Tag Archives: quark gluon plasma

Quark-Gluon Plasma that filled the early Universe investigated by ALICE

The secrets of the Quark-Gluon Plasma that filled the early universe are being unlocked by the ALICE collaboration at CERN with the first measurement of the flow of bottomonium particles.

Quark-Gluon Plasma that filled the early Universe investigated by ALICE (Author's Collection)

Quark-Gluon Plasma that filled the early Universe investigated by ALICE (Author’s Collection)

 

In a paper presented at the European Physical Society’s conference on high-energy physics, the ‘A Large Ion Collider Experiment’ (ALICE) collaboration has documented the first-ever measurement of the flow of a heavy meson particle — bottomonium.

The measurement of particles like bottomonium — a type of ‘upsilon’ particle — helps the researchers understand the Quark-Gluon Plasma (QGP) that filled the hot, dense early universe. 

By observing pairs of ‘heavy electrons’ — known as muons — produced by the decay of bottomonium, the team discovered bottomonium particles have small values of elliptic flow — a measure of how uniform energy and momentum is distributed across the particles when viewed from the beamline. 

This is quite unexpected as all other hadrons investigated thus far have exhibited significant elliptic flow. 

David Evans, a professor of high energy physics at the University of Birmingham, leads the UK participation in ALICE.

He says: “ Elliptical flow measurements in ALICE show that the Quark-Gluon Plasma flows like an almost perfect liquid, with the light quarks (up, down, strange, and charm) flowing with the system.”

“The fact that no significant elliptical flow is seen for the bottomonium suggests b-quarks are only produced in the initial collision of the lead ions, before the QGP is formed.”

The ALICE team’s results seem to support existing theories that bottonium and other upsilon particles split into their constituents during the early stages of their interactions with plasma.

This gives the researchers a better insight at the conditions in the early moments of the universe when it was filled with a plasma composed of free quarks and gluons. 

Evans continues: “This makes b-quarks (and particles made up of b-quarks) an ideal probe for studying the QGP as they experience the entire evolution of the system.”

Bottomonium particles — probing the Quark-Gluon Plasma at the dawn of the universe

Taking a trip through the ‘particle zoo’ to discover what these bottomonium particles actually are, helps us understand their role in the ALICE experiment. 

The six known types of quarks on the left and how they come together to form protons and neutrons on the right
The six known types of quarks on the left and how they come together to form protons and neutrons on the right

Bottonium is a heavy meson — particles which consist of a quark and its own antiparticle. In the case of bottomonium, a bottom (or beauty quark) — b quark — and its antiparticle counterpart.

These subatomic particles are extremely unstable, existing for short periods of time and only at high energies before decaying into other particles. When bottomonium decays it leaves behind a pair of ‘heavy electrons’ called muons. 

Bottomonium particles — which are formed in the LHC by the violent collision of heavy lead-lead ions — provide an excellent probe of the Quark-Gluon Plasma which filled the universe just a few millionths of a second after the big bang. 

Being produced so early in the collision event means bottomonium particles ‘experience’ the entire evolution of the plasma — from the moment it is produced to the moment it cools down and enters a state in which hadrons can form.

This extremely early stage in the universe’s evolution would have been the only time in history that quarks and gluons existed freely in plasma and not bound together in a state called ‘confinement’ in protons, neutrons and other hadrons.

These particles were only able to exist in this free state because of the incredible heat in the universe at this point. In our era, quarks and gluons are never observed as free particles.

Thus, it takes a tremendous amount of energy, to recreate these huge temperatures. At the moment, the Large Hadron Collider (LHC) is the only piece of apparatus on Earth capable of doing this, with collisions in the LHC able to generate temperatures a 100 thousand times hotter than the Sun. 

At these temperatures, protons and neutrons ‘melt’, freeing quarks and gluons from confinement, thus creating a Quark-Gluon Plasma and allowing them to form short-lived, unstable particles like bottonium. 

