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seen Jun 5 '11 at 17:25

Jul
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awarded  Curious
Nov
9
awarded  Nice Question
Jun
5
awarded  Critic
Jun
5
comment Is string theory a quantum theory of gravity?
@MBN When people say that the SM is part of string theory they typically mean that there exist explicit string constructions reproducing the SM particle spectrum, usually as part of a SUSY GUT. This fact in itself is highly non-trivial, although computing all the couplings would take it to a whole new level. Anyway, the only textbook that I'm aware of where modern string-theoretic constructions of the Standard Model are discussed is this one: amazon.com/String-Theory-Particle-Physics-Phenomenology/dp/… .
Jun
5
comment Is string theory a quantum theory of gravity?
@MBN, well, if that were the case, someone would have already received the Nobel Prize for such an achievement, don't you think?
Jun
4
comment Is string theory a quantum theory of gravity?
@MBN, Just to add, there has been some impressive quantitative progress in local F-theory models with regard to computing the Yukawa couplings in the Standard Model: arxiv.org/abs/0910.0477
Jun
4
comment Is string theory a quantum theory of gravity?
@MBN, I gave you some example constructions that contain the SM spectrum with three chiral families as part of a SUSY GUT. Computing all the coupling constants explicitly currently presents a technical challenge, mainly due to the lack of computational tools on the geometric side. Fot the most part, this is just a difficult mathematics problem that is kind of separate from string theory. Example: computing the metric on a Calabi Yau manifold in the Heterotic compactification cited above is one such problem that needs to be addressed before any questions about SM couplings are asked.
Jun
4
comment Is string theory a quantum theory of gravity?
@Cosmonut, a short response to your original question is that in string theory there is no singularity inside the black hole. Instead, the black hole is made of a very large number of branes that wrap the internal dimensions and effectively produce a fuzzy geometry, a so-called "fuzzball". Here is a nice review of this story: arxiv.org/abs/hep-th/0510180
Jun
4
comment Is string theory a quantum theory of gravity?
@MBN, current efforts are aimed at constructing a viable SUSY GUT model, that includes the Standard Model, as opposed to just the Standard Model. Here are a couple of references: arxiv.org/abs/hep-th/0512177 , arxiv.org/abs/0811.2936 , arxiv.org/abs/0905.1968 .
Apr
24
comment Neutralino Dark Matter Detection
Note that while it has a rather high annihilation cross-section, wino-type neutralino dark matter candidate has a very small LSP-nucleon scattering cross-section - in perfect agreement with the recent non-discovery in the direct detection experiment by Xenon100.
Apr
24
comment Neutralino Dark Matter Detection
Dear @Michael Luciuk, the gamma rays from present day dark matter annihilations may have already been detected. There is a well-motivated top-down non-thermal scenario featuring a simple wino-type LSP dark matter, which is perfectly consistent with the data collected by the Pamela satellite detector: arxiv.org/abs/0906.4765
Mar
31
comment How many fundamental forces could there be?
Dear @Nigel Seel, The Higgs can also be associated with a force, although not a gauge-type force. The strength of the Higgs interaction depends on the corresponding Yukawa coupling and particle interactions via Higgs exchange are as important as those mediated by the gauge bosons.
Mar
14
comment What if LHC finds SUSY?
Discovery of superpartners at the LHC will unambiguously imply some version of N=1 D=4 supergravity coupled to the MSSM. In fact, in a very popular class of models, so-called gauge mediation scenarios, the lightest supersymmetric partner (LSP) is the gravitino - the spin $3/2$ partner of the graviton.
Mar
14
comment What if LHC finds SUSY?
Dear @Jerry Schirmer, the reason for "why on earth does SUSY imply that gravity is also supersymmetric?" is very simple. We already know from GR that in the real world the Poincare symmetry cannot be global, it can only be a local one. Furthermore, it is impossible to simultaneously have the supersymmetric extension of the Poincare symmetry being global, while keeping the usual Poincare transformations local. Therefore, super-Poincare symmetry in the real world must be local and local super-Poincare transformations automatically lead to supergravity.
Mar
11
comment Tachyonic complex structure directions in flux vacua
Therefore it is necessary to check explicitly for each choice of fluxes that the resulting potential has a minimum.
Mar
11
comment Tachyonic complex structure directions in flux vacua
However, if we move one modulus, for concreteness the dilaton, away from its stabilised value, the resulting positive definite contribution $\frac{e^{K_{cs}}}{V^2}G^{\tau\bar\tau}D_{\tau}W\bar D_{\bar\tau}\bar W$ is only of the same order as the minimum. Moving the dilaton alters the value of $e^{K_{cs}}$ and thus may increase the numerator of the negative term. As the positive and negative contributions are of the same order, we see that depending on the magnitude of $G^{\tau\bar\tau}D_{\tau}W\bar D_{\bar\tau}\bar W$, this may in general decrease the overall value of the potential.
Mar
11
comment Tachyonic complex structure directions in flux vacua
Dear @Matt Reece, thank you very much for your answer. What you say is basically the standard folklore but this is not as straightforward as it sounds. Let me give you a direct quote from iopscience.iop.org/1126-6708/2005/03/007: The scalar potential is $V=\frac{e^{K_{cs}}}{V^2}\left(G^{\mu\bar\nu}D_{\mu}W\bar D_{\bar\nu}\bar W-3|W|^2\right)$. If $D_{\mu}W=0$ for all moduli, the potential is negative at $O(1/V^{\,\,2})$.
Mar
10
accepted $B\mu$ from $\tan\beta$ and $\mu$
Mar
10
comment $B\mu$ from $\tan\beta$ and $\mu$
Thank you very much @Matt Reece ! I guess I could just take the SOFTSUSY output at the scale $M_S=\sqrt {m_{\tilde t_1}m_{\tilde t_2}}$ and use it to extract $B\mu$ at that scale but what I'm really interested in is the value of $B\mu$ at the unification scale. I'll see if I can find the corresponding RGE to estimate it. Thanks again!
Mar
7
comment What if the LHC doesn't see SUSY?
Dear @LuboŇ° Motl, you gave a rather comprehensive answer but I would add another strong point in favor of SUSY. Namely, Radiative Electroweak Symmetry Breaking (REWSB), i.e. the up-type Higgs turning tachyonic (due to the large top Yukawa) and thereby triggering the $SU(2)_{L}\times U(1)_{Y}\rightarrow U(1)_{em}$ symmetry breaking.