Friday, June 28, 2013

Tetraquark Or Two Meson Molecule Discovered

A new particle, called Zc(3900) is either a tetraquark (an elusive hadron made up of four quarks in a single composite particle) or a molecule made up of two mesons (which are composite particles made of two quarks each).  Either possibility would be the first confirmed sighting of a particle of that type.  This particle is one of the decay products of another unknown particle called Y(4260).

The numbers refer to the estimated particle mass in MeV/c^2 (mega-electron-volts/speed of light squared; the large letter code for general properties displayed by the particle and the c subscript denotes the presence of a charm quark in the particle.

All particles made of quarks found to date (and under the Standard Model) are "confined" to composite particles which are strong force color neutral (i.e. they have "red", "green" and "blue" flavored quarks, or have a quark that is, for example "red" and "antired").  So far, all known composite particles made of quarks come in two quark varieties called mesons, and three quark varieties called baryons.  Prior to this report and some previously unpublished conference reports, no tetraquarks, pentaquarks or larger number of quarks have been observed so far.

But, the rules of quantum chromodynamics (QCD), that governs the strong force interactions of quarks, does not forbid composite particles made up of more than three quarks so long as their are strong force color neutral, which is theoretically possible for any number of quarks two or greater if the structure can be small enough to allow the short range strong nuclear force to operate between all of the quarks involved.

Likewise while baryons (made up of three quarks, such as protons and neutrons) have been observed to form atomic nuclei which in turn form molecules bound electromagnetically, no composite structures made of multiple mesons have previously been observed even though they aren't prohibited by the rules of QCD either.
The Belle and BESIII teams were both studying an odd particle called  when they realized that it decayed to make another interesting particle, Zc(3900). Its mass, says Poling, who is part of the BESIII team, suggests that it is an electrically neutral meson made up of two quarks with opposite charges, called charm and anticharm. But surprisingly, both teams found that Zc(3900) has an electrical charge.
In fact, Poling says no two-quark or three-quark combinations can explain Zc(3900)’s charge and mass. That is leading physicists to the more exotic and exciting conclusion that the particle consists of four quarks: a charm and an anticharm along with an up and an antidown, which are extremely light and create a net positive charge. “The particle’s charge makes it a smoking gun for a four-quark state,” says Tomasz Skwarnicki, a physicist at Syracuse University in New York.
Assuming the evidence for a four-quark arrangement holds up, the big question will be how those quarks are arranged. Zc(3900) could be a single entity of four quarks, Skwarnicki says, but it could also be a coupling of two mesons, analogous to two atoms linking up to form a molecule.
Previous conference report papers on possible tetraquark sightings have tended to favor the two meson molecule explanation, which is the less revolutionary and dramatic of the two possibilities.

Of course, Zc(3900), like all composite particles made of quarks other than protons and neutrons, is a highly unstable particle with a mean lifetime measured in fractions of a second that almost immediately decays to simpler particles. 

So, while its discovery is relevant to clarifying the fine details of what is possible in QCD, and perhaps to the dynamics of the period immediately following the Big Bang, the discovery has essentially no practical technological applications and it will not explain any important phenomena observed outside of intense man made particle accelerator conditions.

If it is a meson molecule, rather than a true tetraquark, the discovery might actually tend to support the theory that there is some unarticulated and unknown law of physics pertinent to QCD that forbids the creation of unitary composite quark structures with more than three quarks, even though none of the current rules of QCD clearly impose this limitation.  This would be as much of a major breakthrough as a finding that unitary composite quark structures with more than three quarks are possible.

A meson molecule model would imply that something similar to the strong force interactions that in diluted form bind atomic nuclei together applies to mesons as well.  The experimentally measured lifetime of Zc(3900) as an indirect measure of this indirect version of the strong force, could help to better pin down experimentally the speed of strong force interactions generally.  These would also be valuable discoveries.


Andrew Grant, "First four-quark particle may have been spotted: Finding might shed light on how nucleus is held together", Science News (June 21, 2013) citing M. Ablikim et al., "Observation of a charged charmoniumlike structure in e+e−→π+π−J/ψ at √s=4.26 GeV.", Physical Review Letters (June 17, 2013) and Z. Q. Liu et al., "Study of e+e−→π+π−J/ψ and observation of a charged charmoniumlike state at Belle.", Physical Review Letters  (June 17, 2013).

