The pole masses of the top quark, Higgs boson, Z boson, W boson, tau lepton, muon, and electron are relatively straight forward to measure directly, and are known to decent to excellent relative precision.
In contrast, the bottom, charm, strange, down, and up quarks are always confined in a hadron. This means that their masses can't be measured directly and instead have to be reverse engineered from hadron properties according to some self-consistent scheme, one in which the pole masses of isolated particles is not even necessarily well defined.
The most common scheme for determining the masses of the five less massive quarks is the MS-bar mass a.ka. the modified minimal subtraction scheme. But this isn't the only scheme for determining their masses. Another one is the "on-shell mass" of bottom and charm quarks which can be determined more or less exactly and to almost the same precision as the MS-bar mass upon which a lot of good data has been assembled. And, it more fundamental, and hence more appropriate to use for theoretical purposes (although the "on-shell" mass of the three lightest quarks is ill-defined). As explained in the introduction of the linked paper:
In perturbative QCD (pQCD) theory, two schemes are frequently adopted for renormalizing the quark masses, e.g. the on-shell (OS) scheme and the modified minimal subtraction (MS) scheme.
The OS mass, also known as the pole mass, offers the advantage of being grounded in a physical definition which is gauge-parameter independent and scheme independent. It ensures that the inverse heavy-quark propagator exhibits a zero at the location of the pole mass to any order in the perturbative expansion.
On the other hand, the MS scheme focuses solely on removing the subtraction term 1/ǫ+ln(4π)−γE from the quantum corrections to the quark two-point function. And by combining this with the bare mass, one can derive the expression for the renormalized MS mass.
In high-energy processes, the MS mass is preferred for its lack of intrinsic uncertainties. It has been found that for the high-energy processes involving the bottom quark, such as the B meson decays, when their typical scales are lower than the bottom quark mass, the using of MS mass becomes less suitable and the OS mass is usually adopted.
Practically, the perturbative series using the OS mass is plagued by renormalon ambiguities, resulting in a perturbative series with poor convergence. Thus for precision tests of the Standard Model, accurate determination of the OS mass is important.
It is noted that the OS mass can be related to the MS mass by using the perturbative relation between the bare quark mass (mq,0) and the renormalized mass in either the OS or MS scheme, where q denotes the heavy charm, bottom, and top quark, respectively.
The MS-bar mass of the bottom quark is 4.18 + 0.03 - 0.02 GeV. A new paper determines that this is equivalent to an on-shell mass of the bottom quark of 5.36 + 0.10 - 0.07 GeV. The new preprint that makes this conversion and its abstract are as follows:
Shun-Yue Ma, Xu-Dong Huang, Xu-Chang Zheng, Xing-Gang Wu, "Precise determination of the bottom-quark on-shell mass using its four-loop relation to the MS bar scheme running mass" arXiv:2406.18025 (June 26, 2024).
Why care which definition of quark mass is used?
One reason is that this is relevant to a hypothesized relationship between the masses of the Standard Model fundamental particles and the Higgs vacuum expectation value (Higgs vev) known as the LP & C relation.
This hypothesis holds that the sum of the square of the fundamental particle masses is equal to the square of the Higgs vev. This is equivalent to saying that the Higgs field Yukawas of the fundamental particles in the Standard Model add up to exactly one.
Putting best fit measurements into this formula comes up just a little short in a way that is principally due to the top quark mass being too light, when MS-bar scheme running masses are used for the other five quarks.
The current best fit measurement of the top quark mass is 172.690 ± 0.3 GeV.
But, to make the LP & C relation work with the best fit masses of all of the other fundamental particles, the preferred value is 173.615 GeV, which is about a 3.1 sigma tension.
If the on-shell mass of the bottom quark is used instead, however, this eases up these tensions somewhat.
For example, fitting the top quark mass alone requires a top quark mass of 173.583 GeV when using the on-shell mass of the bottom quark, which is a little bit less than a 3.0 sigma tension. Using the on-shell masses of the charm quark as well would require a top quark mass of 173.570 GeV, which is a bit more than a 2.9 sigma tension.
Another way to make the numbers fit while reducing the tensions for the mass measurements of individual particles is to use top quark masses and Higgs boson masses. In the case of the Higgs boson, the current world average mass is 125.25 ± 0.17 GeV.
If both the top quark mass and Higgs boson mass are increased above their best fit measurements by equal numbers of standard deviations, which reduces the tension to about 2.6 sigma, and that could be reduced to a tension of about 2.5 sigma or less for both the top quark and the Higgs boson, using on-shell masses, which is significantly more mild than a 3.1 sigma tension in the top quark mass.
With all three adjustments, the LP & C relation could fit with a top quark mass of 173.44 GeV and the Higgs boson mass of 125.675 GeV, neither of which is a huge stretch, which means that the LP & C relation is still a viable theory, even though it is not perfectly consistent with the latest mass measurements.
Using the MS-bar mass rather than the on-shell masses of the three light quarks turns out to be immaterial in evaluating the LP & C relation, in which the uncertainties are dominated by the uncertainties in the largest absolute fundamental particle masses.
Alternatively, the LP & C relation could hold because the list of Standard Model fundamental particles is not complete, in which case it estimates the sum of the square of the masses of the missing fundamental particles, subject to the relative uncertainties in the known fundamental particle masses, in a global test of the completeness of the Standard Model.
The best fit to this gap, if concentrated in a single particle, would be a particle with a mass of about 17.5 GeV, but with a great uncertainty, mostly due to the uncertainties in the top quark and Higgs boson masses, and to a lesser extent the W boson mass uncertainty.
