Measurement of the photon polarization in $\Lambda_{b}^{0}$ $\to$ $\Lambda$ $\gamma$ decays

Information

LHCb-PAPER-2021-030
CERN-EP-2021-230
arXiv:2111.10194 [PDF]

(Submitted on 19 Nov 2021)
Phys. Rev. D105 (2022) L051104
Inspire 1971920

Tools

Abstract

The photon polarization in $b \to s \gamma$ transitions is measured for the first time in radiative b-baryon decays exploiting the unique spin structure of $\Lambda_b \to \Lambda \gamma$ decays. A data sample corresponding to an integrated luminosity of $6\;fb^{-1}$ collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13\;TeV$ is used. The photon polarization is measured to be $\alpha_{\gamma}= 0.82^{ + 0.17 + 0.04}_{ - 0.26 - 0.13}$, where the first uncertainty is statistical and the second systematic. This result is in agreement with the Standard Model prediction and previous measurements in b-meson decays. Charge-parity breaking effects are studied for the first time in this observable and found to be consistent with $CP$ symmetry.

Figures and captions

Distribution of (left) mass, $m( p \pi\gamma )$, and (right) $\cos{\theta_p }$ for $\Lambda ^0_ b \rightarrow \Lambda \gamma $ candidates. The latter shows the subset of candidates in the signal region $m( p \pi\gamma ) \in [5387.1, 5852.1]\text{ Me V} $, delimited by dashed vertical lines in the former. The results of the fits are overlaid as well as the individual contributions.	Fig1left.pdf [20 KiB] HiDef png [286 KiB] Thumbnail [253 KiB] *.C file
	Fig1right.pdf [16 KiB] HiDef png [222 KiB] Thumbnail [205 KiB] *.C file
Distribution of $\cos{\theta_p }$ for selected (left) $\Lambda ^0_ b \rightarrow \Lambda \gamma $ and (right) $\overline{ \Lambda} {}^0_ b \rightarrow \overline{ \Lambda} \gamma $ candidates as well as the individual contributions.	Fig2left.pdf [16 KiB] HiDef png [201 KiB] Thumbnail [175 KiB] *.C file
	Fig2right.pdf [16 KiB] HiDef png [200 KiB] Thumbnail [177 KiB] *.C file
Profile of the negative log likelihood along the (top) combined, (bottom-left) $\Lambda ^0_ b \rightarrow \Lambda \gamma $ and (bottom-right) $\overline{ \Lambda} {}^0_ b \rightarrow \overline{ \Lambda} \gamma $ photon polarization values ( $\alpha_{\gamma }$ , $\alpha_{\gamma }^{-}$ and $\alpha_{\gamma }^{+}$ , respectively).	Fig3top.pdf [14 KiB] HiDef png [138 KiB] Thumbnail [131 KiB] *.C file
	Fig3left.pdf [14 KiB] HiDef png [115 KiB] Thumbnail [104 KiB] *.C file
	Fig3right.pdf [14 KiB] HiDef png [113 KiB] Thumbnail [102 KiB] *.C file
Constraints at the $1 \sigma$ level on the new physics contributions to the (top) left- and right-handed Wilson coefficients, \Ceff7 and \Cpeff7, and on the (bottom) real and imaginary parts of \Cpeff7. In the latter, the \Ceff7 coefficient is fixed to its SM value. The measurements of the inclusive branching fraction, $\mathcal{B} ( B \rightarrow X_s \gamma )$, and the $ B ^0 \rightarrow K ^0_{\mathrm{S}} \pi ^0 \gamma $ mixing-induced $ C P$ asymmetry by the Belle and BaBar experiments [5,6] are shown in blue and yellow, respectively, the $ B ^0_ s \rightarrow \phi \gamma $ and $ B ^0 \rightarrow K ^{*0} e ^+ e ^- $ measurements by the LHCb experiment [7,8] in purple and red, respectively, and the $\alpha_{\gamma }$ measurement presented in this paper in green. The global fit is shown in dashed red (green) lines including (excluding) this measurement.	Fig4top.pdf [525 KiB] HiDef png [477 KiB] Thumbnail [285 KiB] *.C file
	Fig4bottom.pdf [454 KiB] HiDef png [282 KiB] Thumbnail [200 KiB] *.C file
Animated gif made out of all figures.	PAPER-2021-030.gif Thumbnail

Supplementary Material [file]

Supplementary material full pdf	supple[..].pdf [288 KiB]
This ZIP file contains supplemetary material for the publication LHCb-PAPER-2021-030. The files are: supplementary.pdf : An overview of the extra figures supplementary_app.tex : Latex file supplementary.tex : Main latex file .pdf, .png, .eps, .C : The figures in variuous formats	Fig1-S.pdf [16 KiB] HiDef png [207 KiB] Thumbnail [166 KiB] *C file
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