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Tuesday, November 22

  1. page Week Three Talks edited ... 2.) I can make my first foray into open notebook science. First, let me apologize to Andrei P…
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    2.) I can make my first foray into open notebook science.
    First, let me apologize to Andrei Parnachev. We were supposed to split a talk, with 30 minutes each. I went first and took a whole hour, and then walked off with the microphone, leaving Andrei with a grumpy laptop that didn't work right. I actually requested to hear him talk, so I felt like a jerk. Anyway, my apologies to him.
    First,As for physics, let me first address a
    Number one is the deficit angle. In any theory of dynamical gravity, any codimension-two object with a nonzero tension will produce a deficit angle. That happens for cosmic strings in (3+1) dimensions and for D7-branes in (9+1) dimensions. Crucially, the size of the angular deficit is given just by dimensional analysis:
    \Delta \theta \propto G_N T
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    e^{-\phi} = g_s^{-2} =- \frac{1}{2\pi} \log z,
    where z is the complex coordinate in the plane transverse to the D7-brane. Close to the D7-brane, meaning near z=0, the logarithm diverges to negative infinity. That is good: that means the dilaton and hence the string coupling is small near the D7-brane.
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    with the D3-branes. We thenD3-branes, and find some
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    some scale, meaning some radial position, throw out theeverything above that position (the UV part of the geometry,geometry), approximate the
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    very many D3-branes.D3-branes and if you only ask about low-energy physics.
    Equivalent statements apply in the field theory. N=4 SYM by itself is conformal. Adding flavors makes the coupling run, and indeed produces a Landau pole. The beta function has a coefficient Nf/Nc, however, so if we make that small, then we can pick some scale, throw out the UV physics, and restrict ourselves to questions only about IR physics, meaning far below the Landau pole. In that regime the beta function is approximately zero. Equivalently, for small coupling we can say that we discard all diagrams with flavor loops. In that case the coupling doesn't run. The deficit angle corresponds to an anomaly that also disappears. (Notice that dropping those loops disturbs 't Hooft anomaly matching: if we drop those loops, the IR theory need not have the same anomaly as the UV theory, where we don't drop the loops).
    In short, to quote Clifford Johnson, "It's like QED." Yes, the theory has a Landau pole, but as long as we only ask about IR physics, we don't care. Essentially 100% of the papers using probe D7-branes only ask about physics far below the Landau pole, and all of them give results consistent with the field theory, for example the supersymmetric meson spectrum.
    Also like QED, we can confront the Landau pole and ask about the UV completion. In the UV the dilaton will eventually diverge. We're in type IIB, however, so we can do an S-duality and make the dilaton small. In the field theory, we're on the other side of the Landau pole, and we switch the degrees of freedom we use to describe the system to some new, dyonic, degrees of freedom (some (p,q)-strings) that are weakly coupled. At least, in principle we can. Carlos Nunez told me that doesn't work in practice. He said he tried doing that, and found that some components of the metric become negative, which is clearly unphysical. He said ultimately you must lift the system to M-theory to obtain a good UV completion.
    MukundNow let's move to other topics.
    In my talk I gave a brief review of extremal AdS-Reissner-Nordstrom (AdS-RN), and the fact that it has nonzero entropy at zero temperature, which suggests that the system may be unstable. For example, if a charged scalar is present, and has sufficiently large charge and small mass, it may condense, sucking the charge out of the horizon and eventually forming a domain wall. The new ground state has no horizon and hence zero entropy. Veronika Hubeny asked an obvious question, though one often overlooked: doesn't that violate area-increase theorems for black holes? Sean Hartnoll answered this, although in fact the answer appeared in the very last paragraph of a paper by Horowitz and Roberts, 0908.3677. The black hole area-increase theorems apply for fixed energy and varying temperature. For the holographic superconductors, we usually work with fixed temperature and varying energy, hence we evade the area-increase theorems. The point is that we are supposed to imagine fixing the temperature, and then lowering the temperature through the transition and then eventually to zero temperature, where we will see that the black hole with scalar hair has lower energy than extremal AdS-RN.
    Mukund
    Rangamani also noted that
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    hydrodynamics fails toin extremal AdS-RN
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    just like AdS-Reissner-Nordstrom (AdS-RN).AdS-RN.
    That raised
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    As Keun-young Kim and Andreas
    g_s^o = g_s \left ( \frac{\textrm{det}(g + (2\pi\alpha')F)}{\textrm{det} g}\right)^{1/2}
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    the D7-brane. (Carlos Hoyos also suggested that perhaps in the open string sector you could switch from "string frame" to "Einstein frame," in which case the Einstein-frame metric would presumably exhibit AdS2.)
    The next question: if we have AdS2, do we see branch cuts in correlators? Here the answer is subtle, and took about six weeks to answer, with some work from Kristan Jensen, Shu Lin, and Jonathan Shock. The first important statement is that in the IR we obtain a neutral, massless scalar in AdS2. That is true for all D7-brane fluctuations. A neutral, massless scalar in AdS2 is a very special case where in fact the IR Green's function exhibits no branch cut whatsoever. The retarded Green's function is simply
    G_R(\omega) = i \omega
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    in a (3+1)-dimensional CFT: see
    Sean Hartnoll also asked the following question: in AdS-RN, for a fluctuation with finite momentum, the momentum appears in AdS2 as part of the mass. Does that happen for the probe D7-brane? The answer is no. In the near-horizon limit, the terms involving momentum scale with the wrong powers of r to describe a mass in AdS2. The same applies for the angular momentum that the fluctuation has on the S5 (the KK momentum).
    Mukund also asked whether the AdS2 is related to the finite entropy at zero temperature. Our usual intuition is that the entropy is associated with a horizon, and so is the AdS2. The main issue here is that for the probe D7-brane the entropy comes from open string degrees of freedom, not closed string degrees of freedom. It is order Nc, not order Nc^2, and it is not associated with any horizon, i.e. is not really tied to geometry, but to the flux on the brane (open string degrees of freedom). The open string metric, for example, exhibits no horizon. (An electric field produces one, but that is a different story.) Indeed, as noted by Karch, Kulaxizi, and Parnachev, for very large quark mass the entropy of the D7-brane is simply the entropy of a single string times some number of strings on the order of Nc. What is curious is that a single string, or in field theory language a single heavy quark, has an entropy of order \root{lambda}. Anyway, the entropy and AdS2 are connected only in the sense that whenever one appears, so does the other. At the moment I will not commit to any deeper connection.
    (view changes)
    11:21 am

