The luminosity monitor of Beijing spectrometer

Nuclear Instruments and Methods in Physics Research A 336 (1993) 542-551 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

The luminosity monitor of Beijing spectrometer H.L. Ni, H .F. Guo, C.H . Jiang, W. Liu, J.G. Xu, H .S . Zhou and Q .M. Zhu Institute of High Energy Physics, Chinese Academy of Sciences, Beging 100039, China

Received 19 April 1993 and in revised form 2 July 1993 A luminosity monitor (LM) was constructed at Beijing Electron Positron Collider (BEPC) responsible for measuring the colliding luminosity of BEPC . Being a part of the Beijing spectrometer (BES), which is a large major detector of the BEPC, the LM is also capable of providing the integrated luminosity for physics study in BES. By various means, such as fine design and careful manufacture of various counters, precise positioning of the LM, employing multifold and delay coincidences, etc., the LM is able to measure a relatively precise luminosity with a significantly suppressed background, e.g . the first colliding of BEPC in 1988 was detected and announced in time by the LM with a luminosity of 5 X 10 28 /cm2 s. Besides, some useful information was provided by the LM to assist the proper operation of the BEPC. In this paper, the construction, operation, experimental results and the error analysis of the LM are presented in detail . 1. Introduction This article describes the construction, operation and experimental results of the LM of BES. The LM is a part of BES which is a large major detector for multipurpose experiments conducted and to be conducted at BEPC in China [1]. There are two colliding regions in BEPC. BES was constructed at one of the colliding regions and the other region has not yet been utilized so far. The LM of BES is therefore a unique one in BEPC . The major goal of the LM is to measure the colliding luminosity of BEPC and to study the integrated luminosity of BES. The LM of BES is required to provide accurate luminosity data in time and as fast as possible, and to provide the luminosity after off-line analysis for physics study in BES. These are the characteristic requirement to the LM of BES. In the following sections, the design, construction and performance of the LM and the readout system are described. The operation of the LM during the early BEPC testing and the off-line analysis are presented . Besides, the results of the error analysis are also given . 2. Principle of operation The LM of BES is based on the detection of Bhabha scattering at small angle like those of other laboratories [2]. Since both of the acceptance and the cross section for small angle Bhabha scattering can precisely be calculated, the counting rate of Bhabha events is a

very accurate measure of the absolute luminosity of the BEPC storage ring. The schematic arrangement of the LM is shown in fig. 1. The P counters are defining counters, and the C counters are complementary counters . Two particles from Bhabha scattering have an identical energy with opposite direction of motion . If a P counter detects an electron (or a positron) of a Bhabha event, a diagonal C counter will detect a positron (or an electron). S counters are shower counters for measuring the energy of e + or e -. The counting rate R, at which Bhabha events are detected by P counters, and the luminosity have the following relation : R = Lo-,

where Q is the cross section of Bhabha scattering integrated over the aperture of the P counter, and L is the luminosity of the storage ring . The features of the LM with the designed arrangement shown in fig. 1 are as follows: The sum of the event rates in four P counters provides a luminous measurement that is insensitive to the displacement and rotation of the luminous region, and is also insensitive to the change in the longitudinal profile of the luminous region . The individual rate changes in four P counters are mutually related, and reflect the degree of displacements in the X and Z directions or of the rotation around the Y axis . In view of the geometrical condition, the precision of the measured luminosity can be considered, in principle, as a geometry problem of defining the acceptance of the four P counters and of determining the transverse

0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

HL . Ni et al. / Luminosity monitor

54 3

x S1

D D S4

C1

D

C4

S2

S3

Fig. 1 . The arrangement of the luminosity monitor.

separation between P1 and P2 or P3 and P4 and the longitudinal separations among P counters . In order to identify Bhabha events in the presence of the background radiation, it is required that the P counters should coincide with the opposite complementary C counters, and both of them should coincide with the respective S shower counters . The time and the energy spectra of the particles detected by the counters are recorded . In order to provide the on-line luminosity data with a suppressed background, the delay coincident trigger is used to measure the random accidental background . By all these means a relatively precise on-line luminosity can be measured . Besides, some useful data can be obtained, which are of help for adjusting the operation parameters during the early test of BEPC. 3. Detectors 3.1 . Construction of the detectors

