I recently read that CERN planned on performing photon/photon collisions in 2015. I can't seem to find the results. I'm not a scientist so not too familiar with finding CERN documentation.

Any thoughts on how I can find the results? The experiment is known as the CMS-TOTEM Precision Proton Spectrometer (CT-PPS).


In order to make this answer more generally useful, I'll give some background on the experiment. The aim of CT-PPS is to study $pp \to pXp$ where the protons do not dissociate. One such case is when each proton radiates a photon and those two photons collide while the two protons fly away from the collision zone, intact, more or less in the direction of the beam pipes. But the other case is very interesting too: gluon-gluon fusion in a singlet state, where each proton radiates a gluon. A tracking and timing detector is placed about 200m each side of the CMS dectector, so as to reconstruct the mass and momentum of the centrally produced system $X$, as well as the position of the vertex, by measuring the position and the momentum of the intact protons. Centrally means that the momentum of that system is at an angle closer to 90 degrees with respect to the beam pipe.

The interest of this method is that the whole proton momenta is used. This is not the case in usual $pp$ collision at LHC, where the collision is from a parton (i.e. a quark or a gluon) taking a fraction of one proton momentum and a parton taking a fraction of the other proton momentum. This leads to a two-fold advantage:

  • a higher mass region can be probed for the particles produced in the collision, from 300 GeV to 2 TeV;

  • the momentum of the initial state is exactly known. For collisions between partons, their momenta is not known: the cross-sections have to be averaged with a weight giving the probability to find a given parton that takes a given fraction momentum of the proton [*].

The problem in the case of photon-photon collisions is that the cross-section is much smaller than for the usual parton-parton collisions, as electromagnetism is at play for the former, which is much weaker than the strong interaction at play in the latter. But since LHC has a super high luminosity, the number of events could still be large enough to extract interesting results.

This seems very promising. The collaboration has published this summer the proceedings of a talk about their first results [1].

[1] Berretti, Mirko, Very forward measurements at the LHC, arXiv:1708.09853

[*] It should be noted that photon-photon collisions are not exempt of this issue, as the most probable outcome is that each photon will oscillate into a $q\bar{q}$ pair and that one quark from one photon will collide with one quark from the other photon, in which case we do not know the momentum of the colliding particles. But there are also direct $\gamma\gamma$ collisions, which can be a bonus.

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