The trouble is, of course, that this mass range is well-explored, has produced no fundamental particles in this mass range, and would wildly throw off the observed branching fractions of the Higgs boson, Z boson, and W boson if it did exist, so it probably doesn't.
On-shell masses also make sense to use when exploring generalizations of Koide's rule to quark masses.
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Fossilized Bone of Neanderthal With 'Down Syndrome' Challenges Ideas of Prehistoric Care
Humans
27 June 2024
By Rebecca Dyer
The discovery of a Neanderthal child with what appeared to be significant health complications indicative of Down syndrome has swayed debate over the origins of communal healthcare within our species.
In a new study, a research team from Spain says the fact the child survived to at least age six – despite severe hearing loss and balance issues – demonstrates the complexity of social care among our closest evolutionary relatives, the Neanderthals (Homo neanderthalensis).
The child's survival relied on more support than a mother could give, suggesting assistance from a wider group in challenge of ideas that prehistoric caregiving only extended to immediate family or those who could reciprocate the favor.
Neanderthals have long been known to care for the sick and injured in their communities, but there's debate around the motivations behind this behavior.
https://www.sciencealert.com/fossilized-bone-of-neanderthal-with-down-syndrome-challenges-ideas-of-prehistoric-care
Cool. Thanks for the heads up.
sure
so where do Neanderthal and Denisovan comes from ?
From H. Heidelbergensis (a species which migrated out of Africa) or a post-H. Heidelbergensis common ancestor of Neanderthals and Denisovans. They probably diversified into Neanderthals and into Denisovans respectively from H. Heidelbergensis in Eurasia in the Middle Paleolithic. They are essentially West Eurasian and East Eurasian branches of the same clade. H. Heidelbergensis is also the dominant hominin ancestor of modern humans.
n Science Advances today, Alan Rogers, a population geneticist at the University of Utah, Salt Lake City, and his team identified variations at matching sites in the genomes of different human populations, including Europeans, Asians, Neanderthals, and Denisovans. The team tested eight scenarios of how genes are distributed before and after mixing with another group, to see which scenario best simulated the observed patterns. They conclude that the ancestors of Neanderthals and Denisovans—whom they call Neandersovans—interbred with a "super-archaic" population that separated from other humans about 2 million years ago. Likely candidates include early members of our genus, such as H. erectus or one of its contemporaries. The mixing likely happened outside of Africa, because that's where both Neanderthals and Denisovans emerged, and it could have taken place at least 600,000 years ago.
"I think the super-archaics were in the first wave of hominids who left Africa," Rogers says. "They stayed in Eurasia, largely isolated from Africans, until 700,000 years ago when Neandersovans left Africa and interbred with them.
Ghosts in the family tree
At least two super-archaic "ghost" hominins interbred with the ancestors of Neanderthals, Denisovans, and modern humans (red lines). Later, those three groups also met and mingled (blue lines), leaving complex traces in each other's genomes. (Split times are rough estimates; timeline is not drawn to scale).
https://www.science.org/content/article/mysterious-ghost-populations-had-multiple-trysts-human-ancestors
isn't mixing with super-archaics "Likely candidates include early members of our genus, such as H. erectus or one of its contemporaries"
multi regional
H sapiens also included super-archaics "ghost" hominins
Re the 2020 article:
* Neanderthals and Denisovan branch separates from modern human branch ca. 700 kya. Neanderthals and Denisovans admix with H. Erectus ca. 600 kya. Neanderthals and Denisovans break into separate branches by 400 kya, and admix with with another superarchaic ghost ca. 450 kya just before splitting.
* Neanderthals admix with modern humans ca. 125k kya and again 75kya to 30 kya (some extra in Europe in populations wiped out in Ice Age that peaked 20kya).
* Denisovans probably admix with modern humans sometime between 75 kya and 30 kya.
* The 2020 article argues that there is super-archaic ghost population admixture direct into modern humans ca. 90 kya, prior to division into West Eurasian, East Eurasian, Melanesian and Africa. I'm rather skeptical that this is not either indirect or an artifact of population structure in African modern humans.
The story of this new work begins in northern Spain. There, a group of Spanish researchers at the site of Sima de los Huesos teamed up with geneticists from the Max Planck Institute for Evolutionary Anthropology to examine the oldest known hominin DNA sample, which comes from a 400,000-year-old Homo heidelbergensis thigh bone. They sequenced the bone’s mitochondrial DNA (mtDNA), which is passed from mother to child. “What we were expecting to see was Neanderthal mitochondrial DNA,” says Matthias Meyer of the Max Planck Institute, as Neanderthals would later occupy that part of Europe and might be expected to carry genetic material from the previous inhabitants. Surprisingly, the mtDNA is instead more closely related to that of a hominin who lived more than 50,000 years ago in Siberia’s Denisova Cave than it is to that of Neanderthals. The Denisovans were related to, but genetically distinct from, Neanderthals.
According to Meyer, the Sima de los Huesos sample is old enough that it could represent an ancestor to both Denisovans and Neanderthals. However, it is also possible that H. heidelbergensis is not ancestral to either group, but later interbred with the Denisovan lineage. Studies of nuclear DNA, which contains genetic information from both parents, will be needed to clarify the relationship, Meyer believes.
https://archaeology.org/issues/march-april-2014/digs-discoveries/human-dna-homo-heidelbergensis-denisovan-lineage/
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