Standard Model of Particle Physics

The LHCb collaboration has added four new exotic particles to the growing list of hadrons discovered so far at the LHC. The team announces the discovery of two tetraquarks with new quark content (cc̄us̄) in a paper published on the arXiv preprint server: a broader one, Zcs(4220)+ and a narrow one, Zcs(4000)+. X(4685) and X(4630), two new tetraquarks with a quark content of ccss(Common Core State Standards), were also discovered.

Since it began colliding protons – particles that make up the atomic nucleus along with neutrons – in 2009, the LHC has discovered a total of 59 new particles, in addition to the Nobel Prize-winning Higgs boson. Although some of these new particles were predicted, others were unexpected.

The LHC’s mission is to investigate the structure of matter at the shortest distances and highest energies ever measured in a laboratory, putting our current best theory of existence, the Standard Model of Particle Physics, to the test. And the LHC has delivered – it has allowed scientists to discover the Higgs boson, the model’s final missing component. Nonetheless, the principle is still not well known.

Its description of the strong force that holds the atomic nucleus together is one of its most perplexing features. Protons and neutrons, which are each made up of three tiny particles called quarks, make up the nucleus (there are six different kinds of quarks: up, down, charm, strange, top, and bottom). All matter would disintegrate into a soup of loose quarks if we turned off the strong force for a second – a condition that occurred for a brief moment at the beginning of the universe.

Over the last century, the standard model of particle physics has gradually been built, and this standard model of particle physics defines the fundamental particles and their interactions. The theory has been incredibly effective, accurately explaining almost all scientific data with just a few minor inconsistencies. 

In the Standard Model, there are two types of particles: The fermions, which have half-integer spins (±½, ±1½, ±2½, etc.) and where every fermion has an antimatter (anti-fermion) counterpart. The other type is the bosons, which have integer spins (0, ±1, ±2, etc.) and are neither antimatter nor matter. The bosons are exactly what they are: 1 boson (photon) for the electromagnetic force, 1 Higgs boson, 8 gluons for the strong force, and 3 bosons (W+, W- and Z) for the weak force.

The force-carrying particles that enable the fermions to interact are the bosons. The fermions (and anti-fermions) have fundamental charges that determine the forces (and bosons) they are influenced by. Although quarks are affected by all three forces, leptons (and anti-leptons) are unaffected by the strong force, and neutrinos (and anti-neutrinos) are unaffected by the electromagnetic force.

But perhaps the most perplexing aspect of the Standard Model is that, unlike bosons, fermions have “copies.” In addition to the fermionic particles that make up the stable or quasi-stable matter we’re familiar with: protons and neutrons (made of bound states of up-and-down quarks along with the gluons), atoms (made of atomic nuclei, which is made of protons and neutrons, as well as electrons), and electron neutrinos and electron antineutrinos (created in the nuclear reactions that involve building up to or decaying down from pre-existing nuclear combinations).

For each of these, there are two additional generations of heavier particles. There are charm-and-strange quarks, as well as top-and-bottom quarks, in addition to antiquarks and up-and-down quarks in three colors each. There are also the muon and muon neutrino, as well as the tau and tau neutrino, in addition to the electron neutrino, electron, and their antimatter counterparts.

Fermionic particles appear in the Standard Model in three copies, or generations, for some reason. The heavier forms of these particles do not emerge naturally from standard particle interactions but can appear at extremely high energies. You can make any particle-antiparticle pair you like in particle physics as long as you have enough available resources.

One can make any particle-antiparticle pair they choose in particle physics as long as one has enough accessible energy. How much energy is required? One will need enough energy to create both their particle and its antiparticle, regardless of its mass. They can be made as long as there is enough energy, according to Einstein’s E = mc2, which details the conversion between energy and mass.

Similarly, if you make one of these unstable quarks or leptons (apart from neutrinos and antineutrinos), there’s always the chance that they’ll decay into a lighter particle due to weak interactions. Since all Standard Model fermions couple to the weak force, any of the following particles — charm, bottom, strange, or top quarks, as well as the tau leptons or muon— decay down to that stable first generation of particles in a fraction of a second.

The number of generations of particles available can be determined. It turns out that one out of every 30 Z-bosons decays to muon/anti-muon, electron/positron, and tau/anti-tau pairs, whereas one out of every five Z-bosons decays to nothing. According to the Standard Model and our explanation of particles and their interactions, one out of every fifteen Z-bosons (6.66% odds) will decay into one of the three types of neutrinos.

