Final Report Summary - HIGGSWBF (A path to understanding: precision studies of the Higgs boson through Weak Boson Fusion)
The project objectives, work towards them, and main results achieved are summarised below. The initial aim was to measure associated W+jets production in high-energy hadronic collisions in a kinematic region sensitive to the presence of the class of particle-interactions known as weak boson fusion processes. The aim of these studies were to study the theoretical modelling of the kinematics of the produced particles and investigate the differences in jet emission dynamics between strongly-induced and electroweak-induced W+jets production. In doing so, improvements and corrections to theoretical models and their implementation in Monte Carlo generator programs would need to be developed. Darren Price published the first studies of hadronic jet activity in the rapidity interval between two jets produced in association with a vector boson in Phys.Rev. D88 (2013) 092001. The jet activity in this interval is a key feature used to discriminate strongly-mediated (QCD) W+2 jet production from electroweakly-mediated (EWK) W+2 jet production, which has not yet been measured. This latter class of process holds the potential to be sensitive to a wide-range of new physics phenomena. These initial studies found that the best theoretical predictions currently available were not able to simultaneously describe both the jet and W boson kinematics and the jet emission dynamics observed in the data. The conclusion of this study was that data-driven approaches would be needed to isolate QCD production from EWK production.
Work then began towards a first observation and measurement of the electroweak production of two jets in association with a W boson using the ATLAS experiment at the LHC. Methods used to model QCD-initiated production for study of Z+2jet data were refined and further developed for use in the W+2jet analysis. Two quantities, denoted “jet centrality” and “lepton centrality”, related to the location of any additional jets, and the decay products of the W boson respectively, relative to the two highest transverse momentum jets produced, were defined and used to define orthogonal control regions where background processes that mimic electroweak W+2jets production could be constrained and measured directly from the data without reliance on theoretical inputs. The dominant systematic uncertainties on the extraction and measurement of the electroweak component were assessed, and data-driven techniques using the centrality observables were developed to provide in-situ constraints on experimental uncertainties which improve both the significance of the observed electroweak signals and sensitivity to new physics contributions. These developments allowed for (1) the extraction of a signal strength for the electroweak signal relative to the best theoretical predictions currently available, (2) the detailed precision measurements of the sum of QCD and electroweak W+2j production in a range of observables in weak boson fusion (WBF) enhanced and WBF-suppressed kinematic phase spaces, (3) the first observation of W boson WBF electroweak production, (4) first differential cross-section measurements of electroweakly-produced dijet plus W boson production, (5) the first anomalous triple gauge coupling measurements from W WBF production and the most stringent constraints to-date on certain classes of new physics models. These techniques and the results are described in the ATLAS note ATL-COM-PHYS-2014-413 and in the ATLAS publication (in internal review, to be published shortly): “Measurement of the electroweak production of dijets in association with a W boson and constraints on anomalous triple gauge couplings using the ATLAS detector” [ATLAS-STDM-2014-11]. In the process of developing these measurements, Darren Price realised that these techniques could be generalised to the measurement of any colour-neutral final state (replacing the W boson) produced in association with two jets in weak boson fusion topologies. These colour-neutral electroweakly-interacting final states include, but are not limited to, a search for a doubly-charged Higgs indicative of an extended Higgs sector, Majorana neutrinos, weakly-interacting dark matter, and lepton flavour violating phenomena. In the final months of this project, Darren Price has refocused efforts on developing these novel search strategies building on earlier research in the project, as these new avenues of research offer increased potential for discovery of new physics than the initially suggested Standard Model Higgs boson extensions of the project, that are also more precise than any current approaches to searching for these same phenomena.. Darren Price has developed links with theorists from other research centres within the EU and in the US to produce phenomenology papers that describe the feasibility and discovery potential for using the techniques developed in this project for searching for new phenomena in weak boson fusion production topologies, and how the tensor structure of the production vertex can provide key information on the charge-parity properties and particle-content of the produced new particles (such as dark matter candidates, or doubly-charged Higgs bosons). The findings will be published in two papers to be published at the end of this year. On the experimental side, Darren Price has developed a multi-institute analysis team to begin novel searches for these new phenomena using the ATLAS detector, building on the analysis techniques developed for the search for and measurement of W boson weak boson fusion.
Work then began towards a first observation and measurement of the electroweak production of two jets in association with a W boson using the ATLAS experiment at the LHC. Methods used to model QCD-initiated production for study of Z+2jet data were refined and further developed for use in the W+2jet analysis. Two quantities, denoted “jet centrality” and “lepton centrality”, related to the location of any additional jets, and the decay products of the W boson respectively, relative to the two highest transverse momentum jets produced, were defined and used to define orthogonal control regions where background processes that mimic electroweak W+2jets production could be constrained and measured directly from the data without reliance on theoretical inputs. The dominant systematic uncertainties on the extraction and measurement of the electroweak component were assessed, and data-driven techniques using the centrality observables were developed to provide in-situ constraints on experimental uncertainties which improve both the significance of the observed electroweak signals and sensitivity to new physics contributions. These developments allowed for (1) the extraction of a signal strength for the electroweak signal relative to the best theoretical predictions currently available, (2) the detailed precision measurements of the sum of QCD and electroweak W+2j production in a range of observables in weak boson fusion (WBF) enhanced and WBF-suppressed kinematic phase spaces, (3) the first observation of W boson WBF electroweak production, (4) first differential cross-section measurements of electroweakly-produced dijet plus W boson production, (5) the first anomalous triple gauge coupling measurements from W WBF production and the most stringent constraints to-date on certain classes of new physics models. These techniques and the results are described in the ATLAS note ATL-COM-PHYS-2014-413 and in the ATLAS publication (in internal review, to be published shortly): “Measurement of the electroweak production of dijets in association with a W boson and constraints on anomalous triple gauge couplings using the ATLAS detector” [ATLAS-STDM-2014-11]. In the process of developing these measurements, Darren Price realised that these techniques could be generalised to the measurement of any colour-neutral final state (replacing the W boson) produced in association with two jets in weak boson fusion topologies. These colour-neutral electroweakly-interacting final states include, but are not limited to, a search for a doubly-charged Higgs indicative of an extended Higgs sector, Majorana neutrinos, weakly-interacting dark matter, and lepton flavour violating phenomena. In the final months of this project, Darren Price has refocused efforts on developing these novel search strategies building on earlier research in the project, as these new avenues of research offer increased potential for discovery of new physics than the initially suggested Standard Model Higgs boson extensions of the project, that are also more precise than any current approaches to searching for these same phenomena.. Darren Price has developed links with theorists from other research centres within the EU and in the US to produce phenomenology papers that describe the feasibility and discovery potential for using the techniques developed in this project for searching for new phenomena in weak boson fusion production topologies, and how the tensor structure of the production vertex can provide key information on the charge-parity properties and particle-content of the produced new particles (such as dark matter candidates, or doubly-charged Higgs bosons). The findings will be published in two papers to be published at the end of this year. On the experimental side, Darren Price has developed a multi-institute analysis team to begin novel searches for these new phenomena using the ATLAS detector, building on the analysis techniques developed for the search for and measurement of W boson weak boson fusion.