b-jets are cool

Today I am writing about jets for my proceedings paper. And b-quark-originated jets in particular. So I feel inspired to describe to anybody who has time to read what a b-jet is and why we care about it.

In proton-antiproton collisions of high energy such as those occurring at the Tevatron, inside the CDF and D0 detectors, what is actually happening is that a quark (or a gluon, the other constituent of the proton) inside the proton hits another quark (or gluon) inside the antiproton.

It is as if by smashing together very hard two garbage bags containing a lot of tissue paper and a few glass bottles one occasionally managed to get one bottle inside a bag hitting another bottle in the other bag. This is what we call a hard collision: one that does not just bounces the bags off each other, nor one that produces a lot of tissue paper all around, but one that yields a lot of broken glass.

Quarks do not actually break apart as glass bottles do. Rather, they scatter off each other. But, as they get kicked off the bag, they do not oblige. A quark cannot live outside of a proton – it does not like to travel naked. It will "dress up" by creating a hadron – actually, a whole stream of particles will be created, which retains memory of the energy and direction of the originating quark: this is what we call a JET.

(For a simple explanation of the process of jet creation I wrote something simple in last year's blog, at http://qd.typepad.com/6/2005/02/color_radiation.html). 

Quarks come in six species, but they are not equally common. u and d quarks (up and down) are what stuff is made of, while the more "exotic" c and s quarks (for charm and strange) are heavier and only produced in hard collisions, and the still fancier t and b quarks (top and bottom) are even rarer and heavier – the top quark, the heaviest of all, weighs more or less like a whole gold atom.

The bottom quark is "only" as heavy as five protons, but it is a very interesting one, because if one can identify jets produced by a b quark, one can then use them to select a wealth of interesting phenomena, such as the decay of a Higgs boson – a particle physicists have been hunting for thirty years, and have not yet found.

So how does one distinguish a b-quark-originated jet from a generic jet ? In the bag collision model, it is the equivalent of telling a bottle of Jim Beam from a bottle of Jack Daniels by examining the scattered glass. Not easy, but if you find a label in the debris, with "Jim Beam" written on it, you are done… Duh!

The label is what we call a tag: a b-tag is a measured feature of the jet which allows us to claim that it was indeed a b-quark what produced the jet. Since the particle where the b-quark is hiding after the jet creation lives long enough to travel for a few millimeters before breaking apart in yet more lighter particles, by reconstructing the trajectory of the particles included in the jets, and by observing whether some of them come from a common point inside the jet, we can tell whether a b-quark was present in the jet. And sure enough, if there was one in there, it was the originating one.

Here is a picture of tracks reconstructed in a jet, with some of them coming from the secondary vertex – which is our b-tag:

The interaction point is marked I.P.: it is the point where the proton and antiproton collided. From this collision, three separate streams of particles emerge (disregard the green arrow). Jet2, the stream of tracks pointing toward the right of the picture, contains two red tracks. They have been colored that way because their intersection lies far from the interaction point I.P.: the explanation is that they originated from the decay of a particle of long lifetime, which traveled 2.1 millimeters – from I.P. to the vertex of the two red tracks – before decaying.