- Email: [email protected]
Status of BES-III
Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357 www.elsevierphysics.com
Status of BES-III A. Zhemchugova∗ a
Joint Institute for Nuclear Research, Dubna, Moscow reg., 141980 Russia
The BES-III experiment started operation in June, 2008, after major upgrade of the detector setup and accelerator BEPC. Luminosity of 1.2 × 1032 cm−2 c−1 , which is 1/8 of the design one, has been achieved. Detector performance is close to the design one. Physics data taking started, at the center-of-mass energy of 3.686 GeV. At the moment, about of 10M of ψ(2S) decays have been recorded. Main goals of the experiment are the studies of charmonium physics, physics of charmed mesons, tau leptons and light hadron spectroscopy. Tau physics program is focused on the high-precision tau mass measurement, accurate determination of tau branching ratio and study of Lorentz structure of the weak charged current in lepton decays of tau.
1. Introduction The Beijing electron positron collider (BEPC-II ) together with the associated detector BES-III produced first collisions on July 2008, after several years of upgrade. Main goals of the experiment are the studies of charmonium physics, physics of charmed mesons, tau leptons and light hadron spectroscopy. Tau physics program is focused on the high-precision tau mass measurement, accurate determination of tau branching ratio and study of Lorentz structure of the weak charged current in lepton decays of tau. 2. Status of BEPC-II and BES-III 2.1. The Beijing Electron Positron Collider BEPC-II BEPC-II is a two-ring electron-positron collider, which is designed to run in the energy region between 2.0 and 4.2 GeV. Luminosity is expected to reach 1033 cm−2 c−1 at a beam energy of 1.89 GeV. Electrons and positrons collide at the interaction point, with crossing angle of 11 mrad and bunch spacing of 8 ns. Installation was finished in October 2006, and the first collisions made in March 2007. The BES-III detector has been moved into the interaction region in June 2008, and on July, 19 of 2008 the first physics ∗ Supported
by the RFBR-NSFC grant No. 06-02-39009
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.03.057
event was detected. Currently, luminosity of 1.2×1032cm−2 c−1 is achieved, which corresponds to 12% of design value. Main machine parameters are shown in Table 1. 2.2. The BES-III detector The BES-III detector consists of beryllium beam pipe, a drift chamber (MDC), an electromagnetic calorimeter (EMC), time-of-flight (TOF) and muon identification (MUC) detectors[1]. Central part of the detector is placed inside a superconducting solenoidal magnet with a field of 1 Tesla. The MDC is main tracking sub-detector of the BES-III setup. It is designed to determine precisely momentum of charged particles, as well as specific energy deposit (dE/dx) for particle identification. The drift chamber is 2.4 meter in length, with inner radius of 60 mm, and outer radius of 800 mm, thus covering the polar angle |cosϑ| < 0.93. Altogether there are 43 cylindrical layers of drift cells that are coaxial with the beam pipe. The average half-width of a drift cell is about 6 mm in the inner chamber and 8.1 mm in the outer chamber. Helium-propane gas mixture is used to decrease re-interaction and multiple scattering in the gas. The TOF detector is made of plastic scintillation counters and placed between the drift chamber and the electromagnetic calorimeter. It consists of barrel and end-cap parts. The polar angle
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
Table 1 BEPC-II in collision mode Machine parameters
Design
Energy (GeV) Beam current (mA) Bunch current (mA) Bunch number Tunes (βx /νy ) Injection Rate (mA/min) Luminosity (×1033 cm−2 c−1 )
1.89 910 9.8 93 6.54/5.59 200 e− /50 e+ 1
coverage of the barrel TOF is |cosϑ| < 0.83, while that of the end-cap is 0.85 < |cosϑ| < 0.95. The EMC plays an important role in the BES-III detector, providing precise measurement of position and energy of electrons and photons. It comprises one barrel and two end-cap sections. In total, the EMC contains 6272 CsI(Tl) crystals. The typical crystal size is 5 × 5cm2 on the front face and 6.4 × 6.4cm2 on the rear face. Crystal length is of 28 cm, which corresponds roughly to 15 radiation lengths. The MUC is designed to distinguish muons from hadrons by the characteristic hit patterns they produce when penetrating the return yoke of the BES-III magnet. The muon counter is made of resistive plate chambers (RPC) interleaved by iron absorbers. Design parameters of the BES-III detector are summarized in Table 2. A preliminary calibration using cosmic rays was carried out. The measured MDC single-wire resolution proved out to be 10% worse than design one. The TOF time resolution is 30%, and the EMC energy resolution is 40% worse than design values. Further improvement is expected by calibration using Bhabha and dimuon events2 . Design efficiency of the MUC (90%) is already reached.