Tuesday, June 18, 2013

An Ancient Wonder Of The World Misplaced

One of the seven wonders of the ancient world was the Hanging Garden of Babylon. 

Map via the Wikipedia entry on Mesopotamia

One problem.  Contrary to the account of 1st century CE Jewish historian Josephus (in his defense, probably due to inaccuracies in the sources he relied upon), they weren't built by Nebuchadnezzar and they weren't in Babylon in what is modern day Iraq a little bit to the South of Bagdad.

Old research showing an absence of any archaeological evidence or historical writings in places where they should exist strongly disfavor this location.  Instead, new research including ancient written sources (although not yet archaeological evidence) now strongly supports the conclusion that the Hanging Gardens did exist, but that they were located instead in Nineveh, which in ancient times was near the modern city of Mosul, and were built by Assyrian King Sennacherib, a century before the reign of King Nebuchadnezzar in Babylon.

Notably, the new location is also geographically closer to all of the other seven wonders of the ancient world, while Babylon had been a geographic outlier of the seven.


Wednesday, June 5, 2013

A Dialog On Non-Denisovan Archaic Admixture In Denisovan DNA (Postscript added)

The dialog after the jump (edited to remove discussions of other issues) is from the comments to a post from last month at this blog and I've elevated it to a new post for the benefit of readers not following the comments on old posts closely (some of the emphasis in quoted material is mine specific to this post).

Denisovan autosomal DNA was found archaic hominin remains in a Siberian cave a few years ago and is a match for traces of archaic admixture found in modest percentage in modern Australian aborigines and Melanesian populations (and populations derived from these populations), but in no other modern human populations, as shown in the following map (via John Hawks blog).

Recent analysis has determined that the Denisovan DNA is admixed with Neanderthal DNA (who were contemporaneous with the population whose remains were found in Siberia from ca. 30,000 years ago) and not too far to west of them and in the same cave at a later date), just as non-African modern humans are today.  More intriguingly, there is also evidence in the Denisovan autosomal genome of admixture with another unidentified archaic hominin species. They were 17% Neanderthal and 4% unknown archaic hominin.

The $64,000 question is which species of archaic hominin account for the core of the Denisovan DNA and the unidentified archaic admixture in it, respectively.

Tuesday, June 4, 2013

A Style Note For Would Be Einsteins Regarding The Term "Aether"

Many lay persons who want to talk about their ideas for "new physics" beyond general relativity to explain observations made by astronomers use the term "aether" to describe their ideas, as someone leaving a comment at one of my recent posts did. 

I am making this post as a public service to discourage aspiring non-professional scientists from doing so (professional scientists already know better).   Most people use this term do so out of ignorance of its modern connotations.

The central notion of the idea of "aether" is that space-time is not just "nothing", and that the vacuum itself has physical properties that make it in some sense a real substance.  As discussed below, this core concept is not dead.

But, the term "aether" jumped the shark a couple of decades before the Happy Days episode in 1977 that gave rise to this expression aired.  Despite the fact that Albert Einstein used the term himself in published works as late as 1930 and that Paul Dirac published a paper using the term to describe a similar concept as late as 1951, this term is no longer in current usage as a scientific term used by legitimate physicists and hasn't been since the late 1950s.

Today, this term is very strongly associated with a very particular kind of 19th century luminiferous aether theory that was definitively disproven with a many experiments conducted by multiple investigators that were replicated with increasing precision from 1810 to 1935.  As a result, the luminiferous aether theory is synonymous in contemporary physics writing by professional physicists with pseudoscience.  "Aether" is to physicists what Young Earth Creationism is in the fields of biology and geology.  Thus, from a P.R./marketing/credibility perspective, it is hard to imagine a worse choice of name for the medium of space-time than aether by anyone trying to seriously and sincerely advance a scientific hypothesis about physics.

