Luminosity Measurements (authors:  W. Lohmann and M. Woods)

Overview

The luminosity of a linear collider will be measured with a precision of 10^-3 or better by measuring the Bhabha rate in the polar region from 30-90 mrad in the LUMICAL detector.[1]  At 500 GeV center-of-mass energy, the expected rate in this region is ~10 Bhabhas per bunch train.  At smaller polar angles of 5-30 mrad the rate or energy deposition of beamstrahlung e⁺e^- pairs can be measured in the BEAMCAL detector for a fast luminosity diagnostic.  The expected rate in BEAMCAL is 15,000 pairs (and 50 TeV energy deposition) per bunch crossing. 

BEAMCAL is expected to be useful for machine tuning and can be used for the fast IP feedback[2] planned to stabilize the colliding beams.  In addition to its total energy or rate signals for a luminosity diagnostic, the spatial distributions of pairs in BEAMCAL may be useful for determining some of the beam collision parameters such as spotsizes and bunch lengths.[3]

LumiCal and BeamCal are positioned inside the Detector just in front of the first quadrupole magnets.[4]  LUMICAL is planned to be a segmented silicon-tungsten calorimeter.  BEAMCAL must be very radiation hard and a finely segmented diamond-tungsten calorimeter is planned, though other technologies are also studied.  BEAMCAL should also provide good hermeticity and efficiency for detecting high energy electrons; these are needed to suppress brackgrounds from copious 2-photon events in new particle searches (ex. SUSY).  A possible layout of the very forward region of an ILC detector,[5] in this case designed for head-on collisions or a small crossing angle, is shown in Figure 1.

R&D Required

• LUMICAL:  compare precision achievable for different crossing angle geometries
• BEAMCAL:  study possible systematics for its use determining beam parameters and how this is affected by a crossing angle
• BEAMCAL:  check that optimizing the pair signal also optimizes the e+e- luminosity for different aberration tuning scans
• BEAMCAL:  detector requirements and capabilities for electron identification given the high flux and energy deposition of beamsstrahlung pairs
• BEAMCAL:   detailed detector design and prototype:  radiation-hard, hermetic, electron id
• Other Detectors:  radiative Bhabha[6] and wide-angle beamsstrahlung photon detectors[7]

References

1.  Luminosity Measurement via Bhabha Scattering Precision Requirements for the Luminosity Calorimeter, A. Stahl, LC-DET-2005-004, 2005
2.  Interaction Point Feedback System R&D, Philip Burrows, presentation at LCWS05.
3.  Reconstruction of IP Beam Parameters at the ILC from Beamstrahlung, G. White, SLAC-PUB-11341, 2005; 
     Diagnostics of Colliding Bunches from Pair Production and Beam Strahlung at the IP, A. Stahl, LC-DET-2005-003, 2005. 
     Beam Profile Monitor, H. Yamamoto, presentation at SLAC MDI Workshop, 2005. 
2.  K. Moenig, talk at the Workshop "`Instrumentation of the Forward Region of a Linear Collider Detector"', August 2004, DESY-Zeuthen, Germany. 
5.  H. Abramowicz et al., IEEE Trans. Nucl. Sci. 51, 2983 (2004).
6.  Luminosity Monitor Options for TESLA, O. Napoly and D. Schulte, CERN-OPEN-2000-135, 2000. 
7.  Measurement of Colliding Beam Parameters with Wide Angle Beamstrahlung, G. Bonvicini, D. Cinabro, and E. Luckwald, Phys.Rev.E59:4584-4593,1999.
     e-Print Archive: physics/9812020.
     Diagnostics with Beamstrahlung Electrons and Photons, C. Grah, presentation at SLAC MDI Workshop, 2005.