Photons are important probes because they couple to all charged particles. The coupling strength is determined by measuring photon splitting into particle-antiparticle pairs. These strengths have been previously measured for electrons, for quarks, and for one type of meson – the pion. In a paper published in Physical Review Letters,, the ALICE Collaboration reports on the first measurement of coherent photoproduction of a charged kaon pair (K+K–). The data allows us to compare the behavior of heavier mesons, containing strange quarks (kaons) with lighter mesons composed only of up and down quarks (pions). The study found that the coupling of kaons and pions was comparable, after accounting for the large mass difference.
The data came from ultra-peripheral collisions (UPCs) of relativistic lead nuclei, produced at CERNs Large Hadron Collider (LHC). In UPCs, the two nuclei do not physically collide, but interact at long range, via photon exchange. A photon from one nucleus can fluctuate to a virtual (evanescent) K+K– meson pair (particle and antiparticle) and then one of the kaons can scatter elastically (and coherently) from the other nucleus, emerging as a real K+K– pair. The final state kaon pairs have a very low transverse momentum (pT), as is seen in the figure. This showed that the kaon-nucleus scattering was coherent (elastic), with coupling to all of the protons and neutrons in the target. The analysts – the Relativistic Nuclear Collision program’s Minjung Kijm, Spencer Klein and Mateusz Ploskon – applied stringent particle identification cuts to select a pure sample of kaon pairs.
With that, the ALICE collaboration found that the couplings were similar to those of pions (with only ¼ the mass), after adjustment for the different meson mass, showing that different mesons behave similarly, despite difference in quark content and mass. Looking ahead, with the upgraded ALICE data acquisition system, during LHC Runs 3 and 4, ALICE should collect more than 100 times more UPC events than were used here, allowing for precision measurements of kaon pairs, and also allowing us to look at pairs of heavier mesons, and even baryon pairs. A more comprehensive set of results will allow us to study trends based on meson masses and other properties.
Figure 1: The transverse momentum (pT, left) and daikon mass (right) spectra for the ALICE event sample. The pT spectrum shows that the production is coherent over the entire target. The production rate is above expectations from the ϕ meson alone.