Beyond its rich programme in flavour physics based on proton–proton collisions, LHCb opened the door in 2015 to a new domain of physics exploration related to cosmic-ray and heavy-ion physics. Due to its forward coverage, the detector has access to a unique kinematic range in colliding-beam physics. In addition, using a system developed for precise luminosity measurements based on the beam-gas imaging method, neon, helium and argon gas has been injected during some periods into the interaction region to exploit the LHC proton and ion beams for fixed-target physics at the highest available energies.
The measurement of proton–helium collisions has been motivated by recent results from AMS and other space detectors, which suggest that the antiproton yield in cosmic rays may exceed the expected value from secondary production in the interstellar medium. The accuracy of such predictions is limited by the poor knowledge of the proton–helium cross-section for proton energies at the TeV scale. By measuring proton–helium collisions, LHCb mimics the conditions for secondary production, and has the potential to help in the interpretation of these exciting results.
In proton–argon collisions, a nucleon–nucleon centre-of-mass energy of 110 GeV is generated, which is in between those achieved in experiments at the SPS in the 1980s and 1990s and those probed at RHIC more recently. While the produced energy densities are too low to create quark–gluon plasma (QGP), they allow the study of cold-nuclear-matter (CNM) effects, which are crucial to determine QGP formation.
During the last weeks of the LHC physics programme of 2015, the LHCb collaboration also participated in the heavy-ion run, taking data in both fixed-target mode by recording lead–argon collisions at a centre of mass energy of 69 GeV, and in colliding-beam mode, collecting lead–lead collisions at 5 TeV. In both modes, the energy densities are large enough to create a QGP, however lead–argon collisions have lower multiplicities than lead–lead collisions, and are therefore easier to analyse. The experiment is able to reconstruct lead–lead collisions up to a centrality of about 50%. The rapidity coverage by the LHCb detector in fixed-target mode in the nucleon–nucleon centre-of-mass frame is about –3 < y < 1; in colliding-beam mode, the range between 2 < y < 5 is covered. The experiment has precise tracking, vertexing, calorimetry and powerful particle identification over the full detector acceptance.
Comparison of collisions in the various configurations allows QGP effects to be disentangled from CNM effects. The various beam configurations are summarised in the diagram.
The focus of LHCb measurements will, on the one hand, be on hard probes such as open heavy-flavour states and quarkonia, which can be carried out down to very low pT. On the other hand, open questions in the soft sector of QCD can be addressed, which cannot be treated perturbatively. LHCb is looking forward to exciting measurements in a variety of beam configurations in the years ahead.