The location and the space for installing the LM were restricted by some other components of BES and BEPC . In determining the detector size, the profile of BEPC luminous region along the X, Y and Z directions was taken into account. In the design document of BEPC, the beam parameters are as follews: ox is 0.9 mm, cry is 0.2 mm and o-z is about 5.2-6 cm . The size and the position of the detectors are given in table 1. The P and C counters consist of plastic scintillator NE102, with a thickness of 3 mm and 5 mm respectively . As the dimension precision of the P counters affects the systematic errors of the luminosity, the P

counter scintillators were first machined to nominal dimension and then carefully polished . The exact shape of the P scintillator was measured by a universal tool microscope . The light guides of P counters were trips with a size of 15 mm X 3 mm X 280 mm . The light guides of the C counters were mixtures of twisted and plate light guides with a total length of about 300 mm . The rather long light guides were chosen to reduce the direct excitation on photocathodes by background radiation and to keep the photomultiplier far away from the strong magnetic leakage region of BES near the iron gate . The photomultipliers of P and C counters were XP2020. The S shower counters consist of lead-plastic scintillator sandwiches with a total thickness equal to 11 radiation lengths. Each cell in these counters consists of a lead plate of 5.6 mm thick and a plastic scintillator NE110 plate of 6 mm thick. There were 11 cells in each S counter. In order to obtain a good uniformity of the S counter, the light coming from the plastic scintillator plate was collected by upper and lower BBQ wavelength shifters of 8 mm thick [3-5]. The BBQs Table 1 The nominal dimension and the center position of the detectors Counter

Dimension [mm] Width

Height

P

Thick ness

15 55 87

60 90 120

3 5 130

C S

Center position [mm] X Y Z 94 97 .5 101 .5

0 0 0

1926 1966 1986

544

HL . Ni et al. / Luminosity monitor

Table 2 The ratio DE/E of the P and C counters Moment m [GeV] AE/E [%] t1E/E [%]

P C

1.0

40.4 34.7

1 5 42 .6 40 .4

2.0

41 .3 40 .5

w w b 2.5

44.4 43 .2

3.0

40 .6 37 .7

were coupled with XP2020 through the twisted light guides . The parameters of the S shower counters were calculated by the Monte Carlo EGS program [6]. 3.2 . Performance of the detectors The properties of the detectors were measured at the 12 GeV proton synchrotron accelerator at KEK in Japan. The dE/dX spectra for P and C counters were measured at several energies . After fitting the dE/dX spectra by a Landau function, the peak position energy E and the half height width DE (FWHM) were obtained . Table 2 lists the ratio of 0 E/E (FWHM) of the P and C counters at five measured energies . The signal amplitude uniformity of the P and C counters along the height direction is about 10%. The time resolutions of the P and C counters are 320 ps and 230 ps respectively . The energy resolution and linearity of the S shower counters are shown in fig. 2. As the BBQ wavelength shifters were used to collect the light, a better uniformity of the S shower counters was obtained and the output amplitude change is about 10% in the incident region of the Bhabha scattering particles . The time resolution is 470 ps . 3.3 . Setting and position measurement of the detectors Four groups of detectors were set on two adjustable frames, which were set at the east and west sides of the BES respectively . The defining counters PI (or P3) and P2 (or P4) were fixed on a small positioning plate with two targets on which a cross signal is in the center . The setting and position measurement of PI and P2 were proceeded on a jig boring machine with some special precautions ; however, the whole procedure was convenient to handle by experienced persons. The precision of the distance between P1 and P2 (or P3 and P4) along the X direction was ±0 .060 mm, and the position precision of the Y direction of the P counters was ±0 .4 mm . The plates were fixed on the adjustable frames with three cross targets. The frames were settled on the east and west two plates attached to the Q magnets near the two respective sides of the BES. The position precision requirement of the frames along the X, Y and Z directions to the colliding region center are ± 1 mm, + 1 mm and ± 2 mm, respectively. In setting, the targets of the frames were surveyed by

c0

â 0

m

T L N C

w

U 0

0 c.

