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CERN | Geneva | Switzerland

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The Standard Model: a beautiful but flawed theory

This is the first part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explain what is the Standard Model and show its limitations. Then I introduce supersymmetry and explain how it would fix the main flaws of the Standard Model. Finally, I will review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider at CERN.

The Standard Model describes what matter is made of and how it holds together. It rests on two basic ideas: all matter is made of particles, and these particles interact with each other by exchanging other particles associated with the fundamental forces.

The basic grains of matter are fermions and the force carriers are bosons. The names of these two classes refer to their spin – or angular momentum. Fermions have half-integer values of spin whereas bosons have integer values as shown in the diagram below.

2000px-Standard_Model_of_Elementary_Particles.svg

Fermions come in two families. The leptons family has six members, with the electron being the best known of them. The quarks family contains six quarks. The up and down quarks are found inside protons and neutrons. The twelve fermions are the building blocks of matter and each one has a spin value of ½.

These particles interact with each other through fundamental forces. Each force comes with one or more force carriers. The nuclear force comes with the gluon and binds the quarks within the proton and neutrons. The photon is associated with the electromagnetic force. The weak interaction is responsible for radioactivity. It comes with the Z and W bosons. All have a spin of 1.

The main point is: there are grains of matter, the fermions with spin ½, and force carriers, the bosons with integer values of spin.

The Standard Model is both remarkably simple and very powerful. There are complex equations expressing all this in a mathematical way. These equations allow theorists to make very precise predictions. Nearly every quantity that has been measured in particle physics laboratories over the past five decades falls right on the predicted value, within experimental error margins.

So what’s wrong with the Standard Model? Essentially, one could say that the whole model lacks robustness at higher energy. As long as we observe various phenomena at low energy, as we have done so far, things behave properly. But as accelerators are getting more and more powerful, we are about to reach a level of energy which existed only shortly after the Big Bang where the equations of the Standard Model start getting shaky.

This is a bit like with the laws of physics at low and high speed. A particle moving at near the speed of light cannot be described with the simple laws of mechanics derived by Newton. One needs special relativity to describe its motion.

One major problem of the Standard Model is that it does not include gravity, one of the four fundamental forces. The model also fails to explain why gravity is so much weaker than the electromagnetic or nuclear forces. For example, a simple fridge magnet can counteract the gravitational attraction of a whole planet on a small object.

This huge difference in the strength of fundamental forces is one aspect of the “hierarchy problem”. It also refers to the wide range in mass for the elementary particles. In the table shown above, we see the electron is about 200 times lighter than the muon and 3500 times lighter than the tau. Same thing for the quarks: the top quark is 75 000 times heavier than the up quark. Why is there such a wide spectrum of masses among the building blocks of matter? Imagine having a Lego set containing bricks as disparate in size as that!

The hierarchy problem is also related to the Higgs boson mass. The equations of the Standard Model establish relations between the fundamental particles. For example, in the equations, the Higgs boson has a basic mass to which theorists add a correction for each particle that interact with it. The heavier the particle, the larger the correction. The top quark being the heaviest particle, it adds such a large correction to the theoretical Higgs boson mass that theorists wonder how the measured Higgs boson mass can be as small as it was found.

This seems to indicate that other yet undiscovered particles exist and change the picture. In that case, the corrections to the Higgs mass from the top quark could be cancelled out by some other hypothetical particle and lead to the observed low Higgs boson mass. Supersymmetry just happens to predict the existence of such particles, hence its appeal.

Last but not least, the Standard Model only describes visible matter, that is all matter we see around us on Earth as well as in stars and galaxies. But proofs abound telling us the Universe contains about five times more “dark matter”, a type of matter completely different from the one we know, than ordinary matter. Dark matter does not emit any light but manifests itself through its gravitational effects.  Among all the particles contained in the Standard Model, none has the properties of dark matter. Hence it is clear the Standard Model gives an incomplete picture of the content of the Universe but supersymmetry could solve this problem.

Pauline Gagnon

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9 Responses to “The Standard Model: a beautiful but flawed theory”

  1. Ben Meade says:

    Hi Pauline,

    I love reading about this sort of stuff, and as a non scientist this has helped me understand a lot! I really enjoyed reading this, so thank you.
    I have a question about the mass of the particles you talk about.
    In the table it says that an electron has a mass of 0.511 MeV/c², a muon has 105.7 MeV/c², and a tau has 1.777 GeV/c². What is confusing me is you then state ‘we see the electron is about 200 times lighter than the muon and 3500 times lighter than the tau.’ To me the mass numbers and how much lighter each particles are than each other doesn’t add up? Would you care to explain why this is?

    • Bruce Elliott says:

      Ben Meade – Measured in MeV/c^2, the masses are approximately .5, 100, and 1700. Those values produce the ratios of 200 and 3500, as stated.

  2. Uncle Al says:

    The Standard Model is a curve fit. No patch is the last patch. Theory describes massless boson photons to 14+ significant figures. Fermionic matter (quarks; hadrons) is parity violations, symmetry breakings, chiral anomalies, Chern-Simons repair of Einstein-Hilbert action. Vacuum is observed trace chiral anisotropic toward hadrons.

    Noether’s theorems couple exact vacuum isotropy to angular momentum conservation. Vacuum trace chiral anisiotropy toward matter leaks Milgrom acceleration (arXiv:1310.4009, 0906.0668) as the Tully-Fisher relation. Planck: 13.82 Gyr; 68.3% dark energy, 26.8% dark matter, 4.9% baryonic matter. Dark matter inside Saturn’s orbit is less than 1.7×10^(−10) solar mass (arXiv:1306.5534).

    Physical theory must postulate observed vacuum symmetries toward matter. No SUSY, MSSM…. Vacuum trace chiral anisotropy toward hadronic matter is trivially measured by existing bench top apparatus in 90 days using commercial materials. Science is empirical. Look

  3. Virginia Velez Thaxton says:

    Thank you! This first installment of the Standard Model and Supersymmetry was such a great summary, can’t wait to read the rest. I hope I can visit CERN some day :)

  4. All well-known elementary bosons are gauge. Apparently, the found by LHC 125-126 particle represents some hadron multipoet.

    Every physics event is interpretted by particles which similar well-known elementary particles – leptons, quarks and gauge bosons. Therefore, if anybody will claim that he had found Higgs then not believe – this is not Higgs. http://arxiv.org/abs/physics/0302013v3

    “You shall not find any new physics, because all physical events are interpreted well-known (leptons, quarks, and gauge bosons) and forces which have long known (electroweak, gravity, strong interactions).” http://arxiv.org/pdf/0708.2322.pdf
    Quznetsov G 2013 Logical foundation of fundamental theoretical physics (Lambert Academic Publ.)

  5. Dr.K.R.Balasubramaniyan says:

    excellent introduction to explain to students even in B.Sc level.

  6. Dr.K.R.Balasubramaniyan says:

    excellent introduction to explain even to B.Sc students

  7. Bruce Elliott says:

    Very nice overview-thank you. I’m always curious about why diagrams of the fundamental particles don’t indicate color charge, however. The numeric value could be included in each square easily enough, and to show the three colors of the quarks, you could stack three squares on top of each other like cards.

    • Jaco says:

      I think this is because only quarks and gluons have color charge, it is always one, and the three colours are indistinguishable.

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