cern.ch

Analysis of the resonant components in $\overline{B}^0 \to J/\psi \pi^+ \pi^-$

[to restricted-access page]

Abstract

Interpretation of CP violation measurements using charmonium decays, in both the B0 and Bs systems, can be subject to changes due to "penguin" type diagrams. These effects can be investigated using measurements of the Cabibbo-suppressed B0->J/\psi pi+pi- decays. The final state composition of this channel is investigated using a 1.0/fb sample of data produced in 7 TeV pp collisions at the LHC and collected by the LHCb experiment. A modified Dalitz plot analysis is performed using both the invariant mass spectra and the decay angular distributions. An improved measurement of the B0->J/\psi pi+pi- branching fraction of (3.97 +/-0.09+/- 0.11 +/- 0.16)x10^{-5} is reported where the first uncertainty is statistical, the second is systematic and the third is due to the uncertainty of the branching fraction of the decay B- -> J/\psi K- used as a normalization channel. In the J/\psi pi+pi- final state significant production of f0(500) and rho(770) resonances is found, both of which can be used for CP violation studies. In contrast evidence for the f0(980) resonance is not found, thus establishing the first upper limit on the branching fraction product B(B0->J/\psi f0(980) x B(f0(980)-> pi+ pi-) < 1.1x10^{-6}, leading to an upper limit on the absolute value of the mixing angle of the f0(980)$ with the f0(500) of <31 degrees, both at 90% confidence level.

Figures and captions

(a) Tree level and (b) penguin diagram examples for $\overline{ B }{} ^0 $ decays into $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^0_{\rm\scriptscriptstyle S} $.

feyn4.pdf [56 KiB]
HiDef png [85 KiB]
Thumbnail [19 KiB]
*.C file
feyn4.pdf

(a) Tree level and (b) penguin diagram for $\overline{ B }{} ^0 $ decays into $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$.

feyn3.pdf [58 KiB]
HiDef png [96 KiB]
Thumbnail [21 KiB]
*.C file
feyn3.pdf

Distributions of the BDT classifier for both training and test samples of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ signal and background events. The signal samples are from simulation and the background samples are from data.

overtr[..].pdf [33 KiB]
HiDef png [286 KiB]
Thumbnail [61 KiB]
*.C file
overtrain_BDT.pdf

Invariant mass of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ combinations. The data are fitted with a double-Gaussian signal and several background functions. The (red) solid double-Gaussian function centered at 5280 $\mathrm{\,MeV}$ is the $\overline{ B }{} ^0 $ signal, the (brown) dotted line shows the combinatorial background, the (green) short-dashed shows the $B^-$ background, the (purple) dot-dashed line shows the contribution of $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ decays, the (black) dot-long dashed is the sum of $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \eta'(\rightarrow \rho \gamma)$ and $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \phi(\rightarrow \pi^+\pi^-\pi^0)$ backgrounds, the (light blue) long-dashed is the $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^- \pi^+$ reflection, and the (blue) solid line is the total.

fitmas[..].pdf [44 KiB]
HiDef png [416 KiB]
Thumbnail [87 KiB]
*.C file
fitmass-vetoks.pdf

Invariant mass of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^-$ combinations. The data points are fitted with a double-Gaussian function for signal and a linear function for background. The dotted line shows the background, and the (blue) solid line is the total.

Bu2JpsiK.pdf [34 KiB]
HiDef png [226 KiB]
Thumbnail [52 KiB]
*.C file
Bu2JpsiK.pdf

Distribution of $m^2(\pi^+\pi^-)$ versus $m^2( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+)$ for $\overline{ B }{} ^0 $ candidate decays within $\pm20$ $\mathrm{\,MeV}$ of the $\overline{ B }{} ^0 $ mass.