While many "new physics" modern gravitation/dark energy/dark matter theories proposed by professional physicists (and indeed general relativity itself) treat the fabric of space-time as something that has properties rather than being "nothing", using the term "aether" for that medium is the rhetorical equivalent of calling yourself a crackpot.  This is the perception that people who naively use this term are unintentionally broadcasting to people who read their writings about physics. 

Sometimes it makes sense to embrace a word that has become a slur (e.g. "gay" and "lesbian"), but this is not one of those times.

Among professional physicists anywhere in the world in the twenty-teens of the current era, the word "Aether" has connotations that make it basically a dirty word in reputable discussions of physics. Using the word "aether" sincerely, to describe a possible real physical phenomena in a published physics paper or blog post or physics forum comment, is a roughly equivalent writing an official government report or think tank paper or newspaper article describing 2010 census data that uses the word "Nigger" describe African-Americans, the word "Savages" to describe Native Americans, the phrase "Wetback" to describe Hispanics, and the word "Gook" to describe Asian Americans. It destroys your credibility before you have a chance to make a point about the legitimate physics idea that you want to express.  Use of the term "aether" brands you as a backward anti-scientific idiot even if this isn't really who you are at all.  If you want to make a positive impression you should not make this rookie mistake.

When professional physicists wish to express the notion that space-time itself may give rise to gravitational or inertia effects or otherwise fundamentally re-think of the foundational mechanism of effects observed by astronomers and physicists, they have a great many alternative ways of saying what they mean.   A non-professional physicist (or a professional physicist) who wants to express similar ideas should do likewise.  Physicists who talk about a theory in which the physical properties of a vacuum or space-time or empty space that are not nothing, often speak of the properties of space-time, manifolds, branes, the physical properties of the vacuum, the inherent curvature of space-time, the background, the cosmological constant, vacuum energy, dark energy, quintessence, scalar fields such as the Higgs field and the inflaton, superfluid vacuum theory,  the Dirac sea, Le Sage gravity, a Bose-Einstein condensate, and Mach's principal.  But, legitimate physicists almost never use the term "aether" in any sense other than a derogatory or dismissive one.

Given the term aether's illustrious history of use back to the days of Aristotle that it shares with terms like "atom" that have endured the test of time better, this may seem to a would be lay physicist to be a crying shame.  But, life is not fair and this is the sociological reality of modern academia and physics. Any amateur physicist who uses the term should be forewarned about this linguistic connotation reality, and should expect the treatment that they will receive when they use it from professional physicists and more sophisticated amateur physicists alike.

Monday, June 3, 2013

Duly Noted

There have been notable papers published on population genetics in Italy (which Maju also analyzes well), on Mesopotamian influences on the Maikop culture in the Caucasus mountains, which was one of the first prehistoric metal age cultures (also here), a big picture view of the spread of uniparental genetic markers in Asia and Europe, East Asia's Y-DNA history, the ancient mtDNA of the Japanese Jomon people (also here), on Balkan genetics and Armenian origins, providing open access to a lengthy study on Indo-European origins (of course, controversial since there is no uniform consensus on the issue), the climate impact of the Toba eruption in Africa, the cause of the Younger DryasMiddle Stone Age Paleoclimate, Tibetian genetics, evidence for colonization across the Wallace line as ancient as 60,000 years ago or earlier, the dating of Acheulean lithic tools in SW Europe, the taxonomy and origins of the Uralic languages, ancient Siberian mtDNA, and ancient Minoan DNA.

Collectively, the story is that the precisions and resolution of our knowledge of population genetics and prehistory is getting finer especially in the critical area of ancient DNA.  Individually, none of these papers announce definitive paradigm shifting results.  But, the source materials for an increasingly empirically backed and nuanced story of human prehistory and ancient history are being assembled and are ready to be synthesized into something more comprehensible than raw data accessible only specialists and intense hobbyists (like prehistory bloggers).  The prospects for resolving alternative hypotheses about key issues in prehistory look increasingly good.

Maju also notes a story I saw in the newspaper and didn't get a chance to blog: the tragic destruction of a Maya pyramid to get road gravel.  People still do stupid stuff motivated by mild greed and great ignorance.

I also have a separate post in progress about the methodological flaws, but useful factual points made in a recent paper on property right and the early Neolithic era.