News and Gossips from the LHC

Since two weeks Planck stands first of all for a satellite CMB observatory, but it's also the name for an annual series of conferences on physics beyond the standard model. This year's edition is taking place in the furnace of Padova. Since Tommaso is around, he will surely describe everything in great detail, including color of the tie of each speaker, while I should later write a summary of the interesting ideas discussed here in case there is any. But for now I'd like to share a handful of interesting facts about the progress of the LHC that I learned from a supercool talk delivered here by Jörg Wenninger. I guess most of what's below is not new and should be familiar to those closely following the LHC saga.

One interesting fact I was not aware of: a quench (a phase transition from superconductivity to normal conductivity) of an LHC magnet can be induced by just a few milijoules of energy. That energy may be provided by a bunch of strayed protons from the beam . To avoid quenching, LHC cannot lose more than a millionth part of its beam. For comparison, the Tevatron loses about one thousandth of its beam during acceleration. In that respect, Jörg was very convincing that the LHC would ever work ;-) But then, miracles do happen, sometimes.

Another interesting part of the talk was the explanation why the LHC will run at 10 TeV in the center of mass, instead of the nominal 14 TeV. The story goes as follows. Before installing, the LHC magnets have to be "trained", that is to say, to undergo a series of quenches to let their coils settle down at stable positions. After being installed in the tunnel they are supposed to come back to their test performance with no or few quenches. It turns out that the magnets provided by one of the three manufacturing companies need an extraordinary number of quenches to settle down. Although the company in question was not pointed at, everybody knows that the name is Ansaldo. In the case of that company, the number of quenches required for stable operation at 7 TeV per beam is currently unknown, it is probably somewhere between a hundred and a thousand. At the moment it is not clear if the LHC will ever reach 14 TeV; 12-13 TeV might be a more realistic goal.

The talk gave also a detailed account of the incident of September 19 known as the 9/11 of particle physics. Although the evidence has evaporated, one can quite reliably outline the sequence of events. An abnormally large resistance in one of the magnets acted as a heat source that quenched the superconducting cable at one interconnection. In case of a quench the current should start flowing or a few minutes through the copper bus-bar that encloses the cable until the energy stored in the magnet is removed. However, due to bad soldering of an interconnection the current could not flow normally and an electric arc was created. This melted copper, punctured the helium enclosure which led to spilling of 6 tons of helium into the tunnel. The logo of the company that made the faulty magnet is always erased in the pictures, although everybody knows that the name is Ansaldo.

So what's next? The repairs of the damaged sector are almost finished. The current plan is to head for collisions this year (with a caveat "depends how one defines collisions"). Beam commissioning is scheduled for September/October and the first collisions could happen in November. The schedule is very tight and, moreover, the quality control of has revealed problems like bad soldering or reduced electrical contact in a number of places, including sectors that are already cold. The rumor is that some of the LHC magnets in reality turned out to be electric kettles.

The slides here.

Bon Voyage, Planck

The rumors of new imminent delays at the LHC imply that this year we need to look for action elsewhere. Fortunately, experimental astroparticle physics is truly enjoyable these days. FERMI and PAMELA are probing high energy cosmic rays, and there is still a plenty of hope that telltale signals of dark matter will be uncovered. That celebrity couple may soon be outshined by another competitor at shorter wavelengths - the Planck cosmic microwave background observatory. Contrary to my fears, the Ariane rocket which carried Planck into space was not shot down by an Imperial cruiser. Planck is now on its way to the L2 point and in one and half year or so we will bath in a plenty of new precise cosmological data.

Planck is the third in a row, after COBE and WMAP, to chase after small anisotropies of the CMB. At first sight the mission comes close to Lord Kelvin's nightmare: Planck will measure what its predecessors have measured, but more precisely, with better resolution, and in more colors. From the propaganda plot on the right one can see that one practical virtue of Planck is the access to higher multipoles of the CMB temperature anisotropy. Probing more acoustic peaks and the damping tail will allow us to precisely determine the cosmological parameters and put the currently ruling Lambda-CDM cosmological model to a thorough test.

Doesn't sound too exciting? Of course, there is always a good chance that something unexpected will emerge from the data. However, I'm going to argue that even confirming the boring cosmological standard model may provide us with extremely interesting pieces of information. In particular, there is one important question to which Planck, with a little bit of luck, may provide an answer: what is the scale of inflation?

