Basic Collision Experimental
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JuanCastillo  25 Mar 2008
Black Event: Event without zero suppression or any online data preprocessing.
Collision energy: Referred as
in a center of mass reference frame (Heavy Ion Collisions), or energy per nucleon in fixed target experiments. Some ranges:
 AGSBNL: 14.5 AGeV. Berkeley National Lab Alternating Gradient Synchrotron.
 SPSCERN: 200 AGeV. Super Proton Synchrotron @ CERN
 RHIC: = 200 GeV. Relativistic Heavy Ion Collider @ BNL.
CP violation: Observed during K meson disintegration.
Dalitz decay: 3body decay, dielectron source. In general:
.
It generates a continuum, in opposition with the twobody meson decay,
that generates 2 peaks.
EX:
Dalitz decay branch (1.2%)
More info about
decay you find at the
NIKHEF page
Dalitz plot: Technique to represent all essential kinematical variables, of any threebody final state in a scatter plot or twodimensional histogram. For a example with masses and the
decay check
COSY11 web. In general,you could do that if you have relation of the type
.
DrellYan (DY) process: A quark and an antiquark are annihilated creating a photon and the photon decays into a leptop pair. For proton proton deep inelastic scattering it can be written like:
Being X an hadron. For two quarks, in general:
Here you find
the DY cross section
Elliptic Flow: Nucleusnucleus collisions produce an elliptic region of quarkgluon medium. More energetic hadrons squirt out within the plane of interaction than at perpendicular angles, that is, there is a flow in the plane of interaction. We define the
elliptic flow streght as the second coefficient of the Fourier expansion of the Lorentz invariant particle distribution (
). For a thermal Boltzmann source model:
Infrarred Limit: When a parameter is tending to zero.
For
=
, when
,
(infrared stability)
Invariant Mass: Measurement or calculation of the mass that is the
same in all reference frames. Here W is the invariant mass of the system of particles, equal to the mass of the decay particle:
More details
here
Jet: Intensity in a cone of radius R=0.7 in pseudorapidityazimuthal angle space (
cone). If the cone axis is (
), a particle i with momentum p is in the jet if:
.
The
transverse enegy of the jet is thus definend as:
Jet Quenching:.
pp hard scattering produces back to back particle jets, but in AA (nucleusnucleous) collisions, we have a dense quarkgluon medium and the jets are quenched like a shot in the water, and only one usually survives.
Light Cone Variables: Considering the reaction
Using the notation for the 4momentum of a particle
, we define:
 : forward light cone momentum
 : backward light cone momentum
 : forward light cone variable (Lorentz invariant)
 : backward light cone variable (Lorentz invariant)
All of them can be expressed as functions of the rapidity or pseudorapidity, or viceversa.
Minimum bias (MB) events: Two definitions:
 Experimental: nonsingle diffractive (NSD) inelastice interactions. Usually low pt parton scattering events.
 Theoretical: nondiffractive inelastic interactions.
Both definitions differ on around 15% at current collider energies. From the
Glasgow university site
Multiplicity: Total number N of particles produced in a collision. Since most of the detection methods are only sensitive to charged particles, we speak about
charged multiplicity. After Thome, it can be parametrized like:
It is common to speak about
rapidity distribution of the produced charged particles (charged multiplicity), expressed as
where
,
are functions of the rapidity. Since the rapidity is dependent of 2 kinematic variables and the pseudorapidity of only one (angle with the beam axis) and both are equivalent for large momentums, it is more frequently used the
pseudrapidity distribution of the produced charged particles.
From the pseudorapidity distibution, using its dependence with the momentum, we obtain the
transverse momentum distribution. See also rapidity, pseudorapidity and transverse momentum distribution.
Protonproton cross section: It is defined depending of the channel. Schematically it can be written like:
Where
are PDF for the gluon and a quark (depending on "Bjorken x"), and
the fourmomenta of the colliding protons. The sum is over the quarks. It depends on the cross section of the quarkgluon scattering (for example, one term is q= u, therefore of
)
Pseudorapidity: From Special relativity ( that is, p = (E,
p ) = (p
_{0},
p) is a 4vector)
 : angle with beam axis.
 : particle momentum in beam direction (logitudinal).
 : particle momentum (3D).
When the particle is travelling close to the speed of light it is numerically close to the rapidity (
and
)
Some values:

0 
5 
10 
20 
30 
45 
60 
80 
90 

infinite 
3.13 
2.44 
1.74 
1.31 
0.88 
0.55 
0.175 
0 
Rapidity: From Special relativity ( that is, p = (E,
p ) = (p
_{0},
p) is a 4vector)
 = : particle momentum in beam direction (logitudinal).
 E = , energy of the particle.
Used because:
 Particle production is a constant as a function of rapidity( o pseudorapidity).
 Difference in rapidity of two particles is independent of Lorentz boosts along the beam axis ( )
 Rapidity distribution is Gaussian.
Taking antilogarithms and from the definition of
cosh, its relation with the enery of the particle is:
with
the transverse mass of the particle.
Rapidity gap: Diffractive events are characterized by the absence of significant hadronic particle activity over a large region of rapidity or pseudorapidity. This empty region is called a rapidity gap and can be used as an experimental signature for diffraction.
Scattering length: Scattering amplitude at zero momentum. If scattering amplitude T(q) = a + bq + cq
^{3} + ... , then scattering length T(0) = a. In a square well, it is equivalent to the radius of the square well, and the cross section
Shadowing: the modification of the free nucleon parton density in the nucleus. At the lowmomentum fractions, x, observed by collisions at the LHC, shadowing results in a decrease of the multiplicity.
Soft Physics: Physics associated with soft particles in the final state (
Thermal Boltzmann Source: Using the relation rapidityenergy, from the Boltzmann distribution we reach:
Or in the transversal direction:
Where
V0 vertex: Historically, shape of the verted created after a K0 decay onto a
and
. Usually, weak decay of neutral particles, generating 2 charged tracks.
W boson: W is from
weak nuclear force. W
^{−} charge 1, W
^{+} charge +1. Lifetime: 3 x 10
^{−25} s. Rest mass : 80.4 GeV ( 100 proton mass). Spin 1 (as a boson)
Weak force: Changes the flavior of a quark. Present in beta decay. n> p + e
^{} +
Since n=udd, p=uud, we saw d > u + W
^{−}.
Weinberg angle: .Related with symmetry spontaneous violation. g = coupling constant
Z boson: AKA Z
^{0}. Like a W boson but with charge 0. Rest mass: 91.2 GeV.