The Standard Model Of Particle Physics
The Standard Model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, and not including the gravitational force) in the universe, as well as classifying elementary particles we know today.
Questions are great! because they have always helped us to understand everything that we know today. I have seen that kids happen to ask the best questions. Most of the time it is these very simple questions that really make us think deeply. And let me give you an example of what sort of question I’m talking about.
- Why is it that we can move our hands so freely in the air but when to try to put one hand through the other it claps. Now obviously it might not seem to be a great deal of questions to many, but it did make me think for a while because you could really go deep to answer this.
- Why is it that even though there is 99% space free in an atom but still we can’t walk through walls since everything is made out of atoms.
- How can it possibly be that steam, water, and solid ice are exactly the same thing? as they seem to have totally different properties!
- why does fire glow the way it does and not like anything else? and what exactly is fire?
I could just go on and on asking such questions about basic things around us, but that’s not the topic for today. However today’s topic does deal with the true fundamental building blocks of our present known universe. All these questions can be essentially boiled down to one simple question!! The Standard Model of Particle Physics helps us to understand these.
“What are the ultimate building blocks of reality?”
and questions like this have kept humanity perplexed for centuries. From the four elements of nature like fire,water,earth and wind to the more modern ideas of chemistry we have by now developed an understanding that have helped us explain the behavior of atoms and stars , and so we have a name for this understanding.
“Its called the Standard Model of Particle Physics”
By now we all know that any matter we see are made up of some fundamental particle. In the beginning humans thought it was just the four elements of nature i.e fire,earth water and wind. Later as time passed we went on to know that electrons , protons and neutrons were the most fundamental building blocks of our nature and interestingly just by adding more and more protons and neutrons we got different elements. A Very comprehensive article on big bang and how the basic particles came into existence has been written in the post “Did the Universe start with a Big Bang?”
In principle we could theoretically build a simple universe with just proton, neutrons and electrons!!
But things became complicated in the beginning of the 20th century when scientists began to find many new more particles from the cosmic rays. There wasn’t any system established to organize the zoo of particles. So these particles were initially being called as pie,sigma,delta and so on, but soon these symbols were falling too short to name them. And it was a mess!!!
So these particles had to be organized in terms of their properties like :
Soon to simplify the picture new fundamental particles like quarks were predicted and the whole zoo of the particles could be categorized based on the combinations of these quarks, and this was the birth of the standard model of particle physics.
We all have a modern picture of an atom now.
which actually looks like this
i.e a proton and neutron forming the nucleus and the electrons swirling around in a probabilistic cloud around the nucleus. We saw that quarks were predicted to tame the whole zoo of particles into proper categories for better understanding. It was soon found that these quarks constituted the protons and the neutrons! therefore they no longer remained the most fundamental particles.
Quarks comes in various flavours , they are
a proton as we know today is made up of
a neutron is made up of
and they have:
therefore the total charge of a proton is : +2/3 + 2/3 – 1/3 = +1
and the total charge of neutron is : -1/3 – 1/3 + 2/3 = 0
Now it is also found that protons sometimes converts itself into a neutron and vice verse by something called the “weak interaction”. This simply involves the exchange of an up-quark with a down-quark or vice versa. Now in order to balance the charge and energy in this conversion process other two particles are produced.
positron is the anti-particle pair of the electron and neutrinos are these very curious particles produced in millions in the stars like our sun and travel right past us every second without any interaction.
This thing called the “weak interaction ” is what converts the protons and the neutrons that enables the sun to burn and produce energy.
The sun and any star in that matter is fundamentally comprised of hydrogen. At high temperatures inside the sun , the electrons has some much energy that it can escape from the atom leaving behind just the proton. As we saw above that, upon weak interaction this proton converts into neutron and these protons and neutrons combine to form helium.
The helium nucleus has 2 protons and 2 neutrons. However it has been found that the mass of the particles which make up the nucleus is slightly less than the mass of the individual protons and neutrons separately.
This mass difference is converted into energy by the famous equation E=mc^2 , where m is the mass difference. This process is going on all the time and is the source of heat and light for the sun and the reason why life exists here on earth.
So pretty much everything we observe today in the universe can so far be said to made up of electrons, protons, neutrons and further protons and neutrons by quarks.
For some reason only the smallest size among the various sizes these particles exist is found to be stable enough to exist. The larger sizes quickly decay into smaller sizes.
All the particles mentioned have their anti-particle pair which are exactly having the same characteristics as these particles but have opposite charges. When a particle collides with its anti-particle pair, energy is given off.
Therefore now is the time to arrange all the final particles we have discussed into a proper table of the standard model, which looks like this:
The second and the third column is simply the larger size variations of the same species i.e the quarks and the leptons.
The Gauge bosons are nothing but the force carrier particles. Without force these particles would just wonder around without any interaction. Which is bad. If something doesn’t stick the quarks and leptons together there would be no atoms or element and no universe as we know it.
As mentioned in the last article regarding Big Bang , that how by the end of one second since the formation of the universe we had the four fundamental forces of nature in action. These four fundamental forces are:
- Strong Nuclear
- Weak Nuclear
- The force that keeps us on the ground and decides the trajectory of planets and moons and causes various other major cosmological phenomenons.
- Since energy and mass are equivalent, all forms of energy (including photons and light) cause gravitation and are under the influence of it.
- The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars – and for the stars to group together into galaxies – so gravity is responsible for many of the large scale structures in the Universe.
- Gravity has an infinite range of action
- it is however the weakest among the forces
- In general relativity, the effects of gravitation are ascribed to space-time curvature instead of a force.
- Gravity is not well understood in the quantum realm.
- n the decades after the discovery of general relativity, it was realized that general relativity is incompatible with quantum mechanics.It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required.
