## particle physics glossary

**meson:** a particle composed of a quark and an antiquark.

**rapidity**: y = arctanh(v) = (1/2)ln((p_0+p_z)/(p0-pz))

here v is in units of c (speed of light), tanh(v) = sinh(v)/cosh(v), p_0 =
E = m_t*cosh(y), p_z = momentum in beam direction (z) = m_t*sinh(y)

**pseudo-rapidity**: eta = -ln(tan(theta/2)), with theta being the
angle between the produced particle's track and the beam
axis.

**y and eta relation**: sinh(eta) = (m_t/p_t)*sinh(y)

**asymptotic freedom**: in QCD, the effective strong coupling constant becomes smaller at shorter distances. quarks can freely move around in a small region (~ 1 fm cube), consisting of colorless groups (mesons or baryons).

**away-side jet suppression**: jets are fast hadrons produced from a collision. consider a situation when two back-to-back jets are produced in transverse directions. if the vertex is near the surface of the collision, the jet pointing out should be able to escape. but there partner jet will be absorbed inside the medium. the disappearance of away-side jets signals the existence of dense, colored medium (possibly a QGP).

**avalanche**: when a high-energy electron creates electron-ion pairs near the anode wire. the number of electron-ion pairs is proportional to the energy lost by the traversing particle, allowing the dE/dx measurement for particle identification.

**baryon:** a particle composed of three quarks.

**calorimeter**: absorption detector. also called shower detector because
showers are generated. interaction energy turns into heat. heat is
measured not in terms of temperature but the degrees of excitation and
ionization. first layers are the electromagnetic calorimeter, which
detect electrons and photons. the showers are short and compact. the
second set of layers are the hadronic calorimeter. it's used to
absorb the incident hadrons. hadronic showers have a wideer shape.

**cerenkov counter**: light slows down in dielectric medium (a poor electricity conductor but an efficient supporter of electrostatic fields) by a factor of n, v = c/n. n is called the index of refaction. when velocity of a charged particle exceeds the light velocity in such medium, excited atoms become polarized and coherently emit radiation at a characteristic angle theta (radiation direction vs particle direction). cos(theta) = 1/(beta n) (the faster the particle, the narrower the cone). used in combination with momentum tracker, one can find the particle's mass.

**scintillator**: charged particle pass through medium. atoms get excited. when they fall back to ground states, photons are emitted. used in combination with photomultipliers, scintillator can detect particles from the current pulses.

**radiation length**: let m be the number of radiation lengths (r). if one particle enters the medium, there will be 2^m particles after distance m*r. the average energy will be the original energy divided by the number of shower particles.

**bubble chamber**: liquid is heated by high speed particle, bubbles are produced.

**proportional chamber**: charged particles ionize gas inside electric field. free electrons and positive ions drift towards electrodes. if the field is very strong, the electrons can cause secondary ionization, and the signal is amplified.

**drift chamber**: multiwire chamber which measures times electrons take to drift toward the anode wire

**trigger**: the process of picking out interesting events to save. there are many different triggers at STAR, based on what one wants to study, such as ultra-peripheral or minimum-bias events

**chemical potential (mu)**: the energy cost that the reservoir has to pay to put a particle into the system. equals to dE/dN (partial derivative). intensive variable (doesn't change with system size).

**Fermi energy (Ef)**: at absolute zero, the probability of finding a fermion is 1 for E < Ef and 0 for E > Ef.

**boost invariance**: phenomenon when the multiplicites (dN/dy) is
unchanged in rapidity space. (boost = a change in rapidity).
physics is invariant in the plateau region.

**stopping** refers to the degree of converting longitudinal (forward rapidity) energy into transverse degrees of freedom. the opposite of stopping is transparency, which occurs in highly relativistic collisions, where particles pass through each other.

**transverse mass**: m_t=sqrt(m_0^2+p_t^2)

m_0 is the rest mass, p_t is the momentum transverse direction

**thermalization**: multiple scattering of particles that cause the
system to be in thermal equilibrium, where the energy of particles are
distributed according to Boltzmann-like statistics, with a common
temperature T.

**color glass condensate**: at very high energies, the number of
partons (particularly gluons) in a nucleus grows rapidly and become
saturated. "color" is used because gluons have colors.

**Cronin effect**: enhancement of high pT hadrons due to multiple
scattering inside nuclear matter.

**parton saturation model**: at small x the parton (q, qbar,
g) densities increase rapidly. at some point, parton-parton screenings
stop the increase of cross sections.

**excitation function**: energy dependence of any observable

**Bjorken x**: momentum (total) fraction carried by a struck
parton

**Feynman x**: momentum (longitudinal) fraction carried by a struck
parton

**Poisson distribution**: probability of *a* occurences in a given
time interval is: *P(n = a) = (exp(-mean)*mean^a)/(a!)*, where
*mean* = average number of occurence in the specified
interval.