Higgs Boson and Top Quark Interactions Under Investigation

In high-energy physics, scientists study the fundamental particles that make up our universe. One of the key players in this field is the Higgs boson, discovered at the Large Hadron Collider (LHC) in 2012. This particle is important because it gives mass to other particles. In this research, we look at a special kind of interaction known as flavour-changing neutral currents (FCNC) involving the top quark and the Higgs boson, particularly in events that result in multiple leptons, which are particles like electrons and muons.

#What are Top Quarks and Higgs Bosons?

Top quarks are one of the six types (or "flavours") of quarks, and they are known to be the heaviest. They play a crucial role in many interactions in particle physics. The Higgs boson, on the other hand, is often referred to as the "God particle." It is a fundamental component of the Standard Model, which is the theory describing how fundamental particles interact.

#FCNC Interactions

Flavour-changing neutral current interactions involve particles changing their type without changing their electric charge. In the context of this research, we are looking for events where the top quark can interact with the Higgs boson and another up-type quark, resulting in interesting decay patterns.

These types of interactions are rare in the Standard Model and are typically suppressed by a mechanism known as the Glashow-Iliopoulos-Maiani (GIM) mechanism. Because they are so rare, finding evidence of such events could indicate new physics beyond what we currently understand.

#The ATLAS Detector

The ATLAS detector is one of the main instruments used at the LHC to observe collisions between protons at an energy of 13 TeV. It is designed to detect a wide range of particles and measure their properties. The detector includes components that track charged particles, measure energy from electromagnetic and hadronic particles, and identify different particle types.

#Collision Data Analysis

The data analyzed in this research comes from proton-proton collisions recorded between 2015 and 2018. The total amount of data corresponds to an integrated luminosity of 140.1 fb^-1, which measures how much collision data has been collected over time. By examining particular events with multi-lepton final states, researchers can search for FCNC processes.

#Final States of Interest

In this search, we focused on final states that contain either two same-charge leptons (like two electrons or two muons) or three leptons in total. These conditions help narrow down the events of interest and make the analysis more manageable.

#Event Selection

Selecting the right events is crucial for ensuring that the analysis is focused on the processes we want to study. Events must have at least one same-charge lepton or three leptons, as well as one -tagged jet. A -tagged jet indicates that it originates from a bottom quark, which is a key component in the decay processes under investigation.

#Background Processes

In any experiment, there are many background processes that can mimic the signals we are looking for. These include various Standard Model processes that produce similar final states. It is essential to correctly estimate and account for these backgrounds to extract meaningful results.

#Data Reconstruction

To analyze the data, various computational techniques are used to reconstruct the events. This involves identifying the particles produced in the collision and measuring their properties, such as energy and momentum. The relationships between these particles are crucial for distinguishing between signal and background events.

#Neural Networks for Event Classification

To improve the separation of signal from background events, neural networks are employed. These are machine learning algorithms that can learn complex patterns in the data and are trained to classify events based on their features. This helps pinpoint the rare FCNC processes in a sea of other interactions.

#Results of the Analysis

After conducting the analysis and employing various techniques to suppress background noise, no significant excess beyond expectations from the Standard Model was observed. This leads to setting upper limits on the branching ratios of the FCNC processes under study.

#Implications of the Results

The results suggest that the interactions we studied are even rarer than anticipated. This has implications for theories beyond the Standard Model, as it indicates that any new physics related to FCNC interactions with the Higgs boson might be well hidden or suppressed within our current understanding.

#Conclusion

In summary, this research represents a detailed exploration of flavour-changing neutral currents involving the top quark and the Higgs boson using data collected from the ATLAS detector. While the search did not reveal new evidence for these interactions, it did establish stringent limits on their occurrence. Ongoing analyses and future data collection will continue to refine our understanding of the fundamental particles and forces at play in the universe.