How do they know that they've found the Higgs boson and not something else?

[Quartz]

Apparently, CERN have seen glimpses of the Higgs boson. But how can they tell that it’s the particle they seek and not another particle which just happens to have the mass for which they’re testing?

[isaiahrobinson]

I don’t think they’re just searching for any particles in a certain mass range, and if they find one assuming it’s the Higgs. They’re searching, in certain mass ranges, for signals particular only to the Higgs. It looks like they’ve sighted some those ‘special’ signals at the mass range of 125 GeV. I don’t know what those signals are though, hopefully someone with more knowledge can explain.

[Chronos]

It’s not out of the question, as similar things have happened before. For instance, in the 30s, Yukawa predicted the existence of a particle (now called the pion or pi meson) which would be involved in the strong nuclear force, along with an estimate of its mass. A year later, a particle was detected with that mass, which was assumed to be Yukawa’s meson. Further studies showed, though, that that particle didn’t actually have anything to do with the strong force, and had the wrong angular momentum, and in fact behaved very much like an electron, except with a higher mass (we now call these particles mesons). Then, the actual pion was discovered, at a slightly higher mass. The unexpected discovery of this “heavy electron”, which didn’t fit neatly into any of the frameworks of particle physics at the time, led Rabi to famously quip “Who ordered that?”.

Now, we understand a lot more about particle physics now than we did in the 1930s, and we think we’d be able to recognize a similar-mass particle based on differences in angular momentum, baryon number, lepton number, and other conserved quantities. But of course, it’s always possible that we’re having another “who ordered that?” moment.

[Grey]

I was going to open a new thread but if no one minds I’ll just post here.

Since CERN seems to be progressively excluding regions of energy for the Higgs Boson and we seem to be settling down to it actually existing and existing with a likely mass of ~125GeV.

So this gives us 2 things to work from - its existence and its mass relative to quarks.

1. What does it mean if the Higgs is less massive than the top quark? The top seems to have a mass of ~173 GeV.
2. What models does a 125 GeV Higgs exclude?

[MikeS]

@Chronos

Further studies showed, though, that that particle didn’t actually have anything to do with the strong force, and had the wrong angular momentum, and in fact behaved very much like an electron, except with a higher mass (we now call these particles mesons).

You mean “muons”, right?

[DHMO]

@MikeS

You mean “muons”, right?

Muon = Mu + Meson as explained in thiswiki article.

Muons were discovered by Carl D. Anderson and Seth Neddermeyer at Caltech in 1936, while studying cosmic radiation. Anderson had noticed particles that curved differently from electrons and other known particles when passed through a magnetic field. They were negatively charged but curved less sharply than electrons, but more sharply than protons, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron but smaller than a proton. Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for “mid-”. Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. To differentiate between different types of mesons, the mesotron was in 1947 renamed the mu meson (the Greek letter μ (mu) corresponds to m).

[Chronos]

I did mean muons; my bad. And even though “mu + meson” is the etymology for the name “muon”, the muon is not a meson.

[The_Second_Stone]

@Chronos

It’s not out of the question, as similar things have happened before. For instance, in the 30s, Yukawa predicted the existence of a particle (now called the pion or pi meson) which would be involved in the strong nuclear force, along with an estimate of its mass. A year later, a particle was detected with that mass, which was assumed to be Yukawa’s meson. Further studies showed, though, that that particle didn’t actually have anything to do with the strong force, and had the wrong angular momentum, and in fact behaved very much like an electron, except with a higher mass (we now call these particles mesons). Then, the actual pion was discovered, at a slightly higher mass. The unexpected discovery of this “heavy electron”, which didn’t fit neatly into any of the frameworks of particle physics at the time, led Rabi to famously quip “Who ordered that?”.

Now, we understand a lot more about particle physics now than we did in the 1930s, and we think we’d be able to recognize a similar-mass particle based on differences in angular momentum, baryon number, lepton number, and other conserved quantities. But of course, it’s always possible that we’re having another “who ordered that?” moment.

