⚛️ phenomenology

Forward Searches for Heavy Neutrinos and $Z'$ Bosons at FCC-hh

This paper investigates the discovery potential of the Forward Physics Facility at the 100 TeV FCC-hh for heavy neutral leptons and $Z'$ bosons within anomaly-free chiral $U(1)$ extensions of the Standard Model, demonstrating that the facility can significantly extend sensitivity to light, long-lived particles beyond current and proposed experiments through various production and decay channels.

Original authors: ShivaSankar K. A., Souvik Das, Arindam Das, Sanjoy Mandal

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: ShivaSankar K. A., Souvik Das, Arindam Das, Sanjoy Mandal

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 as a giant, bustling city. For decades, physicists have been mapping this city using a blueprint called the "Standard Model." This blueprint explains how most of the city's residents (particles like electrons and quarks) behave. But there's a mystery: the blueprint says neutrinos (tiny, ghost-like particles) should have no weight, yet experiments show they do. They are the "ghosts" of the city that somehow gained a tiny bit of mass.

To fix this blueprint, scientists propose adding new, hidden neighborhoods. This paper explores a specific proposal: a new neighborhood governed by a hidden force (a new gauge symmetry) and populated by heavy, invisible neighbors called "Heavy Neutral Leptons" (or Heavy Neutrinos).

Here is a simple breakdown of what the paper investigates, using everyday analogies.

1. The New Neighborhood: The $Z'$ Boson and Heavy Neutrinos

Think of the Standard Model as a city with only one type of mail carrier (the photon) and one type of heavy truck (the W/Z bosons). The paper suggests there is a new, secret mail carrier called the $Z'$ boson.

The $Z'$ Boson: This is a new particle that acts like a messenger for a hidden force. It's like a new type of delivery truck that can carry packages to places the old trucks can't go.
Heavy Neutrinos: These are the "heavy" versions of the ghost-like neutrinos we know. They are like the "big brothers" of the standard neutrinos. They are so heavy and shy that they rarely interact with normal matter, but they are crucial for explaining why the light neutrinos have mass (using a mechanism called the "seesaw," where a heavy weight on one side of a seesaw lifts a light weight on the other).

2. The Location: The "Forward Physics Facility" (FPF)

The paper looks at a future giant particle collider called the FCC-hh. Imagine this collider as a massive, high-speed train station where protons (particles) crash into each other at incredible speeds.

The Problem: When these protons crash, they create a shower of particles. Most fly straight ahead or scatter wildly. The "heavy" particles we are looking for are like ghosts that slip through the walls. They don't hit the main detectors because they are too shy or too heavy to be caught by the usual sensors.
The Solution (FPF): The paper proposes building a special "ghost trap" called the Forward Physics Facility (FPF). This is a massive, empty tunnel located 1.5 kilometers (about 1 mile) away from the crash site, buried deep underground.
- The Shield: Between the crash site and the tunnel, there is a thick wall of rock and concrete. This wall blocks all the "loud" particles (like regular protons and electrons) but lets the "ghosts" (long-lived particles) pass through.
- The Trap: Inside the tunnel, the ghosts might finally decide to "show themselves" by decaying into visible particles (like flashes of light or charged tracks) that the detectors can see.

3. The Four Ways to Catch the Ghosts

The paper analyzes four different ways these hidden particles might appear in the FPF tunnel:

The Direct Ghost (Heavy Neutrinos from Mesons):
Imagine a heavy truck (a "meson") breaking down in the city. When it breaks, it might accidentally drop a heavy neutrino. Because this neutrino is shy, it travels all the way to the FPF tunnel before it finally "decays" (breaks apart) into visible pieces. The paper calculates how many of these we might catch.
The Messenger that Stops Short (Long-lived $Z'$ ):
Sometimes, the heavy truck breaks down and releases the new messenger ( $Z'$ ). If the messenger is heavy enough, it might travel the whole mile to the tunnel and then break apart into visible particles inside the tunnel. This happens if the messenger is too heavy to turn into the heavy neutrinos.
The Messenger that Drops a Package (Long-lived $Z'$ to Heavy Neutrinos):
The messenger ( $Z'$ ) travels to the tunnel, but instead of breaking apart itself, it drops off a pair of heavy neutrinos. These heavy neutrinos are so shy that they might fly right past the tunnel without being seen. However, if they are just the right amount of shy, they might decay inside the tunnel, leaving a visible trail.
The Instant Messenger (Short-lived $Z'$ ):
Sometimes, the messenger ( $Z'$ ) is very unstable and breaks apart immediately at the crash site. But, it breaks apart into a pair of heavy neutrinos. These heavy neutrinos are the "ghosts" that travel the mile to the tunnel and then decay inside, leaving a visible signal.

