Baryons

Baryons are massive particles which are made up of three quarks in the standard model. This class of particles includes the proton and neutron. Other baryons are the lambda, sigma, xi, and omega particles. Baryons are distinct from mesons in that mesons are composed of only two quarks. Baryons and mesons are included in the overall class known as hadrons, the particles which interact by the strong force. Baryons are fermions, while the mesons are bosons. Besides charge and spin (1/2 for the baryons), two other quantum numbers are assigned to these particles: baryon number (B=1) and strangeness (S), which in the chart can be seen to be equal to -1 times the number of strange quarks included.
The conservation of baryon number is an important rule for interactions and decays of baryons. No known interactions violate conservation of baryon number.
Recent experimental evidence shows the existence of five-quark combinations which are being called pentaquarks. The pentaquark would be included in the classification of baryons, albeit an "exotic" one. The pentaquark is composed of four quarks and an antiquark, like a combination of an ordinary baryon plus a meson.

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pion

Particle	Symbol	Anti- particle	Makeup	Rest mass MeV/c2	S	C	B	Lifetime	Decay Modes
Pion	π+	π-	ud	139.6	0	0	0	2.60 x10-8	μ+νμ
Pion	π0	Self		135.0	0	0	0	0.83 x10-16	2γ

The neutral pion decays to an electron, positron, and gamma ray by the electromagnetic interaction on a time scale of about 10^-16 seconds. The positive and negative pions have longer lifetimes of about 2.6 x 10^-8 s.
The negative pion decays into a muon and a muon antineutrino as illustrated below. This decay is puzzling upon first examination because the decay into an electron plus an electron antineutrino yields much more energy. Usually the pathway with the greatest energy yield is the preferred pathway. This suggests that some symmetry is acting to inhibit the electron decay pathway.
The symmetry which suppresses the electron pathway is that of angular momentum, as described by Griffiths. Since the negative pion has spin zero, the electron and antineutrino must be emitted with opposite spins to preserve net zero spin. But the antineutrino is always right-handed, so this implies that the electron must be emitted with spin in the direction of its linear momentum (i.e., also right-handed). But if the electron were massless, it would (like the neutrino) only exist as a left-handed particle, and the electron pathway would be completely prohibited. So the suppression of the electron pathway is attributed to the fact that the electron's small mass greatly favors the left-handed symmetry, thus inhibiting the decay. Weak interaction theory predicts that the fraction of muons decaying into electrons should be 1.28 x 10^-4 and the measured branching ratio is 1.23 +/- 0.02 x 10^-4.
The pion, being the lightest meson, can be used to predict the maximum range of the strong interaction. The strong interaction properties of the three pions are identical. The connection between pions and the strong force was proposed by Hideki Yukawa. Yukawa worked out a potential for the force and predicted its mass based on the uncertainty principle from measurements of the apparent range of the strong force in nuclei.
Being composed of an up and an antidown quark, the positive pion would be expected to have a mass about 2/3 that of a proton, yet it's mass is only about 1/6 of that of the proton! This is an example of how hadron masses depend upon the dynamics inside the particle, and not just upon the quarks contained.

The pion is a meson. The π+ isconsidered to be made up of anup and an anti-down quark. The neutral pion is considered to be a combination

Pions interact with nuclei and transform a neutron to a proton or vice versa:
The pions π⁺ and π^- have spin zero and negative intrinsic parity (Rohlf Sec 17-2).

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Mesons

Mesons are intermediate mass particles which are made up of a quark-antiquark pair. Three quark combinations are called baryons. Mesons are bosons, while the baryons are fermions. Recent experimental evidence shows the existence of five-quark combinations which are being called pentaquarks.

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The Higgs Boson

artwork: CERN

When you get on the scale in the morning, you may be hoping that it registers a smaller number than the day before -- you may be hoping that you've lost weight. It's the quantity of mass in you, plus the force of gravity, that determines your weight. But what determines your mass?

