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    The Quote Of The Week - How Quarks Came To Be Accepted
    By Tommaso Dorigo | May 30th 2014 06:55 AM | 7 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    "After the 1974 London Conference, with its strong confirmation of the quark model, a general change of view developed with regard to the structure of hadrons. [...] the quark structure of hadrons became the dominant view for developing theory and planning experiments. A crucial element of this change was the general acceptance of QCD, which eliminated the last paradox, namely, why are there no free quarks ? The conjectured infrared slavery mechanism of QCD provided a reason to accept quarks as physical constituents without demanding the existence of free quarks. The asymptotic freedom property of QCD also provided a ready explanation of scaling [...]. There were a number of other important experimental results reported in1974 and in the latter half of the decade that provided further strong confirmations of the quark model. Among these were the discovery of charmonium, and its excited states [...]. The quark model, with quark interactions described by QCD, became the accepted basis for the structure of hadrons."

    Jerome Friedman, "Deep-Inelastic Scattering and the Discovery of Quarks", in "The Rise of the Standard Model", ed. L.Hoddeson, L.Brown, M.Riordan, M.Dresden, Cambridge UP 1997, p.584.

    So, I use to teach my students that it was the discovery of the J/ψ at SLAC and Brookhaven what tilted the balance, but Friedman says people were already convinced of the existence of quarks before that... I guess the truth is in the middle.

    Comments

    Some people still weren't convinced by the discovery of the J/ψ. In 1978, while investigating the possibility of primordial black hole explosions producing observable radio pulses, I got to spend a few days at NRAO Greenbank. During one of the daily coffee breaks I was introduced to a physicist whose name I can't recall. Part of the discussion at the table was about my work and concerned how the quark model might affect the evaporation and explosion of a black hole, and the production of any radio pulses. He severely admonished for believing this "quark nonsense" and assured me that the bootstrap model would prevail.

    dorigo
    HI George,

    thank you very much for your input! Indeed, new ideas require new thinkers. I am sure someone continued to think that quarks were imaginary entities even later, but of course the important thing is when scientific progress is not hindered anymore by the minority view.

    Cheers,
    T.
    Vladimir Kalitvianski
    It is funny that without theoretical solutions at that time QCD was "accepted" with hopes that someday the confinement would be obtained within QCD.

    By the way, do you think that electrons can be free? How about their permanent interaction with the electromagnetic degrees of freedom?
    dorigo
    Yeah, let's fight for the freedom of electrons Vladimir! ;-)
    I think QCD was accepted slowly but steadily in the seventies, but until we knew that quarks and gluons were inside hadrons for real, it was not itself elevated to part of the SM...

    Cheers,
    T.
    Vladimir Kalitvianski
    No, frankly, quarks were invented to describe quantum numbers. Later on, QCD turned out to deal with a sea of quarks, not with two or three inside hadrons. Also those charged gluons...

    I do not fight for the freedom of electrons. On the contrary, I say they are never free - they are "coupled in oscillators", roughly speaking.
    I don't disagree that charmonium was a dramatic demonstration both of quarks and esp. QCD ... and stimulated an astonishing outpouring of synthesis ..... and yet
    As a green grad student in 1972, and having worked for 2 years prior at the Cornell electron machine, the deep inelastic experiments 1968-71 were already very convincing, even indirectly indicating the existence of a strange quark, as was the R-ratio that helped support the notion of color. Even before that, the dramatic prediction of the existence of the Omega minus was hard to simply ignore. The sequence of the meson masses and the baryon masses also supported the quark model by the late 1960's - it provided a convincing explanation of the masses, as did the differences in strong cross-sections for strange versus non-strange particles (like K vs pi, or Lambda vs p and n). Hadron weak decay phenomenology always provided strong evidence for quarks, and the 1973 confirmation of the prediction of neutral currents, which, together with the charged current neutrino cross sections and including indirect evidence of strange quarks in the sea being promoted to charm by prompt dimuon production in neutrino scattering, convinced the great majority of physics researchers before the Nov 1974 AD revolution.

    Cheers & enjoy all this high spring fever...

    dorigo
    Thanks for this summary David. Indeed, the amount of positive clues was very large before the J/ψ, and the latter only tipped the balance. Of course physicists like to be sceptical, and that is a quite healthy thing. But sometimes too much of a good thing is not so good... As an example, by the time there was a 3-sigma effect of Higgs decays at the same mass both in Atlas and CMS data, I got convinced, but many insisted that it could be anything, misinterpreting the reason for the 5-sigma criterion...

    Cheers,
    T.