Did they ever find the Higgs singlet?

So they found the Higgs Boson. Did they ever find the Higgs singlet? Or reject it? Wikipedia has an article on singlets, but doesn't mention the Higgs one.

What do you mean by 'Higgs singlet'?

Algr said:

So they found the Higgs Boson. Did they ever find the Higgs singlet? Or reject it? Wikipedia has an article on singlets, but doesn't mention the Higgs one.

I thought you meant stranglet. I found this, a little bit above my pay grade!
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.075016

Vanadium 50 said:

What do you mean by 'Higgs singlet'?

https://www.smh.com.au/technology/t...ent-to-the-past-or-future-20110322-1c4qq.html

Around 2008, when the LHC was constantly breaking down, someone produced a theory that the LHC had produced a particle "so abhorrent to reality" that the past was changing to prevent it from ever existing. This caught attention because the idea was formulated well enough that physicists couldn't disprove it out of hand. That was the Higgs singlet.

Obviously now that we have found the Higgs Boson, we know that wasn't true. But what was the formal resolution to the theory? Is such a situation still possible?

https://www.pbs.org/wgbh/nova/article/retrocausality-could-send-information-back-to-the-future/

Algr said:

This caught attention because the idea was formulated well enough that physicists couldn't disprove it out of hand.

So there must be some peer-reviewed, scientific paper about it. Can you find one?

weirdoguy said:

So there must be some peer-reviewed, scientific paper about it. Can you find one?

We discuss the current status of theoretical and experimental constraints on the real Higgs singlet extension of the standard model.

For the second neutral (non-standard) Higgs boson we consider the full mass range from 1 GeV to 1 TeV accessible at past and current collider experiments. We separately discuss three scenarios, namely, the case where the second Higgs boson is lighter than, approximately equal to, or heavier than the discovered Higgs state at around 125 GeV.

We investigate the impact of constraints from perturbative unitarity, electroweak precision data with a special focus on higher-order contributions to the 𝑊 boson mass, perturbativity of the couplings as well as vacuum stability. The latter two are tested up to a scale of ∼4×10¹⁰ GeV using renormalization group equations.

Direct collider constraints from Higgs signal rate measurements at the LHC and 95% confidence level exclusion limits from Higgs searches at LEP, Tevatron, and LHC are included via the public codes HiggsSignals and HiggsBounds, respectively. We identify the strongest constraints in the different regions of parameter space. We comment on the collider phenomenology of the remaining viable parameter space and the prospects for a future discovery or exclusion at the LHC.

Robens, T., Stefaniak, T. "Status of the Higgs singlet extension of the standard model after LHC run 1." Eur. Phys. J. C 75, 104 (2015) (open access). https://doi.org/10.1140/epjc/s10052-015-3323-y

@Algr was asked what paper he was talking about, not whether other people could Google a paper with the words "Higgs" and "singlet" in the title, which is decidedly unhelpful.

Vanadium 50 said:

@Algr was asked what paper he was talking about, not whether other people could Google a paper with the words "Higgs" and "singlet" in the title, which is decidedly unhelpful.

Nonetheless, based upon the paper referenced, and other results of a search for a paper about a Higgs singlet, I think that some of the other descriptions of what theory was being advanced in this thread under that name were mistaken.

The Higgs singlet proposal appears to be a truncated version of the widely analyzed two Higgs doublet theory.

In the two Higgs doublet theory, in addition to the ordinary Standard Model Higgs boson, there are four other Higgs bosons: a positively charged Higgs boson (H⁺), its antiparticle a negatively charged Higgs boson (H^-), a pseudoscalar Higgs boson (A⁰), and a second scalar Higgs boson (with the two scalar Higgs bosons being labeled h⁰ and H⁰, for the lower and higher mass scalar Higgs bosons respectively).

Charged Higgs bosons have been excluded by searches at the LHC for masses of less than about 1.1 TeV. Exclusions for the second scalar Higgs boson and the pseudoscalar Higgs boson from the LHC, however, are trickier to characterize because they are more model dependent.

The Higgs singlet theory proposes solely the existence of h⁰ and H⁰ without the other three components of the two Higgs doublet theory, and remains agnostic about whether the Standard Model Higgs boson is the heavier or the less massive of the two Higgs bosons in this model. See, e.g., Dawson, et al. (2021), providing an update on the search for this particle at the LHC. The abstract of this paper states:

The scalar singlet model extends the Standard Model with the addition of a new gauge singlet scalar. We re-examine the limits on the new scalar from oblique parameter fits and from a global fit to precision electroweak observables and present analytic expressions for our results.

For the case when the new scalar is much heavier than the weak scale, we map the model onto the dimension-six Standard Model effective field theory (SMEFT) and review the allowed parameter space from unitarity considerations and from the requirement that the electroweak minimum be stable.

A global fit to precision electroweak data, along with LHC observables, is used to constrain the parameters of the high scale singlet model and we determine the numerical effects of performing the matching at both tree level and 1-loop.

The Higgs singlet theory isn't as well motivated, theoretically, as the two Higgs doublet theory, by analogy to the Standard Model electroweak unification theory.

