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.

Countdown to Planck

These days the CERN Theory Institute program is focused on the interplay between cosmology and LHC phenomenology. Throughout July you should expect overrepresentation of cosmology in this blog. Last Wednesday, Julien Lesgourgues talked about the Planck satellite. Julien is worth listening to. First of all, because of his cute French accent. Also, because his talks are always clear and often damn interesting. Here is what he said this time.

Planck is a satellite experiment to measure the Cosmic Microwave Background. It is the next step after the succesful COBE and WMAP missions. Although it looks like any modern vacuum cleaner, the instruments offer 2*10^(-6) resolution of temperature fluctuations (factor 10 better than WMAP) and 5' angular resolution (factor 3 better than WMAP). Thanks to that, Planck will be able to measure more precisely the angular correlations of the CMB temperature fluctuations, especially at higher multipoles (smaller angular scales). This is illustrated on this propaganda picture:

Even more dramatic is the improvement in measuring the CMB polarization. In this context, one splits the polarization into the E-mode and the B-mode (the divergence and the curl). The E-mode can be seeded by scalar gravitational density perturbations which are responsible for at least half of the already observed amplitude of temperature fluctuations. For large angular scales, the E-mode has already been observed by WMAP. The B-mode, on the other hand, must originate from tensor perturbations, that is from gravity waves in the early universe. These gravity waves can be produced by inflation. Planck will measure the E-mode very precisely, while the B-mode is a chalenge. Observing the latter requires quite some luck, since many models of inflation predict the B-mode well below the Planck sensitivity.

Planck is often described as the ultimate CMB temperature measerument. That is because its angular resolution corresponds to the minimal one at which temperature fluctuations of cosmological origin may exist at all. At scales smaller than 5' the cosmological imprint in the CMB is suppresed by the so-called Silk damping. 5' corresponds roughly to the photon mean free path in the early univere so that fluctuations at smaller scales get washed out. However, there is still room for future missions to improve the polarization measurements.

All these precision measurements will serve the noble cause of precision cosmology, that is a precise determination of the cosmological parameters. Currently, the CMB and other data are well described by the Lambda-CDM model, which has become the standard model of cosmology. Lambda-CDM has 6 adjustable parameters. One is the Hubble constant. The other two are the cold (non-relativistic) dark matter and the baryonic matter densities. In this model matter is supplemented by the cosmological constant, so as to end up in the spatially flat universe. Another two parameters describe the spectrum of gravitational perturbations (the scalar amplitude and the spectral index). The last one is the optical depth to reionization. Currently, we know these parametes with a remarkable 10% accuracy. Planck will further improve the accuracy by a factor 2-3, in most cases.

Of course, Planck may find some deviations from the Lambda-CDM model. There exist, in fact, many reasonable extensions that do not require any exotic physics. For example, there may be the already mentioned tensor perturbations, non-gaussianities or the running of the spectral index, which are predictions of certain models of inflation. Planck could find the trace of the hot (relativistic) component of the dark matter. Such contribution might come from the neutrinos, if the sum of their masses is at least 0.2 eV. Furthermore, Planck will accurately test the spatial flatness assumption. The most exciting discovery would be to see that the equation of state of dark energy differs from w=-1 (the cosmological constant). This would point to some dynamical field as the agent responsible for the vacuum energy.

Finally, the Planck will test models of inflation. Although it is unlikely that the measurement will favour one particular model, it may exclude large classes of models. There are two parameters that appear most interesting in this context. One is the spectral index nS. Inflation predicts small departures from the scale invariant Harrison-Zeldovich spectrum corresponding to nS=1. It would be nice to see this departure beyond all doubt, as it would further strengthen the inflation paradigm. The currently favoured value is nS = 0.95, three sigma away from 1. The other interesting parameter is the ratio r of the tensor to scalar perturbations. The current limit is r < 0.5, while Planck is sensitive down to r = 0.1. If the inflation takes place at energies close to the GUT scale, tensor perturbations might be produced at the observable rate. If nothing is observed, large-field inflation models will be disfavoured.

Planck is going to launch in July 2008. This coincides with the first scheduled collisions at the LHC. Let's hope at least one of us will see something beyond the standard model.

