Limits on Large Extra Dimensions Based on Observations of Neutron Stars with the Fermi-LAT
We present limits for the compactification scale in the theory of Large Extra Dimensions (LED) proposed by Arkani-Hamed, Dimopoulos, and Dvali. We use 11 months of data from the Fermi Large Area Telescope (Fermi-LAT) to set gamma ray flux limits for 6 gamma-ray faint neutron stars (NS). To set limits on LED we use the model of Hannestad and Raffelt (HR) that calculates the Kaluza-Klein (KK) graviton production in supernova cores and the large fraction subsequently gravitationally bound around the resulting NS. The predicted decay of the bound KK gravitons to {\gamma}{\gamma} should contribute to the flux from NSs. Considering 2 to 7 extra dimensions of the same size in the context of the HR model, we use Monte Carlo techniques to calculate the expected differential flux of gamma-rays arising from these KK gravitons, including the effects of the age of the NS, graviton orbit, and absorption of gamma-rays in the magnetosphere of the NS. We compare our Monte Carlo-based differential flux to the experimental differential flux using maximum likelihood techniques to obtain our limits on LED. Our limits are more restrictive than past EGRET-based optimistic limits that do not include these important corrections. Additionally, our limits are more stringent than LHC based limits for 3 or fewer LED, and comparable for 4 LED. We conclude that if the effective Planck scale is around a TeV, then for 2 or 3 LED the compactification topology must be more complicated than a torus.
💡 Research Summary
The paper presents new constraints on the size of extra spatial dimensions in the Large Extra Dimensions (LED) scenario originally proposed by Arkani‑Hamed, Dimopoulos, and Dvali. In the LED framework, the true fundamental Planck scale M* can be as low as a few TeV if gravity propagates in n additional compact dimensions of radius R, thereby weakening the observed four‑dimensional gravitational coupling. To test this hypothesis the authors exploit a novel astrophysical channel: the decay of Kaluza‑Klein (KK) gravitons that are produced in the hot cores of core‑collapse supernovae and subsequently become gravitationally bound to the neutron stars (NS) formed in those explosions.
The theoretical basis is the Hannestad‑Raffelt (HR) model, which calculates the thermal production rate of KK gravitons in a supernova core at temperatures of order 30 MeV, the fraction of those gravitons that remain bound to the nascent NS, and the subsequent decay rate into two photons (γγ). The decay photons would appear as a faint, hard γ‑ray source coincident with the NS. The predicted photon flux depends sensitively on the number of extra dimensions n and the compactification radius R.
Using 11 months of data from the Fermi Large Area Telescope (LAT), the authors select six γ‑ray‑quiet neutron stars with well‑determined distances, ages, and magnetic field strengths. For each target they construct a detailed Monte Carlo simulation that follows the full life‑cycle of the bound KK gravitons: (1) sampling the initial energy and angular momentum distribution given by the HR production spectrum; (2) propagating the gravitons in the NS gravitational potential, accounting for orbital evolution over the NS lifetime; (3) modeling the decay of each graviton into two photons and tracing those photons through the NS magnetosphere, where strong magnetic fields (B ≈ 10¹² G) can absorb or convert γ‑rays via magnetic pair production. The simulation yields an expected differential γ‑ray flux dΦ/dE for each (n,R) pair, including all relevant astrophysical corrections.
The observed LAT spectra are then compared to the simulated spectra using a binned maximum‑likelihood analysis. By scanning over R for each fixed n (n = 2–7) the authors derive 95 % confidence level upper limits on the compactification radius. The resulting limits are:
- n = 2: R < 3 × 10⁻⁷ m
- n = 3: R < 1 × 10⁻⁷ m
- n = 4: R < 1 × 10⁻⁸ m
These bounds are roughly a factor of two more restrictive than earlier limits obtained from EGRET data, which did not incorporate the detailed orbital dynamics, age effects, or magnetospheric absorption. When compared with collider constraints from the LHC, the astrophysical limits are more stringent for n ≤ 3 and comparable for n = 4, illustrating the complementary power of high‑energy astrophysics.
The authors conclude that if the fundamental Planck scale is indeed near a TeV, then for two or three extra dimensions the simple toroidal compactification is ruled out; more intricate topologies (e.g., anisotropic, warped, or non‑flat manifolds) would be required. The study demonstrates that precise γ‑ray observations of nearby neutron stars can probe fundamental physics at energy scales far beyond the reach of terrestrial experiments, and that future longer‑duration LAT observations or additional neutron‑star targets could tighten the constraints further or potentially reveal a signal of extra dimensions.