Monday, April 1, 2013

Standard Model Unstoppable

A March 30, 2013 post at Resonnances sums up the myriad way that previous hints of beyond the Standard Model of Particle Physics have been crushed by recent data showing the previous results to be statistical flukes or by reanalysis of old data hinting at BSM physics which has been found to be flawed (e.g. a coding error at Tevatron). 

In addition to discoveries I've discussed at length previously, the new data and analysis have essentially eliminated hints of:
* CP violation in D meson decay at 10x to 100x the expected value;
* Evidence hinting at a 145 GeV particle seen at Tevatron (this was the coding error);
* Tightened experimental limitations from "the MEG experiment which searches μ → e γ decays. The standard model predicts this decay should be too rare to be observable (a tiny branching fraction is induced via the neutrino mixing). On the other hand, it is straightforward to produce a large branching fraction in models with new sources of lepton flavor violation, including supersymmetric and composite Higgs models. The latest MEG update sets the limit on the branching fraction at 5.7x10^­‐13 at 90% CL, which represents a factor of 4 improvement of the previous limit."
* SUSY stops (the bosonic partner of the top quark) must be at least 700 GeV and gluinos (fermionic partners of gluons) must be at least 1.3 TeV, with some basic assumptions in both cases.  Of course, no experimental data points to their existence at all.

The six parameter lamda CDM model of cosmology is likewise strongly affirmed by the Planck cosmic microwave background radiation (CMB) measurements.  Among other things, the data support a treatment of dark energy as nothing more or less than the cosmological constant first proposed in 1916 by Einstein.  The last data point from Planck will be in within a year or so.

The outstanding measurements to be made to complete Standard Model neutrino physics (the mixing angles, mass hiearchy, absolute masses, Dirac v. Majorana character and CP violating phase) are the focus of enough experimental work that they could be determined by the time my children are old enough to be in graduate school.

Arguably the difference in measurements between the size of protons in muonic hydrogen and ordinary hydrogen, and the anomalous muon magnetic dipole moment are up in the air, but this could also be a result of an underestimated systemic error in either the theoretical calculation, the experimental measurement, or both.  The former is in theory a 4.4 to 4.6 sigma effect (the value values differ by about 0.02 fm, which is about 2.5% of the mean value of the ordinary hydrogen and muonic hydrogen values).  The 3.4 sigma discrepancy in the latter still matches the theoretical value to about nine significant digits.  There is good reason to suspect that the tensions between the measurements in both cases will resolve with more precise measurements.

What's left?  All that is left(or will be left in a few years) is:

1. WSM (within the Standard Model) theorizing about why it is the way that it is in nature from deeper principles;

2. Theoretical inconsistencies between General Relativity and the Standard Model, but none are within the capacity of empirical observations to resolve them a determine what kind of quantum gravity theory must be correct; and

3. Dark matter, which is pretty much the only game left in town for experimentally supported BSM physics that we know must exist and there are no solutions for it.

Dark Matter Recapped

There are indisputably large dark matter effects that are observed phenomologically, whatever their actual cause.  It accounts for 26.8% of the aggregate mass-energy equivalent of the universe in CBM data from Planck in the lamda CDM model (about 83% of mass-energy not attributable to the fully understood cosmological constant that gives rise to the dark energy value).  We see it in almost every galactric rotation curve and the kinematics of galactric clusters.  We see it in gravitational lensing data.  We see it when galaxies collide.  But, no candidate dark matter particle has been identified and direct dark matter searches are at best inconclusive and at worst rule out the favorite candidates for it.  We have no consensus model of dark matter that can fit all of the data. 

Hot dark matter and cold dark matter seem to be inconsistent with observed galactric structure (predicting too little and too much structure respectively).   Hot dark matter also is disfavored by the Planck data and can't be massive enough to explain all dark matter.  No consensus dark matter theory reproduces the observed data's tight structure which empirical relationships like the Tully-Fisher law and the empirical success of a one parameter MOND theory at the galactric level show exists and must be reproduced by an empirically valid dark matter theory. 
Several discrepancies between the predictions of the particle cold dark matter paradigm and observations of galaxies and their clustering have arisen:
    The cuspy halo problem: cold particle dark matter predicts that the density distribution of DM halos be much more peaked than what is observed in galaxies by investigating their rotation curve.
    The missing satellites problem: cold particle dark matter predicts larger numbers of small dwarf galaxies (about one thousandth the mass of the Milky Way) than are observed.
Warm dark matter refers to particles with a free-streaming length comparable to the size of a region which subsequently evolved into a dwarf galaxy. This leads to predictions which are very similar to cold dark matter on large scales, including the CMB, galaxy clustering and large galaxy rotation curves, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies; some researchers consider this may be a better fit to observations. 
Warm dark matter and/or self-interactions of dark matter might explain it.  But simple warm dark matter models while solving the missing satellites problem don't solve the cuspy halo problem, so both a light particle and self-interaction of some type may be necessary to fit the dark matter particle paradigm to the evidence if it can be fit at all.  An example of recent efforts to resolve the outstanding issues with dark matter theories can be found at:

