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Unformatted text preview: only for s-channel processes mediated by W exchange, dðl e ! l eÞ 2me G2 E 1 F ¼ 2 dy  ð1 þ 2me E y=MZ Þ2  ½g2 þ g2 ð1 À yÞ2 Š; L R dðl e ! l eÞ 2me G2 E " " 1 F ¼ 2 dy  ð1 þ 2me E y=MZ Þ2  ½g2 þ g2 ð1 À yÞ2 Š: R L When compared to that of neutrino-nucleon scattering or even nonresonant neutrino-lepton scattering, e scattering " dominates. Such high cross sections can often cause the Earth to be opaque to neutrinos in certain energy regimes and depart substantially from standard model predictions if new physics is present (Gandhi et al., 1996). A. Uncertainties and projections For a more accurate prediction of the cross section, a wellformulated model of the relevant quark structure functions is needed. This predictive power is especially important in the search for new physics. At such high energies, the neutrino cross section can depart substantially from the standard model prediction if new physics is at play. Study of such high-energy neutrinos can be a possible probe into new physics. Direct neutrino scattering measurements at such extreme energies are, of course, unavailable. Therefore, predictions rely heavily on the existing knowledge of parton distribution functions and, as the reader can imagine, extrapolation can introduce substantial uncertainties to these predictions. The best constraints on the relevant parton distribution functions stem from data collected from high-energy ep scattering experiments such as Hadron-Electron Ring Accelerator (HERA) (Chekanov et al., 2003). The challenge rests on the ability to fit existing data to as low values of x as possible. At high energies, the propagator term limits the maximum Q2 to the MW;Z mass. The relevant range for x then falls inversely with neutrino energy, x$ FIG. 29 (color online). Neutrino electron and nucleon scattering in the ultra-high-energy regime (E ! 104 GeV). Shown are the electron interactions  eÀ !  eÀ (crosses),  eÀ !  eÀ (dia" " " " " monds), e eÀ ! e eÀ (hollow circles), e eÀ !  eÀ (filled " circles), and the nucleon charged current (cross markers) and neutral-current (filled triangles) interactions. The leptonic W resonance channel is clearly evident (Butkevich et al., 1988; Gandhi et al., 1996). Rev. Mod. Phys., Vol. 84, No. 3, July–September 2012 (92) MW E (93) which, for EeV scales, implies x down to 10À8 or lower. The ZEUS Collaboration has recently extended their analysis of parton distribution function data down to x ’ 10À5 , allowing a more robust extrapolation of the neutrino cross section to higher energies (Cooper-Sarkar and Sarkar, 2008). Uncertainties in their parton distribution function translate into Æ4% uncertainties for the neutrino cross section for center-of-mass energy of 104 GeV and Æ14% uncertainties pffiffiffi at s ¼ 106 GeV. An equal factor in the precise evaluation of these cross sections is the selection of an adequate PDF its...
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This document was uploaded on 09/28/2013.

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