Experiments have revealed a large number of hadrons, of which only the proton appears to be stable. Indeed, even if the proton is not absolutely stable, experiments show that its lifetime is at least in excess of 5.9 × 10³³ years. In contrast, a single neutron, free from the forces at work within the nucleus, lives an average of nearly 15 minutes before decaying. Within a nucleus, however—even the simple nucleus of deuterium, which consists of one proton and one neutron—the balance of forces is sufficient to prolong the neutron’s lifetime so that many nuclei are stable and a large variety of chemical elements exist.

Some hadrons typically exist only 10⁻¹⁰ to 10⁻⁸ second. Fortunately for experimentalists, these particles are usually born in such high-energy collisions that they are moving at velocities close to the speed of light. Their timescale is therefore “stretched” or “slowed down” so that, in the high-speed particle’s frame of reference, its lifetime may be 10⁻¹⁰ second, but, in a stationary observer’s frame of reference, the particle lives much longer. This effect, known as time dilation in the theory of special relativity, allows stationary particle detectors to record the tracks left by these short-lived particles. These hadrons, which number about a dozen, are usually referred to as “stable” to distinguish them from still shorter-lived hadrons with lifetimes typically in the region of a mere 10⁻²³ second.

The stable hadrons usually decay via the weak force. In some cases they decay by the electromagnetic force, which results in somewhat shorter lifetimes because the electromagnetic force is stronger than the weak force. The very-short-lived hadrons, however, which number 200 or more, decay via the strong force. This force is so strong that it allows the particles to live only for about the time it takes light to cross the particle; the particles decay almost as soon as they are created.

These very-short-lived particles are called “resonant” because they are observed as a resonance phenomenon; they are too short-lived to be observed in any other way. Resonance occurs when a system absorbs more energy than usual because the energy is being supplied at the system’s own natural frequency. For example, soldiers break step when they cross a bridge because their rhythmic marching could make the bridge resonate—set it vibrating at its own natural frequency—so that it absorbs enough energy to cause damage. Subatomic-particle resonances occur when the net energy of colliding particles is just sufficient to create the rest mass of the new particle, which the strong force then breaks apart within 10⁻²³ second. The absorption of energy, or its subsequent emission in the form of particles as the resonance decays, is revealed as the energy of the colliding particles is varied.

Baryons and mesons

The hadrons, whether stable or resonant, fall into two classes: baryons and mesons. Originally the names referred to the relative masses of the two groups of particles. The baryons (from the Greek word for “heavy”) included the proton and heavier particles; the mesons (from the Greek word for “between”) were particles with masses between those of the electron and the proton. Now, however, the name baryon refers to any particle built from three quarks, such as the proton and the neutron. Mesons, on the other hand, are particles built from a quark combined with an antiquark. As described in the section The strong force, these are the only two combinations of quarks and antiquarks that the strong force apparently allows.

The two groups of hadrons are also distinguished from one another in terms of a property called baryon number. The baryons are characterized by a baryon number, B, of 1; antibaryons have a baryon number of −1; and the baryon number of the mesons, leptons, and messenger particles is 0. Baryon numbers are additive; thus, an atom containing one proton and one neutron (each with a baryon number of 1) has a baryon number of 2. Quarks therefore must have a baryon number of 1/3, and the antiquarks a baryon number of −1/3, in order to give the correct values of 1 or 0 when they combine to form baryons and mesons.

The empirical law of baryon conservation states that in any reaction the total number of baryons must remain constant. If any baryons are created, then so must be an equal number of antibaryons, which in principle negate the baryons. Conservation of baryon number explains the apparent stability of the proton. The proton does not decay into lighter positive particles, such as the positron or the mesons, because those particles have a baryon number of 0. Neutrons and other heavy baryons can decay into the lighter protons, however, because the total number of baryons present does not change.

At a more-detailed level, baryons and mesons are differentiated from one another in terms of their spin. The basic quarks and antiquarks have a spin of 1/2 (which may be oriented in either of two directions). When three quarks combine to form a baryon, their spins can add up to only half-integer values. In contrast, when quarks and antiquarks combine to form mesons, their spins always add up to integer values. As a result, baryons are classified as fermions within the Standard Model of particle physics, whereas mesons are classified as bosons.