Cosmic-Ray Scattering Experiment

• Cosmic-Ray Scattering Experiment is a controlled study using advanced detectors to analyze high-energy collisions of cosmic rays with various materials.
• It utilizes a range of technologies—from calorimeters and air shower arrays to radar detection—to capture energy deposition and identify exotic scattering events.
• Empirical results refine interaction models and enhance our understanding of cosmic-ray composition, with implications for dark matter and high-energy astrophysics.

A cosmic-ray scattering experiment is a controlled or observational setup designed to paper how high-energy cosmic rays interact with material—either in laboratory detectors, dense matter (such as ice or air), or via rare or exotic targets (e.g., dark matter, axion-like particles). These experiments aim to characterize the primary and secondary products of such scatterings to illuminate particle physics, astrophysics, or broader questions such as dark matter detection, cosmic-ray composition, and hadronic interaction mechanisms.

1. Experimental Approaches and Detector Technologies

A range of specialized detector architectures and methodologies has been developed to investigate cosmic-ray scattering, capitalizing on the extreme energies and unique event topologies associated with primary cosmic-ray interactions. Among the most significant laboratory implementations are those at high-energy colliders, dedicated air shower observatories, and emerging radio/radar-based detection techniques.

At the CMS experiment at the LHC, small-angle calorimetric detectors—CASTOR (Centauro And STrange Object Research) and the Zero Degree Calorimeter (ZDC)—were installed to capture hadronic and electromagnetic energy deposition from particles produced at very forward rapidities (0.7°–0.08°, or 5.2 < η < 6.6), mirroring the kinematic phase space typical of primary cosmic-ray atmospheric collisions (Norbeck et al., 2010). CASTOR, with its layered tungsten/quartz sampling architecture and 224-channel photomultiplier tube (PMT) readout in 16 azimuthal segments, is optimized to record deep-penetration and highly-baryonic event classes ("Centauro", "long penetrating"), discriminated based on the electromagnetic-to-hadronic (EM/HAD) signal ratio and the longitudinal "stopping curve."

Similarly, the ZDC registers neutral remnants (e.g., neutrons or hypernuclei with Z/A < 0.2) that continue undisturbed along the beamline. Its high-granularity tungsten/quartz fiber design provides spatial localization and discrimination of event classes inaccessible with conventional large-angle collider detectors.

In parallel, air shower observatories such as the Telescope Array (TA) employ hybrid detection, combining surface scintillator arrays (sensitive to charged particle lateral distributions) and fluorescence telescopes (measuring longitudinal energy deposition), enabling precision energy and composition reconstruction at ultra-high energies (Jui, 2011). Innovations in radio detection (e.g., Radar Echo Telescope for Cosmic Rays, RET-CR (Prohira et al., 2021); TARA (Abbasi et al., 2016); macroscopic radar modeling (Santiago et al., 2023)) exploit the radar reflection from plasma created by cosmic-ray cascades in ice or air, introducing new methods for volumetric probing of rare interactions.

2. Event Types, Analysis Techniques, and Signal Discrimination

The small-angle and forward-region detectors are inherently sensitive to rare, anomalous event classes historically identified in emulsion chamber cosmic-ray experiments (e.g., Centauro, observed at Chacaltaya/Pamir). These are defined by:

• Deeply penetrating energy profiles with multiple maxima in the "stopping curve" (energy deposition vs. depth), distinct from ordinary hadronic showers.
• Abnormally high baryon-to-meson ratios, reflected in atypical EM/HAD signal ratios.
• Long-range "spectator" fragments such as strangelets and hypernuclei that may escape the collision region with low Z/A ratios.

Discrimination is realized by reconstructing the energy- and charge-deposition topology in fine-grained longitudinal and azimuthal detector segments. Matched filtering techniques, as employed in radar echo searches (matched template convolution with data traces), are pivotal for extracting short-lived, chirped radar signals against dominant backgrounds (Abbasi et al., 2016, Santiago et al., 2023).

In statistical terms, signal significance can be quantified via

$$S = \frac{N_{\text{observed}} - N_{\text{background}}}{\sqrt{N_{\text{background}}}}$$

where signatures are defined in the multi-dimensional space of EM/HAD fraction, stopping depth, and temporal evolution.

