The search for ‘big news’ continues

The big news this summer was on the new Higgs-like boson and how the hint of an excess in last year’s 7 TeV data from the LHC became an observation with this year’s 8 TeV data. Yet there were many other search results, first presented at the International Conference on High-Energy Physics (ICHEP) in Melbourne, which benefited greatly from the new higher-energy data. The search for hypothetical heavy partners of the Standard Model W and Z bosons – the W’ and Z’ – were the CMS collaboration’s priorities for analysis with the 8 TeV data, both because the 7 TeV data included a hint of a high-mass excess and because the 8 TeV data provide a large boost in sensitivity at high mass. Searches for other heavy particles, such as the supersymmetric partners of the gluon and quarks (the gluino and squarks) were similarly priorities that benefited from the increased LHC energy.

Building on last year’s interesting results, the collaboration searched for narrow high-mass Z’ resonances decaying to pairs of electrons or muons in the 8 TeV data collected between April and June this year. At the same time, a search was conducted for a W’, which should decay to a neutrino and a single lepton (electron or muon). Because the Z’ and W’ can be massive, the searches require the identification of highly energetic leptons and a detailed understanding of their behaviour in the detector. The figure shows the spectra for the decay of the Z’ to electron pairs, for the 7 TeV and 8 TeV data combined. It illustrates the importance of understanding the high masses – just a few events appearing there may indicate a discovery.

The search for supersymmetric particles also relies on the production of a few events with massive particles, e.g. gluinos or squarks. These typically undergo cascading decays culminating in multi-jet final states with apparent momentum nonconservation in the detector, owing to the production of two neutral, weakly interacting particles at the end of the cascades that escape detection. (These particles would serve as excellent dark-matter candidates). Decays involving multiple b quarks, photons or same-sign dileptons were all priority search modes with the 8 TeV data. Each benefited from last year’s methods to measure backgrounds from control samples in the data. They also benefited from the rarity of Standard Model processes with such high-mass and complex final states. One particularly interesting background that affects the same-sign dilepton search is the production of a W or Z boson in association with top quarks, which leads to spectacular final states. A first measurement of these processes – obtained with the 8 TeV data – was also presented at ICHEP.

These high-mass searches have found the data to be consistent with Standard Model processes and have significantly improved limits on the range of possible masses for these hypothetical particles. The W’ and Z’ searches set 95% CL limits at 2.85 TeV and 2.59 TeV, respectively, and the gluino/squark searches excluded their masses up to 1.0 TeV. These results correspond to large increases in sensitivity, thanks to the LHC’s energy increase and improved analysis of the new data. At CMS, the search for more “big news” continues.