## Wednesday, July 04, 2012

### Higgs has been discovered: a chat box

Executive summary: CMS has discovered a new particle fully compatible with the Standard Model Higgs boson (so far: all channels are within 1 sigma from the Standard Model) at 5.0 sigma at diphoton and ZZ channels; when all the other channels are included, the significance drops to 4.9 sigma. Their Higgs mass is 125.3 ± 0.6 GeV.

ATLAS has made a 5-sigma discovery, too. The same channels, the mass is around 126.5 GeV. All ATLAS' channels exactly agree, within 0.5 sigma, with SM+Higgs predictions except for diphoton channel that is almost 100% above predictions. Gianotti hasn't been scared by Comic-Sans haters and used the same font again. Papers with details by the end of July.

Congratulations to everyone involved! Lisa Randall just wrote me that I was right about the coming discovery, so small congratulations to me, too. ;-)

At 9:00 Central European Summer Time (midnight California Daylight Saving Time), a 2-hour-long seminar followed by a press conference starts at webcast.CERN.ch. You should open the video in another window and participate in the chat below, too. You will be able to ask and answer questions over there.

The discovery of the Standard-Model-like Higgs boson is one of the deepest confirmations of a theoretical prediction in the history of science. While the theory isn't too difficult, it's very far from being trivial, too. Imagine you were only given the list of particles found before the Higgs boson, their spins, and their masses.

Without a lot of theoretical work, you couldn't say anything about new particles.

Nevertheless, it is possible to predict the existence of new particles. And in the case of the Higgs boson, it is a completely new kind of a particle because it is the first spinless elementary particle we have discovered in Nature. Its difference from the other elementary particles we know is more qualitative than the differences between any pairs of others.

1. Chat does not work on iPad

2. Sorry for that, you're right. Flash is banned with mobile Apple...

3. congratulations!

4. About friggin' time! Wow! Congratulations to all involved, a truly fantastic achievement.

But you're gonna have to add an extra cubelet to your "rubik's cube" model of the Higgs if the Atlas results are accurate.

Nice to see Peter Higgs was personally present for the announcement. In a BBC interview afterwards he said he was surprised that the particle had been discovered in his lifetime. (Well actually, I think it would have been discovered a decade ago if the government's of the world weren't so backwards regarding science funding)

5. Ah ha http://www.thedailymash.co.uk/news/science-technology/higgs-boson-becomes-first-celebrity-particle-2012070432966

6. That this is a spinless boson follows from it looking like the SM Higgs and is not a direct inference from measurement, right?

7. Congratulation to you Lumo for so confidently foreseeing this discovery!
I hope you receive the money that was bet against your forecast of this finding.

8. Thanks, Peter! It may sound incredible but John R. actually contacted me two hours ago with a friendly e-mail asking me for IBAN or PayPal contacts....

9. Dear Arun, the fact that it's a boson follows directly from the observations because it decays to 2 photons or 2 Z-bosons, among other things. So it's a boson. Its spin must be integer.

Actually, even without any references to detailed theory, its spin must be an even integer, as was explicitly noticed during the talks, too. One may easily prove a theorem that an odd-spin e.g. j=1 particle couldn't decay to two photons. So the spin must be j=0 or j=2. Higher j is also impossible because it couldn't decay to two photons at a sizable rate, either - conservation of angular momentum again.

Now, j=2 or higher is really impossible for theoretical reasons because j=2 particles must get rid of their negative-norm timelike modes predicted by the Lorentz symmetry, and the only gauge invariance that can do it is diffemorphism symmetry of gravity: j=2 elementary particles must be gravitons and nothing else. But even without this theory argument, the proof that the new particle isn't a j=2 particle will be completed rather abruptly. For example, a j=2 particle couldn't decay to two j=1/2 fermions and these decays will also become statistically significant by themselves soon. So roughly by the end of 2012, it will be a direct experimental proof of all the things you find impossible to find out.

10. Dear Lubos, in the former post you suggested it should not be really compatible with SM at 125GeV, But here one reads it IS compatible. What went "wrong?" Thanks

11. Hi, I thought I was very careful what I was saying. The data is compatible with the Standard Model as an effective theory describing everything up to 200 GeV or something like that. There's no known significant deviation, except for some hints I don't want to analyze.

But the Standard Model with a 125 GeV Higgs boson, however successful it is in describing the LHC data, may be proved not to be the complete theory of non-gravitational forces. It can't be valid up to the Planck scale where quantum gravity surely kicks in because of the instability. So we know that this theory isn't the whole story. It's not just a feeling, an aesthetic impression; the theory actually becomes inconsistent - inconsistent as a tool to answer questions we can't experimentally test but even if we can't test them, we know that the Standard Model with 125 GeV Higgs predicts nonsense about them which is bad.

125 GeV Higgs is consistent with the Standard Model as an approximate (we say "effective") theory for the LHC so far; however, it is inconsistent with the Standard Model as the complete theory of forces in Nature.

12. Dear Lumo, cool that John R. will pay his dept, and congratulations from me too ;-)
With the money you can throw a nice higgs party tonight about which I want to read in the New York Times tomorrow :-D

13. Good on ya - yet again, then! :)

14. Fun story, Brian. Good that Rincon wrote a good presentation. On the other hand, the censorship could have been expected given some articles by him:

Climate change has killed neanderthals, is made worse by ozone, must be fought against by submarines etc. Good collection. But he also wrote about "stricter control urged for the IPCC", at least something not entirely preposterous.