2 During the preparation of this write-up a detector calibration using the physics data has been carried out. The MDC and EMC resolution nearly reached the design values
Achieved e
−
e+ ring 1.89 550 > 10 93 6.540/5.596 > 50
ring
1.89 550 > 10 93 6.540/5.599 > 200 0.1
3. Tau physics research program The tau lepton physics has seen a great progress since its discovery, thanks to high statistics data samples and high quality detectors, especially at LEP and B-factories (KEK-B and PEP-II). While the efforts in tau lepton physics at high energy e+ e− colliders have been quite successful, there still exist both unique and complementary possibilities at lower energy facilities running close to the threshold of the tau pair production. The main advantage with respect to high energy machines is a possibility to make study under very clean experimental conditions. The cross section turns on rapidly at the threshold of the tau pair √ production, reaching a maximum of 3.6 nb at s=4.25 GeV, while the background is small and experimentally measurable. Running below the charm threshold suppresses completely c quark background. Also, the maximum energy of a tau decay product is significantly less than the beam energy, which makes easy to reject backgrounds from Bhabha and dimuon pair. These advantages are of ultimate importance for more precise branching fraction measurements and analysis of the Lorentz structure of the weak charged current. BEPC-II /BES-III with its high luminosity and a good detector is able to provide fruitful results for the tau physics. Detailed BES-III physics research program, including extensive discussion on tau physics, can be found at [2].
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
Table 2 The BES-III detector properties, compared with ones of BES-II and CLEO-c BES-III (design) BES-II MDC σxy (μm) 130 250 δP/P at 1 GeV (%) 0.5 2.4 dE/dx resolution (%) 6 8.5 EMC δE/E at 1 GeV (%) 2.5 20 δϑ at 1 GeV (mrad) 5 25 TOF Time resolution (barrel) (ps) 100 180 Time resolution (end-cap) (ps) 110 350 MUC 9 layers 3 layers Magnetic field (Tesla) 1.0 0.4
3.1. Tau mass measurement The mass of the tau lepton is a fundamental parameter of the Standard Model, which was measured by many experiments. BES-III has a potential to improve even further the current precision. Scan near tau production threshold was adopted to measure tau mass at BES-III . The optimal measurement energy is located near the point with the largest derivative of the cross section with respect to energy. To obtain a data sample large enough is not a problem, taking into account BEPC-II luminosity. An integrated luminosity of 63 pb−1 , which corresponds approximately to one week of data taking, would provide a statistical accuracy that is better than 0.1 MeV. The main obstacle to overcome is systematic error. Most significant contribution to the systematics is precision of calibration of beam energy scale. To eliminate it, a beam energy calibration system based on Compton back-scattering technique is being constructed [3]. The expected total relative error is at the level of 5.8 × 10−5 . The absolute calibration of energy scale will be crucial for further improvement of the accuracy of the mτ measurement. If the laser back-scattering technique is applied at BES-III , an ultimate systematic uncertainty
CLEO-c 90 0.5 6 2
— 1.0
of around 0.09 MeV could be achieved. Detailed description of BES-III strategy of tau mass precise measurement can be found at [4]. 3.2. Tau branching measurement Running at threshold provides powerful possibilities to measure branching of two body decays, since one can select different hadronic decay channels from kinematics only [5]. Preliminary Monte-Carlo studies demonstrate, that an inte−1 grated luminosity of 196 √ pb , equivalent to two days of data taking at s=3.6 GeV, decrease statistical uncertainty down to 0.01% for tau pair decay τ − → e− νν, τ + → μ+ νν. For hadronic mode τ − → π − ν, τ + → π + ν one can count on 0.1% statistical uncertainty at the same integrated luminosity, taking an advantage of kinematic decay identification. Unfortunately, for K + K − final state this approach would require unsatisfactory long time - 0.1% statistical uncertainty will be reached after 551 day of running. However to measure tau pair decay to Kπ looks rather realistic. More detailed studies are in progress. Another topic close to tau branching determination is measurement of hadronic spectral functions in tau decays at BES-III . Running at an energy slightly below the ψ(2S) resonance would allow a combination of high statistics and ex-
