The main decay of $ J / \psi $ is $ c \bar{c} \to ggg $. How can this reaction channel be observed? 3 gluons will hadronize immediately, or not?
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2$\begingroup$ it just means that it mainly decays into hadrons. Yes gluons are not free particles but bound within hadrons. $\endgroup$– anna vAug 21, 2019 at 10:01
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$\begingroup$ But here en.wikipedia.org/wiki/J/psi_meson#Decay_modes it is separated. It even reads such that J/Psi has to decay first into a photon and then into hadrons. But all feynman graphs I've seen do not depict that. $\endgroup$– BenAug 21, 2019 at 10:04
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$\begingroup$ And another question: How was it experimentally possible to decover this main decay? I mean 3 gluons..? $\endgroup$– BenAug 21, 2019 at 10:05
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1$\begingroup$ Related post by OP: physics.stackexchange.com/q/497618/2451 $\endgroup$– Qmechanic ♦Aug 21, 2019 at 10:29
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1$\begingroup$ @Ben by fitting models to data, that is the standard way in particle physics. One fits the data and until proven wrong if the fit is statistically good the model stands . arxiv.org/abs/0711.4556 . That is how particle physics progresses, gathers data, fits models, models predict new behaviors new experiments to validate or falsify models. $\endgroup$– anna vAug 21, 2019 at 12:11
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1 Answer
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If you look at the data from the PDG, you see that the decay into three gluons is part of the decay mode into hadrons: $\Gamma_1 \approx \Gamma_2 + \Gamma_3 +\Gamma_4$.
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$\begingroup$ And how are three gluons measured and despite from that they cannot occur freely? $\endgroup$– BenAug 21, 2019 at 17:06
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$\begingroup$ They are not measured directly, but indirectly. Imagine it like this: there are three "categories" of hadrons that can come out of a $J/\psi$ decay. From theoretic (!) considerations, we know that category 1 can only be realized via a virtual photon, category 2 can only be produced via 3 gluons, and category 3 with two gluons and a photon. If you then count how many category-1-hadrons you see, you can deduce that the $J/\psi$ has had to decay into three gluons before. $\endgroup$ Aug 22, 2019 at 1:58