BES

Nuclear Instrumentsand Methods in Physics Research A 379 (1996) 349-353

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Section A

ELSEVIER

Status and upgrade of BEPC/BES Yanan Guo Institute of High Energy Physics, Beijing, I?O.Box 918, Beijing 100039, China

Abstract The performance and physics results of the BEPCYBES are presented. prospect for the future Beijing Tau-Charm Factory.

1. Operation status 1.1. Machine performance In 1981 it was proposed to build an e+e- collider, and the proposal was approved in 1982. In October 1984, the building of the infrastructure of the Beijing Electron Positron Collider (BEPC) started. In October 1988, e+e- collisions were realized. At the same time, the BES detector underwent a cosmic ray test 8 m away from the interaction point. In the summer of 1989, the BES was moved to the IP, and J/q data taking started. Some key parameters of the BEPC are: circumference of the storage ring: 240 m; single e+, e- bunch collision period: 802 ns; peak luminosity: 6 x 1030 cm-2s-1 at EC,,, = 4 GeV, linac injection energy: 1.3 GeV. In the past four running years the following data have been collected: 9 x lo6 Jl+ events; 3.4 x lo6 I/’ events; 5 pb-’ events around the T threshold and a total of 10’ r at 3.7 and 4.03 GeV, 22.3 pbb’ events at 4.03 GeV (for D, and r physics). From October 14, 1994 to May 31, 1995 BEPC operated for 5042 hours, which was shared between BES running, synchrotron radiation experiments and machine study. The total operation efficiency is 94.1%.

Its upgrade status is introduced

together with the

scintillators at each end. TOF covers 97% of 47~ stereo angle. The shower counter is a gas sampling calorimeter with 24 layers of Pb absorber totaling 12 r.1. The barrel shower counter has 560 cells in the p direction. With finer spatial resolution, it can distinguish better between y and 71”. The solenoid produces an axial magnetic field of 0.4 T. There are 20 K channels of electronics in total. The trigger rate after three levels of hardware trigger is about 15 Hz. The event data size is 3 Kbyte on average. The performance parameters of the BES are: MDC: u,/p = 1.7% x dm, dEldx resolution: 8.5%; TOF: gt = 330 ps; BSC: se/E = 22%/,/m; uz = 3 cm; ESC: ~-E/E = 21%/dm, a, = 1.5 cm, a, = 1,.7 cm; MUC: up = 3 cm, a, = 5 cm. Muon Counters

1.2. The BES detector BES is the only detector at BEPC. It is composed of the Central Drift Chamber (CDC), the Main Drift Chamber (MDC), TOF, the Barrel and the End-cap Shower Counter (BSC and ESC), and the Muon Counter (MUC). Fig. 1 shows a longitudinal cross-section of the BES [l] CDC surrounds the beam-pipe with 4 layers x 48 cells. MDC is a 0 230 x 220 cm cylindrical jet type drift chamber. There are 10 layers with 48-108 cells per layer. The wires of the even layers are axial, while those of the odd layers are stereo for z measurement. The barrel TGF has 48 scintillators read out from both ends and the endcap TOF has 24 016%9002/96/$15.00 Copyright @ PUSO168-9002(96)00533-5

TOF agnet lke Fig. 1. Longitudinal view of the BES detector

1996 Elsevier Science B.V. All rights reserved I. OVERVIEWS

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Instr. and Meth. in Phys, Res. A 379 (1996) 349-353

Table 1 New results and new decay channels of t(2230) Decay

Mt (MeV)

Experimental

rc (MeV)

x Br(z$ +x)

?T?T

2235f4f6

19+_$12

5.6t$1.4

Pi;

2235f4f5

lSf_%9

1.5~~~f0.5

K+K-

2230+6 -7 k12

2of:p f 12

2.3+_;+.1

KSKS

2232+8 -7 f15

20:;;

2.7T’d.i f 1.O

zk10

1

t

nr(J/@ -+ ~5)

mode

Ratios of v( 2s) to J/v

x 10’

30

In the spring of 1991, 20 American physicists from six institutes and universities participated in the BES collaboration (now 40 from 9 units).

