For the most part, the abundance of chemical elements in the universe closely matches the predictions of Big Bang Nucleosynthesis (BBN), providing one of the strongest points of "solid ground" in the history of the universe after which we can fairly claim a good understanding of much of the key physics driving everything that came afterwords.
BBN supposes that shortly after the Big Bang, in a universe composed predominantly of free protons and neutrons, that these nucleons randomly collided giving rise to nuclear fusion reactions that produced a specific mix of atomic elements (including the mix of isotopes of those elements which are atoms with the the number of protons in their nuclei that makes them a particular kind of element, with varying numbers of neutrons that establish which isotope of the element it is, with an isotope generally identified by a whole number equal to the number of protons plus the number of neutrons in the atomic nucleus of the isotope). In the conventional chronology of the Universe, this even takes about twenty minutes and begins about ten seconds after the Big Bang.
Then, BBN theory adjusts this initial prediction to reflect known processes by which atomic nuclei decay in nuclear fission, experience nuclear decay, or are merged in nuclear fusion reactions, since the initial BBN, to produce the current mix of chemical elements (and isotopes) in the universe.
BBN is very successful at describing the observed mix of chemical elements and isotopes in the universe with reasonable precision (with relative errors of similar magnitude to the uncertainties in the measurements which for some of the key quantities are on the order of 1%), with one main exception, which is an incorrect proportion of the Lithium-7 isotope (which is present at a fraction of about 1.6 parts per 10,000,000,000 in metal poor stars, which is much smaller than any other of the other detectable BBN products) that is smaller by a factor of about three than the proportion of about 5 parts per 10,000,000,000 expected at the time of initial BBN. This discrepancy has historically been in strong (roughly 4 sigma) tension with the expected value.
But there is only a discrepancy if the proportion of Lithium-7 in a star doesn't decline due to the effect of nuclear processes in the star over many billions of years, as assumed in the models giving rise to the Lithium Problem. This assumption was well supported by early astronomy observations, but those observations have now been superseded with larger volumes of higher precision data.
A new paper, reflecting an improved understanding of post-BBN processes and of the current mix of chemical elements and isotopes in the universe, as a result of a greater availability of more and more precision astronomy observations, concludes that there might not be a Lithium Problem with BBN after all.
In a nutshell, there is strong circumstantial evidence that proportion of Lithium-7 in metal poor stars falls over time due to nuclear reactions in situ within those stars which can explain the Lithium Problem, which would explain why the proportion of Lithium-7 in metal poor stars that is observed is lower than the unmodified BBN expected value.
Basically, almost no Lithium-6 should be created in BBN. Instead, it is created in metal poor stars by cosmic rays (mostly from supernova) in a well understood process that should produce more Lithium-6 than is observed. This is strong evidence of nuclear processes in metal poor stars that deplete Lithium-6 levels in those stars. This observation, in turn, allows astronomers to make rough estimates of the amount by which Lithium-7 levels in those stars was depleted, because Lithium-6 can be transformed into other isotopes more easily than Lithium-7 can. And, this analysis, combined with other data, makes it possible to make a ballpark estimate of the amount of Lithium-7 depletion from the expected BBN value that is consistent with the observed amount Lithium-7 in these stars (although it isn't precise enough to get an expect prediction for current Lithium-7 levels yet).
If this hypothesis is correct, one way to confirm it is to show that the proportion of Lithium-7 in the interstellar medium (ISM) should be higher than it is in the halo stars of typical galaxies, because the Lithium-7 deletion which the paper reasons occurs in stars should not occur in the ISM where the processes that could cause Lithium-7 depletion in stars are absent.
BBN predictions are one of the most powerful and robust constraints on a variety of speculative high energy physics theories beyond the Standard Model. If its predictions are on target, then it is a good global indicator that we have a pretty much complete understanding of the process of nuclear isotope formation in the period of time from ten seconds after the Big Bang to the present, leaving little or no room for a wide away of beyond the Standard Model physics proposals that would otherwise operate in that time frame.
Complete confirmation of BBN predictions also pretty much fixes in place a set of "initial conditions" of the universe as of ten seconds after the Big Bang, putting a lower bound on the energy scales at which significant beyond the Standard Model physics can exist (i.e. 1,000,000,000 Kelvin a.k.a. 100 keV) and also placing a boundary condition on what the effect of those beyond the Standard Model physics theories can be, since those physics have to reproduce conditions at ten seconds after the Big Bang consistent with the assumptions of BBN.
