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Equation 2 · Part 13 · What a Particle Detector Actually Measures

Denominator: Bell^2

s=qBℓ28p,σpp  ∝  pBℓ2.s = \frac{qB\ell^{2}}{8p}, \qquad \frac{\sigma_p}{p} \;\propto\; \frac{p}{B\ell^{2}} .
Bℓ2B\ell^{2}

What this part means

The complete quantity below the fraction bar; it must be nonzero for this division.

Its job in the formula

Bell2l^2 is an input to the expression that computes the quantity on the left.

The passage around this formula

Momentum is not measured. Geometry is measured, and momentum is computed from it under an assumed field map. A charged particle in a solenoidal field follows a helix, and the deviation of its path from a straight line over a chord — the sagitta — carries the momentum information: s=qBℓ28p,σpp  ∝  pBℓ2s = \frac{qB\ell^{2}}{8p}, \qquad \frac{\sigma_p}{p} \;\propto\; \frac{p}{B\ell^{2}} . Here s is the sagitta, B the field, ℓ\ell the path length in the field and p the momentum [ 2 ] . Two consequences follow directly and are worth stating plainly, because they explain most of the architecture of a collider detector.

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Learn the underlying idea

A fraction a/b means a divided by b. The top number is the numerator; the bottom number is the denominator, and it cannot be zero.

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Sources cited in the surrounding passage

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