Parton Distribution Functions, Part 1. Daniel Stump. Department of Physics and Astronomy Michigan State University

Transcription

1 Parton Distribution Functions, Part 1 Daniel Stump Department of Physics and Astronomy Michigan State University A. Introduction B. Properties of the PDFs C. Results of CT10-NNLO Global Analysis D. Uncertainties of the PDFs E. Applications to LHC Physics A 01 1

2 Introduction A. Introduction. QCD and High-Energy Physics QCD is an elegant theory of the strong interactions the gauge theory of color transformations. It has a simple Lagrangian L [ i ga m ] 4 1 F a F a F a A a A a gf abc A b A c A A a T a sums over flavor and color are implied Parameters: g; m 1, m, m 3,, m 6 A 01

3 L [ i ga m ] 4 1 Fa Fa However, the calculation of experimental observables is quite difficult, for reasons: i there are divergent renormalizations; the theory requires regularization perturbation theory a singular limit GeV or, a 0 fm or, n 4 ii quark confinement; the asymptotic states are color singlets, whereas the fundamental fields are color triplets. Introduction A 01 3

4 Introduction Nevertheless, certain cross sections can be calculated reliably inclusive processes with large momentum transfer i.e., short-distance interactions The reasons that QCD can provide accurate predictions for short-distance interactions are asymptotic freedom sq ~ const./lnq / as Q the factorization theorem d hadron ~ PDFs C where C is calculable in perturbation theory. The PDFs provide a connection between quarks and gluons the partons and the nucleon a bound state. A 01 4

5 Global Analysis of QCD and Parton Distribution Functions d hadron ~ PDFs C sum over flavors implied The symbol ~ means asymptotically equal as Q ; the error is Om /Q where Q is an appropriate high momentum scale. The C s are calculable in perturbation theory. The PDFs are not calculable today, given our lack of understanding of the nonperturbative aspect of QCD binding and confinement. But we can determine the PDFs from Global Analysis, with some accuracy. Introduction The symbol means "convolution"; / A 01 5

6 next A 01 6

7 Properties of the PDFs B. Properties of the PDFs -- Definitions First, what are the Parton Distribution Functions? PDFs The PDFs are a set of 11 functions, f i x,q where 0 Q x ~ 1 GeV longitudinal momentum fraction momentum scale,,,,, parton index f 0 = gx,q the gluon PDF f 1 = ux,q the up-quark PDF f -1 = ux,q - the up-antiquark PDF - f = d and f - = d f 3 = s and f -3 = - s etc. Exercise: What about the top quark? B1 01 1

8 Properties of the PDFs Second, what is the meaning of a PDF? We tend to think and speak in terms of Proton Structure ux,q dx = the mean number of up quarks with longitudinal momentum fraction from x to x + dx, appropriate to a scattering experiment with momentum transfer Q. ux,q = the up-quark density in momentum fraction This heuristic interpretation makes sense from the LO parton model. More precisely, taking account of QCD interactions, d proton = PDFs C. B1 01

9 B f i x,q = the density of parton type i w.r.t. longitudinal momentum fraction x valence up quark integral, valence down quark integral, valence strange quark integral Properties of the PDFs longitudinal momentum fraction, carried by parton type i 5 5 i 1 0 i 1 0 i 1 Q x f x Q x f x,, 0 dx Q x s Q x s 1 dx Q x d Q x d dx Q x u Q x u ],, [ ],, [ ],, [ Momentum Sum Rule Flavor Number Sum Rules

10 Properties of the PDFs Example. DIS of electrons by protons; e.g., HERA experiments Kinematic Variables / in lowest order of QCD summed over flavors! B1 01 4

11 Properties of the PDFs But QCD radiative corrections must be included to get a sufficiently accurate prediction. The NLO approximation will involve these interactions From these perturbative calculations, we determine the coefficient functions C NLO i, and hence write ~ Approximations available today: LO, NLO, NNLO B1 01 5

12 The Factorization Theorem For short-distance interactions, ~ Properties of the PDFs ~, and the PDFs are universal! We can write a formal, field-theoretic expression, Correlator function fi x, 4 1 dy e iy p p i 0, y, 0 T i 0, 0, 0 T p although we can t evaluate it because we don t know the bound state p>. B1 01 6

