BES-III Workshop Oct.2001,Beijing The BESIII Luminosity Monitor High Energy Physics Group Dept. of Modern Physics,USTC P.O.Box 4 Hefei,

BES-III Workshop Oct.2001,Beijing Side view of the near IP region

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+cos 2  ) 2 /[8E b 2 (1-cos  ) 2 ]

BES-III Workshop Oct.2001,Beijing Event rate estimation for 3  regions ( Assume L = cm -2 s -1, E beam =1.55GeV,  : 2  covered ) Extreme Forward Region (5 o to 16 o ) Event Rate : Hz End Cap Region (21 o to 34 o ) Event Rate : 412 Hz Barrel Region (34 o to 146 o ) 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 angles Al 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: mm Corresponding to a 7% of error in event count

BES-III Workshop Oct.2001,Beijing Track distribution 20 cm away from the Al surface Effective Al thickness = 35 mm

BES-III Workshop Oct.2001,Beijing Track deflection by the Al Effective thickness = 45 mm

BES-III Workshop Oct.2001,Beijing Secondary charge track numbers due to Al Effective Al thickness = 45 mm

BES-III Workshop Oct.2001,Beijing Error estimate for track smearing and 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 II Zero 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 (m e /E b ) with cylindrical symmetry, where E b and m e is energy of beam 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 k max is 1550MeV if E beam = 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 Ns B (k t ) is measured; the luminosity can be obtained by

BES-III Workshop Oct.2001,Beijing Photon energy spectrum with different K t

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 position 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 k t

BES-III Workshop Oct.2001,Beijing k t –dependence of F AC (k t ). 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

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 photons Assuming 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 k t 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*15cm 3 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 S 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