[Paper Review] Ultra-high energy cosmic rays and new physics

S. Sarkar|ArXiv.org|Feb 3, 2002

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper explores exotic physics beyond the Standard Model to explain ultra-high energy cosmic rays (UHECRs) above the GZK cutoff, proposing that they originate from decaying massive relic particles—'wimpzillas'—in the galactic halo. These hypothetical dark matter particles decay into nucleons, producing a spectrum and anisotropy consistent with observed UHECR data, offering a testable alternative to astrophysical sources or new interactions.

ABSTRACT

Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin `cutoff' at $\sim 4 imes 10^{10}$ GeV pose a conundrum, the solution of which requires either drastic revision of our astrophysical understanding, or new physics beyond the Standard Model. Nucleons of such energies must originate within the local supercluster in order to avoid excessive energy losses through photopion production on the cosmic microwave background. However they do not point back towards possible nearby sources, e.g. the active galaxy Cen A or M87 in the Virgo cluster, so such an astrophysical origin requires intergalactic magnetic fields to be a hundred times stronger than previously believed, in order to isotropise their arrival directions. Alternatively the primaries may be high energy neutrinos, say from distant gamma-ray bursts, which annihilate on the local relic background neutrinos to create ``Z-bursts''. A related possibility is that the primary neutinos may initiate the observed air showers directly if their interaction cross-sections are boosted to hadronic strength through non-perturbative physics such as TeV-scale quantum gravity. Or the primaries may instead be new strongly interacting neutral particles with a longer mean free path than nucleons, coming perhaps from distant BL-Lac objects or FR-II radio galaxies. Yet another possibility is that Lorentz invariance is violated at high energies thus suppressing the energy loss processes altogether. The idea that has perhaps been studied in most detail is that such cosmic rays originate from the decays of massive relic particles (``wimpzillas'') clustered as dark matter in the galactic halo. All these hypotheses will soon be critically tested by the Pierre Auger Observatory, presently under construction in Argentina, and by proposed satellite experiments such as EUSO.

Motivation & Objective

• To resolve the paradox of ultra-high energy cosmic rays (UHECRs) exceeding the GZK cutoff, which challenges standard astrophysical models.
• To investigate whether UHECRs originate from decaying massive relic particles (wimpzillas) rather than distant astrophysical sources.
• To test the viability of wimpzillas as dark matter candidates by comparing predicted UHECR spectra and anisotropies with observational data.
• To assess the potential for future experiments like Pierre Auger and EUSO to distinguish between astrophysical and new physics origins of UHECRs.

Proposed method

• Uses DGLAP evolution equations to evolve fragmentation functions from LEP energies to the mass scale of decaying wimpzillas.
• Models the hadronic decay products of wimpzillas using QCD fragmentation functions to predict the energy spectrum of nucleons.
• Simulates the angular distribution of UHECRs by modeling the galactic dark matter halo with various density profiles (cusped, isothermal, triaxial, tilted).
• Calculates the expected anisotropy power spectrum and compares it with observational constraints on UHECR arrival directions.
• Evaluates photon component from decays against Haverah Park limits, accounting for possible attenuation in the radio background.
• Assesses the detectability of clustering in UHECR arrival directions due to clumpy dark matter distribution in the halo.

Experimental results

Research questions

• RQ1 Can the observed UHECR spectrum above the GZK cutoff be explained by the decay of massive relic particles in the galactic halo?
• RQ2 What is the expected anisotropy pattern in UHECR arrival directions if they originate from decaying wimpzillas with different halo morphologies?
• RQ3 How do the predicted photon and nucleon components from wimpzilla decay compare with observational limits, especially from Haverah Park?
• RQ4 Can the clustering of UHECR events be used as a signature to distinguish wimpzilla decay from isotropic astrophysical sources?
• RQ5 To what extent can future experiments like Pierre Auger and EUSO distinguish between wimpzilla decay and other UHECR origin hypotheses?

Key findings

• The decay of wimpzillas with a mass of $5 \times 10^{12}\,{\rm GeV}$ produces a nucleon spectrum that matches the 'flat' component of UHECRs above the GZK cutoff.
• The predicted anisotropy amplitude is $\sim 0.5$ for a cusped halo, decreasing to $\sim 0.3$ for an isothermal halo, with maximum power in the direction of the Galactic Centre.
• The model predicts a photon component that is $\sim 2$ times more abundant than the observed limit, suggesting potential conflict with Haverah Park data unless photon attenuation in the radio background is significant.
• The halo of M31 is not bright enough to provide conclusive evidence for the wimpzilla hypothesis, ruling out a simple association with M31.
• The model predicts observable clustering in UHECR arrival directions due to the clumpy structure of the dark matter halo, offering a falsifiable signature.
• The model can be ruled out if UHECRs are found to be heavy nuclei, as the wimpzilla decay model predicts a nucleon-dominated composition.