Polarimetry

This page discusses the polarimetry in EIC. The hadron polarimetry and electron polarimetry will be discussed in more detail below

Hadron polarimetry

Introduction

The Electron-Ion Collider will be the first high energy collider to use both polarized electron beams and polarized hadron beams. This will offer a unique opportunity for studies of the proton and nuclei structure and other fundamental QCD studies. The uncertainties on the polarization of the beam particles translate directly into the uncertainties of final physics quantities to be extracted in physics data analyses. Hence, a good control of these uncertainties is critical for the success of the EIC.

Contrary to the case of electron beam polarimetry, which uses physical processes derived from first principles that allow a relatively simple extraction of the electron beam polarization, for hadron beams, no such process is available, and the best currently used methods rely on the process of elastic scattering in the Coulomb-Nuclear Interference (CNI) region, of which there are only effective models with parameters that are still not known.

The experience obtained from proton polarimetry from RHIC, the only high energy collider of polarized proton beams, is extremely important. However, the EIC presents specific challenges, as the shorter spacing between bunches. For this reason, simulations that can describe the data obtained at RHIC and predict measurements at the EIC are relevant. On the other hand, tests can be foreseen while RHIC is still running, to measure rates and, if possible, analyzing powers of the reactions relevant for light ion (p, d, h) polarimetry at the EIC.

Requirements

• Large polarization values, longitudinal and transverse, with flexible bunch polarization orientation
• Small uncertainty in the polarization measurement, less than 1%
• Measurement of the bunch polarization profile in space (x, y and z) and time (extracting the polarization lifetime)
• Measurement of the polarization per bunch (assuming 2 detectors, not all bunches will collide at each interaction point), or at least per set of similar bunches

Challenges

• Short spacing between bunches (8.9 ns instead of the 106 ns at RHIC)
• Non-elastic scattering background to the elastic scattering events may contaminate the preceding bunch
• Luminosity measurement may depend on the polarization σ_Brems: = σ₀(1 + aP_eP_h)
• Pioneering light ion beam polarization measurements at high energies

Electron polarimetry

Introduction

There are three common techniques for measuring the electron polarimetry. The first one is the Mott polarimeter, electrons are scattered by the Coulomb field of a heavy nucleus. Mott scattering is the only practical way to measure electron beam polarization at the beam energy typical of electron guns ( ~50 to 100 keV) and electron injectors ( a few MeV). The second one is the Moller Scattering Polarimeter. The electrons are scattered from other polarized electrons in a target. The process is usually destructive and can offer rapid and precise measurement for electron beam from MeV to GeV. The third one is Compton polarimeter. It uses laser photons scattered from electron beam. It is the easiest for high energy electron beam and is non-destructive.

Regarding the Rapid Cycling Synchrotron (RCS) transmission, a polarimeter needs to cover beam energies from 1 to 18 GeV. Moller polarimeter would be a good candidate from the point of view of cost, technological difficulty, operation, to measure polarization during RCS ramp in, or at exit of, the RCS.

In the Storage Ring, a polarimeter needs to cover beam energies from 5 GeV to 18GeV. The best solution is a Compton scattering polarimeter. It would be located in IR12 (IR6 is considering).

Requirements

• The location for the Compton polarimeter is place at IR12 (Other location is being considered).
• Need to measure both longitudinal and transverse components
  • requires highly segmented pre-shower and ECal with good energy resolution for gamma ;
  • highly segmented ECal with good energy resolution and position resolution for recoil electron;
• Need to measure bunch-by-bunch polarization;
• The measurement precision should be less than 1%;
• Moller polarimeter in RCS (interceptive);
• Compton scattering polarimeter in storage ring (non-interceptive);

Polarization measurement

The asymmetry of the energy spectra measured with left and right helicity have been used to measure the longitudinal component of the polarization.

Energy spectra of scattered photon for Pz=+1.

Energy asymmetry of scattered photon for Pz=+1.

Energy vs theta.

The transverse polarization can be measured by measuring the spatial asymmetry. The horizontal smearing of the scattered photon is much larger than the vertical smearing making the measurement of the x dependence of the cross sections more difficult than of the y dependence.

Vertical position of scattered photon for Py=+1.

Vertical position asymmetry of scattered photon for Py=+1.

Location of the polarimeter in IR

We want the position of the polarimeter to be as close as the interaction point, unfortunately the IR6 is crowded ( considering IR6 ). IR12 has enough space for the polarimeter. We need dipole to bend the electron beam so we can separate the scattered photon. The polarimeter would be placed near 11o’clock side where the outer side of the electron Storage Ring has more space than the inner side. The following plot is the layout of the polarimeter.

The interaction point of Compton scattering is at (0.274464m, 85.73037m) in IR12, just between DB23 and QD12; The entrance window of laser is near QF11 and the exit window is near QF13; The laser would go through five magnets from QF13 and QF11; The scattered photons can not pass through the magnets if the inner radius is too small;

The requirements for the magnets

Layout of IR12 and the requirements for the magnets.

Estimation of time for measurement

The Sokolov-Ternov [15] effect will depolarize these electron bunches with a time constant of 30 min (at the highest energy of 18 GeV). In order to maintain high spin polarization, each of the bunches with their spins parallel to the main dipole field (of which there are 145 at 18 GeV) is replaced every six minutes. The polarization lifetime is larger at lower beam energies and bunch replacements are less frequent.

At 18 GeV operation, a bunch replacement rate of once per six min for each individual bunch in the storage ring is required to keep the time-averaged level of polarization at a level of ≈ 70 %, assuming an initial polarization of the freshly injected bunches of 85 %. Thus the injector has to provide a new bunch about every second to maintain good polarization of all 290 bunches. At 10 GeV and 5 GeV, the depolarization time is much longer and the bunch replacement rate can be reduced by a factor of at least five.

time vs stat. error

The requirements for the beam parameters

time vs beta function