Who discovered single top production ? March 5, 2009Posted by dorigo in news, physics, science.
Tags: CDF, DZERO, Tevatron, top quark
Both CDF and DZERO have announced yesterday the first observation of electroweak production of single top quarks in proton-antiproton collisions. Both papers (this one from CDF, and this one from DZERO) claim theirs is the first observation of the long sought-after subatomic reaction. Who is right ? Who has more merit in this advancement in human knowledge of fundamental interactions ? Whose analysis is more credible ? Which of the two results has fewer blemishes ?
To me, it is always a matter of which one is the most relevant question. And to me, the most relevant question is, Who cares who did it ? ... with the easy-to-guess answer: not me. As I have had other occasions to say, I am for the advancement of Science, much less for the advancement of scientific careers, leave alone to which experiments those careers belong.
The top quark is interesting, but so far the Tevatron experiments had only studied it when produced in pairs with its antiparticle, through strong interactions. Electroweak production of the top quark is also possible in proton-antiproton collisions, at half the rate. It is one of those rare instances when the electroweak force competes with the strong one, and it is due to the large mass of the top quark: producing two is much more demanding than producing only one, due to the limited energy budget of the collisions. The reactions capable of producing a single top quark are described by the diagrams shown above. In a), a b-quark from one of the projectiles becomes a top by intervention of a weak vector boson; in b), a gluon “fuses” with a W boson and a top quark is created; in c), a W boson is produced off-mass-shell, and it possesses enough energy to decay into a top-bottom pair.
Since 1995, when CDF and DZERO published jointly the observation of the top quark, nobody has ever doubted that electroweak processes would produce single tops as well. Not even one article, to my knowledge, tried to speculate that the top might be so special to have no weak couplings. The very few early attempts at casting doubt on the real nature of what the Tevatron experiments were producing died quickly as statistics improved and the characterization of the newfound quark was furthered. So what is the fuss about finding out that the reaction resulting from the Feynman diagrams shown above can indeed be directly observed ?
There are different facets in a thorough answer to the above question. First of all, competition between CDF and DZERO: each collaboration badly wanted to get there first, especially since this was correctly predicted from the outset to be a tough nut to crack. Second, because seeing single top production implies having direct access to one element of the Cabibbo-Kobayashi-Maskawa mixing matrix, the element , which is after all a fundamental parameter in the standard model (well, to be precise it is a function of some of the latter, namely of the CKM matrix parameters, but let’s not split hairs here). Third, you cannot really see a low-mass Higgs at the Tevatron if you did not measure single top production first, because single top is a background in Higgs boson searches, and one cannot really discover something by assuming something else is there, if one has not proven that beforehand.
So, single top observation is important after all. I am a member of the CDF collaboration, and I am really proud I belong to it, so my judgement on the whole issue might be biased. But if I have to answer the question that gave the title to this post, I will first give you a very short summary of the results of the two analyses, deferring to a better day a more detailed discussion. This will allow me to drive home a few points.
The two analyses: a face-to-face summary
- Significance: both experiments claim that the signal they observe has a statistical significance of 5.0 standard deviations.
- CDF uses 3.2 inverse femtobarns, and finds a 5.0-sigma-significance signal of single top production. The sensitivity of the analysis is better measured by the expected significance, which is quoted at 5.9-sigma.
- DZERO uses 2.4 inverse femtobarns, and finds a 5.0-sigma-significance of single top production. The sensitivity of the DZERO analysis is quoted at 4.5-sigma.
- Cross-section: both experiments measure a cross section in agreement with standard model expectations.
- CDF measures , a relative uncertainty of about 24%.
- DZERO measures , a relative uncertainty of about 23%.
- Measurements of the CKM matrix element: both experiments quote a direct determination of that quantity, which is very close to 1.0 in the SM, but cannot exceed unity.
- CDF finds , a 12% accuracy.
- DZERO finds , a 11% accuracy.
- Data distributions: both experiments have a super-discriminant which combines the information from different searches. This is a graphical display of the power of the analysis, and should be examined with care.
1. CDF in its paper shows the distribution below, as well as the five inputs that were used to obtain it. The distribution shows the single-top contribution in red, stacked over the concurring backgrounds. At high values of the discriminant, the single top signal does stick out, and the black points -the data- follow the sum of all processes nicely.
2.DZERO in its paper has only the distribution shown below. I was underwhelmed when I saw it. Again, backgrounds are stacked one on top of the other, the top distribution is the one from single top (this time shown in blue), and the data is shown by black dots. It does not look like the data prefer the hypothesis of backgrounds+single top over the background-only one all that much!
Maybe I am too partisan to really make a credible point here, and since I did not follow in detail the development of these analyses -from their first publications as evidence for single top, to updates, until yesterday’s papers- I may very well be proven wrong; however, by looking at the two plots above, and by knowing that they both appear to provide a 5.0-sigma significance, I am drawn to the conclusion that DZERO believes their background shapes and normalization much better than CDF does!
Now, believing something is a good thing in almost all human activities except Science. And if two scientific collaborations have a very different way of looking at how well their backgrounds are modeled by Monte Carlo simulations (which, at least as far as the generation of subatomic processes is concerned, are -or can be- the same), which one is to praise more: the one which believes the simulations more to extract their signal, or the one which relies less on them?
The above question is rethorical, and you should have already agreed that you value more a result which is less based on simulations. So let us look into this issue a bit further. CDF bases its result on a total sample of 4780 events, where the total uncertainty is estimated at +-533 events. DZERO bases its own on a sample of 4651 events, with a total uncertainty estimated at +-234 events! What drives such a large difference in the precision of these predictions ?
The culprit is one of the backgrounds, the production of W bosons in association with heavy flavor quarks – an annoying process, which enters all selection of top quarks and Higgs bosons at the Tevatron. CDF has it at 1855 events, with an uncertainty of 486 -or 26.2%; it is shown in green in the CDF plot above. DZERO has it at 2646 events, with an uncertainty of 173, or 6.5%; it is also shown in green in the DZERO plot. Do not be distracted by the different size of the contribution of W+heavy flavor in the two datasets: different selection strategies drive the numbers to differ, and besides, it is rather the total number of events of the two analyses which is similar by pure chance. The point here is the uncertainty.
Luckily, the DZERO analysis does not appear to rely too much on the background normalization -this is not a simple counting experiment, where the better you know the size of expected backgrounds, the smaller your uncertainty on the signal; rather, the shapes of backgrounds are important, and the graphs above show that the data appears indeed well-described by the discriminant shape. And of course, background shapes are checked in control samples, so both experiments have many tools to ensure that the different contributions are well understood. However, the issue remains: how much do the different estimates of the W plus heavy flavor uncertainty impacts the significance of the measurements ? The DZERO paper mentions that one of their largest uncertainties arises from the modeling of the heavy flavor composition of W+jet events, but it does not provide further details.
I would be happy to receive an informed answer in the comments thread about the points I mention above…