2. From Twistors to Gauge-Theory Amplitudes WHEPP, Bhubaneswar, India January 7 January 7, 2006

3. WHEPP, Bhubaneswar, India, January 7, 2006 2 The Storyline An exciting time in gauge-theory amplitude calculations Motivation for hard calculations Twistor-space ideas originating with Nair and Witten Explicit calculations led to seeing simple twistor-space structure Explicit calculations led to new on-shell recursion relations for trees, and parts of loops Combined with another class of nonconventional techniques, the unitarity-based method for loop calculations, we are at the threshold of a revolution in loop calculations

4. WHEPP, Bhubaneswar, India, January 7, 2006 3 D0 event

5. WHEPP, Bhubaneswar, India, January 7, 2006 4 Guenther Dissertori (Jan ’04)  De Roeck’s talk

6. WHEPP, Bhubaneswar, India, January 7, 2006 5 Precision Perturbative QCD Predictions of signals, signals+jets Predictions of backgrounds Measurement of luminosity Measurement of fundamental parameters (  s, m t ) Measurement of electroweak parameters Extraction of parton distributions — ingredients in any theoretical prediction Everything at a hadron collider involves QCD  Del Duca’s talk  Harlander’s talk

7. WHEPP, Bhubaneswar, India, January 7, 2006 6

8. 7 A New Duality Topological B-model string theory (twistor space)  N =4 supersymmetric gauge theory Weak–weak duality Computation of scattering amplitudes Novel differential equations Nair (1988); Witten (2003) Roiban, Spradlin, & Volovich; Berkovits & Motl; Vafa & Neitzke; Siegel (2004) Novel factorizations of amplitudes Cachazo, Svrcek, & Witten (2004) Indirectly, new recursion relations Britto, Cachazo, Feng, & Witten (1/2005)

9. WHEPP, Bhubaneswar, India, January 7, 2006 8 Supersymmetry Most often pursued in broken form as low-energy phenomenology "One day, all of these will be supersymmetric phenomenology papers."

10. WHEPP, Bhubaneswar, India, January 7, 2006 9 Exact Supersymmetry As a Computational Tool All-gluon amplitudes are the same at tree level in N =4 and QCD Fermion amplitudes obtained through Supersymmetry Ward Identities Grisaru, Pendleton, van Nieuwenhuizen (1977); Kunszt, Mangano, Parke, Taylor (1980s) At loop level, N =4 amplitudes are one contribution to QCD amplitudes; N =1 multiplets give another

11. WHEPP, Bhubaneswar, India, January 7, 2006 10 Gauge-theory amplitude  Color-ordered amplitude: function of k i and  i  Helicity amplitude: function of spinor products and helicities ±1  Function of spinor variables and helicities ±1  Support on simple curves in twistor space Color decomposition & stripping Spinor-helicity basis Half-Fourier transform

12. WHEPP, Bhubaneswar, India, January 7, 2006 11 Spinors Want square root of Lorentz vector  need spin ½ Spinors, conjugate spinors Spinor product (½,0)  (0, ½) = vector Helicity  1:  Amplitudes as pure functions of spinor variables

13. WHEPP, Bhubaneswar, India, January 7, 2006 12 Complex Invariants These are not just formal objects, we have the explicit formulæ otherwise so that the identity always holds for real momenta

14. WHEPP, Bhubaneswar, India, January 7, 2006 13 Complex Momenta For complex momenta  or but not necessarily both!

15. WHEPP, Bhubaneswar, India, January 7, 2006 14

16. WHEPP, Bhubaneswar, India, January 7, 2006 15 Let’s Travel to Twistor Space! It turns out that the natural setting for amplitudes is not exactly spinor space, but something similar. The motivation comes from studying the representation of the conformal algebra. Half-Fourier transform of spinors: transform, leave alone  Penrose’s original twistor space, real or complex Study amplitudes of definite helicity: introduce homogeneous coordinates  CP 3 or RP 3 (projective) twistor space Back to momentum space by Fourier-transforming 

17. WHEPP, Bhubaneswar, India, January 7, 2006 16 Differential Operators Equation for a line ( CP 1 ): gives us a differential (‘line’) operator in terms of momentum- space spinors Equation for a plane ( CP 2 ): also gives us a differential (‘plane’) operator

18. WHEPP, Bhubaneswar, India, January 7, 2006 17 Even String Theorists Can Do Experiments Apply F operators to NMHV (3 – ) amplitudes: products annihilate them! K annihilates them; Apply F operators to N 2 MHV (4 – ) amplitudes: longer products annihilate them! Products of K annihilate them;

19. WHEPP, Bhubaneswar, India, January 7, 2006 18 What does this mean in field theory?

20. WHEPP, Bhubaneswar, India, January 7, 2006 19 Cachazo–Svrček–Witten Construction

21. WHEPP, Bhubaneswar, India, January 7, 2006 20 Recursion Relations Berends & Giele (1988); DAK (1989)  Polynomial complexity per helicity