The past missions have collected some shreds and pieces of information about inflation. First of all, we know that the highly primitive model of inflation - a single scalar field slowly rolling down its potential - perfectly describes all available data. That is to say, the power spectrum of the primordial density fluctuations that ultimately produced the CMB temperature fluctuations can be explained by quantum fluctuations of that scalar field. Furthermore, we know something about the potential that provides for vacuum energy driving the accelerated expansion during inflation. In particular, the overall scale of the potential can be inferred from the amplitude of the temperature fluctuations observed by COBE and WMAP. This yields
$(V/\epsilon)^{1/4} \sim 3 \cdot 10^{16}$ GeV
where $\epsilon = (V'/V)^2/2M_{Pl}^2$ is one of the so-called slow-roll parameters. The slow-roll parameters must be small during inflation, of order 0.01 or less, which sets the upper bound on the scale of inflation. But, in principle, there's no lower limit on $\epsilon$, and at this point we cannot make a definitive statement about the magnitude of V.

Planck has a good chance to ultimately pinpoint the scale of inflation. The hopes are based on Planck's fantastic ability to measure the CMB polarization. Thomson scattering at the last scattering surface results in linear polarization of the CMB photons. The polarization can be decomposed into the E-mode (gradient) and the B-mode (curl), each of which is then decomposed into multipoles, much as the temperature fluctuations. The lower multipoles of the E-mode have already been detected by WMAP; the B-mode is more tricky and it is waiting for Planck.

The importance of the B-mode follows from the fact that, at the linear level, it is not produced by scalar density perturbations, but only by tensor perturbations, that is by the primordial gravity waves. The amount of tensor perturbations is directly related to the scale of the inflationary potential. The larger V, the higher is the ratio of tensor to scalar primoridial perturbations. As an example, Planck's sensitivity to the primordial B-mode for the tensor-to-scalar ratio = .1 is plotted on the right. If the tensor-to-scalar ratio is high enough for Planck to detect the primordial B-mode, then we will have the first evidence of the existence of a very high-energy scale in particle physics. (who said neutrinos? it's not certain if they're really Majorana, and besides who cares about neutrinos anyway).

But of course the tensor-to-scalar ratio can be too small for Planck to measure. In the worst case scenario Planck may share the tragic fate of LEP: a successful experiment without much success. Let's cross our fingers.

More info in Planck Bluebook.

All eyes on Denver

Tomorrow (Saturday) morning, the FERMI/GLAST collaboration is going to announce their first results at the APS meeting in Denver. FERMI/GLAST is a satellite cosmic gamma-ray observatory, but it also has capabilities to measure the electron+positron spectrum. The latter is eagerly awaited by the particle physics community. Last year, the measurements of the cosmic ray positron fraction by PAMELA and of the combined electron+positron flux by ATIC have sparked some 150 theory papers and one paparazzi affair. Recall that PAMELA sees an excess of positrons over the background (whatever the background means) in the 10-100 GeV range, while ATIC claims there is a clear bump in the spectrum at around 700 GeV. One tantalizing interpretation of these data is that the excess positrons originate from annihilation or decay of TeV scale dark matter particles.

If you can't wait till tomorrow have a look at the plot extracted from a theory paper of two weeks ago. The solid black line on that plot by sheer accident reproduces pretty well the FERMI/GLAST data to come. The sexy ATIC bump is gone and is replaced with milder features: a shallow deep around 100 GeV followed by a mild rise toward 800 GeV, and then a steep decline consistent with the earlier HESS measurements. These new results neither exclude (there's still an excess) nor significantly support the dark matter cause (there's no smoking gun features). Next week arXiv will be flooded with papers refitting the earlier theoretical models to the new data.

Update: FERMI/GLAST has revealed the new measurement of the e+e- cosmic ray spectrum but the plot is not available yet - it will be published coming Monday. According to those who saw Denver's talk, the spectrum is indeed similar to the one plotted above, although the low energy (20-80 GeV) data points lie slightly below the background curve and the dip is even less pronounced. Also, FERMI's data stop at 1 TeV so the high-energy decline cannot be clearly seen. So at this point everything is clear: it's either dark matter or a pulsar or an alien civilization or maybe the galactic propagation model needs refining ;-). More insight should come from FERMI's measurement of the diffuse photon spectrum which is expected by late summer.