A

30 25 20 15 10 0 1000 800 600

ca r

400

U

Z

5

E

3 C

19875 at KEK x TS beam eo ir2 beam e+ Monte-Carlo

200 0

0

1 2 3 4 Momentum of electron ( Gev/c )

5

Fig. 2 . The energy resolution and linearity of the S counter. the targets of the Q magnets, of which the precision was +_ 0.1 mm for the BEPC orbital center . The error contributed by such position precision to the luminosity was less than 1% . In fact, the real position precision obtained was better than the above mentioned requirement . After setting, the frames were locked by several locating pins for the convenience of resetting whenever maintenance is required . Table 3 lists the position precision requirement of the P counters . Table 4 lists the position data of the P counters . Fig. 3 shows the construction of the LM of BES. 4. Readout system and data acquisition The block diagram of the readout system is shown in fig. 4. The features of the readout system are to have a powerful ability to restrain the background, to obtain more accurate on-line luminosity data and to provide some useful information of the beam parameters and background level of BEPC. In order to restrain the background, some conventional techniques, such as Table 3 The position precision requirement of the P counters Direction

Pl and P2 (or P3 and P4) on the positioning plate The frames to colliding region center

X [mm]

Y [mm]

Z [mm]

+10

+1 .0

+2 .0

±0 .060

±0 .4

HL. Ni et al. / Luminosity monitor

54 5

Fig. 3. The construction diagram of the luminosity monitor . 1 - P counter, 2 - positioning plate, 3 - C counter, 5 - S counter. gate control, discrimination, coincidence, veto coincidence and delay coincidence were used in the readout system . Time, amplitude and rate information of each detector, and the rate and the status of each type of coincidence were acquired . These data provide sufficient infomation for on-line and off-line analyses . There are three distinct triggers in the hardware of the LM . (a) Normal event trigger: The normal event trigger required that P and S counters fired simutaneously in coincidence with the C and S counters at diagonally opposite position, e.g . (P1 - S1) - (C3 - S3). This coinci-

Table 4 The coordinates (X, Y, Z), widths, heights, thicknesses and areas of the P counters . All dimensions are m mm Counter X P1 P2 P3 P4

93 .492 94.486 94.065 93 .943

Y

0.093 0.013 0.192 0.219

Z

1926 .09 1926 .66 1926 .67 1926 .20

Height Width Area 59 .240 59 .264 59 .257 59 .274

14.692 14.713 14.716 14.705

870.39 872.01 872.06 871.63

dence corresponds to a good event candidate. There are four kinds of coincidences for normal event trigger. Because some random backgrounds can also induce such coincidence, this kind of trigger includes random background . These four kinds of coincidence corresponded to status numbers 1, 2, 4 and 8 recorded by a model 2351 INPUT register, respectively. (b) Delay coincidence trigger: While passing the LM, the positron and electron beam bunches in the storage ring can produce random signals in coincidence, which is one of the background sources. Because the distance between the colliding region and the detectors is about 2 m, the random signals of two groups occurred at about 6.7 ns before and after the colliding, respectively. After analyzing the time spectrum of the detectors, the early background before colliding was readily rejected . The rejection of the backgrounds after colliding was carried out by the off-line analysis . The delay coincidence trigger was used to obtain some accidental background information in on-line. The delay coincidence trigger required that the P and S counters fired in coincidence with the C and S counters at the diagonally opposite positions with a

546

HL. Ni et al / Luminosity monitor 428E

SCAL .

SCAL .

SCAL .

715 gate

C

428F

623B

P

DELAY

S

gate

S

428F

COIN .

-~

star

T^^

stop

P 5

COIN .

C S

PSCS PSCS'

IN-SR strobe 10 6

PSCS

C COIN .

P S

rn,u

PSCS'

to computer dead tune

ADC

DELAY

Fig. 4. The block diagram of the readout system . 428F : fan-in/fan-out, 62313 - discriminator, 715 timing discriminator, 2323 . dual gate and delay generator, IN-SR: 2351 INPUT register, OUT-SR : 3251 OUTPUT register. time delay of 802 ns, such as (PI - S1) - (C3 S3)d e , ay Hoe  S . The time of 802 ns was the orbital period of BEPC. This type of coincidence indicated the accidental coincidence level in the operation of the BEPC . The four delay coincidences corresponded to status numbers 16, 32, 64 and 128, respectively. (c) Random sample trigger: The readout system acquired automatically once the information recorded by all the detectors after the beam cycled 10 6 times in the ring . The purpose of acquisition was to sample the random background in all different counters and to study any possible correlation among the probabilities of seeing the background in different counters . This trigger corresponded to a status number of 256. Table 5 lists the trigger types. The trigger system of the LM was parallel to the trigger system of BES. Because of the high trigger rate of the LM, a LAM request for VAX-785 was given by every ten event triggers of the LM . In operation, the random sample event has not been acquired because of the limited data acquisition time for the LM . The signal from each detector passed through a cable of 39 m long and was sent to 428F FANIN/

