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1 C4 Lectures Dynamic Quarks and (introduction to) QCD. 5 lectures HT 2013 Tony Weidberg – Feedback very much welcomed! – Please ask questions in lectures – Come and find me in DWB 629 – – Corrections for draft chapter (prize) See handbook for suggested textbooks Slides and draft chapter on C4 website

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Outline Review of evidence for quarks from static quark model. How to study particle structure – Review of Rutherford scattering and how to measure size of nucleus (form factors). – Scattering off nucleons in nuclei Simple Quark Parton Model – ve scattering – q scattering – p scattering – ep and p scattering – Predictions of the QPM direct evidence for spin ½ fractionally charged partons quarks are real! Gluons and extension to QCD – Scaling violations gluon distribution – Hadron – hadron interactions. 2

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Static Quark Model SU(3) and quark multiplets – Allowed understanding of bizarre number of “elementary particles”. – SU(3) badly broken so predictions approximate. Charmonimum and bbar States – Spectra strikingly similar to positronium Can we get more direct evidence for quarks? – Yes we can with DIS These lectures! 3

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Rutherford Scattering (1) Alpha –gold scattering; prototype for all scattering experiments. Use Fermi Golden Rule H’ is Coulomb interaction Plane waves for initial and final states 4

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Rutherford Scattering (2) Matrix element Sub for V(r) and define momentum transfer Use spherical polar r axis // q q.r=qr cos(θ) 5

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Rutherford Scattering (3) Integral is divergent (cf classical result) – Modify potential by factor exp(-r/a) and let a ∞ Let a ∞ gives matrix element 6

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Rutherford Scattering (4) Density of states Cross section and transition rates: T= F and flux F=v. Ignore nuclear recoil: Gives Rutherford scattering cross section 7

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Form Factor (1) Rutherford scattering cross section doesn’t depend on any fixed scale in problem. – We had assumed point like nucleus. What happens for finite sized nuclei? Coulomb potential modified: Matrix element modified 8

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Form Factor (2) Matrix element modified compared to pure Coulomb by form factor Form factor is Fourier transform of charge distribution in 3D space to momentum space. In principle: measure deviations from pure Coulomb potential F(q 2 ) inverse FT gives ρ(r). – Why doesn’t this work in practice? 9

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10 Form Factor (3) Requires probe with λ >1 (high energy!). Assume toy model for charge distribution ie Gives form factor (work it out yourself!) q << m F(q 2 ) ~ constant because resolution too poor. q >> m F(q 2 ) ~ 1/q 4 and σ suppressed by 1/q 8 10

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11 Form Factor (4) Real example! Data and fit to charge distribution. 153 MeV e- scattering on Au Large difference between data and point nucleus. Fits to two models for charge distribution. 11

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12 Approximate fit to uniform charge distribution (sharp edge) Good fit to realistic charge distribution

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13 Summary so far … Low energy scattering shows – Scaling cross section evidence for point like nucleus (within resolution = h/p) Higher energy scattering shows – Fixed scale in cross section evidence for finite size nucleus – Scattering data form factors charge distribution in nucleus 13

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14 Nucleon Scattering (1) Next steps into the nucleus … lets look for evidence of nucleons inside the nucleus from scattering experiments … Conservation of 4 momentum and energy

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15 Nucleon Scattering (2) Recoil invariant mass W and 4-momtum transfer Elastic scattering off entire nucleus W=M 15

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16 Nucleon Scattering (3) e - He scattering fixed angle 45 0, E e =400 MeV Sharp peak: elastic scattering off nucleus smeared peak: elastic scattering off p (Fermi motion) What do we expect with partons inside proton?

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17 Quark-Parton Model Assume QPM is correct predictions for DIS compare with data. (0) Kinematics for DIS (1) (2) (3) Generalise to quarks (4) Generalise to nucleon (5) Extend to e/ nucleon (6) Compare with data!

