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 (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.
Higgs
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.
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