Rare top quark production and top quark properties in ATLAS and CMS

This paper reviews recent analyses by the ATLAS and CMS Collaborations on rare top quark production processes, which, despite their low cross sections, offer unique sensitivity to top quark couplings and potential physics beyond the Standard Model.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful, high-speed particle smasher. Inside this giant ring, scientists smash protons together to recreate the conditions of the universe just after the Big Bang. When these protons collide, they sometimes create a "Top Quark."

Think of the Top Quark as the heavyweight champion of the subatomic world. It is the heaviest known elementary particle, so heavy that it decays (falls apart) almost instantly. Because it's so massive, it's a unique window into understanding how the universe works and if there are hidden rules (New Physics) beyond our current understanding.

This paper is a report card from two giant teams of scientists, ATLAS and CMS, who are watching these collisions. While they see millions of "standard" Top Quark pairs every day, this report focuses on the rare, weird, and exotic ways these particles show up.

Here is a breakdown of their findings using simple analogies:

1. The "Rare" vs. The "Common"

Usually, Top Quarks come in pairs (like a dance couple). This is the "common" mode. But sometimes, nature gets creative:

• Multiple Tops: Instead of a pair, you get a trio or a quartet (like a dance troupe).
• Top + Boson: A Top Quark shows up with a "guest" particle, like a photon (light), a Z boson, or a W boson.

These rare events happen very infrequently (like finding a four-leaf clover in a field of grass), but because they are so rare, they are extremely sensitive to any "glitches" in the laws of physics. If the Standard Model (our current rulebook) is wrong, these rare events are where the errors will show up first.

2. The "Scattering" Event (Electroweak t¯tW)

The Analogy: Imagine two billiard balls (Top Quarks) hitting a third ball (a W boson) and scattering off each other in a very specific way.
The Science: The ATLAS team looked for a specific interaction where a Top Quark and a W boson scatter off each other. It's a messy process with many background noises (other collisions happening at the same time).
The Result: They successfully isolated this rare scattering event. The number of times they saw it matched the predictions of the Standard Model perfectly. It's like checking a complex math equation and finding the answer is exactly what the textbook said it should be. They also used this to test "Effective Field Theory," which is like checking if there are invisible forces tugging on the particles that we can't see directly.

3. The "Lepton Party" (Top Pairs + Extra Leptons)

The Analogy: Imagine a party where you expect two guests (Top Quarks). Suddenly, you see three or four extra guests (leptons like electrons or muons) showing up out of nowhere.
The Science: The ATLAS team looked for Top Quark pairs produced alongside extra leptons. They were specifically looking for "contact interactions"—a fancy way of saying, "Do these particles talk to each other in a way we didn't expect?"
The Result: They found the "party" was exactly as predicted. No surprise guests. This puts strict limits on theories that suggest new particles might be hiding in the shadows, interacting with Tops and Leptons in secret.

4. The "Flashlight" Effect (Top + Photon)

The Analogy: Imagine a Top Quark running around and suddenly flashing a bright light (a photon) in its hand.
The Science: The CMS team measured two things:

1. t¯tγ: A pair of Tops flashing a light.
2. tqγ: A single Top flashing a light (this is the first time CMS has officially "seen" this).
   They used a "Boosted Decision Tree" (a smart computer algorithm) to act like a bouncer, separating the real "flashlight" events from the background noise.
   The Result: They measured exactly how often this happens and how the light is distributed. The results matched the Standard Model predictions perfectly. It's like measuring the brightness of a flashlight and confirming it's exactly the wattage the manufacturer promised.

5. The "Double Trouble" (Top + Boson Pairs)

The Analogy: This is the "VIP section" of the particle zoo.

• ATLAS found a Top pair with two photons (two flashlights).
• CMS found a Top pair with a W and a Z boson (a heavy, chaotic duo).
  The Science: These are incredibly rare. Finding a Top with two photons is like finding a unicorn. Finding a Top with a W and Z is like finding a dragon.
  The Result:
• ATLAS confirmed the "Two-Photon Top" exists and matches predictions.
• CMS confirmed the "WZ Top" with such high confidence (5.8 standard deviations) that it counts as a formal observation. They even used a new AI-like tool (PartT) to separate the signal from the background noise, proving they could tell the difference between the "dragon" and the "background noise."