No slided as usual.

Nima's Marmoset

Here is one more splinter of Nima Arkani-Hamed's CERN visit. Apart from a disappointing seminar for theorists, Nima gave another talk advertising his MARMOSET to a mostly experimental audience. OK, I know it was more than two weeks ago, but firstly it's summertime, and secondly, i'm still doing better with the schedule than the LHC.

MARMOSET is a new tool for reconstructing the fundamental theory from the LHC data. When you ask phenomenologists their opinion about MARMOSET, officially they just burst out laughing. Off the record, you could hear something like "...little smartass trying to teach us how to analyze data..." often followed by *!%&?#/ ^@+`@¦$. I cannot judge to what extent this kind of attitude is justified. I guess, it is partly a reaction to overselling the product. To my hopelessly theoretical mind, the talk and the whole idea appeared quite interesting.

In the standard approach, the starting point to interpreting the data is a lagrangian describing the particles and interactions. From the lagrangian, all the necessary parton level amplitudes can be calculated. The result is fed to Monte Carlo simulations that convolute the amplitudes with the parton distribution functions, calculate the phase space distributions and so on. At the end of this chain you get the signal+the SM background that you can compare with the observations.

Nima pointed out several drawbacks of such an approach. The connection between the lagrangian and the predicted signal is very obscure. The lagrangians have typically a large number of free parameters, of which only a few combinations affect the physical observables. Typically, the signal, e.g. a pT distribution, has a small dependence on the precise form of the amplitude. Moreover, at the dawn of the LHC era we have little idea which underlying theory and which lagrangian will turn out relevant. This is in strong contrast with the situation that has reigned in the last 30 years, when the discovered particles (the W and Z bosons, the top quark) were expected and the underlying lagrangian was known. Nima says that this new situation requires new strategies.

Motivated by that, Nima&co came up earlier this year with a paper proposing an intermediate step between the lagrangian and the data. The new framework is called an On-Shell Effective Theory (OSET). The idea is to study physical processes using only kinematic properties of the particles involved. Instead of the lagrangian, one specifies the masses, production cross sections and decay modes of the new particles. The amplitudes are parameterized by one or two shape variables. This simple parameterization is claimed to reproduce the essential phenomenology that could equally well be obtained from more complicated and more time-consuming simulations in the standard approach.

MARMOSET is a package allowing OSET-based Monte Carlo simulations of physical processes. As the input it requires just the new particles + their production and decay modes. Based on this, it generates all possible event topologies and scans the OSET parameters, like production and decay rates, in order to fit the data. The failure implies necessity to add new particles or new decay channels. In this recursive fashion one can extract the essential features of the underlying fundamental theory.

This sounds very simple. So far, the method has been applied under greenhouse conditions to analyze the "black boxes" prepared for the LHC olympics. Can it be useful when it comes to real data? Proffesionals say that MARMOSET does not offer anything they could not, if necessary, implement within half an hour. On the other hand, it looks like a useful tool for laymen. If a clear signal is discovered at the LHC, the package can provide a quick check if your favourite theory is able to reproduce the broad features of the signal. Convince me if I'm wrong... Anyway, we'll see in two years.

The video recording available here.

Xenon10 taking the lead

It is a bit belated news but i still find it worthy to point out. Two weeks ago the XENON collaboration posted a paper with new limits on dark matter from direct detection. The paper follows earlier announcements and a note in Nature.

There must be a flux of dark matter passing constantly through the Earth. If the dark matter particle belongs to the WIMP category, it is supposed to have weak strength interactions with the Standard Model matter. In such a case, once in a while it should collide with an atom and we may observe a recoil energy transferred to the nucleus. The XENON10 expreriment uses xenon as a target. They apply some clever techniques to reduce the background from other particles (e.g. neutrons or photons) penetrating the tank. Interested engineers and hobbyists may find the details in the paper. For all the rest, the important stuff is summarized in this plot:
It shows the limit on the spin-independent scattering cross section of dark matter on nucleons as a function of the dark matter particle mass. The new limits are better by a factor of six than the previous ones from the CDMS-II experiment. I guess that at this point the controversial DAMA detection signal can be forever buried in oblivion. For reference, the plot shows expected cross sections in the constrained MSSM scenario.

