Thursday, February 27, 2020

Galaxy Formation Simulations Disfavor Warm Dark Matter

A new paper compares simulated galaxy formation in cold dark matter (CDM), self-interacting dark matter (SIDM), and warm dark matter (WDM) models. In the simulation, SIDM models produce galaxies at about the same time as CDM models, while WDM models produce galaxies much later.

Empirically, galaxies are observed to form significantly earlier than predicted in CDM models. This is called the "impossible early galaxies" problem. But, according to this stimulation, self-interacting dark matter models do not to solve the slow galaxy formation problem found in CDM models, and warm dark matter models have the same late galaxy formation problem as CDM models, but one of that is much worse in magnitude, because galaxies form more slowly in WDM models than in CDM or SIDM models. 

This is also a big problem for the dark matter paradigm generally, because the CDM model does not produce the galaxy scale inferred dark matter distributions that are observed. This is the main problem that the SIDM and WDM models were devised to solve and is why these two dark matter theories are the most promising dark matter particle theories currently being considered. 

Of the two, WDM was more attractive than SIDM in many respects, because WDM requires just one kind of dark matter particle, while SIDM requires both a new fundamental dark matter particle and a new dark matter interaction force carrying particle and more free parameters. Also, WDM might have been possible to tie into neutrino physics and the hints of a possible sterile neutrino in measurements of neutrinos oscillations at nuclear reactors.

But, each of these solutions to the galaxy scale problems of CDM (and neither SIDM nor WDM really do a great job of solving that) fails to address the impossible early galaxies problem of CDM. SIDM provides no improvement on this front, and WDM makes the impossible early galaxies problem of CDM significantly worse. So, this paper is a meaningful blow to WDM theories and provides no signs of encouragement for SIDM theories, which an optimist who had not seen the simulation data might have wishfully hoped could have solved the impossible early galaxies problem in addition to improving the galaxy scale behavior of CDM theories.

The paper and its abstract are as follows:

Local Group star formation in warm and self-interacting dark matter cosmologies

Mark R. Lovell (1,2), Wojciech Hellwing (3), Aaron Ludlow (4), Jesús Zavala (1), Andrew Robertson (2), Azadeh Fattahi (2), Carlos S. Frenk (2), Jennifer Hardwick (4) ((1) University of Iceland, (2) ICC Durham, (3) Warsaw, (4) ICRAR/UWA)
The nature of the dark matter can affect the collapse time of dark matter haloes, and can therefore be imprinted in observables such as the stellar population ages and star formation histories of dwarf galaxies. In this paper we use high resolution hydrodynamical simulations of Local Group-analogue (LG) volumes in cold dark matter (CDM), sterile neutrino warm dark matter (WDM) and self-interacting dark matter (SIDM) models with the EAGLE galaxy formation code to study how galaxy formation times change with dark matter model. We are able to identify the same haloes in different simulations, since they share the same initial density field phases. We show that the stellar mass varies systematically with resolution by over a factor of two, in a manner that depends on the final stellar mass. The evolution of the stellar populations in SIDM is largely identical to that of CDM, but in WDM early star formation is instead suppressed. The time at which LG haloes can begin to form stars through atomic cooling is delayed by 200~Myr in WDM models compared to CDM. 70~per~cent of WDM haloes of mass >108M collapse early enough to form stars before z=6, compared to 90~per~cent of CDM and SIDM galaxies. It will be necessary to measure stellar ages of old populations to a precision of better than 100~Myr, and to address degeneracies with the redshift of reionization, in order to use these observables to distinguish between dark matter models.
Comments:17 pages, 13 figures. To be submitted to MNRAS. Contact:
Subjects:Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)
Cite as:arXiv:2002.11129 [astro-ph.GA]
 (or arXiv:2002.11129v1 [astro-ph.GA] for this version)

CKM Matrix Elements Determined With High Precision

Some of the fundamental constants of the Standard Model of Particle Physics are the four parameters from which the values of the nine element CKM matrix, which describes the probability of quarks turning into different kinds of quarks via the weak force, can be determined.  See generally, this Particle Data Group review regarding the CKM matrix.

The square of the absolute value of each element is equal to the probability of the transition described in the element to the second type of quark in the subscript taking place, given that a W+ boson has been emitted by a quark of the type of the first type of quark shown in the subscript. (This version of the CKM matrix can be converted mathematically to the matrix for W- boson emissions if the complex number values of the matrix elements that reflect the charge parity (CP) violating phase of the matrix are used, rather than merely their absolute values shown in this post.)