Update 2: and here is the original plot from Fermi's today paper on arXiv:
Also HESS got its foot in the door and just published new results for the electron+positron flux above 340 GeV, consistent with those of FERMI and inconsistent with ATIC.

Higgs Was At LEP

Everybody knows that the LEP experiment set a stringent limit on the Higgs boson mass - it has be larger than 114.4 GeV. The common expectation is that Higgs is just around the corner, and will be hunted down and roasted alive at the LHC or at the Tevatron. But this is not the only conceivable scenario that future may unfold: the world can be Higgsless, or the Higgs may be too wide or too invisible or too whatever to be detected at a hadron collider. There is yet another possibility that is definitely a bit crazy but is nevertheless not completely excluded. Namely, it is possible that the Higgs is lighter than 115 GeV and therefore kinematically available at LEP but ... we missed it.

How could Higgs have been missed? The point is that the 114.4 GeV limit strictly applies to a particle that walks, talks and couples just like the Standard Model Higgs boson. If we meddle with the Higgs couplings then, with a bit of skill, we can make Higgs effectively invisible to LEP. One obvious way to achieve that is to suppress the production rate. At LEP, Higgs would be dominantly produced by the process petnamed Higgsstrahlung where the e+e- collision first produces a Z boson which then radiates a Higgs boson. The LEP limits can be relaxed by suppressing the Higgs-Z-Z vertex by a factor of 3-4, see the black line on the plot below. However, this is not a theoretically plausible direction, as the electroweak precision observables suggest the existence of a light Higgs particle whose coupling to W and Z bosons is not suppressed. Besides, on the more philosophical side, a particle with a reduced coupling to Z should not be called Higgs (but rather, a scalar particle that slightly mixes with the Higgs). So let's leave the Higgs-Z-Z vertex alone. In that case we can still try to hide the Higgs from LEP by meddling with the Higgs decays.

The LEP collaboration was not that stupid and they also searched for the Higgs decaying in a non-standard way. You could think that Higgs could be hidden by making it invisible, that is to say, it could decay to some light, almost non-interacting particles that leave the detector undetected. This does not work: the signature involving a Z boson plus missing energy (carried out by the invisible stuff) is not easy to miss in a lepton collider, and in consequence the limit on the invisible is 114 GeV, almost as strong as that on the standard Higgs. Thus, paradoxically, to make Higgs invisible one must make it decay into something visible. LEP has concluded the following:

• Higgs decaying to a pair of jets of any flavor (rather than dominantly into b-jets as the standard Higgs) has to be heavier than 113 GeV
• Fermiophobic Higgs decaying dominantly to off-shell WW and ZZ has to be heavier than 110 GeV
• Higgs decaying dominantly into two photons has to be heavier than 117 GeV

All in all, Higgs lighter than 110 GeV decaying into a two-body final state is excluded. But the situation is far less clear if the final state contains more particles. For example, the Higgs can undergo a cascade decay: it first decays into a pair of light scalars or pseudoscalars which subsequently decay into a pair of quarks or leptons each. In that case we deal with a four-body final state, for example with four b-quarks or four tau-leptons (typically, the pseudoscalars decays into the heaviest quark or lepton that is kinematically available). This is of course impossible in the Standard Model, while in the MSSM it occurs only in an obscure corner of the parameter space. But in several popular extensions of the Standard Model, for example in the NMSSM (MSSM adorned by a singlet superfield) or in little Higgs theories such cascade decays appear often and willingly.

The possibility of avoiding the LEP bounds via the cascade decays was first pointed out by Radovan Dermisek and Jack Gunion in the context of NMMSM. In that model, there are new pseudoscalar states in the Higgs sector which can naturally be light and to which the true Higgs (the one that couples to Z with the largest strength) can decay. These pseudoscalars then decay into a pair of b quarks each, or into tau quarks if the pseudoscalar is lighter than twice the b-quark mass. The former possibility was excluded by a subsequent LEP analysis - the limit on the Higgs decaying into four b-jets is now 110 GeV - but the four-tau or the four-light-jet final states allow for a much lighter Higgs particle. See the exclusion limits for the case of four-tau cascade decay - the allowed region on this plot is almost non-existing but there is no limit above the Higgs mass of 85 GeV. The reason why that analysis stopped at 85 GeV is not physical but psychological: in the MSSM there is no parameter space that would allow to consider Higgs heavier than 85 GeV. This is a clinical case of the damage that happens when experimenters take theorists and their theories too seriously (following this logic, if the MSSM did not allow for a light Higgs one could completely skip the LEP experiment).

