A New Source of Millicharged Particles: Secondary Showers in the LHC Forward Absorber

This paper identifies and quantifies a significant new source of millicharged particles at the LHC, demonstrating that secondary showers in the forward TAXN absorber can enhance the expected signal yield for the proposed FORMOSA detector by approximately 50% for light masses, thereby establishing downstream production as a critical component for realistic sensitivity projections in future high-luminosity searches.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed train station where particles are smashed together at incredible speeds. Usually, scientists look for new, tiny particles (called "millicharged particles" or mCPs) that are created right at the moment of the crash, the "interaction point." They expect these particles to fly straight down the tracks, like arrows shot from a bow, and hit a detector waiting far away.

This paper argues that scientists have been missing a huge source of these particles. It turns out the LHC isn't just a crash site; it's also a giant beam dump (a place where energy is absorbed).

Here is the story of what the paper found, explained simply:

1. The "Ghost" Particles and the Wall

When protons collide, they create a spray of debris. Most of this debris is charged and gets steered away by giant magnets. However, some debris is neutral (like neutrons and photons). These "ghost" particles don't care about the magnets; they fly straight down the beam pipe until they hit a giant wall of copper called the TAXN absorber, located about 130 meters down the line.

2. The Snowball Effect (Secondary Showers)

The paper's main discovery is what happens when these ghost particles hit that copper wall.

• The Old View: Scientists thought the wall just stopped the particles.
• The New View: When a high-energy neutron or photon hits the copper, it doesn't just stop. It explodes into a cascade (a shower) of hundreds of new, smaller particles. Think of it like throwing a single snowball at a wall of snow; it doesn't just stop; it shatters and creates a massive avalanche of smaller snowballs.

These new "secondary" particles (electrons, positrons, and other mesons) are created inside the wall. Because they are created there, they can also produce the mysterious millicharged particles (mCPs) right at the wall, not just at the original crash site.

3. Why This Matters: The "Bonus" Signal

The researchers used powerful computer simulations to count how many mCPs come from the original crash versus how many come from this "avalanche" in the copper wall.

• The Result: For lighter particles (those with a mass less than 0.1 GeV), the "avalanche" in the wall produces about 50% to 60% more millicharged particles than the original crash does.
• The Analogy: Imagine you are trying to catch fish in a river. You set a net at the source of the river (the crash site). This paper says, "Hey, there's a huge waterfall 130 meters downstream that is also churning up fish!" If you ignore the waterfall, you miss half your catch.

4. The Detector (FORMOSA)

There is a new detector being designed called FORMOSA, which is meant to catch these millicharged particles. The paper shows that if the scientists building FORMOSA ignore the "avalanche" effect in the copper wall, they will underestimate how many particles they should expect to find.

• By including this new source, the detector's ability to find new physics becomes much stronger.
• The paper provides a "menu" of the particles created in these showers (a public data set) so other scientists can use it for their own research.

Summary

The paper claims that the LHC acts like a beam dump where neutral particles hit a copper wall and create a massive secondary explosion of particles. This explosion creates a significant number of millicharged particles—enough to boost the expected signal for future experiments by about half. Ignoring this "secondary shower" would mean missing a major part of the potential discovery.

What the paper does NOT claim:

• It does not claim to have found these particles yet; it only predicts where they should be.
• It does not discuss medical applications or how this helps treat diseases.
• It does not claim this changes the laws of physics, only that we need to look harder in a specific place to find them.

Technical Summary

Technical Summary: A New Source of Millicharged Particles: Secondary Showers in the LHC Forward Absorber

Problem Statement
Millicharged particles (mCPs), hypothetical particles carrying a tiny electric charge $Q = \epsilon e$ (where $\epsilon \ll 1$), are a well-motivated target for searches at the Large Hadron Collider (LHC), particularly in the far-forward kinematic region. Current sensitivity projections for dedicated far-forward detectors, such as the proposed FORMOSA experiment, primarily rely on mCPs produced directly at the ATLAS interaction point (IP) via the decays of forward-going neutral mesons (e.g., $\pi^0, \eta$). However, the LHC effectively operates as a beam-dump experiment for neutral particles. High-energy neutral particles (photons and neutrons) produced at the IP travel down the beam pipe and strike the neutral particle absorber (TAXN in the High-Luminosity LHC era). While previous studies have focused on primary production at the IP, the potential for significant mCP production via secondary cascades within these downstream absorbers has not been fully quantified. This paper addresses the gap in understanding the contribution of secondary hadronic and electromagnetic showers in the TAXN absorber to the total mCP flux.

