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Particle mass as a result of strings...
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Heinrich Neumaier
science forum beginner


Joined: 07 Mar 2005
Posts: 12

PostPosted: Sat Mar 12, 2005 11:40 pm    Post subject: Re: Particle mass as a result of strings... Reply with quote

Urs Schreiber <Urs.Schreiber@uni-essen.de> wrote message
news:<39au3mF5qbk8eU1-100000@individual.net>

and gave an excellent overview of the issue. Thank you!

I need to ask one question, as I want to understand the following
part:

Quote:
Coming back to string theory the interesting question is: Given we find a
background such that the spectrum of strings in that background reproduces
the massless particle spectrum of the standard model. How do strings give
rise to the Higgs particle?

There is one suggestive and interesting possibility: The Higgs field phi
itself must have a potential which has a local *maximum* at phi=0. That's
because we want it to have a vacuum expectation value and hence in its
lowest energy state it must be away from phi=0 (at every point of space) and
hence the minima of the potential must not be at phi=0.

But this means that the Higgs is actually a tachyon when we look at
perturbation theory around the naive vacuum phi=0.

One nice way to get such tachyons from string theory is from open strings
stretched between brane-antibrane pairs. Such pairs want to mutually
annihilate which means that as a background structure they are unstable,
which again gives rise to a tachyonic mode of open strings stretched between
them, which again indicates in perturbation theory how the brains disappear
as time goes by.

Such tachyons are a possible stringy canditiate manifestation of the Higgs.

If the Higgs is a long string between a brane and an antibrane, I have
the following question: what is the difference of a brane and an
antibrane?

To put it in another way, what is the difference between a string
connecting two branes and a string connecting a brane and an
antibrane?

This is really to understand the issue - I like to put questions
as simply as possible. Thanks for any help!

Heinz
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PostPosted: Sat Apr 02, 2005 9:35 am    Post subject: Re: Particle mass as a result of strings... Reply with quote

Ulrich Thiel wrote:
Quote:
On 2005-03-07, Heinrich Neumaier <heinrich_neumaier@yahoo.com> wrote:

Has anybody ever given a string argument why masses are
so much lighter than Planck masses?

It's better when I quote a part of Brian Greene's book "The elegant
universe" because I can't say it in my own words:

"[the Planck energy] correspond[s] to masses that are on the order
of ten billion billion times (1E19) that of a proton. This [...]
mass [...] is known as the Planck mass.
[...] This raises a crucial question [...]: If the "natural" energy
scale of string theory is some ten billion billion times that of a
proton, how can it possibly account for the far-lighter particles
[...]? The answer, once again, comes from quantum mechanics. The
uncertainty principle ensures that nothing is ever perfectly at
rest. [...] This holds true for the loops in string theory as well;
no matter how placid a string appears it will always experience
some amount of quantum vibration. The remarkable thing [...] is
that there can be energy cancellations between these quantum jitters
[...]. In effect, through the weirdness of quantum mechanics, the
energy associated with the quantum jitters of a string is negative,
and this reduces the overall energy content of a vibrating string
by an amount that is roughly equal to the Planck energy. [...]"


I hope it's an answer to your question...
U. Thiel

I have some vague idea of what quantum jitters of an electron might
look like, but no idea for a vibrating string. I haven't a clue how
the uncertainty principle would apply either. Any intuition?
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