Of course, the importance of the XENON10 results goes far beyond constraining the parameter space of some obsolete models. The exciting point is that we are really probing the cross sections expected for WIMP particles. In the coming years we either confirm the WIMP hypothesis or make it implausible. No need to tell that an eventual positive signal would have a huge impact on the LHC program and science fiction literature.

Nima's Horizons

Nima Arkani-Hamed is yet another soul that has fallen to the spell of the landscape. Landscape is a perfect framework for predicting things we already know. Nima is a bit more clever than that. He is concerned with a bigger picture. I mean, with conceptual questions associated with the existence of multiple vacua in a gravity theory.

Nima shared some remarks on the subject yesterday in a theory seminar. He drew an analogy between the present situation in particle physics and the early days of quantum mechanics. In the latter case, quantum effects turned out to imply loss of predictivity concerning the results of individual measurements. Now, he believes, we should again accept certain loss of predictivity due to the landscape and the anthropic selection.

That was an introduction. The bulk of the talk was about the Standard Model landscape. It turns out that the landscape pops up in the minimal Standard Model coupled to Einstein gravity with a small cosmological constant. Such a system has, of course, a unique 4D de Sitter vacuum, but there exist many more vacua with compactified spatial dimensions. One example is the class of vacua with the AdS3xS1 geometry. The radius of the circle is stabilized by the interplay of the small cosmological constant and the Casimir energy induced by the photon, the graviton and the light neutrinos. The funny accident is that these vacua would not exist if the cosmological constant were a factor of 10 larger or if the solar neutrino mass squared difference were a factor of 10 smaller.

Nima went on discussing some more technical details of this setup:

• A near-moduli space of the photon Wilson line wrapping the circle.
• Black string solutions interpolating between 4D and 3D vacua.
• The two-dimensional CFT dual to the Standard Model.

All that you can find in his recent paper.

Up to this point, i've been almost fair. Now it's time for a few snide remarks. Listening to this talk was like visiting a flea market. It was colourful and entertaining, but most of the things on display seemed utterly useless. Nima forgot to say what can the Standard Model landscape teach us about the big questions he had addressed at the beginning. Well, certainly the landscape may be present in far simpler setups than string theory. But in this very example it seems totally irrelevant, both from the theoretical and the experimental points of view. I had this guilty feeling that i wouldn't even bother to listen if the name of the speaker were different. Judging from the looks on the others' faces, i wasn't all alone...

Transparencies not available, as usual, though this time it isn't so much of a waste.

Wilczek's Phantoms

Last Thursday we had a colloquium by Frank Wilczek here at CERN. Frank has made some impressive contributions to such different areas as astrophysics, particle physics and condensed matter physics. He has also provided a lot of beautiful insight into quantum field theory (asymptotic freedom, fractional statistics, color superconductivity). This was a good sign. On the other hand, Frank is also a Nobel-prize winner. This was a bad sign. Nobel-prize winners tend to fill their talks with banal statements written in large font to make a more profound impression. In the end, we observed a fight between good and bad. The latter being the winning side, i'm afraid.

Snide remarks aside, the colloquium had two separate parts. In the first one, Frank was advertising the possibility of phantoms appearing at the LHC. Phantoms refer to light scalar fields that are singlets under the Standard Model gauge group. It is impossible to write renormalizable interactions with the Standard Model fermions (except for the right-handed neutrino), which might be a good reason why we haven't observed such things so far. We can write, however, renormalizable interactions with the Higgs. Therefore the phantom sector could show up once we gain access to the Higgs sector.

Various better or worse motivated theories predict the existence of phantoms. Probably the best motivated phantom is the one especially dear to the speaker: the axion. This was the bridge to the second part of the talk, based on his paper from 2005, where Frank discussed the connection between axions, cosmology and ...the anthropic principle. Yes, Frank is another stray soul that has fallen under the spell of the anthropic principle.

Axions have been proposed to solve the theta-problem in QCD. As a bonus, they proved to be a perfect dark matter candidate. Their present abundance depends on two parameters: the axion scale f where the Peccei-Quinn symmetry is broken and the initial value of the axion field theta_0. The latter is usually expected to be randomly distributed because in the early hot universe no particular value is energetically favoured. With random theta_0 within the observable universe, there is the upper bound f <> 10^12 GeV.