FANOUT . The output signal was then sent to a model 715 timing discriminator and a model 623 discriminator with a gating control signal of 50 ns in width. The gating control signal should be given at a proper moment so the two signals produced in any detector of the LM before and after colliding were within this gate width. The discriminators were set at a threshold of 60 mV . The output signals after discriminating were sent to model 365L coincidence units. When coincidence occurred in one of the three types of triggers, the control unit would provide many control signals, such as the gate signal of the ADC, the common start signal of the TDC, the strobe signal of the INPUT register, the inhibiting gate signal and the LAM request signal for the computer . The inhibiting gate signal prohibited the input signal of the readout system to ensure the conversion time of the ADC and the TDC. After finishing data acquisition, the clear signal and gate signal were given by the model 3251 OUTPUT register to start another round of data acquisition . The T, signal of the control unit was the beam bunch signal from BEPC and its period was 802 ns . An on-line program realized the communication

Table 5 Trigger types and their status numbers Normal event trigger Coincidence type (P l -S1)-(C3 -S3) (P2-S2)-(C4-S4) (P3-S3)-(C1-S1) (P4-S4)-(C2-S2)

Status 1 2 4 8

Delay coincidence trigger Coincidence type

(P1-S1)-(C3-S3)' (P2- S2)- (C4 - S4)' (P3-S3)-(C1-S1)' (P4-S4)-(C2-S2)'

" W3-S3)' =(C3-S3)d,,"y,d8O2n, and the same for other three coincidences

Status 16 32 64 128

Random sample Status 256

54 7

.L. Ni et al. / Luminosity monitor H

between the VAX-785 and the CAMAC by a VCC (VAX-CAMAC-CHANNEL) interface . The VAX-785 computer performed on-line sample analysis and displayed the important luminosity parameters as well as the spectrum information for inspecting the operation status of BEPC . The luminosity data after subtracting the random background were given by on-line analysis . The rate of each detector and various coincidences were displayed. The correlation among these rates provided very useful information . The ratio of four-fold coincidence and delayed four-fold coincidence was closely related to the beam status of BEPC. The change of the ratio of four-fold coincidence and eight-fold coincidence reflected the change of the colliding region profile. 5. LM operation and its useful information for BEPC adjustment The colliding luminosity of the first BEPC colliding on October 17, 1988 was given by the LM of BES. At the beginning, the luminosity was 5 X 10 28 /cm2 s at 1 .6 GeV colliding energy . During early an test experiment and normal operation, the LM also provided some useful information reflecting the operation status of BEPC in addition to the colliding luminosity . A luminosity curve in the early test of BEPC is shown in fig. 5. The curve indicates that the luminosity increases suddenly to a high value and the lifetime of the beam bunch becomes longer after colliding for about half hour . The average ratio of signal and noise, which is defined as the ratio of normal event trigger numbers and delayed coincidence trigger numbers, is shown in fig. 5. From fig. 5 the ratio of signal and noise becomes better in the high luminosity region . Another ratio of the eight-fold coincidence and the four-fold coincidence, such as the ratio of (PI - S1) - (C3 - S3) (P3 - S3) - (Cl - S1) and (P1 - SO - (C3 - S3), displayed a slight increase . This means that the beam bunch may have been improved . As shown by an arrow in the figure, another operation mode of BEPC has a lower ratio of signal and noise. During the early operation of BEPC, the data from LM sometimes were used to judge whether BEPC was operated properly or not. For example, various trigger event numbers in table 6 showed that the normal event trigger numbers of status 2 and 8 were very high and on the same order of the delayed coincidence numbers of status 32 and 128, respectively, indicating that the status 2 and 8 events were actually the background events . After the BEPC had its maintenance in the summer of 1992, various parameters and the spectra provided by LM were unnormal for a long time . For instance, the time spectra of P and S counters and the energy