6. The "Impossible Trio" (Search for t¯tt)

The Analogy: Imagine looking for a specific, forbidden dance move where three Tops appear together. In the Standard Model, this move is almost impossible to do.
The Science: The CMS team looked for a process called $t\bar{t}t$ (two Tops and one anti-Top). This is so rare that if they found it, it would be a huge sign of "New Physics" (like Flavor-Changing Neutral Currents).
The Result: They didn't find it. They set a limit, saying, "If this dance move exists, it happens less than once in every 25 trillion collisions." This is a very strict rule, effectively closing the door on many theories that predicted this would happen more often.

The Big Picture

What does this all mean?
Think of the Standard Model as a nearly perfect map of a city. These scientists are driving to the very edge of the map, looking for roads that shouldn't exist or buildings that don't match the blueprints.

• Did they find new physics? No.
• Is that bad? No! In fact, it's great news. It means our current map is incredibly accurate. Every time they measure a rare event and it matches the prediction, it proves our understanding of the universe is robust.
• What's next? By ruling out where the "new physics" isn't, they are narrowing down the search. They are telling future theories: "Don't look here; the answer isn't in the Top Quark's rare interactions."

In short, ATLAS and CMS have successfully hunted down the rarest, most exotic Top Quark behaviors, and they all behaved exactly as the universe's rulebook predicted. The Top Quark remains the most obedient, yet most mysterious, heavyweight champion of the subatomic world.

Technical Summary

1. Problem Statement and Motivation

While top quark pair ($t\bar{t}$) production is the dominant mode at the Large Hadron Collider (LHC) and allows for precise Standard Model (SM) measurements, rare top quark production processes offer unique sensitivity to the structure of top quark couplings and potential physics beyond the Standard Model (BSM).

These rare processes, characterized by cross-sections typically below 1 pb, include:

• Multiple top quark production: $t\bar{t}t\bar{t}$ and $t\bar{t}t$.
• Associated production with electroweak bosons: $t\bar{t}Z$, $t\bar{t}W$, $t\bar{t}\gamma$, $t\bar{t}\gamma\gamma$, and $tWZ$.
• Single top production with photons: $tq\gamma$.

The primary challenge lies in the low event rates and the complexity of backgrounds (e.g., misidentified leptons/photons, non-prompt leptons, and irreducible processes like $t\bar{t}Z$). The paper reviews recent analyses by the ATLAS and CMS collaborations aimed at measuring these processes, constraining Effective Field Theory (EFT) operators, and searching for anomalous couplings.

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2. Methodology and Key Analyses

The paper summarizes five distinct categories of analyses, employing advanced statistical techniques, machine learning (ML), and Effective Field Theory interpretations.

A. Electroweak $t\bar{t}W$ Production (ATLAS)

• Objective: Investigate mixed electroweak and QCD corrections, specifically $tW \to tW$ scattering diagrams which are sensitive to new physics.
• Methodology:
  • Selection: Events with same-sign dileptons (electrons or muons) and additional hadronic activity, including a light-flavor jet (spectator quark).
  • Discrimination: Utilized the pseudorapidity difference ($|\Delta\eta_j|$) between the most forward non-b-tagged jet and the jet forming the largest invariant mass with it.
  • Analysis: A fit to the $|\Delta\eta_j|$ distribution combined with control regions to constrain backgrounds ($t\bar{t}Z$, $t\bar{t}H$, fake leptons).
  • Interpretation: Constraints were placed on EFT operators affecting the $tW$ interaction.

B. Top Pairs Associated with Leptons (ATLAS)

• Objective: Search for four-fermion operators inducing $t\bar{t} + \ell\ell$ production.
• Methodology:
  • Selection: Events with high jet activity and three leptons, including an opposite-sign same-flavor (OSSF) pair with invariant mass near the $Z$ boson mass.
  • Backgrounds: Dominated by off-shell $t\bar{t}Z$ and $t\bar{t}W$. Dedicated control regions validated the background modeling.
  • Interpretation: Limits were set on Wilson coefficients for contact interactions, testing scenarios with lepton flavor universality violation.

C. $t\bar{t}\gamma$ and $tq\gamma$ Production (CMS)

• Objective: Perform the first differential measurement of $t\bar{t}\gamma$ and the first observation of $tq\gamma$ by CMS.
• Methodology:
  • Selection: Single lepton, isolated photon, and hadronic activity.
  • Discrimination: A Boosted Decision Tree (BDT) separated $t\bar{t}\gamma$ and $tq\gamma$ signals from backgrounds (misidentified photons, dibosons, $t\bar{t}\gamma$ dilepton decays).
  • Measurement: A simultaneous fit to a 2D distribution of the BDT output and angular separation $\Delta R(\ell, \gamma)$.
  • Differential: Cross-sections measured as a function of photon transverse momentum ($p_T$).

