125 GeV Higgs and Vacuum Instability

[Coin]

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:

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.

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.

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)?

 

[tom.stoer]

If I understand the FRG analysis correctly, a 125 GeV Higgs and therefore the whole SM is perfectly valid w/o any 'beyond-the-standard-model-physics'.

 

[Coin]

Wait-- I'm now double checking the things I claim above (that 125 GeV Higgs implies vacuum instability) and am having trouble finding claims that this is the case but that do not originate with Philip Gibbs or Vixra. Further checking Gibbs seems to think of himself as a bit of an "outsider" and does not appear to be currently affiliated with an academic institution. This of course does not mean he is not right, but above I take things said on his blog at absolute face value where perhaps I should have been treating them as one person's informed opinion...

On a cursory check for real references: This published paper from 1989 seems like it would contain relevant information, but is behind a paywall:

http://www.sciencedirect.com/science/article/pii/0370157389900616

This paper was submitted to the Arxiv yesterday (!), it appears to have a good conventional academic pedigree but is obviously unpublished.

Higgs mass implications on the stability of the electroweak vacuum

This paper I will need to read more closely, but it looks to be quite good and possibly the answer to my question (1) above (i.e. where to find a good technical overview of how the vacuum instability stuff works). It agrees with what Gibbs' claims from his blog but cites a much higher (orders of magnitude higher) number for at what energy the instability would come into play.

We have analysed the stability of the Standard Model vacuum with special emphasis on the hypothesis that the Higgs mass, mh, is in the following range: 124 GeV < mh < 126 GeV, as hinted by recent ATLAS and CMS data.

Given the upper bound on the Higgs mass of 127 GeV, we conclude that the Standard Model ground state is very likely to be metastable. In the preferred range of mh the deeper minimum of the potential occurring at very high energies is sufficiently long-lived compared to the age of the Universe. Full stability is unlikely and would require mt to be closer to its lower allowed range, as summarized in fig. 3.

The scale where the Higgs potential becomes unstable is very high, around 10¹¹ GeV for mh = 125 GeV and central values of mt ans αs.

A question I maybe should have asked earlier: Is the vacuum stability problem with a 125 GeV Higgs indeed something scientific consensus would likely treat as "real", or as a problem that needs solving?

(EDIT: This post was written before seeing Tom's reply.)

 

[torquil]

Even with the "maybe", it is an overstatement to say that the "Higgs has been found (maybe)".

At the recent seminar, they explicitly stressed that it has not been found and that it has not been excluded. They have reduced the range of allowable energies.

 

[Haelfix]

The answer is unfortunately somewhat ambiguous.

A relevant paper is the following:
http://arxiv.org/abs/0906.0954v2

If we were to assume that what we are seeing is indeed the standard model Higgs at 125 +- 2 GeV -and that's far from given-, the stability of the vacuum is going to be a problem that a lot of people will look at very seriously in the next few years.

The problem is that determining the exact details is quite fuzzy in the 120-130 GeV range, and we will need to make more accurate measurements of the top Quark mass and several other Electroweak parameters before we can say with great confidence what the story is.

Anyway, almost all new physics (and even understanding old physics more accurately) will change the story significantly. For instance, whatever it is that explains dark matter will probably alter the conclusions.

 

[marcus]

... 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:
...

Coin, Here is a part of what you quoted. I was pleased to see the reference to the Shaposhnikov and Wetterich paper!
==Gibbs excerpt==

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.
... 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 ...
==endquote==

Shaposhnikov and Wetterich predicted higgs mass 126 GeV back in 2009, from a model where gravity is asym. safe and no new physics beyond SM is needed. They proposed the 126 figure as a signature. If that figure is confirmed next year, or anything close, I expect their idea ("read my lips, no new physics up to Planck scale") is one that is bound to be taken seriously.

This is in line with Tom's post referring to Functional Renormalization Group analysis.

If I understand the FRG analysis correctly, a 125 GeV Higgs and therefore the whole SM is perfectly valid w/o any 'beyond-the-standard-model-physics'.

The way Shaposhnikov and Wetterich express this is to describe SM + asym.safe gravity working as a fundamental (rather than merely effective) theory all the way to Planck scale.

I remember both Hermann Nicolai and Steven Weinberg discussing this scenario in Summer of 2009.
Nicolai in the XXV Max Born symposium at Wroclaw and Weinberg in a talk he gave at CERN in early July. It seems to me very likely that these people and others like them will be further studying this possibility now that hints are seen of the 126 GeV signature discussed earlier.

 

[Coin]

Thanks all for the explanations/responses.

 

[Haelfix]

So there have been several papers in the past few days on this subject. The newest was this one today:

http://arxiv.org/abs/1112.3112

Which argues for an instability at the 10^9-10^11 GeV scale, which is roughly consistent with what you can eyeball from the Ellis paper or from the results of the Cern paper in this thread (which is excellent).

So the standard model is probably broadly inconsistent if you run it up all the way to the Planck scale by the renormalization group, however the obvious way out of this is to argue that the vacuum is metastable (which is the scenario which the CERN paper favors) and doesn't decay for at least the lifetime of the universe.

Anyway, the punchline is that this analysis is woefully incomplete. We need better resolution on the electroweak parameters, and ideally we need lattice studies.

 

[abledoc]

Big bang event was a case of one pseudo-vacuum tunnelling into a lower energy (?pseudo)vacuum and we know that this sort of thing happened. May be we are at the lowest energy level. Thinking this way, this sort of Higgs mass was destined from an anthropic viewpoint

 

[kneemo]

http://arxiv.org/abs/1404.4709


The Higgs vacuum is unstable

Archil Kobakhidze, Alexander Spencer-Smith

(Submitted on 18 Apr 2014 (v1), last revised 22 Apr 2014)

Abstract:

So far, the experiments at the Large Hadron Collider (LHC) have shown no sign of new physics beyond the Standard Model. Assuming the Standard Model is correct at presently available energies, we can accurately extrapolate the theory to higher energies in order to verify its validity. Here we report the results of new high precision calculations which show that absolute stability of the Higgs vacuum state is now excluded. Combining these new results with the recent observation of primordial gravitational waves by the BICEP Collaboration, we find that the Higgs vacuum state would have quickly decayed during cosmic inflation, leading to a catastrophic collapse of the universe into a black hole. Thus, we are driven to the conclusion that there must be some new physics beyond the Standard Model at energies below the instability scale ΛI∼109 GeV, which is responsible for the stabilisation of the Higgs vacuum.

 

[member 11137]

What is exactly meant with vacuum instability? Do we have to understand that pair of words as if it would be related to a kind of chemical potential? As it would be related to a kind of thermodynamic (metastable) state?

Thanks in advance for elaborating a little bit around these questions (level: amateur) - and if it is not the correct forum to ask such question, please feel free to move it somewhere else!