The antiproton, p, is the antiparticle of the proton. The proton has quark composition uud, with quark charges +2/3e for the up quark and -1/3e for the down quark. (a) State the general relationship between the mass of any particle and the mass of its corresponding antiparticle,…

Answer: ((a)) A particle and its corresponding antiparticle have identical mass but opposite charge ((b)) The antiproton is composed of two anti-up quarks and one anti-down quark, u u d ((c)) Total charge = (-2/3e)+(-2/3e)+(+1/3e) = -1e, which is opposite in sign (but equal in magnitude) to the proton's charge of +1e, consistent with (a) ((d)) The antiproton is built from three (anti)quarks, the same number of constituents as a baryon (rather than two, as in a meson), so it belongs to the baryon family of hadrons. Specifically, it is an antibaryon. Part (a): Mass and charge of a particle-antiparticle pair For any particle and its corresponding antiparticle: the two have identical mass, but opposite charge (equal in magnitude, opposite in sign). This is a general property of all parti…

A Level Physics 9702: Antiproton Composition, Charge and Classification (Structured)

The antiproton, $\bar{p}$ , is the antiparticle of the proton. The proton has quark composition $uud$ , with quark charges $+\frac23e$ for the up quark and $-\frac13e$ for the down quark.

(a) State the general relationship between the mass of any particle and the mass of its corresponding antiparticle, and the general relationship between their charges. [2]

(b) State the composition of the antiproton in terms of antiquarks. [1]

(c) Given that each antiquark carries the exact opposite charge to its corresponding quark, calculate the total charge of the antiproton. Show that this result is consistent with your answer to (a). [3]

(d) The antiproton is built from three antiquarks rather than a quark-antiquark pair. State, with a reason, whether this places the antiproton in the same family of hadrons as the proton (baryons) or in the family of mesons. [2]

Part (a): Mass and charge of a particle-antiparticle pair

For any particle and its corresponding antiparticle: the two have identical mass, but opposite charge (equal in magnitude, opposite in sign). This is a general property of all particle-antiparticle pairs, including the proton and antiproton.

Part (b): Quark composition of the antiproton

The proton has quark composition $uud$ (two up quarks and one down quark). The antiproton, being the antiparticle of the proton, is composed of the corresponding antiquarks: $\bar{p} = \bar u\bar u\bar d$ that is, two anti-up quarks and one anti-down quark.

Part (c): Total charge of the antiproton

Each antiquark carries the exact opposite charge to its corresponding quark: $\text{anti-up} = -\left(+\frac23e\right) = -\frac23e, \qquad \text{anti-down} = -\left(-\frac13e\right) = +\frac13e$

Summing the charges of the two anti-up quarks and one anti-down quark: $Q = \left(-\frac23e\right) + \left(-\frac23e\right) + \left(+\frac13e\right)$

Combining the two anti-up contributions first: $-\frac23e - \frac23e = -\frac43e$

Then adding the anti-down contribution: $Q = -\frac43e + \frac13e = -\frac33e = -1e$

The antiproton’s total charge is $-1e$ . Since the proton’s charge is $+1e$ , the two charges are equal in magnitude but opposite in sign. Exactly the relationship stated in part (a), confirming the result is consistent.

Part (d): Classifying the antiproton

The antiproton is built from three (anti)quarks, two anti-up quarks and one anti-down quark, rather than a quark-antiquark pair. Since it is the number of constituent (anti)quarks that distinguishes the two families of hadrons (three for baryons, a quark-antiquark pair for mesons), the antiproton belongs to the baryon family, not the meson family. More precisely, because all three of its constituents are antiquarks, it is classified as an antibaryon. The antiparticle of the proton, which is itself a baryon.

Final answers

(a) Identical mass; opposite (equal-magnitude, opposite-sign) charge.
(b) $\bar{p} = \bar u\bar u\bar d$
(c) $Q = \boxed{-1e}$ , opposite in sign to the proton’s $+1e$ , consistent with (a).
(d) Baryon family (an antibaryon), built from three (anti)quarks, not a quark-antiquark pair.