FLC Polarimetry Group


The International Linear Collider (ILC) is planned as an electron-positron accelerator with polarised beams. So far, a polarisation of 80% is forseen for the electron beam, while 40% seems to be achievable for the positron beam. A later upgrade could provide a positron polarisation of up to 60%.

At the ILC beam diagnostic systems of high precision must complement the interaction region detectors to pursue an ambitious physics programme. Measurement and control of beam parameters to permille level precision is especially important in view of electroweak processes for which the absolute normalisation of expected event rates depends not only on the luminosity, but also on the polarisation of both beams [1,2]. It has been shown repeatedly and for a variety of "New Physics" models that polarised beams can be used to suppress (SM-)background processes while enhancing signal processes and thus helping significantly in revealing the properties (especially spin-related ones) of new new particles [2].

The luminosity will be measured to a precision of 10-3 to 10-4, while for the polarisation average an accuracy of 10-3 seems achievable. While for beam energy and luminosity measurements these precision goals have already been achieved at previous colliders, polarimetry needs to be improved by at least a factor of two compared to the up to now most precise measurement of the SLD polarimeter [3].

Our Work

Our goal is to achieve a permille-level precision (ΔP/P = 0.25%) for the polarisation measurement. In order to realize this ultimate goal, we are optimising the design of the polarimeter magnetic chicane and of the Cherenkov detectors using simulations.

We have also designed, simulated and constructed a prototype detector to test the basic functionality of this design, as well as the readout chain and the entire experimental setup. This two-channel prototype has then been operated successfully in different testbeams (at DESY-II in Hamburg and at ELSA in Bonn). The first results from these "prototype testbeams" are published in an article in the Journal of Instrumentation [4]. Apart from a description of the design, simulation and construction of our small prototype, it also includes an analysis of measurements from the first testbeam campaign at ELSA in 2009.

In addition, we investigate an alternative detector conpept using quartz bars instead of gas-filled channels. [5]

To study different types of photodetectors (esp. linearity) and determine which one is best suited for an ILC polarimeter Cherenkov detector, a testing facility has been setup in a laboratory at DESY (HERA hall WEST). [6]

We are also performing simulations of the beamline to be able to predict how the electrons' spins (and thus the beam polarisation in general) change during their approximate 2 km journey from the upstream polarimeter to the interaction point and further to the downstream polarimeter (UP, IP, DP, respectively). The first outcomes of these studies were recently published. [7]

You can find an overview of the different aspects of our work in the section activities.

[1] The ILC Collaboration, ILC Reference Design Report, Volumes 1-4
     Volume 1: Executive Summary, J. Brau, Y. Okada, N. Walker (eds.) [arXiv:0712.1950 physics.acc-ph];
     Volume 2: Physics at the ILC, A. Djouadi, J. Lykken, K. Moenig, Y. Okada, M. Oreglia, S. Yamashita (eds.) [arXiv:0709.1893 physics.acc-ph];
     Volume 3: Accelerator, N. Phinney, N. Toge, N. Walker (eds.) [arXiv:0712.2361 physics.acc-ph];
     Volume 4: Detectors, T. Behnke, C. Damerell, J. Jaros, A. Miyamoto (eds.) [arXiv:0712.2356 physics.acc-ph].
[2] G.A. Moortgat-Pick et al., The Role of Polarised Positrons and Electrons in Revealing Fundamental Interactions at the Linear Collider,
     Phys. Rept. 460 (2008) 131, [arXiv:hep-ph/0507011].
[3] The ALEPH, DELPHI, L3, OPAL and SLD Collaborations, the LEP and SLD Electroweak Working Groups and the SLD Heavy Flavour Group,
     Precision electroweak measurements on the Z resonance, Phys. Rept. 427 (2006) 257; [arXiv:hep-ex/0509008].
[4] C. Bartels, J. Ebert, A. Hartin, C. Helebrant, D. Kaefer and J. List, Design and Construction of a Cherenkov Detector for Compton Polarimetry at the ILC,
     JINST 7 (2012) P01019, January 2012, DESY 10-225, [arXiv:1011.6314 physics.ins-det].
     JINST 9 (2014) P07003, July 2014, [arXiv:1405.2156 physics.acc-ph].
[5] J. List, A. Vauth and B. Vormwald, A Quartz Cherenkov Detector for Compton-Polarimetry at Future e+e- Colliders,
     JINST 10 (2015) P05014, [arXiv:1502.06955 physics.ins-det].
[5] J. List, A. Vauth and B. Vormwald, A Calibration System for Compton Polarimetry at e+e- Linear Colliders,
     [arXiv:1509.03178 physics.ins-det].
[7] M. Beckmann, J. List, A. Vauth and B. Vormwald, Spin Transport and Polarimetry in the Beam Delivery System of the International Linear Collider,
     JINST 9 (2014) P07003, [arXiv:1405.2156 physics.ins-det].