Automated Luminosity Optimization for Future Electron-Positron Colliders

Project Overview

In this project, originally conceived as part of Stanford’s AA 222/CS 361 - Engineering Design Optimization and awarded the best final project award for the 2023-2024 version of the course, we developed a novel optimization framework for maximizing the collision rate, or “luminosity”, at future electron-positron colliders while maintaining controlled background levels.

Technical Approach

In this study, we developed a new luminosity optimization strategy based on the following:

Surrogate Modeling

• Developed a Gaussian Process surrogate model to approximate the beam-beam enhancement factor $H_{D}$, which is a non-analytic contribution to the luminosity.
• Trained the on PIC (Particle-In-Cell) simulation points with careful hyperparameter optimization

Optimization Framework

• Constructed a constrained optimization problem over six beam parameters:
  • Bunch population
  • Horizontal/vertical emittances
  • Horizontal/vertical beta functions
  • Bunch length
• Implemented both single-objective constrained optimization and multi-objective approaches
• Applied bounds based on technical feasibility and background constraints

Key Results

The methodology demonstrated significant improvements across multiple proposed collider designs:

• Achieved luminosity gains of 22-35% for most collider scenarios while maintaing or reducing beam-induced background levels
• Identified optimal operating points within technically feasible parameter ranges
• Discovered a luminosity saturation effect at specific background levels

Impact

The optimization framework provides:

• Automated tool for collider parameter optimization
• Potential for increased physics reach through higher collision rates
• Framework for evaluating design trade-offs between luminosity and backgrounds across different collider proposals (e.g. CLIC, ILC, C$^3$)

Future Developments

We are still working on improving the analysis, focusing on:

• Further validation against full detector simulations
• Integration with detailed beam dynamics studies
• Extension to additional collider scenarios and constraints
• Refinement of the surrogate model with additional training data

Beam-Induced Background Studies for Future Electron-Positron Colliders

Project Overview

This ongoing study aims to evaluate the impact of beam-induced backgrounds on detector performance for future electron-positron colliders, with particular focus on validating the Silicon Detector (SiD) concept’s compatibility with the Cool Copper Collider (C³) beam parameters.

Technical Approach

In this analysis, we implement the following simulation chain:

Background Generation

• Simulation of electron-positron pair production using the GUINEA-PIG PIC simulator
• Modeling of photoproduced hadrons from beam-beam interactions with a dedicated generator
• Full characterization of background particle distributions and kinematics

Detector Simulation

• Detailed modeling of the SiD detector geometry and response using the Key4HEP software stack
• Tracking the simulated background particles trough the SiD detector and registering energy deposits (or “hits”) in the various detector subsystems.

Performance Evaluation

• Quantification of hit densities and occupancy levels in:
  • Vertex detector
  • Silicon tracker
  • Electromagnetic and hadronic calorimeters
• Assessment of timing distributions of background hits
• Analysis of background energy deposition patterns

Key Objectives

The study aims to:

• Validate the compatibility of the SiD detector, originally developed for the International Linear Collider (ILC), with the beam parameters envisaged for C³.
• Evaluate timing requirements for background rejection
• Assess impact on trigger and readout systems
• Guide detector design optimizations

Expected Outcomes

With this work nearing completion, we eventually aim to provide:

• Quantitative assessment of background levels throughout SiD
• Validation of detector technologies for C³ conditions
• Guidelines for detector timing and readout specifications
• Input for future detector optimization studies

The results will be crucial for demonstrating the feasibility of the SiD concept for future electron-positron colliders and guiding potential design modifications.

Environmental Impact Analysis of Future Particle Colliders

Research Context

In this study we evaluated the environmental impact of next-generation particle physics facilities, focusing on the Cool Copper Collider (C³) proposal, addressing a critical challenge in modern physics: balancing scientific advancement with environmental responsibility.

Key Findings and Impact

The study demonstrated several significant advantages of the C³ design:

• Compact 8-kilometer footprint significantly reduces embodied carbon
• Cut-and-cover construction method offers substantial environmental benefits compared to deep tunneling

We also developed a new metric for comparing different collider proposals based on:

• Physics research potential
• Energy consumption requirements
• Carbon footprint from construction
• Operational environmental impact

Future Implications

This work contributes to the broader discussion of sustainability in big science projects and helps establish environmental impact as a key metric in future accelerator design decisions.

Our research demonstrates how modern accelerator design can balance scientific ambition with environmental responsibility. The framework developed in this study provides a quantitative basis for evaluating and comparing different collider proposals from a sustainability perspective, while maintaining focus on their primary physics goals.

