JRA1-LHC-Combine: Inter-experiment combination of heavy-ion measurements at the LHC

The first (2010-12) and second (2016-18) runs of the Large Hadron Collider at CERN provide a wealth of results from heavy-ion collisions. With 25 times more energy, the quark-gluon plasma signatures observed at the Relativistic Heavy Ion Collider (jet quenching, collective flow…) are again observed, and new probes become accessible (reconstructed jets, b quark hadrons, electroweak bosons…). Surprisingly, LHC reveals that some of the features expected to arise from the quark-gluon plasma in nucleus-nucleus (AA) collisions are also observed in proton-nucleus (pA) and even in selected proton-proton (pp) collisions. The four large LHC collaborations, ALICE, ATLAS, CMS and LHCb, contribute to this programme with very different and complementary capabilities, both in terms of angular coverage and particle identification. Up to now, the collaborations worked very independently, in a healthy competition. We believe that the time has come to improve communication between the four collaborations in the heavy-ion field, and establish an LHC data-combination working group. These objectives can be split into two tasks: The animation of a common forum (task 1) to ensure a regular communication between the four collaborations; and cross-experiment combination work (task 2), such as detailed comparisons of techniques or optimized statistical combination of results, leading to common publications. Task 2 will be split in different projects in the first months of the project, after a kick-off meeting leading to a roadmap (deliverable 1). The range of topics we want to span is as broad as possible. They will concern AA collisions, obviously, as well as pA and pp (both for references and high-multiplicity studies, with the experiments having very different bandwidth). Below are a few examples of actions we foresee, although there is no a priori restriction to the scope to be covered:

• Constrain nuclear parton distribution function (nPDF): Combining the complementary measurements of electroweak bosons in pPb collisions, at mid (ATLAS, CMS) and forward (ALICE, LHCb) rapidities, as well as di-jet measurements, will provide the most precise constrains possible to nPDF modellers.
• Light-by-light scattering: An evidence (13 events leading to 4.4 sigmas), published in Nature by ATLAS, was based on 2015 PbPb collisions. Combined with CMS, it would be a discovery (>5 sigmas). With the 2018 data, more precision will be achieved and combining results will be key in reaching sensitivity to effects beyond the SM.
• Open charm cross section: Combining all measurements of hadrons containing charm quarks in the various acceptance of the LHC experiments will allow assessing the total charm cross section, as well as its dependencies on broad ranges of transverse momentum and rapidity, in pp, pPb and PbPb collisions. These are crucial ingredients to energy loss and charmonium regeneration models, providing key information to the charm quark dynamics and QGP properties. Same work will also be carried out for open beauty, though with lower precision because of limited statistics.
• Quarkonia: Another interesting example is the charmonium case, for which most of the expertise is based in Europe, with the four experiments having very complementary acceptances. Combination for both the high-precision J/ψ and the statistically challenging ψ(2S) would already provide better understanding of the balance of several effects on these complicated probes (dissociation, recombination, energy loss, etc.). This last example is likely to serve as a proof-of principle early in the project (it is to be noted that ALICE and LHCb already have common public notes about the reference quarkonium cross sections in pp collisions). It also extends to bottomonia for which ALICE and CMS have results at different rapidities.