look at the data.



It is about taking seriously the ideas of http://dx.doi.org/10.1016/0370-2693(71)90028-1" [Broken] ): the fermion in the dual model is susy to gluonic strings. So now all you need is to terminate the gluonic string. Regretly in 1971 there were only three states available to terminate the string: u, d, and s. Now we have the full history, and the experimental data tell us that we can terminate the gluonic string with five and only five different states: u, d, s, c, b.



So just count, please, just do the SU(5) global flavour game, and count. How many states do you get of charge +1? six, by terminating with particle and antiparticle. How many of charge +2/3? six of each colour, by terminating with an antiparticle at each end of the string. How many of -1/3? six. How many +1/3, -1, -2/3? Same: six, six, six. And how many neutrals? of course, twelve: the other half of the 24 of SU(5).



BONUS: Does it means that string theory, given as input the 3-2-1 gauge theory of the SM, predicts three generations? No exactly; only if we require that the neutral leptons must be produced too. If we only look at the quark sector, then any pairing of [itex](2^{p})[/itex] "up quarks" with [itex](2^{p+1} -1)[/itex] "down quarks" will produce equal number, [itex]2^p (2^{p+1} -1)[/itex] of up and down combinations, and p=1 is just the simplest case. Numerically minded people will notice that p=4 amounts to 496, but a theory with 16 light "down" quarks, 31 light "up" quarks and a total of 248 generations seems not to be the object that Nature has offered us.



EDIT: Allow me a correction to this remark: Of course, the quark sector condition works for any integers [itex]q[/itex] and [itex]2 q -1[/itex], with [itex]q[/itex] an even number, not necessarily a power of two. But that the powers of two are an interesing subset was noted by Peter Crawley in other thread time ago and I am kind of obsessed with this, because it could constitute the way to reconnect with usual string models, via the above p=4 case.



EDIT: other references using "fermion-meson": L. Brink and D. B. Fairlie http://dx.doi.org/10.1016/0550-3213(74)90529-X [Broken] Nuclear Physics B Volume 74, Issue 2, 25 May 1974, Pages 321-342 ; Edward Corrigan and David I. Olive http://www.slac.stanford.edu/spires/find/hep/www?j=NUCIA,A11,749 [Broken] Nuovo Cim.A11:749-773, 1972. Modernly, there are some papers, in the framework of SQCD and also in Holography, that work with "mesinos", generic susy partners of mesons. But note that phenomenologists restrict the name "mesino" to the composite combination of squark and quark