The Higgs Boson -didn’t- show up. There’s evidence for a particle with mass consistent with the expected mass of the Higgs Boson, but this is not the same thing as a particle with the expected properties of the Higgs Boson.
However, there’s an issue: They didn’t know what the mass was supposed to be. Saying that the mass was consistent with the expected mass becomes less meaningful when you add that they spent the first two years of the LHC’s operation excluding possibilities for what that mass would be. It’s also less meaningful when you consider that they had already detected spikes at that mass range.
So what they -really- found is a particle with a mass consistent with particles they had already observed in the LHC. They’re -calling- it the Higgs Boson, but have not in fact observed the -properties- expected of the Higgs Boson.
We have observed a particle whose properties match what we would expect to see from the Higgs Boson at our current levels of data and sensitivity. This is not based on the value of the mass. We saw three decay modes with consistent excesses at a specific mass, and those decay modes match expected Higgs decays (H → γ γ, H → W W, H → Z Z). There are other decays that don’t show an excess (H → τ τ, H → b b), but we don’t expect nearly as much sensitivity in those channels yet. We excluded the other mass ranges in the same way: the sensitive decay modes in those regions did not show any excess.
There are many properties still to be analyzed, such as spin and branching ratios. There’s a good post at Quantum Diaries about possible values for the spin. I actually hope the branching ratios are different from Standard Model expectations, since that would indicate a massive new particle which could be a dark matter candidate or lead to other new physics.
Stupid question. How the devil does a particle of mass 125 GeV decay to two particles of mass around 80 GeV each? What are the actual observed end products?
That’s not a stupid question at all. Basically, the W and Z bosons are just virtual particles here, that decay very quickly, so that Heisenberg’s uncertainty principle (∆E * ∆t ≥ ℏ/2) is satisfied. The observed end products are four leptons (i.e. electrons, muons or taus plus the associated neutrinos), which add up to a mass much less then 125 GeV – the rest is in their kinetic energy.
Okay. So they’re actually talking about the 4l channel, which on theoretical grounds must involve an intermediate heavy (and virtual) boson. I opine that when the boson is virtual, ie there’s no mass peak in the two-lepton spectra, you ought not to say that you’ve observed the ‘2W’ channel, even if that’s the Feynman diagram you draw to explain the observation.
One exception though: In the Quantum Diaries post Dreaded_Anomaly mentioned, I’m fine with them talking about H → W W and H → Z Z decays, because they’re only talking about spin conservation at that one point – no need to mention the end products there. (But that wasn’t what you were talking about, I guess.)
I haven’t seen the data, or even firsthand accounts; the only thing I’ve seen thus far are second and thirdhand accounts, which conflict on whether or not the decay modes seen are the decay modes expected. They match in type, but not probability, and AFAIK this discrepancy, if not resolved, is a major problem in classifying the particle as the Higgs Boson; if further/better observations resolve this discrepancy, then the decay modes become evidence for it; as they exist right now, it’s mild evidence against it. My inclination is to “Wait for further evidence.”
(I’m accustomed to the “Higgs Boson” being evidence for Higgs Field Theory. If it turns out Higgs Field Theory, and Standard Theory more generally, is wrong, then I’d be reluctant to call it the Higgs Boson even if it’s otherwise exactly the particle predicted, but generated for different reasons.)
If it turns out Higgs Field Theory, and Standard Theory more generally, is wrong, then I’d be reluctant to call it the Higgs Boson
There’s different “kinds of wrong” in science, as noted by Asimov in his essay The Relativity of Wrong:
“When people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together.”
In the Higgs mechanism, the W and Z acquire mass by pairing up with degrees of freedom from a Higgs field. (Fermions get their mass differently; a purely left-handed massless fermion and a purely right-handed massless fermion pair up into a single massive fermion via their interactions with the Higgs field, but the massive fermion does not incorporate degrees of freedom from the Higgs field into itself, the way that the massive gauge bosons do.)
A Higgs boson comes from the unused degrees of freedom of a Higgs field. In the long run, the new particle will be called a Higgs boson if, and only if, it continues to look like a degree of freedom left over from a Higgsing. So it can still be a Higgs without being a standard model Higgs; for example, it might be one of several Higgses, or its couplings to the other particles might be different from the standard model values. The fermions could get their masses in some other way entirely (e.g. by being composite), but this particle would still be a Higgs so long as it’s the residue from the acquisition of masses by the W and Z. That’s the aspect which has to be false, if it is not a Higgs.
I haven’t seen the data, or even firsthand accounts
Here are the press releases from CERN, CMS, and ATLAS. They show several plots directly, and they have links to the very thorough PDFs presented in the July 4 seminar which took place before the press conference.
