Shaulov et al. (2022) proposed that the anomalous effects observed in X-ray emulsion chambers and extensive-air-shower cores at 1–100 PeV — the knee energy range — are produced by a nonnuclear primary component of strange quark matter (strangelets), and that this same component shapes the knee itself. The proposal currently rests on qualitative estimates: no microscopic theory at the level of QCD (quantum chromodynamics, the theory of the strong interaction) of strangelet–air interactions exists, the cooling and evaporation rates of strange quark stars (which set the strangelet source abundance) are undetermined, and no strangelet has ever been confirmed in a cosmic-ray detector. The problem is open.

Background

The all-particle spectrum of Galactic cosmic rays steepens at a few PeV — the 'knee', first seen by Kulikov and Khristiansen in 1958 — and the origin of this feature is one of the longest-standing questions of cosmic-ray physics. Cosmic rays in this energy range are measured indirectly through extensive air showers (EAS), cascades of particles produced when a primary particle strikes the atmosphere. The primary composition is inferred only model-dependently from shower observables, and dedicated forward-hadron-production measurements at the Large Hadron Collider (LHC) and the Relativistic Heavy-Ion Collider (RHIC) — the LHCf and RHICf experiments (e.g. Adriani et al. 2016) — show that hadron production obeys Feynman scaling (the approximate energy independence of particle-production spectra when plotted against the longitudinal momentum fraction) at equivalent laboratory energies of 1–100 PeV, implying that the nuclear interactions themselves change little across the knee region.

Against this background, a set of long-reported anomalies in X-ray emulsion chambers (detector stacks exposing photographic films at mountain altitudes, sensitive to the high-energy hadrons in the cores — the central, hadron-rich regions — of air showers) has resisted standard interpretation: an increase in the absorption length of hadron showers, violation of scaling in the spectra of secondary hadrons, an excess of muons in EAS accompanied by gamma-ray families (groups of many high-energy gamma rays registered close together in a chamber, produced by a common ancestral hadron), 'halos' (saturated dark spots on emulsion), the 'alignment' of energy centroids of produced particles along a straight line, and delayed EAS fronts (shower fronts with substructure arriving hundreds of nanoseconds late). Shaulov et al. (2022) argue that, since accelerator data exclude new nuclear processes at these energies, these anomalies must be astrophysical in origin, and propose that the knee is formed by a nonnuclear component of cosmic rays consisting of strangelets.

Strangelets are droplets of strange quark matter (SQM) — hypothetical matter containing roughly equal numbers of up, down, and strange quarks. Witten (1984) and Farhi and Jaffe (1984) proposed that SQM, rather than ordinary nuclear matter, could be the true ground state of the baryon-rich universe. Small strangelets are unstable, but for sufficiently large baryon number AA the three quark flavors equilibrate, the droplets become almost neutral, and they may be absolutely stable for AA up to stellar values; a strangelet with A∼102A\sim 10^{2}–10810^{8} would carry a small positive charge (Z≈30Z\approx 30–10001000 for the range invoked by Shaulov et al.), giving it a high rigidity (momentum-to-charge ratio, which governs magnetic confinement and acceleration) that allows diffusive shock acceleration (the standard Fermi-type process in which particles gain energy by repeatedly crossing astrophysical shock fronts) up to 101710^{17} eV. The idea has a lineage: Bjorken and McLerran (1979) invoked 'explosive quark matter' to explain the Centauro events (unusually photon-poor, baryon-rich showers) in emulsion chambers. Alcock and Farhi (1985) analyzed the evaporation of small strangelet droplets from the early-Universe quark–hadron transition; Madsen (2005) computed strangelet propagation and the cosmic-ray flux expected from neutron-star surface emission; Monreal (2007) and Kankiewicz et al. (2017) modeled how strangelets would appear in the atmosphere and in ground-based detectors. A single balloon event (Saito et al. 1990) has long been discussed as a strangelet candidate, but no confirmed detection exists.

The SQM scenario of Shaulov et al. (2022) connects the anomalies to a population of strange quark stars — compact objects made of SQM — formed either at the early-Universe phase transition, in supernovae, or injected by compact-object coalescence. However, as the authors themselves state, the interpretation is at present only qualitative — no calculations at the level of QCD (quantum chromodynamics, the theory of the strong interaction binding quarks into hadrons) exist: 'Calculations of the interaction between strangelets within QCD are absent because they are complicated'; 'the problem of the rate of cooling and evaporation of strange quark stars is still unsolved'; and 'to finally solve the problem of existence of strange quark matter in nature, it is necessary to launch large-aperture telescopes into open space.' These three gaps — shower microphysics, source abundance, and observation — are the substance of the problem. Adjacent literature does not fill them: compact-star cooling studies (e.g. Zapata et al. 2022 on thermal relaxation of quark stars with strangelet crusts) address X-ray-observable cooling of isolated quark stars, not cosmological cooling-to-CMB, evaporation, or cosmic-ray injection rates.

Problem Statement

Can the strangelet (strange quark matter) hypothesis quantitatively account for the anomalous effects observed in the 1–100 PeV cosmic-ray knee region, and does strange quark matter exist in nature as a cosmic-ray component? Answering this requires, jointly and at the source's full generality:

  1. A microscopic, quantitative theory of strangelet interactions with air nuclei (and with detector media), replacing the present qualitative Bjorken–McLerran-type estimates — i.e. a treatment grounded in the QCD description of large-baryon-number strange quark matter (its mass–charge relation, binding, and stability window for baryon number A≈102A\approx 10^{2}–10810^{8}) and matched to a hadronic transport description (Monte Carlo tracking of the individual particle collisions in the shower cascade), sufficient to predict strangelet-initiated shower development from first principles.

  2. A determination of the cooling and evaporation rates of strange quark stars, and of strangelet production and survival from the possible channels — early-Universe phase transition, supernova formation, and compact-object coalescence — sufficient to predict the strangelet source abundance and injection spectrum in cosmic rays.

  3. A confrontation of the resulting predictions with the ensemble of knee-region measurements: the emulsion-chamber and EAS-core anomalies (increased hadron-shower absorption length, scaling violation in secondary hadron spectra, muon excess in EAS with gamma-ray families, halos, alignment, delayed EAS fronts), the all-particle spectrum shape and its cutoff near 101710^{17} eV, and direct strangelet searches — at a level of quantitative significance that either establishes the strangelet interpretation of the anomalies or excludes it, and that states, for whichever outcome is reached, what it implies for the existence of strange quark matter as a cosmic-ray component.

References

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