The Sigma_b discovery plots October 20, 2006Posted by dorigo in news, physics, science.
Ok, I have been careless lately… I failed to report on the new arrival in CDF. I will try to give a short report on the new discovery of Sigma_b and Sigma_b* baryons just minutes before Petar’s seminar starts…
With a little more than one inverse femtobarn statistics of proton-antiproton collisions, and the SVT (silicon vertex tracker) – a really wonderful trigger which can measure in a few microseconds the impact parameter of tracks with an accuracy of a few tens of microns – finding new b-quark hadrons is CDF business!
The analysis that brought to the discovery of these new baryons is based entirely on the precise reconstruction of charged particles as they traverse the silicon detector and the central tracking chamber, which constitute the core of CDF.
As charged particles traverse matter (silicon, or an appropriate mixture of gases) they ionize it, and the electron-ion pairs can be collected in a high electric field, yielding a tiny electronic signal which identifies the position of the ionization deposit. Through the processing of the hundreds of thousands of signals, the tracks can be reconstructed, and their momentum be measured given the bending of the track in the strong magnetic field inside which the tracking detectors are located.
Once charged tracks are precisely reconstructed, their point of origin can be identified, and a hypothesis on their common origin can be made. If the hypothesis is correct, one will be able to reconstruct the total mass of the originating body.
Sigma_b baryons are particles composed of three quarks, two of which are a pair of light quarks of the same kind (two +2/3 charge u quarks, or two -1/3 charge d quarks), while the third is a -1/3 charge b-quark. The (uub) composition gives the Sigma_b(+) baryon, a particle of positive charge, and the (ddb) composition is the Sigma_b(-). They decay via the strong interaction by emitting a charged pion (the positive pion, (u anti-d), or the negative pion, (anti-u d), and thus transmuting into a Lambda_b baryon, which has the (udb) quark composition.
Lambda_b baryons are easier to reconstruct and they are well known. Since they decay via the weak interaction, they live longer – and their mass peak is easier to reconstruct. The b-quark inside the Lambda_b disintegrates by creating a c-quark, so that the Lambda_b becomes a Lambda_c. In the process, the baryon emits another charged pion.
Then the Lambda_c also decays by weak interaction, when the charm quark disintegrates into a strange quark. There are several final states available, and CDF reconstructs effectively the one in which three charged tracks are emitted: a proton, a charged kaon, and a charged pion.
To summarize, Sigma_b baryons have been reconstructed through the decay to Lambda_b baryons and a charged pion. The Lambda_b baryon in turn decays to a Lambda_c baryon and a pion, and the Lambda_c can finally yield a proton (the final, lowest mass, stable baryon!), a kaon, and a third pion. All in all, the full reconstruction of the decay chain involves the successive reconstruction of three resonances, or four in the case of the excited state Sigma_b*, whose decay produces a further charged pion and yields the Sigma_b.
The full reconstruction of these states allows a perfect understanding of a long cascade of disintegrations, something that is not very common in present day particle physics. You can see the signal of Sigma_b and Sigma_b* baryons as two successive bumps in the otherwise smooth background distribution in the plots below, which represent the two oppositely charged signals of positive and negative Sigma_b particles.
What is plotted in the abscissa is not directly the mass of the particle, but rather the mass difference between the combined mass of Lambda_b and pion and the sum of pion and Lambda_b masses – something that is called Q-value of the decay, and which is constant if the Lambda_b and pion were the product of a resonance decay.
The measured mass of these new bodies are:
M[Sigma_b(+)] = 5816 MeV
M[Sigma_b(-)] = 5808 MeV
M[Sigma_b*(+)] = 5837 MeV
M[Sigma_b*(-)] = 5829 MeV
all with 2-3 MeV uncertainties.
These new bodies provide yet another small ingredient to the wonderful picture of the organization of quarks into hadrons: objects bound by the strong force, but obliging to the rules of weak, strong, and electromagnetic interactions, which dictate that heavy bodies successively disintegrate into lighter ones, until stable matter is achieved: protons, electrons, photons, neutrinos.