Searching for Long-Lived Particles at ATLAS

The study of Long-Lived Particles (LLPs) has become an important area in particle physics. These particles can live much longer than typical particles before decaying. The search for LLPs is crucial because they can help scientists better understand the fundamental laws of nature, potentially revealing new physics beyond what we currently know.

The ATLAS Detector at CERN's Large Hadron Collider (LHC) is one of the primary tools used to search for these LLPs. In this research, we focus on neutral LLPs that decay into jets of particles, which can be detected as displaced jets in the ATLAS calorimeter. This study uses data from proton-proton collisions at high energies.

#What Are Long-Lived Particles?

Long-lived particles are predicted by various theories in particle physics. They can appear in models that aim to address some of the biggest questions in science, such as the nature of dark matter or the imbalance between matter and antimatter in the universe.

Because of their unique properties, LLPs may have different decay patterns compared to other particles. Some models predict that LLPs can be produced in particle collisions and can decay into common particles like quarks or gluons. Detecting these jets from LLPs can provide insights into their properties and the underlying physics.

#The ATLAS Detector

The ATLAS detector is designed to observe and measure the products of high-energy particle collisions. It has many components, including a tracking system, calorimeters, and a muon detector. The tracking system helps trace the paths of charged particles, while calorimeters measure the energy of both electromagnetic and hadronic particles.

The calorimeters have distinct sections that specialize in measuring different types of particles. For this research, the focus is on hadronic calorimeters, which detect jets resulting from the decay of particles into hadrons.

#Data Collection

The data for this analysis were collected between 2015 and 2018. The LHC produced 140 fb⁻¹ of collisions, a measure of the total data collected. This data was used to search for LLPs under various conditions and potential models.

During data collection, certain quality checks were in place to ensure that stable beams were being used, and all parts of the detector were functioning correctly. Different triggers were used to capture the events that may lead to LLP detection.

#Analysis Channels

This study examines three different channels to search for LLPs:

1. CalRatio + Two Jets (CalR+2J): This channel looks for pairs of LLPs produced in collisions, one decaying into a single displaced jet and the other producing two resolved jets.

2. CalRatio + W Boson (CalR+W): In this channel, the LLPs are produced alongside a W boson, which then decays into leptons.

3. CalRatio + Z Boson (CalR+Z): Similar to the previous channel, but here, the LLPs are produced with a Z boson that decays into a pair of leptons.

Each of these channels has specific characteristics that guide the selection criteria for the events being studied.

#Identifying Displaced Jets

Displaced jets originating from LLP decays are identified through specific techniques. When an LLP decays, its products may not travel directly to the detector, leading to jets that have a delayed arrival time.

One tool used in identifying these jets is the "CalRatio," which measures the energy deposited in the hadronic versus electromagnetic calorimeters. Jets from LLP decays typically show lower energy in the electromagnetic section due to their nature.

#Background Estimation

In particle physics experiments, it's crucial to distinguish between signal events (which suggest the presence of LLPs) and background events (which arise from standard model processes).

The analysis uses a data-driven method for background estimation. This involves categorizing events based on their features and using statistical methods to predict how many background events are expected.

#Systematic Uncertainties

While measuring and estimating backgrounds, uncertainties are taken into account. These can arise from various factors, including how well the detector responds to particles, the accuracy of simulations used, and variations in event conditions.

The effects of these uncertainties are minimized through careful calibration and validation steps during data analysis.

#Results

The analysis results show that no significant excess of events was observed in any of the channels studied. This suggests that LLPs, if they exist within the range considered, do not appear as frequently as some theories might predict.

Constraints were placed on the production cross-sections of various LLP models. For example, branching fractions of certain models were found to be below 1%, indicating that these models are less likely given the collected data.

#Conclusion

This research sheds light on the search for long-lived particles through the use of advanced particle detectors and data analysis techniques. The findings help refine our understanding of potential theories in particle physics and set new limits on their properties.

As investigations continue, new methods and datasets can further enhance the search for LLPs, offering more possibilities for groundbreaking discoveries in the realm of particle physics. Understanding these elusive particles could ultimately lead to significant insights into the universe's fundamental structure.