[Paper Review] Rare exclusive hadronic W decays in a t-tbar environment

Michelangelo L. Mangano, Tom Melia|arXiv (Cornell University)|Oct 28, 2014

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper proposes measuring rare exclusive hadronic W decays, such as $W\to\pi\gamma$ and $W\to\pi\pi\pi$, in the $t\bar{t}$ environment at the LHC using single particle jet isolation to suppress QCD backgrounds. By leveraging high $t\bar{t}$ statistics and tagging on leptonic $W$ decays and $b$-jets, the method enables access to exclusive final states previously unmeasurable at hadron colliders, with a projected signal significance of $\sim 5\sigma$ for $W\to\pi\gamma$ at HL-LHC and $\sim 10\sigma$ at a 100 TeV collider.

ABSTRACT

The large cross section for t-tbar production at the LHC and at any future hadron collider provides a high-statistics and relatively clean environment for a study of W boson properties: after tagging on a leptonic decay of one of the Ws and the two b-jets, an additional W still remains in the event. We study the prospect of making the first exclusive hadronic decay of a fundamental boson of the standard model, using the decay modes W to pi gamma and W to pi pi pi, and other related decays. By using strong isolation criteria, which we impose by searching for jets with a single particle constituent, we show that the three particle hadronic W decays have potential to be measured at the LHC. The possibility of measuring an involved spectrum of decay products could considerably expand our knowledge of how the W decays, and experimental techniques acquired in making these measurements would be useful for application to future measurements of exclusive hadronic Higgs boson decays.

Motivation & Objective

• To overcome the challenge of overwhelming QCD backgrounds in measuring exclusive hadronic $W$ decays at hadron colliders.
• To exploit the high $t\bar{t}$ production cross section at the LHC and future hadron colliders to create a clean environment for studying the leftover $W$ boson after tagging on one leptonic $W$ decay and two $b$-jets.
• To demonstrate the feasibility of measuring rare exclusive hadronic decays such as $W\to\pi\gamma$ and $W\to\pi\pi\pi$ using single particle jet isolation as a novel jet substructure technique.
• To provide a proof-of-principle for future precision measurements of exclusive hadronic Higgs decays by developing experimental techniques in the $W$ boson sector.
• To improve the experimental reach for rare $W$ decays beyond current PDG upper limits, particularly for $W\to\pi\gamma$ and $W\to D_s^+\gamma$.

Proposed method

• Uses a $t\bar{t}$ event environment where one $W$ decays leptonically and the two $b$-jets are tagged, leaving one $W$ boson in the event for study.
• Applies single particle jet isolation: requiring jets with only one hadronic or electromagnetic constituent to identify exclusive final states like $W\to\pi\gamma$ or $W\to\pi\pi\pi$.
• Implements strong isolation criteria based on the rarity of single-parton jets in QCD, reducing background from generic hadronic activity.
• Performs a particle-level Monte Carlo study to simulate signal and background, focusing on $W\to\pi\gamma$ and $W\to\pi\pi\pi$ decay modes.
• Uses operator product expansion (OPE) and effective field theory to compute the amplitude for $W\to\pi\gamma$, retaining leading-twist operators and expanding in $\omega = -2p\cdot q/q^2$.
• Compares the amplitude calculation with existing results (e.g., AMP) and confirms consistency after accounting for form factor normalization and higher-order corrections.

Experimental results

Research questions

• RQ1 Can exclusive hadronic $W$ decays such as $W\to\pi\gamma$ be measured at the LHC despite overwhelming QCD backgrounds?
• RQ2 Is single particle jet isolation an effective technique to identify rare exclusive $W$ decay final states in a high-multiplicity hadronic environment?
• RQ3 What is the expected significance of observing $W\to\pi\gamma$ at the HL-LHC and a future 100 TeV collider using the $t\bar{t}$ tagging method?
• RQ4 How do theoretical predictions for $W\to\pi\gamma$ based on OPE and effective field theory compare with existing results in the literature?
• RQ5 Can the $t\bar{t}$ environment at hadron colliders serve as a viable platform for measuring rare $W$ decays, analogous to $Z$ boson decays at $e^+e^-$ colliders?

Key findings

• The $t\bar{t}$ environment at the HL-LHC offers approximately $O(10^9)$ triggerable $W$ bosons after $b$-jet and leptonic $W$ tagging, enabling high-statistics studies.
• The single particle jet isolation technique effectively suppresses QCD backgrounds, making exclusive hadronic $W$ decays measurable at the LHC.
• A signal significance of $\sim 5\sigma$ is achievable for $W\to\pi\gamma$ at the HL-LHC, rising to $\sim 10\sigma$ at a 100 TeV collider with 10 ab$^{-1}$ data.
• Theoretical amplitude calculations for $W\to\pi\gamma$ via OPE agree with existing results (e.g., AMP) after correcting for form factor normalization and higher-order terms.
• The decay rate for $W\to\pi\gamma$ is estimated at $\sim 10^{-9}$ GeV, consistent with expectations from effective field theory and QCD dynamics.
• The study establishes a proof-of-principle for measuring exclusive hadronic decays of fundamental bosons at hadron colliders, with implications for future Higgs boson decay studies.