⚛️ phenomenology

Exploring $\widetilde{R}_2$ Leptoquarks and Majorana Neutrinos via same-sign dimuons at the HL-LHC

This paper investigates the phenomenology of scalar leptoquarks coupled to right-handed Majorana neutrinos at the High-Luminosity LHC, demonstrating that the distinctive same-sign dimuon and multi-jet signature allows for the exploration of lepton-number violating processes and extends the discovery reach for multi-TeV leptoquarks beyond current constraints through a comprehensive analysis of both pair and single production modes.

Original authors: Subham Saha, Arvind Bhaskar, Manimala Mitra

Published 2026-03-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Subham Saha, Arvind Bhaskar, Manimala Mitra

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

Imagine the universe is a giant, bustling kitchen where particles are the ingredients. For decades, physicists have been trying to figure out the recipe for everything we see. They have a "Standard Model" cookbook that works great for most dishes, but there are a few weird ingredients (like why neutrinos have mass) that don't fit the recipe.

This paper is about hunting for two new, exotic ingredients that might solve these mysteries: Scalar Leptoquarks and Majorana Neutrinos.

Here is the story of their hunt, explained simply.

1. The New Ingredients: The "Leptoquark" and the "Ghost"

The Leptoquark (LQ): Think of this as a "universal translator" or a "bridge." In the Standard Model, there are two distinct families of particles: Quarks (which build protons and neutrons) and Leptons (like electrons and neutrinos). They usually don't talk to each other. A Leptoquark is a hypothetical particle that can turn a quark into a lepton and vice versa. It's like a magical chef who can instantly turn a potato into a fish.
The Majorana Neutrino: Neutrinos are tiny, ghostly particles that barely interact with anything. Usually, we think of them as having a "left-handed" version and a "right-handed" version. But what if the "right-handed" version is its own antiparticle? That's a Majorana neutrino. It's like a coin that is both heads and tails at the same time. If you flip it, it's the same coin. This is a huge deal because it breaks a fundamental rule of physics called "Lepton Number Conservation."

2. The Scenario: The Heavy Chef and the Light Sous-Chef

The authors of this paper are looking at a specific scenario involving a Scalar Leptoquark (let's call it the "Heavy Chef") and a Right-Handed Neutrino (the "Light Sous-Chef").

The Old Way: Usually, scientists look for Leptoquarks by smashing them together and seeing them split into a standard electron and a quark. It's like looking for a broken toy by seeing it fall apart into its standard plastic pieces.
The New Twist: This paper suggests a different path. What if the Heavy Chef is heavier than the Light Sous-Chef?
- Instead of splitting into standard pieces, the Heavy Chef might decay into the Light Sous-Chef and a jet of energy (a "jet" is just a spray of particles).
- Because the Light Sous-Chef is a Majorana particle (the coin that is both heads and tails), it can decay in a very weird way. It can turn into a muon (a heavy cousin of the electron) and jets.
- The Magic Trick: Since the Sous-Chef is its own antiparticle, it has a 50/50 chance of turning into a positive muon or a negative muon.

3. The "Smoking Gun" Signature: The Same-Sign Dimuon

This is the most exciting part. If you produce two Heavy Chefs at the Large Hadron Collider (LHC), and they both decay into the Light Sous-Chef, here is what happens:

Chef A turns into Sous-Chef A $\rightarrow$ decays into a Positive Muon.
Chef B turns into Sous-Chef B $\rightarrow$ decays into a Positive Muon.

Result: You get two positive muons flying out at the same time, accompanied by a bunch of jets.

Why is this special?
In the normal universe (the Standard Model), nature is very picky. It almost never produces two particles with the same electric charge (like two positives) in this kind of collision. It's like flipping two coins and getting "Heads-Heads" every single time you do it, while the rest of the world only gets "Heads-Tails."

If the scientists at the High-Luminosity LHC (HL-LHC)—the super-charged version of the current collider—see a pile of these "Same-Sign Dimuon" events, it would be a smoking gun. It would prove two things at once:

Leptoquarks exist.
Neutrinos are Majorana particles (they are their own antiparticles).

