Has the Higgs Boson Particle Been Discovered at Cern?

[tom.stoer]

You may be right, the audience still does not understand what they are doing and they could invest more time in explanations.

Depends on the audience.

Please note my intention to practise politeness "may", "could", ... if you ask me the level of information is OK.

 

[geordief]

Well would this field be evenly distributed then in the same way as I was wondering about the particle- or might it be stronger in different regions of the universe than in others?

Or can the field be dynamic? Can it feed off interreacting particles and become stronger?



These questions do not make sense.

Doesn't the first question make sense?
Is the higgs field the same everywhere in the universe?
Is there a higgs field in the middle of a supernova? (I am guessing no one knows about inside black holes)

 

[mfb]

Is the higgs field the same everywhere in the universe?

Please define "is".
It is the same everywhere, if "is" means its quantum-mechanical properties.

In a similar way, every electron behaves the same everywhere, but the question "is the electron the same everywhere" is not well-defined.

 

[ApplePion]

How do we know what was found was really the Higgs?

The theory predicted a certain mass range in order for the wek force coupling to work out. Yet no "Higgs" could be found in that range. So they switched the range. So isn't there now a big problem with the weak force?

And what was found did not have the correct decay modes from the Higgs theory.

So something was found that does not seem much to resemble the putative Higgs.

 

[TrickyDicky]

There are no other SM particles which could give this observation.

I guess you mean that no other particle is expected within the SM besides the Higgs.
And this much we all surely agree about.

Another question, does the SM Higgs boson have to be an elementary particle or it could be a composite boson and still be an SM Higgs?
I mention it because I'm reading physicist Matt Strassler' Higgs FAQ, and he says that since we know so little about the Higgs field: "we don’t know whether the Higgs is an elementary field, as is the electron field, or a composite of more elementary fields, as is the proton field."

 

[mfb]

The theory predicted a certain mass range in order for the wek force coupling to work out. Yet no "Higgs" could be found in that range. So they switched the range. So isn't there now a big problem with the weak force?

Can you give any source for that?

For example, look at the famous http://ppd.fnal.gov/conferences/nlc/minutes/000426.html (newer version). While 126 GeV is above the best fit, it is less than 2 standard deviations away.

And what was found did not have the correct decay modes from the Higgs theory.

Source? The tau channel has less events than expected, but it is within the statistic uncertainty.

 

[geordief]

Please define "is".
It is the same everywhere, if "is" means its quantum-mechanical properties.

In a similar way, every electron behaves the same everywhere, but the question "is the electron the same everywhere" is not well-defined.

Thanks.
Yes I think that is what I meant by "is" (not that I would recognise a quantum mechanical property unless it introduced itself politely first)

So I mean if one was in a position to make a prediction on the consequences of an event anywhere at all in the universe would it be fair to say that this could not be done without including the effect of the Higgs field at that point in SpaceTime?

(although I suppose interactions between massless particles are unaffected by the field?-or am I betraying my ignorance again)

 

[ApplePion]

"There are no other SM particles which could give this observation""

I don't know what "SM" means, but I doubt that matters.

The thing discovered decayed into 2 photons. A meson made of a quark and its own anti-quark, could decay into 2 photons--indeed the J/psi particle (made of a charmed quark and an anti-charm quark) decays into 2 photons. So how do we know that the new particle is not a combination of a very heavy newly encountered quark and its anti-partner?

 

[ApplePion]

mfb, these are the best sources I can find on short notice.

Regarding the mass being off, the best I can find right now is the following from wikipedia (sorry about wikipedia)

<<The Standard Model does not predict the mass of the Higgs boson.[28] If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV).[citation needed] Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model.[citation needed] The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes>>

Regarding decay modes, it is supposed to have an W+/W- mode (see link below) but this has not been observed for the new particle:

http://arxiv.org/abs/hep-ph/0608174

 

[ApplePion]

Ooops, wrong quote from wikipoedia--serves me right for quoting wikipedia. This is what I wanted to quote:

<<In theory, the mass of the Higgs boson may be estimated indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons. Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of July 2011, the precision electroweak measurements tell us that the mass of the Higgs boson is lower than about 161 GeV/c2 at 95% confidence level (CL). This upper bound increases to 185 GeV/c2 when including the LEP-2 direct search lower bound of 114.4 GeV/c2.[27] These indirect constraints rely on the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 185 GeV/c2 if it is accompanied by other particles beyond those predicted by the Standard Model>>

 

[kloptok]

So why do you say that the mass is "wrong"? 125 GeV is certainly within the range of possible masses. This is also in the Wiki quote, so I don't see your point.

 

[ranrod]

Why did Stephen Hawking concede? He flat out said that it looks like the Higgs particle has been observed:
http://www.inquisitr.com/269906/stephen-hawking-loses-100-bet-over-higgs-boson-discovery-video/

 

[ApplePion]

kloptok , you are correct. I messed up on the quote--it's been a long day.

