3.3. The r<strong>at</strong>ional pieces 99parton multiplicities. It is simple, however, to generalise the methods described <strong>in</strong>the follow<strong>in</strong>g sections to <strong>in</strong>clude <strong>in</strong>creas<strong>in</strong>g numbers <strong>of</strong> partons. When calcul<strong>at</strong><strong>in</strong>gthe r<strong>at</strong>ional terms it is simplest to <strong>in</strong>clude the cut-completion terms <strong>with</strong> C n , wedef<strong>in</strong>ed the follow<strong>in</strong>g r<strong>at</strong>ional termŝR n = R D n + O n − Inf A n , (3.108)merg<strong>in</strong>g the rema<strong>in</strong><strong>in</strong>g r<strong>at</strong>ional terms <strong>with</strong> the cut-constructible piecesĈ n = C n + CR n . (3.109)In the above equ<strong>at</strong>ions Inf A n , represents the pieces <strong>of</strong> the amplitude which do notvanish as z → ∞ (where z is the BCFW shift parameter). In our calcul<strong>at</strong>ion we willf<strong>in</strong>d th<strong>at</strong> CR n contributes an <strong>in</strong>f<strong>in</strong>ite piece <strong>of</strong> this sort. In the follow<strong>in</strong>g sections wewill analyse each <strong>of</strong> these r<strong>at</strong>ional contributions before putt<strong>in</strong>g the whole r<strong>at</strong>ionalpiece together.3.3.1 The cut-completion termsThe basis-set <strong>of</strong> logarithmic functions <strong>in</strong> which eq. (3.103) and eq. (3.105) are writtenconta<strong>in</strong>s unphysical s<strong>in</strong>gularities, which we remove by add<strong>in</strong>g <strong>in</strong> r<strong>at</strong>ional pieces, theso-called cut completion terms. The new basis is given by the transform<strong>at</strong>ion,L 1 (s, t) = ˆL 1 (s, t),L 2 (s, t) = ˆL 1 12 (s, t) +2(s − t)(t + 1 ),s(L 3 (s, t) = ˆL 1 13 (s, t) +2(s − t) 2 t + 1 ). (3.110)sFrom the breakdown <strong>of</strong> our amplitude it is clear th<strong>at</strong> only A φSnneeds to be completed.When consider<strong>in</strong>g the overlap terms <strong>in</strong> the next section it proves most convenientto write the cut-completion terms <strong>in</strong> the follow<strong>in</strong>g form,CR n (φ, 1 − , . . .,m − , . . .,n + ) = Γ n[m∑n∑i=2 j=m+1( 1ρ j,i−1m1 (P (i,j−1)) + 1 ) m−1∑−s i,j−1 s i,jn∑i=2 j=m( 1ρ i,jm1 (P (i+1,j)) + 1 )s i+1,j s i,j
3.3. The r<strong>at</strong>ional pieces 100+m−1∑∑n+1i=2 j=m+1( 1ρ i,j−11m (P (j,i−1)) + 1 ) m−1∑−s j,i−1 s j,<strong>in</strong>∑i=1 j=m+1( 1ρ j,i1m (P (j+1,i)) + 1 )].s j+1,i s j,i(3.111)The factor Γ n is given by,and<strong>with</strong>ρ a,bm1(P (i,j) ) =Γ n =2Π n α=1〈m| P(i,j) a| 1 〉 3N P3 〈 a |P (i,j) | a ] 2 Aab m1 +〈α α + 1〉,(3.112)〈m |P(i,j) a| 1 〉 22 〈 a |P (i,j) | a ] Kab m1, (3.113)A ab 〈m a〉 〈b 1〉m1 = − (b → b + 1),〈a b〉(3.114)Km1 ab 〈b 1〉 2〈a b〉 2 − (b → b + 1). (3.115)We have also <strong>in</strong>troduced the short-hand not<strong>at</strong>ion,(N P = 2 1 − N )f. (3.116)N cUltim<strong>at</strong>ely we will require the cut-completion terms for the four parton amplitudeA (1)4 (φ, 1 − , 2 + , 3 − , 4 + ),CR 4 (φ, 1 − , 2 + , 3 − , 4 + ) = N P 12 〈1 2〉 〈2 3〉 〈3 4〉 〈4 1〉[(〈3| 2 4 |1〉3 〈3 4〉 〈2 1〉 〈3| 2 4 |1〉2 〈3 4〉 2 〈2 1〉 2 )( 1× − −3(s 234 − s 23 ) 2 〈4 2〉 2(s 234 − s 23 ) 〈4 2〉 2 + 1 )]s 23 s 234+(2 ↔ 4) + (1 ↔ 3) + (1 ↔ 3, 2 ↔ 4). (3.117)3.3.2 The recursive termsWe make a complex shift [106, 140, 141, 161–163] <strong>of</strong> the <strong>two</strong> neg<strong>at</strong>ive gluons suchth<strong>at</strong>|ˆ1〉 = |1〉 + z|3〉, |ˆ3] = |3] − z|1], (3.118)ensur<strong>in</strong>g th<strong>at</strong> overall momentum is conserved s<strong>in</strong>cep µ 1 (z) = pµ 1 + z 2 〈3 |γµ | 1], p µ 3 (z) = pµ 3 − z 2 〈3 |γµ | 1]. (3.119)