and the GEM foils are mechanically stretched as they are glued to Permaglass rings at their ends. The Permaglass rings are used only outside the active area and operate as gas sealing structure and gap spacers. A sandwich of PMI foam, called Rohacell, and kapton is used to provide mechanical rigidity to the anode and cathode electrodes. Rohacell is a very light material that limits the material budget to 0.3% of the radiation length (
$ X_0 $ ) per layer.
Figure 1.5. (color online) Cross-section of the triple GEM detector used in BESIII CGEM-IT.
Figure 1.4. (color online) Relative gain decrease of the cells in the MDC layers for each year of operation.
The readout anode circuit is manufactured with a 5
$ \mu $ m copper clad, 50$ \mu $ m thick polyimide substrate. Two foils with copper segmented strips are used to provide two-dimensional readout. The strip pitch is 650$ \mu $ m, with 570$ \mu $ m wide X-strips parallel to the CGEM axis providing the$ r-\phi $ coordinates. The V-strips, having a stereo angle with respect to the X-strips, are 130$ \mu $ m wide and together with the other view, give the z coordinate. The stereo angle depends on the layer geometry. A jagged-strip layout is used to reduce the inter-strip capacitance up to 30%. An innovative readout based on analogue information and data-pushing architecture has been developed. A dedicated ASIC has been developed to provide time and charge information from each strip.In order to verify that CGEM-IT can reach the required performance, an extensive series of beam tests has been conducted in the last few years as part of the test beam activities of the RD51 Collaboration at CERN. The tests were performed both of the 10
$ \times $ 10$ {\rm cm}^2 $ planar GEM chambers and of the cylindrical prototype with the dimension of the second layer of the final CGEM-IT [19]. All tests were performed in the H4 line of the SPS, in the CERN North Area. Since CGEM-IT will operate in a magnetic field, all test chambers were placed inside Goliath, a dipole magnet that can reach up to 1.5 T with both polarities. Pion and muon beams with a momentum of 150 GeV/c were used. Two scintillators were placed upstream and downstream of the magnet and operated as a trigger. A typical setup using the cylindrical prototype is shown in Fig. 1.6.
Figure 1.6. (color online) Sketch of the setup of the CGEM test beam.
The performance of the planar GEM chambers in a magnetic field was studied with the charge centroid method. The presence of an external magnetic field induces a deformation of the avalanche shape at the anode due to the Lorentz force: the performance of the charge centroid method degrades almost linearly with the magnetic field strength, as shown in Fig. 1.7. It is still possible to improve the performance by a proper optimization of the drift field, as shown in Fig. 1.8. With the proper choice of the gas mixture (Ar/iC
$ _4 $ H$ _{10} $ (90/10)) and drift field (2.5 kV/cm), it is possible to achieve a resolution of 190$ \mu $ m in a 1 T magnetic field.
Figure 1.7. (color online) Resolution as a function of the magnetic field strength for Ar/iC4H10(90/10) and Ar/CO2(70/30).
Figure 1.8. (color online) Resolution as a function of drift field in the 1 T magnetic field for two drift gaps: 3 mm drift gap and 5 mm drift gap.
$ \mu $ -TPC is another available method for track reconstruction. It is an innovative approach that exploits the drift gap of a few millimeters as a Time Projection Chamber. Indeed, the time of arrival of the induced charge on the strip can be used to reconstruct the first ionization position in the drift gap, and thus improve the spatial resolution. The$ \mu $ -TPC method can improve and overcome the limits of the charge centroid method, resulting in a spatial resolution lower than 200$ \mu $ m for a large angle interval, as shown in Fig. 1.9. Further studies are ongoing. By merging the two methods it will be possible for the spatial resolution of CGEM-IT to satisfy the requirements of BESIII.
Figure 1.9. (color online) Spatial resolution of the charge centroid and μ-TPC methods as a function of the incident angle of the track in the 1 T magnetic field.
After the completion of CGEM-IT, a long term cosmic-ray test will be performed to evaluate the performance of the whole CGEM-IT before the replacement of the inner chamber of MDC.
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In addition to the construction of CGEM-IT, an improved inner MDC has been built that can replace the aged inner part of the MDC if needed [20].
The new inner MDC is designed with multi-stepped end-plates. Each step contains one sense-wire layer and one field-wire layer, which can shorten the wire length which exceeds the effective detection sold angle, and minimize the ineffective area in the very forward and backward region, thus reducing the background event rate of all cells, as shown in Fig. 1.10. The maximum reduction of the rate of background events is more than 30% for the first layer cells. With this design, the new inner MDC is expected to have a longer lifetime and improved performance due to the lower occupancy.
Figure 1.10. Overview of the mechanical structure of the inner MDC. (a) The old inner chamber. (b) The new inner chamber.
