In general, within these scenarios additional contributions to particle interactions appear that are due to exchange of gravitons with the bulk (the extra dimensions). Certain scenarios try to solve the hierarchy problem of particle physics by relating the 4-dimensional Planck scale to a fundamental scale M in the TeV range in the higher dimensional space. In such scenarios the new cross section contribution can be as high as 4pi*s/M**4, where s is the squared center of mass energy. Lower limits on M which turn out to be in the interesting TeV range have been derived from additional cooling of type II supernovae due to additional energy loss by graviton radiation into the bulk. Ultrahigh energy neutrinos provide a complementary probe at much higher center of mass energies of 1.4 x 10**5 GeV (E/10^19 eV)**0.5, where E is the lab frame neutrino energy. Neutrino fluxes predicted from nucleons interacting with the cosmic microwave background together with the non-detection of deeply penetrating air showers already provide limits on the neutrino-nucleon cross section and upcoming experiments can improve these limits or provide signatures for new interactions, as discussed in this paper in collaboration with Craig Tyler and Angela Olinto (U of C).
In string theory scenarios the new fundamental scale M in the TeV range would probably be related to the string scale. In this context there is currently no agreement in the literature about the cross section behaviour above the string scale. Whereas amplitudes for single states are exponentially suppressed which can be interpreted as a result of the finite spatial extension of the string states, the number of states may grow exponentially. It is currently unclear and it may be model dependent which effect dominates the total cross section. Thus, the study of penetrating giant showers can probe the neutrino-nucleon cross section for center of mass energies in the TeV and above and could thus constrain string-inspired models of TeV scale extra dimensions.
The figure on the left shows typical neutrino interaction depths
relevant for future atmospheric as well as water/ice based
experiments within the Standard Model,
as compared to a new contribution of the form 4pi*s/M**4.
Ultra-high energy cosmic rays may also test supersymmetry.
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