Searching for New Physics Through Paired Dijet Resonances at the LHC

Project Overview

As an undergraduate student, I contributed to a search for ultra-heavy new particles with the full Run 2 data of the CMS experiment, representing 138 fb⁻¹ of proton-proton collisions at 13 TeV. In that analysis, we looked for new massive particles beyond the Standard Model, in the form of TeV-scale resonances $Y$ decaying to two identical heavy particles $X$, with each one of them decaying hadronically to two jets - collimated sprays of particles that are the hallmark signatures of quarks and gluons. This creates a distinctive four-jet signature: $pp \rightarrow Y \rightarrow XX \rightarrow (jj)(jj)$. A few candidate events were observed in the data, with a maximum local (global) significance of 3.9(1.6)$\sigma$.

Technical Innovations

The analysis employed sophisticated techniques to:

• Distinguish signal from the overwhelming QCD background
• Handle complex four-jet final states
• Develop model-independent search strategies
• Set limits on both resonant and non-resonant production scenarios in low-statistics regimes, where asymptotic appproximations are no longer valid

Results

This search achieved several significant milestones:

• First LHC limits on resonant pair production of dijet resonances via high-mass intermediate states
• Extended previous limits on supersymmetric particle scenarios
• Excluded diquark masses below 7.6 TeV in specific model scenarios
• Placed new constraints on top squark pair production in R-parity-violating supersymmetry

The analysis also revealed some tantalizing potential hints of new physics:

• Two remarkable events were observed with four-jet masses of 8 TeV and average dijet masses of 2 TeV
• These events showed a local significance of 3.9 standard deviations (reduced to 1.6σ when accounting for the look-elsewhere effect)
• Another interesting excess was observed at an average dijet mass of 0.95 TeV, with a local significance of 3.6σ (2.5σ global)

Evaluating Detector Performance for Future Electron-Positron Colliders

Research Context

This ongoing study investigates detector capabilities for future electron-positron colliders, focusing on identifying collimated sprays of particles (or “jets”) originating from strange quarks (“strange-tagging”). This capability is crucial for measuring how strongly the Higgs boson interacts with strange quarks, one of the fundamental particles of the second generation of fermions in the Standard Model.

Technical Challenge

A key aspect of particle identification (PID) at future colliders is the ability to distinguish between different types of particles (kaons, pions, and protons) carrying momenta up to tens of GeV. This study compares different detector concepts in order to evaluate the effect of PID on strange-tagging.

Methodology

In order to evaluate jet-tagging capabilities for different detector models, we employ the following methodology:

• Generate typical particle physics events with jets of different flavors and perform fast-detector simulation using Delphes to model the interaction of the jet constituent particles with various detector configurations
• Train ParticleNet, a Graph Neural Network (GNN) architecture that uses low-level information of the jet constituents, for multiclass classification of the jets based on the flavor of the initiating quark.

Preliminary Results

Our initial findings :

• verify the well-known fact that PID capabilities significantly improve strange-tagging performance
• indicate that enhanced calorimeter resolution shows particular benefits for distinguishing strange quarks from charm and bottom quarks
• quantify the differences in mistag rates across different detector configurations

Future Work

Our ongoing priorities in this project include:

1. Refining our estimates of improved calorimeter resolution and timing capabilities
2. Systematically evaluating different subdetector systems’ contributions
3. Validating fast-simulation results against more accurate, full-detector simulation

This work contributes to the broader study program for future colliders and aims to inform detector design decisions for future electron-positron colliders.

Studying High-Energy Higgs Bosons in Association with Vector Bosons

Analysis Overview

During my first research rotations at Stanford, and working with the SLAC ATLAS group, I contributed to the first measurement attempt of highly energetic (“boosted”) Higgs bosons produced alongside $W$ or $Z$ bosons (collectively termed as vector bosons) with the ATLAS experiment at the Large Hadron Collider, focusing on events where all particles in the final state manifest as collimated sprays of hadrons, known as jets.

Technical Approach

The analysis employs several advanced experimental techniques to identify and measure these rare processes:

Particle Reconstruction

• The decay products of both the Higgs and vector bosons are captured within single large-radius jets due to their high momentum
• Sophisticated jet substructure techniques analyze the internal patterns of these jets to distinguish between signal and background processes
• Specialized algorithms identify jets containing b-quarks from Higgs boson decays

Background Estimation

• The dominant background comes from generic quantum chromodynamics (QCD) multijet production
• These background processes are measured directly from collision data rather than relying on simulation
• A likelihood fit to the jet mass distribution is used to separate signal from background

Measurement Strategy

The analysis measures the production rate:

• Inclusively across all kinematic regions
• Differentially in three ranges of Higgs boson transverse momentum:
  • 250-450 GeV
  • 450-650 GeV
  • Above 650 GeV

Results and Future Work

The measurement yields a signal strength relative to the Standard Model prediction of $\mu = 1.4+1.0-0.9 $, which translates to a production cross-section of $3.1 \pm 1.3(\mathrm{stat})^{+1.8}_{-1.4}(\mathrm{syst}) \ \mathrm{pb}$.

