Using an amplitude analysis to measure the photon polarisation in decays

V. Bellée; P. Pais; A. Puig Navarro; F. Blanc; O. Schneider; K. Trabelsi; G. Veneziano

doi:10.1140/epjc/s10052-019-7127-3

EPJ

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2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2019) 79: 622
https://doi.org/10.1140/epjc/s10052-019-7127-3

Regular Article - Experimental Physics

Using an amplitude analysis to measure the photon polarisation in $B \rightarrow K\pi \pi \gamma$ decays

V. Bellée¹^*, P. Pais¹, A. Puig Navarro², F. Blanc¹, O. Schneider¹, K. Trabelsi³ and G. Veneziano¹

¹ Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
² Physik-Institut, Universität Zürich, Zürich, Switzerland
³ LAL, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France

^* e-mail: violaine.bellee@epfl.ch

Received: 26 March 2019
Accepted: 10 July 2019
Published online: 25 July 2019

Abstract

A method is proposed to measure the photon polarisation parameter $\lambda _{\gamma }$ in ${{b}} \!\rightarrow {s} {\gamma }$ transitions using an amplitude analysis of $B\!\rightarrow K\pi \pi {\gamma }$ decays. Simplified models of the ${K} {\pi } {\pi }$ system are used to simulate ${{{B} ^+}} \!\rightarrow {{K} ^+} {{\pi } ^-} {{\pi } ^+} {\gamma }$ and ${{B} ^0} \!\rightarrow {{K} ^+} {{\pi } ^-} {{\pi } ^0} {\gamma }$ decays, validate the amplitude analysis method, and demonstrate the feasibility of a measurement of the $\lambda _{\gamma }$ parameter irrespective of the model parameters. Similar sensitivities to $\lambda _{\gamma }$ are obtained with both the charged and neutral hadronic systems. In the absence of any background and distortion due to experimental effects, the statistical uncertainty expected from an analysis of ${{{B} ^+}} \!\rightarrow {{K} ^+} {{\pi } ^-} {{\pi } ^+} {\gamma }$ decays in an LHCb data set corresponding to an integrated luminosity of 9 $\,\hbox {fb}^{-1}$ is estimated to be 0.009. A similar measurement using ${{B} ^0} \!\rightarrow {{K} ^+} {{\pi } ^-} {{\pi } ^0} {\gamma }$ decays in a Belle II data sample corresponding to an integrated luminosity of 5 $\hbox {\,ab}^{-1}$ would lead to a statistical uncertainty of 0.018.