I do seriously remember reading that to make the weak force theory work out quantitatively correctly the Higgs boson had to be within a certain mass range. When it was not found there, they just went to a different mass range. Hopefully I can find a reference.

 

[kloptok]

As far as I know, a mass of 125 GeV is not a problem. What we knew before the LHC started was that the Higgs couldn't be too heavy because this would mess up other parameters (the Higgs enters other processes in loop corrections and if the mass is too high the values for these other processes don't agree with experiment), but then we're talking of masses on the order of 1 TeV and higher. Also, previous measurements from LEP and Tevatron excluded all masses below 115 GeV (LEP) and a band around ~160-175 GeV I think (Tevatron), see the exclusion plot below from February 2012 (note that the exclusion is today for the entire plot except just at 125 GeV where the line still is above 1, signifying the excess of events there). At the LHC one has looked for a broad range of Higgs masses, up to ~hundreds of GeV. I don't know exactly why but the region around 120 GeV is often quoted as the most difficult mass region to find the Higgs, which is probably why one left it for last since it would be easier to find the particle elsewhere. This might be wrong, but the difficulty to find it there might have to do with the fact that there are a lot of possible decays there, see e.g. the plot of branching ratio vs. higgs mass below. But as I said, this might not be the full story.

 

[mfb]

"There are no other SM particles which could give this observation""

I don't know what "SM" means, but I doubt that matters.

The thing discovered decayed into 2 photons. A meson made of a quark and its own anti-quark, could decay into 2 photons--indeed the J/psi particle (made of a charmed quark and an anti-charm quark) decays into 2 photons. So how do we know that the new particle is not a combination of a very heavy newly encountered quark and its anti-partner?

SM=Standard Model
The SM contains three quark families. A forth family would be new physics and therefore beyond the SM. Apart from this:
- This quark would form other combinations, too. None of them was observed.
- It would have a mass of ~63 GeV, and therefore have been within the range of LEP (as the process e- e+ -> q anti-quark is quite likely, if the energy allows it) and Tevatron.


A high Higgs mass would have given some trouble.
However, at 126 GeV it is quite "boring" - if it is the Higgs, it is in the expected region.

About your second link:

The decay of a Higgs boson into a pair of W bosons h --> W^+W^-, is a dominant mode for Higgs boson masses above 135 GeV.

The discovered boson does not have a mass above 135 GeV. Below, the sensitivity of this channel is quite bad.

I don't know exactly why but the region around 120 GeV is often quoted as the most difficult mass region to find the Higgs

The decay channel b b-bar is dominant, and this has a lot of background. WW has the missing energy issue with neutrinos, gluon gluon is spammed by background, tau tau has the same neutrino issue again. The easier channels have a smaller branching fraction.
The observation is mainly based on the 2 photon channel now, with a branching fraction of a few permille.

 

[PAllen]

The observation is mainly based on the 2 photon channel now, with a branching fraction of a few permille.

I would say the observation, at the same mass, of excess events consistent with H->Z,Z(*)-> 4 leptons, by both CMS and Atlas adds a great deal to the reliability of the finding, and the probability that it is 'Higgs like'.

 

[Vanadium 50]

Can someone explain to me how they know this particle is a boson?

A boson has integral spin. The photon and Z also have integral spins, so their sum must also. You might say, but what if there is an unseen fermion in the decay and it's really gamma+gamma+neutrino? The answer is that would be a 3-body decay and wouldn't give you a sharp peak: it would have a sharp right edge but extend down to 0. This would render it invisible to searches.

So it must be a boson, otherwise it wouldn't be observed.

 

[Vanadium 50]

Thanks (I missed this was already answered in #77 and #78).
I know there is enough data to say it is an integer spin (boson), and that it is expected that it is 0 (scalar). How would it alter the state of things if it was found out its spin is 1 instead? Could it be the Higgs in that case?

A spin-1 particle cannot decay to two identical spin-1 particles. So the fact that it is seen in gamma+gamma precludes this.

 

[rodsika]

What's the highest mass possible for gravitons? if the ~126 Gev unknown particle detected In the LHC is not higgs.. and it's not gravitons.. what other particles have spin 0?

 

[Vanadium 50]

So that would mean the CMS 5 sigma from the combination of 2 channels is illegitimate, as is the 5 sigma from ATLAS, since that seems not to have used all the relevant channels?

The ATLAS result used all channels from 2011, and all the channels where the analysis was finished from 2012. Only the two most sensitive channels were completed by July 4th.

The CMS result included two numbers: just the two most sensitive channels, and all the channels.

Each experiment has a significance of better than one in a million, so the overall probability is something like one in a trillion. At this level, I think it's fair to say that this is not a statsitical fluctuation. It might be a mistake, but it's not a statistical fluctuation.

