08 October 2014
Two new particles have been discovered by the LHCb experiment at CERN’s Large Hadron Collider near Geneva, Switzerland. One of them has a combination of properties that has never been observed before.
The particles, named DS3*(2860)– and DS1*(2860)–, are about three times as massive as protons.
Physicists analyzed LHCb observations of an energy peak that had been spotted in 2006 by the BaBar experiment at Stanford University in California, but whose cause was still unknown.
“Our result shows that the BaBar peak is caused by two new particles,” says Tim Gershon of Warwick University, UK, lead author of the discovery.
The force is strong
Mesons are particles that contain two quarks – subatomic particles that make up matter and are thought to be indivisible. These quarks are bound together by the strong force, one of the four fundamental forces that also keeps the constituents of nuclei together within atoms. This force is one of the less well-understood parts of the standard model of particle physics, the incomplete theory that describes how particles interact.
The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
Significantly, DS3*(2860)– also has a spin value of 3, making this discovery the first ever observation of a spin-3 particle containing a charm quark.
In other mesons, the quarks can be configured in one of several different ways to give the particle an overall spin value less than three, and this makes the quarks’ exact properties ambiguous. However, for a spin value of three there is no such ambiguity, making DS3*(2860)–’s precise configuration clear.
Combined with the particle’s charm quark, this may make DS3*(2860)– a key player for exploring the strong force, because the calculations involved are more straightforward for heavy quarks than for lighter ones.
The LHCb team used a technique known as Dalitz plot analysis to untangle the data peak into its two components, a complex technique that had never before been used on LHC data.
The technique helps separate and visualise the different paths a particle can take as it decays. Now that it has been used successfully on the LHCb dataset, says Gershon, it can hopefully be applied to more LHC data to help discover further particles and understand how they are bound together.
“This is a lovely piece of experimental physics,” says Robert Jaffe of the Massachusetts Institute of Technology in Cambridge. “Although it doesn’t probe the limits of the standard model, it may shine light on the dynamics of quarks and gluons. The fact that LHCb was able to use Dalitz plot methods is a testimony to the quantity and high quality of the data they’ve accumulated. We can look forward to other similar discoveries in the future using this method.”
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