Thursday, 29 August 2013

SUSY 2013 Live Blog: Day Four Session Two

The post-coffee break session returns to more very theoretical topics.

10:45am: Henrik Johansson, "Towards Determining the UV Behaviour of Maximal Supergravity"

Text-book calculations of perturbative (weak-field) gravity are horrible.  Even the cubic self-coupling has over 100 terms!  And their are infinitely many due to non-renormalisability.  There are some simplifications on-shell, e.g. the polarisation tensor looks like the product of two Yang-Mills polarisation tensors.  Similar simplifications arise in higher amplitudes, which can be expressed as direct products of Yang-Mills terms.  This is conjectured to hold to all orders.

What of off-shell?  Can we do the same thing there?

First, consider the usual UV problem of gravity.  The naive behaviour of the amplitudes is horribly divergent, based on simple power counting.  To get finite results as in Y-M you need ridiculous cancellations.  But this happens in N = 8 supergravity in all known amplitudes.

Surprising fact: no known divergence in pure d = 4 supergravity has been found to date.  A consequence of SUSY is that no 1, 2 loop divergences possible.  Still surprising that no divergences arise at higher loops; N = 8 tested to six loops!

Is this theory UV-finite?  If so, points towards some extra yet-unknown structure/symmetry.

Color-kinetmatics duality: kinematics in Y-M involving cubic couplings have same symmetry properties as the colour structure of those diagrams, specifically antisymmetry and the Jacobi identity.  This is related to the [Gravity] = [Gauge]2 behaviour; in going from the gauge to the gravity case, you replace the colour factors by the kinematic ones.

11:20am: Jaume Gomis, "Exact Results in Supersymmetric Field Theories and Applications"

Need coffee...

11:55am: Zohar Komargodski, "N=1 Supersymmmetry on Curved Spaces"

Simply replacing partial derivatives with covariant derivatives in a flat SUSY theory does not reproduce a SUSY theory in curved space.  You must turn on some background fields as well.  The associated ambiguities can be dealt with systematically.

By just trying everything and seeing what works?  That seems to be the message here.

12:30pm: Gordy Kane, Bonus Talk

This is billed the "After Dinner Talk before Lunch", and I'm definitely hungry.  This seems to be a Yay, String Theory! talk.  But without the Yay!

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