The Large Hadron Collider (LHC) has also already explored circumstances up to about 1,000,000,000,000,000 Kelvin a.k.a. 150 GeV (temperature stated in eV units does not correspond exactly to the more familiar collision energies at the LHC which are currently up to about 13,000-14,000 GeV), largely ruling out new physics at those energy scales as well, which takes us back to the first 1/1,000,000,000,000th of a second or so after the Big Bang in which we know that the Standard Model of Particle Physics should apply, thus shrinking the proportion of the history of the universe where beyond the Standard Model physics aren't ruled out dramatically.
The paper and its abstract are as follows:
The primordial Lithium Problem is intimately connected to the assumption that 7Li observed in metal-poor halo stars retains its primordial abundance, which lies significantly below the predictions of standard big-bang nucleosynthesis.Two key lines of evidence have argued that these stars have not significantly depleted their initial 7Li: i) the lack of dispersion in Li abundances measured at low metallicity; and ii) the detection of the more fragile 6Li isotope in at least two halo stars. The purported 6Li detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of 6Li. This concordance left little room for depletion of 6Li depletion, and implied that the more robust 7Li largely evaded destruction.
Recent (re)-observations of halo stars challenge the evidence against 7Li depletion: i) lithium abundances now show significant dispersion, and ii) sensitive 6Li searches now reveal only firm upper limits to the 6Li/7Li ratio. The tight new 6Li upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial 6Li depletion has occurred--by factors up to 50.We show that in stars with 6Li limits and thus lower bounds on 6Li depletion, an equal amount of 7Li depletion is more than sufficient to resolve the primordial 7Li Problem. This picture is consistent with stellar models in which 7Li is less depleted than 6Li, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas.
6 comments:
Thoughts on this?
https://www.livescience.com/heavy-w-boson-measurement-cracking-standard-model
@Ryan
First, thanks for the heads up. I appreciate it. The bottom line is that the CDF experiment from the old Tevatron collider (long ago decommissioned) combed through ten years of old data to get a W boson mass value of The CDF collaboration measured the value of the W boson to be 80,433 ± 9 MeV.
"The result differed from the Standard Model prediction of the W boson's mass, which is 80,357 ± 6 MeV/c2. The uncertainties in that calculation (the "±") come from uncertainties in the measurement of the Higgs boson and other particles, which must be inserted into the calculation, and from the calculation itself, which relies on several approximation techniques.
The differences between the results aren't very large in an absolute sense. Because of the high precision, however, they are separated by seven standard deviations, indicating the presence of a major discrepancy.
The new result also disagrees with previous measurements from other collider experiments, which have been largely consistent with the Standard Model prediction. It's not clear yet if this result is caused by some unknown bias within the experiment or if it's the first sign of new physics."
This is wrong. There is no "Standard Model prediction of the W boson's mass". It is an experimentally determined free parameter. More precisely, the W boson mass, the Z boson mass, the electromagnetic coupling constant, the weak force coupling constant, and the Higgs vacuum expectation value are five related physical constants with three degrees of freedom and you can take your pick to some extent which you treat as input parameters and which you treat as derived.
The 80,357 ± 6 MeV value is not a "prediction of the Standard Model" it is an electroweak fit of the Standard Model physical constants that also makes some beyond the Standard Model assumptions. See, e.g., https://link.springer.com/article/10.1140/epjc/s10052-018-6131-3 The same procedure suggested that the Higgs boson had a mass of 90 ± 20 GeV when the inverse error weighted global average of the measured real value is 125,250 ± 170 MeV (https://pdglive.lbl.gov/Particle.action?node=S126&init=0) with contributing estimates from data used in the fit that ranged from 35 GeV to 463 GeV.
In short, electroweak fits are tea leaf reading, but not to be taken too seriously and are certainly nothing that cases any doubt on the Standard Model.
Also, the CDF value and all of the other values except the global fit are really about 20 MeV too high due to a definitional issue in how the W boson mass is extracted from the experimental data. See http://dispatchesfromturtleisland.blogspot.com/2022/03/rethinking-w-and-z-masses.html
The disagreement with prior experiments, however, is real. See https://pdglive.lbl.gov/DataBlock.action?node=S043M See also a narrative explanation at https://pdg.lbl.gov/2021/web/viewer.html?file=https://pdg.lbl.gov/2021/reviews/rpp2021-rev-w-mass.pdf
The inverse error weighted global average of best nine most recent independent measurements of the W boson mass is 80,379 ± 12 MeV. Two of those nine measurements are from CDF (80,433 ± 79 MeV in 2001 and 80,387 ± 19 in 2012) and two more are from CDF's sister experiment from Tevatron called D0 (80483 ± 84 from 2002 and 80375 ± 23 from 2014), with the older values in each case made at 1.8 TeV and the newer values in each case made at 1.96 TeV. The four data point inverse error weighted combined Tevatron average was 80387 ± 16 MeV. Three more superseded W boson masses from CDF and D0 were ignored and ranges from 80367 MEV to 80413 MeV.