4. The Results: What Can We Find?

The authors ran simulations to see what this "ghost trap" could discover:

Better than current plans: They found that this facility (especially the larger version, FPF2) could spot these heavy neutrinos and new messengers much better than any existing or proposed experiment (like SHiP or FASER).
The Sweet Spot: It is particularly good at finding particles that are "light" (not too heavy) but "long-lived" (they travel far before disappearing).
The Reach:
- For Heavy Neutrinos: It could detect mixing (how much they interact with normal matter) as low as 1 in a billion ( $10^{-9}$ ) for certain masses.
- For the $Z'$ Messenger: It could detect incredibly weak forces (couplings as small as $10^{-9}$ ) for light messengers.

Summary

In short, this paper argues that building a special, long tunnel 1.5 km away from a future super-collider is the best way to catch the "ghosts" of the universe. These ghosts are heavy neutrinos and new force-carrying particles that could finally explain why neutrinos have mass. The paper shows that with the right setup, we could see these particles for the first time, opening a new window into the hidden laws of physics.

Technical Summary: Forward Searches for Heavy Neutrinos and Z′ Bosons at FCC-hh

Problem and Motivation
The observation of non-zero neutrino masses necessitates physics beyond the Standard Model (BSM). The type-I seesaw mechanism, which introduces heavy neutral leptons (HNLs) or right-handed neutrinos (RHNs), is a primary theoretical framework for explaining these masses. A well-motivated realization of this mechanism involves extending the Standard Model gauge symmetry with an additional anomaly-free chiral $U(1)$ group. Such extensions predict a new neutral gauge boson ( $Z'$ ) and RHNs. While these particles are often long-lived and weakly interacting, making them difficult to detect at conventional collider detectors, they can be produced in high fluxes in the far-forward region of high-energy proton-proton collisions. The proposed Forward Physics Facility (FPF) at the 100 TeV Future Circular Collider (FCC-hh) offers a unique environment to search for these feebly interacting particles (FIPs) and long-lived particles (LLPs). This paper investigates the discovery potential of the FPF for heavy neutrinos and $Z'$ bosons within a broad class of anomaly-free chiral $U(1)$ extensions.

Methodology
The authors analyze the production and decay of heavy neutrinos and $Z'$ bosons at the FCC-hh ( $\sqrt{s} = 100$ TeV) with an integrated luminosity of 30 ab $^{-1}$ . The study focuses on two detector configurations proposed for the FPF, located 1.5 km downstream of the interaction point (IP):

FPF1: A 5 m $\times$ 5 m $\times$ 50 m decay volume.
FPF2: A larger 20 m $\times$ 20 m $\times$ 400 m decay volume.

The analysis considers two classes of anomaly-free $U(1)$ models:

Case-I (Minimal): Three generations of SM-singlet RHNs with universal $U(1)$ charges and one SM-singlet scalar. This includes specific realizations like $U(1)_{B-L}$ , $U(1)_R$ , and others defined by specific charge assignments.
Case-II (Alternative): An extended scalar sector with non-universal RHN charges, realizing a neutrinophilic two-Higgs-doublet model (2HDM) where only two RHNs participate in the seesaw mechanism, while the third serves as a dark matter candidate.

The study evaluates four complementary production and decay scenarios:

Meson-decay HNLs: Heavy neutrinos produced directly from the decays of pseudoscalar mesons ( $B, D, K, \pi$ ) via active-sterile mixing ( $V_{\ell N}$ ).
Long-lived $Z'$ (Visible): $Z'$ bosons produced via meson decays and proton bremsstrahlung that decay visibly inside the detector, assuming the $Z' \to NN$ channel is kinematically forbidden ( $M_N > M_{Z'}/2$ ).
Long-lived $Z'$ (HNL escape): Long-lived $Z'$ bosons decaying into heavy neutrino pairs ( $Z' \to NN$ ) where the heavy neutrinos are sufficiently long-lived to escape the detector, leaving only the visible $Z'$ decay products.
Prompt $Z'$ (HNL visible): Promptly decaying $Z'$ bosons producing long-lived heavy neutrino pairs that subsequently decay visibly within the detector volume.

