String Theory question...

[maddog]

How is it that the supersymmetric particle of a gravitino is thought to have a spin

of 3/2 yet be massless ? I was reading Michio Kaku's book, "Parallel Worlds" and

he was giving an excellent history of what Supergravity was when he layed this

bomb on me that the Gravitino (spin = 3/2) was massless... !

 

How could that be ? Half integral spin particles are matter particles and integral

spin particles are force particles (or so I thougt). What gives.

 

If your going throw around some String Theory (Super or otherwise) or some

M-Theory. Better be prepared to explain everything. I am ready... :) :)

 

Maddog

[Bo]

(half-integer spin particles are called fermions)

(integer spin particles are called bosons)

why would a fermion be massive? and a boson massless?

 

even in the standard model the neutrino (spin 1/2) was thought for decades to be massless. And the bosonic force particles that carry the weak interaction (the so-called intermediate vector bosons: W(+/-) and Z...) are extremely heavy. The reason people heavent found the higgs BOSON yet, is because it is so heavy :)

 

In other words: spin says nothing about the mass of a particle. As Kaku probably explained, supersymmetry is a symmetry that leaves everything invariant, except spin. So the superpartner of the spin 2, massless graviton is the spin 3/2 massless gravitino.

 

Bo

[paultrr]

Since the graviton has spin 2, the gravitino has spin 3/2, and can be seen in some way as the gauge field of local supersymmetry. Breaking supersymmetry means giving mass to the gravitino.

 

As with a gauge boson, the gravitino can gain mass when the ground state of the scalar potential breaks the symmetry of the action. In the bosonic Higgs scenario, the massless Goldstone modes of the scalar field end up as the extra longitudinal components that make the massless gauge boson massive. In the supersymmetric case, in addition to Goldstone bosons, there are massless fermionic states called Goldstinos, and they provide the longitudinal modes that give mass to the gravitino and break supersymmetry.

 

With supergavity, there is the possibility of breaking supersymmetry through gravitational couplings. With this model, the gravitino acquires a mass by "eating" a massless Goldstino, but because of the minus sign in the scalar potential, the total vacuum energy can be tuned to be zero. However, there is a slight assumption in all this that's actually based upon observation, that being that the real total vacuum energy is nearly zero. Quantum theory yields a prediction very much higher, some 120 powers higher. But observation tells us its while not exactly zero very low. The total vacuum energy gives the cosmological constant of the theory. Its already known that the cosmological constant may be a time variable itself with the issue of an accelerated expansion. So there are open questions in all this at present.

[paultrr]

There has been some recent experiments that have yielded the suggestion that the Higg's particle is actually above what was originally thought. Its more in the range of the new collider system coming out. That thought has to do with a discovery that the heavest particle discovered thus far was a bit heavier than the Standard Model predicted.

[maddog]

Paul,

 

Does this Goldstino come from Supergravity or from String Theory ? If string would it be considered

an open or closed string and in 11 dimensions I assume ? :cup:

 

Maddog

[maddog]

Bo,

 

I knew the Z0 particle had a mass (even though 0 spin). However, I didn't know that the

W- & W+ particles had mass (I thought they didn't). :cup:

 

Maddog

[Bo]

for example: http://www.infoplease.com/ce6/sci/A0817026.html

W+- do have mass...

 

as for golstino's:

they arise from a supersymmetric version of the standard higgs effect. So any supersymmetric theory can have these particles, be it supergravity, superstringtheory, or something completely different. I wouldn't dare saying in what spectrum of the string they could arise.

 

Bo