Thanks. That seems to me to be understandable. Or at least in English with accessible
analogies.

As for the announcement, I’m pleased with the caution and non-fanfare nature way they are giving us this update. It is very sciency.

[Pasta]

For the minimal Standard Model Higgs search (which is what you’re hearing about right now), one can look for many rather specific features. If you see evidence of a new particle at a particular mass, then if its the Higgs, the Standard Model tells you…

…the total probability of producing it, per proton-proton collision
…the probability of producing it together with a Z boson
…the probability of producing it together with a W boson
…the probability of producing it together with <…>
…the fraction of time it will decay into two photons
…the fraction of time it will decay into two Z bosons
…the fraction of time it will decay into four muons
…the fraction of time it will decay into <…>
…the directions of the particles produced in the decays
…etc.

In other words, the Higgs has a very distinct “signature”. So far, the hints (or, maybe, hints of hints) that have been seen are consistent with the full suite of Standard Model expectations for the Higgs. However, the above features can’t be tested just yet individually, as there haven’t been enough data collected. At the moment, the hints come from looking at all of these questions together, in a statistical way, to say that “All of the various pieces of information, when taken together, are a little more consistent with the presence of a Higgs particle than with the absence of a Higgs particle.” The statistical tests could have said, “All of the various pieces of information, when taken together, suggest that the “nothing new” hypothesis isn’t good, but the Standard Model Higgs hypothesis doesn’t work any better.” But, they didn’t say that. Rather, the data are indeed better explained by allowing a Standard Model Higgs, though only at a small level of significance so far.

[Lumpy]

In hindsight, mass turned out to not be a meaningful way of classifying particles. “Meson” is now used to refer to any of the particles consisting of a quark/antiquark pair (some of which are more massive than protons or neutrons), and the unrelated heavy electron is now called a muon.

[dzero]

2 quick questions (probably with long answers)

1. I’ve heard that the Higgs gauge field (???) can exist w/o the particle so I’m guessing that this is quantization of the Higgs field - if that even made any sense.

2. How did the Higgs contribute to inflation and is it related to the dilaton?

[Half_Man_Half_Wit]

@Grey

1. What does it mean if the Higgs is less massive than the top quark? The top seems to have a mass of ~173 GeV.

I’m not sure it means anything… There are theories that relate the Higgs and the top quark (such as top condensate, in which there is no fundamental Higgs, but a bound state of top-antitop that plays the role of the Higgs), but I’m not aware of bounds this sets for the Higgs mass.

2. What models does a 125 GeV Higgs exclude?

Here’s a review article that collects Higgs mass predictions; the only thing I can think of straight away (other than Higgsless models, of course) is Alain Connes’ noncommutative geometry, which predicted a Higgs around 178 GeV if memory serves. But since that’s been excluded for a while, I think they’ve already issued a revised prediction (and I expect that’s going to be a general trend).

@dzero

1. I’ve heard that the Higgs gauge field (???) can exist w/o the particle so I’m guessing that this is quantization of the Higgs field - if that even made any sense.

Basically, yes. The Higgs boson is the quantum of the Higgs field. The field itself is always present, but the Higgs boson isn’t.

2. How did the Higgs contribute to inflation and is it related to the dilaton?

The Higgs boson can act as the driver of inflation, and this was indeed how inflation was first proposed; however, if was later found that the Higgs field as it was needed to break electroweak symmetry had different properties than were needed to drive inflation, and thus, the two are now ascribed to different scalar fields, the Higgs field and the so-called inflaton field. I don’t know about any relation to the dilaton, which essentially arises in theories with compactified higher dimensions and a varying gravitational constant (?), IIRC.

[Half_Man_Half_Wit]

Of course, just after writing the above, I stumble across a paper discussing the viability of Higgs inflation in a modern context – apparently, it’s not as dead as I thought it was!

[dzero]

Thank you for taking the time to explain.