Overview The Structure of the Proton Lecture 1- Chs1/2

1. Overview The Structure of the Proton Lecture 1- Chs1/2 Quark-Parton Model Lecture-1 Chs1/2 QCD and the QCD improved parton model Lectures 2/3 Chs 3/4 DGLAP resumming LL NLL in ln(Q2) Lecture 3 Ch 4 Parton Distribution Functions and their Uncertainties Lectures 4/5/6 Ch 6,7,8 QCD at low-x, low-Q2 Lecture 6/7/8 Ch 9 Importance for LHC physics Lecture 9/10 Ch 10 but not always in a book! Devenish and Cooper-Sarkar ‘Deep Inelastic Scattering’ OUP 2004

2. Scattering: elastic, inelastic,deep-inelastic For a charge g from a potential charge Zg through the mediation of a quantum of mass μ. Assume free waves for r>>1/μ where The Matrix Element has the form of a coupling constant squared times a propagator Then using the Golden Rule Which can describe Rutherford Scattering as μ→0, What happens if the scattering centre has finite size? The form factor for a proton is ~ Such a form is NOT point-like.

3. Now consider Inelastic scattering q=k-k’, p2=mN2, PX2=MX2 k+p=k’+pX, pX2 = p2 +q2 +2p.q, so Q2 =-q2= 2p.q + mN2 –mX2 defines the positive momentum transfer squared. If the target is at rest and the incident and scattered energy of the lepton probe is E and E’ respectively then If the final state hadronic mass is known- as for elastic scattering OR the production of specific hadron resonances like N* and Δ - then E’ and θ are not independent But if we integrate over final states – ‘inclusive’ cross-sections- then they are and we usually use the two variables Q2 and x=Q2/2p.q to describe the process. (For elastic scattering x=1). We will then need form factors or ‘structure functions which depend on x and Q2 The figures show inclusive spectra for E’ at low and slightly higher energies. The peaks correspond to the nucleon resonances. One can see the importance of these decreasing as the energy increases- we are moving into the deep-inelastic region. What is the origin of region B? Could it be scattering from constituents of the proton? Wouldn’t this lead to more peaks? Not if these constituents have momentum~200MeV -due to their confinement

4. PDFs were first investigated in deep inelastic lepton-nucleon scatterning -DIS Leptonic tensor - calculable 2 Lμν Wμν dσ~ Hadronic tensor- constrained by Lorentz invariance E Ee Ep q = k – k’, Q2 = -q2 s= (p + k)2 x = Q2 / (2p.q) y = (p.q)/(p.k) Q2 = s x y This is the scale of the vector boson probe These are 4-vector invariants s = 4 Ee Ep Q2 = 4 Ee E sin2θe/2 y = (1 – E/Ee cos2θe/2) x = Q2/sy The kinematic variables are measurable

6. Without assumptions as to what goes on in the hadron the double differential cross-section for e± N scattering can be written as d2(e±N) = [ Y+ F2(x,Q2) - y2 FL(x,Q2) ± Y_xF3(x,Q2)], Y± = 1 ± (1-y)2 dxdy Leptonic part hadronic part F2, FL and xF3 are structure functionswhich express the dependence of the cross-section on the structure of the nucleon (hadron)– The Quark-Parton Model interprets these structure functions as related to the momentum distributions of point-like quarks or partons within the nucleon AND the measurable kinematic variablex = Q2/(2p.q)is interpreted as the FRACTIONAL momentum of the incoming nucleon taken by the struck quark We can extract all three structure functions experimentally by looking at the x, y, Q2 dependence of the double differential cross-section- thus we can check out the parton model predictions (xP+q)2=x2p2+q2+2xp.q ~ 0 for massless quarks and p2~0so x = Q2/(2p.q) The FRACTIONAL momentum of the incoming nucleon taken by the struck quark is the MEASURABLE quantity x

9. Now consider neutrino scattering Let’s see the early data on Bjorken scaling And on FL=0

10. There is clearly a need for the qbar term This leads to the idea of 3-valence quarks PLUS a q-qbar Sea Where does the Sea come from? q→q g g→q-qbar

13. So hopefully you now believe that the Quark Parton Model has some basis in fact End lecture-1