https://indico.ihep.ac.cn//event/9864/

The JC-105 paper describes the topic of ttbar multi-differential cross sections where data sets were recorded by the CMS experiment in pp collisions at sqrt(s)=13 TeV.

1. Introduction.

  • ttbar is mainly created via gluon fusion at p-p collider, therefore, the information of cross section is deeply related to the PDF (parton distribution function) in the protons, as well as the strength of its coupling.

  • Top quark decays before the hadroniation, thanks to its heavy mass. However, calculating its mass from the decay products, originated from a W boson and a b quark, is only robust estimation. That is because the top quark has a color whereas the final hadrons are all color singlet.

  • Question from Xin: what does the “top quark pole mass” represent ?

    • A. The definition of the pole mass is that the mass term in the propagator, where the renormalization is applied.

  • There is lots of discussion on this pole mass, though. For example, it is not easy as the QED to deduce higher order correction, because of the strong coupling.

2. CMS ( The Compact Muon Solenoid )

  • At 15 meters high and 21 meters long, it really is quite Compact for all the detector material it contains.

  • It is designed to detect particles known as Muons very accurately.

  • It has the most powerful Solenoid magnet ever made.

3. Event Selection

  • Signals consist of two jets originating from b quarks and two leptons originating from W bosons. e+e-, mu+mu-, e+mu- and e-mu+ combinations are allowed in this analysis. The event reconstuction has been performed as normal. ( see the detail in the slide )

  • The single variable distribusions, PT(t), y(t) (=rapidity of top), M(tt), and Njet are shown and compared with Monte Carlo simulation results.

4. Results: differetial cross section

  • Cross section is obtained using so-called “response matrix” which converts the experimental raw results into that for full phase space at parton level. Then, the double-differential cross sections and triple-differential cross sections are deduced and compared with MC expectations. Especially, the data exhibit softer PT distributions than the theoretical predictions (MC).

  • Subsequently, simultanous fit results for PDF, the coupling constant (alpha), and the pole mass is shown at the end.