The elusive single top November 7, 2006Posted by dorigo in news, physics, science.
CDF has recently blessed two new results on the search for the process called “single top production”, which is nothing else but the production of one top quark not accompanied by its antiparticle.
At the Tevatron proton-antiproton collider, top quarks do not always get produced in particle-antiparticle pairs, in fact. Pair production is the only way you can create top quarks through the strong interaction of quarks of gluons in the incoming projectiles, but weak interaction processes are also possible, and they may produce events with a single top quark together with a bottom quark, because weak interaction transmutates quarks of one kind into another, or produces odd pairs.
If you do not know the real difference between the two forces, it is just as easy as this: the weak interaction is responsible for radioactive decay and few other observable processes, and operates through the exchange of very massive particles, the vector bosons W and Z, sensitive to the “weak charge” of quarks and leptons. The strong interaction is instead the one that binds hadrons together, keeping quarks from manifesting themselves as free particles. It operates through the exchange of gluons, which are massless particles that carry themselves the strong color charge they are sensitive to.
Normally, weak interaction processes are way less frequent than strong ones when you collide hadrons together: this reflects the different “strength” of the two interactions. At theTevatron, for instance, you can produce an event with two jets of 40 GeV energy roughly every thousand collisions via the strong interaction, while less than once in a million collisions via the weak interaction.
However, when asked to produce two massive top quarks, even the strong force balks: a large portion of the energy of the incoming projectiles has to be spent to create the new matter, and that is a rare occurrence, since it requires that the quarks involved in the interaction be carrying a large fraction of the proton they live within. Therefore, a top quark pair is created only every ten billion times via the strong interaction.
Weak processes which yield one top quark and one bottom quark have it easier: they only have one massive body to produce. Therefore they are less suppressed for once, and as many as one every twenty billion times a single top quark is produced via weak interactions.
So, only a factor of two less. And we have hundreds of top quark pairs in our bag in CDF and D0 data. So surely, we have seen single top production too, haven’t we ? NO. Not so far.
The trouble is that, being less energetic, the weak process producing a single top quark is also less discernible from the backgrounds. Quite a bit less so, in fact. All searches for the elusive single top quark production have been insuccessful so far. But… We thought that one inverse femtobarn of available CDF data would do the trick. In fact, I predicted that by the end of 2006 we would have found that process, six months ago.
Not yet. In a careful analysis of “W+2jet events” – ones containing a lepton, missing energy originated by a neutrino, and two jets, at least one of which originated from b-quark hadronization, CDF can not measure the production of single top yet, and actually is in the awkward position of excluding its production according to the predictions of the Standard Model.
Nobody really believes that single top production is not there: it must be. It probably is just a unlucky downward fluctuation of our data. But still, it starts to be embarassing! Here you can see a plot of the event likelihood, constructed with all available kinematic information, for the set of events where the search has been conducted. On the right, the single top production processes (actually mostly the t-channel one in this particular plot, see below) should contribute sizeably (green and blue histograms), but they do not, and in fact we observe a deficit of events with respect to all concurrent background processes.
The last plot then shows that the most probable value of the cross section for single top production via “s-channel” and “t-channel” processes (two different mechanisms that should both yield a single top signal, which have been searched independently in the data) is very close to zero, and that the standard model prediction (the red point on the right) is actually excluded!
I have no doubt that more statistics will allow us to nail down the elusive single top production, but for today… More work is in order!