New strange and charming particles

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 (arxiv.org/abs/1407.7574 and arxiv.org/abs/1407.7712).
The unique properties of one of these particles could help us explore the strong force”
The distinctive characteristics of these particles should help us find out more about a
fundamental force of nature known as the strong force. Back in 2006, physicists at the
BaBar experiment in Stanford University in California spotted an energy peak given off when
smashing electrons and positrons together, which might indicate the presence of a particle. A team
at the LHC found a similar peak in Flames that never die traced to their source IN SOUTHERN Turkey, there are fires that never go out. The flames have
been alight for millennia, but the source of the methane that fuels them was a mystery – until now.
The seeping gas feeds dozens of half-metre-high flames at the site, called Yanartas, Turkish for “flaming stone”. The flames are believed to
have inspired Homer to create the fire-breathing Chimera in his Illiad.
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CERN’s data, and have been trying to pinpoint the cause. “Our result shows that the BaBar peak is caused by two new particles,” says Tim Gershon of Warwick University, UK, lead
author of the LHC work. The pair are both mesons, and so contain two quarks – subatomic
particles that make up matter and are thought to be indivisible. These quarks are bound together
by the strong force. Quarks come in six different flavours known as up, down,
strange, charm, bottom and top, in order from lightest to heaviest. The new particles each contain one charm antiquark and one strange quark. Quarks also have a property
called spin, which can be +1/2 or -1/2. When they come together to form mesons, there is an extra
spinning effect due to the exact arrangement of the quarks. This adds to the individual spins to give
a “total angular momentum”. Different quark arrangements can give the same total angular
momentum, so a particle’s exact configuration is often ambiguous. However, DS3*(2860) – is a special case: its total angular momentum is three, a value for which there is no ambiguity and so it’s clear how the quarks are arranged. This is the first particle containing a charm quark seen to have such properties. The unique characteristics of this particle could help us explore
the strong force, one of the four fundamental forces along with gravity, electromagnetism and
the weak force. This is partly because the calculations involved are more straightforward for
heavy quarks than lighter ones. The LHCb team used a method known as Dalitz plot analysis,
which had never before been used on LHC data, to untangle the peak into its two components. This 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 may be
applied to more LHC data. “This is a lovely piece of experimental physics,” says Robert Jaffe of the Massachusetts Institute of Technology. “The fact that LHCb was able to use Dalitz
plot methods is a testimony to the quantity and high quality of the data they have accumulated. We
can look forward to other similar discoveries in the future.” ■