Hiding the Higgs is a nice prank in itself, but there are also some theoretical and phenomenological motivations for playing this game. Firstly, the electroweak precision observables are best fitted by a fairly light Higgs mass with the central value of order 80 GeV,
and the light Higgs of 90-100 GeV would alleviate the tension. Secondly, LEP saw a 2.3 sigma excess of Higgs-like bbar events around the mass of 100 GeV. That cannot be interpreted as the standard Higgs (the number of events would have been five times much higher), but can be perfectly explained by the Higgs decaying most of the times into four light quarks or leptons and one fifth of the times into the b quarks. Recall that in the final year of LEP a smaller excess created much larger theoretical activity.

Of course, a light elusive Higgs is a nightmare for the LHC. Fortunately, theories that motivate such a scenario typically predict a lot of new phenomena at the TeV scale to provide enough fun for the LHC experiment. Just that some people will have to wait a bit longer for their Nobel prize.

Here is the review of the non-standard Higgs decays.

Inelastic

As for today, only the DAMA experiment in Gran Sasso claims to have detected dark matter particles. The claim is based on observing the annual modulation of the number of scattering events in DAMA's sodium-iodine detector. Such an effect could arise due the motion of the Earth around the Sun that implies the annual variation of the Earth's velocity with respect to the sea of dark matter pervading our galaxy.

The experimental community is divided about DAMA. One half considers them ignorants who have no idea what they're doing, whereas the other half thinks that they deliberately rigged their results. Theorists, on the other hand, are by construction more open-minded (or maybe just bored) and they sometimes entertain the possibility that the DAMA signal might actually be dark matter. The challenge is then to explain why other, in principle more sensitive detection techniques have yielded null results. There has been several, less or more contrived proposals to reconcile DAMA with the stringent limits from other direct detection experiments like CDMS, XENON, CRESST, ZEPLIN and KIMS. The DAMA signal can be explained by the standard WIMP dark mater scattering on the sodium atoms if the dark matter particle has a fairly small mass of order 5 GeV (although there is some controversy about this interpretation). This post is about another scenario called inelastic dark matter, iDM in short. It was originally proposed quite some time ago, but recently it is becoming more and more fashionable.

A typical WIMP particle scatters elastically on the target nucleons, that is to say, it retains its identity in the process. In the iDM scenario, on the other hand, the cross section for elastic scattering is assumed to be suppressed. Instead, the dark matter particle scatters inelastically into a slightly heavier partner. If the mass splitting between the two dark matter particles is of order 100 keV - the typical kinetic energy in the dark matter sea - the DAMA signal can be, with a bit of luck, reconciled with the bounds from other experiments.

The way it works is the following. All direct detection experiments attempt to measure the recoil energy of a nucleon that has been hit by a passing dark matter particle. In the iDM scenario, the minimal velocity of the incoming dark matter particle needed to produce the recoil $E_R$ is given by the formula
$v_{min} = \frac{\delta+ m_N E_R/\mu_N }{\sqrt{2 m_N E_R}}$,
where $\mu_N$ is the reduced mass of the dark matter + nucleon system and $\delta$ is the mass splitting between the two dark matter states. As long as the splitting term dominates, heavier targets require lower velocity to give them a kick. DAMA's target contains pretty heavy iodine (A=127) (as compared to CDMS germanium with A=73). The sea of dark matter is expected to have the Maxwellian distribution of velocities that rapidly fall above the peak velocity which is of order $v \sim 0.001$, so that even a small change of the minimal velocity may significantly affect the number of events. Also for that reason, the oscillation signal studied by DAMA is enhanced, because the small summer/winter variation of the dark matter velocity distribution (in the Earth reference frame) may lead to a large variation of the signal. All in all, there remains some allowed parameter space, as can be seen in the example plot borrowed from this paper. For a fixed dark matter mass, the DAMA region in the mass splitting - cross section plane is marked in magenta, while black lines are the current bounds, the most stringent coming from CDMS (solid) and CRESST (dashed).