But the Higgs singlet theory has the virtue of proposing only one new particle not found in the Standard Model instead of four. It also has the "virtue" of being much harder to distinguish experimentally from the Standard Model because its only new particle is electrically neutral in could be in the same order of magnitude of mass as the Standard Model Higgs boson at about 125 GeV of mass. Its ability to evade detection until needed to explain some new experimental anomaly is also facilitated because the couplings of the additional Higgs singlet are model dependent, and can be adapted to help explain any anomaly seen at the LHC for which a massive scalar gauge boson could be an explanation.

The paper referred to in the "time travel" press release which appears to have spawned all of the other main stream media stories (yet another example of the tendency of university PR staff to produce stories that mislead the public about science, IMHO, by granting undue credence to a highly speculative new paper proposing a theory that isn't taken seriously by the vast majority of scientists in the field) which was released as a preprint about a year before the SM Higgs boson was discovered is:

Chiu Man Ho, Thomas J. Weiler. "Causality-Violating Higgs Singlets at the LHC." arXiv.org (2011).

The abstract of this paper is as follows:

We construct a simple class of compactified five-dimensional metrics which admits closed timelike curves (CTCs), and derive the resulting CTCs as analytic solutions to the geodesic equations of motion. The associated Einstein tensor satisfies all the null, weak, strong and dominant energy conditions. In particular, no negative-energy "tachyonic" matter is required. In extra-dimensional models where gauge charges are bound to our brane, it is the Kaluza-Klein (KK) modes of gauge-singlets that may travel through the CTCs. From our brane point of view, many of these KK modes would appear to travel backward in time.

We give a simple model in which time-traveling Higgs singlets can be produced by the LHC, either from decay of the Standard Model (SM) Higgs or through mixing with the SM Higgs. The signature of these time-traveling singlets is a secondary decay vertex pre-appearing before the primary vertex which produced them. The two vertices are correlated by momentum conservation. We demonstrate that pre-appearing vertices in the Higgs singlet-doublet mixing model may well be observable at the LHC.

The comments at the arXiv entry for the paper state in relevant parts:

Version updated to include in single manuscript the contents of Erratum [Phys. Rev. D 88, 069901(E) (2013)], Reply [Phys. Rev. D 88, 068702 (2013)], Comment [Phys. Rev. D 88, 068701 (2013), arXiv:1302.1711], and original published article [Phys. Rev. D 87, 045004 (2013), arXiv:1103.1373]. Positive conclusions remain unchanged

The abstract of the 2013 comment paper by Steffen Gielen referenced above states:

The spacetime of Ho and Weiler [Phys. Rev. D 87, 045004 (2013)] supposedly admitting closed timelike curves (CTCs) is flat Minkowski spacetime with a compactified coordinate and can only contain CTCs if the compact direction is chosen to be timelike. This case of a "periodic time" is probably the simplest example of a causality-violating spacetime; it trivially satisfies all energy conditions usually assumed in general relativity, and its geodesics are just straight lines. Its relevance for phenomenology of the LHC, on the other hand, depends on consistency with observational constraints on gravity, as is mentioned in general but not discussed in any detail by Ho and Weiler. We verify a basic consistency check for stationary sources.

There is, of course, no credible evidence whatsoever for causality-violating Higgs Singlets at the LHC. It is not a theory with many advocates for it at this point (and probably never had many advocates for it in the scientific community at any point).

Alright... One often hears about symmetries in physics. These can refer to "passive transformations" like rotating the axis of your coordinate system, or changing the way you label the possible charges of a particle, or they can be "active transformations", as when an object actually rotates, or an interaction actually changes its properties.

Something is a "singlet", relative to a particular symmetry or a particular transformation, if it is unchanged or unaffected. (Well, it might experience a kind of phase shift - I am thinking of U(1) symmetries.) On the other hand, you might have a "doublet" if there are two descriptions or two states connected by a transformation, a "triplet" if there are three, and in general there is a whole branch of math called representation theory, describing ways that a symmetry group can act upon objects, which is used intensively throughout physics (and also a bit in chemistry, where it can describe crystal structures).

This term "Higgs singlet" is a bit barbarous or ambiguous, because "Higgs" isn't a symmetry group. So without context, you can't really tell what it means. Using Google Scholar, I found a dozen papers from the 1970s that refer to "Higgs singlets". They seem to mean Higgs fields that are only charged under U(1) symmetries.

On the other hand, the "Higgs singlet" model above, seems to refer to mixing between the standard model Higgs boson (which comes from an electroweak doublet representation) and a new scalar field that is a singlet under the entire standard model gauge group. I presume that the "causality violation" paper is talking about this kind of field, but let me emphasize that most "Higgs singlet" models do not involve time travel or backwards causation. This is a feature only of that particular paper, which added some extra stuff about shortcuts through a fifth dimension. Without that, the particles in a Higgs singlet model behave normally.

One more historical detail. The media attention to the idea, that some kind of time loop was conspiring to prevent the LHC from starting up, actually derives from another series of papers, which didn't talk about Higgs singlets at all. One such paper is Nielsen and Ninomiya (2009); as you can see, arxiv records "24 blog links" to the paper, which actually include stories at CNN and the New York Times. As far as I know, these papers by Nielsen (who writes a lot of weird papers) are the true origin of this Terminator-meets-CERN plotline, and their proposed physical mechanism (a "complex action principle") was quite different to the mechanism in the "causality-violating Higgs singlets" paper, which came out a few years later - just before the Higgs was discovered, without blowing up the Earth.