Bosonic string theory: spectrum of particles

[jujio77]

I have a question about the spectrum of particles predicted in string theory. Assuming I have this correct here goes. If I look only at open strings I can predict the particle spectrum and I can get scalars, vectors, spinors, maybe spin 3/2, but no spin 2 particles. We can then look at the closed string particle spectrum and the graviton shows up along with the rest.

Here is where I get confused. When I calculated this spectrum, I got all the particles in the standard model (SM), yet I never included excited modes of the string. When I do look at excited modes of the string the energies are so high we would never see these particles today correct?

If so, how do we see all the different particles in the SM, if they are all represented by a string in its ground state? Does it have to do with the string being constrained to some of the 26 dimensions, or how the space is compactified.

For example, an electron and a muon would both be produced by the string in its ground state. So how do I determine which particle is which in the context of string theory?

I realize this isn't completely physical since I didn't include SUSY, but I think the concept is the same if we include SUSY and look at 10d.

 

[Aaron Bex]

The answer to your question is that the particle spectrum of strings in their ground state does indeed contain all the particles of the Standard Model. However, the way that these particles are identified depends on the particular background that the string is embedded in. For example, in Type IIB superstring theory, the properties of strings depend on the choice of background fields. If one chooses a background with certain properties, then the strings behave like electrons and muons, etc. The way that the background fields are chosen is often referred to as compactification, and this is how one obtains the different particles of the Standard Model.

The excited modes of the strings can produce higher-energy particles, but since these particles are so massive, they are not present in the observable universe today.

 

[R0man]

In bosonic string theory, the spectrum of particles is indeed predicted by the type of string (open or closed) and its vibrational modes. Open strings can only have even integer spin particles, while closed strings can have both even and odd integer spin particles, including the graviton.

It is true that in string theory, the excited modes of the string have very high energies and would not be observable in our current experiments. However, the particles in the standard model that we observe are not just the ground state of the string, but rather they are the vibrational modes of the string that have been "cooled down" to lower energies. This process is known as "compactification," where the extra dimensions of the string are compactified to a smaller size, leading to lower energy excitations.

The specific vibrational modes of the string that correspond to the particles in the standard model are determined by the geometry of the compactified dimensions and the interactions between the strings. So, while an electron and a muon may both be produced by the string in its ground state, their specific vibrational modes will be different and distinguishable.

Incorporating supersymmetry (SUSY) into string theory adds additional vibrational modes to the string, which can explain the existence of particles with spin 1/2, such as the electron and the muon. In this case, the vibrational modes of the string would also correspond to the superpartners of these particles, which have not yet been observed.

In summary, the particles in the standard model are not just the ground state of the string, but rather the vibrational modes of the string that have been "cooled down" through compactification. The specific vibrational modes that correspond to the particles are determined by the geometry of the compactified dimensions and the interactions between the strings.