BES-III Workshop Oct.2001,Beijing

The BESIII Luminosity Monitor

High Energy Physics Group

Dept. of Modern Physics,USTCP.O.Box 4 Hefei, 230027

BES-III Workshop Oct.2001,Beijing

Side view of the near IP region

BES-III Workshop Oct.2001,Beijing

BES-III Workshop Oct.2001,Beijing

Channels to Measure Luminosity

• The Bhabha Channel

e+e- → e+e- (γ ) at small angle with

respect to the IP and beam

Lowest Order Diff. Cross Section

d /dcos = 2(3+cos2 )2/[8Eb2(1-cos)2]

BES-III Workshop Oct.2001,Beijing

Event rate estimation for 3 regions(Assume L = 1033cm-2s-1, Ebeam=1.55GeV, : 2 covered)

• Extreme Forward Region (5o to 16o )

Event Rate : 12743 Hz

• End Cap Region (21o to 34o ) Event Rate : 412 Hz

• Barrel Region (34o to 146o)

Event Rate 423 Hz

BES-III Workshop Oct.2001,Beijing

LUM Type I Extremely Forward Luminosity Monitor

• The Defining and Complimentary Counter

Dimension of θ : Scintillation fiber

or Silicon Strips

Dimension of φ : Plastic scintillator

• The Calorimeter

BGO / PWO Crystal

BES-III Workshop Oct.2001,Beijing

Requirement on space resolution

• The precision requirement on the inner edge of the tracker part should be 160 m for

a tracker put at Z = 41.6cm To make the Bhabha event accepted within a 1% change

(This also sets installation precision of the micro-beta magnet If it is around 1 mm, error of luminosity measured > 6%)

BES-III Workshop Oct.2001,Beijing

Arrangement of the EFLM

BES-III Workshop Oct.2001,Beijing

LUM Arrangement(Tracker not plotted)

BES-III Workshop Oct.2001,Beijing

Front View of Defining/Complimentary Counter

BES-III Workshop Oct.2001,Beijing

Cross Section of Fiber Bunch

BES-III Workshop Oct.2001,Beijing

Separation Power of the Calorimeter

BES-III Workshop Oct.2001,Beijing

Effects of the support Al structure of the

MDC Effective thickness of the Al plate and tube

~ 25 to 50 mm for different anglesAl plate : 20 mm , Al tube surround the beam pipe :2mm

R.M.S of the track smearing for the case

of 45mm thick Al case: 0.905 mm

Corresponding to a 7% of error in event count

BES-III Workshop Oct.2001,Beijing

Track distribution 20 cm away from the Al surfaceEffective Al thickness = 35 mm

BES-III Workshop Oct.2001,Beijing

Track deflection by the AlEffective thickness = 45 mm

BES-III Workshop Oct.2001,Beijing

Secondary charge track numbers due to AlEffective Al thickness = 45 mm

BES-III Workshop Oct.2001,Beijing

Error estimate for track smearingand installation precision

• Track smearing due to Al :

~6%

• Assuming a 1mm error in the installation precision of the micro-beta magnet:

~6%

• Total effect :

> 8%

BES-III Workshop Oct.2001,Beijing

LUM Type IIZero Degree Luminosity Monitor

Luminosity Monitor Based on e － (e ＋） single Bremsstrahlung(SB)

The photons are emitted along the e － (e ＋ ) direction within a cone of total aperture of (me/Eb) with cyl

indrical symmetry, where Eb and me is energy of bea

m and mass of electron respectively.