There is also a purely sociological reason why the bounds from other experiments get relaxed: iDM has not really been searched for...The nature of iDM leads to a very peculiar nucleon recoil spectrum. Whereas for the standard WIMP the number of events grows exponentially at low recoil energies, the recoil spectrum in the iDM scenario is suppressed at low energies and displays a "resonant" shape. Most experiments derive their bounds assuming the standard recoil spectrum and they do not optimize their search strategies to probe non-standard scenarios. For this reason, the idea of iDM is relevant for dark matter searches irrespectively of DAMA. It is a phenomenologically distinct possibility that should be taken into account, and one may easily miss the Nobel prize by restricting to the standard WIMP paradigm.

From the theoretical point of view, models of iDM are not difficult to write down. One simple possibility is the dark matter particle being a Dirac fermion with a large mass of order 100 GeV spiced up by a small 100 keV Majorana mass. The later leads to the required splitting between the two Majorana mass eigenstates. Furthermore, if the Dirac fermion has vector interactions the vector couples non-diagonally in the eigenstate basis, and the elastic scattering is suppressed with respect to the inelastic one. Another simple realization of iDM is a complex scalar whose two real components are split by a small "holomorphic" mass term. There is no obstacles to embed iDM into mainstream theories beyond the Standard Model. For example, in the MSSM, the Standard Model neutrino is partnered by a sneutrino who is a complex scalar, and the mass splitting could originate from a small lepton-violating term $(L H)^2$ in the superpotential.

So, just keep our fingers crossed while waiting for the new results from CRESST, XENON-100, LUX, KIMS and many others.

See also this post on Dirac Sea.

Inconstant

The funniest April Fools prank was definitely the one about time variation of $\pi$. That idea is of course absurd because the Bible unambiguously sets the value of $\pi$ to be equal three. But the physical constants like the QCD scale or the Fermi constant are not mentioned in the Bible which suggests that they might not be constants. Recently, Harald Fritzsch put on ArXiv a neat status report of various theoretical and experimental pursuits of varying fundamental constants.

For almost a century the idea of varying fundamental constants has been attracting most brilliant minds and complete crackpots alike. At the theoretical level the mechanism is easy to imagine: the physical constants can be set by a vacuum expectation value of a scalar field that evolves on cosmological timescales. In high-energy theory we already have one evolving scalar field for inflation and sometimes another one for quintessence, so that introducing yet another one for varying constants is not that difficult to swallow.

At the beginning of this century the idea has received renewed attention due to some experimental claims that the electromagnetic constant $\alpha$ may vary in time. A group of astrophysicists studying absorption spectra of very distant quasars concluded that 10 billion years ago $\alpha$ was smaller than today by $\Delta \alpha/\alpha \sim 10^{-5}$, corresponding to a time variation of order $10^{-15}$ per year. This claim is very controversial because of various assumptions involved in the determination $\alpha$ and, most of all, because other groups did not confirm this result. A more recent claim that the proton-to-electron mass ratio was different 10 billion years ago also remains highly controversial.

Yet another reason why the above claims are taken with a huge grain of salt is that the so-called Oklo bounds imply a slower variation of $\alpha$. 2 billion years ago, when the Earth was young and beautiful, the uranium-235 isotope was five times more abundant than today. Thanks to that fact and some other lucky coincidences, near the river Oklo in today's Gabon nature could create a fully organic nuclear reactor which operated for 100 million years. The uranium fission produced many rare isotopes, and the particular ratio of Samarium-149 to Samarium-147 can be used to constrain variation of the fundamental constants. The point is that the cross-section for the neutron capture on Samarium 149 is accidentally enhanced by a presence of resonance just 0.1 eV above the threshold. From the fact that the position of this resonance could not migrate by more than 0.1 eV one can set the bound $\Delta \alpha/\alpha \sim 10^{-7}$ (assuming that only the electromagnetic constant is varied) corresponding to a time variation $10^{-16}$ per year. If $\alpha$ was changing faster than that (as suggested by some astrophysical results) it had to stabilize at least two billion years ago.

In the neat future there is hope for more progress from precision measurements in a controlled laboratory environment. Experiments in quantum optics have recently reached a similar sensitivity to varying constants as the astrophysical observations. In particular, Theodor Haensch's group in Munich is running an experiment that studies time variation of the frequency of the 1s-2s transition in atom hydrogen (review here). The measurements from different years are related to the hyperfine transitions of Cesium-133 and to another precision measurement of quadrupole transitions in Mercury, which allows them to constrain the variation of both the electromagnetic constant and the QCD scale. The results published several years ago constrain the variation of both at the level of few times $10^{-15}$ per year.

