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  (2009) 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.


By Frank Close
Prospect Magazine, October 2013

Edited by Andy Ross

Nobel laureate Peter Higgs is used to delays. He waited for 48 years, until on July 4, 2012, scientists announced they had found the Higgs boson. Back in 1964, at the same time as five other physicists who worked independently of him, he had proposed a theory saying the universe is filled with weird stuff now known as the Higgs field.

The theory explains why the sun converts hydrogen into helium slowly, over billions of years. It also explains how electrons and quarks form atoms rather than flying around freely like light. But it had to be tested experimentally. Among the "gang of six" theorists, Higgs alone saw a way to test it. He pointed to the boson predicted by the theory: Find it and measure it.

Physicist Ben Lee first referred to the "Higgs" boson at a conference in Chicago in 1972. After it, in the Edinburgh University staff club, Peter Higgs met his colleague Ken Peach, who had been there. Ken: "Peter! You're famous!"

The Large Hadron Collider is a ring the size of the Circle Line on the London Underground. It took 20 years to design and build. The total cost of the LHC project is estimated at €10 billion. The public think it was all to find the Higgs boson.

In 2008, the LHC was complete. It was started up. Then a component failed. It was repaired, and in 2010 the data began to pour in. If all went well, a clear answer should emerge by the fall of 2012.

In the summer of 2012, Higgs was in Erice, Sicily, in the Restaurant Venus. His colleague Alan Walker had been told of a meeting at CERN on July 4. During lunch the phone rang. Alan: "It's John Ellis saying we should go to CERN." Higgs: “If John Ellis says that, then we should go.”

Two teams at CERN announced their results to a packed auditorium. When the first team announced strong evidence for the Higgs boson, the audience burst into prolonged applause. When the second team said they too had strong evidence, independently, there was cheering. Higgs: "It was very moving. I burst into tears."

Peter Higgs had waited a long time. He had to wait another year for the joint award of the Nobel Prize to himself and François Englert.


By Dave Goldberg
Slate October 2013

Edited by Andy Ross

The Higgs boson gives mass to other particles. The Standard Model is a mathematical description of every fundamental particle and interaction of nature except gravity. The Higgs is the final particle to be discovered in the Standard Model.

The Higgs boson also explains why the weak nuclear interaction, the interaction that controls (among much else) the fusion reactions inside the sun, is confined to atomic nuclei. In quantum theory, the fundamental interactions of physics arise when two particles exchange a mediator particle. For electromagnetism, the mediator is a photon, which is massless. In the Standard Model, all of the mediators are supposed to massless.

For the strong force that holds protons and neutrons together, the mediators are massless too, but the weak interaction is different. Its mediator particles are massive W and Z bosons, which are confined to atomic nuclei. The Higgs allows the W and Z particles to interact with themselves rather than with other particles. It is responsible for the mass of W and Z particles and of electrons and quarks.

It is easy to overstate the importance of the Higgs discovery. Ordinary stuff is made of protons and neutrons, but those particles are made of quarks. Quarks get their masses from the Higgs, but protons are much more than the sum of their parts. The vast majority of your mass comes from the fact that everything inside your atomic nuclei is flying around at nearly the speed of light.