Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a new discovery at CERN for our blog. A new Science Museum exhibition about the Large Hadron Collider will open in November 2013, showcasing particle detectors and the stories of scientific discoveries.
In 2003 physicists at the Belle experiment in Japan reported they had discovered a brand new particle.
Adding a new entry to the big book of particle physics is certainly satisfying, but not usually cause for much excitement. The discovery of the Higgs-like boson last year was an exception. After all, hundreds of particles have shown up in experiments over the last century. So many in fact, that they were often referred to, rather derisively, as a “zoo”.
But the particle found at Belle was different.
It didn’t fit neatly into the picture painted by theory and there was no clear explanation for its origin. It was a bit of an enigma, and earned a suitably enigmatic name: the X particle.
Professor Val Gibson from the University of Cambridge told me that she and her colleagues “have been mesmerized” about the identify this mysterious particle for the last ten years.
The Particle Zoo
The vast majority of the particles that make up the particle zoo are not fundamental; in other words they are made up of smaller things and these things are fundamental particles called quarks. Six different types of quark have been discovered and they can form a large number of different combinations, explaining the particle zoo.
However, quarks only bind together in very specific ways. Two ways in fact. One option is a ménage à trois known as a baryon. Baryons include the proton and the neutron, the building blocks of the atomic nucleus. The other option is where a quark and an antiquark couple up to form a meson.
The X didn’t fit easily into either of these pictures. This generated a lot of excitement and there was speculation as to whether it could be an ordinary meson, or some new exotic combination involving four quarks, a tetraquark, or a “molecule” of two mesons stuck together.
If this were true it would be the first time such an exotic state had been definitively seen in nature.
The only way to tell would be to measure the quantum numbers of the X, three properties that give a clue to its internal structure. This hadn’t been possible, until now.
Exciting, Exotic X
Amid the hundreds of trillions of collisions generated by the Large Hadron Collider over the past three years physicists at the LHCb experiment (the experiment I work on) managed to pick out about 300 X particles.
This week, they presented the first full measurement of the quantum numbers of the X, at a conference at La Thuile in Italy. The result was emphatic – the X is not a meson, it is something altogether more exotic.
LHCb physicist Dr Matt Needham told me that “this measurement is a great step forward in understanding this mysterious X” and a “very exciting result”. However, there is still work to be done.
“The real nature (of the X) is still unclear”. Whether it’s a tetraquark, meson molecule or something else entirely must now be determined.
His colleagues at LHCb will now search for signs of the X decaying in new ways to try to separate out the various different options. Although the Large Hadron Collider has now shut down for two years physicists at LHCb will have no shortage of data to work with. An unprecedented sample was collected during 2012, corresponding to 180 trillion collisions, each one producing hundreds of particles.
The true nature of this enigmatic particle may soon be known. Whatever the result, we have now had our first glimpse of an altogether new state of matter. Finding out exactly what the X is will bring us deeper understanding of nature’s fundamental building blocks and the forces that bind them together.
Visitors to the Science Museum will have a chance to get up close and personal with the LHC at a new exhibition opening in November 2013. The exhibition will showcase real pieces of the LHC, including an intricate particle detector from the heart of the LHCb experiment.