4. The Hunt Strategy: Pair vs. Single

The paper does a deep dive into how to catch these elusive particles. They realized that catching them depends on how heavy they are:

The "Pair" Strategy (For lighter Leptoquarks): If the Leptoquarks aren't too heavy (around 1-2 TeV), the best way to find them is to smash the collider beams together hard enough to create two of them at once. It's like trying to catch two fish by casting a wide net. This is the "Pair Production" method.
The "Single" Strategy (For heavier Leptoquarks): If the Leptoquarks are very heavy (3-4 TeV), creating a pair is too hard; the energy isn't there. Instead, the scientists propose looking for a single Leptoquark appearing alongside other particles. It's like spotting a single rare bird flying past a tree, rather than waiting for a flock.

The paper shows that by combining these two strategies, the HL-LHC can probe a much wider range of possibilities than current searches.

5. The Conclusion: Why This Matters

The authors ran complex computer simulations to see if this signal could be seen amidst the "noise" of the collider (the background events that look like normal physics).

They found that:

The "Same-Sign Dimuon" signal is very clean. The background noise is extremely low, making it easy to spot if the signal is there.
Even if the Leptoquarks are very heavy (beyond what current experiments can see), the HL-LHC has a good chance of finding them using the "Single Production" method.
This search fills a huge gap. Current experiments are looking for the "standard" decay of Leptoquarks. If the Leptoquarks decay into these "ghost" neutrinos instead, current searches would miss them entirely. This paper provides a new map for the treasure hunt.

In a nutshell:
This paper is a guidebook for the next generation of particle physicists. It says, "Stop looking only for the standard broken toys. Look for the magical ones that turn into ghosts and produce two matching coins. If you find them, you will rewrite the rules of the universe."

1. Problem Statement and Motivation

Context: Leptoquarks (LQs) are hypothetical color-triplet bosons that couple quarks and leptons, appearing in various Beyond Standard Model (BSM) theories (e.g., Pati-Salam, GUTs, R-parity violating SUSY).
The Gap: Current LHC searches for scalar leptoquarks (sLQs) are optimized for "conventional" decay modes where the LQ decays into a charged lepton and a quark ( $LQ \to \ell q$ ). These searches place stringent mass limits but are weakly constrained in scenarios where the LQ couples to Right-Handed Neutrinos (RHNs).
Specific Scenario: The paper investigates the scalar leptoquark doublet $\tilde{R}_2$ (specifically the component $\tilde{R}_2^{2/3}$ ) coupled to a Majorana RHN ( $N$ ).
The Challenge: If the sLQ is heavier than the RHN ( $M_{\tilde{R}_2} > M_N$ ), the decay $\tilde{R}_2 \to N + q$ can dominate over the SM-like mode. The subsequent decay of the Majorana RHN leads to distinctive final states that are currently under-explored. The goal is to determine the discovery potential of the High-Luminosity LHC (HL-LHC) for these specific signatures.

2. Methodology and Model Framework

Theoretical Model:
- The Standard Model is extended with an sLQ doublet $\tilde{R}_2$ (quantum numbers $(3, 2, 1/6)$ ) and three RHNs ( $N$ ).
- Lagrangian: Interactions are governed by two Yukawa couplings:
  - $Y_{12}$ : Coupling between the sLQ, a first-generation quark, and a second-generation lepton (muon).
  - $Z_{11}$ : Coupling between the sLQ, a first-generation quark, and a first-generation RHN.
- RHN Properties: The RHN ( $N$ ) is a Majorana fermion that mixes with the SM muon neutrino ( $\nu_\mu$ ) via an active-sterile mixing angle $V_{\mu N}$ . This mixing allows $N$ to decay into SM particles ( $\mu^\pm, W, Z, H$ ).
Production Mechanisms at HL-LHC ( $\sqrt{s} = 14$ TeV):
The study considers both Pair Production and Single Production, analyzing their contributions to the signal:
1. Pair Production:
  - SM-mediated: $s$ -channel via $\gamma/Z$ or gluons.
  - New Physics-mediated: $t$ -channel exchange of $\mu$ or $N$ (scaling as $Y^4$ or $Z^4$ ).
  - Interference: Between SM and NP diagrams.
  - Unique Channels: Due to the Majorana nature of $N$ , same-sign pair production (e.g., $\tilde{R}_2^{2/3}\tilde{R}_2^{2/3}$ ) becomes possible via $t$ -channel $N$ exchange, which does not interfere with SM processes.
2. Single Production:
  - Processes like $pp \to \tilde{R}_2 N j$ or $\tilde{R}_2 \mu j$ .
  - Becomes increasingly dominant for multi-TeV sLQ masses where pair production is phase-space suppressed.
Signal Signature:
- Final State: Same-sign dimuons plus multiple jets ( $\mu^\pm \mu^\pm + \text{jets}$ ).
- Origin: The Majorana nature of $N$ allows it to decay into $\mu^+$ or $\mu^-$ with equal probability. When two sLQs decay (or one sLQ and one RHN in single production) involving $N$ , same-sign dileptons are generated naturally, violating lepton number.
Analysis Strategy:
- Kinematic Selection: Utilizing the high transverse momentum ( $p_T$ ) of jets and the scalar sum of transverse momenta ( $H_T$ ) to separate signal from background.
- Cuts:
  - $N_{\mu} = 2$ (same charge), $p_T^\mu > 20$ GeV.
  - $N_{\text{jets}} \ge 4$ (with at least 2 jets having $p_T > 200$ GeV).
  - $H_T > 2000$ GeV.
  - $b$ -veto ( $N_b = 0$ ) to suppress $t\bar{t}$ backgrounds.
- Backgrounds: Dominant SM backgrounds include Triboson ($VVV$), Diboson + jets ($VV+jets $),$ t\bar{t}V$, and $t\bar{t}t\bar{t}$ . Charge misidentification is deemed negligible after cuts.
- Statistical Significance: Calculated using the asymptotic formula for likelihood ratio tests ( $Z$ -score) to determine $2\sigma$ exclusion and $5\sigma$ discovery reach.

