Breit-Wigner Enhancement of Dark Matter Annihilation

By M. Ibe, H. Murayama, T. Yanagida. 2008

228 citations recorded by the source corpus.

Dark matter of the universe has been discussed since 1933, yet its nature still remains elusive[1]. Seventy-five years later we only managed to restrict its mass between 10 31 and 10 50 GeV, demonstrating our lack of understanding. However, the thermal relic of a Weakly Interacting Massive Particle (WIMP) is arguably best theoretically motivated because it has the same mass scale as the anticipated new physics that would explain why our universe is a superconductor (electroweak symmetry breaking). Hopes are high to discover WIMPs at the forthcoming LHC experiments, to detect them directly in sensitive underground experiments, as well as to observe signals of WIMP annihilations from the galactic center or the halo in high-energy cosmic rays. Recent observations of the PAMELA[2], ATIC[3], and PPB-BETS[4] experiments strongly suggest the existence of a new source of positrons (and electrons) in cosmic rays. The most interesting interpretation of these results is the annihilation of the dark matter with a mass at the TeV scale. However, such interpretation requires that the annihilation cross section of the dark matter in the galactic halo is much larger (by a factor of O(100)) than the one appropriate to explain the dark matter relic density precisely measured by the WMAP experiment[5]. The enhancement of the dark matter annihilation in the galactic halo is called a “boost factor.” So far, there have been several proposals to explain the origin of the boost factor both from astrophysics such as the enhanced local dark matter density, and from particle physics such as the Sommerfeld enhancement due to an attractive force among the dark matter particles[6]. In this letter, we propose a new explanation of the boost factor. We consider the dark matter which annihilates via a narrow Breit–Wigner resonance just below the threshold. When the resonance mass is just below twice the dark matter mass, the annihilation cross section becomes sensitive to the velocity of the dark matter. In such a case, the time evolution of the dark matter abundance is quite different from the one in the usual non-resonant case of annihilation, and we find that the annihilation cross section in the halo is enhanced compared to what is inferred from the relic abundance. As we will show, the cross section required from the dark matter density can be large enough to explain the PAMELA, ATIC, and PPB-BETS results, and hence, we do not need in our proposal any additional boost factor due to an overdense region in the halo or the Sommerfeld enhancement. Cross Section Just Above a Pole. In this study, we assume that the dark matter with mass m annihilates via a narrow resonance R. For a simplicity, we consider a scalar resonance, although generalization to arbitrary spins is straight forward. The general formula for the scattering cross section via a resonance R is given by σ = 16π E 2 cm ¯ βiβi M 2 2

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/f1297a22a5cc10953d1e4d0293426cf106a80d3c

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In Astronomy and Astrophysics

  1. F. Riva
  2. Jure Zupan
  3. A. Urbano
  4. M. Frigerio
  5. L. Lopez-Honorez
  6. Jonathan L. Feng
  7. T. Schwetz
  8. A. Pomarol

In Nuclear and High Energy Physics

  1. F. Riva
  2. A. Urbano
  3. M. Frigerio
  4. Jure Zupan
  5. A. Pomarol
  6. L. Lopez-Honorez
  7. Jonathan L. Feng
  8. T. Schwetz

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