The new inner MDC consists of two multi-stepped endplates and an inner carbon fiber cylinder. The length of the new inner chamber is 1092 mm, and the radial extent is from 59 mm to 183.5 mm, including 8 stereo sense wire layers, comprising 484 cells in total. Similar to the old chamber, the drift cells of the new chamber have a nearly square shape, as shown in Fig. 1.11. The size of each cell is about 12 mm
$ \times $ 12 mm with a sense wire located in the center, surrounded by eight field wires. The sense wires are 25$ \mu $ m gold-plated tungsten wires, while the field wires are 110$ \mu $ m gold-plated aluminum wires.
Figure 1.11. End view of the new inner chamber and the layout of the cells. (a) End view of the new inner chamber. (b) The drift cells of the chamber.
For the construction of the new inner chamber, two aluminum endplates were manufactured with an eight-step structure for each one. A total of 2096 wire holes with a diameter of 3.2 mm were drilled in each endplate with the mean tolerance of 14
$ \mu $ m. A carbon fiber inner cylinder with a thickness of 1.0 mm, was covered with two layers of 100$ \mu $ m thick aluminum foils on its inner and outer surfaces for electromagnetic shielding. The endplates and the cylinder were assembled with a precision better than 30$ \mu $ m. Wire stringing was performed after the mechanical structure was assembled. Good quality of wire stringing was achieved by monitoring the wire tension and leakage current during the stringing. The non-uniformity of wire tension was less than 10%, and the leakage current was lower than 2 nA for each wire.After the completion of the construction of the new chamber, a cosmic-ray test without magnetic field was carried out to evaluate its performance, shown in Fig. 1.12. The results of the cosmic-ray test showed that the new inner chamber achieves a spatial resolution of 127
$ \mu $ m and a dE/dx resolution of 6.4%, shown in Fig. 1.13 and Fig. 1.14, which satisfy the design specifications. These measurements verified the successful construction of the new chamber. The new inner chamber is now ready to be used if needed.
Figure 1.12. (color online) The cosmic-ray test of the new inner MDC.
Figure 1.13. Residual distribution of the new inner MDC showing the results of a fit with a double Gaussian function.
Figure 1.14. The dE/dx resolution of the new inner MDC showing the results of a fit with a single Gaussian function.
A decision on whether to install CGEM or the new inner MDC will be made according to the results of their beam and cosmic-ray tests.
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BEPCII delivered its first physics data in 2009 at the ψ(3686) resonance. Since then, BESIII has collected about 30
${\rm fb}^{-1} $ of integrated luminosity at different energies, from 2.0 to 4.6 GeV. By using these data samples, the BESIII collaboration has published more than 270 papers, which have made significant contributions to hadron spectroscopy, tests of various aspects of QCD, charmed hadron decays, precision tests of SM, probes of new physics beyond SM, as well as$ \tau $ mass measurement. Nowadays, the BESIII experiment plays a leading role in the study of the$ \tau $ -charm energy region.During the past 10 years of successful running, a better understanding of the machine was achieved. With the increasing physics interest, two upgrade plans of BEPCII were proposed and approved. The first one is to increase the maximum beam energy to 2.45 GeV, to expand the energy territory. The second is the top-up injection to increase the data taking efficiency. The activities related to these two upgrades began in 2017.
Before 2019, the beam energy of BEPCII ranged from 1.0 to 2.3 GeV. In order to extend the physics potential of BESIII, an upgrade project to increase the beam energy to 2.45 GeV was initiated. In order to achieve this goal, some hardware modifications were necessary, including the power supplies of the dipole magnets, power supplies of the special magnets in the interaction region, and the septum magnet and its water cooling system. These hardware modifications were completed during the summer shutdown in 2019, while the commissioning will be finished by the end of 2019. However, it is expected that when the machine is running in the high energy region above 1.89 GeV, the beam current will decrease due to the limitations related to the radio frequency (RF) power and difficulties in controlling the bunch length and emittance. Hence, the peak luminosity decreases when the beam energy is increased, as shown in Fig. 1.15. In the future, it would be interesting to investigate the possibilities of a slight increase of the beam energy to 2.5 GeV and a slight decrease to 0.9 GeV, that are interesting for the studies of
$ \Xi_c $ and nucleon production, respectively.
Figure 1.15. (color online) The estimated peak luminosity of BEPCII in the energy region above 2.1 GeV.
