Spin and Parity Measurements of the Elusive Lambda(1405) Particle

First description of common particle’s properties provides insights into the nature of the universe.

Image courtesy of Jefferson Lab
The CEBAF Large Acceptance Spectrometer in Jefferson Lab’s Hall B was used to carry out the experiment.

The Science

First identified more than 50 years ago, the sub-atomic particle called Lambda(1405) was routinely seen in experiments, yet two of its key characteristics were too difficult to measure. For the first time, scientists measured these descriptors, intrinsic angular momentum and parity.

The Impact

The research describes fundamental characteristics of Lambda(1405), answering basic science questions about the internal structure of these particles and how they interact to ultimately provide a roadmap of matter that will help unlock the secrets of how the universe is put together.


The Lambda(1405) was first identified by its mass in experiments in 1962. However, many of the intrinsic properties of the elusive particle have remained a mystery for more than 50 years. Using data from an experiment conducted at Jefferson Lab for another purpose, scientists extracted the first determinations of the particle’s intrinsic angular momentum and parity, two quantities related to the particle’s spin. The intrinsic angular moment turned out to be one-half, a quantity it has in common with electrons and protons. The second property, parity, determines how a particle's spin behaves – whether the spin remains the same or whether it is flipped to the opposite state when the short-lived Lambda(1405) decays into other particles. The Lambda(1405) has negative parity, unlike the proton. The researchers are continuing to make the most of the data from this experiment, looking for other key findings that lie deep within the data used to make this discovery and in applying the analysis method developed for this particle to other experiments.


Reinhard Schumacher
Carnegie Mellon University


This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177 and work by Carnegie Mellon’s Medium Energy Physics group supported under DOE Grant No. DE-FG02-87ER40315. Further support was provided by the National Science Foundation, the United Kingdom’s Science and Technology Facilities Council, and the Italian Istituto Nazionale di Fisica Nucleare.


K. Moriya, et al. (CLAS Collaboration), "Spin and parity measurement of the Λ(1405) baryon.” Physical Review Letters 112, 082004 (2014). [DOI: 10.1103/PhysRevLett.112.082004]

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