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18 Kinematics (1) Assume quarks have mass m=xM Elastic scattering off quark: Define Q 2 =-q 2 nb only need scattered electron measurement

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19 Kinematics (2) x can also represent fractional proton momentum carried by quark. DIS large Q 2 neglect quark mass compared to energy evaluate quark mass after scattering. x can represent either mass fraction or momentum fraction (infinite momentum frame) p.q LI use lab frame

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20 Kinematics (3) Relation between CMS angle and lab energies. LT from CMS to lab ( =1) for scattered and initial lepton. Define y= /E (0

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21 Anti-neutrino e scattering (1) Consider W contribution – W couples to LH e- and RH anti – high energy chirality = helicity. – Very simple spin structure – Amplitude from projecting m=1/2 to m’=1/2

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22 (Anti) Neutrino e scattering (2) Cross section proportional to |A| 2 Phase space proportional to p 2 =s/4. G F Fermi coupling constant Numerical factors requires Feynman rules

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23 Neutrino e scattering J z =0 so J=0 or J=1 not as simple … Crossing symmetry: anti-particles & amplitudes – Matrix element for crossed diagrams same with p_incoming - p_outgoing. – Compare e and e p1 p3 – Isotropic, phase space + G F + numerical factor e e-e- e-e-

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24 Neutrino Quark Scattering (1) Universality of CC weak interactions. In q scattering: Now write down cross sections for (anti) neutrino on (anti) quark scattering by analogy with (anti) neutrino scattering on e-

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Neutrino quark scattering 25 q and q’ not same (CKM). Also ignore threshold effects

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26 Neutrino Nucleon Scattering QPM: scattering of leptons from nucleon is incoherent sum of scattering off quarks – Assumption only valid for large q 2 >>1 GeV 2 – Justification from QCD and experiment – Let q(x) be the pdf for finding quark with momentum fraction x in a proton. What does q(x) mean? – QPM can’t predict q(x) need fits to data. – Many clear predictions can be checked with data

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27 Neutrino Nucleon Scattering (1) Combine all ingredients cross section for measurable quantities!

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Neutrino Detector High energy beam . – What final state particles do we want to detect? Detector shape and size ? Passive material? Active material? Readout?

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29 Neutrino Nucleon Scattering (2) Compare y distributions with data – If anti-quarks negligible: N : flat y anti- N: (1-y) 2 Data fits combination of flat and (1-y) 2 quarks are spin ½ and interact by a parity violating interactions.

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30 Neutrino Nucleon Scattering (3) Integrate in x and y to get total cross sections. – Assume quarks dominant – y integrals trivial – x integral ~ unity – Neutrino larger than anti-neutrino – See next slide for data – Cross sections scale like s quarks are point like If quarks had finite size form factor suppression.

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31 Neutrino Nucleon Scattering (4) Quarks are point like! S=2mE /E constant vs E

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32 Parton Distribution Functions (1) How to determine the PDFs q i (x) from data? Rewrite QPM prediction to allow fits to data. Compare with formula allowed by Lorentz Invariance

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33 Parton Distribution Functions (2) Fit F 2 and F 3 to data q and anti-q Which quarks are we considering? – Assume nucleons only contain u,d,s and anti-quarks

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34 Parton Distribution Functions (3) Which quarks do see ?

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35 Parton Distribution Functions (4) What about neutrons? – Assume SU(2) symmetry:

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36 Nuclear Targets What about nuclei? – Assume isoscalar target I=0 – Just average n and p and assume s=sbar

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37 Sum Rules (1) Important consistency checks of QPM Divide q(x) into valence and sea 2 valence u + 1 valence d in proton Gross- Llewellyn-Smith sum rule Check with data

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38 GLS Sum Rule Agrees with data at large Q 2 Disagreement at low Q 2 can be explained by higher order QCD corrections.

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39 Addler Sum Rule Addler Sum Rule: Two valence u and one valence d S A =1 – 2S A =2.02 ± 0.4 (D. Allasia, et al., Z. Phys. C 28 (1985) 321.) – Agrees with data!

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40 Momentum Sum Integrate F 2 momentum fraction carried by quarks Naïve expectiation: I=1 but I=0.44 at Q 2 =10 GeV 2. ~ ½ momentum is carried by particles that don’t experience weak force First evidence for gluons! More direct evidence when we look at QCD.