The scenario with a low-scale inflation was the one discussed. Now theta_0 is a parameter randomly chosen by some cosmic accident. One can argue that the resulting probabilistic distribution of dark matter abundance (per log interval) is proportional to the square root of this abundance, favouring large values. Enters the anthropic principle. The observation is that too much dark matter could be dangerous for life. Frank made more precise points about halo formations, black holes, too close star encounters, matter cooling and so on. In short, using the anthropic principle one can cut off the large abundance tail of the probability distribution. One ends up with this plot:
The dotted line is the observed dark matter abundance. The claim is that axions combined with anthropic reasoning perfectly explain dark matter in the universe.

My opinion is that postdictions based on the anthropic principle aren't worth a penny. This kind of results relies mostly on our prejudices concerning the necessary conditions for life to develop. If they prove anything, it is rather limited human imagination (by the way, i once read an SF story about intelligent life formed by fluctuations on a black hole horizon :-) Only impressive, striking and unexpected predictions may count. That's what Weinberg did. That's why some exclaimed "Oh shit, Weinberg got it right". Nobody would ever use a swearword in reaction to the plot above...

For more details, consult the paper. If you are more tolerant to anthropic reasoning, here you can find the video recording.

SN 1987A

Twenty years ago, Sanduleak 69 202a - a blue giant in the Large Magellanic Cloud - turned into a supernova before our very eyes. This very well designed cosmic experiment allowed to peer into supernova dynamics, learn about neutrino properties, put some constraints on physics beyond the Standard Model, and much more. And the pictures are so spectacular :-) Arnon Dar was telling us all about it at the TH seminar last Wednesday. So many things were said that I cannot report them all here. I just pick up a few stories.

Neutrinos: In some 15s, the SuperKamiokande and the IMB neutrino detectors registered a total of 20 neutrinos above the background. This was the first and so far the only detection of neutrinos from beyond the Solar System. Registering the neutrino pulse confirmed that the core collapse model of supernova explosion is roughly valid and that the gravitational energy of the collapse is released mostly into neutrinos. On the quantitative side, the neutrino pulse allowed to estimate the total energy and the temperature of the explosion. For the neutrino community, it provided constraints on the magnetic moment and the electric charge of neutrinos, as well as on their right-handed couplings.

New physics: From the duration of the neutrino pulse we know that other hypothetical light particles cannot release supernova energy too efficiently. This provides bounds on putative theories beyond the Standard Model. For example, one can derive an important upper bound on the axion mass, m < 0.01 eV.




Rings: The rings consist of gas ejected from the progenitor star some 20000 years prior to the explosion. It is not known precisely what shaped this beautiful structure. The rings were lightened by the supernova ejecta only several months after the explosion. The delay allowed to calculate the distance to the supernova: 168,000 light-years. Thus we know quite accurately the distance to the Large Magellanic Cloud, which is the important element in the cosmic distance ladder. In this way, SN1987A contributed to measuring the Hubble constant.

Light curve: The afterglow is powered by radioactive decay of the heavy elements produced during the explosion. The fall of the supernova brightness nicely fits into the picture of the radioactive decay chain: Nickel-56 -> Cobalt-56 -> Iron-56. This confirmed the expectations that the heavy elements in the Universe are produced and scattered by supernova explosions. The amount of Cobalt-56 produced could be quite accurately estimated to be 0.07 solar mass.

Missing remnant: Observations agree very well with models of the core collapsing into a neutron star. However the neutron star remnant has not been observed so far? Is it just veiled by dust or did something else form. A black hole? A hyperstar? A quark star?

Arnon spoke also about the current research in supernova physics. He devoted quite some time to gamma ray bursts and his cannonball model. But that would be too much for one post...

So much fun from such brief fireworks. No new physics was found, but tons of new astrophysics. Unfortunately, due to budget cuts in science, no other nearby supernova has been exploded in recent years. There are however serious projects to explode Eta Carinae in the Milky Way, see here or here. Let's wait and see ;-)

Transparencies not available, as usual.