Another four measurements are from the defunct LEP (linear electron positron collider) from 2006 to 2008 at energies from 161-209 GeV with an error weighted average of 80376 ± 33 MeV. The range of the LEP measurements was 80270 MeV to 80440 MeV.
Many far less precise measurements from 1983 to 2018 were ignored in determining the inverse error weighted world average.
One of the measurements is 80370 ± 18 MeV at an energy of 7 TeV that shares 7 MeV of systemic uncertainty with the Tevatron average.
The newly announced result is exactly the same as the 2001 measurement by the same group but with a claimed uncertainty of 9 MeV instead of 79 MeV.
Rather than overturning the Standard Model, all this result should do is replace the old combined Tevatron value of 80387 ± 16 MeV with a new combined Tevatron value of 80,433 ± 9 MeV which will pull the global average a little higher than it used to be, although to be honest, my intuition is that a claim to shift the combined average from the very same data from the very same machine up by 46 MeV while reducing the uncertainty by 44% (7 MeV) is pretty suspect. I really doubt that our data analysis compared to a previous compilation done in 2013-2014 would change that much even if there were significant innovations in that time frame.
When your result which claims to have only modestly less uncertainty than the prior experiments is a huge outlier with respect to everyone else, it is more likely that you or the scientists who are the source of your data, have done something wrong than it is that you are right and they are wrong (most likely in underestimating the true uncertainty of their measurement). Indeed, in addition to shifting up the global average, this result will probably increase rather than decrease the uncertainty in the overall global average because the contributing data points are now a lot less tightly clustered than they were before relative to their claimed uncertainties, which undermines the assertion that the claimed uncertainties of the new CDF value are correct.
FWIW, the inaccurate claim of tension with the Standard Model is derived from inaccurate claims in Fermilab's April 7, 2022 press release https://www.eurekalert.org/news-releases/948608 and not really the fault of the Paul Sutter at Live Science. The press release does claim that "This result uses the entire dataset collected from the Tevatron collider at Fermilab. It is based on the observation of 4.2 million W boson candidates, about four times the number used in the analysis the collaboration published in 2012." Tevatron stopped collecting data in 2011.
The press release describes the global electroweak fit by stating that: "The collaboration also compared their result to the best value expected for the W boson mass using the Standard Model, which is 80,357 ± 6 MeV/c2. This value is based on complex Standard Model calculations that intricately link the mass of the W boson to the measurements of the masses of two other particles: the top quark, discovered at the Tevatron collider at Fermilab in 1995, and the Higgs boson, discovered at the Large Hadron Collider at CERN in 2012." But it really grossly overstated the extent to which a global electroweak fit is a "prediction" of the Standard Model or in claiming that it is scientific in the way that a Standard Model calculations is scientific. The claim in the abstract that "This measurement is in significant tension with the standard model expectation" is misleading and I would have objected strenuously to the claim if I were a peer-reviewer of the paper. The follow up discussion of the Higgs mechanism, dark matter, and extensions of the Standard Model is likewise total bullshit shenanigans that doesn't belong in a paper reporting a Standard Model constant measurement from 11 year old data.
The paper, as published is at https://www.science.org/doi/10.1126/science.abk1781 which reports 80433.5 ± 6.4 statistical ± 6.9 systemic MeV. The combined uncertainty is ± 9.4 MeV for a combined Tevaton of 80427.4 ± 8.9 MeV, and a combined Tevatron and LEP of 80424.2 ± 8.7 MeV.
They also update the Z boson measurement at "91,192.0±6.4stat±4.0syst MeV [ed. combined error 7.5 GeV] (stat, statistical uncertainty; syst, systematic uncertainty), which is consistent with the world average of 91,187.6±2.1 MeV." This is also suspect. The Z boson uncertainty should be much lower than the W boson measurement uncertainty and instead it is only slightly smaller.
The real story is that the new result is significantly out of step with the other experiments that have measured the same thing, while being in closely in line with CDF's previous reported value that claimed more than eight times as much uncertainty. The claimed difference from a "Standard Model prediction" is a smoke screen designed to shift the narrative away from a result that makes it look, at least at first glance, like CDF screwed up.
Another paper also suggesting Lithium depletion. https://arxiv.org/abs/2204.05820
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