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Measuring the strange sea (quarks) with silicon

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Posted December 19, 2014

Last DZero result at Fermilab began like so:

“The parts inside of a proton are called, in a not terribly imaginative terminology, partons. The partons that we tend to think of first and foremost are quarks — two up quarks and a down quark in each proton — but there are other kinds of partons as well.”

This time, we start in the same place — with those unimaginatively named partons. There are three types.

Sea quarks and gluons, the nonvalence parts of the proton, are continuously interacting with each other, as shown here. Gluons (the springy lines) can split into quark-antiquark pairs that nearly instantly merge again to reform gluons. But if another proton or antiproton collides with this constantly changing system, one of these sea quarks can be broken off and fly out of the proton with great energy. That sea quark might be a bottom flavored quark, or it might be a strange flavored quark that can be converted into a charm flavored quark by a W boson.

Sea quarks and gluons, the nonvalence parts of the proton, are continuously interacting with each other, as shown here. Gluons (the springy lines) can split into quark-antiquark pairs that nearly instantly merge again to reform gluons. But if another proton or antiproton collides with this constantly changing system, one of these sea quarks can be broken off and fly out of the proton with great energy. That sea quark might be a bottom flavored quark, or it might be a strange flavored quark that can be converted into a charm flavored quark by a W boson.

The first type comprises those alluded to above: quarks. The two up quarks and one down quark that make up protons are called valence quarks. They determine the electrical charge of the proton. There are six flavors of quark, and all the different combinations of three out of the six correspond to a particle of a specific type, called a baryon. (Well, almost. Top flavored quarks decay so quickly they never form a particle.)

The second type of parton is the gluon. Gluons hold the quarks inside the proton together and are the mediators of the strong nuclear force. Just as electromagnetic energy comes in point-like units called photons, so energy of the strong nuclear force comes in units of the gluon.

The third type of parton is the sea quark. A gluon can split into a quark-antiquark pair that exists for a fleetingly short time (10-24 seconds or less) before reforming back into a gluon.

Sea quarks can be of any flavor. They very often are up or down quarks, just like the valence quarks. But they can also be strange quarks, and strange quarks do not exist as valence quarks in protons. A reaction with a strange quark in the initial state lets you measure these strange sea quarks in proton collisions.

The reaction involves the collision of a strange sea quark from one proton (or antiproton) with a gluon from an antiproton (or proton) to produce a Wboson and a charm quark. The charm quark, when produced with a large momentum transverse to the direction of the initial collision, will produce a narrow spray of particles all moving in roughly the same direction. Such a particle spray is called a jet. Because the charm quark will travel a few millimeters before decaying, the fact that there was a charm quark producing the jet can be inferred using the silicon based microstrip tracking detector at the very center of the DZero detector.

Silicon technology also helps identify jets produced from bottom flavored quarks. In fact, bottom quark jets are easier to find than charm quark jets. Measuring the production of bottom quark jets in events with a W boson provides important information about the nonvalence partons — specifically, gluons — of the proton.

DZero has recently measured the production of both charm and bottom jets when a W boson is also produced. The new measurement uses more data than earlier analyses, and for the first time, we obtain information about the production (with a W) of charm and bottom jets that are produced with different momenta transverse to the collision axis. How the production varies with the transverse momentum is a valuable measurement tool to understand the various subprocesses at work. This is also the first measurement of charm-W production that relies upon the silicon microstrip tracking technology; previous measurements were based on less effective techniques.

Source: FNAL, written by Leo Bellantoni

 

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