Searching for top-quark pairs produced in photon collisions at the LHC
"The CMS collaboration has just submitted for publication the results of the first search for the so-called central exclusive production of top quark pairs at the LHC. The LIP-CMS group had a leading role in this analysis. "
Top quarks are abundantly produced in pairs at the LHC. The dominant processes at such energies are strong interactions involving a quark in each of the initial proton and result in the production of top quark-antiquark pairs (tt). A different production mechanism is however possible, in which the two protons remain intact after the interaction and merely exchange photons (or, in other words, interact via the electromagnetic force), and a tt pair is subsequently produced in photon interaction. This process, pp → pttp, is called central exclusive production.
While central exclusive production of tt pairs is much rarer than the dominant production modes and its observations is only expected to become possible at the high-luminosity LHC, contributions from physics beyond the SM (for example, the exchange of other particles that we don’t yet know) could enhance the production cross section, making it detectable with the data collected so far.
The search now completed was carried out in proton-proton collisions at a centre-of-mass energy of 13 TeV. The top quark pair decay products were reconstructed using the central CMS detector, while forward protons were measured in the CMS-TOTEM Precision Proton Spectrometer (PPS), placed close to the beam pipe around 200 meters away from the collision point. The observations are compatible with what was expected from SM backgrounds. An upper limit of 0.59 pb at 95% confidence level was set, valid for interactions of protons loosing between 2% and 20% of their momentum.
The LIP-CMS group had a leading role in this analysis, performed over the last few years. The main contributors were the LIP/IST graduate students Matteo Pisano (currently completing his PhD in exclusive processes in CMS) and Beatriz Lopes (who completed her master thesis also in the LIP-CMS group and is now a PhD student at DESY, Germany). The group has major responsibilities in the PPS detector system and contributed to earlier publications on searches of exclusive processes.
The paper, now submitted for publication to the Journal of High Energy Physics (JHEP). is available at: https://arxiv.org/abs/2310.11231