Abstract
Photon bremsstrahlung is proposed to be used to identify elastic proton-proton interactions at the LHC. In addition to a measurement of the elastic pp cross section (assuming that the elastic slope is known), the bremsstrahlung photons will allow the evaluation of the total pp cross section, luminosity and to align the Zero Degree Calorimeters (ZDCs).
Highlights
Elastic proton-proton interactions can be tagged at the LHC by detecting the bremsstrahlung photons. p p’ t s p p’The probability to radiate a soft photon with energy kE in proton-proton elastic scattering is given by a well-known formula (1).4 d k pp el em dk d cos k d exp( k 0 s s s ) d F, (1) where F m k k
Unlike the radiative photon signal from elastic scattering, this background is expected to be uniform over the small area of the Zero Degree Calorimeters (ZDCs), resulting in a flat distribution in the variable r2
A restricted energy range for the radiative photon signal in the ZDC is necessary in order to reduce background
Summary
Elastic proton-proton interactions can be tagged at the LHC by detecting the bremsstrahlung photons (figure 1). Based on a compilation of the most recent theoretical results 6 , the ratio el tot varies at the LHC energy of 14 TeV in the range of 0.0515 to 0.0876 This in spite of the fact that at lower energies all the models describe the measured total and elastic cross sections reasonably well. The ZDCs are located at ~zero degrees to the incident beams on each side of the IP, in-between the two beam pipes beyond the first separation dipole, D1, at a distance, |z| 140 m, see Figure 2 Their transverse size is ~8 cm x 8 cm and they cover the pseudorapidity range, | | > 8.5.
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