Sie sind hier: Startseite Research ATLAS Experiment Higgs to tautau Couplings and Differential Cross-section Measurements

Higgs to tautau Couplings and Differential Cross-section Measurements

Description & Group Activities


With the data from the first run of the LHC, both the ATLAS and CMS experiments observed a Higgs boson with a mass of approx. 125 GeV. A crucial question to answer is whether this Higgs boson behaves as predicted by the Standard Model (SM) -- any deviation would constitute signs of new physics.

In our group we are in particular focusing on the decay of the Higgs boson to two tau leptons. Since the Higgs boson couples to mass, the tau leptons are the leptons expected to have the largest coupling. Using the dataset collected in 2011 and 2012, ATLAS saw evidence, at a significance of 4.5 sigma, of decays of the Higgs boson to two tau leptons - an analysis many members of our group played a decisive role in.

This measurement allowed the Htautau couplings to be measured to an uncertainty of about 20%. Part of the experimental program of our group is to utilise the data from the ongoing, second run of the LHC, using the ATLAS experiment, to:

  1. More precisely measure the Htautau couplings 
  2. Perform a measurement of the differential cross-section in this channel.

Both objectives will test the data for deviations from the SM. In the first case these could manifest in a different coupling value, while in the second case they could manifest in differences in the differential distributions of crucial observables, such as the Higgs-boson transverse momentum.

The tau leptons decay inside of our detector, either to a lighter lepton (electron, muon) and two neutrinos, or to one neutrino and charged and neutral hadrons. For the decays of Higgs bosons to two tau leptons, this means that 42% of the time both taus decay hadronically, 46% of the time one of them decays hadronically and the other leptonically, and finally 12% of the time both decay leptonically. Our group focuses it's efforts on the second and third decay channels.

One of the most important aspects of this kind of analysis is the estimation of background processes, and the most important such background process in the search for H→tautau are Z→tautau decays, since they have the same final-state particles. In order to estimate the contributions of this background, the so-called "embedding" technique is used. It consists of selecting Z→mumu events from data, removing the muons and simulating tau decays with the same kinematics as the removed muons. Then the simulated taus are combined with the rest of the original Z→mumu event. A set of such hybrid events are then used to estimate the Z→tautau background, since all properties of the hybrid events except the well-known tau-decay are modelled directly with data. Our group is responsible for the Embedding procedure inside ATLAS.

Possible thesis topics relating to this, involve the study, development and optimisation of the following analysis components:

  • Multivariate event selection
  • Embedding method
  • Experimental reconstruction of the Higgs-boson mass
  • Comparison of methods for estimating backgrounds from multijet production
  • Identification of hadronic tau decays
  • Determination of sensitivity for future studies
Benutzerspezifische Werkzeuge