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
cellent background conditions to carry out this study. Using leptonic τ -decay tagging in combination with the usual kinematic selection criteria (high missing momenta, acollinearity and acomplanarity, broken PT balance ) will make it is possible to select an extremely clean sample of tau decays, with backgrounds well below 1%. One can estimate, assuming a three-month dedicated tau run and an 80% tagging efficiency, that around a 280000 events of τ → ππ 0 ντ can be selected. Systematics is expected to be the same or better as at LEP experiments. One more attractive possibility is related to the determination of strange spectral functions. At LEP experiments, kaons were identified only on a statistical basis. At BES-III, kaons produced in tau decay will have momenta below 0.8 GeV/c, i.e. in the momentum range where they can be selected with high purity by the TOF and dE/dx measurements. 3.3. Lorentz structure of the weak charged current In general case, the tau decay can be described by different types of interaction: scalar, vector, tensor, left-handed, right-handed. These possibilities are parametrized in terms of Michel parameters (ρ, η, ξ, ξδ)[6]. The Michel parameters have been extensively measured in tau lepton decays by many experiments. The Michel parametrization is based on certain assumptions; namely it assumes the Hamiltonian to be lepton-number conserving, derivative-free, local, Lorentz invariant, and a 4-fermion point interaction. While most of these assumptions are quite natural, there is no fundamental reason to assume that the interaction Lagrangian does not include derivatives. Up to date, an anomalous interaction involving derivatives (which can only be a tensor interaction), was measured in DELPHI experiment only[7] (together with the “standard” Michel parameters), but with a large statistical error and only under the assumption that the “standard” Michel parameters take exactly the Standard Model values. Both the Michel parameters and the constant of the anomalous tensor interaction κ can be measured from the energy spectrum of the tau decay: dΓ dx
∼ x2 (3(1 − x) + ρ(8x/3 − 2) + κx),
(1)
where x = E/Emax is the normalized energy of the tau decay product. The non-SM values of the Michel parameters and of the tensor interaction result in different distortions of the spectrum (see Fig. 1), which allows a simultaneous measurement of both (provided the statistics is sufficient).
Figure 1. The deviation of the spectrum of leptons from tau decays from the Standard Model prediction. Dashed line: for the non-zero value of the anomalous tensor coupling. Solid line: for ρ = 0.75. Horizontal axis shows the relative lepton energy x = E/Emax
Preliminary Monte-Carlo studies show that the BES-III statistics and the detector performance are sufficient to improve the current precision on the Michel parameters can be improved by factors of 2-4, and the limits on the anomalous coupling constant κ can be improved by at least a factor of 10 with a 5 f b−1 data sample collected at Ecm = 3.69 GeV. The large statistics also makes it possible to measure all parameters simultaneously, without assumption that all other parameters take the Standard Model values.
A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
4. Summary Construction and installation of BEPC-II and BES-III has been completed successfully. Luminosity of 1.2 × 1032 cm−2 c−1 , which is 12% of the design one, is currently achieved. Detector performance is close to the design one, no major hardware faults have been found. Data taking already started, about of 10M of ψ(2S) decays have been recorded. Detailed tau physics program is prepared. Data taking plan is not fixed for the forthcoming years yet. However dedicated run for tau physics is unlikely until 2010. REFERENCES 1. BES-III Collaboration, The Preliminary Design Report of the BES-III Detector, Report No. IHEP-BEPCII-SB-13. 2. D. M. Asner et al., arXiv:0809.1869 3. M. N. Achasov et al., arXiv:0804.0159 [physics.acc-ph]. 4. X. H. Mo, Nucl. Phys. Proc. Suppl. 169 (2007) 132. 5. A. Stahl, Int. J. Mod. Phys. A 21 (2006) 5667. 6. L. Michel, Proc. Phys. Soc. A63 (1950) 514; C. Bouchiat and L. Michel, Phys. Rev. 106 (1957) 170. 7. P. Abreu et al. [DELPHI Collaboration], Eur. Phys. J. C 16 (2000) 229 [arXiv:hepex/0107076].
