Hidden-sectors search and probe of discrete symmetries at the REDTOP experiment

This paper evaluates the REDTOP experiment's potential to discover physics beyond the Standard Model and probe discrete symmetries by leveraging its unprecedented dataset of $\eta$ and $\eta'$ mesons to search for hidden sectors via four specific portals while simultaneously refining crucial inputs for the muon anomalous magnetic moment.

The Gist

Imagine the universe as a giant, bustling city. For decades, scientists have been mapping this city using a very detailed map called the Standard Model. This map explains almost everything we see: the buildings (atoms), the traffic (particles), and the laws of the road (forces).

But there are huge gaps in the map. We know there are "ghosts" in the city—things like Dark Matter (invisible stuff holding galaxies together) and Dark Energy (a mysterious force pushing the universe apart)—but we can't see them on our current map. We also don't know why there is more matter than antimatter, or why neutrinos have mass.

Enter REDTOP. Think of REDTOP not as a car, but as a super-powered, high-speed camera designed to take a billion photos of a very specific, tiny, and shy citizen of our particle city: the Eta ($\eta$) and Eta-prime ($\eta'$) mesons.

Here is a simple breakdown of what the paper proposes:

1. The Mission: Catching the "Shy Ghosts"

The Eta and Eta-prime mesons are like rare, shy celebrities. They are special because they are "neutral"—they don't carry an electric charge, making them perfect hiding spots for new, invisible particles.

• The Problem: Previous experiments only took about a billion photos of these mesons. It's like trying to find a specific needle in a haystack by looking at one tiny corner of the hay.
• The REDTOP Solution: REDTOP plans to take 100 trillion photos ($10^{14}$) of Eta mesons. This is like turning on a floodlight and scanning the entire haystack. With this much data, even the tiniest, rarest events (like a ghost appearing for a split second) become visible.

2. The Detective Tools: The REDTOP Detector

To catch these rare events, the team designed a massive, high-tech detector. Imagine it as a multi-layered security system in a stadium:

• The Target (The Stage): A beam of high-energy protons (like a super-fast bullet) hits a stack of thin metal foils (like lithium or beryllium). This collision creates a shower of Eta mesons.
• The Vertex Detector (The Front Row): This is a super-sensitive camera placed right next to the collision point. It needs to see details as small as a human hair's width to spot if a particle decayed a tiny bit away from the main crash. This helps spot "long-lived" particles that travel a bit before disappearing.
• The Central Tracker (The Hallway): A large tube with a magnetic field. As charged particles fly through, the magnetic field bends their paths. By measuring the bend, scientists can tell how heavy and fast the particles are.
• The "Cherenkov" System (The Bouncer): This is a special gate that only lets fast particles (like electrons) pass through while stopping slower ones (like protons). It helps filter out the "noise" of the crowd so the rare signals stand out.
• The Calorimeters (The Energy Meters): These are giant blocks of glass and plastic that catch particles and measure exactly how much energy they have. They are so sensitive they can tell the difference between a photon (light) and a neutron (a neutral particle) just by how they crash into the blocks.

3. The Four "Portals" to the Hidden World

The paper suggests that the "Dark Sector" (the hidden world of dark matter) might talk to our visible world through four specific "doors" or portals. REDTOP is designed to knock on all four:

1. The Vector Portal (The Dark Photon): Imagine a new kind of light that is invisible to us but interacts with dark matter. REDTOP looks for Eta mesons turning into a photon and this "dark photon."
2. The Scalar Portal (The Dark Higgs): A new, light version of the famous Higgs boson. REDTOP looks for Eta mesons decaying into a pion and this new light particle.
3. The Pseudoscalar Portal (The Axion): A hypothetical particle proposed to solve a mystery about why the strong nuclear force doesn't break certain symmetry rules. REDTOP searches for Eta mesons turning into pions and these "axion-like" particles.
4. The Heavy Neutral Lepton Portal: A search for heavy, invisible cousins of the neutrino that might explain why neutrinos have mass.

4. Testing the Rules of the Universe

Beyond finding new particles, REDTOP is a strict referee for the laws of physics:

• CP Violation (The Mirror Test): In our world, if you look in a mirror, physics usually works the same. But sometimes, nature breaks the mirror. REDTOP will check if Eta mesons decay differently than their mirror images. If they do, it could explain why the universe is made of matter and not antimatter.
• Muon Polarization (The Spin Test): When an Eta meson decays into muons (heavy electrons), the paper proposes measuring how these muons "spin." If they spin in a way that shouldn't happen according to current rules, it's a smoking gun for new physics.