22. WHEPP, Bhubaneswar, India, January 7, 2006 21 On-Shell Recursion Relations Britto, Cachazo, Feng (2004) Amplitudes written as sum over ‘factorizations’ into on-shell amplitudes — but evaluated for complex momenta

23. WHEPP, Bhubaneswar, India, January 7, 2006 22 Massless momenta:

24. WHEPP, Bhubaneswar, India, January 7, 2006 23 Proof Ingredients Less is more. My architecture is almost nothing — Mies van der Rohe Britto, Cachazo, Feng, Witten (2005) Complex shift of momenta Behavior as z   : need A(z)  0 Basic complex analysis Knowledge of factorization: at tree level, tracks known multiparticle-pole and collinear factorization

25. WHEPP, Bhubaneswar, India, January 7, 2006 24 C

26. WHEPP, Bhubaneswar, India, January 7, 2006 25 Proof Consider the contour integral Determine A(0) in terms of other poles Poles determined by knowledge of factorization in invariants At tree level

27. WHEPP, Bhubaneswar, India, January 7, 2006 26 Very general: relies only on complex analysis + factorization Applied to gravity Bedford, Brandhuber, Spence, & Travaglini (2/2005) Cachazo & Svrček (2/2005) Massive amplitudes Badger, Glover, Khoze, Svrček (4/2005, 7/2005) Forde & DAK (7/2005) Integral coefficients Bern, Bjerrum-Bohr, Dunbar, & Ita (7/2005) Connection to Cachazo–Svrček–Witten construction Risager (8/2005) CSW construction for gravity  Twistor string for N =8? Bjerrum-Bohr, Dunbar, Ita, Perkins, & Risager (9/2005)

28. WHEPP, Bhubaneswar, India, January 7, 2006 27 Unitarity-Based Method for Loop Calculations Bern, Dixon, Dunbar, & DAK (1994) Use a basic property of amplitudes as a calculational tool Key idea: sew amplitudes not diagrams Proven utility as a tool for explicit calculations –Fixed number of external legs –All-n equation –Formal proofs

29. WHEPP, Bhubaneswar, India, January 7, 2006 28 Unitarity-Based Calculations Bern, Dixon, Dunbar, & DAK (1994) At one loop in D=4 for SUSY  full answer (also for N =4 two-particle cuts at two loops) In general, work in D=4-2 Є  full answer van Neerven (1986): dispersion relations converge Merge channels: find function w/given cuts in all channels ‘Generalized cuts’: require more than two propagators to be present

30. WHEPP, Bhubaneswar, India, January 7, 2006 29 Unitarity-Based Method at Higher Loops Loop amplitudes on either side of the cut Multi-particle cuts in addition to two-particle cuts Find integrand/integral with given cuts in all channels In practice, replace loop amplitudes by their cuts too

31. WHEPP, Bhubaneswar, India, January 7, 2006 30 On-Shell Recursion at Loop Level Bern, Dixon, DAK (2005) Subtleties in factorization: factorization in complex momenta is not exactly the same as for real momenta For finite amplitudes, obtain recurrence relations which agree with known results ( Chalmers, Bern, Dixon, DAK; Mahlon ) and yield simpler forms Simpler forms involve spurious singularities Again, use properties of amplitude as calculational tool

32. WHEPP, Bhubaneswar, India, January 7, 2006 31 Amplitudes contain factors like known from collinear limits Expect also as ‘subleading’ contributions, seen in explicit results Double poles with vertex Non-conventional single pole: one finds the double-pole, multiplied by

33. WHEPP, Bhubaneswar, India, January 7, 2006 32 Eikonal Function

34. WHEPP, Bhubaneswar, India, January 7, 2006 33 Rational Parts of QCD Amplitudes Start with cut-containing parts obtained from unitarity method, consider same contour integral

35. WHEPP, Bhubaneswar, India, January 7, 2006 34 Start with same contour integral Cut terms have spurious singularities, absorb them into ; but that means there is a double-counting: subtract off those residues Cut terms Rational terms Cut terms

36. WHEPP, Bhubaneswar, India, January 7, 2006 35 A 2→4 QCD Amplitude Bern, Dixon, Dunbar, & DAK (1994) Only rational terms missing

37. WHEPP, Bhubaneswar, India, January 7, 2006 36 A 2→4 QCD Amplitude Rational terms

38. WHEPP, Bhubaneswar, India, January 7, 2006 37 Also computed Berger, Bern, Dixon, Forde, DAK

39. WHEPP, Bhubaneswar, India, January 7, 2006 38 All-Multiplicity Amplitude Same technique can be applied to calculate a one-loop amplitude with arbitrary number of external legs

40. WHEPP, Bhubaneswar, India, January 7, 2006 39 Road Ahead Opens door to many new calculations: time to do them! Approach already includes external massive particles (H, W, Z) Reduce one-loop calculations to purely algebraic ones in an analytic context, with polynomial complexity Massive internal particles Lots of excitement to come!