1. Crete Center for Theoretical Physics
A remarkable claim has been made by the OPERA experiment, that takes a neutrino beam from CERN and studies its interactions inside the Gran Sasso laboratory in central Italy.
As described in their paper http://fr.arxiv.org/abs/1109.4897 submitted to the ArXiV, they have measured the velocity of the neutrinos and found it to be 2.5 X 10^(-5) times larger than that of light in vaccum, using a clever technique proposed in http://arxiv.org/abs/0805.0253.
The result is more than 6 sigma away from the expected value of the velocity of neutrinos, namely the velocity of light.
The result is so unexpected that it will take some time untill it passes further scrutiny and is confirmed by other experiments.
It should be noted that Supernova 1987a data already excluded this state of affairs if the phenomenon is energy independed. However as supernova neutrinos had an energy of about 10 MeV while OPERA neutrinos have 10 GeV, it suggests that the effect has quadratic dependence on energy and is therefore due to a non-renormalizable operator.
A potential loophole that has not obviously been considered is the use of general relativistic corrections to the path of neutrinos.
These may correct the result wich takes as the distance the straight line between emission and interaction points.
A simple newtonian simplified solvable model indicates that such corrections give an effective speed that is larger than than the standard one.
In Scwartzschild modeling of the gravitational field f the earth, the (small) parameter that controls the gravitational corrections is x=GM/(Lc^2) where G is Newton’s constant, M the mass of the earth, L is the length of the path (about 750 Km) and c is the speed of light in vacuum.
We expect the result to have a regular expansion in x. Putting the values of the various constants we obtain that x~10^(-8) and is probably too small to affect the result.
There are not many theoretical ideas on how to achieve this state of affairs from first principles assuming that the experimental result is correct. Of course one could just postulate that the speed of neutrinos is different (larger) than c, but this is excluded by supernova data. To make it compatible with such data , Lorentz invariance should be broken.
There however another way that is possible: to have the SM live on a brane-world and the neutrinos to be able to travel in a warped bulk.
This phenomenon was found first as a side effect in a holographic study of the effective potential in N=4 superYM using probe branes by CCTP member E. Kiritsis in hep-th/9906206. It was observed that on a brane, embeded in the background of other black branes, a velocity of light is induced on the brane, that is variable and depends on its position in the bulk. Moreover the velocity of light is smaller that that of the bulk. S. Alexander shortly afterwards provided some brane models that stabilize such a velocity of light to arbirary small values.
This line of ideas lend to the developement of the idea of Mirage Cosmology.
A bit later and probably independently, Chung and Freeze suggested that faster propagation outside of branes could be used to solve the horizon problem in cosmology. This subject was subsequently developed by Caldwell and Langlois.
Finally Herdeiro and Gibbons have shown in full generality that the speed of light on D-branes is always smaller or equal to that of the bulk.
This line of reasoning suggests that models where (a) the SM is living on a brane, and (b) neutrinos can move in a approapriately warpoed bulk can natuarally produce a superluminal speed for neutrinos.
Such models where entertained in the context of orientifold in string theory in hep-ph/0004214 and in more detail in hep-th/0210263 by I. Antoniadis, E. Kiritsis, I. Rizos and T. Tomaras.
In these works the foundations of the bottom-up approach to string theory model building were introduced. The right-handed neutrinos were leaving in the bulk and they were mixing with the doublet neutrinos on the brane. This could potentially provide a first principles model compatible with the OPERA data.
2. Luboš Motl
Superluminal neutrinos from noncommutative geometry
In two previous blog entries, I discussed possible mistakes in the Opera experiment and theoretical reasons why they probably exist. But it seems pretty likely to me that a Fermilab experiment will confirm the Opera result – either because there is new physics or, more likely, because it will be affected by the same glitch in the GPS system 😉 – and theorists will be increasingly pushed to give an explanation. Imagine that we’re really forced to admit that the neutrinos are faster than the photons.
What changes will we apply to our theoretical picture of the world? What’s the most sensible setup to rebuild our understanding of the reality? Does string theory offer some semi-natural tools to account for the different speeds? Well, I will mostly promote the famous 3,000-citation 1999 article by Nathan Seiberg and Edward Witten, «String Theory and Noncommutative Geometry» to get a flavor of some semi-realistic attempts to assign different speeds to the massless and very light particles…..
….. Read more: http://motls.blogspot.com/2011/09/superluminal-neutrinos-from.html