357
Status of BES-III A. Zhemchugova∗ a
Joint Institute for Nuclear Research, Dubna, Moscow reg., 141980 Russia
The BES-III experiment started operation in June, 2008, after major upgrade of the detector setup and accelerator BEPC. Luminosity of 1.2 × 1032 cm−2 c−1 , which is 1/8 of the design one, has been achieved. Detector performance is close to the design one. Physics data taking started, at the center-of-mass energy of 3.686 GeV. At the moment, about of 10M of ψ(2S) decays have been recorded. Main goals of the experiment are the studies of charmonium physics, physics of charmed mesons, tau leptons and light hadron spectroscopy. Tau physics program is focused on the high-precision tau mass measurement, accurate determination of tau branching ratio and study of Lorentz structure of the weak charged current in lepton decays of tau.
1. Introduction The Beijing electron positron collider (BEPC-II ) together with the associated detector BES-III produced first collisions on July 2008, after several years of upgrade. Main goals of the experiment are the studies of charmonium physics, physics of charmed mesons, tau leptons and light hadron spectroscopy. Tau physics program is focused on the high-precision tau mass measurement, accurate determination of tau branching ratio and study of Lorentz structure of the weak charged current in lepton decays of tau. 2. Status of BEPC-II and BES-III 2.1. The Beijing Electron Positron Collider BEPC-II BEPC-II is a two-ring electron-positron collider, which is designed to run in the energy region between 2.0 and 4.2 GeV. Luminosity is expected to reach 1033 cm−2 c−1 at a beam energy of 1.89 GeV. Electrons and positrons collide at the interaction point, with crossing angle of 11 mrad and bunch spacing of 8 ns. Installation was finished in October 2006, and the first collisions made in March 2007. The BES-III detector has been moved into the interaction region in June 2008, and on July, 19 of 2008 the first physics ∗ Supported
by the RFBR-NSFC grant No. 06-02-39009
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.03.057
event was detected. Currently, luminosity of 1.2×1032cm−2 c−1 is achieved, which corresponds to 12% of design value. Main machine parameters are shown in Table 1. 2.2. The BES-III detector The BES-III detector consists of beryllium beam pipe, a drift chamber (MDC), an electromagnetic calorimeter (EMC), time-of-flight (TOF) and muon identification (MUC) detectors[1]. Central part of the detector is placed inside a superconducting solenoidal magnet with a field of 1 Tesla. The MDC is main tracking sub-detector of the BES-III setup. It is designed to determine precisely momentum of charged particles, as well as specific energy deposit (dE/dx) for particle identification. The drift chamber is 2.4 meter in length, with inner radius of 60 mm, and outer radius of 800 mm, thus covering the polar angle |cosϑ| < 0.93. Altogether there are 43 cylindrical layers of drift cells that are coaxial with the beam pipe. The average half-width of a drift cell is about 6 mm in the inner chamber and 8.1 mm in the outer chamber. Helium-propane gas mixture is used to decrease re-interaction and multiple scattering in the gas. The TOF detector is made of plastic scintillation counters and placed between the drift chamber and the electromagnetic calorimeter. It consists of barrel and end-cap parts. The polar angle
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
Table 1 BEPC-II in collision mode Machine parameters
Design
Energy (GeV) Beam current (mA) Bunch current (mA) Bunch number Tunes (βx /νy ) Injection Rate (mA/min) Luminosity (×1033 cm−2 c−1 )
1.89 910 9.8 93 6.54/5.59 200 e− /50 e+ 1
coverage of the barrel TOF is |cosϑ| < 0.83, while that of the end-cap is 0.85 < |cosϑ| < 0.95. The EMC plays an important role in the BES-III detector, providing precise measurement of position and energy of electrons and photons. It comprises one barrel and two end-cap sections. In total, the EMC contains 6272 CsI(Tl) crystals. The typical crystal size is 5 × 5cm2 on the front face and 6.4 × 6.4cm2 on the rear face. Crystal length is of 28 cm, which corresponds roughly to 15 radiation lengths. The MUC is designed to distinguish muons from hadrons by the characteristic hit patterns they produce when penetrating the return yoke of the BES-III magnet. The muon counter is made of resistive plate chambers (RPC) interleaved by iron absorbers. Design parameters of the BES-III detector are summarized in Table 2. A preliminary calibration using cosmic rays was carried out. The measured MDC single-wire resolution proved out to be 10% worse than design one. The TOF time resolution is 30%, and the EMC energy resolution is 40% worse than design values. Further improvement is expected by calibration using Bhabha and dimuon events2 . Design efficiency of the MUC (90%) is already reached.
