Assuming Higgs is found at 125 GeV.Is there any direct or indirect consequence on string theory ? Will it be a blow to string theory or models employing string theory ?


Ps - I am just a curious pure maths student, so forgive me if my question makes no sense ! :)

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    $\begingroup$ There are ways to get a Higgs-Type effect in String Theory. But since the main Issue is still finding a sensible way to get the Standard Model to arise naturally, I believe it's still a bit early to say. I'm also rather interested in what role the Higgs Effect will play in String Theory. $\endgroup$ – Michael Dec 13 '11 at 21:25
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    $\begingroup$ See arxiv.org/abs/arXiv:1112.1059 (stolen from Lubos Motl's blog) for a string scenario in very good agreement with the 125 GeV figure $\endgroup$ – Squark Dec 13 '11 at 22:10
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    $\begingroup$ The Standard Model predicts a Higgs at around this mass. $\endgroup$ – Mateus Araújo Dec 14 '11 at 1:24

I'm going to answer a slightly different question about consequences of a 125 GeV Higgs for low-energy supersymmetry. For internal consistency, supersymmetry seems to be a requirement at very high energies (or perhaps just on the worldsheet) in string theory. So these questions are indirectly (and tenuously) connected.

This also seems to be the approach Luboš Motl took in his answer, but I think he conflates some issues that should be disentangled. The bottom line, in my opinion, is: 125 GeV is an unexpectedly large mass in the MSSM. If supersymmetry at the TeV scale is correct, this Higgs mass strongly hints at one of three things: either very heavy scalar superpartners, out of reach of collider searches (though fermionic ones may be lighter), or a SUSY spectrum with very large mixing among scalar top quarks, or an extension of the minimal model with new interactions for the Higgs boson. Only the last of the three can evade the conclusion of fine-tuning, though one can debate whether Nature cares about what we call tuning.

One typically hears that supersymmetry requires a Higgs mass below 135 GeV. For instance, you can find such a statement in equation 45 of a review article by Carena and Haber (which can also serve as a source for other statements I'll be making in this answer; I won't try to be exhaustive with references to the original literature). Let's unpack that claim a little, which is also related to this other recent question about the little hierarchy problem:

The MSSM: This is the minimal supersymmetric Standard Model. In this model, the Higgs has gauge interactions and the Yukawa interactions it needs to give mass to SM fields, and no other interactions. This is very predictive. At leading order, it predicts $m_h < m_Z$ = 91.1876 GeV. So the MSSM is always in some tension with larger Higgs masses. There are corrections to this Higgs mass formula, however, from quantum corrections arising from its interactions with supersymmetric partners of the top quark (scalar tops or "stops"); roughly, there are corrections going as $m_t^4/v^2 \log m_{\tilde t}^2$ with $m_{\tilde t}$ the stop mass, and corrections going as $m_t^4/v^2~X_t^2/m_{\tilde t}^2$ and $m_t^4/v^2~X_t^4/m_{\tilde t}^4$ where $X_t$ is a measure of mixing between left- and right-handed stops.

So, in the MSSM, a large Higgs mass requires large stop masses. The masses required are a little less large when the stops are highly mixed. Now, the number 135 GeV for the maximal allowed Higgs mass is derived assuming stops are below 2 TeV, although I think a more modern version of the calculation would conclude that even 135 is out of reach. On the other hand, dropping the assumption of stops below 2 TeV, in the MSSM with extremely heavy superpartners the Higgs mass could even be a little above 140 GeV.

The MSSM, with large Higgs masses, is tuned: both the stop mass and the mixing $X_t$ show up in quantum corrections that want to shift the electroweak breaking vacuum. Most people working on the MSSM have studied models with light superpartners, below 1 TeV, so that this tuning is relatively small. On the other hand, some have studied "split" models with heavy scalars, giving up on solving the fine-tuning issue. A 125 GeV Higgs is a better fit to those scenarios, with scalars in the ~ 10 TeV regime, than to "standard" MSSM models. It is still compatible with more standard MSSM scenarios in the limit of large stop mixing, however (though this imposes strong and interesting constraints on the model!).

