Strong Force and Weak Force


Anand Deo

MBBS Final Year

There are four known fundamental forces in Nature:-gravitational, electromagnetic, strong nuclear, and weak nuclear. These forces affect everything in the universe. Each force has a particular property associated with it, such as electric charge for the electromagnetic force, space-time curvature for gravity, color charge for strong nuclear force and weak hypercharge for weak nuclear force.

Strong Force

Quarks and particles made of quarks attract each other through the strong force. The strong force holds the quarks in protons and neutrons together, and it holds protons and neutrons together in the nuclei of atoms. If electromagnetism were the only force between quarks, the two up quarks in a proton would repel each other because they are both positively charged. The up quarks are also attracted to the negatively charged down quark in the proton, but this attraction is not as great as the repulsion between the up quarks. However, the strong force is stronger than the electromagnetic force, so it glues the quarks inside the proton together.

A property of particles called color charge determines how the strong force affects them. The term ‘color charge’ has nothing to do with color in the usual sense; it is just a convenient way for scientists to describe this property of particles. Color charge is similar to electric charge, which determines a particle’s electromagnetic interactions. Quarks can have a color charge of red, blue, or green. Antiquarks can have a color charge of antired (also called cyan), antiblue (also called yellow), or antigreen (also called magenta). Quark types and colors are not linked-up quarks, for example, may be red, green, or blue.

All observed objects carry a color charge of zero, so quarks which compose matter must combine to form hadrons that are colorless, or color neutral. The color charges of the quarks in hadrons cancel one another. Mesons contain a quark of one color and an antiquark of the quark’s anticolor. The color charges cancel each other out and make the meson white, or colorless. Baryons contain three quarks, each with a different color. As with light, the colors red, blue, and green combine to produce white, so the baryon is white, or colorless.


The bosons that carry the strong force between particles are called Gluons. Gluons have no mass or electric charge and, like photons, they are their own antiparticle. Unlike photons, however, gluons do have color charge. They carry a color and an anticolor. Possible gluon color combinations include red-antiblue, green-antired, and blue-antigreen. Colors and anticolors attract each other, so gluons that carry one color will attract gluons that carry the associated anticolor. Because gluons carry color charge, they themselves are affected by strong force; they can attract each other and form an unstable collection of gluons called glueball or gluon ball or gluonium. Recently, such gluoniums were believed to be observed during experiments in particle accelerators.

Gluons carry the strong force by moving between quarks and antiquarks and changing the colours of these particles. Quarks and antiquarks in hadrons constantly exchange gluons, changing colours as they emit and absorb gluons. Baryons and mesons are all colourless, so each time a quark or antiquark changes colour, other quarks or antiquarks in the particle must change colour as well to preserve the balance. The constant exchange of gluons and colour charge inside mesons and baryons creates a colour force field that holds the particles together.

The strong force is the strongest of the four forces in atoms. Quarks are bound so tightly to each other that they cannot be isolated. Separating a quark from an antiquark requires more energy than creating a quark and antiquark does. Attempting to pull apart a meson, then, just creates another meson: The quark in the original meson combines with a newly created antiquark, and the antiquark in the original meson combines with a newly created quark.

In addition to holding quarks together in mesons and baryons, gluons and the strong force also attract mesons and baryons to one another. The nuclei of atoms contain two kinds of baryons: protons and neutrons. Protons and neutrons are colourless, so the strong force does not attract them to each other directly. Instead, the individual quarks in one neutron or proton attract the quarks of its neighbours. The pull of quarks toward each other, even though they occur in separate baryons, provides enough energy to create a quark-antiquark pair. This pair of particles forms a type of meson called a pion. The exchange of pions between neutrons and protons holds the baryons in the nucleus together. The strong force between baryons in the nucleus is called the residual strong force.

The fact that quark and gluons have colour charge and that stable particle can’t have a color charge prevents one from observing isolated quark or gluon. Also, the fact that energy required to separate quarks is enough to create a meson prevents a quark from being identifies individually during experiments and this in turn make the whole notion of quark and gluon metaphysical. However, there is another property of strong nuclear force that makes the concept of quarks and gluons well-defined. At normal energies, the strong nuclear force is indeed strong, and it binds the quarks tightly together. However experiments with large particles accelerators indicate that at high energies the strong nuclear force becomes much weaker, and the quarks and the gluons behave almost like free particles.


