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humanino
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NuTeV "anomaly" : evidence for in-medium nucleon modification
Dear HEP folks,
maybe some of you have missed it :
Isovector EMC Effect and the NuTeV Anomaly
Dear HEP folks,
maybe some of you have missed it :
Isovector EMC Effect and the NuTeV Anomaly
From http://www.jlab.org/news/releases/2009/NuTeV.html [Broken]A neutron or proton excess in nuclei leads to an isovector-vector mean field which, through its coupling to the quarks in a bound nucleon, implies a shift in the quark distributions with respect to the Bjorken scaling variable. We show that this result leads to an additional correction to the NuTeV measurement of sin2W. The sign of this correction is largely model independent and acts to reduce their result. Explicit calculation in nuclear matter within a covariant and confining Nambu–Jona-Lasinio model predicts that this vector field correction may account for a substantial fraction of the NuTeV anomaly. We are therefore led to offer a new interpretation of the NuTeV measurement, namely, that it provides further evidence for the medium modification of the bound nucleon wave function.
Experimenters at Fermilab's NuTeV (Neutrinos at the Tevatron) experiment sent a beam of neutrinos into a steel target and measured the ratio of two types of subatomic particles - neutrinos and muons - that emerged. They found that about one percent fewer neutrino-target collisions produced neutrinos than predicted by the Standard Model.
"Many people were convinced that they had discovered evidence for physics beyond the Standard Model," said Thomas.
He and his colleagues pored over the experimental information and began applying their theories for the EMC Effect to it. They found that one common assumption that was used in the analysis of the NuTeV data involved a correction for a natural imbalance in the number of protons and neutrons in the nucleus of iron, the most common element in NuTeV's steel target.
"The correction made for the extra neutrons involved a subtraction of the structure function of the extra neutrons," Cloët explained. "But according to our theoretical model of the EMC Effect, those extra neutrons generate a force that subtly changes the structure of every proton and neutron in the nucleus."
The theorists went further, combining this newly discovered effect with another correction for the difference in masses of different quarks in the protons and neutrons (charge symmetry violation). When they applied the two corrections to the NuTeV analysis, they found that the experiment showed excellent agreement with the Standard Model.
As a consequence, the NuTeV result may now be interpreted as providing crucial evidence for the idea that the structure of a proton or neutron is fundamentally modified when it is bound in a nucleus.
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