Friday, November 17, 2017

Yet Another Dark Matter Parameter Space Constraint

Simple cold dark matter models still don't work, although theorists don't have good alternatives:

A detection of wobbling Brightest Cluster Galaxies within massive galaxy clusters

A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, Aw, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of Aw<2kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of Aw=11.82+7.33.0kpc, with the observations dis-favouring Aw=0 at the 3σ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero Aw, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.
I explained this in the following way to someone asking about this paper at the Physics Forums:

The really core point, from the abstract is that:
[The] Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. . . . This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters.
Ten years ago, this would have been a really big deal since it contradicts the cold dark matter (CDM) hypothesis as an explanation for dark matter phenomena. But, at this point, it is really just piling onto an abundant collection of evidence showing contradictions between the CDM hypothesis and observation.

One of these contradictions (there are several of them) is known as the cusp-core problem, which is that CDM theories, generically, predict that dark matter halos should have a cuspy density profile, but inferences about the distribution of dark matter from the dynamics of visible matter in galaxies and gravitational lensing observations demonstrate that this is not actually the shape of inferred dark matter distributions in galaxies. Instead, inferred dark matter halos distributions have what is known as an "isothermal" distribution of dark matter within the dark matter halo around a galaxy.

So, this result really just confirms in a novel way something that was widely known from other evidence. This is still important, because it makes the conclusion that there really is a cusp-core problem that is not just an artifact of a flaw in some particular methodology that provides the evidence for the cusp-core problem much more robust. But, it doesn't really change the bottom line from existing data.

To prevent a cuspy density profile from emerging in a halo you need some kind of feedback either between dark matter particles or between ordinary matter and dark matter that spreads it out when it gets too dense.

But, that contradicts the assumption made in early cold dark matter theories that dark matter should be collisionless, which has strong support from the failure to direct dark matter detection experiments to see it, from the absence of a strong dark matter annihilation signal, and from the non-detection of dark matter at the Large Hadron Collider (LHC), and is consistent with the success of the lamdaCDM model of cosmology at scales much larger than galaxies and galaxy clusters, although these methods would often miss detect interactions between dark matter and other dark matter that does not result in annihilation of the interacting dark matter particles and take place at relative small distances relative to those important for cosmology.

Warm dark matter proponents have suggested a quantum effect that only kicks in at masses of dark matter particles on the order of 2 keV/c2 or less. Others have proposed self-interacting dark matter (SIDM) models to address the issue. But, those models have their own problems beyond the scope of this discussion.

Wednesday, November 15, 2017

Most Dark Matter-Neutrino Interactions Can Be Ruled Out

The following new paper places significant new boundaries of the parameter space of particle dark matter candidates that interact with neutrinos to some extent.
Dark matter and neutrinos provide the two most compelling pieces of evidence for new physics beyond the Standard Model of Particle Physics but they are often treated as two different sectors. The aim of this paper is to determine whether there are viable particle physics frameworks in which dark matter can be coupled to active neutrinos. 
We use a simplified model approach to determine all possible renormalizable scenarios where there is such a coupling, and study their astrophysical and cosmological signatures. 
We find that dark matter-neutrino interactions have an impact on structure formation and lead to indirect detection signatures when the coupling between dark matter and neutrinos is sufficiently large. This can be used to exclude a large fraction of the parameter space. 
In most cases, dark matter masses up to a few MeV and mediator masses up to a few GeV are ruled out. The exclusion region can be further extended when dark matter is coupled to a spin-1 mediator or when the dark matter particle and the mediator are degenerate in mass if the mediator is a spin-0 or spin-1/2 particle.
Andres Olivares-Del Campo, et al., "Dark matter-neutrino interactions through the lens of their cosmological implications" (November 14 2017).

Meanwhile, the parameter space of axion-like dark matter theories has once again been greatly reduced. 

LHC Measures Electroweak Mixing Angle

One of the physical constants in the Standard Model of Particle Physics is called the electroweak mixing angle also known as the Weinberg angle.  This is a function of two more fundamental physical constants in the Standard Model. Crudely speaking, the coupling constants of the electromagnetic force and the weak force (g and g'), which more precisely speaking are the

  and  couplings (weak isospin g and weak hypercharge g', respectively).

The cosine of the weak mixing angle is also equal to the mass of the W boson divided by the mass of the Z boson.

Measuring it is complicated somewhat because one has to defined the weak mixing angle at a particular energy scale to produce a numerical value for it since it runs with energy scale, in order to make a consistent measurement.

The CMS experiment at the Large Hadron Collider (LHC) has measured the electroweak mixing angle with twice the precision of any previous LHC measurement. The new measurement is as follows (combining all sources of error into a single uncertainty figure):


Put another way, the plus or minus one standard deviation range that flows from this measurement is 0.23049 to 0.23153.

The central value of the previous state of the art LHC measurement (via the first link in this post) was 0.23142, which is consistent with, but at the high end, of the new measurement and only have a margin of error of about ± 0.00100.

A 2004 global average from a variety of experimental sources (from the same link) was 0.23120 ± 0.00015 which is also consistent with this result.

The CODATA 2014 determination based upon W and Z boson mass measurements (from the same link) was 0.2223(21) implying a one sigma range of 0.22010 to 0.22440, which is below the value of the new measurement and below the 2004 global average at the one standard deviation level, and is just barely consistent at a two sigma level with the new LHC measurement.

The new paper notes that:
The most precise previous measurements of sin2 θ lept eff are reported by LEP and SLD experiments However, the two most precise measurements differ by more than 3 standard deviations. Measurements of sin2 θ lept eff are also reported by LHC and Tevatron experiments. . . . The results are consistent with the most precise LEP and SLD measurements [ed. when they are combined]. 
Meta Footnote

This post brings the number of posts at this blog for the year to an all time record high for a yearly number of posts at the blog, of 223, with 46 days left to go in 2017.