ALICE: Collision queen

The ALICE collaboration — consisting of over a 1000 scientists operating a 10-thousand-tonne, 16m tall detector buried 56m under the Alps — achieves this high-energy feat by slamming together beams of lead ions rather than the proton-proton collisions used in other LHC experiments. 

The decay of upsilon particles — another type of meson — into muon pairs as the result of a lead-lead collision. The red lines track the two muons the orange lines track the other particles produced (CERN)
The decay of upsilon particles — another type of meson — into muon pairs as the result of a lead-lead collision. The red lines track the two muons the orange lines track the other particles produced (CERN)

The benefit of colliding lead ions is that, as ALICE is looking to create Quark-Gluon Plasma, the more quarks available to it, the better the chance of observing something significant. 

A single proton and neutron each contain three quarks but as lead ions contain at least 56 protons and at least 204 neutrons this leaves the team with far more quarks to play with. 

ALICE then measures this plasma as it expands and cools, but it is still unable to measure the particles created directly— instead deducing the presence and properties of QGP from the signatures on pairs of muons it produced by decay. 

One of these signatures is the elliptic flow — the collective movement of the produced particles determined by several factors like particle type, mass, the angle at which the particles meet and the momentum they possess as they collide —  which is what the team measured. The flow is created by the expansion of hot plasma after the collision of the lead ions.

Upgrading ALICE: More collisions. More Quarks. More Results

Perhaps the most promising thing about ALICE’s mission to probe the early universe and its conditions is the fact that the forthcoming high-luminosity upgrade only promises to yield more data for researchers to investigate.

The ALICE Experiment is about to be revitalised (CERN)

The upgrade — which CERN hopes will be operational by 2021— aims to increase the luminosity of the LHC by a factor of 10. Luminosity, as described in reference to particle accelerators, is proportional to the number of collisions that occur. Thus increasing the luminosity also means increasing the number of collisions.

As an example of the usefulness of this upgrade, whereas the LHC produced 3 million Higgs boson particles in 2017, the High-Luminosity LHC is expected to produce 15 million per year.

In addition to the High-Luminosity upgrade — on which work began in 2018 — ALICE will also several other upgrades and improvements. As a result of these improvements, the ALICE team expect an overall gain of 100 times the current results.

Evans says: “With this huge increase in statistics and a new inner detector in ALICE, we will be able to measure particles made of b-quarks with much higher precision.”

As such, the experiment stands a very good chance of significantly improving our knowledge of the quark-gluon plasma and the conditions in the early universe, with these new bottomonium results pointing the way.

Evans concludes: “[The upgrades] allow us to probe the properties of the QGP in much more detail and hence learn more about the evolution of the early universe.”

A three-dimensional view of a p-Pb collision that produced collective flow behavior. The green lines are the trajectories of the sub-atomic particles produced by the collision reconstructed by the CMS tracking system. The red and blue bars represent the energy measured by the instrument's two sets of calorimeters. (CMS Collaboration)

Smallest liquid droplets created at LHC are 100,000th the size of a hydrogen atom

Scientists closely working with the  Large Hadron Collider, the largest and most powerful particle accelerator in the world, have identified evidence of the minuscule droplets produced in the aftermath of high energy proton and lead ions collisions. If their calculations are right, then these are the smallest droplets of liquid ever encountered thus far, just three to five protons in size. That’s about one-100,000th the size of a hydrogen atom or one-100,000,000th the size of a virus. WOW!

“With this discovery, we seem to be seeing the very origin of collective behavior,” said  Julia Velkovska, professor of physics at Vanderbilt who serves as a co-convener of the heavy ion program of the CMS detector, the LHC instrument that made the unexpected discovery. “Regardless of the material that we are using, collisions have to be violent enough to produce about 50 sub-atomic particles before we begin to see collective, flow-like behavior.”