5. The Plan and Cost

• Where: The experiment could be built at Fermilab (USA), CERN (Europe), or other major labs that have powerful proton beams.
• Timeline: If approved, the project would take about 12 years (starting around 2027) to design, build, and run.
• Cost: The estimated cost for the hardware is about $107 million. This is considered very cheap for a major physics experiment, especially because it uses existing infrastructure and proven technologies.

The Bottom Line

The paper argues that we are currently blind to a huge range of the universe's secrets because we haven't looked hard enough at these specific, neutral particles. By building a "super-camera" (REDTOP) that takes trillions of pictures, we might finally see the "ghosts" (Dark Matter), fix the broken mirror (CP violation), and understand the invisible glue holding the universe together. It's a low-cost, high-reward gamble to rewrite the map of the universe.

Technical Summary

Technical Summary: Hidden-sectors search and probe of discrete symmetries at the REDTOP experiment

Problem Statement
The Standard Model (SM) fails to explain several fundamental phenomena, including dark matter, the origin of neutrino mass, and the baryon asymmetry of the universe. While high-energy colliders probe the TeV scale, a significant portion of the parameter space for Light Cold Dark Matter (LDM) and other Beyond the Standard Model (BSM) physics lies in the MeV–GeV mass range. This region is often weakly constrained by direct detection and collider searches, particularly for particles with extremely small couplings ($\sim 10^{-8}$ or lower) to SM fields.

The $\eta$ and $\eta'$ mesons are uniquely suited to probe this regime. As nearly Goldstone bosons with zero electric charge, zero isospin, and negative parity, their decays are flavor-conserving and proceed through neutral currents. Consequently, many decay modes are suppressed or forbidden in the SM, making them highly sensitive to rare processes mediated by hidden sectors. However, existing experiments have collected samples of only $\sim 10^9$ $\eta$ mesons, insufficient to probe the branching fractions ($< 10^{-9}$) required to access the relevant BSM parameter space.

Methodology
The paper proposes the REDTOP (Rare Eta Decays To Observe Physics Beyond the Standard Model) experiment, designed to produce $\mathcal{O}(10^{14})$ $\eta$ and $\mathcal{O}(10^{12})$ $\eta'$ mesons. The methodology involves a comprehensive assessment of the detector concept, beam options, and physics sensitivity through detailed Monte Carlo simulations.

• Detector Design: The proposed apparatus is a low-mass, high-granularity fixed-target detector operating in a high-rate environment (up to 700 MHz inelastic interaction rate). Key subsystems include:

  • Target Systems: Segmented thin foils of Lithium or Beryllium, or a Liquid Deuterium (LDe) target, to minimize material budget and multiple scattering while maximizing production.
  • Vertex Detector: Three layers of High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) based on MuPix technology, providing spatial resolution $< 20$ $\mu$m and timing resolution $\sim 6$ ns to identify detached vertices and reject photon conversions.
  • Central Tracker: A 4D tracking system using Low Gain Avalanche Detectors (LGADs) for precise momentum resolution ($\sigma_{p_T}/p_T^2 \sim 10^{-2}$ GeV$^{-1}$) and timing ($< 30$ ps).
  • Particle Identification (PID): A Threshold Cherenkov Time-of-Flight (Cherenkov-TOF) system using fused-quartz tiles to reject slow baryons (protons/pions) and identify leptons.
  • Calorimetry: A dual-readout (ADRIANO2) and triple-readout (ADRIANO3) calorimeter system. ADRIANO2 uses lead-glass and scintillator tiles for electromagnetic/hadronic separation, while ADRIANO3 adds Gadolinium-doped glass RPCs for neutron identification and hadronic containment.
  • Trigger System: A three-level trigger (L0, L1, L2) designed to reduce the raw 700 MHz rate to a storage rate of $\sim 0.56$ MHz, utilizing fast calorimeter signals and topological filtering.

• Beam Options: Two configurations are evaluated:

  1. Proton Beam: 1.8–2.0 GeV for $\eta$ and 3.5–4.0 GeV for $\eta'$ production. This offers a technically simpler solution with high intensity ($10^{11}$ protons/s).
  2. Pion Beam: 0.83–1.5 GeV. This offers a $\sim 10\%$ reduction in QCD background and a signal-to-noise improvement of nearly an order of magnitude, though it requires a more complex secondary beamline.

