Tuesday, June 26, 2012

New Ideas On Gluon Spin And Other QCD Topics

On Wednesday . . . one [talk] was delivered by Luigi Capozza on behalf of COMPASS collaboration. . . . Their aim is to take a measurement of the components of the proton spin. These are usually divided in three parts: Quark contribution, gluon contribution and orbital contribution. The striking part of the measurement is that the gluon contribution is compatible with zero! From what we know in the high-energy limit, where the gluon concept is well-defined, these are spin-1 objects and so, it is not so straightforward to have zero contribution from them to the proton spin and indeed this is an important open problem in theoretical physics.

From Marco Frasca reporting on a conference session at QCD@Work (with presentation slides available here).

This result seems to echo others in QCD that seem to suggest that there is some sort of phase change that gluons experience between high energy (UV) states and low energy (IR) states, giving rise to seemingly completely different particles in terms of both spin and mass.  At any rate, mysteries in something so basic as the sources of spin in a proton are themselves reminders that we don't know everything yet.

Another interesting account from the same source regarding the same conference is this one:
I heard the talk from Mirko Serino about a really innovative idea. Mirko is a PhD studend at University of Salento and together with Claudio Coriano, that is his professor, Luigi Delle Rose and Antonio Quintavalle are producing an analytical computation of Standard Model in presence of gravity. This kind of computation is highly non-trivial and quite complicated. The striking result they get is that appears a coupling between a scalar degree of freedom and the gauge field and this appears as a rather interesting new proposal for mass generation. I have talked with the students of Coriano and they were really excited by this result that is indeed really interesting and unexpected.
In a nutshell, Serino devises proposed additional terms in the Einstein-Hilbert actions from quantum mechanical first principles that could provide a phenomenological clue to look for if one is looking for quantum gravity effects.  The paper does not explore what this would imply in practice at a level sufficiently down to earth for a mere human like myself to understand, and to be perfectly honest, I'm not sure that I perfectly understand all of the assumptions that go into the calculation, even though I understand most of them.

There are also suggestions in the final session of the conference (Svjetlana Fajfer , "New Physics in B to D* tau nu decay") that the Belle and BaBar experiments are seeing 3 sigma plus deviations from the Standard Model in B meson decays which could suggest Beyond the Standard Model physics, such as additional Higgs bosons (charged and heavy), since the results seem impossible to explain with any single consistent set of Standard Model CKM matrix values. Then again, maybe there are just flaws in how the theoretical expecations are being calculated.  Having been through multiple CKM matrix can't be reconciled exercises only to find the data confirm it again, I'm more that a bit skeptical of the results from such an exotic decay.  But, given how scarce any more than three sigma BSM experimental results are these days, it bears watching.

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