15. Is this THE SM Higgs? What about possibilities of being a new MSSM or NMSSM particle?

16. I see, thanks. However, the most successful scientific theory of all times QED is mathematically inconsistent at too high energies. Is there a real problem here ?

17. Very plausible if not downright likely. At the bottom of

http://motls.blogspot.cz/2012/07/compact-formula-for-all-tree-nnn8-sugra.html

I mentioned a new paper that actually claims that it makes MSSM or NMSSM more likely when we observe an excess of diphotons and deficit of ditaus...

18. It would be a problem if QED were supposed to be a complete theory, too. It's the same question, of course.

QED, unlike QCD, is perturbatively inconsistent. Equivalently, it's inconsistent at very high energies.

There's still a difference from the Higgs instability situation, however. QED would get inconsistent at huge energies greater than the Planck scale - we know that gravity, the Planck scale, becomes relevant before the QED Landau pole, so the Landau pole is rendered moot. We may always believe that some quantum gravity saves us from the Landau pole.

However, the Higgs instability would occur below the GUT/Planck energies so we know that something else aside from gravity has to "save the day".

19. Congratulation to physicists. Truly spectacular triumph of the human intellect

20. A curious question from somebody who is not exactly an expert in experimental physics.

They announced a 5-sigma event. But in order to turn standard error measures into tail probabilities (to have a p-value for the statistical test), we need to know the underlying distribution of events - more precisely, only very special distributions (like Gaussian) allow a bijection between the second moment (controlling for the first) and tail probabilities.

So how is this done?

1) The distribution of the measurement IS indeed Gaussian. I know this is the natural benchmark, arising under classical measurement error and the Central Limit Theorem helps as well. But I do not know if the conditions are satisfied in this case.

2) The distribution is non-Gaussian. The p-value is calculated under the correct observed distribution, and then, merely for the purposes of easy interpretation, it is expressed in the number of sigmas under a hypothetical Gaussian distribution.

Can somebody who understands the nature of the experiments and calculations shed some light on that? I am just curious. Thanks!

21. I don't see why the Higg's field is not considered to be another (fifth) fundamental force - albeit a far less easily identifiable one than the four that is conventionally recognized as fundamental. How could it be wrong to classify it as such?

22. Dear jb, whenever the confidence level becomes as high as 5 sigma in all similar situations, the distribution is indistinguishable from a Gaussian one.

The central limit theorem holds, at least as an excellent approximation, because there are many terms that are simply added or subtracted - numbers of individual collisions, and on the other side, theoretical predictions from different channels or Feynman diagrams etc.

However, for a small significance level, it deviates from the Gaussian. After all, getting events of one kind is a Poisson process so the number of such events is distributed via a Poisson distribution. Again, for large numbers of events, the Poisson distribution exhibits a Gaussian peak, too. Nevertheless, the CERN folks always rigorously calculate with whatever Poisson distribution is relevant for the events and their systematic errors; and they carefully - really nonlinearly - compose this distribution with other distributions for the systematic errors and others.

But again, much of this is really unnecessary for the broader point that the signal is 5-sigma-like because when things get this exact, you are effectively talking about a linearized problem in which the errors are "nearly infinitesimal" and they're added from various contributions in a linear way, and the Gaussianity comes from having a large number of terms that affect the result.

Just sociologically, the folks doing these statistical analyses are among the best practicians in statistics in the world. It's really their business, they have a sufficient math background even for the most advanced aspects of statistics, and they're trained by an immense amount of problem most of which are instantly being checked - comparisons of theory and experiments.

All the best
Lubos

23. I think it's a good point, Peter. Every bosonic field mediates a force - except for the Higgs, we are making this claim about every other fundamental boson whether its spin is 1 or or 2.

One would just have to decide what you exactly mean by the Higgs force. Is it a long-range force? If you mean the exchange of the Higgs excitation only, it would be a short-range force, much like the weak nuclear force itself, because the Higgs is very heavy.

However, you could also view the Higgs mechanism as a result of this fifth Higgs force. In that case, it affects every single particle anywhere, it's more a long-range force than any others!

You know, while electromagnetism induces a force between 2 charged particles, arising from terms like electron.electron*.proton.proton* in the Lagrangian, the Higgs mechanism induces simpler terms, just electron.electron*, for example: it gives the mass to the electron. The Higgs field makes the electron interact with itself, not just with others. This simplicity of the modifications to the environment caused by the Higgs field is kind of unlike the effect of any other bosonic field which may be a reason why it could be cumbersome to classify it as a new force.

But more generally, your point of course has a very good, smart logic.

Cheers
Lubos

24. I'm letting you off the hook so easily. You show a diagram of something v something in which the putative Higgs lies outside the allowable area for compatibility with the Standard Model bu within the limits for the supersymmetric mode..

So what gives? Your older post seems to state that this particle invalidates the Standard Model.

25. I have explained the same thing several times. A 125 GeV Higgs is OK for a Standard Model as an effective theory at low enough energies; but it is inconsistent or at least very dangerous as a more complete theory for all energy scales because it creates an instability.

If you want to see that Nobel prize winning David Gross said the very same thing today, search for his name here:

http://physicsworld.com/blog/2012/07/nobel_laureates_react_to_higgs.html

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27. thanks for posting..

28. gravity isn't energy? then the conservation of energy could appear in the interactions of the gravitational fields,because the deformations geometrics of spacetime contain energy in the systems.