Searching for High-Mass Particles with ATLAS

The ATLAS detector at the Large Hadron Collider (LHC) is used to look for new particles that might exist beyond what we already know in physics. This study focuses on high-mass particles that can decay into a certain type of particle, a lepton, and a neutrino-an invisible partner that carries away energy but does not interact with matter in the same way as normal particles.

#Proton-Proton Collisions

Researchers use proton-proton collisions to find these high-mass particles. In this case, the collisions are happening at a very high energy level of several TeV, where TeV stands for tera-electron volts, a unit of energy. By smashing protons together at such high energies, we can create conditions similar to those just after the Big Bang, allowing us to investigate phenomena that are not seen in our everyday lives.

#Analysis of the Data

The analysis is based on data collected by the ATLAS experiment between 2015 and 2018. This data set is substantial, with a total of 140 fb (femto-barns) of recorded data. To look for the particles, scientists focus on the lepton produced from the decay event and the missing energy that can be attributed to the Neutrinos. By studying how the lepton and the missing energy balance each other out, they can infer details about the possible new particles.

#Finding New Physics

The primary goal of this work is to identify any signs of particles that might suggest new physics, beyond the Standard Model of particle physics, which has explained many particle interactions so far. No unexpected increases in events above the expected levels were found, leading researchers to set limits on how often these new particles can be produced.

#Setting Limits on Particle Production

By analyzing the data, researchers can establish upper limits on how frequently these high-mass particles can be created in collisions. They found that heavy particles, like vector bosons, could not exist at certain masses. Specifically, they found that particles with masses up to 5 TeV could not be produced at the same rate as predicted by the Standard Model, reinforcing our current understanding of particle physics.

#Non-Universal Couplings

The study also investigates theoretical models that suggest different kinds of interactions between particles. This is referred to as non-universal gauge interactions. Some models suggest that the properties of these new particles could be different based on the family of particles they interact with. The work done here also excluded certain models that do not align with the observed data.

#The ATLAS Detector Explained

The ATLAS detector is a complex machine designed to capture data from the particle collisions. It is built in a cylindrical shape and can detect a wide range of particles due to its multi-layered structure. The inner part has tracking devices that measure where particles go after collisions, while various calorimeters measure the energy of these particles.

#Reconstructing Events

To understand the collisions, scientists need to reconstruct the events accurately. This involves identifying the particles created in the collisions and measuring their properties, such as their momentum and energy. Hadronic decays of Leptons are one of the primary focuses, making it critical to track the behavior of particles that emanate from the main collision point.

#Identifying Particle Types

Identifying types of particles is essential. Different types of leptons decay in different ways, and scientists use specific algorithms and machine learning techniques to identify these particles accurately. By analyzing the shape of energy deposits and the patterns in the tracking data, researchers can categorize and identify the particle types involved.

#Background Events Consideration

In any particle search, distinguishing between actual signals from potential new particles and background events from known processes is crucial. For this study, the background events arise from processes like jets-groups of particles produced during collisions-and other particles that can mimic the signal we are looking for.

#Data-Driven Background Estimates

To estimate the background in the data accurately, researchers used a data-driven method. Different regions of the data that help understand the backgrounds were analyzed. Control regions were established, whereby specific selection criteria were applied to identify events that should not contribute to the signal.

#Triggering Events

When data is collected during an experiment, triggers are used to select which events are recorded. Various thresholds based on the expected energies and types of particles are set to reduce the number of uninteresting events, ensuring that the most relevant data is collected for analysis.

#Systematic Uncertainties

In any measurement, uncertainties are inevitable. This study outlines various sources of systematic uncertainties, which can arise from experimental conditions, detector responses, and the theoretical models used for predicting particle behaviors. It's essential to account for these uncertainties to ensure that results are robust.

#Statistical Analysis

A statistical analysis is crucial when interpreting the data collected. In this study, a profile likelihood fit was employed to compare how well the observed data fits the expected background versus the signal. This method allows researchers to systematically examine the contributions of various factors and refine their conclusions about the existence of new particles.

#Conclusion

The search for new high-mass particles using the ATLAS detector has not shown excesses beyond the established Standard Model expectations. Through the detailed analysis of data from proton collisions, researchers have set limits on the production of heavy gauge bosons and explored theories of non-universal gauge interactions.

#Future Studies

Future studies will aim to collect more data and refine analysis techniques, which could improve the sensitivity of searches for new particles. This work highlights the importance of keeping the search for knowledge open, as the universe often reveals unexpected phenomena.