Supersymmetry

By Anil Ananthaswamy
New Scientist, November 11, 2009

Edited by Andy Ross

The standard model describes all known particles, electromagnetism, and the weak and strong nuclear forces. If the Large Hadron Collider finds the Higgs and nothing but the Higgs, the standard model will be sewn up. But then particle physics will be at a dead end. But if theorists are right, the LHC will see the first outline of supersymmetry. SUSY is a daring theory that doubles the number of particles needed to explain the world.

The Higgs came about to solve the fact that fermions and bosons all have mass. Particle masses were loose threads in the theories behind the standard model. In 1964, Peter Higgs of the University of Edinburgh, and François Englert and Robert Brout of the Free University of Brussels (ULB), proposed independently that the mass an elementary particle such as an electron or quark acquires depends on the strength of its interactions with a field called Higgs, whose quanta are Higgs bosons.

Fields like this are key to the standard model as they describe how the electromagnetic and the weak and strong nuclear forces act on particles through the exchange of various bosons. But what is the mass of the Higgs itself? It should consist of a core mass plus contributions from its interactions with all the other elementary particles. When you add them all up, the Higgs mass balloons out of control.

Experimental clues suggest that the Higgs mass should lie somewhere between 114 and 180 giga- electronvolts, easily the sort of energy the LHC can reach. But theory comes up with values 17 or 18 orders of magnitude greater, a discrepancy dubbed the hierarchy problem.

In today's universe, the three forces dealt with by the standard model have very different strengths and ranges. In the 1960s, Steven Weinberg showed with Abdus Salam and Sheldon Glashow that at high energies the weak and electromagnetic forces unify into one force. The expectation was that if you extrapolated back far enough towards the big bang, the strong force would also succumb, and be unified with the electromagnetic and weak force in one single super-force.

Supersymmetry made its appearance in the work of Soviet physicists Yuri Golfand and Evgeny Likhtman. Julius Wess of Karlsruhe University in Germany and Bruno Zumino of the University of California, Berkeley, brought its radical prescriptions to wider attention a few years later. Wess and Zumino were trying to show that the division of the particle domain into fermions and bosons is the result of a lost symmetry that existed in the early universe.

According to supersymmetry, each fermion is paired with a more massive supersymmetric boson, and each boson with a fermionic super-sibling. For example, the electron has the selectron (a boson) as its supersymmetric partner, while the photon is partnered with the photino (a fermion). In the early universe, particles and their super-partners were indistinguishable. Each pair co-existed as single massless entities. As the universe expanded and cooled, this supersymmetry broke down.

Supersymmetry can tame all the contributions from the Higgs interactions with elementary particles. They are cancelled out by contributions from their supersymmetric partners. In 1981, Howard Georgi and Savas Dimopoulos recalculated force reunification with supersymmetry and found that the curves representing the strengths of all three forces intersected exactly in the early universe.

Electrons, photons and the like are all around us, but of selectrons and photinos there is no sign. If such particles exist, they must be extremely massive and long since have decayed into the lightest supersymmetric particles, neutralinos. The neutralino has no electric charge and interacts with normal matter only by means of the weak nuclear force.

When physicists calculated exactly how much of the neutralino residue there should be, they found it was far more than all the normal matter in the universe. Neutralinos seem to fulfill all the requirements for the dark matter that astronomical observations persuade us must dominate the cosmos.

Each of the three questions that supersymmetry seems to solve — the hierarchy problem, the reunification problem, and the dark-matter problem — might have another answer. But one theory is better. Supersymmetry can also explain why quarks are always corralled together by the strong force into larger particles such as protons and neutrons. With supersymmetry, this drops out of the equations naturally.

The best proof for supersymmetry would come if we could produce neutralinos in an accelerator. The mass of the super-partners depends on precisely when supersymmetry broke apart as the universe cooled and the standard particles and their super-partners parted company. The kind of super- symmetry that best solves the hierarchy problem will become visible at the higher energies the LHC will explore. Similarly, if neutralinos have the right mass to make up dark matter, they should be produced in great numbers at the LHC.

The protons that the LHC smashes together are composite particles made up of quarks and gluons, and produce extremely messy debris. Any supersymmetric particles will decay in less than a femto- second into a spray of secondary particles, culminating in lots of neutralinos. Because neutralinos barely interact with other particles, they will evade the LHC detectors. Anything that looks like a neutralino would be very big news indeed. It would tell us that nature is supersymmetric. Most popular variants of string theory start out from supersymmetry.
 

AR  I find supersymmetry an a priori plausible concept and therefore hope we find neutralinos. Perhaps it would even justify my taxpayer contribution to the LHC. I'm holding out for a Higgs and SUSY double whopper — then I shall feel it was money well spent.