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41 Charged Lepton Probes Advantages of e/ – Higher intensity beams – ep (HERRA) access to larger range in x and q 2. Disadvantages? How to calculate the lepton – nucleon ? – Calculate e scattering – Generalise to e q scattering – Generalise to eN scattering (QPM)

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42 e Scattering (1) Work by analogy with elastic e scattering EM interactions, photon massless and strength given by . Need to evaluate spin factor F

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43 e Scattering (2) Spin factor F – Calculate amplitude for each spin configuration – Sum over final spin states – Average over initial spin states For (1) and (2): J Z =0 same angular distribution as e isotropic. For (3) : J Z =1 same rotation matrices as for

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44 e Scattering (3) For (4): J Z =-1 rotation matrices gives Averaging over initial spins and summing over final spin states – EM parity conserving equal amplitudes for LH and RH (high energy –ive and +ive H)

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45 Elastic e Quark Scattering Fractional charge of quark flavour i, is q i Momentum fraction x cms energy squared of e quark:

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46 e/ Nucleon DIS Same assumptions as for DIS: – Incoherent scattering off individual quarks – Momentum distribution functions f i (x) cf general phenomenological formula

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47 Quark Spin (1) Equating powers y QPM and phenomenological formula (Callan-Gross) Data agrees with spin ½ and excludes spin 0. Simple explanation? Quark spin also measured in e + e - 2 jets. /2

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Quark Spin (2) e + e - qq 2 jets Assume jet direction = quark Helicity conservation for EM interaction – Coupling is LR and LR – Jz=+1 or Jz=-1 – Rotation matrix for J=1, m=1,m’=1 and m=-1,m’=-1 amplitude

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Quark Spin (3) Data from e + e - annihilation at Petra Agrees with 1+cos 2 Confirms quark spin=1/2.

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50 Scaling Most critcial prediction of QPM is scaling SF depend on x but not Q 2 at high Q 2 and. Data show approximate scaling. – If quarks had finite size very large form factor suppression. – Slow (log) variations with Q 2 can be explained by QCD. Quarks are point like within resolution of current experiments

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51 Quark Charges (1) Compare eN and N Assume isospin symmetry u p =d n ubar=dbar (?) Isoscalar target

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52 Quark Charges (2) cf N & eN SFs and assuming s negligible (ok at x>~0.2)? Agreement confirms quark charge assignments. Quark charges also confirmed by (see appendix for plot) x 18/5 F 2 (ep) ● F 2 ( p)

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53 Gottfried Sum Rule Compare ep and en DIS – How can we have a n target ??? Split quark = valence + sea Define Gottfried sum

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55 NMC Data Naïve QPM I G =1/3 Lets look at data from p DIS for H and D targets – E =90 & 280 GeV No data at very low x – Why? Extrapolate x=0 I G =0.235 ± Why ??? NMC, Phys. Rev. D 50, R1–R3 (1994)

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DIS Summary Scaling cross section point like partons Quark spin = ½ confirmed. Comparison of and charged lepton data consistent with quark charges. Quarks are real ! Momentum sum: more to proton than quarks: gluons important. Why does the QPM work at all?

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57 Gluons & QCD Direct evidence for gluons Running coupling constant – Asymptotic freedom – Confinement Scaling violations and gluon distribution Hadron-hadron collisions – Drell-Yann: + -, W & Z. – Jet production – Look to the future: LHC

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58 Direct Evidence for Gluons Indirect evidence from momentum sum. Look at e + e - annihilation – Clear 3 jets: gluon bremsstrhalung – Angular distribution for 3 rd jet wrt 2 nd jet.

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59 3 Jet Events Jade experiment at PETRA CMS E ~ 30 GeV

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60 Gluon Spin Measure angle of 3 rd jet sensitive to gluon spin Boost 2 lowest pt jets to CMS and plot angle jets make with thrust axis Consistent with 1.