The parameters are measured by independently assembling experimental data regarding various kinds of top quark and hadron decays in high energy physics experiments, and from beta decays of radioactive element isotypes, that whose data can be used, in principle, to determine particular CKM matrix element values. This is hard, but has become an increasingly precise science with good instrumentation and large data sets reducing both systemic error and statistical error in these measurements to low levels relative to "theoretical error."

Then, this raw data is converted to numerical values of the matrix elements of the CKM matrix using Lattice QCD and other high energy physics calculations. This is the hardest part by far and provides the limiting factor in the precision with which these matrix elements can be determined at this time.

Once this is done, all that is left is comparatively elementary mathematics and statistics that can be done in a moment using simple computer programs on a PC. First, those independent measurements of individual elements are globally fitted to the nine elements of the CKM matrix based upon their raw values, the uncertainty of each measurement value, and the Standard Model's CKM matrix probability sum rules. There are six different unitarity sum rules that apply to the CKM matrix. Then, one globally fits those nine data points to the four Standard Model CKM matrix parameters (something that can be done in any of several widely used parameterization schemes).

To date, these independent CKM matrix parameter measurements using different kinds of decays have been consistent with the Standard Model sum rules to within the range of measurement errors, providing a strong global check that the Standard Model itself is a sound theory. This check shows that the quantities defined are physically meaningful across a robust range of circumstances, and that there do not appear to be "missing beyond the Standard Model quarks" into which W boson transitions outside the Standard Model (e.g. into hypothetical fourth generation up-type and down-type quarks commonly called t' and b', or into supersymmetric particles) occur. The experimental evidence is consistent with all six of the CKM matrix sum rules and also a sum rule related to other CKM matrix parameters, at the two sigma level, and also suggests that some of the margins of error in those measurements have been conservatively overestimated because the fit is "too good" relative to the stated margin of error (a common situation in the case of electroweak measurements in the Standard Model). As the Particle Data Group review article linked above explains:
Using the independently measured CKM elements mentioned in the previous sections, the unitarity of the CKM matrix can be checked. We obtain |Vud| 2 + |Vus| 2 + |Vub| 2 = 0.9994 ± 0.0005 (1st row), |Vcd| 2 +|Vcs| 2 +|Vcb| 2 = 1.043±0.034 (2nd row), |Vud| 2 +|Vcd| 2 +|Vtd| 2 = 0.9967±0.0018 (1st column), and |Vus| 2 + |Vcs| 2 + |Vts| 2 = 1.046 ± 0.034 (2nd column), respectively. 
The uncertainties in the second row and column are dominated by that of |Vcs|. For the second row, a slightly better check is obtained from the measurement of P u,c,d,s,b |Vij | 2 in Sec. 12.2.4 minus the sum in the first row above: |Vcd| 2 +|Vcs| 2 +|Vcb| 2 = 1.002±0.027. 
These provide strong tests of the unitarity of the CKM matrix. With the significantly improved direct determination of |Vtb|, the unitarity checks for the third row and column have also become fairly precise, leaving decreasing room for mixing with other states. 
The sum of the three angles of the unitarity triangle, α + β + γ = (180 ± 7)◦ , is also consistent with the SM expectation.

The margin of error in these measurements comes predominantly from "theoretical error", i.e. mostly in the process of converting the raw experimental data into the numerical value of the matrix elements of the CKM matrix using Lattice QCD and other theoretical adjustments (e.g. determining the correct renormalization adjustments to make to the raw data based upon estimates of jet energies in hadron decays). 

This is so difficult because, except in the case of top quark decays, which are always to b quarks, except in about 1 in 600 +/- top quark decays, it is impossible to directly measure the decay of an individual free quark because all other quarks are only observed confined into hadrons. So, instead, you have to measure the decay of a composite particle called a hadron, which has more than one quark and many gluons in it, or the decays of the even more complex system of the beta decay of a radioactive isotype of a periodic tale element. Then, you have to used very difficult lattice QCD calculations and other high energy physics calculations in order to use that data to infer the behavior of individual quarks within the decaying composite particles whose decays are actually observed.

