My current research involves precision measurements of Higgs boson production and weak boson fusion, using proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider (LHC). I have a long-standing interest in Quantum Chromodynamics and help with the ATLAS calorimeter jet energy scale calibration. I also occasionally work on particle physics phenomenology to help bridge the gap between experimentalists and theorists.
In July 2012, the ATLAS and CMS Collaborations announced the observation of a particle in the search for the Standard Model Higgs boson. Since then, the properties of this new particle have been studied in detail by both collaborations, to see whether this new particle is consistent with the predictions of the Standard Model. Deviations from the Standard Model predictions would indicate that theory is incomplete, and the Higgs boson offers an excellent opportunity to search for new physics beyond the Standard Model.
My team are heavily involved in the effort to probe the properties of this new particle. In 2014, we published the first model-independent measurements of the Higgs boson cross section using the diphoton decay channel. An example of the results is shown in the figure on the right, where the cross section is presented for seven different fiducial regions defined by different cuts on the photon, lepton and jet kinematics. As the results are model independent, they can be compared to a variety of theoretical models. In addition, we published around twenty differential cross sections, probing the kinematics of the Higgs boson in detail.
My team also lead an effort to use these model-independent differential cross section measurements to search for new types of Higgs boson interactions. Of particular interest are interactions that violate charge conjugation and parity, as these would provide a mechanism to explain the matter-antimatter asymmetry in the universe. In 2016, we showed that the differential cross section measurements could be used to search for these interactions. The figure on the left shows the limits placed on anomalous CP-even (x-axis) and CP-odd (y-axis) interactions between the Higgs boson and gluons. Similar constraints were place on anomalous CP-even and CP-odd interactions between the Higgs boson and weak bosons.
The statistical precision of all these measurements will improve by a factor of at least eight using data collected by ATLAS in the LHC Run-II (2015-2018). Our first (preliminary) results were presented at the ICHEP conference in 2016 and can be found here.
Weak boson fusion
Weak-boson fusion is an important process that can be used to study electroweak phenomena. Measurements of Higgs boson production via weak boson fusion are important to test the nature of the Higgs boson interaction with weak bosons and allow new searches for CP-violation in the Higgs sector. A related process, weak boson scattering, can be used to search for anomalous weak boson interactions. These processes are extremely rare and difficult to extract from the large backgrounds in the harsh experimental conditions at the LHC.
Weak boson fusion processes have the characteristic signature of two low-angle "tagging" jets, one on each side of the ATLAS detector, with no additional jet activity in the region between the two tagging jets. We exploited these characteristics to measure Z-boson production via weak boson fusion. The figure on the right shows the excess of data with respect to the background-only model in the tail of the invariant mass distribution of the two tagging jets. This was the first time a weak boson fusion process had been observed at a hadron collider.
In the LHC Run-II, we expect a huge increase in event yields in the region sensitive to weak boson fusion (the tail of the dijet invariant mass spectrum), due to the increase in beam energy. This should open the door to many more measurements of weak boson fusion processes and my team will be heavily involved.