dalitz.pdf [130 KiB]
HiDef png [480 KiB]
Thumbnail [168 KiB]
*.C file
dalitz.pdf

Distribution of (a) $m( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+)$ and (b) $m(\pi^+\pi^-)$ for $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ candidate decays within $\pm20$ $\mathrm{\,MeV}$ of $\overline{ B }{} ^0 $ mass shown with the solid line. The (red) points with error bars show the background contribution determined from $m( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-)$ fits performed in each bin.

m-jpsipi.pdf [39 KiB]
HiDef png [226 KiB]
Thumbnail [63 KiB]
*.C file
m-jpsipi.pdf
m-pipi.pdf [39 KiB]
HiDef png [249 KiB]
Thumbnail [65 KiB] *.C file
m-pipi.pdf

Distributions of $\cos\theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ for the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ simulated sample in (a) the entire dipion mass region and (b) $\rho(770)$ region.

cosH_eff.pdf [35 KiB]
HiDef png [225 KiB]
Thumbnail [54 KiB]
*.C file
cosH_eff.pdf
cosH_e[..].pdf [36 KiB]
HiDef png [244 KiB]
Thumbnail [62 KiB] *.C file
cosH_eff_rho.pdf

Exponential fit to the acceptance parameter $a(s_{12})$ used in Eq. 18.

cosHacc.pdf [11 KiB]
HiDef png [125 KiB]
Thumbnail [52 KiB]
*.C file
cosHacc.pdf

Parametrized detection efficiency as a function of $m^2(\pi^+\pi^-)$ versus $m^2( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+)$ determined from simulation. The $z$-axis scale is arbitrary.

effmodel.pdf [519 KiB]
HiDef png [1 MiB]
Thumbnail [142 KiB]
*.C file
effmodel.pdf

Projections onto (a) $m^2( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+)$ and (b) $m^2(\pi^+\pi^-)$ of the simulated Dalitz plot used to determine the efficiency parameters. The points represent the simulated event distributions and the curves the projections of the polynomial fits.

effx.pdf [38 KiB]
HiDef png [180 KiB]
Thumbnail [47 KiB]
*.C file
effx.pdf
effy.pdf [38 KiB]
HiDef png [183 KiB]
Thumbnail [51 KiB] *.C file
effy.pdf

The $m^2(\pi\pi)$ distribution of background. The (black) histogram with error bars shows the same-sign data combinations with additional background from simulation, the (blue) points with error bars show the background obtained from the mass fits, the (black) dashed line is the partially reconstructed $\overline{ B }{} ^0_ s $ background, and the (red) dotted is the misidentified $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^- \pi^+$ contribution.

bkgcmp.pdf [37 KiB]
HiDef png [196 KiB]
Thumbnail [54 KiB]
*.C file
bkgcmp.pdf

Projections of invariant mass squared of (a) $m^2( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+)$ and (b) $m^2(\pi^+\pi^-)$ of the background Dalitz plot. The points with error bars show the same-sign combinations with additional background from simulation.

x_bkgnew.pdf [39 KiB]
HiDef png [190 KiB]
Thumbnail [60 KiB]
*.C file
x_bkgnew.pdf
y_bkgnew.pdf [38 KiB]
HiDef png [182 KiB]
Thumbnail [55 KiB] *.C file
y_bkgnew.pdf

distribution of the background in $\cos\theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ resulting from $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ candidate mass fits in each bin of $\cos\theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$. The curve represents the fitted function $1+\alpha\cos^2\theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$.

bkg_cosH.pdf [35 KiB]
HiDef png [202 KiB]
Thumbnail [57 KiB]
*.C file
bkg_cosH.pdf

Dalitz fit projections of (a) $m^2(\pi^+\pi^-)$, (b) $m^2( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^{+})$, (c) $\cos \theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ and (d) $m(\pi^+\pi^-)$ for the best model. The points with error bars are data, the signal fit is shown with a (red) dashed line, the background with a (black) dotted line, and the (blue) solid line represents the total. In (a) and (d), the shape variations near the $\rho(770)$ mass is due to $\rho(770)-\omega(782)$ interference, and the dip at the $ K ^0_{\rm\scriptscriptstyle S} $ mass [31] is due to the $ K ^0_{\rm\scriptscriptstyle S} $ veto.