13 Properties of the PDFs Exercise. Suppose the parton densities for the proton are known, f i x, Q for { i 0, 1,,, 5 } A In terms of the f i x,q, write the 11 parton densities for the neutron, say, g i x,q. B In terms of the f i x,q, write the 11 parton densities for the deuteron, say, h i x,q. B1 01 7

14 next B1 01 8

15 B. Properties of the PDFs Q evolution Q evolution Evolution in Q The PDFs are a set of 11 functions, f i x,q where ~ longitudinal momentum fraction momentum scale f i x,q = the density of partons of type i, carrying a fraction x of the longitudinal momentum of a proton, when resolved at a momentum scale Q. The DGLAP Evolution Equations We know how the f i vary with Q. That follows from the renormalization group. It s calculable in perturbation theory. B 01 1

16 B 01 The DGLAP Evolution Equations V.N. Gribov and L.N. Lipatov, Sov J Nucl Phys 15, ; G. Altarelli and G. Parisi, Nucl Phys B16, ; Yu.L. Dokshitzer, Sov Phys JETP 46, Solve the 11 coupled equations numerically. For example, you could download the program HOPPET. G. P. Salam and J. Rojo, A Higher Order Perturbative Parton Evolution Toolkit; download from a library of programs written in Fortran 90. Q evolution,,,, /, ln, z P z P z P z P q q q Q z x q Q z P z dz Q Q Q x q 1 0 S j i 1 x S S S S e.g. NON - SINGLET distribution for a Q evolution

17 Q evolution B 01 3

18 Some informative results obtained using HOPPET Starting from a set of benchmark input PDFs, let s use HOPPET to calculate the evolved PDFs at selected values of Q. For the input not realistic but used here to study the evolution qualitatively: Output tables xu xd v v x x xd x xu x xs x xg x x x 1. 7 x x 1 x x d x 1 x xs x 0. xd x xu x x x x 4 6 Q evolution B 01 4

19 Q evolution The Running Coupling of QCD SQ d S d lnq S b 0 S b 1 3 S b 4 S b 0 33 n f / 1 b n f / 4 b n f 35 n f / B 01 5

20 The QCD Running Coupling Constant Q evolution d d lnq Q ln Q 0 S S 0 S d For Global Analysis, we need an accurate SQ. approx. : N F = 5 massless B 01 6

21 The QCD Running Coupling Constant Q evolution Evolution of S as a function of Q, using the 1-loop beta function red the -loop beta function green the 3-loop beta function blue For Global Analysis, we need an accurate SQ. S MZ SM Z = S. Bethke, G. Dissertori and G.P. Salam; Particle Data Group 01 B 01 7

22 The QCD Running Coupling Constant Q evolution How large are the -loop and 3-loop corrections for SQ? B 01 8

23 Comments on SQ Q evolution An important improvement is to include the quark masses. The central fit has SM Z = Bethke CTEQ also provides alternative PDFs with a range of value of SM Z ; these are called the S-series. Another approach is to use the Global Analysis to determine the value of SM Z. Asymptotic freedom B 01 9

24 How does the u-quark PDF evolve in Q? Q evolution Examples from HOPPET U-quark PDF evolution : Black : Q = Q 0 = GeV Blue : Q = 3.16 GeV 1-loop, -loop, 3-loop Red : Q = GeV 1-loop, -loop, 3-loop B 01 10

25 How does the gluon PDF evolve in Q? Q evolution Examples from HOPPET Gluon PDF evolution : Black : Q = Q0 = GeV Blue : Q = 3.16 GeV 1-loop, -loop, 3-loop Red : Q = GeV 1-loop, -loop, 3-loop B 01 11

26 DGLAP evolution of PDFs Q evolution The structure of the proton depends on the resolving power of the scattering process. As Q increases PDFs decrease at large x PDFs increase at small x as we resolve the gluon radiation and quark pair production. The momentum sum rule and the flavor sum rules hold for all Q. These graphs show the DGLAP evolution for LO, NLO, NNLO Global Analysis. B 01 1

27 Q evolution How large are the NLO and NNLO corrections? a good exercise using HOPPET B 01 13

28 Q evolution next B 01 14

29 CT10-NNLO PDFs Structure of the Proton CTEQ Parton Distribution Functions CTEQ6.6 P. M. Nadolsky, H.-L. Lai, Q.-H. Cao, J. Huston, J. Pumplin, D. Stump, W.-K. Tung, C.-P. Yuan, Implications of CTEQ global analysis for collider observables, Phys.Rev.D78: ; arxiv: [hep-ph] CT10 Hung-Liang Lai, Marco Guzzi, Joey Huston, Zhao Li, Pavel M. Nadolsky, Jon Pumplin, C.-P. Yuan, New parton distributions for collider physics, Phys.Rev.D8: ; arxiv: [hep-ph] CT10-NNLO 01 The results shown below are for CT10-NNLO. B3 01 1

