CMS-PAS-BPH-15-002

CMS-PAS-BPH-15-002
Measurement of the $\Lambda_{b}$ polarization and the angular parameters of the decay $\Lambda_{b}\to J/\psi(\mu^{+}\mu^{-}) \Lambda^{0}(p\pi^{-})$
CMS Collaboration
August 2016

Abstract: We present a measurement of the $\Lambda_{b}$ polarization based on an angular analysis of the decay $\Lambda_{b}\to J/\psi(\mu^{+}\mu^{-}) \Lambda^{0}(p\pi^{-})$, using data from pp collisions at $\sqrt{s} =$ 7 TeV and 8 TeV collected with the CMS detector. A transverse $\Lambda_{b}$ polarization of 0.00 $\pm$ 0.06 (stat) $\pm$ 0.02 (syst) is measured.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRD 97 (2018) 072010. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Definition of the angles used to describe the $\Lambda _{b} \rightarrow J/\psi (\rightarrow \mu ^{+}\mu ^{-})\Lambda ^{0}(\rightarrow p\pi ^{-})$ decay.
png pdf	Figure 2: Distribution of the $p$ vs $\pi $ momentum obtained from the simulation of $\Lambda _{b}\to J/\psi \Lambda (p \pi ^{-})$ (and complex conjugate) decays. Events lie always above the green line, ie., the $p^{+}$ ($p^{-}$) momentum is always greater than $\pi ^{-}$ ($\pi ^{+}$) momentum.
png pdf	Figure 3-a: Invariant mass distribution of $\Lambda_b$ (a and c) and $\overline{\Lambda}_b$ (b and d) candidates in (a-b) 2011 and (c-d) 2012 samples. The solid blue line represents the fit result. The signal (background) projection is shown with a dashed (dot-dashed) red (blue) line.
png pdf	Figure 3-b: Invariant mass distribution of $\Lambda_b$ (a and c) and $\overline{\Lambda}_b$ (b and d) candidates in (a-b) 2011 and (c-d) 2012 samples. The solid blue line represents the fit result. The signal (background) projection is shown with a dashed (dot-dashed) red (blue) line.
png pdf	Figure 3-c: Invariant mass distribution of $\Lambda_b$ (a and c) and $\overline{\Lambda}_b$ (b and d) candidates in (a-b) 2011 and (c-d) 2012 samples. The solid blue line represents the fit result. The signal (background) projection is shown with a dashed (dot-dashed) red (blue) line.
png pdf	Figure 3-d: Invariant mass distribution of $\Lambda_b$ (a and c) and $\overline{\Lambda}_b$ (b and d) candidates in (a-b) 2011 and (c-d) 2012 samples. The solid blue line represents the fit result. The signal (background) projection is shown with a dashed (dot-dashed) red (blue) line.
png pdf	Figure 4-a: Angular efficiencies for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$, $\cos\theta _{\mu }$ (from a to c) obtained from simulated $\Lambda _b \rightarrow J/\psi \Lambda ^0$. The angular efficiencies are parametrized with Chebychev polynomials and are fitted simultaneously to the three angular distributions.
png pdf	Figure 4-b: Angular efficiencies for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$, $\cos\theta _{\mu }$ (from a to c) obtained from simulated $\Lambda _b \rightarrow J/\psi \Lambda ^0$. The angular efficiencies are parametrized with Chebychev polynomials and are fitted simultaneously to the three angular distributions.
png pdf	Figure 4-c: Angular efficiencies for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$, $\cos\theta _{\mu }$ (from a to c) obtained from simulated $\Lambda _b \rightarrow J/\psi \Lambda ^0$. The angular efficiencies are parametrized with Chebychev polynomials and are fitted simultaneously to the three angular distributions.
png pdf	Figure 5-a: Angular background distributions obtained from the mass sidebands are shown from left to right for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$ and $\cos\theta _{\mu }$. The solid blue lines are the results of the fits.
png pdf	Figure 5-b: Angular background distributions obtained from the mass sidebands are shown from left to right for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$ and $\cos\theta _{\mu }$. The solid blue lines are the results of the fits.
png pdf	Figure 5-c: Angular background distributions obtained from the mass sidebands are shown from left to right for $\cos\theta _{\Lambda }$, $\cos\theta _{p}$ and $\cos\theta _{\mu }$. The solid blue lines are the results of the fits.
png pdf	Figure 6-a: Distributions of $m(J/\psi \Lambda )$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\Lambda _b$ candidates in 2011 and 2012 datasets in $m(J/\psi \Lambda )\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 6-b: Distributions of $m(J/\psi \Lambda )$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\Lambda _b$ candidates in 2011 and 2012 datasets in $m(J/\psi \Lambda )\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 6-c: Distributions of $m(J/\psi \Lambda )$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\Lambda _b$ candidates in 2011 and 2012 datasets in $m(J/\psi \Lambda )\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 6-d: Distributions of $m(J/\psi \Lambda )$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\Lambda _b$ candidates in 2011 and 2012 datasets in $m(J/\psi \Lambda )\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 7-a: Distributions of $m(J/\psi \overline{\Lambda })$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\overline{\Lambda }_b$ candidates in 2011 and 2012 datasets in $m(J/\psi \overline{\Lambda })\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 7-b: Distributions of $m(J/\psi \overline{\Lambda })$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\overline{\Lambda }_b$ candidates in 2011 and 2012 datasets in $m(J/\psi \overline{\Lambda })\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 7-c: Distributions of $m(J/\psi \overline{\Lambda })$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\overline{\Lambda }_b$ candidates in 2011 and 2012 datasets in $m(J/\psi \overline{\Lambda })\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).
png pdf	Figure 7-d: Distributions of $m(J/\psi \overline{\Lambda })$, $\cos\theta _{p}$, $\cos\theta _{\Lambda }$, $\cos\theta _{\mu }$ (from a to d) for $\overline{\Lambda }_b$ candidates in 2011 and 2012 datasets in $m(J/\psi \overline{\Lambda })\in [5.56, 5.68]$, with fit results superimposed. The signal (background) component is shown in red (gray).

