Z boson

Above is a picture of the very first Z boson candidate at LHCb, found by Ph.D. student James Keaveney, which made for a nice story in the Irish Times. When a number had been collected and the efficiency and purity of the selection were determined, it was possible to make a measurement of the cross-section i.e. how often in proton-proton collisions the Z boson is produced. This number is a test of the electroweak theory. Furthermore, because the Z is created when quarks inside the proton collide, the measurement is sensistive to the distribution of quarks in the protons.

Paper describing the results at a centre-of-mass energy of 7 TeV was published in the Journal of High Energy Physics JHEP 06 (2012) 58.

Soon after, the LHC increased the beam energy to a centre-of-mass of 8 TeV and delivered more data, allowing us to increase the precision of the result and compare to theory at two energy points. The 8 TeV results are described in this paper and were published in JHEP 01 (2015) 155.

The machine energy was increased to 13 TeV and a measurement with an uncertainty of just 4% (dominated by how well we know the luminosity of the LHC) was obtained. See Ronan Wallace's thesis for details of this measurement that was published in JHEP 09 (2016) 136.

All the results above used dimuon final states, but we also managed to see the Z boson decaying into taus. A comparison of the muon and tau measurements lets us test a very important symmetry in nature called lepton universality that states that the electron muon and tau are truly identical, except for their mass. The tau paper was published in JHEP 01 (2013) 111.

In addition to the Z boson, other heavy particles decay to two tau leptons - in particular, the Higgs! So we looked for it in this channel, as some (supersymmetric) Higgs models have enhanced decays to taus. Such measurements are important in determining the nature of the Higgs boson. Our paper setting stringent limits on Higgs decays to taus was published in JHEP 05 (2013) 132.

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