Searches for VLQs and LQs from the ATLAS Experiment

This paper presents new results from the ATLAS detector at the LHC regarding searches for vector-like quarks and leptoquarks, which are hypothesized particles in extensions of the Standard Model designed to address the hierarchy problem and flavor anomalies.

The Gist

Imagine the universe as a giant, incredibly complex puzzle. For decades, scientists have been fitting pieces together to form the "Standard Model," which is their best picture of how matter works. It explains almost everything we see, but there are still gaps in the picture. The paper you're asking about is a report from the ATLAS experiment (a massive particle detector at the Large Hadron Collider) saying, "We've been looking for the missing pieces, and here is what we found (or didn't find)."

Here is a simple breakdown of their search for three specific types of "missing pieces."

The Three Suspects: VLQs, VLLs, and LQs

The scientists are hunting for three hypothetical particles that don't exist in our current rulebook but might exist in a bigger, more complete version of the universe.

1. Vector-like Quarks (VLQs):

   • The Analogy: Think of regular quarks (the building blocks of protons and neutrons) as dancers who have a specific "handedness." They only dance with their left hand or their right hand, never both at the same time. This is called being "chiral."
   • The Twist: VLQs are like dancers who can use both hands equally. They are "vector-like." Because they are so symmetrical, they don't need the usual "Higgs mechanism" (a cosmic force that gives things mass) to get heavy. They can just be naturally massive.
   • The Search: The ATLAS team smashed protons together to see if they could create these heavy, two-handed dancers. They looked for them appearing alone (single production) and then breaking apart into known particles like a top quark, a W boson, or a Z boson.

2. Leptoquarks (LQs):

   • The Analogy: Imagine a "social butterfly" particle. In our current rules, quarks (which make up matter) and leptons (like electrons and neutrinos) are in different social clubs and rarely interact directly.
   • The Twist: A Leptoquark is a particle that belongs to both clubs. It carries the traits of a quark and a lepton at the same time. If it exists, it would be a bridge allowing these two groups to mix in ways we haven't seen before.
   • The Search: The team looked for a single Leptoquark appearing out of nowhere and immediately splitting into a lepton and a quark (like a muon and a bottom quark).

3. Vector-like Leptons (VLLs):

   • The Analogy: If quarks can have "two-handed" versions (VLQs), why not leptons? These are the heavy, symmetrical cousins of electrons and neutrinos.
   • The Twist: The paper discusses a specific scenario where these heavy leptons decay into a Leptoquark. It's a "Russian nesting doll" scenario: A heavy lepton breaks down into a Leptoquark, which then breaks down into a tau particle and a quark.

The Detective Work: How They Looked

The ATLAS detector is like a giant, high-speed camera that takes pictures of the debris from particle collisions. Since these new particles are too heavy to be seen directly, the scientists look for the "footprints" they leave behind.

• The "Mono-Top" Mystery: In one search, they looked for a single, high-energy top quark flying off alone, accompanied by a huge amount of "missing energy."
  • Metaphor: Imagine a billiard ball hitting a table, and suddenly one ball flies off at high speed, but the other ball that should have bounced off is invisible. The "missing energy" is the clue that something heavy and invisible (like a neutrino) took the other ball away. This suggests a heavy VLQ decayed into a top quark and a Z boson that turned into invisible neutrinos.
• The "All-Hadronic" Hunt: In another search, they looked for collisions where everything turned into jets of particles (hadrons), with no electrons or muons. They used special "triggers" (like a security guard at a club) to spot specific patterns, such as a large jet that looks like a W boson and a smaller jet that looks like a bottom quark.

The Results: The "No-Go" Zones

The most important part of this paper is what they didn't find. In particle physics, not finding something is actually a huge success because it tells us where not to look next.

• Setting the Boundaries: The scientists calculated that if these particles exist, they must be heavier than a certain limit.
  • For the heavy "two-handed" quarks (VLQs), they ruled out any that weigh less than about 1.4 to 2.4 TeV (depending on how strongly they interact).
  • For the "social butterfly" Leptoquarks, they ruled out any lighter than 2.8 to 4.3 TeV.
• The "Exclusion Plot": You can imagine this as a map of a forest. The scientists have walked through the bottom part of the forest (the lighter, easier-to-find particles) and said, "We are 95% sure these particles aren't hiding here." They have pushed the boundary of the "safe zone" deeper into the heavy, high-energy territory.

The Conclusion

The paper concludes that while the Standard Model is incomplete, these specific "missing pieces" (VLQs, VLLs, and LQs) are not hiding in the low-energy, easy-to-find regions of the particle zoo.

If they do exist, they are much heavier and harder to catch than previously thought. The ATLAS team has successfully expanded the "No-Go" zones, forcing theorists to rethink their models or build even more powerful machines to find these elusive particles in the future. They haven't found the new physics yet, but they have successfully cleared the ground to see where it might be hiding.

