This Letter reports the observation of <math><mi>W</mi><mi>Z</mi><mi>γ</mi></math> production and a measurement of its cross section using <math><mn>140.1</mn><mo>±</mo><mn>1.2</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The <math><mi>W</mi><mi>Z</mi><mi>γ</mi></math> production cross section, with both the <math><mi>W</mi></math> and <math><mi>Z</mi></math> bosons decaying leptonically, <math><mi>p</mi><mi>p</mi><mo>→</mo><mi>W</mi><mi>Z</mi><mi>γ</mi><mo>→</mo><msup><msup><mo>ℓ</mo><mo>′</mo></msup><mo>±</mo></msup><mi>ν</mi><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup><mi>γ</mi></math> (<math><msup><mo>ℓ</mo><mrow><msup><mo>(</mo><mo>′</mo></msup><mo>)</mo></mrow></msup><mo>=</mo><mi>e</mi></math>, <math><mrow><mi>μ</mi></mrow></math>), is measured in a fiducial phase-space region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross section is found to be <math><mn>2.01</mn><mo>±</mo><mn>0.30</mn><mo>(</mo><mrow><mi>stat</mi></mrow><mo>)</mo><mo>±</mo><mn>0.16</mn><mo>(</mo><mrow><mi>syst</mi></mrow><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>fb</mi></math>. The corresponding standard model predicted cross section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is <math><mrow><mn>1.50</mn><mo>±</mo><mn>0.06</mn><mtext> </mtext><mtext> </mtext><mi>fb</mi></mrow></math>. The observed significance of the <math><mi>W</mi><mi>Z</mi><mi>γ</mi></math> signal is <math><mn>6.3</mn><mi>σ</mi></math>, compared with an expected significance of <math><mn>5.0</mn><mi>σ</mi></math>.

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Observation of <math><mi>W</mi><mi>Z</mi><mi>γ</mi></math> Production in <math><mi>p</mi><mi>p</mi></math> Collisions at <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> with the ATLAS Detector