10 .0 50

1989 .12 .18

4 :08--5 :55

1989 .12 .18

4-08--5 :55

_

.9 'V a

O

x

1.0 0.5 50

c â m

m

10

!Il

5

ô 0 ro

0

25

50

75

100

125

Time (min) Fig. 5. The luminosity and the ratio of signal and noise in an early test of BEPC . spectra of S counters are shown in fig. 6. Two peaks appear obviously in the time spectra in both P4 and S4 counters. These peaks correspond to the events at two moments before and after colliding. The time interval of the two peaks is about 13 ns . The events corresponding to the front peak, which is very high, must be backgrounds, as these backgrounds were introduced by the electron beam bunch while passing the third and fouth group of detectors in the east side of BES at the moment before colliding. The energy spectra of S Table 6 Various trigger numbers in a Run Status number 1 2 4 8 5 10 16 32 64 128 34 136 40 42 168

Trigger status numbers 1 2 4 8 1,4 2,8 16 32 64 128 2,32 8, 128 8,32 2, 8, 32 8, 32, 128

Event numbers 0 1256 0 630 0 266 0 1265 0 768 108 102 19 26 2

54 8

.L N et al. / Luminosity monitor H loo

ô âé~

l iîÎé,;~ l~IL

ô

É 50

c_o

0

v

1 0 05 0

'ai l

~IU~CwlP~!4!~l~di~Tli :fii~iütlR5i71f.~f1!lIIIN!!!fi/ot ~1 . .. I iW;it 9Yi f tt'to11e I~ ~f~ 1 ~' l'~~ü! aÉ ~W%âààï 3I ' 'l â

f-

TIME (rin)

Fig. 6 . Ratios of the luminosity in each channel versus the average luminosity obtained from Run 3135, Run 3136, and Run 3137 . counters show a large tail at lower energy . These phenomena suggust that there may be a background source in the east side of BES. After careful inspection, a leakage was found in a seal made of ceramics in the statical separator about 6 m from the LM . When the vacuum of BEPC in the east of the colliding region had been improved by solving this leakage problem, the parameters of the LM as well as various spectra became normal . The luminosity by on-line analysis was calculated with subtraction of random coincidence backgrounds . It is important for strong backgrounds . Fig. 7 shows the average luminosity curve, of which the data were obtained from Run 3135, 3136, 3137 of the Ds experiments. Fig. 8 shows the luminosity ratio of each channel and the average luminosity . Four curves represent

50

100

150

200

Time (mm)

Fig. 7 . The luminosity curve of Run 3135, Run 3136, and Run 3137 of the Ds experiments

the data from four groups of detectors, respectively . The ratio curves of signal and noise by on-line analysis are shown in fig. 9. There are lower backgrounds in fig. 9. These random backgrounds were related closely to the beam parameters of BEPC . 6. Off-line analysis Off-line analysis of the LM data was processed by using the information obtained by S counters . As the time and energy spectra of S counters are Gaussion distributions, it was convenient to find a proper cut condition for analyzing the background . Five cut conditions were used for off-line analysis and they are as follows: For example (Pl - Sl) - (C3 - S3) trigger channel DT =TDC(S1)-TDC(S3),

TDC(S1), TDC(S3), ADC(S1), ADC(S3) . The pulse height distribution of S counters was slightly different from that of other trigger types. This resulted

P1

P3 P4 400 Channel of ADC

Fig. 8 . The energy and time spectra of the LM while a leakage occurred in the BEPC . TDC : 50 ps/channel .

549

HL . Ni et al. / Luminosity monitor Events

Events

z

P1

20 10 0

zo

z

S1

10 400

z w

Channel of ADC

800

1200

Channel of TDC

Fig. 11 . The energy and time spectra of P1 and S1 in status number 16 obtained from Run 4545, Run 4546, and Run 4547. TDC: 50 ps/channel .

z

TIME (min)

Fig. 9. Ratios of the signal and noise obtained from Run 3135, Run 3136, and Run 3137 .

from the different incident positions of the particles. This factor was considered in determining the cut conditions . In general, the cut conditions were selected at +_ 3.5 standard deviations for ADC and at + 6 standard deviations for TDC and DT, respectively_ Fig. 10 shows the time and energy spectra of the detectors with trigger type (Pl - Sl) - (C3 - S3). Fig. 11 shows the time and energy spectra of Pl and S1 with trigger type of (Pl - Sl) - (C3 - S3)delay . Fig. 12 shows the time and energy spectra of the events which were rejected by the cut conditions . The distributions in figs. 11 and 12 are very similar, but the event numbers in fig. 12 are higher than those in fig. 11 . The ratio of the delayed coincidence events and the rejected events was about 60%. These three figures were obtained from Events