30 CT10-NNLO PDFs Structure of the Proton The U Quark Q = 10 GeV ; Q = 3.16 GeV Red: ux,q Dashed Red: ūx,q Gray: u valence x,q This log-log plot shows for x 0.1 the valence structure dominates for x 0.01 ū approaches u B3 01

31 CT10-NNLO PDFs Structure of the Proton The U Quark Q = 10 GeV ; Q = 3.16 GeV Red: x ux,q Dashed Red: x ūx,q Gray: x u valence x,q This linear plot shows the momentum fraction for Q = 3.16 GeV = area under the curve integral 0.3, 0.05, 0.7 B3 01 3

32 CT10-NNLO PDFs Structure of the Proton The U Quark Q = 10 GeV ; Q = 3.16 GeV Red: u valence x,q This linear plot demonstrates the flavor sum rule: integral = B3 01 4

33 CT10-NNLO PDFs Structure of the Proton The D Quark Q = 10 GeV ; Q = 3.16 GeV Blue: dx,q Dashed Blue: đx,q Gray: d valence x,q This log-log plot shows for x 0.1 the valence structure dominates for x 0.01 đ approaches d B3 01 5

34 CT10-NNLO PDFs Structure of the Proton The D Quark Q = 10 GeV ; Q = 3.16 GeV Blue: dx,q Dashed Blue: đx,q Gray: d valence x,q This linear plot shows the momentum fraction for Q = 3.16 GeV = area under the curve integral 0.15, 0.06, 0.10 B3 01 6

35 CT10-NNLO PDFs Structure of the Proton The D Quark Q = 10 GeV ; Q = 3.16 GeV d valence = d đ x,q This linear plot demonstrates the flavor sum rule: integral = 1 B3 01 7

36 CT10-NNLO PDFs Structure of the Proton The D and U Quarks Q = 10 GeV ; Q = 3.16 GeV Blue: dx,q Dashed Blue: đx,q Gray: d valence x,q Red: ux,q Dashed Red: ūx,q Gray: u valence x,q This linear plot compares U and D -- quarks and antiquarks; naively, d = 0.5 u, but it s not that simple. B3 01 8

37 CT10-NNLO PDFs Structure of the Proton The D and U Quarks Q = 10 GeV ; Q = 3.16 GeV dx,q / ux,q v x dx,q / ux,q is interesting. Exercise: What scattering processes could provide information on this ratio? B3 01 9

38 CT10-NNLO PDFs Structure of the Proton Gluons Q = 10 GeV ; Q = 3.16 GeV black : gx,q red : ux,q The gluon dominates at small x. The valence quarks dominate at large x. B

39 CT10-NNLO PDFs Structure of the Proton Gluons Q = 10 GeV ; Q = 3.16 GeV black : gx,q red : ux,q blue: dx,q The gluon dominates at small x. The valence quarks dominate at large x. B

40 CT10-NNLO PDFs Structure of the Proton Sea Quarks and Gluons Q = 10 GeV ; Q = 3.16 GeV seax,q and gx,q x vs black: gx,q brown: seax,q sea = db + ub + sb B3 01 1

41 CT10-NNLO PDFs Structure of the Proton Q = 10 GeV ; Q = 3.16 GeV Ratios : db, ub, sb/g Exercises: 1 Identify db, ub, sb. Why are they all equal at small x? B

42 CT10-NNLO PDFs Structure of the Proton Q = 10 GeV ; Q = 3.16 GeV All in One Plot u_valence d valence sea * 0.05 gluon * 0.05 Sea = db + ub + sb u_valence = u ub ; u = u_valence + u_sea ub = u_sea B

43 CT10-NNLO PDFs Structure of the Proton as a function of Q All in One Plot u_valence d valence sea * 0.05 gluon * 0.05 Sea = db + ub + sb u_valence = u ub ; u = u_valence + u_sea ub = u_sea B

44 CT10-NNLO PDFs Exercise: A Use the Durham Parton Distribution Generator online PDF calculator to reproduce these graphs. B Show the Q evolution. B

45 next B