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Searching for Extra Dimensions May 5, 2006

Posted by dorigo in news, physics, science.

Particle physics today is at a crossroad.

The last thirty years have been spent by experimental particle physicists to look and find more and more precise confirmation of the validity of the Standard Model, a theory that explains quite accurately the observed behavior of subatomic particles, and which has allowed precise computations to be made of phenomena not yet observed – and later confirmed.

On the other hand, very little has emerged that was not expected. Even the discovery of neutrino oscillations, a property of these almost-but-not-quite massless particles, although not fitting in the general framework of the existing theory, had been foreseen decades in advance, and a non-zero mass of neutrinos changes little in the general understanding of the field.

Despite these successes, theoretical physicists are restless. Why ? Because the Standard Model is not everything. 

First of all, the SM only explains a part of the properties of subatomic particles: it does not explain gravity, a very weak force but the one which we need to understand most urgently these days, when astrophysicists are puzzled by the observed properties of our universe.

Second, the SM bears some inconsistencies. No big deal, but there are things that smell funny, and hint at the possibility that there is something deeper to discover – something that includes the SM as a first-order explanation.

To clarify, think at Newtonian Mechanics: it is all good, works perfect to determine the trajectory of your tennis ball, at the speed you normally throw it. But if you were to push a tennis ball to half the speed of light, you would need Relativistic Mechanics, and Newtonian Mechanics would seriously misrepresent the reality.

So, one possibility theoretical physicists speculate about is that besides the regular four dimensions we live in, there are more. Small dimensions. Something we would not be make a step into, because they are shrunk to a sub-millimeter scale: that is why we would not be aware of them.

And how do we test that crazy hypothesis ? Because as experimental physicists, we need to give an answer to that speculation, by investigating whether it provides explanation to any observable phenomena.

We can test the hypothesis by studying very high-energy collisions between protons and antiprotons, at the Tevatron collider. If extra dimensions exist, they would be responsible for the occurrence of funny collisions when the protons break apart, emitting a dense and energetic stream of particles in one direction – a jet, that is – and nothing on the opposite side.

That is a quite distinctive signature: only very few competing processes predicted by the Standard Model can produce a energetic jet recoiling against a undetected particle (which, in the case of extra dimensions, is a graviton).

The CDF collaboration studied that signature, looking for events with a jet of large energy (transverse component larger than 150 GeV) and invisible energy above 120 GeV – the energy the graviton might be carrying away unseen.

In 368pb-1 of collisions – the amount collected from 2001 to 2004 – CDF did observe 263 events of the kind described. However, they are exactly what the Standard Model predicts: no need for an extra theory to justify what is seen, that is. Those events are mostly produced when a Z boson is produced together with one energetic jet, and the Z decays to two neutrinos – disappearing, for all practical purposes.

Using the fact that there is consistence between SM prediction and observation, CDF can play "limit the model": place constraints to the extension of these additional dimensions.

A paper has been submitted with the CDF results last month. You can find a preprint at the following address:


The main result of the analysis is shown in the picture below. There, on the x axis you see the number of extra dimensions hypothesized. For each hypothesis, the bars show the minimum value of a parameter that defines the theory of large extra dimensions: it is labeled Md and stands for "(4+n)-dimensional Planck scale". The larger that value is, the less impact the extra dimensions have on our world.

The plot shows the minimum value found by three different experiments: the LEP ones, D0, and CDF. The CDF result limits more than the others the minimum value of Md, if one hypothesizes the existence of 4 or more extra dimensions.



1. diptarka - June 19, 2007

i cannot understand the graph properly, please send a mail to diptarka.das@gmail.com

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