D. Associated Production with Boson Pairs (ATLAS & CMS)

• $t\bar{t}\gamma\gamma$ (ATLAS):
  • Selection: Single lepton, two photons, hadronic activity.
  • Technique: Likelihood fit to a BDT distribution to extract the signal.
• $tWZ$ (CMS):
  • Challenge: Separating signal from the irreducible $t\bar{t}Z$ background.
  • Technique: Utilized a PartT architecture discriminator (a particle-flow based neural network) to distinguish topologies.
  • Data: Combined $\sqrt{s} = 13$ and $13.6$ TeV data.

E. Search for $t\bar{t}t$ Production (CMS)

• Objective: Search for flavor-changing neutral currents (FCNC) enhancing $t\bar{t}t$ production.
• Methodology:
  • Selection: Events with one lepton or same-sign dileptons.
  • Background Handling: Used data-driven methods for non-prompt leptons.
  • Discrimination: A two-stage BDT approach:
    1. Separate $t\bar{t}t\bar{t}$ (background) from signal and other backgrounds.
    2. Separate $t\bar{t}t$ (signal) from remaining backgrounds.

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3. Key Results

Process	Collaboration	Key Result	Significance / Limit
$t\bar{t}W$	ATLAS	Upper limit on cross-section: 251 fb (95% CL).	~5x SM prediction. Consistent with SM; EFT constraints on $tW$ operators resolved degeneracies found in $t\bar{t}Z$ analyses.
$t\bar{t} + \ell\ell$	ATLAS	No excess observed.	Limits set on Wilson coefficients for 4-fermion operators. Enhanced sensitivity to lepton flavor non-universality.
$t\bar{t}\gamma$	CMS	$\sigma = 1445 \pm 80$ fb.	Consistent with SM ($1369 \pm 23$ fb). First differential measurement.
$tq\gamma$	CMS	$\sigma = 236 \pm 17$ fb.	First observation by CMS. Consistent with SM ($207 \pm 9$ fb).
$t\bar{t}\gamma\gamma$	ATLAS	$\sigma = 2.42^{+0.58}_{-0.53}$ fb.	Consistent with LO SM prediction ($1.5^{+0.5}_{-0.4}$ fb, $k \approx 1.7$). Observation reported.
$tWZ$	CMS	Observed significance: 5.8$\sigma$ (Expected: 3.5$\sigma$).	Observation of $tWZ$ production. Simultaneous measurement with $t\bar{t}Z$ improved separation.
$t\bar{t}t$	CMS	Upper limit: 25 fb (Observed), 26 fb (Expected).	Consistent with SM ($\sim$2 fb). More stringent constraints than previous measurements.

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4. Significance and Impact

1. Validation of the Standard Model: All measured cross-sections for rare processes ($t\bar{t}\gamma$, $tq\gamma$, $t\bar{t}\gamma\gamma$, $tWZ$) are consistent with SM predictions, providing robust tests of perturbative QCD and electroweak theory at high energies.
2. Discovery of Rare Processes: The paper highlights the first observations of $tq\gamma$ (by CMS), $t\bar{t}\gamma\gamma$ (by ATLAS), and $tWZ$ (by CMS). These milestones demonstrate the LHC's capability to probe extremely low cross-section processes.
3. BSM Sensitivity: By interpreting results in the framework of Effective Field Theory (EFT), the analyses constrain Wilson coefficients for operators involving top quarks and electroweak bosons. The combination of $t\bar{t}W$ and $t\bar{t}Z$ data is crucial for resolving operator degeneracies that single-channel analyses cannot address.
4. Advanced Analysis Techniques: The paper underscores the critical role of modern machine learning (BDTs, PartT architectures) and data-driven background estimation in isolating rare signals from overwhelming backgrounds.
5. Flavor Physics: The search for $t\bar{t}t$ and $t\bar{t}+\ell\ell$ provides direct constraints on Flavor-Changing Neutral Currents (FCNC) and lepton flavor universality, areas where many BSM theories predict deviations.

In conclusion, this contribution demonstrates that rare top quark production modes have matured from mere searches into precision measurement tools, offering a powerful window into the electroweak sector and potential new physics.