1. 16.451 Lecture 11: Quarks inside the proton 9/10/2003 1 Idea: try to identify a kinematic regime in which the electrons scatter from pointlike constituents inside the proton: “deep inelastic scattering” Microscopic scattering mechanism: “virtual” photon has energy and momentum given by the 4 -momentum transfer from the incoming & outgoing electrons! proton absorbs so much energy that the intermediate excited state flies apart very quickly afterwards. Mass, momentum, and total energy of the recoiling object after the collision

2. First discovery: for large energy transfer, the form factors are independent of Q 2 M. Breidenbach et al., Phys. Rev. Lett. 23, 935 (1969) Ratio of measured cross-section to pointlike prediction for the proton = form factor! Mass of recoiling object is 2 – 3 x the mass of the proton! “Scaling” – in this regime, the form factors are approximately equal and are almost independent of momentum transfer... 2

3. A measure of “inelasticity” – Ratio of momentum and energy transfer... Kinematics result from lecture 10: For elastic scattering: For inelastic scattering: The variable x measures the degree of inelasticity: 3

4. Generalization of the scattering formalism: New form factors F 1 and F 2 are called “structure functions” – they depend on both the 4-momentum transfer and the energy transfer. cross-section: kinematic factor to classify inelasticity: 4 (Note: there are two independent kinematic parameters: Q 2 and. The structure-dependent contributions to the cross section can be accounted for in terms of these two variables, or alternatively in terms of Q 2 and x.) The “scaling” discovery is equivalent to finding that the structure functions only depend on x and not on Q 2 in this regime.

5. Illustration: range of x in electron – proton scattering  increasing energy transfer... structure function F 2 (x,Q 2 ) 5 x = 1: Elastic dashed line: very large Q 2 solid line: much smaller Q 2 “Deep inelastic region” x = 1/3; large and small Q 2 have the same structure function.

6. Evidence of point-like quarks comes from “scaling” of the structure functions Idea: pointlike scattering object has a constant form factor or structure function. The proton structure functions are essentially independent of Q 2 in the deep inelastic regime, indicating scattering from pointlike constituents with mass approx 1/3 the proton mass  u and d quarks! 6 This line is flat!!!

7. Contrast: elastic and inelastic form factors! proton electric form factor for elastic scattering, x = 1, falls off rapidly with increasing Q 2 because the proton is not a pointlike object!!! (the whole proton recoils in this case) proton “F 2 ” structure function, deep inelastic regime, x = 0.25. independent of Q 2 (a pointlike object inside the proton recoils after the collision) 7

8.  Broad peak at x = 1/3... ? “Deep inelastic region” x = 1/3 x = 1: Elastic increasing energy transfer... structure function F 2 (x,Q 2 ) Look again at the features in x: 8

9. Scattering from quarks? Elastic scattering from the proton is a narrow peak at x = 1 Deep inelastic scattering shows a broad peak at x = 1/3... ??? If the quark has mass m = M/3, then we should see a peak at: The peak is broad because the quark is confined in a small space – the quark `target’ is moving in a random direction inside the proton (very fast!!!) Estimate: 9

10. A bit more about “x” Feynman’s “parton” model, 1969: viewed in a frame in which the proton is moving relativistically, deep inelastic scattering (DIS) involves the electron scattering from a single “parton” with a fraction x of the proton’s total momentum. Comparing DIS structure functions for different targets (n, p, 3 He, etc...) allows the individual quark and anti-quark distribution functions to be mapped out, ie, we can measure what fraction of the proton’s momentum is carried by each kind of quark. Observations: 1. only about 60% of the proton’s momentum is carried by quarks 2. only about 30% of the proton’s spin is due to the quarks... so what else is in there ???? 10

11. We don’t know exactly, but there is a great deal of effort underway to find out! Have another look at the NSAC long range plan (web link, lecture 1.) p.14 – 28 –you should be able to understand a lot more of it now!!! (N.B. scattering at very small x is sensitive to the presence of anti- quarks inside the proton – these are important too! (research frontier)... 11