⚛️ high-energy experiments

A search for electroweak $t\bar{t}Wj$ production in multileptonic final states at $\sqrt{s} = 13$ TeV with the ATLAS detector and bounds on effective field theory operators

Using 140 fb⁻¹ of 13 TeV proton-proton collision data collected by the ATLAS detector, this study presents the first direct probe of the $tW$-scattering vertex through a search for electroweak $t\bar{t}Wj$ production in multileptonic final states, establishing a 95% CL upper limit on the cross section and constraining relevant Standard Model Effective Field Theory operators.

Original authors: ATLAS Collaboration

Published 2026-07-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: ATLAS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Hunting for a Rare "Ghost" Collision

Imagine the Large Hadron Collider (LHC) as a massive, high-speed particle smasher. Every time it smashes two protons together, it's like throwing two complex watches at each other at 99.9% the speed of light. Usually, they shatter into a predictable pile of gears and springs (standard particles).

This paper is about looking for a very specific, rare, and slightly "weird" shattering pattern. The scientists are hunting for a collision that produces:

A pair of heavy "top" quarks (the heaviest known particles).
A "W" boson (a force carrier particle).
At least one extra "jet" of debris (a spray of particles).

Specifically, they are looking for the Electroweak (EW) version of this event. Think of it like this:

The Common Version (QCD): This is like two cars crashing and sending a spray of sparks flying everywhere because they hit hard and fast. It happens often.
The Rare Version (Electroweak): This is like two cars crashing, but instead of just sparks, they somehow "bounce" off each other in a way that sends one specific piece of debris flying way forward, far away from the main crash site. This "forward piece" is the key clue.

The Detective Work: Finding the Needle in the Haystack

The ATLAS detector is a giant 3D camera surrounding the crash site. The scientists collected data equivalent to 140 "femtobarns" of collisions (a huge amount of data, but still a tiny fraction of the total universe).

The Strategy:

The "Same-Sign" Trick: Most collisions produce a positive and a negative particle (like a pair of shoes). But the rare event they are hunting for often produces two particles with the same charge (two left shoes or two right shoes). This is a huge clue because it filters out 99% of the "noise" (background events).
The "Forward" Clue: The rare event has a special "forward jet" (a spray of particles) that flies much further out to the side than the common events. The scientists used this to separate the signal from the background, like looking for a specific type of bird that always flies higher than the rest of the flock.

The Results: Did They Find It?

The scientists looked at the data and asked: "Is there more of this rare 'forward jet' event than the Standard Model (our current best theory of physics) predicts?"

The Verdict: They didn't find a "smoking gun" that proves new physics exists. The number of events they saw was consistent with what the Standard Model predicts.
The Limit: However, they set a very strict "speed limit" on how often this rare event could be happening. They concluded that if this process happens, it happens less than 251 femtobarns of the time. (The theory predicts it should happen about 48 times, so they are saying, "We haven't seen it happen more than 5 times the expected amount.")

The "EFT" Connection: Testing the Rules of the Game

The paper also uses this search to test Effective Field Theory (EFT).

The Analogy: Imagine you are playing a game of billiards. You know the rules (Standard Model). But you suspect there might be a hidden rule you don't know about that only kicks in when the balls hit really hard or at very specific angles.
The Test: The scientists used their data to check if the "hidden rules" (called EFT operators) were changing how the top quarks behaved. They looked at two specific "hidden rules" (named $O_{Ht}$ and $O_{HQ}$ ).
The Outcome: They found that the data fits the standard rules perfectly well. They didn't find evidence that these hidden rules are active. However, by combining their results with other types of collisions (specifically top quarks and Z bosons), they were able to rule out certain combinations of these "hidden rules" that would have been impossible to rule out if they only looked at one type of collision.

Why Does This Matter?

Even though they didn't find "new physics" (like a new particle), this is a crucial step.