Tables
png pdf	Table 1: Functions describing the angular distribution of the decay $\Lambda _{b} \rightarrow J/\psi \Lambda ^{0}$, $J/\psi \rightarrow \mu ^+ \mu ^-$, $\Lambda ^{0} \rightarrow p \pi ^-$.
png pdf	Table 2: Correlation Matrix of the fitted parameters.
png pdf	Table 3: Summary of systematic uncertainties.

Summary

Based on an angular analysis of about 6000 $\Lambda_b \rightarrow J/\psi(\rightarrow\mu^{+}\mu^{-})\Lambda(\rightarrow p\pi^{-})$ decays collected by the CMS experiment in 2011 and 2012, we perform a measurement of the $\Lambda_b$ polarization ($P$), the weak asymmetry parameter of the $\Lambda_b$ decay ($\alpha_1$), the $\Lambda^0$ longitudinal polarization ($\alpha_2$), and a measure of the longitudinal/transverse composition of the $J/\psi$ meson ($\gamma_0$). The measured values are

$P= 0.00 \pm 0.06 \text{(stat)} \pm 0.02 \text{(syst)}$,
$\alpha_1 = 0.12 \pm 0.13 \text{(stat)} \pm 0.06 \text{(syst)}$,
$\alpha_2 = -0.93 \pm 0.04 \text{(stat)} \pm 0.04 \text{(syst)}$,
$\gamma_0 = -0.46 \pm 0.07 \text{(stat)} \pm 0.04 \text{(syst)}$

corresponding to the helicity amplitudes

$ |T_{-0}|^2 = 0.51 \pm 0.03\text{(stat)} \pm 0.02\text{(syst)}$,
$|T_{+0}|^2= -0.02 \pm 0.03\text{(stat)} \pm 0.02\text{(syst)}$,
$|T_{--}|^2= 0.46 \pm 0.02\text{(stat)} \pm 0.02\text{(syst)}$,
$|T_{++}|^2= 0.05 \pm 0.04\text{(stat)} \pm 0.02\text{(syst)}$

Our result of the $\Lambda_b$ polarization is compatible with predictions from perturbative QCD calculations [21] for a polarization of ${\sim}$10% at 1.5$\sigma$, but it disfavors the 20% expectation reported in Ref. [22]. Also, as expected [11], our data analysis indicates a maximum longitudinal $\Lambda^0$ polarization $\alpha_2$, or, equivalently, strongly suppressed transitions into the $\lambda_\Lambda =$ 1/2 helicity states of the $\Lambda$ baryon ($T_{+0}$ and $T_{++}$).

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