So, to grossly oversimplify the matter, the main barrier to getting more precise values for these fundamental CKM matrix constants of the Standard Model is mostly a question of QCD calculations being very cumbersome and requiring almost unfathomable computational power in addition to novel analytical break throughs that are needed to simplify subparts of the calculations involved. As the paper explains:
We have seen that in a number of quantities the theory error is the limiting factor in determining a CKM matrix element. Even in cases for which the experimental and theoretical errors are currently comparable, we can expect that BESIII, Belle II, and LHCb will reduce the experimental errors.
The authors of the paper estimate that theoretical error in these measurements can be reduced by only as much as a factor of five over the next ten years. 

The latest values for the absolute value of eight of those nine quantities are as follows (note that both a direct measurement and a global fit value are provided for both of the first two values listed).

The element for the ninth of these CKM matrix elements, the absolute value of V sub tb, is omitted from the list above. 

This element is omitted because there is currently no experiment sufficiently precise in its measurement of this CKM matrix element (and no experiment will be able to do so for the foreseeable future). A precision of about one part per 600 would be required to determine the amount by which the CKM matrix element for the absolute value of V sub tb differs from 1 with a direct measurement, unless beyond the Standard Model physics were present. But, as the chart above shows, the relevant experimental apparatuses (basically only the now concluded Tevatron experiment, and the Large Hadron Collider a.k.a. the LHC) have a relative precision of only a one part per 23 to one part per 59 at this time at the relevant energy scales.

But, we know that the V sub tb element has a value of approximately 0.999. . . based upon a global fit of CKM matrix element values, and the experimental data is not inconsistent with this estimate. 

This global fit value can be estimated, with back of napkin precision, from the other values based upon the Standard Model rule that the sum of the squares of the absolute values of V sub td, V sub ts, and V sub tb, is equal to exactly 1, representing a probability of 100% that when there is a W boson flavor changing transition of a t quark to one of the down-type quarks that are possible in a possible Standard Model W boson mediated flavor change (i.e. a transition to a down quark, a strange quark, or a bottom quark). 

Wednesday, February 26, 2020

Modern Humans Were In India Without Interruption Pre- And Post-Toba

I've been aware of this for quite a few years, but a Nature Communications article published on February 25, 2020 (per CNN) is making a big splash over the fact that modern human tools are present in India both before and after the Toba eruption (ca. 74,000 years ago), with an indication that there is continuity between the before culture and the after one.

I was mentioning this as old news in an August 15, 2012 post at this blog. I also mention it in a September 24, 2010 post at Wash Park Prophet citing a BBC report regarding a find by Dr Michael Petraglia who is the last listed author of the February 25, 2020 paper.

Presumably the new study strengthens the decade old finds, the abstract suggests, by linking it to other contemporaneous finds. 

It is an important measure of Out of Africa modern human expansion that refutes the naive inference from modern human genetics alone that Out of Africa for modern humans dates to only around 50,000 years ago. The reason for the disconnect between the genetic based date and the archaeology based one is unclear, but the most obvious possibility is widespread population replacement of earlier modern human populations by later Out of Africa, or Out of Arabia or India waves of migration.

Multiple other independent lines of evidence (from the Levant, Arabia, Burma, Indonesia, Australia, Neanderthal and Denisovan DNA, and possibly China) also point to an earlier Out of Africa date. The article and its abstract are as follows:
India is located at a critical geographic crossroads for understanding the dispersal of Homo sapiens out of Africa and into Asia and Oceania. Here we report evidence for long-term human occupation, spanning the last ~80 thousand years, at the site of Dhaba in the Middle Son River Valley of Central India. An unchanging stone tool industry is found at Dhaba spanning the Toba eruption of ~74 ka (i.e., the Youngest Toba Tuff, YTT) bracketed between ages of 79.6 ± 3.2 and 65.2 ± 3.1 ka, with the introduction of microlithic technology ~48 ka. The lithic industry from Dhaba strongly resembles stone tool assemblages from the African Middle Stone Age (MSA) and Arabia, and the earliest artefacts from Australia, suggesting that it is likely the product of Homo sapiens as they dispersed eastward out of Africa.
Chris Clarkson, Michael Petraglia, et al., "Human occupation of northern India spans the Toba super-eruption ~74,000 years ago" 11 Nature Communications Article number: 961 (February 25, 2020).