Weak Force

While the strong force holds the nucleus of an atom together, the weak force can make the nucleus decay, changing some of its particles into other particles. The weak force is so named because it is far weaker than the electromagnetic or strong forces. For example, an interaction involving the weak force is 10 quintillion (10 billion billion) times less likely to occur than an interaction involving the electromagnetic force. Three particles, called Vector bosons, carry the weak force. The weak force has a peculiar property called ‘weak hypercharge’ or the ‘flavor’.Weak hypercharge determines whether the weak force will affect a particle. All fermions possess weak hypercharge, as do the vector bosons that carry the weak force.

All elementary particles, except the force carriers of the other forces and the Higgs boson, interact by means of the weak force. But the effects of the weak force are usually masked by the other forces stronger than it- the strong and the electromagnetic force. The weak force becomes significant when an interaction does not involve the strong force or the electromagnetic force. For example, neutrinos have neither electric charge nor color charge, so any interaction involving a neutrino must be due to either the weak force or the gravitational force. The gravitational force is even weaker than the weak force on the scale of elementary particles, so the weak force dominates in neutrino interactions.

One example of a weak interaction is beta decay involving the decay of a neutron. When a neutron decays, it turns into a proton and emits an electron and an electron antineutrino. The neutron and antineutrino are electrically neutral, ruling out the electromagnetic force as a cause. The antineutrino and electron are colorless, so the strong force is not at work. Beta decay is due solely to the weak force.

The weak force is carried by three Vector bosons. These bosons are designated the W+, the W, and the Z0. The W bosons are electrically charged (+1 and –1), so they can feel the electromagnetic force. These two bosons are each other’s antiparticle counterparts, while the Z0 is its own antiparticle. All three vector bosons are colorless. A distinctive feature of the vector bosons is their mass (around 100GeV). The weak force is the only force carried by particles that have mass. These massive force carriers cannot travel as far as the massless force carriers of the three long-range forces, so the weak force acts over shorter distances than the other three forces. Moreover according to the Weinberg-Salam theory these vector bosons exhibit the property of spontaneous symmetry breaking according to which at low energies, all these three different particles are merely different forms of same particle. At high energies of order much higher than 100GeV, all these W+, the W, and the Z0 (and the photon too) would behave in same manner.

When the weak force affects a particle, the particle emits one of the three weak vector bosons-W+, W, or Z0-and changes into a different particle. The weak vector boson then decays to produce other particles. In interactions that involve the W+ and W, a particle changes into a particle with a different electric charge. For example, in beta decay, one of the down quarks in a neutron changes into an up quark and the neutron releases a W boson. This change in quark type converts the neutron (two down quarks and an up quark) to a proton (one down quark and two up quarks). The W boson released by the neutron could then decay into an electron and an electron antineutrino. In Z0 interactions, a particle changes into a particle with the same electric charge.

A quark or lepton can change into a different quark or lepton from another generation only by the weak interaction. Thus the weak force is the reason that all stable matter contains only first generation leptons and quarks. The second and third generation leptons and quarks are heavier than their first generation counterparts, so they quickly decay into the lighter first generation leptons and quarks by exchanging W and Z bosons. The first generation particles have no lighter counterparts into which they can decay, so they are stable.



The Fifth Force?

Physicists in Hungarian Academy of Science last year reported that they have found a reasonable candidate for the fifth force. They detected a brand new super-light boson that was only 17MeV, 34 times heavier than an electron. You might be wondering what does this super-light boson has to do with fifth force. Well, this isn’t the first time researchers have claimed to have detected fifth force. Over the past decade, the search for new forces has ramped up because of the inability of the standard model of particle physics to explain dark matter – an invisible substance thought to make up more than 80% of the Universe’s mass. Theorists have proposed various exotic-matter particles and force-carriers, including “dark photons”, by analogy to conventional photons that carry the electromagnetic force.

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