*Matt Strassler [January 17, 2013]*

Particle physicists have recently discovered a previously unknown, possibly elementary, and certainly important particle. And if you’ve been following along in even the slightest degree, you know the new particle resembles to some degree a type of long-awaited Higgs particle. * (For laypersons, here’s my FAQ about the Higgs field and particle, and here’s an article about why the Higgs particle matters so much.)* The discovery of such a particle, a ripple in the mass-providing Higgs field, apparently confirms that the Higgs field, without which we and all ordinary matter would explode, really does exist in nature.

The discovery was made back in July. With six months having passed, it seems a good moment to step back and take stock, as some of us did at the Higgs Symposium last week in Edinburgh, Scotland, UK. The purpose of this article is to bring non-experts up to date on what we’ve learned over the past six months since the discovery, and to elucidate the questions that lie ahead… using the talks given at the Symposium as source material.

In this article, I want to take a look at what we know so far about the new particle.

- How Higgs-like is this new particle?
- How likely is it that it is the only Higgs particle?
- How likely is it that it is an elementary particle, as the electron is thought to be, as opposed to made from more elementary particles, like a proton is known to be?
- And how likely, if it is elementary and the only type of Higgs particle in nature, is it to be a Higgs particle
*of the simplest possible type*: the so-called?**Standard Model Higgs particle**

Several of the speakers at the Higgs Symposium addressed these issues in one way or another, and I’ll be reviewing what they had to say in a non-technical fashion (except that now and then I’ll mention a few techy details — which you can easily skip over.).

**1. ****Is the Newly Discovered Particle a Higgs Particle of Some Type?**

**2. ****If the New Particle is a Higgs Particle, Might it be Composite?**

**3. ****Might the New Particle be a Higgs of the Simplest Possible Type (the Standard Model Higgs)?**

**5. Supersymmetry: What Does The New Particle Teach Us About This Possibility?**

**6. Even If It’s The Only Higgs Particle, How Might It Surprise Us?**

**7. Suppose the Standard Model is Right; What Then? **[Coming soon: two perspectives, one from Shaposhnikov who offers an argument as to why the Higgs has a mass of close to 125 GeV/c², and one from me, about how we ought to view this situation, if it turns out to be true.]

Hi Matt, big follower, thank you very much for taking time to explain this things. I’m interested in the argument that composite higgs suggests composite quarks and leptons. I’m trying to find a more technical account, but failed so far besides slides for presentations which are kind of hard to follow. Do you have any references that are technical, like any papers or so. Thanks

Did you look at the papers that are referenced in Rattazzi’s talk at the Symposium?

p.s. I was thinking I should explain this, at least briefly. It would take just a few sentences to get the basic point across… the problem is that it needs a good picture to go with it, and I haven’t invented one yet…

I tried to look at some of the papers there referenced, and found some discussion of the partial compositeness cenario, but could not find how one compares the fundamental fermions vs composite fermions supposing a composite higgs. At some point there is a reference to a paper in preparation, I’m not sure if will discuss this point at lenght. In any case I was interested because I’ve seen an argument for a composite higgs in which the fermions must be elementary.

I don’t know any such argument — where did you hear it? You can make some of the lighter fermions elementary (or at least vastly smaller in size than the Higgs) but it is really quite difficult to do this with the top quark. And with what has recently been learned about conformal field theory, it seems that making the lighter fermions elementary without creating large flavor-changing neutral currents is even harder than people used to think. You can look at Luty and Okui, for instance; the type of model they write down (inspired by two decades of work before them, cf. walking techicolor) has a dynamical requirement (one I also wrote about), which Riccardo Rattazzi, Slava Rychkov, Alessandro Vichi. [arXiv:1009.5985 [hep-th]] showed is impossible to satisfy.