1.3. Physics results 0

1.3.1. J/I) physics Study of &(2230) in J/t@ The decay channels J/e + 75, 5 -+ K+K-, K,K, were confirmed. Some new decay channels e.g. J/I+? -+ y&, .$ -+ ~‘+71.-, pji were observed [ 21. The results are depicted in Table 1 and Fig. 2. These results show distictive properties of &( 2230) : Copious production in radiative J/q+ decay. From BES and PS185 results we have: BR( Jl$ -+ ~5) > 3 x 1O-3 which is larger than the branching ratios of other radiative decay channels, e.g. J/q -+ yf2 ( 1270)) J/$ + ye( 1525), Jl$ --+ yf4(2050), J/q? -+ @( 1720) etc. Very narrow width (-20 MeV) . Flavor-symmetric decay to n-+rr-, K+K-. r,+,: l-,+,- N 1 : 1. All these strongly hint that t( 2230) is a candidate glueball. The J/g resonance peak parameters were also measured.

Fig. 3. Experimental ratio of $’ to J/I/ for different hadronic decay modes. The open circle and the dashed bar represent the results before BES; the solid circle and the solid bar are BBS results.

The results are [ 31: T’,J, = 84.4 f 8.7 keV, rh = 74.1 + 8.1 keV, r, = 5.14 & 0.39 key r, = 5.13 f 0.52 keV, r;llr = 0.878 f 0.005; re/r = 0.0609 f 0.0033. 1.3.2. I# decay VP puzzle of $’ [ 41: The previous observed abnormality of 9’ + prr , K+*K- and yr] suppression was confirmed, and some new upper limits were set: BES result

PDG value

Br( 9 - w) Br($’ + K+*K-)

< 3.6 x10-5

(<8.3X 10-S at 90% C.L.)

< 2.4~10--~

Br(9’

< 1.7x10-5

(c2.0 x 10e4 at 90% CL.)

+ ~71’)

Two new suppression Br($’

modes were discovered:

--+ wfs( 1270)) < 9.5

Br(@’ + pas) < (4.8 f 3.3)

x

10m5; x

10m4.

This showed the puzzle of VP decays extended to VT decays. $’ decays to the final states containing a pair of octet barions are: PDG value

BES value

t

I

2

II

Y

I

2.2

MASS Fig. 2. c(2230)

(2.73f0.21f0.39) (2.14f0.31f0.26)

x 1O-4 x IO-4

1.9x 10-4 4.0x 10-h

(l.llf0.38f0.19) (0.83f0.28f0.12)

x10-4 x 1O-4

no no

Y

.

.

1

2.6

2.4

BR(#

(GeV)

decay to m-r, pF, KfK-

and Ksk.

-+ r+r-)

BR(@’ --+ r+r-)

was first measured. The result is: = (3.54 f 0.61 f 0.63) x 10-3.

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Instr. and Meth.

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these events, several D, inclusive semi-leptonic were observed. The branching ratio was

I

(b)

(a)

in Whys.Res.

decay events

Br(D, + eX) = ( 10.4Y$L_ii~~)%. Three pure leptonic decay events of D, were observed. weak decay constant fDs was obtained [ 61:

The

fDs = 434 f 143 & 34 MeV.

?i64cl-3660

W I

The absolute branching sured [7]:

I

I

I

(c)

Br(D, + 4~)

-s -2

,E 14I -6

(MeV)

I

I

O-

36oot36!533554

3660

/

/

,

-6 / 1774

,

/

/

/

/

/

/

\

\

‘_L f 0.5)%.

1.4. The BSRF (Beijing Synchrotroa Radiation Facility)

\ \ \

_ \

I

I

1776

1776 m,

\

WeW

Fig. 4. (a) The 7 production cross section near threshold. (b) Partial enlargement of (a). (c) Two parameter fitting of 7 mass with likelihood method. Dashed line: fit using e@ decay events only; solid line: fit using 6 decay channels.

1.3.3. r physics m7 measurement: 1784.12$- MeVlc2,

was also mea-

\

\

The 4/1’production in eiehas been studied too.

