At the particle detector LHCb, which forms part of the large accelerator ring Large Hadron Collider (LHC), particle physicists from the European Organization for Nuclear Research (CERN) have succeeded in detecting a tetraquark consisting of four identical quarks. That is a particle consisting of two so-called charm quarks and two charm antiquarks.
The researchers presented the discovery at a symposium and published it in an unreviewed preprint on arXiv on July 1.
They have described the findings in detail on the LHCb project website.
Read more: CERN: Large Hadron Collider gets stronger, better, brighter
What does it teach us?
Quarks are the smallest known elementary particles besides leptons and bosons. Knowing how quarks bind together to form larger particles is a step toward solving the huge jigsaw puzzle posed by the great mysteries of physics.
These include the question of what dark matter is and of how we can explain physical observations that do not seem to fit into the standard model of physics — and maybe there is even an answer to the big question: Is there a theory of everything?
Tetra- and pentaquarks are not completely new
Until a few years ago, it had only been certain that quarks typically combine in groups of two or three to form hadrons. But for decades, physicists had been using models to calculate that there must also be particles with four or five quarks.
Then, the first tetraquarks were discovered by Japanese and Chinese researchers in 2013. Researchers at CERN were able to confirm their existence a year later. However, these tetraquarks consisted of different quarks than those making up the particle that has now been found.
"Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type," Giovanni Passaleva, the outgoing spokesman for the LHCb project, explained. "Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type."
In 2017, incidentally, LHCb particle physicists also succeeded in finding a pentaquark. This is a particle consisting of four quarks and one antiquark.
Read more: Boson predictor Peter Higgs: A fundamentally modest physicist
What force holds the atoms together?
Pentaquarks and tetraquarks can help physicists to better understand one of the four basic forces of physics: the strong nuclear force that holds the atoms together.
It binds protons, neutrons and atomic nuclei, which make up the matter we know.
The discovery of the particle could help to develop models to "to explain the nature of ordinary matter particles, like protons or neutrons," Passaleva's successor Chris Parkes said.
Huge crash test center for the tiniest particles
At CERN, researchers are discovering the new particles — including the Higgs boson, whose discovery in 2012 caused a sensation in the science world — by colliding particles at almost the speed of light in a strong magnetic field in a huge ring-shaped accelerator. In the process, they disintegrate into their individual parts.
Using gigantic detectors — similar to the photosensors of a digital camera — the physicists then record in which direction and how far apart the fragments of the atoms fly. From this data, they can reconstruct the properties of the elementary particles.
Read more: The KATRIN Tritium Neutrino experiment: A giant scale for the tiniest particles starts
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Huge digital cameras record tiny particles
Pictures of particles
The ALICE detector, owned by the European Organization for Nuclear Research (CERN) - is located more than 90 meters underneath this colorful building in Geneva. ALICE is a huge digital camera capable of photographing even the smallest building blocks of the universe - the components of an atom's nucleus.
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Huge digital cameras record tiny particles
Helmets required
In addition to ALICE, three other detector cameras, named ATLAS, CMS, and LHCb, keep a record of particle collisions at the LHC. To see them you have to go deep below the rock of the French and Swiss Alps.
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Huge digital cameras record tiny particles
Did puny particles follow the Big Bang?
When protons or lead ions smash together at the speed of light the smallest elementary particles are released - and this is what it looks like to the CMS detector. Scientists believe our universe was created from such particles in the first billionth of a second after the Big Bang.
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Huge digital cameras record tiny particles
On track at high speed
This is where lead ions and hydrogen protons are accelerated. They fly through a vacuum tube with the energy of a speeding train and are kept on track by massive electromagnets. The pipe has a circumference of 27 kilometers and can be accessed through the four large detectors where the particle collisions take place.
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Huge digital cameras record tiny particles
The world’s largest fridge
The electromagnets that keep the particle beam on track are made of superconducting inductors. The cables must be kept at a chilly minus 271.3 degrees Celsius (minus 456 Fahrenheit) so they no longer have any electrical resistance. To cool them down, the collider sends a whole lot of liquid helium through the pipes.
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Huge digital cameras record tiny particles
Precise magnets
The LHC is not a perfect circle but instead consists of long straight stretches interrupted by curves, where magnets redirect the beam. The electromagnets are extremely precise. Just before a collision they focus the beam in exactly the angle so that the probability of two particles colliding is very high. The clash then happens right in the middle of the detector.
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Huge digital cameras record tiny particles
Built like a ship in a bottle
The detectors are as big as multi-level houses. But they all had to be brought into the mountain in smaller parts through narrow shafts like this one. Underneath it is a gigantic cavern where ALICE was put together.
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Huge digital cameras record tiny particles
8,000 photos per second
This is the ALICE detector when it is opened for maintenance. When in operation, ion beams collide in its center. New particles are created, flying off in different directions through several layers of silicon chips, similar to the sensors of a digital camera. The chips and other detectors record the particles' routes. ALICE can capture 1.25 gigabytes of digital data each second.
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Huge digital cameras record tiny particles
Electromagnets make particles identifiable
This blue chunk is another huge electromagnet, an important part of the ALICE detector. It creates a field making it possible to identify particles that are created during the high-speed collisions. Scientists study the direction the new particles travel. For instance, they can determine whether particles were neutral or positively or negatively charged.
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Huge digital cameras record tiny particles
Wings to catch a muon
The ATLAS detector has a special gauge, the so-called muon spectrometer, which lies outside the detector’s heart, just like large wings. With these wings a heavy relative of the electrons - the muon - can be caught. Muons are difficult to find because they only exist for two millionths of a second.
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Huge digital cameras record tiny particles
Watching from a safe distance
All detectors have a control room, just like this one for ATLAS. Once the collider is in operation, no one is allowed to stay inside the underground facilities. An out of control proton beam can melt 500 kilograms of copper and escaped helium could cause frostbite and suffocation. The particle stream could even create radioactivity.
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Huge digital cameras record tiny particles
What to do with the data?
The detectors deliver data 40 million times per second. But because not all collisions are interesting for scientists, the data has to be filtered. In the end, no more than 100 interesting particle collisions per second remain. That’s still more than 700 megabytes of data per second - about what fits on a commercial CD. All data initially lands here in CERN’s data processing center.
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Huge digital cameras record tiny particles
A global computer network
CERN produces an amount of data per year that if it were stored on CD, the pile would be 20 kilometers high. Even though such a tape library can hold a lot of data, it is still not enough. So the data are distributed worldwide. More than 200 universities and research institutes have created a worldwide CERN computer network with their data processing centers.
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Huge digital cameras record tiny particles
Data for everyone
Particle physicists from around the world have access to CERN data. The center sees itself as a service provider for universities and institutes conducting basic research. A common project for everyone's benefit.
Author: Fabian Schmidt / asb