Ruling Out Options: In science, knowing what isn't there is just as important as knowing what is. They have tightened the net, making it harder for theories that predict these rare events to be true.
The "Forward" Advantage: They proved that looking at the "forward" particles is a powerful way to spot subtle differences between how forces work at high energies.
Future Proofing: This analysis acts as a blueprint. As the LHC collects more data in the future (Run 3), scientists will use these same techniques to look for even rarer signals. If new physics exists, it will likely show up in these high-energy, forward-jet events first.

In summary: The ATLAS team took a massive snapshot of particle collisions, filtered out the common noise using a clever "same-charge" trick, and looked for a specific "forward-flying" debris pattern. They didn't find a new particle, but they confirmed our current theories are holding up and set strict limits on how much "new physics" could be hiding in the shadows.

Technical Summary: A Search for Electroweak $t\bar{t}Wj$ Production in Multileptonic Final States

Problem and Motivation
The Large Hadron Collider (LHC) physics program aims to probe the nature of electroweak symmetry breaking (EWSB) within and beyond the Standard Model (SM). While the production of top quark pairs in association with a $W$ boson ( $t\bar{t}W$ ) is a well-known SM process, its electroweak (EW) production component in association with an additional jet ( $t\bar{t}Wj_{EW}$ ) offers a unique testing ground. This specific process contains a $tW$-scattering vertex, which is probed directly for the first time in this analysis. Unlike the dominant Quantum Chromodynamics (QCD) induced $t\bar{t}W$ production, the $t\bar{t}Wj_{EW}$ process is characterized by a "forward" jet with higher pseudorapidity.

Theoretical interest in this channel stems from its sensitivity to Beyond the Standard Model (BSM) physics. The $tW$-scattering vertex exhibits strong energy-enhanced sensitivity to BSM effects, particularly those involving the top quark's coupling to the sector responsible for EWSB. Deviations in these couplings could manifest as energy growth in the cross-section that diverges quadratically with momentum. Furthermore, recent measurements of inclusive $t\bar{t}W$ production at $\sqrt{s}=13$ TeV have shown tensions with SM predictions, which are partially alleviated by including higher-order effects. Precise modeling of $t\bar{t}W$ with additional jets is therefore critical for reducing background uncertainties in searches for rare processes like $t\bar{t}H$ and $t\bar{t}t\bar{t}$ .

Methodology
The analysis utilizes proton-proton collision data collected by the ATLAS detector during Run 2 of the LHC, corresponding to an integrated luminosity of 140 fb $^{-1}$ at $\sqrt{s}=13$ TeV.

Event Selection: The search targets events with a dilepton signature containing two same-charge leptons (electrons or muons) and at least four jets, with at least one $b$ -tagged jet. This topology arises from the decay chain where the $W$ boson decays leptonically, one top quark decays via $W \to \ell\nu b$ , and the other top decays hadronically.
Signal Discrimination: To distinguish the signal ( $t\bar{t}Wj_{EW}$ ) from the dominant QCD-induced $t\bar{t}Wj$ background, the analysis exploits the kinematic properties of the most forward non- $b$ -tagged jet. Specifically, the difference in pseudorapidity ( $\Delta\eta_j$ ) between this forward jet and the jet with which it forms the highest invariant mass is used as a discriminant.
Background Estimation:
- Prompt backgrounds (e.g., $t\bar{t}Z$ , $VV$, $t\bar{t}W$ QCD) are estimated using Monte Carlo (MC) simulations normalized to data via a simultaneous fit.
- Non-prompt and fake leptons (arising from heavy-flavor decays or misidentified jets) are estimated using a data-driven Matrix Method (MM).
- Charge-flip backgrounds (where an electron's charge is misidentified) are estimated using di-electron events near the $Z$ boson mass peak.
Statistical Analysis: A binned profile-likelihood fit is performed simultaneously across a Signal Region (SR) and seven Control Regions (CRs). The CRs are designed to constrain background normalizations (e.g., $t\bar{t}Z$ , dibosons, fake leptons) and are defined based on jet multiplicity, lepton charge, $b$ -jet count, and kinematic variables like $H_T$ and $\Delta\eta_j$ .
EFT Interpretation: The results are interpreted within the Standard Model Effective Field Theory (SMEFT) framework, focusing on dimension-6 operators $O_{Ht}$ and $O^{(1)}_{HQ}$ . These operators modify the top quark's electroweak couplings. The analysis considers the energy-dependent sensitivity of the $t\bar{t}Wj_{EW}$ process, re-binning the SR in leading lepton transverse momentum ( $p_T(\ell_0)$ ) to maximize sensitivity to the quadratic energy growth predicted by these operators. The fit accounts for both linear and quadratic dependencies on Wilson coefficients, utilizing pseudo-datasets to address potential violations of Wilks' theorem due to the quadratic terms.