Events

120 40

1

80

40 0 150 100 50

LeorWLo

Fing P lum

~

(2)

Fr,g and Flm represent the correction factors of the dead time of the BES trigger system and the LM trigger system, respectively . Fig. 13 shows the distribution of Fi ng. and Flom in the mass measurement experiment of the tau lepton . In the physics study of the tau mass, the integrated luminosity was provided by the LM . Table 7 lists the integrated luminosity of the measured energy points [7]. 7. Error analysis The luminosity error was contributed from the calculation of the Bhabha scattering integrated cross section and the measurment of the Bhabha event rate .

750 500 250

80 0 120 80 40

Run 4545, Run 4546 and Run 4547 of the Ds experiments. In calculating the luminosity, the correction of the dead time was taken into acount :

A

1

400

S1

1206

S3

12

800 400

S3

800 400

C3

800

Channel of ADC

0

0

400

800

1200

Channel of TDC 1200

Fig. 10 . The energy and time spectra of the detectors in status number 1 obtained from Run 4545, Run 4546, and Run 4547 . TDC: 50 ps/channel .

Channel of ADC

Channel of TDC

Fig. 12. The energy and time spectra of the events, which were rejected by cut condition, obtained from Run 4645, Run 4546, and Run 4547 . TDC: 50 ps/channel

H L . Ni et al / Lummosity monitor

55 0

Table 7 The integrated luminosity of the measured energy points Number 1

2 3 4 5 6 7 8 9 10 11 12

Energy [MeV1

Luminosity [nb -1 1

1784 .19 1780.99 1772.09 1776 .57 1778 .49 177595 1776 .75 177698 1776 .45 177662 1779 .51 1789 .58

245.8 248.8 232.8 322.9 322.6 296.7 3839 360.8 793.9 1109 .1 494.4 249.8

O-

The integrated cross section is given by the following equation : do,

=I

soled angle

dI~

d,fl .

In calculating the integrated cross section, there are two kinds of errors, i.e . the error of the differential cross section formula and the error of the LM geometrical facter, e.g . the integrated cross section error from the solid angle . The geometrical factors include the LM position precision and the position and profile changes of the colliding region of BEPC . The major error of the differential cross section was from the radiative correction . The differential cross section formula used by on-line analysis can be expressed as do-

a2

d,O

SEZ

(2 - sin 2 0)(4 - sin

20)2

(4)

sin4B

In tau mass measurement experiments, the Physcis Group of the BES calculated the integrated cross section by a Monte Carlo method [81 in which the Bhabha Entries 20 17 15 12 1D 7

Entries 20 .

.

17 .5 -

.5 . .5 . .5

5. 2 .5

15 .

12 .51 D. 7 .5 5.

0 D .98

Fig.

1

1

1

0 .985

n I

1

1

0 .99

F lum

13 .

scattering generator including a3 terms was utilized at E = 1 .77 GeV, while the E Z value was calculated by Monte Carlo without including a a 3 term . The ratio between two E 2 o- values calculated by the MC method without a 3 term and by eq . (4) is 1.002 . After taking the radiative correction into account, and the multiscattering of the beam pipe thin window, the magnet deflection of the Bhabha particles, and the integrated cross section was calculated . The correction coefficient is 1.038 . The cross section error contributed by the beam energy diversity and the beam energy precision is about 0.1% (according to those parameters in the design document of BEPC). The difference between the actual area and the rectangular area of the defining counters used in the calculation is less than 0 .3% . The cross section errors contributed by the position precisions in X, Y and Z directions are 0 .3%, 0.1% and 0.3%, respectively . The setting error of the BES solenoid coil was calculated by the beam parameters, and the result was that the axis of the coil was not in the BEPC beam central orbit, but shifted about 4 mm towards to the south [9]. The BEPC beam parameters were measured by the beam position monitor (BPM), and the result was that the colliding region was outside the design beam central orbit about 1 .5 mm towards to the south. Therefore the colliding region was shifted to the north of the BES axis by about 2.5 mm . Fig. 14 shows the center positions XD , Yo and Z11 of the colliding region from Run 3097 to Run 3995 . From Run 3579 to Run 3737 the data were in operations at the J/1Y energy region, and the other data were at the Ds energy region . The data acquired by BES were filtered, and the primary vertex point of the events, which included four charged particles in the main drift chamber, was found. The distribution of the event vertex point in the X, Y and Z directions for one and a few runs displayed a Gaussion profile. After fitting with a Gaussion function, the center positions of X, l, Y and Z11 and their standard deviations o,,, o,,, and o-, were obtained . XD, Y11 and Z11 are shown in fig. 14 .