The paper by Luty and Okui, together with the last reference is exactly what I was looking for. Need a bit time to diggest it though, but I’ll try to navigate by myself. Somewhat embarassed to missed the Rattazzi, Rychkov and Vichi paper, they indeed were in the slides. As for the argument, I should say I find it a “numerological” one by the gravity folks based on a intriguing coincidence. Basically they argue that every fermion from the SM obeys the inequality for naked singularities and not by the Kerr-Newman black holes, and then postulate a connection between elementary particles and naked singularities. It’s in the conclusions from [arXiv:0905.1077[gr-qc]] by George Matsas et al, if you would like to take a look. Just to be clear, I was not convinced, just intrigued. Anyway, thanks for the references

When you say “A Higgs particle must be a boson, and must not be impacted directly by the strong nuclear force or electromagnetic force (i.e. it must not have electric charge). This is true of the new particle; we know this because it can decay to two photons (which are also bosons with no electric charge and with no direct effects from the strong nuclear force.)”… do you have to also assume that the Higgs is not composite? Given that a neutral pion can decay to two photons, yet is affected by the strong force?

Since you’re the second expert to ask this question, I’ll change the wording so that you won’t complain. The neutral pion is made from particles that carry BOTH electromagnetic AND strong nuclear forces, so yes, WHEN INVOLVED IN SUFFICIENTLY HIGH ENERGY PROCESSES, it can be affected by BOTH strong nuclear forces AND electromagnetic forces. Similarly the Higgs is color-neutral as well as electrically neutral — but if it is composite, then WHEN INVOLVED IN SUFFICENTLY HIGH ENERGY PROCESSES (which would be well above LHC scales, given what Rattazzi had to say) it might be affected in a similar way.

Matt; there’s a problem with trying to write for non-experts without introducing other demons, but here I think one may be digging a hole in trying to draw a conclusion that actually requires rather a lot of deep argument. The Higgs (even a colour neutral elementary Higgs, which is what this beast may well be) couples to quarks (top antitop in particular) which is what drives the two gamma decay. It is the strength of the H to quarks coupling that is what matters. The production and decay rates at LHC are consistent with that coupling being “standard Higgsish” (i.e. not strong); simply seeing it decay to two gammas cannot allow one to draw the conclusion that the beast is not strong. Now I dont know how to go through all that and retain the interest of a general reader (!) but I also am uneasy at drawing inferences without doing so. Its one of the many hazards of popularization (I like your article very much by the way, and this is one of the issues that I would like to find a neat way of explaining myself)

Hmm… I’m honestly really confused. I feel like we’re talking at cross-purposes, but can’t quite figure out why.

My only and entire point about Higgs –> two photons is that it proves the Higgs isn’t colored or charged [i.e., carries no electric monopole or color monopole charge.] Now I know you’re not disagreeing with

that.I think you’re trying to say that Higgs –> two photons does not prove the Higgs isn’t composite; and that if it is composite, what it is made from may carry charge or color. And I of course agree with that. I certainly didn’t say the Higgs

isn’tcomposite and internally-strongly coupled; in fact I have a whole section about that: http://profmattstrassler.com/articles-and-posts/the-higgs-particle/taking-stock-of-the-higgs-jan-2013/2-perhaps-composite/ And in such a scenario, the Higgs is very much like a neutral pion, with similar properties.So when you say “digging a hole in trying to draw a conclusion” — I am puzzled about what conclusion you’re talking about, or how I am digging the hole. I don’t think the conclusions I’m drawing differ from yours. Please clarify… thanks!

What is a top-prime? Is it some excited state of the third-generation top quark, or is it a fourth generation quark, or what? I thought there were strong arguments that there were only three generations of quarks, unless the subsequent generations were REALLY massive?

It’s different from a fourth-generation quark. This is not the appropriate article for me to go into all sorts of details — remember this is an article for

non-experts.If I were an expert, I would probably know what a top-prime is :-) Honestly not trying to nitpick, just trying to understand… take care and thanks

Sorry, I was rushing the last answer and should have done a better job.

The main difference between a top-prime (and as I said that is a provisional name — over the years lots of things have been called “top-prime” for different reason) and a fourth-generation quark is that a fourth generation quark gets its mass from the Higgs field (just the way quarks of the other three generations do) but a top-prime does not. Therefore (a) although the arguments that there are only three generations of quarks are indeed correct (and in fact do NOT allow for very massive generations), it happens that (b) the top-prime is not affected by the arguments in (a).

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