This measurement opens new possibilities for studying the Higgs boson in previously unexplored kinematic regimes and provides crucial insights into the Higgs mechanism at high energies, complementing existing measurements in other decay channels.

Although the sensitivity of this analysis was not sufficient to measure Higgs production in this phase-space with adequate statistical significance, future improvements in the analysis techniques, as well as additional data collected during Run 3 of the LHC, will enable a more precise determination of the Higgs boson properties at high energies.

Search for Boosted Higgs Bosons in Associated Production with Vector Bosons

Project Overview

This analysis builds upon our previous ATLAS study of highly energetic (“boosted”) Higgs bosons produced alongside $W$ or $Z$ bosons, incorporating significant improvements in both dataset size and analysis techniques. The study focuses on events where all final state particles manifest as collimated sprays of particles called “jets”, offering unique sensitivity to new physics effects at high energies.

Technical Advancements

The analysis features several major improvements:

Enhanced Dataset

• Combination of full Run 2 data with partial Run 3 statistics
• More than double integrated luminosity compared to previous analysis

Advanced Reconstruction

• Implementation of latest b-jet identification algorithms with higher background rejection
• Novel b-jet mass regression techniques for improved mass resolution
• Refined jet substructure methods for better signal-background discrimination

Methodology Improvements

• Sophisticated multivariate analysis techniques for event selection
• Enhanced background estimation methods
• Potential inclusion of vector-boson fusion (VBF) production category
• Implementation of dedicated high-transverse-momentum $p_{T}$ regime optimizations

Theoretical Interpretation

• Extended analysis framework incorporating Effective Field Theory (EFT) operators
• Constraints on Wilson coefficients sensitive to high-momentum Higgs production
• Investigation of possible new physics contributions in the high-energy regime

Physics Goals

The analysis aims to:

• Improve precision of $VH(b\bar{b})$ production cross-section measurement
• Probe potential deviations from Standard Model predictions at high energies by constraining relevant EFT operators through the unique sensitivity of the boosted topology

Impact

This measurement will:

• Enhance our understanding of Higgs boson interactions at high energies
• Provide crucial inputs for constraining possible new physics scenarios
• Demonstrate the capability of advanced reconstruction techniques

The analysis leverages the latest experimental and theoretical developments to maximize sensitivity to both Standard Model measurements and potential new physics effects.

Beam Physics Studies for the Cool Copper Collider

Research Significance

A high-energy electron-positron collider is widely recognized as the crucial next step for detailed studies of the Higgs boson and other fundamental particles. As part of my graduate studies, I worked on analyzing the luminosity and beam-beam interaction characteristics of the Cool Copper Collider (C³), a novel linear collider concept.

This study addresses a fundamental challenge in particle physics: achieving high collision rates while managing beam-induced backgrounds in future electron-positron colliders. The work provides crucial validation of the Cool Copper Collider (C³) design’s capabilities relative to other proposed facilities.

Technical Analysis

The research examines several critical aspects of collider performance:

Beam-Beam Interactions

• Analysis of electromagnetic forces between nanometer-scale particle bunches
• Evaluation of beamstrahlung effects on luminosity distribution
• Quantification of beam-induced background particle production
• Impact on detector design and physics measurement precision

Luminosity Optimization

• Comprehensive parameter space exploration for beam characteristics
• Development of optimization strategies for collision rates
• Assessment of trade-offs between luminosity and background levels
• Comparison with other linear collider proposals

Key Findings

In this study we demonstrated that C³ can:

• Achieve equivalent or superior collision rates compared to alternative proposals
• Maintain controlled levels of beam-induced backgrounds
• Provide robust physics performance across its energy range

Broader Impact

This work serves as an additional Validation of the C³ design concept and lays out a common framework for comparing future collider proposals

Our analysis showed that C³ can achieve the same or higher collision rates compared to other proposals, while maintaining lower beam-induced background fluxes. These findings position C³ as a promising candidate for future particle physics research, leveraging technological advances in accelerator physics developed across various linear collider initiatives.