NuTeV anomaly helps shed light on physics of the nucleus

June 29, 2009

NEWPORT NEWS, VA, June 29, 2009 - A new calculation clarifies the complicated relationship between protons and neutrons in the atomic nucleus and offers a fascinating resolution of the famous NuTeV Anomaly.

The calculation, published in the journal Physical Review Letters on June 26, was carried out by a collaboration of researchers from the Department of Energy's Thomas Jefferson National Accelerator Facility, Tokai University and the University of Washington. It grew out of attempts to make sense of the complex environment found in the nucleus of the atom.

The calculation began with the EMC Effect, a famous result in the world of nuclear physics that showed that the structures of protons and neutrons in a nucleus are different from protons and neutrons found outside a nucleus.

"I was at CERN when the EMC Effect was discovered more than 20 years ago," said Anthony Thomas, Jefferson Lab Chief Scientist and an author on the paper. "It's such a fundamental piece of information about the structure of nuclei that I wanted to understand it."

Thomas and his colleagues, Ian Cloet, a JSA Thesis prize winner in 2008, and Wolfgang Bentz, a long term visitor at Jefferson Lab in 2008, had theorized that the internal structures of protons and neutrons are modified by the presence of other protons and neutrons inside the nucleus.

Meanwhile, another landmark result, the NuTeV Anamoly, provided Thomas and his colleagues with another puzzle regarding the nucleus. 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," Cloet 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.

"Now, the next thing is to carry out an experiment to test this explanation directly," Thomas said. "You can make measurements that directly test whether it's right or wrong."
-end-
This research was funded by the Office of Nuclear Physics within the Department of Energy's Office of Science. Jefferson Lab is managed and operated by Jefferson Science Associates, LLC, for the Department of Energy.

Link to the Physical Review Letters paper: http://link.aps.org/doi/10.1103/PhysRevLett.102.252301

Citation: I.C. Cloët, W. Bentz, and A.W. Thomas, Phys. Rev. Lett. 102, 252301 (2009)

DOE/Thomas Jefferson National Accelerator Facility

Related Neutrinos Articles from Brightsurf:

Big answers from tiny particles
A team of physicists led by Kanazawa University demonstrate a theoretical mechanism that would explain the tiny value for the mass of neutrinos and point out that key operators of the mechanism can be probed by current and future experiments.

Physicists cast doubt on neutrino theory
University of Cincinnati physicists, as part of an international research team, are raising doubts about the existence of an exotic subatomic particle that failed to show up in twin experiments.

Exotic neutrinos will be difficult to ferret out
An international team tracking the 'new physics' neutrinos has checked the data of all the relevant experiments associated with neutrino detections against Standard Model extensions proposed by theorists.

Excess neutrinos and missing gamma rays?
A new model points to the coronoe of supermassive black holes at the cores of active galaxies to help explain the excess neutrinos observed by the IceCube Neutrino Observatory.

Where neutrinos come from
Russian astrophysicists have come close to solving the mystery of where high-energy neutrinos come from in space.

Where did the antimatter go? Neutrinos shed promising new light
We live in a world of matter -- because matter overtook antimatter, though they were both created in equal amounts when our universe began.

Strongest evidence yet that neutrinos explain how the universe exists
New data throws more support behind the theory that neutrinos are the reason the universe is dominated by matter.

Why didn't the universe annihilate itself? Neutrinos may hold the answer
New results from an experiment called T2K suggest that physicists are closer than ever before to answering a major mystery: Why didn't the universe annihilate itself in a humungous burst of energy not long after the Big Bang?

T2K insight into the origin of the universe
Lancaster physicists working on the T2K major international experiment in Japan are closing in on the mystery of why there is so much matter in the universe, and so little antimatter.

Radar and ice could help detect an elusive subatomic particle
A new study published today in the journal Physical Review Letters shows, for the first time, an experiment that could detect a class of ultra-high-energy neutrinos using radar echoes.

Read More: Neutrinos News and Neutrinos Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.