A three-dimensional view of a p-Pb collision that produced collective flow behavior. The green lines are the trajectories of the sub-atomic particles produced by the collision reconstructed by the CMS tracking system. The red and blue bars represent the energy measured by the instrument's two sets of calorimeters. (CMS Collaboration)

A three-dimensional view of a p-Pb collision that produced collective flow behavior. The green lines are the trajectories of the sub-atomic particles produced by the collision reconstructed by the CMS tracking system. The red and blue bars represent the energy measured by the instrument’s two sets of calorimeters. (CMS Collaboration)

These tiny droplets “flow” in a manner similar to the behavior of the quark-gluon plasma, a state of matter that is a mixture of the sub-atomic particles that makes up protons and neutrons and only exists at extreme temperatures and densities. Some scientists claim that at the very dawn of the Universe’s existence shortly after the big bang, this primordial cosmic goo was everywhere, because of much higher temperature and density conditions.

These interactions weren’t actually targeted for observation by the LHC researchers, though. Scientists were looking to check the validity of their lead-lead results, and scheduled a proton-lead ion collision for as a simply control run – they ended up with quark-gluon plasma in the process.

“The proton-lead collisions are something like shooting a bullet through an apple while lead-lead collisions are more like smashing two apples together: A lot more energy is released in the latter,” said Velkovska.

Indeed, last September LHC researchers found that in five percent of the  protons and lead nuclei collisions —those that were the most violent – evidence of collective behavior was encountered. In turn, this allowed for the formation of   liquid droplets about one tenth the size of those produced by the lead-lead or gold-gold collisions.  The data gathered then, however, wasn’t enough to discount the influence of particle jets. New experiments in January and February of this year resulted in hundreds of cases where the collisions produced more than 300 particles flowing together.

According to doctoral student Shengquan Tuo, who recently presented the new results at a workshop held in the European Centre for Theoretical Studies in Nuclear Physics and Related Areas in Trento, Italy, only two models were advanced to explain their observations at the workshop. Of the two, the plasma droplet model seems to fit the observations best.

The new observations are contained in a paper submitted by the CMS collaboration to the journal Physics Letters B and posted on the arXiv preprint server.

[source]

Densest material ever created announced at LHC

The Large Hadron Collider (LHC) continues on its quest to find out exactly what happened in the first seconds after the Big Bang, unveiling what is the densest material known so far to man.

Exotic densest substance

Known as the quark-gluon plasma, this amazing exotic substance can exist only at incredibly high temperatures or pressures, and it consists almost entirely of free quarks and gluons; it is possible that the whole universe was filled only with this substance in the immediate aftermath of the Big Bang.

It is about 100 times hotter than the inside of the Sun and denser than a neutron star.

“Besides black holes, there’s nothing denser than what we’re creating,” said David Evans, a physicist at the University of Birmingham in the U.K. and a team leader for the LHC’s ALICE detector, which helped observe the quark-gluon plasma. “If you had a cubic centimeter of this stuff, it would weigh 40 billion tons.”

Quark gluon plasma acts like perfect liquid

By unleashing the fantastic energy of thousands of ultrahigh-speed collisions each second physicists at the LHC are breaking subatomic particles into even denser and hotter forms of matter, hoping to find out what the universe was made of right after the Big Bang; to be more exact, one trillionth of a second after it.

The quark-gluon plasma was also created last year by smashing together lead ions that have been stripped of all their electrons, at a speed very close to the speed of light. As I told you before (and as the name suggests), the substance is made almost entirely out of quarks and gluons. Quarks are almost never found in isolation, due to a process called color confinement; they are the building blocks of particles called hadrons, out of which the most ‘famous’ and stable are protons and neutrons. Gluons are the exchange particles (or gauge bosons) for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles. The quark-gluon plasma is what is called a perfect liquid.

“If you stir a cup of tea with a spoon and then take the spoon out, the tea stirs for a while and then it stops. If you had a perfect liquid and you stirred it, it would carry on going around forever,” Evans explained.

Meanwhile, the LHC is still going at only half of its potential, which can only make you wonder about what they will find once they turn out the full engines.