The second ECFA High Luminosity LHC Experiments Workshop was held in Aix-les-Bains, France, from the 21^st to the 23^rd of October 2014. The first such workshop last year was organized as a follow on from the Update of the European Strategy for Particle Physics, which identified Europe’s top priority to be the exploitation of the full potential of the LHC, including the high-luminosity upgrade of the machine and detectors with a view to collecting ten times more data than in the initial design. The ECFA HL-LHC workshops are an opportunity for an unusually open discussion between the accelerator community, the experimentalists and theorists. In terms of interactions between the experiments and the machine, there was a classic moment of clarification when an LHCb representative managed to get across the point that although they have a “one-sided” detector, nonetheless they need the extra beam protection (TAN) on both sides of the interaction point.

The big challenge for the general purpose detectors comes from the increase in instantaneous (and integrated) luminosity, and I presented techniques for pileup mitigation, in other words how to deal with on average 140 ordinary events in exactly the same bunch crossing as an interesting hard scatter event. ATLAS had some very fresh results showing how the performance degrades going well beyond the nominal HL-LHC luminosity to an average pileup of 200, 250 or even 300 events. In addition, we explored the potential benefits of going from a Gaussian longitudinal beam spot to a long flat beam spot, which could be achieved with the “crab-kissing” scheme. With ttbar events, in which it is relatively easy to reconstruct the primary vertex of interest, there seemed to be little to gain from stretching out the beam spot. This provoked a lot of discussion. However, these studies need to be augmented with more challenging signals, such as a Higgs boson decaying to two photons, with a greater range of pileup scenarios and by both experiments before any firm conclusion can be reached.

A new physics highlight for me was to see the prospects for observing Higgs boson pair production presented by both ATLAS and CMS. Experts from the two experiments had a chance to compare their internal results before they were released. Each experiment projected over 1 sigma sensitivity, with similar numbers of signal and background events. Some remaining differences are under study, but seem to be explained by the different input assumptions on reconstruction efficiency and background rates. This is an elusive final state, so it was encouraging to see that we have scope for investigating it with the HL-LHC.

For more information you can visit: https://indico.cern.ch/event/315626/