 

[Vanadium 50]

So how do we know that the new particle is not a combination of a very heavy newly encountered quark and its anti-partner?

Reason one: if this is a ¹S₀ state of a new quarkonium state (bound state of a quark-antiquark pair), there will also be a ³S₁ state that will decay to e+e- and mu+mu- pairs, at a rate where there should be a hundred thousand or more events by now. Such a thing would have been discovered long ago - probably at the Tevatron or HERA, but certainly by the LHC last year.

Reason two: A 65 GeV quark would completely screw up precision electroweak measurements and would have been discovered indirectly years ago.

Reason three: The decay into ZZ* is way too big. Maybe a million times too big. A second-order weak process competing with strong processes? It should be invisible.

 

[Vanadium 50]

The theory predicted a certain mass range in order for the wek force coupling to work out. Yet no "Higgs" could be found in that range. So they switched the range.

What on Earth are you talking about?

The Higgs Boson has to have a mass between 0+epsilon and 1000 GeV, otherwise it has problems doing the job of the Higgs and properly break electroweak symmetry.

Direct searches ruled out the range below 114 GeV and precision electroweak measurements ruled out the range above 200 GeV or so.

It ended up at 126 GeV. So what's the problem?

 

[ApplePion]

"The SM contains three quark families. A forth family would be new physics and therefore beyond the SM." Clearly when I was asking about it being a quark ant-quark pair with a 7th quark I was discussing the possibility that more than 6 quarks exist."This quark would form other combinations, too. None of them was observed."

Yeah, such as this 7th quark with the anti-quark of an 8th quark. How do you know the 8th quark is not so massive that such a neson could not be found at the energies currently used?

 

[ApplePion]

"if this is a 1S0 state of a new quarkonium state (bound state of a quark-antiquark pair), there will also be a 3S1 state that will decay to e+e- and mu+mu- pairs, at a rate where there should be a hundred thousand or more events by now. Such a thing would have been discovered long ago - probably at the Tevatron or HERA, but certainly by the LHC last year>>

What would the mass of the 3S1 state be? Would it be too high to have been created by current experiments?

 

[ApplePion]

Could someone please clarify something for me. In earlier experiments they were looking for and expecting to find the Higgs in a certain mass range. They didn't find it, of course, and began looking in a different range.

What was the reason why they were originally so convinced it had to be in that first range?

 

[lpetrich]

Look at charmonium and bottomonium states for 1S0 - 3S1 splittings. Eta-c vs. J/psi, eta-b vs. upsilon. They are not very large compared to the masses of the particles.

The relative sizes of the effects parallel the electromagnetic case, though with a much larger "fine structure constant".

Overall mass: m
Orbital energy: m*a²
Spin-orbit and spin-spin energy: m*a⁴

a = "fine structure constant" or g²/(4*pi)

If the particles differ in mass, then:

Smaller: m
Larger: M
Overall mass: M
Orbital energy: m*a²
Spin-orbit energy: m*a⁴
Spin-spin energy: m*(m/M)*a⁴

So if the recently-discovered particle was a 1S0 quarkonium state, then it ought to have a related 3S1 quarkonium state with a very close mass value.

 

[ApplePion]

Thanks for your post, lpetrich.

 

[Vanadium 50]

What would the mass of the 3S1 state be?

5 to 6 MeV (not GeV, MeV) above the 1S0. So also ~125 GeV.

It would surely have been seen previously.

 

[mfb]

I would say the observation, at the same mass, of excess events consistent with H->Z,Z(*)-> 4 leptons, by both CMS and Atlas adds a great deal to the reliability of the finding, and the probability that it is 'Higgs like'.

Related to possible measurement errors: Yes, of course.
Related to the total statistical significance: Not unimportant, but the 2photon-channel dominates the combination.

Clearly when I was asking about it being a quark ant-quark pair with a 7th quark I was discussing the possibility that more than 6 quarks exist.

And my statement was that this option is not part of the SM.

"This quark would form other combinations, too. None of them was observed."

Yeah, such as this 7th quark with the anti-quark of an 8th quark.

Or the 7th quark with up, down, strange and (rare) charm and bottom. None of these combinations was observed.

In earlier experiments they were looking for and expecting to find the Higgs in a certain mass range.

Earlier experiments had less energy and therefore not the sensitivity to look in the whole possible mass range. They just hoped to find it where the experiments were sensitive... you always hope to find something with your detector.

 

[Edgardo]

Here is a video from the Fermilab youtube channel explaining the term sigma:

Searching for the Higgs boson and other particles requires scientists to take into account statistics and probability in their analyses. Fermilab physicist Don Lincoln explains these concepts using simple dice.

http://youtu.be/73JeQ2RZnwc

https://www.youtube.com/watch?v=73JeQ2RZnwc