Particle physics is one of the most exciting branch of physics. It involves an amazing blend of both theory and experiments. The beauty of particle physics lies in the fact that its path converges with cosmology in the longer run. Lisa Randall, an American Physicist, says, "Creativity is the backbone of particle physics and cosmology". Indeed, a deep quote by a particle physicist. However, it must also be mentioned that this field is one of the most difficult field of physics. It involved rigorous mathematics on theoretical front and sophisticated labs and instruments on the experimental front. Let us have a look at a few unsolved problems in particle physics that are way too interesting.
1. The Proton Decay Problem
Is proton decay possible? If yes, then what is its lifetime?
Proton is the positively charged nucleon whose decay was hypothesized by Andrei Sakharov in 1967. Theoretically, a proton must decay into a neutral pion and a positron. However, despite cutting edge technology and rigorous experimental efforts, no proton decay has been detected. The proton's half life is constrained to be 16,700 million trillion trillion years (1.67 x 10^34 years).
Proton decay is an important prediction of the Grand Unified Theory (GUT). A lot of problems in cosmology depend on its solution; one of them being the ultimate fate of the universe.
2. The Existence of Magnetic Monopoles
Did particles that carry "magnetic charge" exist in some past, higher-energy epoch? If so, do any remain today?
When we cut a magnet from its center, we do not get a separate north and south pole. What we get are two smaller magnets with their own pairs of north and south poles. Thus, like electric charges, there are no magnetic charges in nature i.e. magnetic monopoles do not exist.
However, several Grand Unified Theories say that magnetic monopoles should exist in the universe. The first thing that the discovery of magnetic monopoles will change is the century old set of four Maxwell's equations that won the battle against the Newton's laws of motion when it came to formulate special relativity. With the monopoles now in picture, Gauss's law of magnetism and the Faraday's law will change. The divergence of magnetic field will no longer be zero. The only asymmetric part of these equations is that there is no single point source of magnetic field like the electric field. So the discovery of monopoles will now bring an utmost symmetry in the Maxwell's equations. There are only 2 ways to detect the monopole: Invent a magnetic monopole or observe the ones already created in the universe, neither of which has been successful yet.
3. The Heirarchy Problem
"Why is gravity a trillion trillion times weaker than the weak force?"
There are four fundamental forces in nature: Strong, weak, electromagnetic and gravitational. The Heirarchy problem in particle physics seeks an explanation to the question above. In other words, how come our universe is so fine tuned? One answer given by physicists is the anthropic principle. If the universe came to exist by chance, and perhaps vast numbers of other universes exist or have existed, then life capable of physics experiments only arose in universes that by chance had very balanced forces. All the universes where the forces were not balanced, didn't develop life capable of the question. So if a lifeform like human beings are aware and capable of asking such a question, humans must have arisen in a universe having balanced forces, however rare that might be. So when we look, that is what we would expect to find, and if done properly, it is acquired.
4. The Micro Black Holes
"Do particles of Planck mass exist?"
Micro black holes are tiny black holes that are governed by the laws of quantum mechanics. Every object has an associated Schwarzchild radius with it. It is that radius of a sphere, to which, if an object is compressed, will become a black hole. For Sun, it is 3 Km. So if the Sun is compressed to a sphere of radius 3 Km, it will become a black hole. On the other hand, the Compton wavelength, λ = h/Mc, where h is the Planck constant, represents a limit on the minimum size of the region in which a mass M at rest can be localized. Combining these two equations, we get the lower limit for mass of such a black hole: 22 micro-grams. This is a Planck particle.
In 2010, scientists predicted that such particles should be seen in the high energy collisions at the LHC. Detecting them would be extremely difficult as they would evaporate in just 10^-40 seconds. Physics tells us that such particles might have played an important role in the early high energy epochs of the universe. But no micro black hole has ever been detected/made.
5. The Problem of Gravitons
"Is there a carrier particle of gravitational force?"
Three out of four forces of nature have an associated carrier particle. Photons carry electromagnetic force, W & Z Bosons carry weak force and gluons carry the strong force. However, the standard model of particle physics has no description of the carrier of gravity. String theory says, gravitons, the carriers of the weakest force in nature, should exist.
Discovery of gravitons will really change the course of physics. It will show that gravitational field, just like the electromagnetic field, is a quantum field. This won't mean that general relativity is wrong. Steven Weinberg showed this 50 years ago. Also, the existence of gravitons will give the Standard Model of particle physics its first real blow. The theory has a wonderful past filled with successful predictions, the major one being that of Higgs Boson that was discovered earlier in the decade. With gravitons in picture, the theory will have to be extended to include them. However, the experimental confirmation of gravitons is quite difficult and impossible with modern day technology.