Probes of flavour symmetry and violation with top quarks in ATLAS and CMS

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle smasher. Inside, protons crash together at nearly the speed of light, creating a chaotic storm of subatomic particles. Among this debris, the top quark is the "heavyweight champion"—it's the heaviest fundamental particle we know, and because it's so massive, it acts like a unique spotlight. If there are any hidden rules of the universe breaking down, the top quark is the most likely place to see the cracks.

This paper is a report card from two giant detectors, ATLAS and CMS, which act like high-speed cameras capturing these collisions. The scientists are looking at data from 2015 to 2018 (a massive amount of information, like 140 million billion collisions) to see if the top quark behaves exactly as the "Standard Model" (our current rulebook for physics) predicts, or if it's doing something weird that hints at new, undiscovered physics.

Here is a breakdown of their four main investigations, using simple analogies:

1. The "Fairness" Test (Lepton Flavour Universality)

The Concept: The Standard Model says that the universe treats electrons and muons (a heavier cousin of the electron) exactly the same way, like two identical twins wearing different colored hats. They should interact with force-carrying particles (W and Z bosons) with equal strength.
The Experiment: The scientists looked at top quarks decaying into these particles. They compared how often a top quark produced an electron versus a muon.
The Analogy: Imagine a vending machine that is supposed to dispense Coke and Pepsi with equal probability. If you press the button 1,000 times, you expect roughly 500 of each.
The Result: The machine is perfectly fair. The ratio of electrons to muons was measured to be 0.9995, which is incredibly close to 1. This is the most precise test of this "fairness" rule ever done, confirming that, so far, the universe treats these two particles as equals.

2. The "Forbidden Swap" Hunt (Charged Lepton Flavour Violation)

The Concept: In the Standard Model, particles generally don't change their "flavour" (identity) easily. An electron shouldn't just turn into a muon. If it did, it would be a massive rule-breaker, suggesting new physics like "leptoquarks" or supersymmetry.
The Experiment: The teams looked for top quarks that decayed into a mix of different particles that shouldn't go together, like an electron and a muon appearing together from a single top quark event.
The Analogy: Imagine a chef who only cooks burgers. If you suddenly find a burger that has a slice of pizza and a donut stuck to it, you know the chef is using a secret, forbidden recipe.
The Result: They found no forbidden burgers. No evidence of these "forbidden swaps" was seen. However, because they didn't find any, they were able to set very strict limits on how rare these events could possibly be. They essentially told the universe: "If this forbidden swap happens, it must be incredibly, incredibly rare."

3. The "Identity Theft" Check (Baryon Number Violation)

The Concept: In our current understanding, the total number of "baryons" (particles like protons and neutrons that make up matter) is conserved. Matter isn't just created or destroyed out of thin air.
The Experiment: They searched for top quarks decaying in a way that would break this rule, potentially turning a top quark into a lepton and some other particles in a way that violates the conservation of matter.
The Analogy: Imagine a bank where the total amount of money in the vault is supposed to stay constant. The scientists are looking for a teller who somehow manages to withdraw a $100 bill and turn it into a $100 bill plus a $50 bill, creating money out of nothing.
The Result: No "money printers" were found. The universe still seems to keep its books balanced. The scientists set new, much stricter limits on how often this "identity theft" could happen, improving on previous limits by factors of 1,000 to 1,000,000.

4. The "Ghost Particle" Search (Heavy Neutral Leptons)

The Concept: We know neutrinos have mass, but we don't know why. A popular theory suggests there are "Heavy Neutral Leptons" (HNLs)—ghostly, heavy cousins of neutrinos that are hard to detect.
The Experiment: This was a first for ATLAS: looking for these ghost particles specifically inside a top quark decay. They looked for a top quark turning into a heavy neutrino, which then decays into two particles with the same electric charge (like two positive muons).
The Analogy: Imagine a magician pulling a rabbit out of a hat. Usually, you expect a rabbit. But here, they are looking for a specific, heavy, invisible rabbit that leaves a very specific trail of footprints (two same-sign particles) before disappearing.
The Result: They didn't find the heavy ghost rabbit. However, they successfully mapped out exactly where this rabbit could be hiding (in terms of mass and how strongly it interacts) and ruled out a wide range of possibilities, especially for heavier versions of these particles.

The Bottom Line

The ATLAS and CMS teams have performed a rigorous "health check" on the top quark.

• Did they find new physics? No. The top quark is behaving exactly as the Standard Model predicts.
• Is this a failure? Not at all. In physics, "nothing happened" is a huge success because it tells us exactly where not to look.
• What's next? They have tightened the net. They have proven that if new physics exists, it is hiding in a much smaller, more elusive corner than we thought. With more data coming from the next phase of the LHC (Run 3), they will keep looking with even sharper eyes.