That's one of the most-asked, most-hotly pursued questions in physics today. Many of the experiments circulating in the world's particle accelerators are looking into the mechanism that gives rise to mass. Scientists at CERN, as well as at Fermilab in Illinois, are hoping to find what they call the "Higgs boson." Higgs, they believe, is a particle, or set of particles, that might give others mass.

The idea of one particle giving another mass is a bit counter-intuitive... Isn't mass an inherent characteristic of matter? If not, how can one entity impart mass on all the others by simply floating by and interacting with them?

artwork: CERN Click on the image above for a helpful cartoon explanation of the Higgs Mechanism.

An oft-cited analogy describes it well: Imagine you're at a Hollywood party. The crowd is rather thick, and evenly distributed around the room, chatting. When the big star arrives, the people nearest the door gather around her. As she moves through the party, she attracts the people closest to her, and those she moves away from return to their other conversations. By gathering a fawning cluster of people around her, she's gained momentum, an indication of mass. She's harder to slow down than she would be without the crowd. Once she's stopped, it's harder to get her going again. This clustering effect is the Higgs mechanism, postulated by British physicist Peter Higgs in the 1960s. The theory hypothesizes that a sort of lattice, referred to as the Higgs field, fills the universe. This is something like an electromagnetic field, in that it affects the particles that move through it, but it is also related to the physics of solid materials. Scientists know that when an electron passes through a positively charged crystal lattice of atoms (a solid), the electron's mass can increase as much as 40 times. The same might be true in the Higgs field: a particle moving through it creates a little bit of distortion -- like the crowd around the star at the party -- and that lends mass to the particle.

photo: CERN Scientists at CERN use the enormous ALEPH detector in their search for the Higgs particle.

The question of mass has been an especially puzzling one, and has left the Higgs boson as the single missing piece of the Standard Model yet to be spotted. The Standard Model describes three of nature's four forces: electromagnetism and the strong and weak nuclear forces. Electromagnetism has been fairly well understood for many decades. Recently, physicists have learned much more about the strong force, which binds the elements of atomic nuclei together, and the weak force, which governs radioactivity and hydrogen fusion (which generates the sun's energy).

Electromagnetism describes how particles interact with photons, tiny packets of electromagnetic radiation. In a similar way, the weak force describes how two other entities, the W and Z particles, interact with electrons, quarks, neutrinos and others. There is one very important difference between these two interactions: photons have no mass, while the masses of W and Z are huge. In fact, they are some of the most massive particles known.
The first inclination is to assume that W and Z simply exist and interact with other elemental particles. But for mathematical reasons, the giant masses of W and Z raise inconsistencies in the Standard Model. To address this, physicists postulate that there must be at least one other particle -- the Higgs boson.

The simplest theories predict only one boson, but others say there might be several. In fact, the search for the Higgs particle(s) is some of the most exciting research happening, because it could lead to completely new discoveries in particle physics. Some theorists say it could bring to light entirely new types of strong interactions, and others believe research will reveal a new fundamental physical symmetry called "supersymmetry."

photo: CERN CERN scientists were unsure whether these events recorded by the ALEPH detector indicated the presence of a Higgs boson. Check out the links listed below for the latest information on the search for the Higgs Boson.

First, though, scientists want to determine whether the Higgs boson exists. The search has been on for over ten years, both at CERN's Large Electron Positron Collider (LEP) in Geneva and at Fermilab in Illinois. To look for the particle, researchers must smash other particles together at very high speeds. If the energy from that collision is high enough, it is converted into smaller bits of matter -- particles -- one of which could be a Higgs boson. The Higgs will only last for a small fraction of a second, and then decay into other particles. So in order to tell whether the Higgs appeared in the collision, researchers look for evidence of what it would have decayed into.

In August 2000, physicists working at CERN's LEP saw traces of particles that might fit the right pattern, but the evidence is still inconclusive. LEP was closed down in the beginning of November, 2000, but the search continues at Fermilab in Illinois, and will pick up again at CERN when the LHC (Large Hadron Collider) begins experiments in 2005.

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