The controversy concerning the definition of quark and gluon angular momentum: what's it all about and does it matter?
Elliot Leader
Imperial College, London, UK
Vendredi 05/04/2013, 11:00
Bat 703, p 45, CEA Saclay, Orme des Merisiers


 A major controversy has arisen in QCD as to how to split the total angular

momentum of the nucleon into separate quark and gluon contributions, and as

to whether the gluon angular momentum can itself be split, in a gauge-invariant

way, into a spin and orbital part. Recall that textbooks on QED tell you that

you cannot split the angular momentum of a photon into a spin part and an

orbital part in a gauge-invariant way, so you would expect the same to be true

for gluons. Yet we claim to be able to measure the spin  of the gluons in a

nucleon. Several authors have proposed various answers to these questions and

offered a variety of different expressions for the relevant operators. I survey

some of the ideas put forward and try to assess their physical implications.

Contact : Stephane PLATCHKOV


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