= (4.2:;:

ratio of D, ---) $a

annihilation

at EC,,,= 4.03 GeV

From the PDG 90 value the following was deduced:

m7

The BSRF is constructed around the storage ring of BEPC. It has two experiment halls, seven beamlines and ten experimental stations. They are a topography station, an EXAFS station, a small angle X-ray scattering station and a diffraction station, a photoelectron spectrometer station, a lithography station, a fluorescence analysis station, a diffuse scattering station and others. BSRF users are distributed all over China. There are hundreds of scientists from 56 institutions doing various experiments. Interesting results have been obtained. About 150 papers have been published and 100 reports are given in China and abroad.

= 2. Upgrade

of BEPCYBES

g7/gfi = 0.970 f 0.013, which is inconsistent with Iepton universality. In late 1991, the m7 was precisely measured on BEPC/BES. The procedure of the experiment was a data driven scan around the energy region of the efe- + r+rthreshold (Fig. 4). The final result is [ 51: m7 = (1776.9 f 0.2 z!=0.2)MeV/c2. Together with the improvements measurements, we deduct:

in rr and BR( r -+ Zvv)

gr/g&&= 0.999 f 0.003. Comparing with other experiments at Ecm N 10 GeV, the BEPC/BES has a unique advantage, i.e., it can get better result with less event data. m, measurement using the data at 4.03 GeV: The processes r ---f 3~ and r --) KKrr are utilized. The result of m,,, is being finalised. 1.3.4. D, physics Based on 22.3 pbb’ data at Ecm = 4.03 GeV, 155 D, events were reconstructed in four hadronic decay modes. Among

The upgrade of BEPCIBES started in May 1995. The main purpose of the upgrade is to increase its luminosity by a factor of 1.7-2.0. 2.1. Upgrade of BEPC 2.1.1. The storage ring ‘Ike main means for increasing luminosity is a mini-beta optics. A 0 95 mm x 0.5 m permanent quadrupole was inserted only 1.25 m away from the IP. At the same time, the ring impedance was improved. 40 bellows were shielded to reduce the impedance from 2.0 R to 0.68 a, and four kickers from 1.4 0 to 0.3 a; Two additional RF cavities were used with an RF voltage higher than 2.0 MV. The bunch length will be reduced from 7 cm (PC = 8.5 cm) to 3 cm (py* = 3.6 cm). 2.1.2. Linac The power of the klystrons was increased from 20 MW to 65 MW, and that of the modulators from 70 MW to 150 MW. In this way, the injection energy was increased from 1.3 GeV up to 1.75 GeV.

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2.1.3. Control and diagnostic system The original on-line control computer configuration of VAX750+ VCC interface was changed to VAX4500 + MicroVAX + KSC-2922. The control touch panel (Grinnel interface) was replaced by two VAX4090 workstations and two X-terminals. This yields a more powerful and reliable system. 2.2. Upgrade of BES detector The goals of the upgrade of the BES are: to decrease the

background under higher luminosity; to improve the detector performance; and to increase data acquisition speed. In the upgrade, some subdetectors and subsystems are entirely rebuilt. They include: A new Main Drift Chamber (MDC) , which still has 10 layers but with smaller cell size; The HV distribution and feedthroughs are improved; A Vertex Chamber (VC) and Be beam-pipe ( 1.27 mm thick) replace the central drift chamber and the 2 mm Al beam-pipe. The vertex chamber and beam pipe were used for MARKIII and shipped to Beijing for BES now. VC has 640 straw tubes and o;l = 100 ,um. Its front-end electronics were newly designed; The new barrel TOF uses BC408 scintillators and Hamamatsu R2490-05 fine-mesh type PMTs. They can operate in a magnetic field, so the light guides were shortened. at will improve from 330 ps to 135 ps, the attenuation length A = 4.4 m. Fig. 5 shows the configuration of the new DAQ. The previous DAQ was a centralized VAX-VCC-CAMAC architecture. The dead-time was 20 ms/event. We developed an intelligent VME-CAMAC-Branch Driver (VCBD) . Together with the MVME162 event builder, it can perform parallel foreground data collection. The dead-time will reduce down to 10 ms/event. Data will be logged locally as well as at the computer center via a LAN. DEC Alpha3600S replaced VAX-785 as a host. On-line monitoring power is drastically improved. An X-terminal replaced the touch panel and the error display. The new small angle luminosity mon-

computsr ~c.llier

~pp&g..*~ M0c.Q

HDCd

p rr&w

Fig. 5. The configuration

of the new DAQ of BES.

