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5.7: Super Symmetry

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

• Describe some of the concepts the standard model does not describe.
• Describe “super symmetry,” abbreviated SUSY.

Overview

The standard model appears to be incomplete. While it does describe many phenomena and can predict many more, there are a few concepts it does not adequately describe.

Electron Size

According to the standard model, when examining the forces involved in the electron, it cannot be any smaller than 1017m\begin{align*}10^{-17}\;\mathrm{m}\end{align*} due to repulsion in the electron cloud.

According to the standard model, the electron cannot be smaller than in diameter. This is due to the internal forces pushing outwards. However, an electron is approximately in diameter. If you include superparticles, using the concepts of super-symmetry, then this smaller size is allowed by this modified standard model.

Singular “Super Force”

As the universe ages it cools down. To examine the conditions of the universe when it was young, it must be heated up. One current belief is that in the beginning all the forces acted as one “super” force. Perhaps the electroweak and the strong forces combined to create a single super force. If the universe is heated up, physicists have shown that their strengths change. Electroweak get weaker and the strong force also gets weaker. However, according to the standard model, these forces don’t converge as the universe gets hotter.

Comparison of the Two Standard Models

In a super collider such as the LHC, much of the kinetic energy of the colliding particles is converted into thermal energy. This re-creates the high temperatures believed to have existed at the moment the universe was created. Essentially this is looking back in time to when the universe was young. During a collision the LHC will experience temperatures of 10\begin{align*}10\end{align*} million billion degrees Celsius, or 1×1016 C\begin{align*}1 \times 10^{16}\ ^{\circ}\mbox{C}\end{align*}. This is 500\begin{align*}500\end{align*} million times hotter than the Sun, (www.YouTube.com, LHC accelerator at CERN, 2008).

The Search

Evidence of super symmetry (SUSY) lies in finding tangible evidence of “superpartner” particles. Some evidence has already been found at experiments at Fermilab’s Tevatron, KEK’s KEKB e+e\begin{align*}e+ e-\end{align*} collider in Japan, and PEP II e+e\begin{align*}e+ e-\end{align*} storage ring at Stanford Linear Accelerator Center in the United States (U.C. Department of Science, "Particle Physics as Discovery’s Horizon,” 2006). A "superpartner” is related to the particles in the standard model.

Visual Representation of the Standard Model

In the standard model the particles can be divided into particles responsible for mass and particles responsible for force. The electron, e\begin{align*}e^-\end{align*}, muon, μ\begin{align*}\mu\end{align*}, the tau, τ\begin{align*}\tau\end{align*}, the three neutrinos, νe\begin{align*}\nu_e\end{align*}, νμ\begin{align*}\nu_{\mu}\end{align*}, ντ\begin{align*}\nu_{\tau}\end{align*}, and the quarks are responsible for mass. The photon, γ\begin{align*}\gamma\end{align*}, gluon, g\begin{align*}g\end{align*}, Z\begin{align*}Z-\end{align*}boson, Z\begin{align*}Z\end{align*}, and the W\begin{align*}W-\end{align*}boson, W±\begin{align*}W^{\pm}\end{align*} are responsible for force. In other words, the 12\begin{align*}12\end{align*} quarks and leptons pictured on the left in the standard model’s table are called fermions and are responsible for mass. The four particles in the last column on the right are called bosons and are responsible for all the forces. If the Higgs particle is confirmed in collider experiments, the standard model table could change to look something like the table below (Cox, TED, 2008).

This chart shows how the standard model could change if evidence of the Higgs particle is substantiated.

The super–symmetry model says that matter and force are not separate but somehow connected. Because of this connection, every fermion has a super–symmetric partner boson and for every boson there is a super–symmetric fermion. These super–symmetric particles are called the superpartners for the particles in the standard model. The superpartner particles are different from their counterparts by having half a quantum spin difference. They also have specific names and symbols.

The chart shows how the standard model could change if the superpartners are found.

The symmetrical particles for the fermions are the superpartner bosons. The suffix, “ino,” is added to the name. The symmetrical particles for the bosons are the superpartner fermions. The letter, “s\begin{align*}s\end{align*},” is added in front of their name.

Particle and the Corresponding Symmetrical Particle
Fermion Symmetrical Boson
quark squark
electron selectron
neutrino sneutrino
muon smuon
tau stau
boson symmetrical fermion
photon photonino
gluon gluino
W±\begin{align*}\mathrm{W}^{\pm}\end{align*} Wino±\begin{align*}\mathrm{Wino}^{\pm}\end{align*}
Z\begin{align*}\mathrm{Z}\end{align*} Zino
Higgs Higgsino

Many of the superpartners are very heavy. This means they are short–lived during and after a collision and can only be created by converting a lot of kinetic energy to mass. The LHC could provide enough energy to create these superpartners. One theory has the sneutrino as being responsible for dark matter.

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