Higgs boson coupling measurement at the LHC

n 2012, the CMS and ATLAS experiments announced the discovery of a Higgs boson. So far, all subsequent Higgs-boson-related measurements agree with predictions of our current theory of particles and their interactions, the Standard Model. Yet we know that this theory is only approximately correct since it does not describe phenomena such as dark matter or gravity. Why have we not seen any deviations in Higgs boson measurements, then? According to the most popular theories alternative to the Standard Model, the current precision of the measurements is not sufficient to observe the tiny predicted deviations. Thus the best bet to discover something truly new is to strongly increase the precision of our measurements of Higgs boson properties and then look for small deviations. Where can we expect to find such deviations first? An intriguing answer lies in Higgs boson decays to two tau leptons. The project Higgs boson coupling measurement at the LHC proposes to measure the strength of the interaction between the Higgs boson and tau leptons with unprecedented precision. A first step towards this goal is the discovery of Higgs bosons decays to tau leptons. This is needed because so far, the Higgs boson has not been discovered in decays to fermions the class of particles tau leptons belong to. The subsequent precise measurement of the related interaction strength is of prime interest because several theories predict that for a Higgs boson, deviations with respect to the Standard Model are largest in its decay to fermions. Higgs boson decays to tau leptons are thus ideally suited to challenge our current understanding of particle physics. Technically, we plan to significantly improve the precision by using techniques borrowed from machine learning. This allows to combine all the information about an event recorded by the CMS experiment in a unique way instead of looking at isolated quantities one-by-one as has been done so far. Ultimately this will allow us to triple the current precision of the measurement of the Higgs-tau interaction strength. With such a precision we will be sensitive to the most plausible alternative theories to the Standard Model of particle physics.

The interaction strength of Higgs bosons with other particles has been measured with unprecedented precision. Such a precise measurement is important because many theories which include new particles and interactions predict small deviations of this strength compared to the standard model prediction. The standard model of particle physics currently is our best description of elementary particles and their interactions. However, we know that it is incomplete: for example, it neither describes dark matter nor gravitation. Extending this model hence is the most important task of modern particle physics. Measurements of the Higgs boson interaction strength may lead to indirect evidence about which extension is realised in nature or at least can show the way by excluding candidates for the standard model extension which are not consistent with experiment and hence allow us to focus on the remaining candidates. To reduce the uncertainties of the measurement, two novel methods were introduced in the course of this project with the aim to improve upon existing methods for measuring Higgs boson decays to two tau leptons: On the one hand, machine learning methods, and on the other hand, a novel method to estimate the background of the measurment, called fake factor method. The background consists of those events in which no Higgs bosons have been produced but which are so similar to signal events with Higgs bosons that they cannot be experimentally distinguished. This method has been tested by applying it to events which are the same as Higgs boson events except for one detail: in those, a Z boson is produced in place of a Higgs boson. This kind of event has already been precisely measured in the past and hence it is ideally suited to compare results of new methods with these readily available results. The fake factor method was also used for direct searches for additional Higgs bosons, together with other improvements. The result of this search improved lower bounds on the mass for certain classes of additional Higgs bosons: if such Higgs bosons exist then they are heavier than the value of the corresponding bounds. Finally the now well-tested new methods were used to measure the strength of the interaction of Higgs bosons with other particles with unprecedented precision. Additionally, for the first time the rate of occurence of very rare Higgs bosons events in decays to tau leptons have been measured; for example the production of Higgs bosons with high energies or in association with other particles. The next step will be the application of these methods to the most recent data of the CERN Large Hadron Collider (LHC).