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A Sbottom candidate event in CDF ? April 10, 2007

Posted by dorigo in news, personal, physics, science.

Back to work today, I browsed the slides of the winter conferences on high energy physics held a month ago in the italian Alps, and now available in the respective sites here , here , and here .

One of the talks I looked at first was that of my friend Song Ming Wang, who had presented “Searches for Supersymmetry at the Tevatron” at Moriond QCD. Of course, I pay particular attention to SUSY searches these days, having bet 1,000 US$ on the absence of any such things as supersymmetric particles. And while I usually know what is going on in CDF from the internal meetings, things do escape me now and then.

So today, halfway in Song Ming’s talk, I was startled to learn that CDF has caught a beautiful event which could in principle be a candidate for the production of the sbottom quark, the supersymmetric partner of the less exotic bottom quark we all know and love. A faint image of a $1000 bill floating away briefly materialized in front of my eyes, but I quickly brushed it off. Yes, you could believe that the event is due to sbottom pair production, unless you cynically guesstimated that the probability that the event is a “simple” production of a pair of Z bosons is umpthousand times higher.

So, before I show you the event and discuss it, you are well excused if you ask: What is a sbottom quark anyway, and how did CDF look for it ?

First of all, SUSY. Supersymmetry is a hypothesized symmetry of nature, whereby for each known subatomic particle (quarks, leptons, vector bosons, you name it) there is a super-partner, which has the same properties of the non-supersymmetric particle, but a different spin. Fermions have spin 1/2 (in units of the Planck constant) ? Well then, sfermions have spin 1. W bosons have spin 1 ? Good, then their Susy partners have spin 1/2.  

Things do not stop there, of course. The above symmetry has some useful features from a theoretical standpoint, on which I need not delve today. One thing to note, though, is that in order to believe in SUSY, having seen no clue of the sparticles, we need to accept that there is a mechanism at work that broke the mass degeneracy, making sparticle masses a lot higher than the mass of ordinary particles. This quite opportune mechanism hid them away from the sight of present-day experiments, that is. Higher masses, for sure. But just how much higher, we have no idea. So, searching for them does make sense.

Experimentally, one can search for a signature of the decay of supersymmetric particles produced by particle collisions such as protons hitting antiprotons at the Tevatron. On the decays of SUSY particles, theory does offer some predictions – sort of. For instance, we know that a sbottom quark would probably decay most of the times into a bottom quark and a neutralino – the lightest supersymmetric particle, which would behave just as a neutrino, leaving no trace in our detector. So, if we produced a pair of sbottom quarks, we would be likely to see a pair of bottom quark jets recoiling against nothing, but this nothing would carry away a lot of impulse – and for the law of conservation of impulse, we would know that that nothing was indeed quite something.

Confused ? Don’t be. Look at the diagram on the left: a pair of quarks q collides (lines join from the left), create a gluon of high virtuality (read: a angry, very energetic one, labeled g and shown as a horizontal dashed line), and the latter materializes in a pair of stop or sbottom quarks (t or b with a tilde sign above them), each of them decaying to a more regular quark c, b and a neutralino (labeled x) on the right.

By searching for the striking signature of a pair of b-jets recoiling against -well, quite nothing- CDF found among others the event shown below. Indeed, a very clean one!


In the picture you see a pair of jets – collimated energy deposits in the calorimeter (in blue and magenta), corresponding to streams of charged tracks (in red and green). Although you cannot tell from the picture, analysis shows that both jets contain a secondary vertex, something which tells us they originated from b-quark hadronization. And if you compute the transverse energy that is missing to balance the kinematics, you come up with needing a hefty 145 GeV (shown as the red arrow)!

So, could this be a first evidence of pair production of sbottom quarks ? Naah. For three reasons.

One, the total event yield of the CDF analysis is in good agreement with backgrounds: no excess is seen which one could attribute to the exotic sought process.

Two, the invariant mass of the two jets is 82 GeV. Now, that should not ring any bell, because 82 is far from 91 GeV, the mass of the Z boson. It should not, unless you realize that b-jets get undermeasured in a calorimeter, due to their leptonic content and other characteristics. Indeed, the most likely invariant mass of a pair of b-jets originated from Z boson decay and reconstructed by the CDF detector is about 85 GeV, with a largish spread (that, before we apply special corrections that were not used here). So, those two jets might just be the result of Z production. More than might, I would venture to say: they are quite likely to!

Ok, but what about the missing energy the two b-jets are recoiling against ? The easiest explanation is that it is due to a decay of a second Z boson in a pair of neutrinos.  Yes, neutrinos, not neutralinos. No need to invoke SUSY!

Three, this cannot be a sbottom pair production event, because, you see, if it were, I would lose my bet!

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