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QCD We start from experiment with: – Spin 1 gluons – Number of quark colours = 3. Assume theory is given by SU(3) colour. – The group theory for SU(3) colour identical to SU(3) flavour but colour is conserved exactly (cf electric charge) but SU(3) flavour is approximate. – Quarks come in 3 colours, red, blue, green (nothing to do with colour of light! Just convenient labels). 61

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SU(3) Unitary matrices U † U=UU † =I. Diagonal terms must be real 3 constraints. 6 off-diagonal terms but if U † ij =0 U † ji =0. Leaves 3 complex elements 3*2=6 constraints. Det(U † U)=Det((U T )*) Det(U)=1 Det(U)* Det(U)=1 Det(U)= ±1 62

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SU(3) – continued SU(3): select Det(U)=+1. 3*3 complex matrix: – Start with 2*9=18 numbers – Constraints 3(diagonal) + 6 (off-diagonal) + 1 (det U=+1) – SU(3) Needs 8 parameters 8 generators for SU(3) 8 gluons. Quarks: – fundamental representation of SU(3), triplet of states labeled |r>, |b>, |g> 63

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SU(3) Octet Gluon wave functions (same maths as for SU(3) flavour) 64

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Consequences of SU(3) 8 gluon states (transform into each other under SU(3) rotations). Mathematically we could have a colour singlet gluon How do we know that such a gluon state doesn’t exist? Why is it ok to have colour singlet states for mesons? How do we know that this choice is correct? 65

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Colour Factors (1) We can use gluon wave functions to calculate relative amplitudes from different qq scattering processes (colour factors). qq qq: same colour, chose r (b or g would give same answer): rr rr – qqg vertex requires term g 3 and g 8 – Amplitudes from gluon wave functions (previous page): 66

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Colour Factors (2) Next consider two different colours, arbitrary choice, take rb – Add amplitudes for (1) rb rb and (2) rb br – (1) needs and terms g 3 and g 8 – (2) needs and terms g 1 and g 2

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Colour Factors (3) Antiquarks -ive sign at vertex (cf –ive electric charge in QED). Compare with – Same gluons exchanged, flip sign Compare with Same gluons exchanged, flip sign 68

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Colour Factors Summary 69 Now we’ve done the maths we can do some physics … (1)Learn something about quark binding in hadrons (2)Calculate scattering amplitudes

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Quark Binding in Mesons (1) Now consider colour singlet state of Calculate colour factor for single gluon exchange for singlet using our previous results – 3 terms (colours) like – Gives total = -3*2/9 = -2/3 – – Same term for – Multiply *3 for 3 colours = 3*(-2/3)=-2 Final result for colour singlet a=-2/3-2 =-8/3

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Quark Binding in Mesons (2) Amplitude also contains – coupling “constant” from each vertex – Propagator gives 1/q 2 (4-momentum transfer) In non-relativistic (NR) limit Fourier Transform of potential scattering amplitude. – Inverse FT (amplitude) potential V(r) ~ 1/r – Colour factor =-8/3 negative suggests that colour singlet should be bound state. – Similar calculation for colour octet gives +ive colour factor. – Suggestive of confinement for colour singlets but beware this is NR limit in which coupling constant large so can’t use perturbation theory. 71

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Quark scattering Don’t have free quarks so can’t measure the qq scattering cross sections directly but … We can do hadron-hadron (mainly pp or pp) scattering and if we know quark PDFs we can combine this with these scattering amplitudes to calculate measurable cross sections (see later). 72

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QCD Assume SU(3) colour gauge symmetry. Consider transformation: – g s coupling constant and T a are the generators of SU(3) – Infinitesimal transformation – Add 8 gluon fields to keep gauge invariance f abc are structure constants of SU(3). 73

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QCD (2) Replace derivative with covariant derivative (cf QED) KE term for gluon fields This gives – KE for gluons and qqg vertices (cf QED) and – 3g and 4g vertices (unlike QED). 74

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75 Running Coupling Constant - QED QED: example of running coupling constant Shielding by virtual e + e - pairs increases at shorter distance, higher Q 2. Bare charge & renormalised charge This term depends on number of “active” flavours m

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76 Running Coupling Constant - QCD Similar shielding from g q qbar Anti-shielding from g gg Net result: s (Q 2 ) decreases as Q 2 increases asymptotic freedom. – Explains why naïve QPM works at large Q 2 : s (Q 2 ) small so lowest order calculation gives good approximation. Infrared slavery quark confinement.

