I think I get it. The problem was I was thinking of the energy on the horizontal scale as being the total energy contributed by the collision whereas you are saying it’s the energy measured for a particular ‘signal’ that has been found. Flip said the same thing in his post if only I’d read it more carefully.

1. Indeed, the background is large and can be a very subtle thing. However, physicists work *very* hard to correctly subtract away the background (they also do more sophisticated things such as varying parameters for the background in the fit). At the end of the day we’re just counting how many events the Standard Model predicts versus how many events were actually measured. That being said, it is possible that there is something about the background that was mismodelled. Very good people are working on this aspect. We’ll have to see.

2. *IF* the bump comes from a new particle, then the width is related to the decay properties of that particle. I’m not quite sure what you mean by ‘harmonic’ — at this level it might be best to say that this kind of “resonance” is not quite the same as an acoustic resonance.

3. I’m not quite sure what you mean. The decay process may be spin-dependent, but the plot over energy spectrum should not be. This is conservation of energy. (Again, this is all assuming that the two jets came from a new particle.)

4. At least within the Standard Model, the ‘soup’ that we live in is the ‘vacuum’, and as you correctly point out, our vacuum isn’t empty. In addition to quantum fluctuations, we live in a particular Higgs field background configuration. I will blog about this as soon as I get a chance (unfortunately this CDF result came up!).

Thanks for the comments,
F

But there is still one thing that comes to my mind.
The obvious later onset of the first peak and even a NEGATIVE bump in the substracted data which inspires doubt over our model of the underlying “knowns”.

The second peak is much wider and that is also what bothers me. For electromagnetic resonance – in the second harmonic you might expect double the width of the first peak, but not tripple. If it were straight data, the black wavy tail would also have been significant. Maybe a slight tweak in the underlying model is in order, apart from search for a new particle.

Two more notes. The energy peaks could be spin-dependent, the second for double the spin of the first one — but that is only a sidenote.

Second, I really do believe we all swim in a soup and do not realize that. My explanation of everything is that we live in a physically defined space, the kind that Dirac imagined as the first one. Gamow pictured Dirac with a dolphin as Dirac talks to him. The dolphin does not realize he is in water, that the molecules are all around him, the same way we may not realize that we live in a checkered 3-dimensional grid paper, and this physically defined grid (almost weightless) is what we may be interacting with. I explain it with the fact that even vacuum has physical properties.

Am I wrong?

Another way of saying this is that that the significance level quoted accounts for “known knowns” and “known unknowns.” If there’s something that we really don’t understand about the QCD background, then maybe such a ‘discovery’ isn’t a new particle but but some hitherto misunderstood effect in nonperturbative physics (just to throw out a random possibility).

Hi Jon! I think I understand where your confusion is coming from and I suspect that it might come down to terminology. We use the phrase “resonance” to describe the bump in the mass spectrum, but this does not mean that we expect to see higher “harmonics” of this bump. This is a different kind of resonance than the acoustic resonance that is more commonly known. I need to think a bit more carefully about the precise relation from a physics point of view—but but in general, if we have a particle of mass X which decays into two jets, then we expect to see a bump in the “dijet invariant mass” at X, and not at 2X, 3X, etc.

I should also say again that just because there’s a bump, it doesn’t (yet) mean that it has to be a new particle. Given the big media hoopla, I want to make it clear that the people in the field are being as conservative as possible and trying to make sure that there could be no other possible explanation.

Cheers,
F

Obviously I’m wrong, but what is it I’m missing? Why does it have to be a new particle?

Is there 5 sigma condition for a new particle discovery?

Thanks,

Nick

Thanks for the correction with the t-channel. I’ve made the correction in the post.

Best,
Flip

Great article. You’ve really nailed it to the point and made it really understandable for “normal” people like me. Looking forward to reading more from you.

This is a nice post, but I have a couple of comments. You wrote:

we understand the standard Higgs well enough to know that if it had a mass of 150 GeV, then we would expect about three times more events than were found.

Can you explain this estimate? Here’s my attempt, which gets a rather different answer: eyeballing Fig 3.3 in the big Djouadi review hep-ph/0503172, it looks like the HW cross section at the Tevatron is ~ 50 fb. After asking for the W to go to e or mu, this drops to 10 fb. The data sample was 4 fb^-1, so this is about 40 events, and acceptance effects will cut that down by another factor of 3 or so. But the plots show about 200 events in this bump. So I would say that for a standard Higgs we expect about a factor of 20 fewer events than were found. (Not to mention that a 150 GeV SM Higgs wouldn’t dominantly decay to dijets; asking instead about an MSSM Higgs that would, though, doesn’t help improve the rate.) This is back-of-the-envelope, but I would have to be missing something important for it to be off by much more than a factor of 2 either way.

the CDF result does not appear to be the Higgs, but if there is a new particle responsible for it, it is produced from the same diagram (with the dashed line representing the new particle).

Or, it could be a t-channel process with a quark radiating both a W and the new particle. A few people have already posted such things on the arxiv. (Unfortunately, UA2 didn’t have enough luminosity to exclude a light Z’ coupling only to hadrons, and our current colliders are too high in energy, so that a simple dijet bump search is swamped by QCD background at this low mass.)

Best wishes,
Matt