Search for New Light Particles at LHC

This article discusses a search for light particles that might decay into two Jets, produced with either a high-energy photon or another jet. This search took place at the ATLAS detector, part of the Large Hadron Collider (LHC), where protons are smashed together at very high energy levels, specifically 13 TeV. The focus is on the data collected from 2015 to 2018.

#What was the Search About?

The key objective was to find any unusual signals in the mass distribution of the jets. Scientists think that there could be new types of particles that decay into jets, and these would appear as a peak in the measured data. The search looks at two cases:

1. Photon as Initial-State Radiation: Here, the incoming particle is a photon.
2. Jet as Initial-State Radiation: In this case, the incoming particle is another jet.

In these two cases, they examined both scenarios where there are no specific requirements on the types of jets and where both jets must be identified as containing a certain type of particle called a hadron.

#Findings from the Data

Despite these efforts, no excess or significant deviation was detected beyond the expectations set by existing physics models. As a result, upper limits were established for the likelihood of these new particles being produced. The study effectively extended the limits on light particles decaying into jets, covering a mass range from 200 to 650 TeV.

#Background on Particle Physics

The Standard Model of particle physics is a well-established theory explaining how the known particles interact. However, it does not account for everything observed in the universe, such as Dark Matter. Dark matter is a mysterious substance that does not interact with light, making it invisible and detectable only through its gravitational effects.

In this context, scientists are searching for new types of particles that could potentially link to dark matter. One focus is on particles called Bosons, which might serve as intermediaries in these interactions.

#Importance of Searching for New Particles

New particles may help explain the forces and interactions not covered by the Standard Model. Current searches at the LHC have already produced significant constraints on certain models. However, some hypothetical particles might not interact with all known particles, making them harder to detect.

#Strategies Employed in the Search

Two main strategies were applied in this search:

1. Recording Minimal Information: This involves capturing more data than usual at a lower threshold for events to explore lower mass ranges.
2. High Initial-State Radiation (ISR): Here, the focus is on events where a photon or jet recoils against the jets created by the particle decay. This technique helps access lower masses without triggering biases from the usual selection criteria.

The analysis involved different channels based on the initial-state radiation and the flavor of the decay products.

#Roles of Jets and Photons

Jets are streams of particles produced after protons collide. The search for new particles therefore revolves around analyzing how these jets behave. Identifying the right jets is crucial, especially in cases where the decay products are tagged as containing certain particles to improve signal detection.

#Photon Channels

In the photon channels, events are required to have a photon triggered with certain parameters. The search focuses on the two leading jets, their asymmetry, and how they behave compared to background events.

#Trijet Channels

In the trijet channels, events must contain at least three jets. The challenge is to identify which jets correspond to the decay of the hypothetical particle.

#Detector Used: ATLAS

The ATLAS detector is a complex apparatus designed to capture a wide range of particle interactions. It has several layers of different detectors:

• Tracking Detector: This helps trace the paths of charged particles.
• Calorimeters: These measure the energy of the particles.
• Muon Spectrometer: This identifies muons, which are heavy relatives of electrons.

Together, these components help scientists gather data about high-energy collisions effectively.

#Data Collection and Simulation

The analysis utilized both real data and simulated samples to improve accuracy. They relied heavily on data from proton collisions, with a total luminosity collected during the designated period.

Monte Carlo simulations were used to model both signal and background events. These simulations provided an estimate of what a signal might look like, allowing researchers to compare real data against expected outcomes.

#Event Selection Criteria

Event selection included several requirements to ensure the data were relevant for the search:

• Primary Vertex Requirement: Events must have a primary vertex with specific characteristics.
• Jet Reconstruction: Jets are reconstructed using a specific algorithm integrating various measurements to ensure accuracy.
• Photon Reconstruction: Photons must meet strict energy and isolation criteria to reduce background noise.

#Analyzing the Results

Once events were selected and reconstructed, scientists fit the data to models to extract meaningful information. They looked for localized excesses indicating potential signals from new particles.

A prominent method used is called likelihood fitting, which combines estimates of background and signal to extract information about potential new particles.

#Systematic Uncertainties

Several uncertainties could impact results, including:

• Luminosity Measurements: Any errors here could affect the normalization of results.
• Jet Energy Scale: Variations in the energy scale of jets could alter the mass calculations.
• Photon Identification: Errors in how photons are identified can lead to misinterpretations.

These uncertainties were systematically evaluated and included in the models to ensure reliability.

#Conclusion

The search for light resonances that decay into jets in association with photons or jets did not reveal any significant new particle evidence. Despite this, the study provided a clearer picture of the limits on such particles, helping shape future research directions in particle physics. The work also emphasized the need for continuing investigations into models that might explain unknown phenomena in the universe.

Further experiments at the LHC and beyond may eventually lead to discoveries that challenge or expand our current understanding of particle physics. The search for new particles remains a vital frontier in science, holding the potential to unlock many of the universe's mysteries.