A single-author model based on the HADRON hybrid experiment at the Tien-Shan mountain station proposes that the primary cosmic-ray mass composition changes sharply at knee energies: above the first knee break at 3⋅1015 eV3\cdot10^{15}\ \mathrm{eV}, stable lumps of strange quark matter (strangelets) gradually replace ordinary nuclei, and above 1017 eV10^{17}\ \mathrm{eV} strangelets become the dominant component, with the ultimate cosmic-ray cutoff attributed to their charge vanishing at baryon number A≃107A\simeq10^{7}. The author declares the most reliable test to be a direct measurement of the strangelet flux in outer space at primary energies E0>1017 eVE_0>10^{17}\ \mathrm{eV}. The paper also discusses, qualitatively and without prescribing them as tests, evidence whose absence it acknowledges: independent core-sensitive hybrid air-shower measurements of the reported spectral breaks, hadron-spectrum hardening, and muon-core correlations; a microscopic predictive model of strangelet atmospheric cascades (the strangelet-air interaction is treated only via qualitative Bjorken-McLerran-type arguments); and its explanation of why strangelet manifestations are invisible to experiments that do not register shower cores. The prescribed measurement has not been performed and none of these complementary steps has been carried out: the problem remains open.

Background

Cosmic rays reaching Earth are measured indirectly through extensive air showers (EAS) — cascades of particles produced when a primary cosmic ray strikes the atmosphere; the shower size NeN_e (total charged-particle count) and the number of muons NμN_\mu are the basic observables, and the primary energy E0E_0 is inferred from cascade models. The all-particle spectrum steepens at the 'knee' around 3⋅1015 eV3\cdot10^{15}\ \mathrm{eV}, discovered by Khristiansen and Kulikov in 1958, and shows a further slope change (the 'second knee') slightly below 1017 eV10^{17}\ \mathrm{eV}. Their origin is conventionally attributed to rigidity-dependent cutoffs — rigidity being a particle's momentum per unit charge, so that nuclei of each charge steepen at a characteristic energy — of the partial spectra of nuclei of increasing charge, but this remains unproven. A strangelet is a hypothetical compact lump of strange quark matter (deconfined up, down, and strange quarks) that could be stable or long-lived for baryon numbers A≳103A\gtrsim10^{3}, an idea going back to Bodmer (1971); for 103<A<10710^{3}<A<10^{7} strangelets would be stable and positively charged with Z≃0.03 AZ\simeq0.03\,A, and their large interaction cross-section with small inelasticity (inelasticity being the fraction of primary energy lost per collision) would make the showers they induce resemble those of mid-weight CNO nuclei (carbon, nitrogen, oxygen) (Bjorken and McLerran, 1979, who invoked strange matter to explain the anomalous 'Centauro' cosmic-ray event). Madsen (2005) computed strangelet galactic propagation and flux expectations as a prerequisite for direct searches. Against this background, S. Shaulov (Lebedev Physical Institute), analyzing data of the HADRON hybrid experiment at the Tien-Shan station (690 g/cm2^2 atmospheric depth), which combined an EAS array with a large X-ray emulsion chamber (XREC — a detector of stacked lead plates and photographic emulsion/foils that resolves the dense cascade 'core' of a shower and individual gamma-ray cascades from π0→2γ\pi^0\to2\gamma with energies above the TeV range), reported three coupled anomalies in the knee region: (a) two breaks in the EAS size spectrum, at Ne=106N_e=10^{6} (E0≃3⋅1015 eVE_0\simeq3\cdot10^{15}\ \mathrm{eV}) and at Ne∼108N_e\sim10^{8} (the source's linear estimator E0≃2Ne GeVE_0\simeq2N_e\ \mathrm{GeV} maps this second break to E0∼2⋅1017 eVE_0\sim2\cdot10^{17}\ \mathrm{eV}; the conventional second knee, slightly below 1017 eV10^{17}\ \mathrm{eV}, is a separate feature); (b) an 'abnormal scaling violation' — instead of the softening expected from a heavier composition, the spectra of the most energetic gamma rays in so-called gamma-families (groups of cascades with common arrival direction, attributed to one parent shower) become harder in the range Ne=106N_e=10^{6}–10710^{7}, contradicting the quark-gluon-string-based model MQ1n; and (c) an excess of muons in exactly those EAS that contain the most energetic hadrons, which no nuclear composition can produce simultaneously with hard gamma spectra (light nuclei give hard gammas but few muons, heavy nuclei many muons but no energetic gammas). Shaulov concludes that the penetrating, non-nuclear component must be stable quark matter: above the knee, strangelets with A=103A=10^{3}–10710^{7} progressively replace nuclei; the least stable ones (A≃103A\simeq10^{3}) decay to hundreds of hyperons and produce the hard hadron spectra; at Ne∼108N_e\sim10^{8} the strangelets become so stable that decays stop and spectra soften again; and the cosmic-ray cutoff corresponds to Z=0Z=0 at A≃107A\simeq10^{7}. The author notes that strangelets would be invisible to classical EAS arrays and fluorescence telescopes because these do not resolve shower cores, that only three hybrid XREC+EAS experiments have ever existed, and that the strangelet-atmosphere interaction picture relies on qualitative Bjorken-McLerran-type arguments rather than a quantitative cascade model.

Problem Statement

Determine whether the primary cosmic-ray flux contains a strangelet component (stable strange quark matter with baryon numbers A=103A=10^{3}–10710^{7}), as proposed in the strangelet-based model of the cosmic-ray spectrum built on the HADRON hybrid (XREC+EAS) data: whether strangelets progressively replace ordinary nuclei above the first knee break at 3⋅1015 eV3\cdot10^{15}\ \mathrm{eV} and become the dominant component above 1017 eV10^{17}\ \mathrm{eV}, with the ultimate cosmic-ray cutoff attributed to their charge vanishing at A≃107A\simeq10^{7}. The source states a single most reliable test: a direct experimental measurement of the flux of strangelets in outer space at primary energies E0>1017 eVE_0>10^{17}\ \mathrm{eV}, or an upper limit at those energies. Resolving the task requires a quantitative determination, referenced to the model's predicted strangelet flux level, of whether strangelets are the dominant primary component at E0>1017 eVE_0>10^{17}\ \mathrm{eV} — that is, whether their contribution dominates the primary flux at those energies rather than being merely present at a subdominant level; the source's prescribed outer-space flux measurement is one route to such a determination, and performing that measurement is not a prerequisite for a solution. The source additionally discusses, as qualitative background rather than as prescribed tests, evidence whose absence it notes and which bears directly on the same question: independent confirmation, by a core-sensitive hybrid experiment other than HADRON, of the reported EAS spectral breaks (at Ne=106N_e=10^{6} and Ne∼108N_e\sim10^{8}), of the hardening of high-energy gamma-ray/hadron spectra over the range Ne=106N_e=10^{6}–10710^{7}, and of the muon excess correlated with the most energetic hadron-containing showers; a microscopic model of strangelet interaction with air and atmospheric cascade development, which currently rests only on qualitative Bjorken-McLerran-type arguments; and the paper's explanation of why strangelet signatures are not visible in non-core-sensitive cosmic-ray experiments.

References

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