String Theory Demystified

CHAPTER 10 A Summary of Superstring Theory 191
supercharges has N = m supersymmetry. Finally, we can characterize each
superstring theory by the gauge symmetry that it admits. All superstring theories
eliminate the tachyon from the spectrum and include a graviton, so superstring
theory naturally describes gravity.
TYPE I SUPERSTRING THEORY
Type I superstring theory can be characterized as follows:
• It includes both open and closed strings.
• It describes unoriented strings.
• It has N = 1 supersymmetry.
• It has SO(32) gauge symmetry.
In addition, type I superstrings can have charges attached to their ends called
Chan-Paton factors, a topic we will explore in a later chapter.
TYPE II A
Type II A theory describes closed, oriented superstrings. We can summarize the
theory as follows:
• It only includes closed strings.
• It has N = 2 supersymmetry.
• It has a U(1) gauge symmetry.
Since this theory only has a U(1) gauge symmetry, it is not large enough to
describe all the particle states seen in nature. It can describe gravity and
electromagnetism, but cannot describe the weak or strong forces. The theory has
1 2
two supercharges, and Θ and Θ have opposite chirality. Practically speaking, this
means that each fermion has a partner state with opposite chirality.
TYPE II B
Type II B theory also describes closed strings, also oriented. Although it includes
fermionic states because it is a superstring theory, it has no gauge symmetry and so
can only describe gravity. Like type II A theory, it has N = 2 supersymmetry, but
1 2
Θ and Θ have the same chirality. This remedies the diffi culty in type II A theory
in that the fermions described in type II B theory do not have partners of opposite
chirality. But the lack of a gauge group indicates the theory cannot be the whole
story as far as a unifi ed theory of physics.