Methodology
The authors employ a multi-stage simulation approach to quantify the secondary mCP flux:

1. Primary Flux Generation: Forward particle spectra (neutrons and photons) within an angular acceptance of $\theta < 3$ mrad are generated using four dedicated Monte Carlo event generators: EPOS-LHC, QGSJET, SIBYLL, and a forward-tuned Pythia 8. These spectra are validated against LHCf data and scaled to an integrated luminosity of $3 \text{ ab}^{-1}$ (HL-LHC era).
2. Shower Development: The interaction of these primary neutral particles with the TAXN absorber (modeled as a $1 \text{ m} \times 1 \text{ m} \times 4 \text{ m}$ solid copper block located $\sim 130$ m from the IP) is simulated using Geant4 (version 11.3) with the FTFP_BERT physics list. This generates secondary electromagnetic (EM) and hadronic cascades.
3. Secondary Particle Spectra: The resulting energy spectra of secondary particles (photons, $e^\pm$, and neutral mesons like $\pi^0, \eta, \rho^0$) are extracted. The authors note that while secondary mesons are generally softer than primary ones, their multiplicity is high.
4. mCP Production Channels:
   • Hadronic Showers: mCPs are produced via the decays of secondary neutral mesons ($\pi^0, \eta \to \gamma \chi\bar{\chi}$ and $\rho^0 \to \chi\bar{\chi}$).
   • Electromagnetic Showers: mCPs are produced via electron-positron annihilation ($e^+e^- \to \chi\bar{\chi}$) and bremsstrahlung ($e^\pm N \to e^\pm N \chi\bar{\chi}$).
5. Detection Simulation: The propagation of mCPs from the IP and the TAXN to a benchmark detector geometry ($1 \text{ m} \times 1 \text{ m} \times 4 \text{ m}$, representing the FORMOSA concept) is simulated. Detection efficiency is calculated using the Bethe-Bloch formula for ionization energy deposition in plastic scintillator (BC-408), accounting for the accumulation of soft Coulomb interactions.

Key Contributions

• Identification of a New Source: The paper identifies and quantifies secondary production in the TAXN absorber as a significant source of mCPs, distinct from the primary IP production.
• Comprehensive Simulation Framework: The authors combine forward-tuned MC generators with detailed Geant4 shower modeling to produce simulated secondary spectra for both hadronic and electromagnetic cascades.
• Public Data Release: To facilitate future forward physics studies, the authors make the simulated secondary hadron and electromagnetic spectra publicly available via a GitHub repository.

Results

• Flux Enhancement: For mCP masses $m_\chi \lesssim 0.1$ GeV, the inclusion of secondary production enhances the expected signal yield by approximately 50% to 60% relative to the primary production baseline alone.
• Generator Dependence: The magnitude of the enhancement depends on the choice of the primary event generator. For the EPOS-LHC generator, the secondary-to-primary ratio reaches $\sim 40\%$, while for the forward-tuned Pythia 8, it exceeds $60\%$. The SIBYLL generator predicts a lower relative contribution due to a higher predicted rate of primary $\eta$ production.
• Energy Spectrum Shift: Secondary mCPs exhibit a spectrum shifted toward lower energies compared to primary mCPs. This is due to the energy partitioning in shower development. However, the high multiplicity of secondary particles compensates for the lower average energy per particle.
• Detector Geometry Dependence: The relative importance of the secondary component increases with larger detector transverse sizes. For a $2 \text{ m} \times 2 \text{ m}$ geometry, the secondary contribution can enhance the signal by up to 90% relative to the primary baseline, as secondary production occurs over a broader angular range than the highly collimated primary flux.
• Sensitivity Projections: When secondary production is included, the projected exclusion bounds on the kinetic mixing parameter $\epsilon$ improve significantly. The shift in sensitivity caused by secondary production exceeds the width of the theoretical uncertainty band associated with the primary-only prediction in the low-mass regime.

Significance
The paper concludes that secondary production in downstream infrastructure is an essential ingredient for realistic sensitivity projections and new-physics searches at the High-Luminosity LHC. The impact of these secondary showers is a dominant effect that is not obscured by current theoretical modeling uncertainties. Consequently, neglecting this source could lead to an underestimation of the discovery potential for millicharged particles and other light, weakly coupled Beyond the Standard Model (BSM) scenarios in far-forward experiments. The framework developed is generalizable to other light BSM particles, and the provided data aims to refine future forward signal simulations.