Autumn Institute: Precision physics to achieve the LHC accuracy goals

Tuesday 17 Oct 2017, 14:30 → 17:30 Europe/Rome

Aula B1 (Laboratori Nazionali di Frascati (Rome), Italy)

Gennaro Corcella (LNF)

Aim of the workshop:
The Large Hadron Collider has so far performed extremely well and led in 2012 to the milestone discovery of the Higgs boson; however, no signal of new physics has shown up yet. For the sake of carrying out searches for physics Beyond the Standard Model and reliable estimates of the backgrounds, precise calculations and Monte Carlo generators have been of paramount importance. In this mini-workshop, we shall present novel improvements in the matching of parton showers with exact NLO matrix elements, as well as progresses in the fits of parton distribution functions, including small-x resummation. New strategies to measure the top-quark mass and CP violation in top events at the LHC will also be presented.

Speakers:

Benjamin Fuks  (Université Pierre et Marie Curie, Paris)
"Next-to-leading order calculations matched to parton showers for supersymmetry and dark matter"

Umberto De Sanctis  (Università di Roma `Tor Vergata')  
"Top physics with soft muons in ATLAS"   

Marco Bonvini  (Università di Roma `La Sapienza')
"Small-x resummation in PDF fits"  

1 Next-to-leading order calculations matched to parton showers for supersymmetry and dark matter

The ATLAS and CMS collaborations are extensively investigating many different channels in order to get hints for new physics. Many of these searches are currently based on Monte Carlo simulations of the signals where leading-order matrix elements of different partonic multiplicities are matched to parton showers and merged. More sophisticated differential theoretical predictions are however always helpful for setting more accurate exclusion limits, possibly refining the search strategies, and measuring the model free parameters in case of a discovery. In this talk, I will discuss how the MadGraph5_aMC@NLO framework can provide a general platform for computing (differential) observables matching next-to-leading order accurate predictions to parton showers. This will be illustrated through specific examples in the context of dark matter simplified models and supersymmetry.

Speaker: Benjamin Fuks

2 Top Physics with soft muons in ATLAS

Top Physics in ATLAS is one of the crucial sector of the physics programme of the ATLAS experiment at the LHC.  Data collected, are usually analysed to explore top properties or search for heavy resonances, in final states characterised by the presence of jets, lepton(s) and missing transverse momentum. Nevertheless, the abundance of top quarks produced at LHC gives the possibility to focus also on other specific decay channels, such as the ones involving the presence of muons (called "soft-muons") coming from the semileptonic decays of the b-quarks that in turn come from top/antitop decays. In this seminar I will explore the possibility to use these "soft-muons" to measure quantities such as the top mass and the amount of CP violation in the semileptonic b-decays in Run2 data.

Speaker: Dr Umberto De Sanctis

3 Small-x resummation in PDF fits

I present a new determination of parton distribution functions (PDFs) which includes the resummation of large logarithms at small momentum fraction x, relevant at high energies. The description of the HERA data at small x, small Q improves considerably. The new PDFs, obtained from a global fit, exhibit smaller uncertainty at small x, and differ significantly to their fixed-order counterparts in this region. These results demonstrate how the exploration of novel dynamical regimes of QCD, in this case the BFKL regime, represents a fundamental ingredient of the physics program of future high-luminosity and high-energy colliders.

Speaker: Marco Bonvini