2 During the preparation of this write-up a detector calibration using the physics data has been carried out. The MDC and EMC resolution nearly reached the design values
Achieved e
−
e+ ring 1.89 550 > 10 93 6.540/5.596 > 50
ring
1.89 550 > 10 93 6.540/5.599 > 200 0.1
3. Tau physics research program The tau lepton physics has seen a great progress since its discovery, thanks to high statistics data samples and high quality detectors, especially at LEP and B-factories (KEK-B and PEP-II). While the efforts in tau lepton physics at high energy e+ e− colliders have been quite successful, there still exist both unique and complementary possibilities at lower energy facilities running close to the threshold of the tau pair production. The main advantage with respect to high energy machines is a possibility to make study under very clean experimental conditions. The cross section turns on rapidly at the threshold of the tau pair √ production, reaching a maximum of 3.6 nb at s=4.25 GeV, while the background is small and experimentally measurable. Running below the charm threshold suppresses completely c quark background. Also, the maximum energy of a tau decay product is significantly less than the beam energy, which makes easy to reject backgrounds from Bhabha and dimuon pair. These advantages are of ultimate importance for more precise branching fraction measurements and analysis of the Lorentz structure of the weak charged current. BEPC-II /BES-III with its high luminosity and a good detector is able to provide fruitful results for the tau physics. Detailed BES-III physics research program, including extensive discussion on tau physics, can be found at [2].
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
Table 2 The BES-III detector properties, compared with ones of BES-II and CLEO-c BES-III (design) BES-II MDC σxy (μm) 130 250 δP/P at 1 GeV (%) 0.5 2.4 dE/dx resolution (%) 6 8.5 EMC δE/E at 1 GeV (%) 2.5 20 δϑ at 1 GeV (mrad) 5 25 TOF Time resolution (barrel) (ps) 100 180 Time resolution (end-cap) (ps) 110 350 MUC 9 layers 3 layers Magnetic field (Tesla) 1.0 0.4
3.1. Tau mass measurement The mass of the tau lepton is a fundamental parameter of the Standard Model, which was measured by many experiments. BES-III has a potential to improve even further the current precision. Scan near tau production threshold was adopted to measure tau mass at BES-III . The optimal measurement energy is located near the point with the largest derivative of the cross section with respect to energy. To obtain a data sample large enough is not a problem, taking into account BEPC-II luminosity. An integrated luminosity of 63 pb−1 , which corresponds approximately to one week of data taking, would provide a statistical accuracy that is better than 0.1 MeV. The main obstacle to overcome is systematic error. Most significant contribution to the systematics is precision of calibration of beam energy scale. To eliminate it, a beam energy calibration system based on Compton back-scattering technique is being constructed [3]. The expected total relative error is at the level of 5.8 × 10−5 . The absolute calibration of energy scale will be crucial for further improvement of the accuracy of the mτ measurement. If the laser back-scattering technique is applied at BES-III , an ultimate systematic uncertainty
CLEO-c 90 0.5 6 2
— 1.0
of around 0.09 MeV could be achieved. Detailed description of BES-III strategy of tau mass precise measurement can be found at [4]. 3.2. Tau branching measurement Running at threshold provides powerful possibilities to measure branching of two body decays, since one can select different hadronic decay channels from kinematics only [5]. Preliminary Monte-Carlo studies demonstrate, that an inte−1 grated luminosity of 196 √ pb , equivalent to two days of data taking at s=3.6 GeV, decrease statistical uncertainty down to 0.01% for tau pair decay τ − → e− νν, τ + → μ+ νν. For hadronic mode τ − → π − ν, τ + → π + ν one can count on 0.1% statistical uncertainty at the same integrated luminosity, taking an advantage of kinematic decay identification. Unfortunately, for K + K − final state this approach would require unsatisfactory long time - 0.1% statistical uncertainty will be reached after 551 day of running. However to measure tau pair decay to Kπ looks rather realistic. More detailed studies are in progress. Another topic close to tau branching determination is measurement of hadronic spectral functions in tau decays at BES-III . Running at an energy slightly below the ψ(2S) resonance would allow a combination of high statistics and ex-
