The challenge of monochromatization

The FCC-ee could measure the electron Yukawa coupling in a dedicated run at $$\sim $$ ∼ 125 GeV collision energy, provided that the center-of-mass (CM) energy spread can be reduced by means of monochromatization, e.g., through introducing nonzero horizontal dispersion of opposite sign at the interaction point (IP), for the two colliding beams. If the IP dispersion is nonzero, beamstrahlung blows up the horizontal emittance, and self-consistent IP parameters need to be determined. Two configurations are being studied. The first uses crab cavities to establish effective head-on collisions. The second configuration maintains the standard FCC-ee crossing angle, which, together with the IP dispersion, introduces a correlation between the local collision energy and the longitudinal location inside the detector, thereby allowing for an integrated scan of the Higgs resonance curve. We compare both approaches.

Spin Experimentation with Unpolarized Colliding Beams at the LHC

A brief recollection of the problems related to a significant hyperon polarization observed in pp-collisions is given with an emphasis on the general role of spin in the dynamics of hadron interactions. The old, unsolved problem of the observation of a significant hyperon polarization can provide new insights as a result of the measurements of energies at the LHC; in combination with other measurements, these can be used to tag the QGP formation in pp-collisions with colliding beams. Polarization studies in the processes of hyperon production do not require the use of polarized beams or targets and can be performed in the existing experimental environment at the LHC. Model predictions based on the chiral dynamics and pictures of the impact parameter are presented for the illustration of a possible dynamical mechanism that leads to a hyperon polarization.

Massless Composite Bosons Formed by the Coupled Electron-Positron Pairs and Two-Photon Angular Correlations in the Colliding Beam Reaction e-e+→Bγγ with Emission of the Massless Boson

The approach in which the electron and positron are treated as ordinary, different particles, each being characterized by the complete set of the Dirac plane waves, is examined. This completely symmetric representation that is beyond the standard QED makes it necessary to choose another solution of the Dirac equation for the free particle propagator as compared to that used currently. The Bethe-Salpeter equation with these particle propagators is solved in the ladder approximation. A new solution has been found represented by the massless composite bosons formed by the coupled electron-positron pairs with the coupling equal to the fine structure constant. It has been demonstrated that (1) the massless boson states have normalizable complex wave functions which are transversely compressed plane waves; (2) the transverse radius of the wave functions diverges as the boson energy goes to zero; that is, the composite bosons cannot be at rest; (3) increasing the boson energy results in an extension of the transverse wave function in the momentum space and a corresponding contraction of the real space coordinate wave function. The new reaction e-e+→Bγγ is investigated with the products composed of the massless composite boson and two photons. The cross-section of this reaction is derived for nonrelativistic colliding beams of spin-polarized electrons and positrons. In this case the 2γ angular correlation spectrum is characterized by a narrow peak with the full-width-at-half-maximum not exceeding 0.2 mrad. It is shown that in order to distinguish between the conventional annihilation of the singlet electron-positron pair with the two-photon emission and the new examined reaction yielding the three particles, experiments are proposed with the extremely nonrelativistic colliding beams.

A merged quadrupole-calorimeter for CEPC

The luminosity [Formula: see text] of colliding beams in a storage ring such as CEPC depends strongly on [Formula: see text], the half-length of the free space centered on the intersection point (IP). [Formula: see text] is also the length from the IP to the front edges of the two near-in quadrupoles that are focusing the counter-circulating beams to the IP spot. The detector length cannot, therefore, exceed [Formula: see text]. Since [Formula: see text] increases strongly with decreasing [Formula: see text], there is incentive for reducing [Formula: see text]; but this requires the detector to be shorter than desirable. This paper proposes a method for integrating these adjacent quadrupoles into the particle detector to retain (admittedly degraded) active particle detection of those forward going particles that would otherwise be obscured by the quadrupole. A gently conical quadrupole shape is more natural for merging the quadrupole into the particle detector than is the analytically exact cylindrical shape. This is true whether or not the calorimeter is integrated. It will be the task of accelerator physicists to determine the extent to which deviation from the pure quadrupole field compromises (or improves) accelerator performance. Superficially, both the presence of strongest gradient close to the IP and largest aperture farther from the IP seem to be advantageous. A tentative design for this merged, quadrupole-calorimeter is given.