These results show that if a fourth (or more) generation of particles exists, every one of them, including neutrinos and leptons, will have a mass of more than 45 GeV/c2, a threshold that only the Z, W, Higgs, and top particles are perceived to surpass.

The hadrons (quark-containing particles) about 70% of the time, neutral leptons about 20% of the time,  Z-boson decays to charged leptons about 10% of the time, according to the final results of many various particle accelerator experimental studies. This makes sense if there are only three generations of particles.

Nothing is stopping the fourth generation from developing and being much, much heavier than any of the particles we’ve seen thus far; it’s theoretically possible. However, these collider results aren’t the only thing limiting the number of generational forms in the Natural world; another limitation is the abundance of light elements generated during the Big Bang’s early stages.

The theory of the strong interaction, ostensibly known as “quantum chromodynamics,” has a strong foundation. It explains how quarks interact with each other through the strong force by exchanging gluons. Gluons are analogs of the more well-known photon, a light particle that also serves as a carrier of electromagnetic force.

The strong force, on the other hand, behaves very differently from electromagnetism due to the way gluons interact with quarks. While the electromagnetic force weakens as two charged particles are separated, the strong force strengthens as two quarks are separated. As a result, quarks are trapped inside hadrons, which are particles made up of two or more quarks that contain protons and neutrons. Unless, of course, you crack them open at breakneck speeds, like Cern is doing.

To make matters even more complicated, in the standard model, all particles have antiparticles that are virtually identical to them but have the opposite charge (or other quantum property). If you take a quark from a proton, the force will ultimately be strong enough to form a quark-antiquark pair, with the newly formed quark going back into the proton. You get a proton and a brand new “meson,” which is a particle made up of quarks and antiquarks. Particles may emerge out of empty space, according to quantum mechanics, which governs the world on the smallest of scales.

Calculations of what would otherwise be a straightforward electromagnetism process can become impossibly complicated due to the theory of the strong force. As a result, we can’t (yet) prove that quarks can’t live on their own. Worse, we have no way of knowing which quark combinations will be viable in nature and which would not.

LHCb tetraquark
Charmed existence: conceptual illustration of a tetraquark. (Courtesy: CERN)

When quarks were first found, scientists realized that there could be many potential combinations in theory. Pairs of quarks and antiquarks (mesons); two quarks and two antiquarks (tetraquarks); three antiquarks (antibaryons); three quarks (baryons); and four quarks and one antiquark (pentaquarks) were all allowed, as long as the number of quarks excluding antiquarks in each combination was a multiple of three.

Only mesons and baryons were seen in experiments for a long time. The Belle experiment in Japan, however, found a particle that didn’t match anywhere in 2003. It was discovered to be the first in a long line of tetraquarks. Two pentaquarks were discovered at the LHCb experiment in 2015. The four new particles discovered recently We recently discovered four new particles, all of which are tetraquarks with a charm quark pair and two other quarks. In the same way, as the neutron and proton are particles, both of these objects are particles. Quarks and electrons, on the other hand, are the real building blocks of matter.

Fifty-nine new hadrons have been discovered by the LHC. This includes the most recently discovered tetraquarks, as well as new mesons and baryons. Heavy quarks like “charm” and “bottom” are found in both of these new particles.

These hadrons are fascinating to investigate. They tell us what nature accepts as a bound quark combination, even if just for a relatively short time. They often reveal what nature despises. Why, for example, do all tetra- and pentaquarks (with one exception) possess a charm-quark pair? Also, why aren’t there any particles with strange-quark pairs? There is no clarification at this time.

An artist’s rendering of what a pentaquark structure might look like. (CERN)

Another mystery is how the strong force binds these particles together. They are compact particles, like the proton or neutron, according to one school of thought. Others compare them to “molecules” made up of two loosely connected hadrons. Experiments on each newly discovered hadron will determine its mass and other properties, which provide insight into how the strong force behaves. This helps to close the gap between theory and experiment. We can tune the models to the experimental facts if we can find more hadrons.

These models are essential for the LHC to achieve its ultimate objective of discovering physics beyond the standard model. Despite its achievements, the standard model is far from complete in terms of particle comprehension. It is, for example, incompatible with cosmological models describing the creation of the universe.

The LHC is on the lookout for new elementary particles that could explain these anomalies. These particles may be visible at the LHC, but they may be hidden by particle interactions in the background. Alternatively, they could appear as minor quantum mechanical effects in well-known processes. To find them in either case, a better understanding of the strong force is required. With each new hadron, we gain a deeper understanding of nature’s laws, allowing us to better describe matter’s most fundamental properties.




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