itor is smaller in size and closer to the IP to make room for insertion quadrupoles. Other components are partially improved, e.g. the dead channels of the Barrel and End-Cap Shower Counter were fixed and a new HV distribution was implemented; new trackers for the new MDC and VC trigger were designed, which use FPGA and consist of 75 new modules; the off-line processing power is greatly increased from 176 MIPS to 960 MIPS by buying seven HP workstations. The storage capacity increased from 19.2 Gbyte to 53.2 Gbyte. Now there is a better network in IHEP. It includes DEChubs and a FDDI campus ring, and a better connection to Internet. CU SeeMe were used in LP95 to broadcast over the world. Operating systems were shifted from VMS to UNIX. Software adaptation to the hardware modifications has been performed; BES electronics have also been upgraded. Five LRS 1879 TDCs and 1821 controller are used for VC readout. 2.3. BSRF upgrade Six beam lines and seven experimental stations were improved to better performance and higher efficiency. Some new facilities are going to be set up. 2.4. Budget The total budget for the BEPC/BES upgrade is RMB30 M from the Chinese government of which RMB 10 M is for BES, and RMB6 M for BSRF. We also got 0.4 M (= RMB3.3 M) in kind contribution from U.S. collaborators, which includes: manufacturing of MDC endplates and feedthroughs; recovery of the vertex chamber of MARK-III; readout electronics and data acquisition for VC; a new luminosity monitor; and a laser monitor of TOE

3. Feasibility (BTCF)

study of Beijing

Tau-Charm

Factory

The BTCF is a logical extension of BEPC/BES. The Beijing Tau-Charm Factory Workshop was held on February 5-9, 1996 in Beijing [ 81. The preliminary parameters of BTCF are: Beam energy: 1.5-2.5 GeV, optimized at 2.0 GeV; Luminosity: 1 x 1O33cm-*s-l; Beam crossing period: 38 ns. The research field covers r physics, charmonium and charm physics, the precise research and test of the standard model. The physics results from BES show that r, charmonium and charm physics are interesting and have unique advantages: - At the energy region of E,, = 3-5 GeV there are resonancepeaks of J/e, @‘, and $“, the production thresholds of 7, D’,D* and the maximum production cross section of r and charm.

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Instr. and Meth. in Phys. Rex A 379 (1996) 349-353

Operating near threshold completely excludes backgrounds from b quarks. Other background can be directly measured either above or below &&hold. The method of single tag could be used to reduce the background and biases. The small Lorentz boost would make the r/K/p separation easier. The J/I) and I)’ resonance peaks offer a good calibration of energy. The tentative schedule is: 1995-1996: feasibility study; 1997-1999: R&D for BCTF; 2000-2002: construction if the project is approved.

The total cost is estimated to be $120 M. It calls for international

353

References [I] J.Z. Bai etal., Nucl. Instr. [21 J.Z. Bai et al., Phys. Rev. [3] J.Z. Bai et al., Phys. Lat.

and Meth. A

344 (1994) 319.

Lett., to be published. B 355 (1995) 374. [41 J.Z. Bai et al., A new anomalous suppression mode in +(2s) hadronic decavs, HEP and Nucl. Phvs. 19 (1995) 93 (in Chinese): Stud; of 9(2s) decay to-7 leptk p&s, HEP and N&l. Phys. 19 (1995) 577 (in Chinese). [51 J.Z. Bai et al., Phys. Rev. Lett. 69 (1992) 3021, Phys. Rev. D 53 (1996) 20. [6] J.Z.Bai et al., Phys. Rev. Lett. 74 (1995) 4599.

[71 J.Z. Bai et al., Phys. Rev. D 52 (1995) 3781. [ 81 FeasibilityStudy Report on Beijing Tau-Chum Factory,IHEP-BTCF Report-01,Dec. 1995.

collaboration.

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