Technical Summary

Technical Summary: Probes of Flavour Symmetry and Violation with Top Quarks in ATLAS and CMS

Problem and Scope
This paper presents a comprehensive overview of six new analyses conducted by the ATLAS and CMS collaborations using proton-proton collision data at a center-of-mass energy of 13 TeV. The analyses utilize the full Run 2 datasets, corresponding to integrated luminosities of 140 fb⁻¹ (ATLAS) and 138 fb⁻¹ (CMS). The primary objective is to validate Standard Model (SM) predictions within the top quark sector and to search for deviations indicative of Beyond-the-Standard-Model (BSM) physics. Specifically, the work targets three categories of potential new physics: Charged Lepton Flavour Violation (cLFV), Baryon Number Violation (BNV), and the existence of Heavy Neutral Leptons (HNLs). Additionally, the paper reports a precision measurement of Lepton Flavour Universality (LFU).

Methodology
The analyses employ model-independent Effective Field Theory (EFT) approaches where applicable, assuming a new physics mass scale ($\Lambda$) significantly larger than the LHC energy scale. This allows for the extraction of limits on Wilson coefficients ($|C|$) for dimension-6 operators.

• Lepton Flavour Universality (LFU): The ATLAS analysis measures the ratio of $W$ boson branching fractions to muons and electrons ($R_{\mu/e}^W$) using $t\bar{t}$ events in dileptonic channels ($ee, e\mu, \mu\mu$). To mitigate systematic uncertainties related to lepton identification, the analysis exploits the kinematics of $Z \to \ell^+\ell^-$ decays and calculates a double ratio ($R_{\mu/e}^{WZ}$). Reweighting factors and in-situ isolation efficiency measurements are applied to align electron and muon kinematics.
• Charged Lepton Flavour Violation (cLFV): Three distinct analyses search for cLFV in single-top production and $t\bar{t}$ decay:
  • CMS $e\mu$ Trilepton: Uses Boosted Decision Trees (BDT) trained in invariant mass regions ($m(e\mu) > 150$ GeV for production, lower masses for decay) to separate signal from background.
  • ATLAS $\mu\tau$ Trilepton: Selects hadronic $\tau$ decays ($\tau_{had}$) and utilizes the scalar sum of transverse momenta ($H_T$) as the discriminating variable. This analysis also interprets results in the context of a scalar leptoquark ($S_1$) model with hierarchical couplings.
  • CMS $\mu\tau$ Hadronic: Focuses on events where the SM $W$ boson decays hadronically. It employs kinematic reconstruction via $\chi^2$ minimization and a multiclass Deep Neural Network (DNN) trained on 28 kinematic variables to distinguish cLFV signals from $t\bar{t}$ backgrounds.
• Baryon Number Violation (BNV): The CMS analysis searches for $tqq'\ell$ vertices (violating both baryon and lepton number) in single-top and $t\bar{t}$ processes. It considers various quark flavor combinations ($q \in [d,s,b], q' \in [u,c]$) and uses a single BDT to extract signals in $ee, e\mu,$ and $\mu\mu$ channels.
• Heavy Neutral Leptons (HNL): The ATLAS analysis searches for HNLs ($N$) produced in $t\bar{t}$ decays via the chain $t \to N\ell, N \to W\ell$, resulting in same-sign dileptons ($ee$ or $\mu\mu$) and hadronic $W$ decays. Signal separation is achieved using separate BDTs for low ($15-50$ GeV) and high ($>50$ GeV) HNL masses, with profile likelihood fits applied to the data.

Key Results

• LFU Measurement: The ATLAS measurement yields $R_{\mu/e}^W = 0.9995 \pm 0.0045$. This result is consistent with the SM assumption of LFU and represents the most precise test of $e$-$\mu$ universality to date, improving upon the previous PDG average.
• cLFV Limits: No significant excess over SM expectations was observed in any channel.
  • Limits on Wilson coefficients $|C|/\Lambda^2$ and branching fractions $B(t \to q\ell\ell')$ were established for $u$ and $c$ quarks.
  • The ATLAS $\mu\tau$ analysis improved previous indirect limits by factors of 7–41.
  • The CMS $\mu\tau$ hadronic analysis improved previous Wilson coefficient limits by a factor of two.
  • In the leptoquark interpretation, ATLAS excluded couplings $\lambda_{LQ}$ from 1.3 to 3.7 for masses $m_{S1}$ between 0.5 and 2.0 TeV.
• BNV Limits: The CMS analysis found no excess, setting upper limits on BNV branching fractions that improve upon previous 8 TeV results by factors of $10^3$ to $10^6$.
• HNL Limits: The ATLAS analysis extended existing limits on HNL mixing with muon neutrinos in the mass range above 50 GeV, utilizing the $t\bar{t}$ decay topology for the first time.

Significance and Claims
The paper claims that these analyses constitute an extensive programme of BSM searches in the top quark sector, achieving typical branching ratio limits in the range of $10^{-6}$ to $10^{-8}$. The authors highlight the utility of the EFT approach for model-independent constraints on dimension-6 four-fermion interactions. Furthermore, the precision measurement of LFU via top processes serves as a stringent validation of the SM. The paper concludes that the current results, derived from the full Run 2 dataset, provide the most stringent constraints to date on these specific violation modes, with future Run 3 datasets expected to further probe top quark interactions with improved precision and new techniques.