What is present inside a proton????????? , a neutron??? , an electron???
Answers
During the 1960s, everybody came to agree that protons and neutrons contain three quarks. According to the Standard Model, there is no other kind of massive particles inside the proton. A few years later, when it turned out that these quarks carry less than half of the proton’s linear momentum (indicating that there had to be other massive particles inside the proton), QCD fans proclaimed that this missing momentum was actually carried by the gluons. It is important to note here that gluon behavior is very different from that of one other massless particle, the photon, related to the electromagnetic interactions of a particle’s bound state.
In this article we’ll see that there is much more than an additional massive particle in the proton. As modern particle accelerators reach increasingly higher energy levels, this fact becomes more and more obvious.
An impactful quantum mechanical experiment
In 1913, James Franck and Gustav Hertz conducted one of the first experiments showing that the atom could not have any arbitrary energy state, but only specific energy levels. The results of this experiment supported Bohr’s atom model. Bohr’s model was later abandoned, but the results of this experiment gave a major boost for the development of the nascent Quantum Mechanics.
What did Franck and Hertz do? They fired electrons into a tube containing Argon gas or Mercury vapors, and measured the value of the electric current at the other end of the tube. The electrons were accelerated by means of an electric field.
Franck and Hertz could control the electrons’ velocity by changing the electric field. At first, the current increased consistently with the increase of electron speed. But when the electrons’ velocity reached a certain threshold, the current at the other end of the tube suddenly dropped. When they further increased the electric field and the acceleration of the electrons, the current at the end of the tube raised again, until a certain threshold, and then, when electrons’ speed crossed this new threshold, the current level dropped again.
What are the experiment’s conclusions? The atom can only have specific energy levels. When the electrons were moving slowly and their energy was too low to excite the atom, which means, to bring the atom to a higher energy level, the electrons just traveled across the atoms and reached the end of the tube with no energy loss. But when the electrons were accelerated and reached a certain velocity (and kinetic energy), their energy was high enough to excite the atom, and was transferred to the atom. This explains the first drop in current intensity. The second drop in current intensity corresponds to the loss of electrons’ kinetic energy when exciting atoms again, and so forth.
Analogous results are regularly observed in today’s particle accelerators as well, but with energy levels extremely higher than those used a century ago. The analysis of these experimental results are based on what physicists call “the cross section curve”, which refers to particle collision rates and is related to the probability to observe a specific event as a result of the collision. It turns out that the cross section depends not only on the type of particles involved in the collision but on the process energy as well.
The study of the proton structure
In the Franck-Hertz experiment, electrons colliding with atoms interacted with the atomic electrons. Exploring the proton requires much more energetic particle beams, and that for two main reasons. One reason is that in Quantum Theory, a particle’s location is not point-like, the way it had been considered in classical physics, but is described by a wave. A slowly moving particle has a relatively large wavelength. When a particle’s wavelength is larger than that of the proton size, it does not collide with an isolated quark, but rather with the whole proton. In order to study quarks, the incident particle’s wavelength should be significantly smaller than the proton’s size, and this is only possible when the incident particle moves at a very high energy.
And indeed, during the last decades scientists managed to accelerate many different particles, electrons, positrons, muons and protons reaching very high energies which significantly shortened the particle’s wavelength and allowed particles of these beams to interact with a single quark within the proton.