Key Contributions

First Direct Probe of $tW$-Scattering: This work presents the first direct experimental probe of the $tW$-scattering vertex embedded within the $t\bar{t}Wj_{EW}$ process.
Separation of EW and QCD Contributions: The analysis successfully isolates the electroweak contribution from the dominant QCD background using forward jet kinematics, providing a cleaner measurement of the EW process.
EFT Complementarity: The study demonstrates the utility of combining $t\bar{t}Wj_{EW}$ and $t\bar{t}Z$ measurements. While $t\bar{t}Z$ production is sensitive to the same EFT operators, it suffers from degeneracies between $O_{Ht}$ and $O^{(1)}_{HQ}$ . The inclusion of $t\bar{t}Wj_{EW}$ , with its distinct quadratic energy dependence, helps lift these degeneracies, providing tighter constraints on the Wilson coefficients.
Data-Driven Background Control: The analysis employs sophisticated data-driven techniques, particularly the Matrix Method for fake leptons, which is tuned to account for its dependence on the $t\bar{t}W$ cross-section, thereby reducing modeling uncertainties.

Results

Cross-Section Measurement: The observed signal strength is $\mu_{t\bar{t}Wj_{EW}} = 0.9^{+2.4}_{-3.0}$ , consistent with the Standard Model expectation of 1.0.
Upper Limits: A 95% confidence level (CL) upper limit on the $t\bar{t}Wj_{EW}$ cross-section is set at $\sigma_{t\bar{t}Wj_{EW}} < 251$ fb (expected limit: 230 fb), to be compared with the SM prediction of $47.7^{+1.9}_{-1.4}$ fb.
EFT Constraints:
- Limits are set on the Wilson coefficients $c_{Ht}/\Lambda^2$ and $c^{(1)}_{HQ}/\Lambda^2$ (with $\Lambda = 1$ TeV).
- When fitting both operators simultaneously, the 95% confidence intervals are determined to be $c_{Ht} \in [-10, -0.96] \cup [1.5, 7.3]$ and $c^{(1)}_{HQ} \in [-12, 0.33] \cup [3.4, 12]$ (in units of TeV $^{-2}$ ) when considering both $t\bar{t}Wj_{EW}$ and $t\bar{t}Z$ processes.
- The analysis confirms that the combined fit of $t\bar{t}Wj_{EW}$ and $t\bar{t}Z$ effectively closes the degenerate directions present when analyzing $t\bar{t}Z$ alone.
Systematic Uncertainties: The measurement is dominated by systematic uncertainties, primarily the normalization and modeling of the $t\bar{t}W$ QCD background, jet modeling uncertainties, and the estimation of fake-lepton contributions.

Significance
The paper concludes that the search is consistent with Standard Model expectations, with the measurement limited by systematic uncertainties rather than statistical power. The primary significance lies in the demonstration of the $t\bar{t}Wj_{EW}$ process as a powerful probe for EFT operators affecting top-quark electroweak couplings. The analysis highlights the importance of energy-dependent sensitivity and the necessity of multi-process, multi-operator EFT interpretations to disentangle new physics effects. The authors note that while current sensitivity is limited by event statistics at high $p_T$ , future LHC Run 3 data is expected to enhance this sensitivity, particularly in exploring the interplay between processes with and without significant high-energy growth.

The Big Picture: Hunting for a Rare "Ghost" Collision

The Detective Work: Finding the Needle in the Haystack

The Results: Did They Find It?

The "EFT" Connection: Testing the Rules of the Game

Why Does This Matter?

More like this