Saturday, 5 September 2009

Hidden Photons and New Forces - Latest Limits

Standard physics claims the world it built from just 4 forces, electromagnetism, the weak and strong nuclear forces, and gravity. This has been the known state of the act
of physics since the late forties. Is it still possible there are more unknown forces at work in the universe. Certainly cosmologist have found known matter to be just 5% of the universe, and dark matter and dark energy could easy interact with unknown forces of which we could say little. Here however I want to see whether current measurement allow for any extra force acting on the sort of matter we find on earth, i.e. acting on neutrons, protons or electrons.

Since I've been making a claim of a new force in my paper and on this blog, axitronics, it behoves to me the duty, of every so often checking physics papers to see if any new experiments rule out my force. Fortunately I'm not the only one claiming new forces. String theory generically predicts many extra U(1) (electromagnetism like) interactions which could have any range or strength. A recent paper, Naturally Light Hidden Photons in LARGE Volume String Compactifications, looks at both the theory and experimental search for such forces. I'll ignore the theory, and concentrate on the experimental limits. The diagram above is taken from that paper. The horizontal (log scale) axis represents the mass of the force carrying particle (given the force a maximum range of around hbar/mc), while the vertical axis represents how much the force mix with ordinary light, given ordinary electronics a fractional charge in whichever new force. The yellow regions are places where a new force might be useful is explaining cosmological data, and the other colour regions are excluded bye existing experiments. In fact very large parameter space exists where new forces can exists.

Which regard to my axial force nothing in the above data excludes it, firstly conversion of parity absolute forbids mixing of the axi-force carrier, the axi-photon, with ordinary photons in the vacuum, and secondly I've assumed the force to be massless in the vacuum. However to the axial force, vacuum is very hard to find, there will always be some neutrinos around, and this leads to an effective mass for axi-photon depending on the neutrino density. For the earth based experiments this gives any effective mass (based on the neutrino density in the atmosphere and presumably in vacuum in earth based labs), of about 100eV), leading to a range of about 5nm. However in deep space and based on the ordinary estimates of neutrino density, the effective mass will be about 0.3 neV (nano eletron volts), and a range of about 7mm. Thus the axial force is a natural chameleon changing with its environment, just like chameleon theories of dark energy suggest.

The most recent experiment looking for a dark energy chameleon particle, is the GammeV experiment. Which looked for light converting to other small mass particles, inside a very strong magnet. Its excluded particles up to 2meV. A much smaller mass than our prediction, so at present, it seems the axial force, dark energy theory, is still very viable.

Perphaps the best test of a fifth force, and certainly one like the axial force, is in neutron interactions. A recent measurement of the ultra cold neutrons which limits any force with range greater than about 1 micrometer, and strength stronger 10^12-10^15 times gravity or 10^24 times weaker than electromagnetism. This is a very stringent limit, but once again the chameleon type, screening of the axial force by neutrinos on the earth seems to hide the axial force very effectively. It seems to me that the axial force is still allowable to present experimental tests. This is not so true of many other forces, only the screening by the neutrino background, allows the force to hide so effectively.

1 comment:

Dj said...

Hello Dr Adams! I must say that is a very very impressive blog you've got! By the way, i am not an established theoretical physicist (but there isn't anything that i wouldn't give to become one! :) ) I'm just an 18 year old kid trying to understand such concepts on my own. Such stuff is really challenging and above all it is truly amazing. I am in the middle of my exams now.. Will read your blog in detail after they get over.
Dhananjay K.R