• Physics Analysis: Sensitivity studies were performed using full detector simulations for four BSM "portals": Vector (Dark Photon), Scalar (Higgs-mixing), Pseudoscalar (Axion-like Particles), and Heavy Neutral Leptons (HNL). Additionally, tests of discrete symmetries (C, CP, T) and Lepton Flavor Universality (LFU) were analyzed.

Key Contributions and Results

1. Sensitivity to Hidden Sectors:

   • Vector Portal: REDTOP is projected to probe kinetic-mixing parameters $\epsilon^2$ down to $10^{-11}$–$10^{-12}$ for dark photons ($A'$) in the mass range of 10–500 MeV, significantly extending current limits.
   • Scalar Portal: Sensitivity to hadrophilic scalar mediators ($H$) coupling to up-quarks is projected to reach $g_u \sim \text{few} \times 10^{-6}$, improving upon KLOE limits by several orders of magnitude.
   • Pseudoscalar Portal: The experiment can probe branching ratios for Axion-Like Particles (ALPs) down to $\mathcal{O}(10^{-9})$ in channels like $\eta \to \pi\pi a$, offering constraints on the Peccei-Quinn scale.
   • Heavy Neutral Leptons: Sensitivity to branching ratios of $\mathcal{O}(10^{-7})$ for processes involving HNLs is estimated, probing specific 2HDM scenarios.

2. Tests of Discrete Symmetries:

   • CP Violation: The experiment aims to measure the charge asymmetry in the Dalitz plot of $\eta \to \pi^+\pi^-\pi^0$ with statistical uncertainties of $\sigma(\alpha) \sim \mathcal{O}(1)$ GeV$^{-6}$ and $\sigma(\beta) \sim \mathcal{O}(10^{-3})$ GeV$^{-2}$. This represents an order-of-magnitude improvement over KLOE-2 and BESIII, potentially probing new physics scales up to $\Lambda \sim 1$ TeV.
   • Muon Polarimetry: REDTOP proposes measuring longitudinal ($P_L$) and transverse ($P_T$) muon polarization in decays like $\eta \to \mu^+\mu^-$ and $\eta \to \pi^0\mu^+\mu^-$. A non-zero $P_T$ would be a direct signature of T-violation (and thus CP-violation) beyond the SM.
   • LFU: Precision measurements of ratios like $\Gamma(\eta \to \gamma\mu^+\mu^-)/\Gamma(\eta \to \gamma e^+e^-)$ are projected to reach $10^{-6}$ precision, testing lepton universality at the per-mille level.

3. Nonperturbative QCD:

   • The high-statistics dataset will provide critical inputs for Chiral Perturbation Theory ($\chi$PT), refining the determination of light quark mass ratios and the $\eta$-$\eta'$ mixing angle.
   • Improved measurements of transition form factors will reduce uncertainties in the hadronic light-by-light contribution to the muon anomalous magnetic moment ($g-2$).

Significance and Claims
The paper claims that REDTOP represents a unique opportunity to explore the "intensity frontier" in the MeV–GeV mass range, a region largely unexplored by current high-energy colliders. By targeting $\eta$ and $\eta'$ mesons, the experiment offers a complementary approach to heavy-flavor and high-energy experiments, uniquely suited for probing rare, symmetry-violating processes and hidden-sector particles with weak couplings.

The authors assert that REDTOP is among the few facilities capable of probing all four theoretical portals (Vector, Scalar, Pseudoscalar, and Neutrino) connecting the SM to the dark sector. The proposed sensitivity to branching fractions below $10^{-9}$ (and down to $10^{-11}$ for many modes) would allow for competitive or superior constraints on LDM models compared to direct detection and collider searches. Furthermore, the experiment claims the potential to uncover new sources of CP violation in flavor-conserving processes, addressing the Sakharov conditions for baryogenesis in a regime distinct from EDM measurements.

The paper concludes that the REDTOP detector design, leveraging advanced technologies like HV-MAPS, LGADs, and triple-readout calorimetry, is technically feasible and cost-effective (estimated at $\sim 107$ M$). It outlines a roadmap for realization, suggesting a timeline of approximately 12 years from R&D to the completion of the physics program, positioning REDTOP as a pivotal experiment for future discoveries in dark matter, discrete symmetries, and nonperturbative QCD.