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The Xi-cc++ Particle

CERN’s LHCb collaboration has announced the discovery of a new “charming” particle, thought to be instrumental to the strong force – the Xi-cc++.  Another particle.  So…?

They always discover particles at CERN Large Hadron Collider, you may think…

Exotic things.  Like the pentaquark for example.

So, what’s really new?

 

The Matter of Elementary of Particles

The discovery, announced by the LHCb collaboration at the European Physical Society Conference on High Energy Physics (EPS-HEP) in Venice on 6 July 2017, will help researchers learn more about the so-called “strong force” which holds atomic nuclei together.

The existence of the Xi-cc++ particle (Ξcc++, pronounced Ksī-CC plus-plus) was theoretically predicted by the Standard Model.

But it is the first time it has been identified.

This result is based on 13 TeV data  recorded during run 2 at the Large Hadron Collider, and confirmed using 8 TeV data from run 1.

The LHCb collaboration submitted a paper in the Physical Review Letters reporting these findings.

 

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Baryonic Matter

 

Baryonic matter is made up with three quarks: Up, Down and Strange. Source: AnimatedPhysics.com

Nearly all matter in and around us is made of molecules, atoms and elementary particles.

We ARE made up of elementary particles.

Baryonic matter is “normal” day-to-day ordinary matter: electrons, neutrons and protons…

 

Neutrons and protons form the centres of atoms.  They are made up of quarks.

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Many Flavours of Quarks

Six different types of quarks combine to form other kinds of heavier particles.

They have somewhat unusual names: “Up”, “Down”, “Strange”, “Charming”, “Top” and “Bottom”.  Particle Physics for you…

Quarks can be either light or heavy.

Neutrons and protons are made up of three types of quarks.

 

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Quarks and their Known Properties

 

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Those inside neutrons and protons are called “Up” and “Down”.  These quarks are held together by the nuclear strong force.

The particles that have been detected so far contain at the most, one heavy quark.

The Xi-cc++ baryon contains two.

 

A graphic representation of the Xi-cc++ baryon – the new particle observed by LHCb.  It contains two charm quarks and one up quark.  Image: Daniel Dominguez/CERN

Properties of the Xi-cc++ Baryon

Physicists have been looking for such baryons with two heavy quarks for many years.  For the first time, researchers have unambiguously detected the particle at LHCb, and confirmed the existence of a particle with two heavy quarks.

Two stars of similar mass are in circular orbits about their centre of mass. Image: Zhatt

Unlike other particles of this type, in which three quarks follow elaborate paths around each other, a particle with two heavy quarks is expected to act like a planetary system.

The motion of those heavy quarks may be compared to that of stars in a binary system, each one orbiting around the other, with the lighter quark orbiting around it.

The mass of the newly identified particle is about 3621 MeV, which is almost four times heavier than the most familiar baryon – the proton

This unusual property arises from its doubly-charmed quark content, which gives it two positive charges.

 

The Strong Force

The study was carried out at the LHCb experiment by Dr Patrick Spradlin and his colleague, Prof Paul Soler, from Glasgow University.  Their discovery will probably shed new light on the longstanding puzzle that is the strong nuclear force.

The strong force is one of the four fundamental forces in Nature.  It acts between the protons and neutrons.  Neutrons and protons, both nucleons, are affected by the nuclear force almost identically.

Since protons have a positive charge e+, they experience an electromagnetic force that tends to push them apart.  At short range, however, the attractive nuclear force is strong enough to overcome the electromagnetic force.

The strong force binds nucleons into atomic nuclei.

 

Physicists have a theory called quantum chromo-dynamics for how the strong force works.   QCD is a type of quantum field theory.

However, using it to make predictions requires very complex calculations.

 

Looking for More Double-Heavy Particles

Research will now be carried out to measure the properties of the Xi-cc++ and establish how this new arrangement of quarks behaves.

Measuring the properties of the Ξcc++ particle will help to establish how a system of two heavy quarks and a light quark behaves.  Important insights can be obtained by measuring production and decay mechanisms precisely, as well as the lifetime of the new particle.

The observation of this new baryon proved to be challenging, But it was made possible owing to the high production rate of heavy quarks at the LHC.  It is also the result of the unique capabilities of the LHCb experiment, which can identify decay products with excellent efficiency.

The Ξcc++ baryon was identified via its decay into a Λc+ baryon and three lighter mesons K, π+and π+.

Ξcc++ → Λc+ + K + 2π+

 

The observation of the Ξcc++ in LHCb raises the expectations to detect other representatives of the family of doubly-heavy baryons.  Researchers will now be searching for more double-heavy quark particles at the LHC.

The team will also aim to determine how the strong force holds the system together.

The discovery will herald the beginning of an exciting new field of investigation.

We have just opened a new frontier in understanding the strong nuclear force…