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ERC

COLORTTH Report Summary

Project ID: 335696
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Mid-Term Report Summary - COLORTTH (The Higgs: A colored View from the Top at ATLAS)

The aim of particle physics is the understanding of the most fundamental constituents of matter and its interactions. Over the last century, many advancements in this understanding have been achieved, and a standard model of particle physics was established. Despite its success, the standard model leaves many questions open. For example it is not fully clear yet how particles acquire their mass, why we have more matter than antimatter in the universe, and what the nature of dark matter is. With the recent discovery of the Higgs boson in 2012, one question got answered, but it also opened up many new questions. The currently running LHC proton-proton collider at CERN aims to improve our understanding with precision measurements and searches for physics beyond the standard model. The dedicated study of the Higgs boson, and its connection to the heaviest known elementary particle, the top quark, are particularly promising avenues towards deepening our understanding of nature.

The main aim of the COLORTTH project is two-fold: (1.) the development and establishment of a new tool utilising QCD-colour connections between jets, which can be used to distinguish colour-octet from colour-singlet states and thus be useful for many searches for new physics; and (2.) the search for Higgs bosons produced in association with a top quark pair (ttH), with the ultimate goal to measure the top-Higgs Yukawa coupling and probe hints for new physics. At the mid-term of the project, already first steps have been achieved, in accordance with the research plan: A first measurement of the colour-flow observable (jet pull) has been done in top antitop quark events, showing world's first evidence for non-octet behaviour of the two jets from the W boson from the top quark. This result has been published in PLB. Furthermore, the search for ttH with new LHC data at 13 TeV collision energy was performed, increasing the sensitivity compared to any other previous searches. This result was one of the highlights at ICHEP 2016, which is one of the two most important bi-annual particle physics conferences (together with EPS), documented in a public conference note (ATLAS-CONF-2016-080).

Besides these two major achievements, various studies are ongoing that are important inputs to achieving the main goals of the project. In particular, we played a leading role in measuring the top antitop quark pair production cross section with the new 13TeV data in 2015, being major highlights at the EPS 2015 conference (documented in ATLAS-CONF-2015-049 and ATLAS-CONF-2015-03). A measurement of differential distributions of top antitop quark pair production in events with identified b-jets has been shown at ICHEP2016 (ATLAS-CONF-2016-040), and is currently prepared for publication.
Along the way, various tools have been developed as part of this project, as for example improved methods to determine the QCD-fake background and a multivariate reconstruction technique for ttH.
Furthermore, several PhD and undergraduate students are trained, and postdoctoral researchers achieve very good visibility within the collaboration and the field as part of the project.

Currently, several avenues are explored as part of the project to further strive towards its goal. In particular, we will study double-top-double-b-quark events, which are the most important background to ttH, and which has not been measured yet precise enough in data. In collaboration with theorists, we also study models with multiple Higgs bosons, which would result in a similar signature as the ttH final state we explore. We also collaborate with theorists on how to use the colour flow observable in tills like top-tagging. Furthermore, improvements of the measurement of the colour flow observable, its application in ttH and the general improvement of the sensitivity to ttH are ongoing.

In summary, the COLORTTH project is well underway towards its goals, with already first major achievements having been reached.

Contact

Liz Fay, (EU Funding and Development Manager)
Tel.: +441612757114
E-mail
Record Number: 191556 / Last updated on: 2016-11-21