Unfortunately, experimental detection of Hawking radiation from real, massive astrophysical black holes seems impossible, since the corresponding temperature, as seen by an outside observer, is tiny, e.g. 10^{-7} Kelvin for a solar mass black hole. For comparison, the temperature of the cosmic microwave background photons—the relic radiation from the big bang—is 2.7 Kelvin and thus 10 million times larger.

This is different for hypothetical microscopic black holes: they would evaporate within a very short time in a particle radiation flash. According to the Standard Model of particle physics, such microscopic black holes can be produced in principle in particle scattering experiments with center-of-mass energies of order 10^{19} GeV, about 15 orders of magnitude larger than what will be available at the CERN's Large Hadron Collider (LHC) in around 2007.

Recently, however, it was pointed out that in extensions of the Standard Model of particle physics, which involve, in addition to the usual three spatial dimensions, a number of extra spatial dimensions in which gravity can act, above-microscopic black hole production may occur copiously already at center-of-mass energies available at the LHC or in cosmic ray and neutrino interactions.

Therefore, if these extensions of the Standard Model are right, they open a window to study black hole production and evaporation experimentally for the first time and within the next decade.

In May 2001, my graduate student Huitzu Tu started to work on her Ph.D. thesis, "Ultrahigh energy cosmic neutrinos and physics beyond the Standard Model," under my supervision. The intention of her thesis was and still is to explore the possibilities of studying exotic processes beyond the Standard Model at cosmic ray facilities such as the Pierre Auger Observatory and neutrino telescopes such as AMANDA/IceCube, complementary to laboratory studies at the LHC.

The seminal papers of Dimopoulos & Landsberg and Giddings & Thomas on black hole production at the LHC appeared on the ArXiv in June 2001, the paper of Feng & Shapere on the detection possibility at the Pierre Auger Observatory for cosmic rays in September 2001. Huitzu and I were just perfectly prepared to also make a significant contribution in this field.

Apart from improving the expected event rate estimates in comparison to Feng & Shapere by taking into account more recent estimates of the flux of cosmic neutrinos impinging on the Earth's atmosphere, we presented the first limits on black hole production and thus on extensions of the Standard Model with extra spatial dimensions from the non-observation of black hole-initiated air showers by the Fly's Eye cosmic ray collaboration.

Even stronger constraints on black hole production arising from the non-observation of black hole-initiated air showers by the AGASA cosmic ray collaboration have been worked out (Anchordoqui, Feng, Goldberg, and Shapere).

Recently, we have derived strict lower bounds on the cosmic neutrino flux (Fodor, Katz, A.R., Tu) which are presently exploited to derive very robust cosmic ray constraints to black hole production (Anchordoqui, Fodor, Katz, A.R., Tu, in preparation).

In 10 years, we should know whether such extensions of the Standard Model with large extra spatial dimensions are realized in Nature, both from laboratory experiments (LHC) as well as from cosmic ray and neutrino observatories (Auger, IceCube).

Andreas Ringwald
DESY
Hamburg, Germany