⚛️ phenomenology

Leptonic CP asymmetry and heavy neutrino searches in seesaw scenario

This paper investigates the prospects for detecting heavy Majorana neutrinos at the 14 TeV LHC within the type-I seesaw framework, demonstrating that collider sensitivity to light-heavy neutrino mixing is strongly correlated with the ratio of same-sign to opposite-sign dilepton events and the associated CP asymmetry, particularly for neutrino masses between 50 and 100 GeV.

Original authors: Arindam Das, Wei Liu, Supriya Senapati

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Arindam Das, Wei Liu, Supriya Senapati

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 Mystery: Why is the Universe Full of Stuff?

Imagine the universe as a giant party. For a long time, physicists have been puzzled by a strange fact: the party is full of "matter" (us, stars, planets), but there is almost no "anti-matter" (the mirror image of us). According to the basic rules of physics (the Standard Model), the party should have started with equal amounts of both, and they should have canceled each other out, leaving nothing but empty space.

This paper investigates a theory that explains how we ended up with a matter-filled universe. It suggests that heavy, invisible particles called Majorana Neutrinos might be the key. These particles are like "ghosts" that can break the rules of symmetry, creating a slight imbalance that eventually led to everything we see today.

The Cast of Characters

To understand the paper, think of the universe as having two types of neutrinos:

The Light Neutrinos: These are the ones we know exist. They are tiny, ghostly, and zip through everything. They are the "lightweights."
The Heavy Neutrinos: These are the new characters the paper is looking for. They are much heavier (like a sumo wrestler compared to a feather) and are predicted by a theory called the Seesaw Mechanism.

The Seesaw Analogy:
Imagine a playground seesaw. On one side, you have the light neutrinos (very light). On the other side, you have the heavy neutrinos (very heavy). The theory says that for the light ones to be so incredibly light, the heavy ones must be incredibly heavy. It's a balancing act.

The Hunt: Catching Ghosts at the LHC

The authors are planning how to catch these heavy neutrinos at the Large Hadron Collider (LHC), a giant particle accelerator in Europe that smashes protons together at near-light speed.

How they expect to find them:

The Collision: When protons smash, they might create a heavy neutrino along with a charged lepton (like an electron or muon).
The Decay: The heavy neutrino is unstable and immediately falls apart (decays). It usually breaks into a charged lepton and two jets of particles (like debris from a crash).
The Signature: Because these heavy neutrinos are "Majorana" particles, they are their own anti-particles. This leads to a very specific, rare outcome:
- Same-Sign (SS) Events: You get two leptons with the same charge (e.g., two positive electrons). This is like finding two left-handed gloves in a box that should only have right-handed ones. It's a "smoking gun" that proves the particle is a Majorana ghost.
- Opposite-Sign (OS) Events: You get two leptons with opposite charges (one positive, one negative). This is the "normal" outcome you'd expect from regular particles.

The Secret Ratio: $R_{\ell\ell}$

The paper focuses on a specific number called $R_{\ell\ell}$ . Think of this as a balance scale between the "Same-Sign" (ghostly) events and the "Opposite-Sign" (normal) events.

If $R_{\ell\ell} = 1$ : The scale is perfectly balanced. You see equal numbers of Same-Sign and Opposite-Sign events. This means the heavy neutrino is a pure "Majorana" ghost.
If $R_{\ell\ell} = 0$ : The scale tips completely. You only see Opposite-Sign events. This means the particle acts like a normal "Dirac" particle (not a ghost).
If $R_{\ell\ell}$ is somewhere in between: The particle is a mix of both.

Why does this matter?
The paper argues that this ratio is directly connected to CP Asymmetry. In simple terms, CP Asymmetry is the "bias" in nature that allowed the universe to choose matter over anti-matter. The paper shows that if you measure this ratio ( $R_{\ell\ell}$ ) at the collider, you can actually calculate how strong that bias was in the early universe.

What the Paper Found (The Results)

The authors ran computer simulations to see what the LHC could find if it ran at full power (14 TeV).