3. Key Contributions

Comprehensive Production Analysis: Unlike previous studies focusing solely on pair production, this work rigorously combines pair and single production mechanisms. It demonstrates that single production is crucial for probing sLQ masses beyond 2 TeV.
Majorana Nature Probe: The study highlights that the same-sign dimuon signature is a "smoking gun" not only for sLQs but specifically for the Majorana nature of the RHN, as it relies on lepton-number violation.
Decay Topology Classification: The authors categorize signals into Symmetric (both sLQs decay to $N+j$ ) and Asymmetric (one to $N+j$ , one to $\mu+j$ ) modes for both pair and single production, calculating the cross-sections and efficiencies for each.
Parameter Space Mapping: The analysis maps the sensitivity in the $Y_{12} - Z_{11}$ coupling plane for sLQ masses ranging from 1 TeV to 4 TeV, comparing results against existing ATLAS/CMS direct and indirect constraints.

4. Results

Mass Dependence:
- 1–2 TeV Range: Pair production dominates the sensitivity. The cross-section is largely driven by QCD (SM) processes, making the sensitivity contours linearly dependent on couplings up to $Y_{12} \approx 1$ .
- > 2 TeV Range: Pair production cross-sections drop significantly. Single production becomes the dominant discovery channel. The combined sensitivity contours merge with the single-production contours, allowing the HL-LHC to probe masses up to 4 TeV.
Coupling Dependence:
- For low couplings, sensitivity is driven by the branching ratios of the decays.
- For high couplings ( $Y_{12} \gtrsim 1$ ), Yukawa-induced production terms ( $Y^4$ ) become significant, altering the shape of the exclusion contours.
Discovery Reach:
- The HL-LHC can probe sLQ masses up to 4 TeV in the $Y_{12}-Z_{11}$ plane.
- The same-sign dimuon channel accesses regions of parameter space excluded by neither current direct LQ searches, SUSY-inspired gluino searches, nor indirect dilepton limits.
Background Suppression: The $H_T > 2$ TeV cut and $b$ -veto effectively suppress SM backgrounds, resulting in a very clean signal region with low background contamination.

5. Significance

Complementarity: The study establishes a strong complementarity between pair and single production modes. Relying solely on pair production would miss the heavy sLQ parameter space; single production is essential for a complete exploration.
New Physics Window: It identifies a "blind spot" in current LHC searches where sLQs decaying to RHNs are weakly constrained. The proposed search strategy opens a unique window to test this specific BSM scenario.
Majorana Verification: The observation of a statistically significant excess of same-sign dimuons would provide simultaneous evidence for sLQ dynamics and the Majorana nature of neutrinos, a fundamental question in particle physics.
Future Guidance: The paper provides a concrete, optimized search strategy (kinematic cuts, event selection) for experimental collaborations (ATLAS/CMS) to target these specific signatures during the HL-LHC run.

In conclusion, this paper demonstrates that the HL-LHC has the unique capability to discover or constrain heavy scalar leptoquarks coupled to Majorana right-handed neutrinos through the clean same-sign dimuon channel, significantly extending the reach of current experimental limits.