- it is the most common force in the universe and it wont be an exaggeration to say that for virtually every force we experience in our day to day lives apart from gravity , is electromagnetism is nature.
- it also the reason behind light! which is the most important component of the universe.
- we live in an electromagnetic world.
- the whole of chemistry depends on it.
- the force of friction, the “normal” force that keeps us falling through the floor and all the chemical forces that binds the molecules together.
- it also explains why a paper clip sticks to a magnet and how mobile phones work.
- Electromagnetism also works on a large range.
- Although electromagnetism is considered one of the four fundamental forces, at high energy the weak force and electromagnetic force are unified as a single electroweak force. In the history of the universe, during the quark epoch the unified force broke into the two separate forces as the universe cooled.
- PHOTONS are the force carriers of this field.
3.Strong Nuclear Force
- strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force or nuclear strong force).
- At the range of 10−15 m (1 femtometer), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 1038 times as strong as gravitation.
- this is what keeps the quarks glued together.
- strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron.
- If you consider that the nucleus of all atoms except hydrogen contain more than one proton, and each proton carries a positive charge, then why would the nuclei of these atoms stay together?
- The protons must feel a repulsive force from the other neighboring protons. This is where the strong nuclear force comes in. The strong nuclear force is created between nucleons by the exchange of particles called mesons. This exchange can be likened to constantly hitting a ping-pong ball or a tennis ball back and forth between two people. As long as this meson exchange can happen, the strong force is able to hold the participating nucleons together.
- The nucleons must be extremely close together in order for this exchange to happen.
- GLUONS are the force carriers if this field.
4. Weak Force
- it is the mechanism of interaction between sub-atomic particles that causes radioactive decay and thus plays an essential role in nuclear fission.
- The weak force, or weak interaction, is stronger than gravity, but it is only effective at very short distances.
- It acts on the subatomic level and plays a crucial role in powering stars and creating elements.
- W and Z BOSONS are the force carries of this field.
The strong and the weak nuclear forces are kind of like the velcro that keep things together only when they touch. The same analogy goes for these forces as they work only at distances nor more than an atom wide. At greater distances these forces don’t exist.
Also as we have seen above that the strength of these forces has to do it with their range of action.
Let us assume the strength of the strong nuclear force be = X
then the electromagnetic force would be = X/100
the weak force would be = X/100,000
and the gravitational force will be =X/10^40
This weakness of gravity is still a mystery and is the reason why it cannot be studied in the particle accelerators.
Therefore so far we have understood the forces and its carriers and how it fits the standard model as the gauge bosons.
Now the question arises that how does this forces work in the subatomic level?
in the subatomic scale forces are experienced by transfer of their particles from one another.
Imagine you are standing on a boat and someone throws a ball at you. On receiving the ball you would have experienced a force and the boat would move away slightly. Similarly when you would throw the ball back, you yourself would have experienced a similar force causing you boat to further move back. Clearly we can see by exchanging this ball of certain mass force is being experienced. The same goes on inside the sub-atomic world.
Now lets see a chart that sums up all we have learnt so far:
Hadrons are just combination of the quarks, which are of two main types ,i.e baryons and ,mesons.
Baryons being combination of 3 quarks which actually forms the protons and the neutrons. Mesons are just a combination of one quark and one anti-quark.
Masses of these particles are expressed in MeV (mega electron volts) or GeV(gega electron volts), due to their mass energy equivalence.
Lastly we are left with the Higgs Bosons which were referred as the God Particle. The Higgs boson is an elementary particle in the Standard Model of particle physics. It is the part of the scalar bosons. It is the quantum excitation of the Higgs field. The presence of the field, now confirmed by experimental investigation, explains why some fundamental particles have mass. The Higgs Bosons were predicted way back in the 1960’s but in 2012 the discovery of a new particle with a mass between 125 and 127 GeV/c2 was announced at CERN; physicists suspected that it was the Higgs boson. Named after Peter Higgs , this particle is a boson with spin zero, no electric charge and no colour charge. It is said that particles while moving through this ever prevailing Higgs field experiences a resistance. This resistance is said to be the reason why particles have a mass.f Higgs particle theories are valid, then a Higgs particle can be produced much like other particles that are studied, in a particle collider. This involves accelerating a large number of particles to extremely high energies and extremely close to the speed of light, then allowing them to smash together. However it is extremely rare to find these Higgs Bosons.
On 14 March 2013 CERN confirmed that:
- “CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and even parity [two fundamental criteria of a Higgs boson consistent with the Standard Model]. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson.”
This also makes the particle the first elementary scalar particle to be discovered in nature.
There are phenomenons like the:
- what is Dark Matter?
- and what exactly is the Super-Symmetry?
- how many dimensions are there and what are these Extra Dimensions?
- what is the Grand Unified Theory?
- why is gravity so different compared to the other forces? and how well does it fit into the four forces?
and there are plenty more of them. These are great questions just like the ones in the beginning of this article.
So lets keep asking questions and lets keep exploring the universe around us with our curious minds.
More articles on individual particles will be coming soon.
please leave a comment below or ask any question you want to regarding the article “The Standard Model of Particle Physics”.
I would highly recommend few books that would really help you know much in depth about the standard model and much more regarding the cosmos:
- THE THEORY OF EVERYTHING
- A BRIEF HISTORY OF TIME
- GEORGE AND THE BIG BANG
- A Brief History of the Universe: From Ancient Babylon to the Big Bang (Brief Histories)
- The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics (Sterling Milestones)
- THE BIG BANG THEORY
- Relativity: The Special and the General Theory (Routledge Classics)
- Black Holes: The Reith Lectures
- The Oxford Companion to Cosmology (Oxford Quick Reference)