Thursday, 12 July 2007

Minimalistic Dark Matter

These days, cosmologists, astrophysicists and all that lot fill every nook and crannie of CERN TH. They also fill the seminar schedule with their deep dark matter talks. I have no choice but to make another dark entry in this blog. Out of 10^6 seminars i've heard this week i pick up the one by Marco Cirelli about Minimal Dark Matter.

The common approach to dark matter is to obtain a candidate particle in a framework designed to solve some other problem of the standard model. The most studied example is the lightest neutralino in the MSSM. In this case, the dark matter particle is a by-product of a theory whose main motivation is to solve the hierarchy problem. This kind of attitiude is perfectly understandable from the psychological point of view. By the same mechanism, a mobile phone sells better if it also plays mp3s, makes photographs and sings lullabies.

But after all, the only solid evidence for the existence of physics beyond the standard model is the observation of dark matter itself. Therefore it seems perfectly justified to construct extensions of the standard model with the sole objective of accommodating dark matter. Such an extension explains all current observations while avoiding the excess baggage of full-fledged theoretical frameworks like supersymmetry. This is the logic behind the model presented by Marco.

The model is not really minimal (adding just a scalar singlet would be more minimal), but it is simple enough and cute. Marco adds one scalar or one Dirac fermion to the standard model, and assigns it a charge under SU(2)_L x U(1)_Y. The only new continuous parameter is the mass M of the new particle. In addition, there is a discrete set of choices of the representation. The obvious requirement is that the representation should contain an electrically neutral particle, which could play the role of the dark matter particle. According to the formula Q = T3 + Y, we can have an SU(2) doublet with the hypercharge Y= 1/2, or a triplet with Y = 0 or Y = 1, or larger multiplets.

Having chosen the representation, one can proceed to calculating the dark matter abundance. In the early universe, the dark matter particles thermalize due to their gauge interactions with W and Z gauge bosons. The final abundance depends on the annihilation cross section, which in turn depends on the unknown mass M and the well known standard model gauge couplings. Thus, by comparing the calculated abundance with the observed one, we can fix the mass of the dark matter particle. Each representation requires a different mass to match the observations. For example, a fermion doublet requires M = 1 TeV, while for a fermion quintuplet with Y = 0 we need M = 10 TeV.

After matching to observations, the model has no free parameters and yields quite definite predictions. For example, here is the prediction for the direct detection cross section:We can see that the cross sections are within reach of the future experiments. The dark matter particle, together with its charged partners in the SU(2) multiplet, could also be discovered at colliders (if M is not heavier than a few TeV) or in the cosmic rays. There are the usual indirect detection signals as well.

The model was originally introduced in a 2005 paper. The recent paper corrects the previous computation of dark matter abundance by including the Sommerfeld corrections.


Anonymous said...

Physics is a reductionist science. The string is hypothesized to be the smallest particle of which everything is made. The goal of reductionist physics is to find a few simple principles that underlie complex phenomena. The string theorists invent astonishing physical and mathematical complexity as the end point of reductionism. Well, quite obviously, the end point of reductionism is a theory as Einstein stated that we can teach to the kids and quite obviously not a theory that nobody can understand. When the end point of reductionism is the greatest complexity imaginable it is just plain absurd.

In 2000 an independent scientist working alone sent a copy of his book, The N-particle Model, to all the physics graduate students at Berkeley. Now he’s back and on the Reference Frame. He claims the universe is made of a single elementary particle that he now names the Ö particle and that particle is neither created nor destroyed. He claims its energy is 2.68138x10^-54 J. He claims the small size of the Ö particle is the reason electric, magnetic, photon and gravity fields appear continuous.

There is the question about lemmings when they get to the edge of the cliff. Do they choose to jump off or are they responding only to herd instinct or maybe aerodynamically drafted. It looks to me like the theorists are right on the edge.

aaacss said...

There is an even simpler quantum gravity theory that proposes that all that exists is one continous space-time fluid with no particular particle size.

Of course the plank scale would be the smallest measureable part of the fluid. This means empty space is filled with the same stuff as a car, or a tree, or you or me. It's only the qualities of that stuff (charge, mass, etc.) that make it appear different to each of us.

Therefore, our perception is like measuring the temperature of boiling water. One doesn't measure the actual stuff of the water itself, only one of it's qualities.

Gphillip, Houston TX

Jester said...

oh no...i was away for a few days and i find my blog overridden by crackpots :-( Go away. There's no use. Nobody's reading this blog anyway.