The Sweet Spot: They found that if the heavy neutrino weighs between 60 and 80 GeV (about 60 to 80 times the mass of a proton), the LHC is most likely to find it.
The Improvement:
- With current data, the LHC can set limits on how "mixed" these particles are.
- With future data (High-Luminosity LHC), they expect to improve their sensitivity by 10 to 100 times. They could detect much weaker signals than before.
The Catch: The ability to find these particles depends heavily on that ratio ( $R_{\ell\ell}$ $R_{ℓℓ}$ ).
- If the ratio is close to 1 (pure Majorana), the search is very powerful.
- If the ratio is small (closer to Dirac), the search becomes much harder, and the LHC might miss them.

The Bottom Line

This paper is a roadmap for a treasure hunt. It tells physicists:

Where to look: The 60–80 GeV mass range.
What to look for: A specific mix of "Same-Sign" and "Opposite-Sign" particle pairs.
Why it matters: Finding these particles and measuring their ratio ( $R_{\ell\ell}$ ) won't just prove they exist; it will help us solve the mystery of why the universe is made of matter instead of nothing.

The authors conclude that the next generation of the LHC (HL-LHC) is our best bet to either find these heavy neutrinos or rule out this specific version of the theory, bringing us one step closer to understanding the origin of the universe.

Technical Summary: Leptonic CP Asymmetry and Heavy Neutrino Searches in Seesaw Scenario

Problem Statement
The origin of tiny neutrino masses and the baryon asymmetry of the Universe (BAU) remain unexplained within the Standard Model (SM). The type-I seesaw mechanism, involving heavy Majorana right-handed neutrinos (RHNs), offers a unified explanation for both phenomena. However, a tension exists in standard seesaw scenarios: achieving observable heavy neutrino production rates at colliders typically requires large light-heavy mixing angles, which would generate unacceptably large light neutrino masses unless fine-tuned cancellations occur. This tension is naturally resolved if the heavy neutrino sector respects an approximate generalized lepton number symmetry. In this limit, lepton number violation (LNV) is suppressed, and the residual violation is quantified by the ratio $R_{\ell\ell}$ (the ratio of same-sign to opposite-sign dilepton events). This paper investigates the prospects for probing these heavy Majorana neutrinos at the 14 TeV Large Hadron Collider (LHC) and the High-Luminosity LHC (HL-LHC), specifically exploring the correlation between collider sensitivity, the parameter $R_{\ell\ell}$ , and the leptonic CP asymmetry ( $\epsilon$ ) relevant for resonant leptogenesis.

Methodology
The authors employ a type-I seesaw framework extended by three generations of gauge-singlet RHNs. Two generations are assumed to be nearly degenerate at the GeV scale ( $50 \lesssim M_N \lesssim 100$ GeV), while the third is decoupled at a high scale ( $\sim 10^{13}$ GeV).

Model Parameters: The Dirac Yukawa couplings are constructed using the Casas-Ibarra parametrization to reproduce neutrino oscillation data (NuFit 6.0 global fit) while satisfying non-unitarity constraints. The complex orthogonal matrix $R$ and CP-violating phases are varied within phenomenologically allowed ranges.
Production and Decay: Heavy neutrinos are produced via the charged-current process $pp \to \ell^\pm N$ mediated by an $s$ -channel $W$ boson. The dominant decay mode considered is $N \to \ell^\pm jj$ (via off-shell or on-shell $W$ bosons), leading to final states with two leptons and two jets.
Observable Definition: The study focuses on the ratio $R_{\ell\ell} = N_{SS}/N_{OS}$ , where $N_{SS}$ and $N_{OS}$ are the event counts for same-sign (LNV) and opposite-sign (LNC) dileptons, respectively. This ratio is linked to the mass splitting ( $\Delta M$ ) and decay width ( $\Gamma$ ) of the nearly degenerate neutrinos.
Simulation and Analysis: Event generation is performed using MadGraph5, followed by hadronization (PYTHIA8.3) and detector simulation (Delphes with CMS card). A cut-based analysis is conducted for both SS and OS channels at $\sqrt{s} = 14$ TeV. Selection criteria include transverse momentum and pseudorapidity cuts for leptons and jets, invariant mass windows around the heavy neutrino mass, and missing transverse energy vetoes.
Backgrounds: Dominant backgrounds for the SS channel include $W^\pm W^\pm jj$ and charge-misidentified $t\bar{t}$ events. For the OS channel, backgrounds include $t\bar{t}$ , $Z$ +jets, and $W^+W^-$ production. $b$ -jet vetoes are applied to suppress top-quark backgrounds.
Sensitivity Calculation: Statistical significance is estimated using the standard $S/\sqrt{S+B}$ metric for integrated luminosities of $140 \text{ fb}^{-1}$ (current LHC) and $3 \text{ ab}^{-1}$ (HL-LHC).