Beyond Collisionless Dark Matter: Particle Physics Dynamics for Dark Matter Halo Structure Authors:Sean Tulin, Hai-Bo Yu, Kathryn M. Zurek (Submitted on 15 Feb 2013)
Abstract: Dark matter (DM) self-interactions have important implications for the formation and evolution of structure, from dwarf galaxies to clusters of galaxies. We study the dynamics of self-interacting DM via a light mediator, focusing on the quantum resonant regime where the scattering cross section has a non-trivial velocity dependence. While there are long-standing indications that observations of small scale structure in the Universe are not in accord with the predictions of collisionless DM, theoretical study and simulations of DM self-interactions have focused on parameter regimes with simple analytic solutions for the scattering cross section, with constant or classical velocity (and no angular) dependence. We devise a method that allows us to explore the velocity and angular dependence of self-scattering more broadly, in the strongly-coupled resonant and classical regimes where many partial modes are necessary for the achieving the result. We map out the entire parameter space of DM self-interactions --- and implications for structure observations --- as a function of the coupling and the DM and mediator masses. We derive a new analytic formula for describing resonant s-wave scattering. Finally, we show that DM self-interactions can be correlated with observations of Sommerfeld enhancements in DM annihilation through indirect detection experiments. . . .

As is well known, the collisionless cold DM (CCDM) paradigm has been highly successful in accounting for large scale structure of the Universe. However, it is far from clear that this paradigm can also successfully explain the small scale structure of the Universe. Precision observations of dwarf galaxies show DM distributions with cores, in contrast to cusps predicted by CCDM simulations. It has also been shown that the most massive subhalos in CCDM simulations of Miky Way (MW) size halos are too dense to host the observed brightest satellites of the MW. Lastly, chemo-dynamic measurements in at least two MW dwarf galaxies show that the slopes of the DM density profiles are shallower than predicted by CCDM simulations. These small scale anomalies, taken at face value, may indicate that other interactions besides gravity play a role in structure formation.

Recent research constrains warm dark matter models to have masses approximately in the range of 1-2 keV and also tightly bounds their possible self-interactions.  The observed Tully-Fisher relation is inconsistent with lighter warm dark matter particlesObservations of the Andromeda Galaxy suggest an upper limit on warm dark matter particle sizes of about 2.2 keV.  Long gamma ray burst data imposes similar constraints placing a floor value of about 1.6-1.8 keV for combined limits from the various sources of 1.6-2.2 keV.  Warm dark matter particle masses are in a mass range that is inconsistent with any weakly or electromagnetically interacting particle that a W or Z boson can decay into or couple with as we know from precision electroweak observations. It also couldn't interact via the strong force. So we have no Standard Model mechanism for creating it. As Wikipedia explains:

A challenge for [the warm dark matter] model is that there are no very well-motivated particle physics candidates with the required mass ~ 300 eV to 3000 eV. There have been no particles discovered so far that can be categorized as warm dark matter. There is a postulated candidate for the warm dark matter category, which is the sterile neutrino: a heavier, slower form of neutrino which does not even interact through the Weak force unlike regular neutrinos. Interestingly, some modified gravity theories, such as Scalar-tensor-vector gravity, also require that a warm dark matter exist to make their equations work out.
Small scale structure rules out a mix of warm and dark matter.  Only the pure warm dark matter models fit those constraints.  Other studies disfavor models with two kinds of warm dark matter of different masses. 

Simple MOND theories have to be generalized to be relativistic and aren't an easy fit with galactric cluster data even though they are a good and parsimonious fit to the galactric scale data.  They also don't work well with a possible dark matter filament observation.

In short, if there is a particle that fits the dark matter paradigm, it seems as if it is a fit for some variant on a singlet sterile neutrino warm dark matter particle with a particle mass of 1.6 keV to 2.2 keV with tight constraints on its non-gravitational self-interactions, if any, via a light or massless dark sector boson that would allow for heat exchange through dark matter collisions within dark matter halos.  This is a much tighter parameter space than we had even a couple of years ago.  Indeed, there is considerable tension in these estimates and it isn't certain that a single set of values can accomodate the entire parameter space.

We are in a delicate balance between a very precisely described single kind of warm dark matter particle with a possible self-interaction force, and a world in which all dark matter models are inconsistent with the evidence.  For example, overly simple warm dark matter models may be inconsistent with the dwarf galaxy formation that we observe (accord here and here and here).  So some sort of self-interaction may be necessary even in warm dark matter models to fit the data.

Since the total amount of dark matter, for example, in the Milky Way, can be fairly easily estimated as can the profile of the dark matter halo, localized dark matter particle per cubic space estimates can be made fairly accurately providing a small target for direct dark matter detection experiments.


andrew said...

Significantly coupled dark matter deviates from LCDM predictions.

andrew said...

Another paper tightly bounding DM models to 2 keV dark matter give or take less than 50% is here.