Luboš is advocating the split models, and they are interesting; nature may not care what we consider to be "fine-tuned." Also, one sometimes encounters claims that the fine-tuning is reduced in some versions of these models. Trying to quantify precisely what is meant by tuning is a can of worms I'll avoid opening in this response. I don't agree that these slightly split models with 10 TeV scalars are more "stringy" than "QFT-like," since all of the calculations are done in effective supergravity theories, although they do lend themselves to solving the moduli problem that arises in string theories (see this and this, as well as the more recent work of Gordy Kane that Luboš cites).

Beyond the MSSM: Supersymmetric models can also accommodate a Higgs with more than the minimal set of interactions. In that case, new contributions to its mass can arise already at the leading order, and the tension with fine-tuning described above is much smaller. There are too many conceivable versions of this to classify in this answer, and their implications for colliders can depend on details. But it's certainly interesting to consider models without tuning and with new physics beyond the MSSM.

Bottom line: It's still too early to say definitively if the hints at the LHC are evidence of a 125 GeV Higgs. If so, then the next few years--possibly even 2012--could tell us if we are in one of these three scenarios (split MSSM, MSSM with highly mixed stops, beyond the MSSM) or if supersymmetry is completely absent at the weak scale.

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    $\begingroup$ A nice quasi stock review paper just came out today regarding the naturalness of SuSY with Mh at 125 GeV. arxiv.org/abs/1112.2703 $\endgroup$ – Columbia Dec 14 '11 at 15:36
  • $\begingroup$ It's not just a review, they point out an important bit of physics in the $\lambda$SUSY scenario that earlier papers had missed (that the Higgs is made relatively light by mixing with the singlet). I'll have my own mini-contribution (with Draper, Meade, and Shih) focusing on just the MSSM and how this constrains certain classes of mediation mechanism on the arxiv tonight. A whole flood of these things should be coming in.... $\endgroup$ – Matt Reece Dec 14 '11 at 15:46
  • $\begingroup$ Plus one, Matt. I think that the MSSM is still being given too much credibility relatively to other SUSY theories/limits but your answer was ordered, anyway. $\endgroup$ – Luboš Motl Dec 15 '11 at 6:49
  • $\begingroup$ "I don't agree that these slightly split models with 10 TeV scalars are more "stringy" than "QFT-like," since all of the calculations are done in effective supergravity theories" - Well, unless extra dimensions are large or very warped, all physics below $10^{18}$ GeV or so will always be dominated by calculations based on QFT and SUGRA. This doesn't mean that all of these calculations are equally stringy or non-stringy, does it? String/M-theory simply has different "naturalness" standards than QFTs. One may even say that things like grand unification are "more stringy" than bare MSSM. $\endgroup$ – Luboš Motl Dec 15 '11 at 8:40
  • $\begingroup$ @Luboš: SUSY models other than the MSSM are definitely growing much more interesting in light of the data. It's just that I don't know how to explain all the options in an answer of reasonable length here! $\endgroup$ – Matt Reece Dec 15 '11 at 14:05

125 GeV is below 135 GeV, which means that it makes supersymmetry, a key component of string theory, more likely than not. Moreover, 125 GeV is the boundary between "simple and visible QFT-like SUSY" (below 125) and "hidden, complicated, or extended SUSY" like stringy SUSY (above 125), if I am a bit approximate.

So there are lots of grand unified supersymmetric models and string/M-theory-based models that predict Higgs of this mass.

On the contrary, the Standard Model predicts that it's unlikely that the Higgs mass is below 135 GeV or so – well, the Standard Model would prefer masses that are larger by orders of magnitude and even given the known and measured vev, it would prefer masses closer to 500 GeV or more. Moreover, the Standard Model with the Higgs mass below 126 GeV or so – the number is known plus minus a few GeV (which is unfortunate because the newly measured Higgs mass is very close to this critical value) – would ultimately cause the vacuum to be unstable at an energy scale beneath the Planck scale which would probably be an inconsistency. This inconsistency has to be fixed by adding new fields and particles to the Standard Model, anyway.

To summarize, 125 GeV makes pure Standard Model much less likely, it makes SUSY more likely, among SUSY models, it makes extended and stringy models somewhat more likely than the simple field-theoretical ones, but no definitive and fully reliable statement about the right theory of the Universe may be extracted out of this single number, of course.



and articles linked in it for more comments on this issue.

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