Quarks
Now, before I get to that, let me also introduce the protagonist of this play to those of you who do not really know much about it. The top quark is the heaviest of the subnuclear constituents of hadronic matter, i.e. protons, neutrons, atomic nuclei as well as more ephemeral particles we can produce in particle accelerators.
The top quark is the sixth and the last one to be discovered. This happened at the end of a thirty-year quest that started in 1964, when Murray Gell-Mann and George Zweig imaginatively classified baryons and mesons until then discovered into representations of the SU(3) symmetry group, and independently hypothesized that what were the truly elementary constituents of matter were fractionary-electric-charge, point-like and massive bodies contained in triplets inside baryons (of which protons and neutrons are examples), and in particle-antiparticle pairs inside mesons (the simplest of which is the pion, a unstable particle with a mass equal to a sixth of that of the proton).
Gell-Mann and Zweig identified the need for three quarks to account for all until-then known hadrons: the up, the down, and the strange quark. As quarks cannot be observed as individual entities (you cannot extract them from within their parent hadrons) they remained more like a mathematical entities rather than ones with acknowledged physical reality until 1974, when two competing experiments led by Samuel Ting at Brookhaven and Burton Richter at SLAC both observed a particle - the J/Psi meson - which could only be explained as a bound state of a quark-antiquark pair. This was understood to be a fourth element of the set, the charm, which had in fact been hypothesized by Glashow, Iliopoulos and Maiani four years later to explain some unexplained properties of neutral mesons called kaons.
Following the charm discovery, which was paid back by the Nobel committee with no less than five Nobel prizes, the fifth element, bottom, was also identified in 1977 as the constituent of a new meson, this time the Upsilon. That discovery, along with accurate theoretical predictions, convinced everybody that a sixth quark, heavier than the others, ought to exist.
The race to the top
The hunt for that particle went underway. Initially it involved electron-positron colliders, which were however unable to produce very heavy particles, and then the UA1 and UA2 experiments at the CERN SppS collider which had served Rubbia his discovery of W and Z bosons in 1983.
The CDF experiment at the Fermilab Tevatron collider, which went in operation in 1985 and collected a first meaningful set of proton-antiproton collisions in 1987, was the first to see an evidence of the production process of a top-antitop quark pair. In April 1994 the CDF collaboration published an article on Physical Review Letters, where they showed how they had isolated 12 events with the required characteristics, corresponding to a 2.7-standard-deviations excess over background predictions. Tellingly, seven of those events allowed a coherent estimate of the top quark mass, at a value of 174 GeV which is still extremely close to the currently accepted world average for that quantity.
In 1994 the "five-sigma criterion" which is now an established standard for claiming discovery of a new effect in fundamental physics was not such, and the experiment could very well have decided to claim to have discovered the particle there and then. But CDF was hostage in those years of the very conservative mindset of some of its collaborators, who had clear in their mind the potential damage to their careers coming from hasty discovery claims. Knowing that more data were coming in, they published an extremely accurate, valuable scientific work but half-assedly named it, indeed, an "evidence" for the top quark.
One year later, they had enough additional statistics to reach the five-sigma statistical significance, and as per a previous internal agreement with the competing experiment across the ring, DZERO, they communicated to them with a two-weeks advance their intention to publish an observation claim for the particle. DZERO had of course also been looking for the top quark, and they thus had the time to put together their findings, also claiming a five-sigma significance. The two publications appeared back-to-back in Physical Review Letters, and the two experiments were jointly credited with the discovery of the last quark.
It should also be mentioned here, for its importance in the following discussion, that both CDF and DZERO published several other "interim" articles reporting on their ongoing progress with the search of the top quark, besides the three above. Those publications mainly discussed lower limits at 90 or 95% confidence level on the mass of the quark, based on not observing it in experimental data.
So what's the matter with Wikipedia?
Wikipedia works by freely allowing its users to edit its contents. Hence it should not be real news to observe that something is incorrect or imprecise there. However, I came across the following intriguing bit. If you look for the entry "top quark", you find that the references curiously do not point to the two discovery papers:
" The top quark was discovered in 1995 by the CDF[2] and DØ[3] experiments at Fermilab."[...]
- F. Abe et al. (CDF Collaboration) (1995). "Observation of Top Quark Production in
p
p
Collisions with the Collider Detector at Fermilab". Physical Review Letters. 74 (14): 2626–2631. arXiv:hep-ex/9503002. Bibcode:1995PhRvL..74.2626A. doi:10.1103/PhysRevLett.74.2626. PMID 10057978. - ^ Jump up to:a b S. Abachi et al. (DØ Collaboration) (1995). "Search for High Mass Top Quark Production in
p
p
Collisions at √s = 1.8 TeV". Physical Review Letters. 74 (13): 2422–2426. arXiv:hep-ex/9411001. Bibcode:1995PhRvL..74.2422A. doi:10.1103/PhysRevLett.74.2422. PMID 10057924.
So while the claim is that the discovery is credited to CDF and DZERO in 1995, the reference for DZERO points to a 1994 article! This is quite strange, especially since one would have if anything guessed that the CDF 1994 "evidence" paper would be cited.
This could just be an erratic glitch, but if one visits the Wikipedia page of the DZERO experiment, one finds the same pattern:
"On February 24, 1995, DØ and CDF submitted research papers to Physical Review Letters describing the observation of top and antitop quark pairs produced via the strong interaction.[16] On March 2, 1995, the two collaborations jointly reported the discovery of the top quark at a mass of about 175 GeV/c2 (nearly that of a gold nucleus). [17] [18] [19]"[...]
- T.M. Liss; P.L. Tipton (1997). "The Discovery of the Top Quark" (PDF). Scientific American. 277 (3): 54–59. doi:10.1038/scientificamerican0997-54.
- ^ F. Abe et al. (CDF Collaboration) (1995). "Observation of Top Quark Production in
p
p
Collisions with the Collider Detector at Fermilab". Physical Review Letters. 74 (14): 2626–2631. arXiv:hep-ex/9503002. Bibcode:1995PhRvL..74.2626A. doi:10.1103/PhysRevLett.74.2626. PMID 10057978. - ^ S. Abachi et al. (DØ Collaboration) (1995). "Search for High Mass Top Quark Production in
p
p
Collisions at √s = 1.8 TeV". Physical Review Letters. 74 (13): 2422–2426. arXiv:hep-ex/9411001. Bibcode:1995PhRvL..74.2422A. doi:10.1103/PhysRevLett.74.2422. PMID 10057924.
Above, ref.17 is a review article, and ref.18 is the original discovery paper by CDF, but ref.19 is again the 1994 paper by DZERO, which in no way is one which claims discovery for the top quark.
If one just looks for the word "quark" on Wikipedia one still gets a text that explains how CDF and DZERO jointly discovered the top, but again the DZERO reference is to the 1994 article.
On the other hand, if one visits the CDF experiment page on Wikipedia, one observes a rather strange asymmetry. That page is rather poorly written and it does not contain as much material as the DZERO one, e.g.; also, it does not cite directly any scientific paper by the collaboration, only relying on reviews as references. Interestingly, the text does not mention DZERO in any way when it explains that CDF discovered the top quark:
"Finally in February 1995, CDF had enough evidence to say that they had "discovered" the top quark.[7]"My "if you think bad you're probably going to be correct" explanation for the strange pointing to an interim (but early) publication by DZERO is that some DZERO members decided they would hack the pages and replace the 1995 "Discovery of the top quark" article by DZERO with the earlier study. But I cannot really claim this action may have any other consequence than confusing Wikipedia readers....
I will welcome comments on this topic in the comments thread!
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