Key Contributions and Results

Correlation between $R_{\ell\ell}$ and CP Asymmetry: The authors derive an analytical relationship showing that the leptonic CP asymmetry $\epsilon$ is maximized when $R_{\ell\ell} \approx 0.5$ (resonant condition) and suppressed as $R_{\ell\ell} \to 0$ (Dirac limit) or $R_{\ell\ell} \to 1$ (pure Majorana limit). They demonstrate that for $50 \text{ GeV} \lesssim M_N \lesssim 100 \text{ GeV}$ , the collider reach is strongly correlated with $R_{\ell\ell}$ and the associated CP asymmetry.
Projected Constraints on Mixing: The study derives projected exclusion limits on the light-heavy mixing parameters $|V_{\ell N}|^2$ $∣ V_{ℓ N} ∣^{2}$ as a function of the heavy neutrino mass $M_N$ $M_{N}$ .
- Optimal Sensitivity: The strongest sensitivity is found in the mass range $M_N \simeq 60-80$ GeV.
- Impact of $R_{\ell\ell}$ : For $R_{\ell\ell} \simeq 1$ (near-pure Majorana), the sensitivity at $140 \text{ fb}^{-1}$ improves upon current LHC bounds by approximately one order of magnitude. The HL-LHC ( $3 \text{ ab}^{-1}$ ) is projected to improve this by another order of magnitude, reaching mixing strengths of $|V_{\ell N}|^2 \sim 10^{-6} - 10^{-7}$ .
- Sensitivity Degradation: For smaller values of $R_{\ell\ell}$ (e.g., $R_{\ell\ell} = 0.33$ or the Dirac limit $R_{\ell\ell}=0$ ), the sensitivity decreases substantially, dropping by up to two orders of magnitude compared to the $R_{\ell\ell} \simeq 1$ case.
Flavor Dependence: The analysis covers electron and muon channels ( $|V_{eN}|^2$ , $|V_{\mu N}|^2$ ) and the mixed-flavor quantity $|V_{eN}^* V_{\mu N}|$ . The results indicate that future experiments will be similarly sensitive to heavy neutrinos coupling to either flavor, with the mixed-flavor channel providing direct insight into the flavor structure of the neutrino mass model.
Benchmark Scenarios: Specific benchmark points are analyzed using Dirac CP phases derived from T2K and NO $\nu$ A data. The study notes that while T2K and NO $\nu$ A central values yield similar predictions for the combined dataset, the T2K-only central value results in significantly smaller CP asymmetries due to cancellations in the Yukawa sector.

Significance and Claims
The paper claims that the search for heavy Majorana neutrinos in the GeV mass range at the LHC is not merely a test of the seesaw mechanism but a probe of the specific dynamics responsible for the BAU via resonant leptogenesis.

Interplay of Parameters: The authors emphasize that the collider sensitivity is inextricably linked to the underlying CP-violating dynamics. The ratio $R_{\ell\ell}$ serves as a bridge between collider observables (SS vs. OS rates) and low-energy CP asymmetry.
Motivation for Future Measurements: The results provide additional motivation for future measurements of $R_{\ell\ell}$ and heavy-neutrino oscillations at colliders. Specifically, determining $R_{\ell\ell}$ in the $50-100$ GeV mass range is crucial for understanding the CP asymmetry required for leptogenesis.
Limitations: The authors modestly note that for $M_N \gtrsim 100$ GeV, radiative stability arguments suggest $R_{\ell\ell} < 1/3$ . In this region, the projected HL-LHC sensitivity does not surpass current experimental bounds, highlighting the specific importance of the sub-100 GeV mass window for this scenario.

In conclusion, the work establishes a direct, experimentally testable connection between heavy neutral lepton searches, the flavor structure of light-heavy mixing, and the CP-violating mechanisms of the early Universe, with the HL-LHC offering the potential to improve current exclusion limits by up to two orders of magnitude in the most favorable scenarios.