Blessed!!!!!!!!!!!!

Just back from the Joint Physics Group meeting... My PR plot of the Z-> bb signal was blessed with no real fight....

So here it is: You are the very first to see it, since a minor cosmetic change was requested half an hour ago, and I just made it and copied from my computer to the blog area. The invariant mass of pairs of hadronic jets originated from b-quarks is shown in blue points: 85,794 events from 21,723,278 collected by the Z_BB trigger, and selected with requirements aimed at reducing the QCD background and obtaining a high purity of b-quark jets. The black histogram with grey shading is the background shape, and the green histogram is the Z signal content returned by the fitter. The red points show the result of the fit, which is a mixture of 96% background and 4% signal. They fit very well the experimental data (blue points). The data you're looking at is the largest sample of Z->bb decays ever isolated at a hadron collider, and actually just the second one, the first being a 90-event signal isolated by myself during my PhD analysis in 1998 (see here for the old plot).

Why was it so difficult ? Because the tiny Z->bb signal is buried within a background which, before selection, is 1000 times more numerous. So you have to select it carefully. But most importantly, you need a extremely precise model of the shape of the background in the mass distribution (the black histogram with grey shading above), otherwise you will never be able to claim that the insignificant shoulder in the distribution of blue points is actually any different than the rest of the data. But it is! The Z->bb is different than the production of pairs of b-quarks from a gluon, g->bb. The gluon is the carrier of the strong interaction, and from the theoretical point of view it has totally different properties than the Z boson, which is one of the four carriers of the electroweak interaction. Separating the two experimentally, however, it's tough!

Things do not end here. I need to start working seriously now! In fact, what I blessed today is just a plot we will use to boast about our ability to constrain the jet energy scale, and measure the top quark mass with higher accuracy than our cousins in D0. From tomorrow (well, next Monday!) my business will be to really do things carefully, and produce the best possible measurement with the dataset at hand.

They say that to do things well only requires a little bit more effort than doing them sloppily... In this case it is quite untrue. We will need to reprocess about 60 million events (not just the Z_BB dataset, but two others), generate 20 million more events with a simulation of the physics processes we are studying, and study everything systematically and carefully. I predict that by June we will have some solid result. But for today, I am quite happy!

Comments

[Ben]

Congratulations! Could you tell us more about the larger significance of the result (i.e., what it tells us about the Z boson, b quarks and the Standard Model?) Thanks!

[Tommaso Dorigo]

Well, the answer is really simple. It does not tell us anything that we do not as of yet know about the physics. It instead speaks volumes about a hadron collider experiment actually being able to reconstruct hadronically decaying heavy objects, such as is the Z boson. This is not easy due to the limited resolution of the typical detectors to hadronic jets, and due to the typically huge backgrounds due to non-resonant production of jets.
The whole business of the Higgs hunt at the Tevatron revolves around the assumption that we can see a tiny bump on top of a large background - not unlike the one in the plot. The Higgs boson decays to pairs of b quarks with large probability if the Higgs mass is not too large. We hope that we can find it that way, but it will require the hell of a lot of data and the most cunning ideas on our side.

T.

[Ben]

Thank you for the information, and I am enjoying your other posts too! Congratulations again on what sounds like a fantastic technical and intellectual achievement. However, I wanted to ask why you and fellow particle physicists don't (at least publicly) get discouraged that experiments like this don't yield physics information that wasn't already known. It sounds as if you're finding an already known needle in a large haystack, in order to gain the skills to find a yet-unobserved needle. In this World Year of Physics, I have to ask: Didn't you and your particle physics colleagues--among the brightest, hardest-working, most mathematically talented scientists--go into physics to discover more new things than this? From my (uninformed!) point of view, it seems that particle physicists are celebrating very incremental steps like these, rather than worrying that the return of truly significant, worldview-altering scientific knowledge appears to be very low at this point (with no real guarantees that there will be much huger payoffs even with the LHC). Please comment!

[Tommaso Dorigo]

Hi Ben,

I always reply in my blog to people's comments, even the least interesting, but your answer is indeed very interesting and I will paste it to an independent post tomorrow. For now, I answer below.

1) Thanks for your appreciation of this blog and of the results shown above!

2) You are right when you say that we (I am extrapolating other colleague's feelings here, but I am confident on it) study Particle Physics to hit on big new things, which can bring sudden advances to our field and to science in general. However, there are then several constraints. First of all, one is not always free to pick the kind of research one thinks is the most interesting, the most useful, or the most likely to produce a Nobel prize. To first order, after a PhD all but the very brightest (who are able to pick exactly what they want) are typically just happy to be able to stay in the business, and will conform to the research program of the University or Institute that hires them. After a few years, one gets to a tenured position and can then have more freedom to choose... But will one then be still able to have the energy and the stimulus to do groundbreaking research ? Not easy!
The other constraints come from one's background. It is hard to leave one's specific expertise (which is a valuable asset) to move to another more promising research. Finally, the most promising things are the ones where there is more competition, and one cannot always afford to have to fight.
3) Most of the big advancements in Physics, and I would say in Science in general, are indeed pictured as quantum leaps, but are rather the result of the collection of a mass of independent, smallish pieces of information. Then there is a stroke of genius, sure. But if you look closely, the genius was first of all a very well informed person! And without all the work of obscure colleagues, even a Planck would not have figured out how to fit a power spectrum the right way. In order to have the need of hypothesizing a quantization of energy Planck needed a very precise measurement of the emission spectrum of a blackbody to have to fit: and the higher precision of that measurement was - one could argue - a very incremental step from less precise measurements available at that time, to use your words. Another instance of that mechanism is the discovery of parity violation. Lee and Yang produced a earthquake when they speculated that Weak Interaction did not conserve the Parity symmetry, in a paper in 1956. But that paper was just a very detailed, precise and methodical review of all data available to that point: small experiments, tiny bits of information, sometimes imprecise, sometimes incomplete. Incremental steps!

So, am I worried by doing my tiny little bit of Science instead than concentrating on the big issues ? No. I am a soldier, not a general. Maybe one day I will be a general, but I have the feeling I will enjoy Particle Physics less then!

Cheers,
T.

[Ben]

Thank you so much, Tommaso! I look forward to your further posts! As a science writer (whose beat does not currently include particle physics), I've always been very interested in these issues--and you've shed a lot of light on them for me!

And you know what, I think that LHC WILL probably yield lots of great info--I guess I was just feeling that the Standard Model has been so surprisingly unyielding against many of the best experimental tests in recent times! Anyway, thanks for helping me to appreciate and celebrate the importance of "incremental steps"--physics and the public are very lucky to have "soldiers" like you!
Ben