The top-up injection is a highly efficient operation scheme for the accelerator [21], which provides a nearly constant beam current. As there is no stop for beam refilling, the integrated luminosity can be increased by 20% to 30% for long data taking runs. The BEPCII upgrade of the top-up injection for the collision mode has started in September 2017. In order to obtain a stable online luminosity, the beam current fluctuation is controlled within 1.5% with one
$ e^+ $ injection and two$ e^- $ injections every 90 seconds, so that the variation of the instantaneous luminosity is less than 3% of its nominal value. The injection rates of the$ e^+ $ and$ e^- $ bunches must be higher than 60 mA/min and 180 mA/min, respectively. The commissioning of the top-up injection began after the summer shutdown in 2019 and will be finished by the end of the year.There are also discussions on further machine luminosity upgrades. The recently proposed crab-waist collision scheme [22] is believed to be essential for the luminosity challenge of the next-generation high luminosity
$ e^+e^- $ colliders. The possibility of a crab-waist scheme at BEPCII has been considered since 2007. However, it was found impossible if only minor changes on the current design are allowed. A recent upgrade proposal of BEPCII based on the crab-waist scheme was discussed in detail in Ref. [23], which presents an upgrade project with a peak luminosity of$ 6.0\times 10^{33} $ cm$ ^{-2}{\rm s}^{-1} $ . This is 10 times higher than the achieved luminosity of BEPCII at the beam energy of 2.2 GeV. The crab-waist scheme with a large Piwinski angle is suggested to be adopted with modifications of the BEPCII parameters. The$ \beta $ functions at the interaction point are to be modified from 1.0 m/1.5 cm to 0.14 m/0.8 cm in the horizontal and vertical planes, respectively. The emittance is to be reduced from 140 nm to 50 nm with damping wigglers. Regarding this proposal, a detailed design of the crab-waist scheme has been studied, and many physical and technical issues were investigated, such as the injection, dynamic aperture, emittance coupling, high power RF, super-conducting quadrupoles/wigglers, strong crab sextupoles, etc. It was found that the crab-waist scheme is a complicated and time-consuming project which is not practical with the present BESIII detector.Another, more economic, way to increase the luminosity is to augment the beam current, which could potentially provide a factor of 2 improvement of the peak luminosity. For this purpose, bunch lengthening needs to be suppressed, which requires higher RF voltage. The scenario of expected luminosity, beam current and SR power is shown in Fig. 1.16. The RF, cryogenic and feedback systems need to be upgraded to sustain higher beam currents. Nearly all photon absorbers along the ring and some vacuum chambers also need to be replaced in order to protect the machine from SR heating. The required budget is estimated at about 100-200 million CNY, and it will take about 3 years to prepare the equipment for the upgrade and 1 year for its installation and commissioning. The upgrade scheme with higher beam current is at present more realistic than the crab-waist scheme.
Figure 1.16. (color online) Scenario of the BEPCII upgrade based on the increase of the beam current. The lines show the expected performance after the upgrade. The points in the upper plot show the achieved values of the current BEPCII.
Future Physics Programme of BESIII
- Received Date: 2019-12-25
- Available Online: 2020-03-26
Fund Project:
University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt 100 Talents Program of CAS The Knut and Alice Wallenberg Foundation (Sweden) (2016.0157) U. S. Department of Energy (DE-FG02-05ER41374, DESC-0010118, DE-SC-0012069) the Thousand Talents Program of China CAS Key Research Program of Frontier Science (QYZDJ-SSW-SLH003, QYZDJ-SSW-SLH040) The Swedish Research Council Koninklijke Nederlandse Akademie van Wetenschappen (KNAW) (530-4CDP03) German Research Foundation DFG under Contracts Nos. Collaborative Research Center CRC 1044, FOR 2359 Joint Large-Scale Scientific Facility Funds of the NSFC and CAS (U1532257, U1532258, U1732263) the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program National Natural Science Foundation of China (NSFC) (11335008, 11425524, 11625523, 11635010, 11735014, 11822506, 11935018) the Russian Ministry of Science and Higher Education (14.W03.31.0026) INPAC and Shanghai Key Laboratory for Particle Physics and Cosmology the CAS Center for Excellence in Particle Physics (CCEPP)Supported in part by National Key Basic Research Program of China (2015CB856700) National Science and Technology fund CAS PIFI Istituto Nazionale di Fisica Nucleare, Italy Ministry of Development of Turkey (DPT2006K-120470)
Abstract: There has recently been a dramatic renewal of interest in hadron spectroscopy and charm physics. This renaissance has been driven in part by the discovery of a plethora of charmonium-like XYZ states at BESIII and B factories, and the observation of an intriguing proton-antiproton threshold enhancement and the possibly related X(1835) meson state at BESIII, as well as the threshold measurements of charm mesons and charm baryons. We present a detailed survey of the important topics in tau-charm physics and hadron physics that can be further explored at BESIII during the remaining operation period of BEPCII. This survey will help in the optimization of the data-taking plan over the coming years, and provides physics motivation for the possible upgrade of BEPCII to higher luminosity.
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