43. Kinematics 1 43. KINEMATICS Revised January 2000 by J.D. Jackson (LBNL) and June 2008 by D.R. Tovey (Sheffield). Throughout this section units are used in which ~ = c = 1. The following conversions are useful: ~c = 197.3 MeV fm, (~c)2 = 0.3894 (GeV)2 mb. 43.1. Lorentz transformations The energy E and 3-momentum p of a particle of mass m form a 4-vector p =(E, p) whose square p2 E2 p 2 = m2. The velocity of the particle is β = p/E. The energy ≡ ∗ ∗−| | and momentum (E , p ) viewed from a frame moving with velocity βf are given by ∗ E γf γf βf E ∗ ∗ = − , p = p , (43.1) µ p ¶ µ γ β γ ¶ µ p ¶ T T k − f f f k where γ = (1 β2)−1/2 and p (p ) are the components of p perpendicular (parallel) to f − f T k βf . Other 4-vectors, such as the space-time coordinates of events, of course transform in the same way. The scalar product of two 4-momenta p1 p2 = E1E2 p1 p2 is invariant (frame independent). · − · 43.2. Center-of-mass energy and momentum In the collision of two particles of masses m1 and m2 the total center-of-mass energy can be expressed in the Lorentz-invariant form 1/2 2 2 Ecm = (E1 + E2) (p1 + p2) , h − i 1/2 2 2 = m1 + m2 +2E1E2(1 β1β2 cos θ) , (43.2) h − i where θ is the angle between the particles. In the frame where one particle (of mass m2) is at rest (lab frame), 2 2 1/2 Ecm =(m1 + m2 +2E1 lab m2) . (43.3) The velocity of the center-of-mass in the lab frame is βcm = plab/(E1 lab + m2) , (43.4) where p p and lab ≡ 1 lab γcm =(E1 lab + m2)/Ecm . (43.5) The c.m. momenta of particles 1 and 2 are of magnitude m2 pcm = plab . (43.6) Ecm For example, if a 0.80 GeV/c kaon beam is incident on a proton target, the center of mass energy is 1.699 GeV and the center of mass momentum of either particle is 0.442 GeV/c. It is also useful to note that Ecm dEcm = m2 dE1 lab = m2 β1 lab dplab . (43.7) J. Beringer et al.(PDG), PR D86, 010001 (2012) (http://pdg.lbl.gov) June 18, 2012 16:20 2 43. Kinematics 43.3. Lorentz-invariant amplitudes The matrix elements for a scattering or decay process are written in terms of an invariant amplitude iM . As an example, the S-matrix for 2 2 scattering is related to M by − → p′ p′ S p p = I i(2π)4 δ4(p + p p′ p′ ) h 1 2 | | 1 2i − 1 2 − 1 − 2 M (p , p ; p′ , p′ ) 1 2 1 2 . (43.8) 1/2 1/2 ′ 1/2 ′ 1/2 × (2E1) (2E2) (2E1) (2E2) The state normalization is such that p′ p = (2π)3δ3(p p′) . (43.9) h | i − 43.4. Particle decays The partial decay rate of a particle of mass M into n bodies in its rest frame is given in terms of the Lorentz-invariant matrix element M by 4 (2π) 2 dΓ = M dΦn (P ; p ,...,pn), (43.10) 2M | | 1 where dΦn is an element of n-body phase space given by n n 3 4 d pi dΦn(P ; p ,...,pn) = δ (P p ) . (43.11) 1 − i (2π)32E Xi=1 iY=1 i This phase space can be generated recursively, viz. dΦn(P ; p1,...,pn) = dΦj(q; p1,...,pj) 3 2 dΦ (P ; q, p ,...,pn)(2π) dq , (43.12) × n−j+1 j+1 2 2 j 2 j where q =( i=1 Ei) i=1 pi . This form is particularly useful in the case where a − ¯ ¯ particle decaysP into another¯P particle¯ that subsequently decays. ¯ ¯ 43.4.1. Survival probability : If a particle of mass M has mean proper lifetime τ (= 1/Γ) and has momentum (E, p), then the probability that it lives for a time t0 or greater before decaying is given by −t0 Γ/γ −Mt0 Γ/E P (t0) = e = e , (43.13) and the probability that it travels a distance x0 or greater is −Mx0 Γ/|p| P (x0) = e . (43.14) June 18, 2012 16:20 43. Kinematics 3 p1, m1 P, M p2, m2 Figure 43.1: Definitions of variables for two-body decays. 43.4.2. Two-body decays : In the rest frame of a particle of mass M, decaying into 2 particles labeled 1 and 2, M 2 m2 + m2 E = − 2 1 , (43.15) 1 2M p = p | 1| | 2| 2 2 2 2 1/2 M (m1 + m2) M (m1 m2) = − − − , (43.16) £¡ ¢2 ¡M ¢¤ and 1 p dΓ = M 2 | 1| dΩ , (43.17) 32π2 | | M 2 where dΩ = dφ1d(cos θ1) is the solid angle of particle 1. The invariant mass M can be determined from the energies and momenta using Eq. (43.2) with M = Ecm. 43.4.3. Three-body decays : p1, m1 P, M p2, m2 p3, m3 Figure 43.2: Definitions of variables for three-body decays. 