The event rates are calculated using the FORESEE package for event analysis and DarkCast for branching ratios, incorporating realistic detector geometry, decay probabilities, and a visible energy threshold of $E_{vis} \ge 100$ GeV. The analysis assumes a background-free environment with 100% detection efficiency for visible decays.

Key Contributions and Results
The paper derives projected sensitivities for the heavy neutrino mass ( $M_N$ ) and active-sterile mixing ( $|V_{\ell N}|^2$ ), as well as the $Z'$ mass ( $M_{Z'}$ ) and gauge coupling ( $g_X$ ).

Heavy Neutrinos from Meson Decays:
- FPF1 is projected to probe $|V_{\ell N}|^2$ in the range of $10^{-9}$ to $10^{-5}$ for $M_N \sim 0.5\text{--}6$ GeV. This sensitivity is comparable to the projected reach of the SHiP experiment but significantly outperforms SHiP for $M_N \gtrsim 2.5$ GeV due to the enhanced production of $B$ mesons at the FCC-hh.
- FPF2, with its larger volume, extends the accessible parameter space further, surpassing current experimental bounds and future projections for $0.5 \text{ GeV} \lesssim M_N \lesssim 6 \text{ GeV}$ (and down to $0.04$ GeV for muon flavor).
$Z'$ -Mediated Scenarios:
- Long-lived $Z'$ (Visible): For $Z'$ masses in the range $0.1 \text{ GeV} \lesssim M_{Z'} \lesssim 1.5$ GeV, the FPF can probe gauge couplings as small as $g_X \sim 10^{-9}$ to $5 \times 10^{-8}$ . FPF2 improves the reach by approximately one order of magnitude compared to FPF1.
- Long-lived $Z'$ (HNL escape): When the $Z'$ decays into heavy neutrinos, the visible branching fraction is reduced. The "alternative" scenario (Case-II) generally shows broader exclusion regions due to larger $U(1)$ charges for RHNs, which enhance the $Z' \to NN$ branching ratio and suppress visible decays.
- Prompt $Z'$ (HNL visible): This scenario allows probing of small mixing angles ( $|V_{\ell N}|^2 \sim 10^{-9}$ ) for $M_N \sim 0.25$ GeV. The sensitivity depends on the fixed benchmark values of $g_X$ and $M_{Z'}$ .

Significance and Claims
The authors claim that the FPF at the FCC-hh offers a powerful and complementary probe for neutrino-mass models and hidden gauge sectors. The facility's ability to detect light, long-lived heavy neutrinos and light $Z'$ bosons extends the discovery reach significantly beyond existing and proposed experiments (such as SHiP, FASER, and beam-dump experiments).

Specifically, the paper asserts that:

The enormous forward particle flux and enhanced heavy-flavor production at the FCC-hh, combined with the large fiducial decay volume of the FPF (especially FPF2), provide sensitivity well beyond current limits over broad regions of the model parameter space.
The chiral structure of general $U(1)_X$ models plays a crucial role in determining decay widths and branching fractions, leading to model-dependent sensitivities that the FPF is well-suited to explore.
The combination of different production mechanisms (meson decays, bremsstrahlung) and decay topologies (prompt vs. long-lived $Z'$ , visible vs. invisible decays) allows for a comprehensive exploration of the $U(1)$ parameter space, including the gauge coupling, $Z'$ mass, heavy neutrino mass, and mixing angles.

The work concludes that the FPF constitutes a unique facility for testing the origin of neutrino masses and searching for hidden gauge sectors through long-lived particle signatures.

1. The New Neighborhood: The Z′Z'Z′ Boson and Heavy Neutrinos

2. The Location: The "Forward Physics Facility" (FPF)

3. The Four Ways to Catch the Ghosts

4. The Results: What Can We Find?

Summary

More like this

1. The New Neighborhood: The $Z'$ Boson and Heavy Neutrinos