One of the routes towards discoveries is the measurement of known processes with increasing precision and the comparison with precise theoretical calculations looking for possible discrepancies.
Proton collisions at the LHC are described by the Standard Model (SM) and in particular by its strong sector, Quantum Chromo-Dynamics (QCD). QCD is a complex theory, with a rich and partly yet unknown phenomenology. Luckily, processes with large momentum exchange, such as those of interest at the LHC, can be calculated as a perturbative series in the strong coupling constant αs. Theoretical physicists made huge progress in the last decades in calculating QCD processes with increasing precision. In most cases the theoretical predictions are limited by the knowledge of the internal structure of the incoming protons. This structure, that is described by the parton density functions (PDFs), can not be calculated at the moment from first principles, and has to be extracted from the comparison of measurements and theoretical predictions. The figure below shows a collection of cross sections for different SM processes measured by ATLAS, compared with state of the art theoretical calculations. The agreement is very good (unfortunately!).
One of the processes that can be calculated with highest precision is the production of lepton-antilepton pairs (or lepton-antineutrino), known as the Drell-Yan (DY) process. The diagram in the figure shows the production of a muon-antimuon pair at the lowest perturbative order in QCD (in this case ?s0). A quark and an anti-quark, originating from the incoming protons, annihilate into a gamma or Z virtual boson that in turn produces the muon pair.
This process can be exploited to test the SM by measuring some of its fundamental parameters, such as the weak mixing angle or the W boson mass, to test the internal consistency of the model, or to search for deviations from the expected cross sections, for example at large di-lepton invariant masses, that can be caused by new physics. In any case the precision of the test will be limited by the current knowledge of the PDFs. Nevertheless the DY process itself can be used to improve our knowledge of the PDFs.
The Rome ATLAS group has been studying the DY process in the regime of low invariant mass of the lepton pairs (where low means 7 to 60 GeV). In this regime, the data are sensitive to a poorly known region of the PDFs, where quark or anti-quarks carry a very small fraction of the proton momentum (order 10-4). The lowest mass region is also an experimental challenge, since high backgrounds and large rates make the selection and the triggering very difficult. Results from a first measurement, based on the first LHC run at a center of mass energy of 7 TeV, are shown below, compared to the perturbative QCD prediction at order αs2 (FEWZ) using the PDF parametrization MSTW2008.
