Jet Trimming

Abstract: Initial state radiation, multiple interactions, and event pileup can contaminate jets and degrade event reconstruction. Here we introduce a procedure, jet trimming, designed to mitigate these sources of contamination in jets initiated by light partons. This procedure is complimentary to existing methods developed for boosted heavy particles. We find that jet trimming can achieve significant improvements in event reconstruction, especially at high energy/luminosity hadron colliders like the LHC.

• The paper presents a novel jet trimming technique that isolates core jet components by discarding low-transverse momentum subjets, thereby mitigating contamination from ISR, MI, and pileup.
• The method employs an outside-in algorithm which reclusters seed jet constituents and applies a dynamic hard scale to determine which subjets to retain.
• Enhanced mass resolution and improved background rejection in dijet resonance and decay chain analyses demonstrate the method’s potential for precision studies in collider physics.

An Expert Review of "Jet Trimming"

The paper "Jet Trimming," authored by David Krohn, Jesse Thaler, and Lian-Tao Wang, explores an advanced jet reconstruction technique designed to address challenges posed by contamination from initial state radiation (ISR), multiple interactions (MI), and event pileup in high-luminosity hadron collider environments like the LHC. This contamination complicates the ability to accurately reconstruct jets, which are used to infer the kinematics of underlying partonic events. This research introduces jet trimming as a method specifically tailored to improve the reconstruction of jets initiated by light partons, in contrast to techniques developed for boosted heavy particles.

Jet Trimming: Concept and Implementation

Jet trimming builds on prior methods by recognizing the distinct kinematic features of light parton jets. Traditional cleaning procedures for boosted heavy particles involve retaining a fixed number of subjets, each corresponding to a hard parton scattering final state. However, light parton jets typically emit a broad spectrum of radiation without a fixed hard scattering imprint, thus necessitating a different approach.

This study introduces a jet trimming technique that emphasizes the local hardness of calorimeter deposits, leveraging the softer nature of contamination sources like ISR/MI/pileup relative to final state radiation (FSR). The authors propose an "outside-in" algorithm:

1. Identify an initial seed jet using any standard clustering algorithm.
2. Recluster the seed jet constituents into subjets using a smaller radius.
3. Discard subjets that fall below a specified transverse momentum threshold relative to a hard scale of the event.
4. Reassemble the remaining subjets into the trimmed jet.

The choice of the hard scale $\Lambda_{\text{hard}}$ is pivotal, as it affects how subjets are interpreted in the context of different kinematic regimes. The paper explores two determinations: the seed jet's transverse momentum and the event's total transverse momentum.

Results and Analysis

The investigation demonstrates significant improvements in reconstruction using jet trimming, evident across various scenarios. For processes like dijet resonance reconstruction and more complex decay chains, the tailored trimming procedures reveal increased precision in mass resolution over both trimmed algorithms designed for boosted heavy particles and untrimmed alternatives. The benefit of jet trimming is particularly noted when employing a dynamically chosen hard scale $\Lambda_{\text{hard}}$, allowing for contextual adaptation based on the event topology.

The authors present their findings through quantified measures such as the skewed Breit-Wigner distribution fitting, assessing parameters like peak mass $M$ and width $\Gamma$ to evaluate reconstruction fidelity. A notable improvement is observed in trimmed jets' ability to maintain effective rejection of softer contamination sources without increasing sensitivity to background processes.

Implications and Future Directions

This study's contributions to jet trimming offer a substantial advancement in our capability to develop agile experimental methodologies for studying events at hadron colliders. With judicious parameterization and adaptation to an event's kinematic structure, jet trimming can provide enhanced signal clarity while maintaining background suppression. This holds considerable promise for high-precision studies of standard model processes and beyond.

The practical implications extend to significant systematic uncertainties: trimming, by zeroing in on kinematically relevant subjets, might mitigate biases associated with jet pileup and contamination correction factors. Future exploration should explore optimizing $\Lambda_{\text{hard}}$ further, potentially employing machine learning approaches to dynamically adapt trimming parameters in real-time analyses.

Overall, "Jet Trimming" enriches the toolkit available for modern collider physics, pushing towards more robust and efficient jet reconstruction amidst challenging experimental conditions. This work encourages further investigation into sophisticated trimming techniques capable of harnessing the full potential of high-luminosity experiments.