Actually, Harald Fritzsch is spreading wild rumors that the most recent results from Munich imply the time variation of the QCD scale at the level of $3 \cdot 10^{-15}$ per year. Well, I'd rather bet that at the end of the day the constants will once more turn out to be constants. But who knows...in the end the Hubble constant has changed since the nucleosynthesis by some 17 orders of magnitude.

Dark Matter more like Baryons

April Fools is over; I'm staying dead serious for the rest of the year. The most serious things in the months before the first LHC results are dark matter searches high and low. Here is another idea what they might find.

The most popular scenario for dark matter assumes that it consists of weakly interacting massive particles (WIMPs) who were once in thermal equilibrium. In the early hot and dense universe such a particle can efficiently annihilate into familiar particles like photon or electrons, and in this way dark matter is kept in equilibrium with the the rest of the cosmic plasma. The equilibrium ceases to hold when the temperature T of the universe falls below the dark matter particle mass M. In that regime, the number of dark matter particle very quickly decreases - as an exponential $e^{-M/T}$ - and at some point dark matter freezes out: there isn't enough dark matter particles around that they could find each other and annihilate. The surviving particles float around in the universe playing hide and seek with astronomers and physicists alike.

The WIMP scenario is nice and robust but it sheds little light on the surprising fact that the present abundanceof dark matter $\Omega_{DM}$ is very close to that of the ordinary matter who is today dominated by the baryon (proton and neutrons) abundance $\Omega_{B}$. After WMAP data we are confident that the ratio $\Omega_{DM}/\Omega_B$ is roughly five. Of course, one can always cook up the parameters of the WIMP model such that this constraint is satisfied, but nevertheless the proximity of $\Omega_B$ and $\Omega_{DM}$ is intriguing. It may suggests that baryons and dark matter have a common origin. But baryons are definitely NOT a cold relic!

In fact, we don't know for sure what is the origin of baryons in our universe but we have a bunch of ideas that go under the name of baryogenesis. The general idea is that the very early universe contains an equal number of baryons and antibaryons, but at some point in its evolution the fundamental interactions in the plasma produce a tiny $10^{-10}$ asymmetry between matter and antimatter. Once the temperature falls below the baryon mass most of the baryons and anti-baryons annihilate with each other and turn into the sea of photons, leaving only the small unpaired $10^{-10}$ fraction of baryons. These are the protons and neutrons that make galaxies and stars today.

Is it conceivable that dark matter originates in a similar fashion? That is to say, the early universe contains dark matter and anti dark matter particles which almost completely annihilate away leaving only the small asymmetric fraction? Can the dark asymmetry and the baryon asymmetry have the common origin? The answer to these questions is yes, and the first practical realization I'm aware of is due to David B. Kaplan in early nineties. In that model, the dark matter particle carries a charge under an additional U(1) global symmetry who, much as the U(1) baryon symmetry, has a mixed anomaly with the electroweak SU(2) gauge symmetry. Because of the anomalies, non-perturbative electroweak interactions that are effective in the early universe violate both the baryon number and the dark matter number. Then, if some conditions are satisfied, the electroweak phase transition generates the baryon and the dark asymmetries roughly of the same order. At the end of the day on obtains the relation $\Omega_{DM}/\Omega_B \sim m_{DM}/m_{proton}$ and the experimentally measured ratio is recovered if the dark matter particle's mass is around 5 GeV.

Kaplan's original model is long gone for several reasons, but the idea is still floating in the backchannels of model building. The most recent approach in the context of supersymmetry was made by David E. Kaplan et al (David E. Kaplan is a more recent version of David B. Kaplan with more features). In that model, there's no new quantum number invented especially for the dark matter particle; instead, it carries the lepton (or baryon in another version) quantum number. Furthermore, the model does not rely on electroweak baryogenesis but rather it assumes that the the B - L asymmetry is generated at high energies (for example by leptogenesis). That asymmetry is later redistributed between baryons and dark matter by higher-dimensional interactions. When these interactions fall out of equilibrium, dark matter asymmetry is frozen in and one again ends up with $\Omega_{DM}/\Omega_B \sim m_{DM}/ m_{proton}$. All that remains is to find 5-15 GeV dark matter in the sky, or in colliders or by direct detection...