University Quantum Field Theory I · topic UQFTI-14.01
The vertex function: form factors F_1(q^2) and F_2(q^2)
Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.
Further practice
Work through a fresh case before tackling the extension.
The vertex function: form factors F₁(q²) and F₂(q²): predict a finite change
The form factors F₁(q²) and F₂(q²) are the two real functions of the momentum transfer that, by Lorentz invariance, parity and current conservation, fix the whole coupling of an on-shell spin-½ particle to a photon: ū(p′)[γ^μ F₁ + (iσ^μν q_ν/2m)F₂]u(p). Use the supplied topic model Y_out = (f1-pm^2*f2)^2/(1+pm^2), where Y_out is N AT 180° (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. f1 is Dirac form factor F₁, dimensionless. Keep f2 (Pauli form factor F₂) = 1.79; pm (Momentum p/m) = 0.6 fixed. For this exercise use only 0.10625 ≤ f1 ≤ 0.63125 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. Calculate Y_out at f1 = 0.22175 and 0.29525. Find the signed output change ΔY_out and the average sensitivity ΔY_out/Δf1 across this interval. State whether the response increases or decreases; distinguish this interval average from a derivative at one point.
N AT 180° against f1
Predictions from the supplied The vertex function: form factors F₁(q²) and F₂(q²) model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): N AT 180°
Horizontal axis (x): f1: Dirac form factor F₁
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| f1: Dirac form factor F₁ | N AT 180° |
|---|---|
| 0.10625 | 0.21294516360294122 |
| 0.2375 | 0.12174088970588234 |
| 0.36875 | 0.05586979595588233 |
| 0.5 | 0.015331882352941173 |
| 0.63125 | 0.00012714889705882343 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Substitute each value of f1 separately while keeping every other input fixed, then subtract in the same order.
Reveal the worked solution
- At f1 = 0.22175, substitute into Y_out = (f1-pm^2*f2)^2/(1+pm^2): Y₀ = 0.131348.
- At f1 = 0.29525, the same fixed-parameter model gives Y₁ = 0.0896366.
- Signed change: ΔY_out = Y₁ − Y₀ = -0.0417113; the input change is Δf1 = 0.0735.
- Average sensitivity is ΔY_out/Δf1 = -0.5675 1 per unit input. Its negative sign means that this response decreases over the specified interval.
- This is a secant slope between two settings. It equals a point derivative only for a locally linear relation or in a justified small-interval limit.
Answer Y₀ = 0.131348, Y₁ = 0.0896366; ΔY_out = -0.0417113; average sensitivity = -0.5675 1 per unit input.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Extra challenging questions
Work through the linked stages and explain what each result means.
The vertex function: form factors F₁(q²) and F₂(q²): infer an input with uncertainty
The form factors F₁(q²) and F₂(q²) are the two real functions of the momentum transfer that, by Lorentz invariance, parity and current conservation, fix the whole coupling of an on-shell spin-½ particle to a photon: ū(p′)[γ^μ F₁ + (iσ^μν q_ν/2m)F₂]u(p). Use the supplied topic model Y_out = (f1-pm^2*f2)^2/(1+pm^2), where Y_out is N AT 180° (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. f1 is Dirac form factor F₁, dimensionless. Keep f2 (Pauli form factor F₂) = 1.79; pm (Momentum p/m) = 0.6 fixed. For this exercise use only 0.10625 ≤ f1 ≤ 0.63125 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.034912066, with a bounded reading uncertainty ±0.0052177. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown f1 by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0000525, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.
N AT 180° against f1
Predictions from the supplied The vertex function: form factors F₁(q²) and F₂(q²) model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): N AT 180°
Horizontal axis (x): f1: Dirac form factor F₁
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| f1: Dirac form factor F₁ | N AT 180° |
|---|---|
| 0.10625 | 0.21294516360294122 |
| 0.2375 | 0.12174088970588234 |
| 0.36875 | 0.05586979595588233 |
| 0.5 | 0.015331882352941173 |
| 0.63125 | 0.00012714889705882343 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
The model's endpoint predictions bracket the reading. Solve Y_out(f1) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.
Reveal the worked solution
- Endpoint predictions are Y_out(0.10625) = 0.212945 and Y_out(0.63125) = 0.000127149. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives f1_line = 0.10625 + (0.0349121 − 0.212945)(0.63125 − 0.10625)/(0.000127149 − 0.212945) = 0.545439. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
- Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives f1 = 0.4265; substituting back gives Y_out = 0.034912066.
- The interpolation error in the input is |f1_line − f1| = 0.118939. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.029694366 and 0.040129766. Sort the roots even if the response decreases: 0.410784 ≤ f1 ≤ 0.443441.
- The lower and upper input deviations are 0.0157161 and 0.0169415. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.
Answer f1 ≈ 0.4265; compatible interval [0.410784, 0.443441]. Endpoint interpolation gives 0.545439.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.