I

1

0 .995

2 .5

I

0 "o .8

nif 0 .88

o

i

0 .96

i

1 .04

Ftrig

The distributions of the dead time correction factors of the trigger system and the LM system .

55 1

H. L . Ni et al. /Luminosity monitor

From the curves in fig. 14, the value of Xo is about 2-2.5 mm, which is in good agreement with the parameters from BEPC . Y, 1 is in the range of -0 .3 mm to - 0.1 mm, indicating the colliding region is quite close to the designed beam orbital plane. Zo is about 5 mm, i.e . the colliding region is slightly shifted to the east . The above mentioned colliding parameters would cause the luminosity values of the first and fourth groups to decrease and the luminosity values of the second and third groups to increase . Four curves shown in fig. 8 display the results of the luminosity changes. During the operation of Run 3097 to Run 3995, if the colliding region shift along the X direction was -1 .5 mm and along the Z direction 5 mm, these would in turn cause the luminosity errors of ALl, AL2, AL3 and ALA to be -5 .5%, +8 .9%, +5 .9% and -6 .5%, respectively . The average luminosity change of the four groups was only 0.65% . This result was qualitatively in agreement with the curves shown in fig. 8. Table 8 lists the known sources of systematic errors responsible for the LM including those of geometrical origin and beam parameters . The statistical errors of the integrated luminosity by the scaler and ADC in a run were about 0.7% and 2.5% respectively. In the tau mass experiment, the luminosity value of off-line analysis was a little less than that of on-line analysis and the difference was about 4% . The statistical error of the integrated luminosity by off-line analysis was less than 0.30 U X

XO - Position

025

r O

E

_U 0 N

Luminosity [%] < ±0 .3 +0 .44 < ±0 .1 ±0 .65 +0 .85

0.5% for each energy point in the tau mass measurement . 8. Summary The construction and installation of the LM of BEPC were completed in 1988 . The LM has been successfully operated since then for more than five years. During BEPC operation, the LM provided the colliding luminosity as a routine measurement. Besides, because of its relevant precision, the integrated luminosity measured by the LM was also employed for physics studies in BES, e .g . the mass measurement of the tau lepton in 1992 . During the maintenance of BES in 1991, some degraded counters (caused by radiation effects) were replaced and the LM was re-installed and has performed quite well as before .

The authors would like to acknowledge Profs . M.H . Ye, S.X. Fang, and Z.P . Zheng for their encouragement, the Administration of the Institute, and a great number of colleagues for their continuous help and assistance in the construction of the LM .

0 15

E

Source P counter size P counter location Beam energy Colliding region center Total

Acknowledgements

0.20

0.10 002

Table 8 The sources of the systematic errors including the geometrical origin and the beam parameters

YO - Position

000 -002

References

-004

[1] J.Z . Bai et al ., High Energy Phys. and Nucl . Phys . 169 (1992) 769, in Chinese. [2] L.H . O'Neill et al ., Nucl. Instr. and Meth . 216 (1983) 361. [3] W. Hofmann et al., Nucl. Instr. and Meth . 195 (1982) 475. [4] J. Fent et al ., Nucl . Instr. and Meth . 211 (1983) 315. [5] C. Aurouet et al ., Nucl . Instr. and Meth . 211 (1983) 309. [6] L. Ford and W.R . Nelsen, SLAC Report SLAC-210, UC-32 (1978). [7] J.Z. Bai et al ., Phys . Rev. Lett . 69 (1992) 3021 . [8] Y.S . Zhu and P. Wang, Internal Report BIHEP-ARCHIVE-1992-M-Tau-093 . [9] C. Zhang, Internal Report of BEPC, Institute of High Energy Physics (1990).

ZO - Position

1.0 05 0.0 -05 3000

3200

3600 3400 Run number

3800

4000

Fig. 14 . The center position distributions of the colliding region obtained from Run 3135, Run 3136, and Run 3137 of Ds experiments .