For broadcast radar searches, the full signal chain (from shower modeling, electromagnetic response—frequency, pulse duration, polarization, coherence—and matched filter detection) determines sensitivity. The effective radar cross-section is constrained with null results; e.g., TARA sets a 90% C.L. upper limit of σ_RCS ≲ 42 cm², strongly suggesting catastrophic collisional damping in the atmosphere (Abbasi et al., 2016).

3. Empirical Results and Forward Particle Production

At CMS, during p-p runs at √s = 0.9, 2.36, and 7.0 TeV, the mean total charge collected in CASTOR rose from ~2,941 ± 7.3 fC at 0.9 TeV to ~1.195×10⁴ ± 36 fC at 7.0 TeV, reflecting increased forward particle production with collision energy. No candidate Centauro or "long penetrating" events were confirmed in p-p data—forward region backgrounds remain modest, maximizing sensitivity to rare classes should they arise in Pb–Pb runs.

In simulated heavy-ion collisions, significant fractions (up to 30%) of exotic spectator fragments (e.g., strangelets) could fall within the CASTOR geometric acceptance, with highly anomalous energy depositions also possible in the ZDC.

The experimental confirmation of spectral features such as the GZK cutoff in the ultra-high energy cosmic-ray spectrum (TA/HiRes) (Jui, 2011)—a manifestation of CR scattering off the CMB—is a direct consequence of the underlying hadronic scattering physics probed in such experiments.

4. Implications for Cosmic-Ray Showers, Modeling, and Exotic Physics

Precision forward region measurements underpin the tuning of high-energy interaction models (used in EAS Monte Carlo codes such as QGSJET, EPOS, HIJING). Improved constraints on the secondary particle spectra and forward cross sections directly impact the accuracy of air shower simulations, compositional inferences, and energy scale calibrations.

The detection—or exclusion—of event classes such as Centauro or spectator-carrying hypernuclei would have direct bearing on the phases of strongly interacting matter produced in heavy-ion cosmic-ray collisions (e.g., the existence of deconfined quark matter, strangeness distillation, or stable strangelets).

The topological and kinematic characteristics of exotic events, if verified in the controlled collider environment, would clarify their role in the observed cosmic-ray composition anomalies and extensive air shower profiles at the highest energies.

5. Future Instrumentation and Experiment Plans

Multiple future directions are advocated:

• A second CASTOR detector incorporating enhanced tracking could enable detailed topological reconstruction, further suppressing backgrounds and enabling unambiguous exotic event identification (Norbeck et al., 2010).
• Improvements to ZDC spatial and energy resolution would allow separation of conventional spectators from putative hypernuclear signals.
• Synergistic calibration and interpretation efforts with cosmic-ray observatories (e.g., shared parameterizations, absolute cross section measurements) are anticipated to standardize approaches across detection modalities.
• Upgrades in photodetector technologies, faster and lower-noise electronic readouts, and advanced signal processing (e.g., adaptive matched filters) are envisioned to push the sensitivity to lower signal thresholds, particularly for radar-based detection and in high-multiplicity environments.

6. Theoretical and Broader Astrophysical Significance

The direct laboratory reproduction of cosmic-ray–like collisions at the LHC provides critical input for cosmic-ray physics, enabling model-independent tests of hadronic interaction models in the regime previously accessed only via indirect atmospheric measurements. For example, forward cross sections measured at CMS are essential for refining the treatment of the first interaction in the Earth's atmosphere, which seeds the development of all secondary cosmic-ray showers and drives the subsequent electromagnetic and muonic content observed by ground-based detectors.

By demonstrating both sensitivity to, and selectivity against, exotic event classes, collider-based cosmic-ray scattering experiments deliver decisive constraints on both Standard Model and beyond-Standard Model mechanisms relevant to high-energy astrophysics. The detection (or exclusion) of candidate strangelet, hypernucleus, or anomalous baryon/meson events will iteratively inform both microphysical and phenomenological cosmic-ray models.

Emergent experimental designs that combine fine spatial segmentation, longitudinal depth profiling, and cross-calorimetric approaches—especially in concert with microwatt- to kilowatt-scale radar systems—are paving the way for high-statistics exploration of hadronic and exotic scattering channels across the full cosmic-ray spectrum.