Thursday, November 17

  1. page Social Activities edited ... Xi Dong is coming, will need a ride. Tom Faulkner is coming. John McGreevy is coming. Loga…
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    Xi Dong is coming, will need a ride.
    Tom Faulkner is coming.
    John McGreevy is coming.
    Loganayagam
    is coming.
    Previous dinners:
    Thursday September 1
    (view changes)
    5:38 pm
  2. page Social Activities edited ... -the number of additional people coming with you Nabil Iqbal is coming, no car. ... Keeler…
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    -the number of additional people coming with you
    Nabil Iqbal is coming, no car.
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    Keeler is coming, + 1.coming
    Julian Sonner is coming.
    Koushik Balasubramanian is coming.
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    Vijay Kumar is coming + 1.
    Xi Dong is coming, will need a ride.
    Tom Faulkner is coming.
    John McGreevy
    is coming.
    Previous dinners:
    Thursday September 1
    (view changes)
    4:51 pm
  3. page Social Activities edited ... Vijay Kumar is coming + 1. Xi Dong is coming, will need a ride. Tom Faulkner is coming. …
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    Vijay Kumar is coming + 1.
    Xi Dong is coming, will need a ride.
    Tom Faulkner is coming.
    Previous dinners:
    Thursday September 1
    (view changes)
    4:34 pm
  4. page Social Activities edited ... Eric Perlmutter is coming. Vijay Kumar is coming + 1. Xi Dong is coming, will need a ride. …
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    Eric Perlmutter is coming.
    Vijay Kumar is coming + 1.
    Xi Dong is coming, will need a ride.
    Previous dinners:
    Thursday September 1
    (view changes)
    4:31 pm
  5. page Social Activities edited ... Julian Sonner is coming. Koushik Balasubramanian is coming. ... Vegh is coming. coming, …
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    Julian Sonner is coming.
    Koushik Balasubramanian is coming.
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    Vegh is coming.coming, meeting there.
    Alejandra Castro is coming.
    Eric Perlmutter is coming.
    (view changes)
    4:27 pm
  6. page Social Activities edited ... Alejandra Castro is coming. Eric Perlmutter is coming. ... Kumar is coming. coming + 1. …
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    Alejandra Castro is coming.
    Eric Perlmutter is coming.
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    Kumar is coming.coming + 1.
    Previous dinners:
    Thursday September 1
    (view changes)
    4:22 pm
  7. page Social Activities edited ... David Vegh is coming. Alejandra Castro is coming. ... is coming. Vijay Kumar is comin…
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    David Vegh is coming.
    Alejandra Castro is coming.
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    is coming.
    Vijay Kumar is coming.

    Previous dinners:
    Thursday September 1
    (view changes)
    4:19 pm
  8. page Social Activities edited ... Koushik Balasubramanian is coming. David Vegh is coming. Alejandra Castro is coming. Eric …
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    Koushik Balasubramanian is coming.
    David Vegh is coming.
    Alejandra Castro is coming.
    Eric Perlmutter is coming.

    Previous dinners:
    Thursday September 1
    (view changes)
    4:13 pm
  9. page Social Activities edited ... Cindy Keeler is coming, + 1. Julian Sonner is coming. ... is coming. David Vegh is co…
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    Cindy Keeler is coming, + 1.
    Julian Sonner is coming.
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    is coming.
    David Vegh is coming.

    Previous dinners:
    Thursday September 1
    (view changes)
    3:51 pm

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