Large extra dimensions ruled out?

[bcrowell]

Search for Microscopic Black Hole Signatures at the Large Hadron Collider
Authors: CMS Collaboration
http://arxiv.org/abs/1012.3375

A search for microscopic black hole production and decay in pp collisions at a center-of-mass energy of 7 TeV has been conducted by the CMS Collaboration at the LHC, using a data sample corresponding to an integrated luminosity of 35 inverse picobarns. Events with large total transverse energy are analyzed for the presence of multiple high-energy jets, leptons, and photons, typical of a signal expected from a microscopic black hole. Good agreement with the expected standard model backgrounds, dominated by QCD multijet production, is observed for various final-state multiplicities. Limits on the minimum black hole mass are set, in the range 3.5 -- 4.5 TeV, for a variety of parameters in a model with large extra dimensions, along with model-independent limits on new physics in these final states. These are the first direct limits on black hole production at a particle accelerator.

(BTW, slashdot is running this as a falsification of string theory, based on a jokey summary on the Not Even Wrong blog. Not Even Wrong has posted an update explaining their sarcasm to the uninitiated: http://www.math.columbia.edu/~woit/wordpress/?p=3333 )

Personally, I disagree with Not Even Wrong's opinion that large extra dimensions were physically unmotivated. There was a very clear physical motivation for them, which was to make the electroweak unification scale the same as the Planck scale.

 

[Coin]

Surely though they didn't actually even rule out black holes due to LED, they only placed lower bounds on the energy range where they appear?

 

[Coin]

. There was a very clear physical motivation for them, which was to make the electroweak unification scale the same as the Planck scale.

Bcrowell, here is a long term sort of question. Let's say that over the next twenty, thirty years colliders continue looking for micro black holes and find nothing, pushing the lower bound for black hole production up higher and higher. (I'm assuming collider experiments can't exclude LED entirely.) How high does the lower bound have to go before the physical motivation you suggest evaporates?

 

[bcrowell]

Bcrowell, here is a long term sort of question. Let's say that over the next twenty, thirty years colliders continue looking for micro black holes and find nothing, pushing the lower bound for black hole production up higher and higher. (I'm assuming collider experiments can't exclude LED entirely.) How high does the lower bound have to go before the physical motivation you suggest evaporates?

That's a good question, and I don't know the answer. I'm hoping someone here with more particle physics knowledge can answer that. I suppose it might depend on the number of extra dimensions. Wouldn't the largest numbers of extra dimensions be the easiest to rule out?

 

[Haelfix]

The answer depends on who you ask, and can get involved rapidly.

From a phenomenology point of view, large extra dimensions (ADD models, etc) and/or RS scenarios are extremely attractive b/c they answer a host of nagging problems in particle physics that really require immediate answers. Whether its neutrino physics or the hierarchy problem, there is a large portion of the parameter space in these models that has a great deal of unifying explanatory power. The LHC has no chance of ruling out this paradigm, unless for some reason another more compelling answer shows up. So for instance if supersymmetry is detected, the sort of answers LED provides would be mostly redundant. However, if no new physics (other than a scalar Higgs) is observed at the LHC, these sorts of models will rapidly come back into favor b/c unlike supersymmetry, which really only has phenomenology benefits near the electro weak scale, these theories can exist to arbitrarily high energy and still retain some usefullness.

On the other hand, if you ask a theorist, they don't necessarily like these scenarios as much b/c they are somewhat contrived and don't necessarily fall out from pure theory naturally.

Now, as to what is or is not easy to rule out.. Unfortunately, that becomes highly model dependant, and there is of course a great deal of freedom in building such things. At best, the exclusion limits provided by the LHC will become simply another constraint to model builders.

 

[suprised]

Indeed there are no strong theoretical reasons for large extra dimensions; they are a possibility among other possibilities and any experiment that puts bounds is highly welcome.

In fact the idea was heavily promoted by certain people (Randall, Sundrum, Arkani-Hamed, Dimopoulos etc) as an alternative to string theory, only later it was put in a stringy perspective. Many phenomenologists jumped on it, since it allowed to deal with extra dimensions, gravity, black holes and all sorts of fancy stuff without having to learn string theory. A whole industry of extra-dimensional model builders thrives on this, and comprises a major portion of "beyond the standard model phenomenology". Despite much of this is work is quite unmotivated and often on shaky theoretical grounds, this community somehow evades all criticism while the string physicists take, as always, the heat - see those headlines.

 

[marcus]

Thanks all, for raising and sorting out these issues.

Search for Microscopic Black Hole Signatures at the Large Hadron Collider
Authors: CMS Collaboration
http://arxiv.org/abs/1012.3375

(BTW, slashdot is running this as a falsification of string theory, based on a jokey summary on the Not Even Wrong blog. Not Even Wrong has posted an update explaining their sarcasm to the uninitiated: http://www.math.columbia.edu/~woit/wordpress/?p=3333 )

Personally, I disagree with Not Even Wrong's opinion that large extra dimensions were physically unmotivated. There was a very clear physical motivation for them, which was to make the electroweak unification scale the same as the Planck scale.

Indeed there are no strong theoretical reasons for large extra dimensions; they are a possibility among other possibilities and any experiment that puts bounds is highly welcome.

In fact the idea was heavily promoted by certain people (Randall, Sundrum, Arkani-Hamed, Dimopoulos etc) as an alternative to string theory, only later it was put in a stringy perspective. Many phenomenologists jumped on it, since it allowed to deal with extra dimensions, gravity, black holes and all sorts of fancy stuff without having to learn string theory. A whole industry of extra-dimensional model builders thrives on this, and comprises a major portion of "beyond the standard model phenomenology". Despite much of this is work is quite unmotivated and often on shaky theoretical grounds, this community somehow evades all criticism while the string physicists take, as always, the heat - see those headlines.

How could anyone criticize Lisa Randall, she's such a doll. But seriously, this is an enlightening thumbnail sketch of the situation seen by an insider.

 

[JustinLevy]

Excuse my ignorance, but how small does the lattice spacing in Lattice QCD go when they are doing hadron mass calculations?

The fact that QCD is well supported experimentally, wouldn't this be a decent argument that the largest an extra dimension (which matter can go in, so not the Randall or brane ideas) can be is limitted by what these lattice spacing are (and to some extent were extrapolated to in order to get the hadron mass)?

Said another way:
Wouldn't the mass of hadrons be sensitive to TeV scale extra dimensions?
I would think this has already been ruled out.

 

[PAllen]

Indeed there are no strong theoretical reasons for large extra dimensions; they are a possibility among other possibilities and any experiment that puts bounds is highly welcome.

In fact the idea was heavily promoted by certain people (Randall, Sundrum, Arkani-Hamed, Dimopoulos etc) as an alternative to string theory, only later it was put in a stringy perspective. Many phenomenologists jumped on it, since it allowed to deal with extra dimensions, gravity, black holes and all sorts of fancy stuff without having to learn string theory. A whole industry of extra-dimensional model builders thrives on this, and comprises a major portion of "beyond the standard model phenomenology". Despite much of this is work is quite unmotivated and often on shaky theoretical grounds, this community somehow evades all criticism while the string physicists take, as always, the heat - see those headlines.

The bigger the claims, the bigger the target. This group is not on record saying theirs is the only plausible approach to beyond SM physics, nor to claiming prospects for a TOE. Of course, only a few string theorists were vocal on these points, but these were highly visible, and played up in the media. Unsurprisingly, there was a reaction, which also got good play in the media. It is easy to forget that string theory was the pop-sci media darling for quite a while before it became the punching bag de jeur. The sooner this all passes, the better.