The LHC Run 1, famed for its discovery of the Higgs boson, came to a conclusion on Valentine’s Day (14 February) 2013. The little-known fact is that the last three days of the run were reserved neither for the highest energies nor for the heaviest ions, but for relatively low-energy proton–proton collisions at 2.76 TeV centre-of-mass energy. Originally designed as a reference run for heavy-ion studies, it also provided the perfect opportunity to bridge the wide gap in jet measurements between the Tevatron’s 1.96 TeV and the LHC’s 7 and 8 TeV.
Jet measurements are often plagued by large uncertainties arising from the jet-energy scale, which itself is subject to changes in detector conditions and reconstruction software. Because the 2.76 TeV run was an almost direct continuation of the 8 TeV proton–proton programme, it provided a rare opportunity to measure jets at two widely separated collision energies with almost identical detector conditions and with the same analysis software. CMS used this Valentine’s Day gift from the LHC to measure the inclusive jet cross-section over a wide range of angles (absolute rapidity |y| < 3.0) and for jet transverse momenta pT from 74 to 592 GeV, nicely complementing the measurements performed at 8 TeV. The data are compared with the theoretical prediction at next-to-leading-order QCD (the theory of the strong force) using different sets of parameterisations for the structure of the colliding protons. This measurement tests and confirms the predictions of QCD at 2.76 TeV, and extends the kinematic range probed at this centre-of-mass energy beyond those available from previous studies.
Calculating ratios of the jet cross-sections at different energies allows particularly high sensitivity to certain aspects of the proton structure. The main theory scale-uncertainties from missing orders of perturbative calculations mostly cancel out in the ratio, leaving exposed the nearly pure, so-called DGLAP, evolution of the proton parton density functions (PDFs). In particular, one can monitor directly the evolution of the gluon density as a function of the energy of the collisions. This lays a solid foundation for future searches for new physics, for which the parametrisations of the PDFs are the leading uncertainty. Also, the experimental uncertainties cancel out in the ratio if the conditions are stable enough, as indeed they were for this period of data-taking. This principle was proven by ATLAS with 2.76 TeV data collected in 2011 (2013 Eur. Phys. J. C 73 2509), but with a data set 20 times smaller.
The figure demonstrates the excellent agreement of the ratio of 2.76 and 8 TeV data with the QCD predictions, laying a solid foundation for future searches for new physics through smaller QCD uncertainties. This opportunistic use of the 2.76 GeV data by CMS has again proven the versatility and power of the LHC programme – a true Valentine’s Day for jet aficionados.