C

#### Coin

**Detecting large extra dimensions via "mini black holes"?**

Occasionally I come across something explaining that one of the

possible discoveries at the Large Hadron Collider might be the

production of "mini black holes", which if found would demonstrate the

existence of large extra dimensions. The idea, at least as I

understand it, is supposed to be that gravity in an more-than-four-

dimensional universe could be actually much stronger than we measure

it to be; but in a way that only has any effect over very small

distances, because the gravitons leak out into the extra dimensions if

given the chance. The "very small distances", if I'm understanding all

this right, could possibly be large enough that colliding particles at

a modern particle accelerator would be able to trigger the creation of

microscopic, short-lived black holes. The reason I usually see given

for why people are excited about this possibility is that it would

provide strong experimental support for string theory, by showing one

of string theory's more controversial elements-- extra dimensions-- to

be not just a theoretical convenience but actually a physical fact.

An example of a writing on this subject, picked at random from google,

might be:

http://cerncourier.com/cws/article/cern/29199

or:

http://quasar9.blogspot.com/2006/08/new-dimensions-at-lhc.html

The thing that confuses me about this, though, is the frequent

implication that this effect-- the effect that allows for mini black

hole production in particle accelerators, that is-- wouldn't exist in

all theories of more-than-four-dimensional spacetime, or even all

variations of string theory. It's usually left very vague, however,

*which* theories would or wouldn't produce this behavior. This seems

to make it very difficult to evaluate what the detection or non-

detection of these black holes at the LHC would or wouldn't be telling

us. So, I would like to ask:

** What attributes would a string theory have to have, in order for

the extra dimensions to be in principle detectable through black hole

production in a particle accelerator? The one thing everything I find

on this subject seems to agree on is that the spacetime which the

strings inhabit must have more than four non-compact dimensions, but

are there any *other* conditions which either the spacetime, or the

string theory acting within that spacetime, must satisfy for the "mini

black hole" production to be possible? **

I have a few slightly more specific versions of this question I'd also

like to ask, but I unfortunately do not know very much about String

Theory so I am not certain I know how to ask them correctly. So please

excuse me if the following turns out to be gibberish:

1. As far as I'm aware, the preeminent variation of string theory with

"large extra dimensions" would be the "brane cosmology" models, where

ALL 10 or 11 dimensions are taken to be arbitrarily large, but the

perceived universe is lower-dimensional because the movement of

strings is restricted to the surfaces of lower-dimensional "branes".

(I somehow got the impression that the string theories with compact

extra dimensions can be approximated within this model by folding the

branes properly, but I am not sure about this.) Within a model of

braneworld cosmology, is it possible to see mini-black-hole production

at a particle accelerator within some individual brane-world embedded

in the bulk? And if so, would the mini-black-hole effect be *required*

by the use of braneworld cosmology, or is it possible to design a

brane-world where the effect which results in the mini-black-holes

does not exist?

2. If the answer to the last question is "yes, but only in a model

with the correct circumstances", then is the mini-black-hole effect a

property of the theory which describes the branes, or is it only a

property of some individual brane-world described by that theory? In

other words, would it be possible for there to be a higher-dimensional

bulk, contained within which were BOTH brane-worlds which allow the

"mini black hole" production, and also brane-worlds which do not?

3. The specific large-extra-dimensions model I seem to hear about the

most is this Kachru-Kallosh-Linde-Trivedi, or KKLT, construction,

defined by the paper the arXiv has at hep-th/0301240. My *incredibly*

limited understanding of this construction is that it assumes a

braneworld cosmology model (i.e. all ten-ish dimensions are noncompact

but strings live on a lower-dimensional brane) and then describes a

way of configuring branes within the bulk so that you create a nice de

sitter space for the strings to live on. (I somehow got the impression

that the "string landscape" you sometimes hear about specifically

refers to all the different ways of configuring the KKLT construction,

but I am not sure about this.) Does use of the KKLT construction allow

the mini black holes effect to be present within a brane-world? Does

KKLT *require* the effect be present?

Thanks!