Technical Summary

Technical Summary: Searches for VLQs and LQs from the ATLAS Experiment

Problem and Motivation
The Standard Model (SM) of particle physics, while successful, remains incomplete. Extensions to the SM, such as Composite Higgs and little Higgs models, propose the existence of new matter-like particles to address issues like the hierarchy problem and flavor sector anomalies. Specifically, Vector-like Quarks (VLQs), Vector-like Leptons (VLLs), and Leptoquarks (LQs) are hypothesized particles that differ from SM fermions in that their left- and right-handed components share the same quantum numbers. Unlike chiral SM fermions, VLQs do not require mass generation via the Higgs mechanism. While light neutrino constraints and electroweak precision measurements have ruled out a fourth generation of chiral fermions, these vector-like and leptoquark extensions remain viable and unexcluded. This paper presents new search results for these particles using the ATLAS detector at the Large Hadron Collider (LHC).

Methodology
The analysis utilizes the full Run 2 dataset at a center-of-mass energy of $\sqrt{s} = 13$ TeV, with one search incorporating 55 fb$^{-1}$ of Run 3 data. All limits are established at the 95% confidence level. The methodology focuses on single production channels, which are model-dependent and proceed via electroweak (EW) processes, as well as pair production where applicable.

• Vector-like Quarks (VLQs): Searches target VLQs decaying into SM bosons ($W, Z, H$) and third-generation quarks ($t, b$).

  • Single Lepton Channel: A search for singly produced $T$ or $Y$ quarks decaying to $Wb$ utilizes triggers for single leptons ($e/\mu$), requiring missing transverse energy ($E_T^{miss}$), one hard $b$-jet, and a forward jet. The VLQ mass is reconstructed from the lepton, $E_T^{miss}$, and the $b$-jet.
  • All-Hadronic Channel: A complementary search for $T/Y \to Wb$ in the all-hadronic final state employs large-$R$ jet triggers to identify $W$-tagged jets and additional small-$R$ $b$-jets.
  • Mono-top Channel: A search for singly produced $T$ decaying to $Zt$ (where $Z \to \nu\nu$) looks for a boosted top quark and significant $E_T^{miss}$. An XGBoost classifier is used to discriminate signal from background.
  • Combination: Results from single $T$ searches decaying to $tH$, $tZ$, and the mono-top channel are combined to constrain decay widths ($\Gamma$) and branching ratios as a function of relative coupling ($\xi_W$).

• Leptoquarks (LQs) and VLLs:

  • Single Resonant LQ Production: A search for scalar LQs coupling to charged leptons and down-type quarks utilizes the full Run 2 dataset plus partial Run 3 data.
  • VLL Decays: A search is performed for VLLs decaying into vector LQs ($U_1$), which subsequently decay into third-generation leptons/quarks, based on the "4321" model extension.

Key Contributions and Results

1. Singly Produced VLQs ($T/Y \to Wb$):

   • Lepton Channel: Limits on the coupling constant $\kappa$ were set for singlet $T$ ($0.22 < \kappa < 0.52$ for $1150 < m_T < 2300$ GeV) and triplet $Y$ ($0.14 < \kappa < 0.46$ for $1150 < m_Y < 2600$ GeV). Interference with SM backgrounds was found to be negligible.
   • All-Hadronic Channel: Lower mass limits were established for $Y$ (2.0 TeV for $\kappa=0.5$; 2.4 TeV for $\kappa=0.7$) and $T$ (1.4 TeV for $\kappa=0.5$; 1.9 TeV for $\kappa=0.7$). This analysis improves mass limits for $Y$ in the $(B, Y)$ doublet compared to previous ATLAS results.

2. Mono-top Search ($T \to Zt$):

   • For an SU(2) singlet $T$ with a coupling $\kappa_T = 0.5$ and a branching ratio $BR(T \to Zt) = 25\%$, a lower mass limit of 1.8 TeV was established.

3. Combined Single $T$ Search:

   • The combination of searches for $T \to tH$, $T \to tZ$, and mono-top channels provides observed lower mass limits parametrized by the relative coupling $\xi_W$. This allows for constraints across different SU(2) representations (singlet vs. doublet).

4. Leptoquarks (Single Resonant Production):

   • Lower mass limits for scalar LQs were determined based on the Yukawa coupling $y$. Limits include 3.4 TeV for $y_{de}=1.0$, 4.3 TeV for $y_{s\mu}=3.5$, 3.1 TeV for $y_{be}=3.5$, and 2.8 TeV for $y_{b\mu}=3.5$. These results extend previous mass limits for LQs.

5. Vector-like Leptons (VLLs):

   • In the context of the "4321" model, VLL masses between 200 GeV and 910 GeV were excluded for the cross-section predicted by the model.

Significance
The paper claims that these results reinforce and extend the mass coverage and coupling constraints for singly produced VLQs and LQs. By focusing on single production channels, the analyses complement previous pair-production searches, thereby expanding the exclusion limits in both mass and coupling parameter space. The work improves the understanding of the potential properties of heavy particles in scenarios beyond the Standard Model, specifically addressing the hierarchy problem and flavor anomalies through the non-observation of these hypothesized particles. The combination of multiple search channels for single $T$ production further refines the constraints on VLQ decay widths and branching ratios.