2 2 2 2 2 2 2 2 2 Defining pij = pi + pj and mij = pij, then m12 + m23 + m13 = M + m1 + m2 + m3 2 2 2 2 and m12 = (P p3) = M + m3 2ME3, where E3 is the energy of particle 3 in the rest frame of M−. In that frame, the− momenta of the three decay particles lie in a plane. The relative orientation of these three momenta is fixed if their energies are known. The momenta can therefore be specified in space by giving three Euler angles (α, β, γ) that specify the orientation of the final system relative to the initial particle [1]. Then 1 1 dΓ = M 2 dE dE dα d(cos β) dγ . (43.18) (2π)5 16M | | 1 2 June 18, 2012 16:20 4 43. Kinematics Alternatively 1 1 dΓ = M 2 p∗ p dm dΩ∗ dΩ , (43.19) (2π)5 16M 2 | | | 1|| 3| 12 1 3 ∗ ∗ where ( p1 , Ω1) is the momentum of particle 1 in the rest frame of 1 and 2, and Ω3 is the angle of| particle| 3 in the rest frame of the decaying particle. p∗ and p are given by | 1| | 3| 2 2 2 2 1/2 ∗ m12 (m1 + m2) m12 (m1 m2) p1 = − − − , (43.20a) | | £¡ 2m¢ ¡12 ¢¤ and 2 2 2 2 1/2 M (m12 + m3) M (m12 m3) p = − − − . (43.20b) | 3| £¡ ¢2 ¡M ¢¤ [Compare with Eq. (43.16).] If the decaying particle is a scalar or we average over its spin states, then integration over the angles in Eq. (43.18) gives 1 1 dΓ = M 2 dE dE (2π)3 8M | | 1 2 1 1 = M 2 dm2 dm2 . (43.21) (2π)3 32M 3 | | 12 23 This is the standard form for the Dalitz plot. 2 2 43.4.3.1. Dalitz plot: For a given value of m12, the range of m23 is determined by its values when p2 is parallel or antiparallel to p3: 2 (m23)max = 2 ∗ ∗ 2 ∗2 2 ∗2 2 (E2 + E3) E2 m2 E3 m3 , (43.22a) − µq − − q − ¶ 2 (m23)min = 2 ∗ ∗ 2 ∗2 2 ∗2 2 (E2 + E3) E2 m2 + E3 m3 . (43.22b) − µq − q − ¶ ∗ 2 2 2 ∗ 2 2 2 Here E2 = (m12 m1 + m2)/2m12 and E3 = (M m12 m3)/2m12 are the energies − − − 2 2 of particles 2 and 3 in the m12 rest frame. The scatter plot in m12 and m23 is called a Dalitz plot. If M 2 is constant, the allowed region of the plot will be uniformly populated with events [see| Eq.| (43.21)]. A nonuniformity in the plot gives immediate information on M 2. For example, in the case of D Kππ, bands appear when m = m ∗ , | | → (Kπ) K (892) reflecting the appearance of the decay chain D K∗(892)π Kππ. → → June 18, 2012 16:20 43. Kinematics 5 10 + 2 (m1 m2) M−m 2 8 ( 1) ) m2 2 ( ) 6 23 max V − 2 (M m3) (Ge 2 23 4 m 2 (m23)min + 2 2 (m2 m3) 0 0 1 2 3 4 5 2 2 m12 (GeV ) Figure 43.3: Dalitz plot for a three-body final state. In this example, the state is π+K0p at 3 GeV. Four-momentum conservation restricts events to the shaded region. 43.4.4. Kinematic limits : 43.4.4.1. Three-body decays: In a three-body decay (Fig. 43.2) the maximum of p , | 3| [given by Eq. (43.20)], is achieved when m12 = m1 + m2, i.e., particles 1 and 2 have the same vector velocity in the rest frame of the decaying particle. If, in addition, m > m , m , then p max > p max, p max. The distribution of m values possesses 3 1 2 | 3 | | 1 | | 2| 12 an end-point or maximum value at m12 = M m3. This can be used to constrain the mass difference of a parent particle and one invisible− decay product. June 18, 2012 16:20 6 43. Kinematics 43.4.4.2. Sequential two-body decays: cb a 2 1 Figure 43.4: Particles participating in sequential two-body decay chain. Particles labeled 1 and 2 are visible while the particle terminating the chain (a) is invisible. When a heavy particle initiates a sequential chain of two-body decays terminating in an invisible particle, constraints on the masses of the states participating in the chain can be obtained from end-points and thresholds in invariant mass distributions of the aggregated decay products.