Polarized electron and positron beams are foreseen for the future linear collider. High precision physics requires the polarization of both beams to be known with a relative uncertainty of about 0.5% or better. Therefore all possible depolarizing effects that could operate between the polarized sources and the interaction regions have to be under full control.
The ‘heLiCal’ collaboration aims to provide a full `cradle-to-grave’ analysis of all depolarizing effects from the source to the interaction region. The largest depolarizing effect is expected to result from the collision of the two beams at the interaction point(s). Two effects influence the spin motion in electric and magnetic fields: a) spin precession (figure) and b) spin-flip via synchrotron radiation (see figure).
Spin precession is described by the Thomas–Bargmann-Michel-Telegdi (T-BMT) equation. In order to describe all depolarization effects accurately effects from higher-order processes have to be talken into account and the adaption of classical equations for such a strong field regime has to be performed.
Beam polarisation has the potential to discriminate between many physics models and will be a vital tool in unravelling the mysteries of physics Beyond the Standard Model.
One of the most promising candidates for physics beyond the SM is supersymmetry. This new symmetry predicts that every SM particle has a SUSY partner that has the same quantum numbers as their SM partner, with the exception of the spin. Electroweak precision tests predict that at least some SUSY particles should be accessible at sqrt(2)=500 GeV. To really establish supersymmetry experimentally, all model assumptions and implications have to be verified.
For instance, the chiral quantum numbers of the scalar partners of the electron/positron have to be verified. This association can only be directly tested in the production of the pairs sel+L sel-R. Even a highly polarized electron beam may not be sufficient to separate this process from sel+R sel-R, since both can be produced with almost identical cross sections and have the same decay. Applying simultaneously polarized positrons, the pairs get different cross sections, can be isolated and the properties of the particles can be tested separately.
Transversely-polarized beams are sensitive to non-standard interactions, which are not of the current–current type, such as those mediated by spin-2 gravitons or (pseudo)scalar exchanges, even in indirect searches. With transversely-polarized beams (both beams have to be polarized) an azimuthal asymmetry can be constructued that uniquely distinguishes, for instance, different extra-dimension models up to at least 3 TeV.
Representative examples are the models of Randall-Sundrum (RS) and Arkani-Hamed, Dimopoulos, Dvali (ADD). The new asymmetry vanishes for both the SM and the RS scenario, so that a non-zero value unambiguously signals the ADD graviton exchange. Study was done at sqrt(s)=500 GeV.