New, higher precision data that could only have been gathered at the Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Lab) shows the Theta-plus pentaquark doesn’t appear in another place it was expected. This intriguing finding contradicts evidence previously presented by Jefferson Lab researchers that they had sighted a pentaquark, a particle built of five quarks. Volker Burkert, a Jefferson Lab Experimental Hall Leader, will present this preliminary result in a talk reviewing world pentaquark data at Lepton-Photon, the XXII International Symposium on Lepton-Photon Interactions at High Energy, in Uppsala, Sweden, on Friday, July 1.
Researchers sent photons into deuterium nuclei to try to produce pentaquarks. If pentaquarks had been produced, sensitive detectors would have measured a particular mix of Kaons (K-mesons) and protons; neutrons could have been inferred from the data. The researchers did not detect this reaction. Image credit: JLab
The result comes from a very carefully crafted experiment that was designed to repeat Jefferson Lab’s original pentaquark search with a factor of ten higher statistics. Researchers in Jefferson Lab’s CEBAF Large Acceptance Spectrometer (CLAS) collaboration took data with a high-energy photon beam on a deuterium target March 13 – May 16, 2004. Deuterium is an isotope of hydrogen with one proton and one neutron in its nucleus. An earlier probe of this same region by CLAS revealed a possible signal for a pentaquark with mass 1542 MeV.
The new experiment searched for pentaquarks in this same channel at a level of precision at least 10 times higher, or one order of magnitude better, than the previous published result and found no pentaquarks. “The earlier results on the Theta-plus can not be reproduced in the analysis of the high-statistics run,” Burkert says.
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