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Pseudorapidity values shown on a polar plot. In particle physics, an angle of zero is usually along the beam axis, and thus particles with high pseudorapidity values are generally lost, escaping through the space in the detector along with the beam.

As polar angle approaches zero, pseudorapidity tends towards infinity.

In experimental particle physics, pseudorapidity, η{displaystyle eta }, is a commonly used spatial coordinate describing the angle of a particle relative to the beam axis. It is defined as

η≡−ln⁡[tan⁡(θ2)],{displaystyle eta equiv -ln left[tan left({frac {theta }{2}}right)right],}

where θ{displaystyle theta } is the angle between the particle three-momentum p{displaystyle mathbf {p} } and the positive direction of the beam axis.^[1] Inversely,

θ=2arctan⁡(e−η).{displaystyle theta =2arctan left(e^{-eta }right).}

As a function of three-momentum p{displaystyle mathbf {p} }, pseudorapidity can be written as

η=12ln⁡(|p|+pL|p|−pL)=artanh⁡(pL|p|),{displaystyle eta ={frac {1}{2}}ln left({frac {left|mathbf {p} right|+p_{text{L}}}{left|mathbf {p} right|-p_{text{L}}}}right)=operatorname {artanh} left({frac {p_{L}}{left|mathbf {p} right|}}right),}

where pL{displaystyle p_{text{L}}} is the component of the momentum along the beam axis (i.e. the longitudinal momentum – using the conventional system of coordinates for hadron collider physics, this is also commonly denoted pz{displaystyle p_{z}}). In the limit where the particle is travelling close to the speed of light, or equivalently in the approximation that the mass of the particle is negligible, one can make the substitution m≪|p|⇒E≈|p|⇒η≈y{displaystyle mll |mathbf {p} |Rightarrow Eapprox |mathbf {p} |Rightarrow eta approx y} (i.e. in this limit, the particle's only energy is its momentum-energy, similar to the case of the photon), and hence the pseudorapidity converges to the definition of rapidity used in experimental particle physics:

y≡12ln⁡(E+pLE−pL){displaystyle yequiv {frac {1}{2}}ln left({frac {E+p_{text{L}}}{E-p_{text{L}}}}right)}

This differs slightly from the definition of rapidity in special relativity, which uses |p|{displaystyle left|mathbf {p} right|} instead of pL{displaystyle p_{text{L}}}. However, pseudorapidity depends only on the polar angle of the particle's trajectory, and not on the energy of the particle. One speaks of the "forward" direction in a hadron collider experiment, which refers to regions of the detector that are close to the beam axis, at high |η|{displaystyle |eta |}; in contexts where the distinction between "forward" and "backward" is relevant, the former refers to the positive z-direction and the latter to the negative z-direction.

In hadron collider physics, the rapidity (or pseudorapidity) is preferred over the polar angle θ{displaystyle theta } because, loosely speaking, particle production is constant as a function of rapidity, and because differences in rapidity are Lorentz invariant under boosts along the longitudinal axis: they transform additively, similar to velocities in Galilean relativity. A measurement of a rapidity difference Δy{displaystyle Delta y} between particles (or Δη{displaystyle Delta eta } if the particles involved are massless) is hence not dependent on the longitudinal boost of the reference frame (such as the laboratory frame). This is an important feature for hadron collider physics, where the colliding partons carry different longitudinal momentum fractions x, which means that the rest frames of the parton-parton collisions will have different longitudinal boosts.

The rapidity as a function of pseudorapidity is given by

y=ln⁡(m2+pT2cosh2⁡η+pTsinh⁡ηm2+pT2),{displaystyle y=ln left({frac {{sqrt {m^{2}+p_{text{T}}^{2}cosh ^{2}eta }}+p_{text{T}}sinh eta }{sqrt {m^{2}+p_{text{T}}^{2}}}}right),}

where pT≡px2+py2{displaystyle p_{text{T}}equiv {sqrt {p_{text{x}}^{2}+p_{text{y}}^{2}}}} is the transverse momentum (i.e. the component of the three-momentum perpendicular to the beam axis).

Using a second order Maclaurin expansion of y{displaystyle y} expressed in m/pT{displaystyle m/p_{text{T}}} one can approximate rapidity by

y≈η−pL2|p|(mpT)2=η−tanh⁡η2(mpT)2=η−cos⁡θ2(mpT)2,{displaystyle yapprox eta -{frac {p_{text{L}}}{2|mathbf {p} |}}left({frac {m}{p_{text{T}}}}right)^{2}=eta -{frac {tanh {eta }}{2}}left({frac {m}{p_{text{T}}}}right)^{2}=eta -{frac {cos {theta }}{2}}left({frac {m}{p_{text{T}}}}right)^{2},}

which makes it easy to see that for relativistic particles with pT≫m{displaystyle p_{text{T}}gg m}, pseudorapidity becomes equal to (true) rapidity.

Rapidity is used to define a measure of angular separation between particles commonly used in particle physics ΔR≡(Δy)2+(Δϕ)2{displaystyle Delta Requiv {sqrt {left(Delta yright)^{2}+left(Delta phi right)^{2}}}}, which is Lorentz invariant under a boost along the longitudinal (beam) direction. Often, the rapidity term in this expression is replaced by pseudorapidity, yielding a definition with purely angular quantities: ΔR≡(Δη)2+(Δϕ)2{displaystyle Delta Requiv {sqrt {left(Delta eta right)^{2}+left(Delta phi right)^{2}}}}, which is Lorentz invariant if the involved particles are massless. The difference in azimuthal angle, Δϕ{displaystyle Delta phi }, is invariant under Lorentz boosts along the beam line (z-axis) because it is measured in a plane (i.e. the "transverse" x-y plane) orthogonal to the beam line.

Values

A plot of polar angle vs. pseudorapidity.

Here are some representative values:

θ{displaystyle theta }	η{displaystyle eta }	θ{displaystyle theta }	η{displaystyle eta }
0°	∞	180°	−∞
0.1°	7.04	179.9°	−7.04
0.5°	5.43	179.5°	−5.43
1°	4.74	179°	−4.74
2°	4.05	178°	−4.05
5°	3.13	175°	−3.13
10°	2.44	170°	−2.44
20°	1.74	160°	−1.74
30°	1.32	150°	−1.32
45°	0.88	135°	−0.88
60°	0.55	120°	−0.55
80°	0.175	100°	−0.175
90°	0

Pseudorapidity is odd about θ=90{displaystyle theta =90} degrees. In other words, η(θ)=−η(180∘−θ){displaystyle eta (theta )=-eta (180^{circ }-theta )}.

Conversion to Cartesian momenta

Hadron colliders measure physical momenta in terms of transverse momentum pT{displaystyle p_{text{T}}}, polar angle in the transverse plane ϕ{displaystyle phi } and pseudorapidity η{displaystyle eta }. To obtain cartesian momenta (px,py,pz){displaystyle (p_{x},p_{y},p_{z})} (with the z{displaystyle z}-axis defined as the beam axis), the following conversions are used:

px=pTcos⁡ϕ{displaystyle p_{text{x}}=p_{text{T}}cos phi }

py=pTsin⁡ϕ{displaystyle p_{text{y}}=p_{text{T}}sin phi }

pz=pTsinh⁡η{displaystyle p_{text{z}}=p_{text{T}}sinh {eta }}.

Therefore, |p|=pTcosh⁡η{displaystyle |p|=p_{text{T}}cosh {eta }}.

References

• V. Chiochia (2010) Accelerators and Particle Detectors from University of Zurich