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Coin
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125 GeV Higgs and "Vacuum Instability"
So the Higgs has been http://press.web.cern.ch/press/pressreleases/Releases2011/PR25.11E.html (maybe). Nothing "beyond the standard model" about that of course; the Higgs is standard model. Except-- in the leadup to the LHC announcement, I repeatedly saw claims that the exact candidate mass of the Higgs, 125 GeV, is a strong sign that something beyond the standard model is going on, because a Higgs at that mass possibly indicates "vacuum instability". Here's a typical example of the claim:
Here are some things I am wondering. Assuming we don't get lucky (and just happen to get the correct top mass and SM parameters to keep the 125-GeV-Higgs universe stable):
1. Where can I read a more precise explanation of this negative Higgs self-coupling -> unstable vacuum idea?
2. Supersymmetry is usually the first theory cited as benefactor if the SM Higgs is found unstable. What other theories can also fix the problem? Can Little Higgs/Composite Higgs/Technicolor do it? Do the "Asymptotic Safety" models which are cited in other threads in this forum currently as producing a ~125 GeV Higgs have a way of solving the vacuum stability issue? Are there any other candidates?
3. Is there any technical reason, out of the gate, to prefer anyone of these vacuum-stabilizing candidates over the other? My understanding is SUSY would be strongly preferred in any case due to the many other benefits it brings (solves certain mathematical problems, makes string theory possible, is "beautiful") but are there any known technical advantages of SUSY, or any other candidate, for the specific purpose of solving this particular problem (stabilizing the vacuum with a light Higgs)?
4. Again assuming the problem doesn't go away on its own with more accurate measurements of Higgs, top, etc-- what will be the next steps for discerning which of the vacuum-stabilizing candidate theories is real? Have any of the candidates had their parameter space significantly excluded by the LHC work so far, or do any of the candidates have important parameter space the LHC might be able to detect in future?
My very dull impression from blog comment sections is that the next step is to pick a supersymmetry model with a 125 GeV Higgs and start looking for whatever it predicts to be the lightest supersymmetric partner (the word "gluino" keeps getting kicked around)?
So the Higgs has been http://press.web.cern.ch/press/pressreleases/Releases2011/PR25.11E.html (maybe). Nothing "beyond the standard model" about that of course; the Higgs is standard model. Except-- in the leadup to the LHC announcement, I repeatedly saw claims that the exact candidate mass of the Higgs, 125 GeV, is a strong sign that something beyond the standard model is going on, because a Higgs at that mass possibly indicates "vacuum instability". Here's a typical example of the claim:
So, this is exciting. It seems to me most recent physics theories are solutions looking for a problem and now we have a very large problem to solve.Philip Gibbs said:It has been known for about twenty years that for a low Higgs mass relative to the top quark mass, the quartic Higgs self-coupling runs at high energy towards lower values. At some point it would turn negative indicating that the vacuum is unstable. In other words the universe could in theory spontaneously explode at some point releasing huge amounts of energy as it fell into a more stable lower energy vacuum state. This catastrophe would spread across the universe at the speed of light in an unstoppable wave of heat that would destroy everything in its path...
As it turns out a Higgs mass of 125 GeV is quite a borderline case... if the mass of the Higgs turns out to be 120 GeV despite present rumours to the contrary then the stability problem would be a big deal. This would be a big boost for SUSY models that stabilize the vacuum amd mostly prefer the light Higgs mass. If on the other hand the Higgs mass was found at 130 GeV or more, then the stability problem would be no issue. 125 GeV leaves us in the uncertain region where more research and better measurements of the top mass will be required...
At 126 GeV the vacuum might remain stable up to Plank energies (see e.g. Shaposhnikov and Wetterich). If this is the case then there is nothing to worry about, but depending on the precise values of the standard model parameters, instability could also set in at energies around a million TeV. This is well above anything we can explore at the LHC but such energies are found in the more extreme parts of the universe and nothing bad has happened. The most likely explanation would be that some new unknown physics changes the running of the coupling to avert it from going negative. Examples of something that could do this include the existence of a Higgsino or a stop as predicted by supersymmetry, but there are other possibilities.
Here are some things I am wondering. Assuming we don't get lucky (and just happen to get the correct top mass and SM parameters to keep the 125-GeV-Higgs universe stable):
1. Where can I read a more precise explanation of this negative Higgs self-coupling -> unstable vacuum idea?
2. Supersymmetry is usually the first theory cited as benefactor if the SM Higgs is found unstable. What other theories can also fix the problem? Can Little Higgs/Composite Higgs/Technicolor do it? Do the "Asymptotic Safety" models which are cited in other threads in this forum currently as producing a ~125 GeV Higgs have a way of solving the vacuum stability issue? Are there any other candidates?
3. Is there any technical reason, out of the gate, to prefer anyone of these vacuum-stabilizing candidates over the other? My understanding is SUSY would be strongly preferred in any case due to the many other benefits it brings (solves certain mathematical problems, makes string theory possible, is "beautiful") but are there any known technical advantages of SUSY, or any other candidate, for the specific purpose of solving this particular problem (stabilizing the vacuum with a light Higgs)?
4. Again assuming the problem doesn't go away on its own with more accurate measurements of Higgs, top, etc-- what will be the next steps for discerning which of the vacuum-stabilizing candidate theories is real? Have any of the candidates had their parameter space significantly excluded by the LHC work so far, or do any of the candidates have important parameter space the LHC might be able to detect in future?
My very dull impression from blog comment sections is that the next step is to pick a supersymmetry model with a 125 GeV Higgs and start looking for whatever it predicts to be the lightest supersymmetric partner (the word "gluino" keeps getting kicked around)?
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