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A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
cellent background conditions to carry out this study. Using leptonic τ -decay tagging in combination with the usual kinematic selection criteria (high missing momenta, acollinearity and acomplanarity, broken PT balance ) will make it is possible to select an extremely clean sample of tau decays, with backgrounds well below 1%. One can estimate, assuming a three-month dedicated tau run and an 80% tagging efficiency, that around a 280000 events of τ → ππ 0 ντ can be selected. Systematics is expected to be the same or better as at LEP experiments. One more attractive possibility is related to the determination of strange spectral functions. At LEP experiments, kaons were identified only on a statistical basis. At BES-III, kaons produced in tau decay will have momenta below 0.8 GeV/c, i.e. in the momentum range where they can be selected with high purity by the TOF and dE/dx measurements. 3.3. Lorentz structure of the weak charged current In general case, the tau decay can be described by different types of interaction: scalar, vector, tensor, left-handed, right-handed. These possibilities are parametrized in terms of Michel parameters (ρ, η, ξ, ξδ)[6]. The Michel parameters have been extensively measured in tau lepton decays by many experiments. The Michel parametrization is based on certain assumptions; namely it assumes the Hamiltonian to be lepton-number conserving, derivative-free, local, Lorentz invariant, and a 4-fermion point interaction. While most of these assumptions are quite natural, there is no fundamental reason to assume that the interaction Lagrangian does not include derivatives. Up to date, an anomalous interaction involving derivatives (which can only be a tensor interaction), was measured in DELPHI experiment only[7] (together with the “standard” Michel parameters), but with a large statistical error and only under the assumption that the “standard” Michel parameters take exactly the Standard Model values. Both the Michel parameters and the constant of the anomalous tensor interaction κ can be measured from the energy spectrum of the tau decay: dΓ dx
∼ x2 (3(1 − x) + ρ(8x/3 − 2) + κx),
(1)
where x = E/Emax is the normalized energy of the tau decay product. The non-SM values of the Michel parameters and of the tensor interaction result in different distortions of the spectrum (see Fig. 1), which allows a simultaneous measurement of both (provided the statistics is sufficient).
Figure 1. The deviation of the spectrum of leptons from tau decays from the Standard Model prediction. Dashed line: for the non-zero value of the anomalous tensor coupling. Solid line: for ρ = 0.75. Horizontal axis shows the relative lepton energy x = E/Emax
Preliminary Monte-Carlo studies show that the BES-III statistics and the detector performance are sufficient to improve the current precision on the Michel parameters can be improved by factors of 2-4, and the limits on the anomalous coupling constant κ can be improved by at least a factor of 10 with a 5 f b−1 data sample collected at Ecm = 3.69 GeV. The large statistics also makes it possible to measure all parameters simultaneously, without assumption that all other parameters take the Standard Model values.
A. Zhemchugov / Nuclear Physics B (Proc. Suppl.) 189 (2009) 353–357
4. Summary Construction and installation of BEPC-II and BES-III has been completed successfully. Luminosity of 1.2 × 1032 cm−2 c−1 , which is 12% of the design one, is currently achieved. Detector performance is close to the design one, no major hardware faults have been found. Data taking already started, about of 10M of ψ(2S) decays have been recorded. Detailed tau physics program is prepared. Data taking plan is not fixed for the forthcoming years yet. However dedicated run for tau physics is unlikely until 2010. REFERENCES 1. BES-III Collaboration, The Preliminary Design Report of the BES-III Detector, Report No. IHEP-BEPCII-SB-13. 2. D. M. Asner et al., arXiv:0809.1869 3. M. N. Achasov et al., arXiv:0804.0159 [physics.acc-ph]. 4. X. H. Mo, Nucl. Phys. Proc. Suppl. 169 (2007) 132. 5. A. Stahl, Int. J. Mod. Phys. A 21 (2006) 5667. 6. L. Michel, Proc. Phys. Soc. A63 (1950) 514; C. Bouchiat and L. Michel, Phys. Rev. 106 (1957) 170. 7. P. Abreu et al. [DELPHI Collaboration], Eur. Phys. J. C 16 (2000) 229 [arXiv:hepex/0107076].
357