The Higgs Boson -didn’t- show up. There’s evidence for a particle with mass consistent with the expected mass of the Higgs Boson, but this is not the same thing as a particle with the expected properties of the Higgs Boson.
However, there’s an issue: They didn’t know what the mass was supposed to be. Saying that the mass was consistent with the expected mass becomes less meaningful when you add that they spent the first two years of the LHC’s operation excluding possibilities for what that mass would be. It’s also less meaningful when you consider that they had already detected spikes at that mass range.
So what they -really- found is a particle with a mass consistent with particles they had already observed in the LHC. They’re -calling- it the Higgs Boson, but have not in fact observed the -properties- expected of the Higgs Boson.
That’s not what we’re saying.
We have observed a particle whose properties match what we would expect to see from the Higgs Boson at our current levels of data and sensitivity. This is not based on the value of the mass. We saw three decay modes with consistent excesses at a specific mass, and those decay modes match expected Higgs decays (H → γ γ, H → W W, H → Z Z). There are other decays that don’t show an excess (H → τ τ, H → b b), but we don’t expect nearly as much sensitivity in those channels yet. We excluded the other mass ranges in the same way: the sensitive decay modes in those regions did not show any excess.
There are many properties still to be analyzed, such as spin and branching ratios. There’s a good post at Quantum Diaries about possible values for the spin. I actually hope the branching ratios are different from Standard Model expectations, since that would indicate a massive new particle which could be a dark matter candidate or lead to other new physics.
Stupid question. How the devil does a particle of mass 125 GeV decay to two particles of mass around 80 GeV each? What are the actual observed end products?
That’s not a stupid question at all. Basically, the W and Z bosons are just virtual particles here, that decay very quickly, so that Heisenberg’s uncertainty principle (∆E * ∆t ≥ ℏ/2) is satisfied. The observed end products are four leptons (i.e. electrons, muons or taus plus the associated neutrinos), which add up to a mass much less then 125 GeV – the rest is in their kinetic energy.
Okay. So they’re actually talking about the 4l channel, which on theoretical grounds must involve an intermediate heavy (and virtual) boson. I opine that when the boson is virtual, ie there’s no mass peak in the two-lepton spectra, you ought not to say that you’ve observed the ‘2W’ channel, even if that’s the Feynman diagram you draw to explain the observation.
I mostly agree with you.
One exception though: In the Quantum Diaries post Dreaded_Anomaly mentioned, I’m fine with them talking about H → W W and H → Z Z decays, because they’re only talking about spin conservation at that one point – no need to mention the end products there. (But that wasn’t what you were talking about, I guess.)
I haven’t seen the data, or even firsthand accounts; the only thing I’ve seen thus far are second and thirdhand accounts, which conflict on whether or not the decay modes seen are the decay modes expected. They match in type, but not probability, and AFAIK this discrepancy, if not resolved, is a major problem in classifying the particle as the Higgs Boson; if further/better observations resolve this discrepancy, then the decay modes become evidence for it; as they exist right now, it’s mild evidence against it. My inclination is to “Wait for further evidence.”
(I’m accustomed to the “Higgs Boson” being evidence for Higgs Field Theory. If it turns out Higgs Field Theory, and Standard Theory more generally, is wrong, then I’d be reluctant to call it the Higgs Boson even if it’s otherwise exactly the particle predicted, but generated for different reasons.)
There’s different “kinds of wrong” in science, as noted by Asimov in his essay The Relativity of Wrong:
Which of these “wrongs” did you mean?
In the Higgs mechanism, the W and Z acquire mass by pairing up with degrees of freedom from a Higgs field. (Fermions get their mass differently; a purely left-handed massless fermion and a purely right-handed massless fermion pair up into a single massive fermion via their interactions with the Higgs field, but the massive fermion does not incorporate degrees of freedom from the Higgs field into itself, the way that the massive gauge bosons do.)
A Higgs boson comes from the unused degrees of freedom of a Higgs field. In the long run, the new particle will be called a Higgs boson if, and only if, it continues to look like a degree of freedom left over from a Higgsing. So it can still be a Higgs without being a standard model Higgs; for example, it might be one of several Higgses, or its couplings to the other particles might be different from the standard model values. The fermions could get their masses in some other way entirely (e.g. by being composite), but this particle would still be a Higgs so long as it’s the residue from the acquisition of masses by the W and Z. That’s the aspect which has to be false, if it is not a Higgs.
Here are the press releases from CERN, CMS, and ATLAS. They show several plots directly, and they have links to the very thorough PDFs presented in the July 4 seminar which took place before the press conference.