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77 Running Coupling Constant – QCD (2) Experiments measure s (Q) at different values of Q 2. – tau decay BR: how? – Quarkonia: how? – e + e- 3 jets: how? Good fit to QCD prediction with s (M Z )= ± Q (GeV)

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Experimental Tests SU(3) Running of s with Q 2 is implicit test of SU(3) Can we do explicit test of SU(3)? – SU(3) predicts colour factors for qqg and ggg vertices: C F =4/3 and C A =3. – Several variables sensitive to this in e + e - hadrons. Some examples: Charged particle multiplicity in q and g jets Angular distributions in 4 jet events Event shapes, e.g. Thrust 78

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Experimental Fit Fit for C F and C A consistent with SU(3) and inconsistent with other choices for the symmetry. – C A = 2.89 ± 0.21 – C F = 1.30 ± S. Kluth, Tests of QCD at e+e- colliders doi: / /69/6/R04

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80 Scaling Violations Expect log scaling violations in QCD – q q g – g q qbar – g g g As Q 2 increases, resolve more splitting. – Valence q decreases – Sea q and gluons increase at low x

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81 Scaling Violations Need to look over large range of x and Q 2. HERA data gives good coverage (high energy!) – At low x, F 2 increases as Q 2 increases – At high x, F 2 decreases as Q 2 increases – Use these scaling violations to fit the gluon distribution figures

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82 Kinematic Plane

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83 Measurements from HERA + Fixed Target Approximate scaling at intermediate x Low x: F 2 increases as Q 2 increases High x: F 2 decreases as Q 2 increases

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84 PDF Fits Q 2 =10 GeV 2 Q 2 =10,000 GeV 2 LHC : mainly gluons !

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85 Hadron-Hadron Collisions (1) Master Picture p p a b c d ab cd Convolution of PDFs with parton-parton cross section PDFs from DIS fits + QCD evolution to appropriate scale Need to calculate parton-parton cross section ab cd

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86 Hadron-Hadron Collisions (2) Master equation – Convolution integral over parton distribution functions and parton-parton cross section Parton kinematics Q 2 : scale of parton –parton reaction, e.g. M 2

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87 Drell-Yann (1) + - via virtual photon: parton-parton cross section. Master formula: Change variables (appendix) Scaling cross section:

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88 Drell-Yann (2) Scaling

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89 Drell-Yann (3) Use DY (+QCD) predict Z cross sections for pbar p and pp

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W Charge Asymmetry (1) Define asymmetry in terms of rapidity of charged lepton Assume – W + from u valence dbar sea – W - from d valence ubar sea 90

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W Charge Asymmetry (2) Assume sea symmetric ubar=dbar u(x)> d(x) expect A +ive. u(x)/d(x) increasing with x. Rapidity lepton correlated with rapidity W expect A to increase with . 91

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W Charge Asymmetry (3) 92

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Dijet Angular Distributions Dijet LHC: mainly gg gg. Angular distribution dominated by g exchange (cf Rutherford scattering) propagator 1/Q 4 Gives angular distribution ~ 1/sin 4 ( /2) Change variables to get flatter distribution: 93

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Dijet Angular Distribution 94 > % cl d< fm Sensitive to any new contact interaction cf 4 Fermi theory more isotropic distribution excess of events at low .

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95 DIS & QCD Summary Quarks are real! – Scaling point like constituents of proton – Spin ½ and charges from quark model – More to p than uud: sea quarks + gluons. QCD – Running coupling constant, explains success of naïve QPM. – Measurements of s (Q 2 ). – Extension to hadron-hadron collisions

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96 Appendix

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e+e- R

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Crossing Symmetry From the solutions to the Dirac equation we formally represent the states which apparently have negative energy as positive energy states travelling backwards in time. crossing symmetry relates the amplitudes of reactions particle anti-particle. Fortwo crossed diagrams structure of the matrix elements are the same replace the momentum of the incoming (outgoing) particles minus that of the outgoing (incoming) anti-particles.

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