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New ansatz for low-energy QCD

by mitchell porter
Tags: ansatz, lowenergy
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mitchell porter
Jan29-13, 06:52 PM
P: 751
A few years ago, Tamar Friedmann (then at MIT) wrote two papers proposing a new interpretation of the hadron spectrum, and some new phenomenological laws governing hadron size (and see this talk for a synopsis).

According to yesterday's hep-ph update, they're finally going to appear in print, so it might be a good time to discuss them. It would be especially good to have an opinion from people who know something about hadron phenomenology and systematics.

Recall the context: so far, we can't analytically solve QCD (though we can do computationally expensive lattice calculations), so the complicated realities of hadronic physics are understood using rules of thumb, phenomenological parameters, and heuristic justifications.

On the theory side, Friedmann proposes three new rules of thumb, three "laws": there are no radial excitations in low-energy QCD, the radius of a hadron is largest when the hadron is in its ground state, the radius of a hadron decreases when the hadron's orbital excitation increases.

She's also working with a somewhat "stringy" picture of hadrons. The concept of, say, a pion as a quark and antiquark connected by a string of gluon flux should be familiar. Here she builds on the idea that such strings can also terminate with diquarks, quark-quark pairings held together by emergent short-range forces. From this perspective, a baryon may be a quark-diquark string, and some of the heavier mesons may be diquark-antidiquark strings.

Now consider what happens if we take one of these hadrons and keep increasing its orbital excitation. According to her third law, the radius will be decreasing, i.e. the string will be contracting, bringing its ends closer together; eventually they will get close enough to break the string and deconfine.

The resulting sequence of states with increasing spin and mass is an example of a "Regge trajectory". These "trajectories" were observed in the hadron spectrum in the 1960s, and played a big role in pre-QCD thinking (as well as in the genesis of string theory), and it's an outstanding challenge in QCD to derive the existence of the Regge trajectories from the fundamentals. Friedmann's picture implies a new synthesis in which the approach to deconfinement (at least via orbital excitation) is a Regge trajectory. It may be a big clue towards a more profound understanding of QCD, especially (this part is my speculation) in conjunction with the new perspectives on gauge theory arising from the twistor string, that have been heavily promoted by Nima Arkani-Hamed.

Empirically, the absence of radial excitations implies the absence of radial quantum numbers, so hadronic systematics have to be revised, and Friedmann has a new plan here too, though about this part I have nothing to say. But I would urge people who know their mesons and baryons to have a look at her phenomenology paper, and comment.
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Vanadium 50
Jan29-13, 07:48 PM
Vanadium 50's Avatar
P: 16,169
I very much doubt this is correct. This does not explain the relative strengths of quarkonium radiative transitions, it does not explain why potential models can fit a broad range of quarkonium masses with just a few parameters, and it does not explain why quarkonia with high radial quantum numbers are bigger - indeed, it predicts the reverse.

It doesn't seem to answer many questions, and of those it does answer, it gets them wrong.

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