BES-III Workshop Oct.2001,Beijing

Position of the ZDLM

BES-III Workshop Oct.2001,Beijing

Photon energy

• Maximum energy

is the total energy in the center of momentum system. For BES3 of BEPC2, the cone of total aperture of photon radiated is about 0.33 mrad.and kmax is 1550MeV

if Ebeam = 1.55GeV

BES-III Workshop Oct.2001,Beijing

Formula for Luminosity calculation

If a photon detector is located coaxially with the incident beam line and is subtended to IP with a solid angle of D, the counting rate of NsB(kt) is measured; the luminosity can be obtained by

BES-III Workshop Oct.2001,Beijing

Photon energy spectrum with different Kt

BES-III Workshop Oct.2001,Beijing

Angular distribution for different Kt

BES-III Workshop Oct.2001,Beijing

The acceptance and the rate estimation

Suppose a calorimeter is located behind the splitter magnets at the positio

n of 10 meter away from the IP. An aperture of 20 mm lead collimator coaxial with the incident beam line is assembled in the cross sections of the

calorimeter with various photon energy cuts kt

BES-III Workshop Oct.2001,Beijing

kt –dependence of FAC(kt). It’s shown that the total aperture of 2 mrad for the calorimeter is able to accept more than 87% of

the SB-photons for < 1mrad. 66% for <0.5mrad.

BES-III Workshop Oct.2001,Beijing

BES-III Workshop Oct.2001,Beijing

Background

• Beam gas Bremsstrahlung (GB) background. The calorimeter faces the direction of the incident e—beam, so that the beam gas Bremsstrahlung in the IP region (~30meter straight part) is the main background of SB photon–GB-background. GB has a very similar energy spectrum and angular distribution with the SB photon

BES-III Workshop Oct.2001,Beijing

Energy spectrum of GB photonsAssuming 10-7 mmHg vacuum in the 30 m long chamber

BES-III Workshop Oct.2001,Beijing

Background caused by beam lost

The lost beam (BL) hits the vacuum chamber, the spread secondary photons and electrons would be another background source of SB counting. A veto counter, which is sensitive to charged particles in the front of the calorimeter, could effectively suppress the secondary charged particles and make the beam lost background negligible.

BES-III Workshop Oct.2001,Beijing

Calorimeter system

The SB photon rates are so high, It’s difficult to count photons one by one, doing energy analysis is apparently impossible. We could not be able to set kt cut for readout electronics. So absolute luminosity measurement based on SB process is hardly to do. High SB photon flux is an advantage for relative luminosity monitoring, the integrated currents output from the photon calorimeter will be a relative measurement for the real time luminosity.

BES-III Workshop Oct.2001,Beijing

Detector: GSO crystal 5*5*15cm3 coupled with photodiode.

The high flux of SB photons (from 10 to 1550 MeV) will deposit their energies in the crystal and the absorb dose will be up to 0.23 Mrad/day. So that the radiation hardness of GSO should be good.

BES-III Workshop Oct.2001,Beijing

The photo-diode Hamamstsu S3584-09 will be coupled through the air light guide and concave mirror to the GSO like the

Belle design

BES-III Workshop Oct.2001,Beijing

The sensitivity to the parameters of IP, transverse positions (x,y) and crossing angles

Fixing the e+ beam 11mrad relative to z axis and the e- beam –11mrad relative to z axis, the axis of the calorimeter, which faces the IP and subtends a half angle of ,is coincided with the axis of incident e- beam, steering the e- beam’s axis deviated from the original axis with an amount of

BES-III Workshop Oct.2001,Beijing

Factor of photons accepted changes due to crossing angle error (1mrad acceptance)

BES-III Workshop Oct.2001,Beijing

Factor of photons accepted changes due to crossing angle error (0.5mrad acceptance)

BES-III Workshop Oct.2001,Beijing

The relative acceptance changes with the x (1mrad acceptance)

BES-III Workshop Oct.2001,Beijing

The relative acceptance changes with the x (0.5mrad acceptance)

BES-III Workshop Oct.2001,Beijing

Conclusion

• The EFLM can be used as a relative luminosity online monitor for BESIII while

the precise value of luminosity can be completed by end cap and barrel detectors.

• SB photon’s measurement by the ZDLM can be used as a sensitive real time and relative luminosity monitor for BEPC2