Why the Higgs boson has been discovered so late?

[haael]

The top quark has mass around 173GeV. The Higgs boson (probably) has only 125GeV. Why the top quark has been discovered earlier despite it has larger mass? Why did we need more powerful accelerator to detect a lighter particle? Is it possible that there are some other light particles within our accelerators' range that have not been discovered? What mass ranges have actually been searched to exclude the existence of particles in them?

 

[Bill_K]

It's not the rest mass that matters so much as the production rate. The Higgs boson was well within range of the Tevatron, and the LHC could have seen a Higgs with a much larger rest mass. But what made the LHC necessary for the Higgs discovery was its much greater luminosity.

 

[Vanadium 50]

Additionally, about half of the top quark decays are in channels that are easily detectable. For a Higgs its less than a percent.

 

[mfb]

The top cross-section is about 10 times the Higgs cross-section. For each produced Higgs, 10 top/antitop pairs are produced. Those pairs have a clear signature, as (nearly) all top-quarks decay into W + b-quark: Two W decays (each [high-energetic leptons + missing transverse energy] or [2 jets]) plus two high-energetic jets with a b-meson inside.
Produce some top-quarks, and you cannot miss them.

Higgs is much more difficult - the clean decay channels (especially to 2 photons or 4 muons) are extremely rare. The other decay channels are even worse, as they do not allow to measure the mass directly. This gives even more background.

Is it possible that there are some other light particles within our accelerators' range that have not been discovered?

If their coupling to other particles is extremely weak, this is possible. Most dark matter searches focus on particles around ~100 GeV.

 

[lpetrich]

Putting together Vanadium 50's and mfb's comments, the ratio of detectable events is:

(Higgs detectable events) / (top detectable events) = ( (Higgs rate) / (top rate) ) / (Higgs d fraction / top d fraction ) = (1/10)*(0.01/0.5) = 500

So one gets 500 top-event detections for every Higgs-event detection.

 

[FeynmanIsCool]

Higgs has just a 1.56X10^-22 s lifespan before decay

 

[Vanadium 50]

Higgs has just a 1.56X10^-22 s lifespan before decay

How is this relevant?

 

[Bill_K]

Higgs has just a 1.56X10^-22 s lifespan before decay

It hasn't been accurately measured yet. Certainly not to three decimal places.

 

[FeynmanIsCool]

It hasn't been accurately measured yet. Certainly not to three decimal places.

Its speculation, they haven't even said for sure they have found it yet

" Tentatively observed;– a boson "consistent with" the Higgs boson has been observed, but as of August 2012, it has not been conclusively identified as the Higgs boson"

 

[mfb]

Well, that "consistent with" is a bit confusing: You cannot do better. Either you discover some property which is inconsistent with the Higgs (then it is not the Higgs) or all measurements agree with the Higgs hypothesis (then it is "consistent with" the Higgs boson).
In the same way, you can claim that scientists did not observe "the top-quark", but "a particle which behaves like the top-quark in every measured aspect".

It is true that there are just a few measured properties for that new boson. But there won't be some single event which changes evidence from "Higgs-like boson" to "Higgs boson". I expect that ATLAS and CMS will call it "Higgs boson" in spring 2013, based on the expectation that all measurements will agree with the hypothesis that they discovered "the" Higgs-boson.

 

[Vanadium 50]

Again, whu is the lifetime relevant? What point are you trying to make?

 

[Bill_K]

Again, whu is the lifetime relevant? What point are you trying to make?

It's a property predicted by the Standard Model. Three decimal places is not relevant, but if the lifetime/width of the boson was off by an order of magnitude, it would raise suspicion that what they found is not the standard Higgs.

 

[Vanadium 50]

Yes, but a) the experiments do not have sensitivity to an order of magnitude change in the width - or two - or almost three - and b) it has nothing obvious to do with the question "Why the Higgs boson has been discovered so late?"

 

[mfb]

I do not think it can be measured, if it is somewhere close to the SM value.

Lifetime measurement: Even if we ignore all background and other experimental challenges, the vertex resolution for 4 muons (probably the best channel for that) is something like ~10µm under ideal conditions. With a Higgs boost of ~5, this corresponds to ~10^-14s. This is for a single Higgs, but even 10^6 Higgs would not give anything better than ~10^-17s.

Decay width measurement: Even with perfect alignment, I would expect that the energy resolution is ~1 GeV in the 4-muon channel (probably the best channel here, too), but let's add electrons to get more events. This gives (optimistic) 1 fb of cross-section*branching ratio*selection efficiency. The upper estimates for the total integrated LHC luminosity are in the range of a few inverse attobarn. If they get 10/ab, both experiments will see ~10000 Higgs in lepton-only channels. Assuming a perfect analysis, this would allow to see the decay width if it is at least ~10 MeV. That corresponds to 7*10^(23)s. A factor of 50 above the SM prediction - and my assumptions are really optimistic.

I am sure that the collaborations will set limits - but I would be very surprised to see an actual determination of the lifetime (without an uncertainty of a factor of 10).

 

[FeynmanIsCool]

Yes, but a) the experiments do not have sensitivity to an order of magnitude change in the width - or two - or almost three - and b) it has nothing obvious to do with the question "Why the Higgs boson has been discovered so late?"

Im not going to argue with you since you have your PhD in physics, and I dont.

 

[Vanadium 50]

I'm not trying to argue - I am asking you what your point is, because I don't understand it.

 

[Bill_K]

it has nothing obvious to do with the question "Why the Higgs boson has been discovered so late?"

It really does. Part of the reason is that the Higgs mass of 125 GeV puts it in a range where it is more difficult to detect. A light Higgs decays primarily into b-bar, and this is hard to resolve from the QCD background. Correspondingly the width for a light Higgs is quite narrow, best estimates at about 6 MeV. But as the mass is increased, decays into WW and ZZ open up and the width grows rather abruptly, to Γ = 1 GeV at M = 160 GeV. The Tevatron could have seen a heavier Higgs.

the experiments do not have sensitivity to an order of magnitude change in the width - or two - or almost three

This paper, for example, claims the LHC will be able to eventually determine the Higgs width indirectly to within 10-20 percent.

 

[Vanadium 50]

Bill, if he had said "the mass is 125 GeV", I would have understood the point he was making. But the width/lifetime is relevant only insofar as it predicts the mass. In the SM it does, but in other models it does not. In any event, if he meant "the mass was 125 GeV", wouldn't he have said it?

 

[ofirg]

This paper, for example, claims the LHC will be able to eventually determine the Higgs width indirectly to within 10-20 percent.

Yes, The basic idea is to measure the higgs couplings to the W boson through the measurement of the higgs production cross section in the VBF channel (which includes the vertex $W^{+}W^{-}->h$ but you can also have instead $ZZ->h$ so I guess you would have to assume the SM relationship bewteen them holds). Then use that coupling to calculate the partial width $\Gamma[h->W^{+}W^{-}]$ and divide by the measured branching ratio to get the total width