y.pdf [40 KiB]
HiDef png [221 KiB]
Thumbnail [59 KiB]
*.C file
y.pdf
x.pdf [40 KiB]
HiDef png [204 KiB]
Thumbnail [61 KiB] *.C file
x.pdf
cosH.pdf [37 KiB]
HiDef png [232 KiB]
Thumbnail [65 KiB] *.C file
cosH.pdf
mpp.pdf [39 KiB]
HiDef png [276 KiB]
Thumbnail [67 KiB] *.C file
mpp.pdf

Helicity angle distributions of (a) $\cos \theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ ($\chi^2$/ndf =15/20) and (b) $\cos\theta_{\pi\pi}$ ($\chi^2$/ndf =14/20) in the $\rho(770)$ mass region defined within one full width of the $\rho(770)$ mass. The points with error bars are data, the signal fit to the best model is shown with a (red) dashed line, the background with a (black) dotted line, and the (blue) solid line represents the total.

cosH_rho.pdf [35 KiB]
HiDef png [210 KiB]
Thumbnail [58 KiB]
*.C file
cosH_rho.pdf
cospp_rho.pdf [36 KiB]
HiDef png [220 KiB]
Thumbnail [61 KiB] *.C file
cospp_rho.pdf

Background subtracted and efficiency corrected helicity distributions of (a) $\cos \theta_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ ($\chi^2$/ndf =20/20) and (b) $\cos\theta_{\pi\pi}$ ($\chi^2$/ndf =13/20) in the $\rho(770)$ mass region defined within one full width of the $\rho(770)$ mass. The points with error bars are data and the solid blue lines show the fit to the best model.

cosH_r[..].pdf [35 KiB]
HiDef png [255 KiB]
Thumbnail [63 KiB]
*.C file
cosH_rho_corr.pdf
cospp_[..].pdf [35 KiB]
HiDef png [257 KiB]
Thumbnail [64 KiB] *.C file
cospp_rho_corr.pdf

Animated gif made out of all figures.

PAPER-2012-045.gif
Thumbnail
thumbnail_PAPER-2012-045.gif

Tables and captions

Efficiency parameters to describe the acceptance on the signal Dalitz-plot.

Table_1.pdf [44 KiB]
HiDef png [108 KiB]
Thumbnail [16 KiB]
tex code
Table_1.pdf

Parameters for the background model used in Eq. 23.

Table_2.pdf [45 KiB]
HiDef png [125 KiB]
Thumbnail [19 KiB]
tex code
Table_2.pdf

Possible resonances in the $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ decay mode.

Table_3.pdf [38 KiB]
HiDef png [119 KiB]
Thumbnail [18 KiB]
tex code
Table_3.pdf

Breit-Wigner resonance parameters.

Table_4.pdf [54 KiB]
HiDef png [102 KiB]
Thumbnail [17 KiB]
tex code
Table_4.pdf

Values of $\chi^2/\text{ndf}$ and $\rm -ln\mathcal{L}$ of different resonance models.

Table_5.pdf [39 KiB]
HiDef png [77 KiB]
Thumbnail [11 KiB]
tex code
Table_5.pdf

Fit fractions and significances of contributing components for the best model, as well as the fractions of the helicity $\lambda=0$ part. The significance takes into account both statistical and systematic uncertainties.

Table_6.pdf [54 KiB]
HiDef png [70 KiB]
Thumbnail [10 KiB]
tex code
Table_6.pdf

Interference fractions ${\cal{F}}_{\lambda}^{RR'}$ (%) computed using Eq 25. Note that the diagonal elements are fit fractions defined in Eq 24.

Table_7.pdf [35 KiB]
HiDef png [55 KiB]
Thumbnail [8 KiB]
tex code
Table_7.pdf

Resonant phases from the best fit.

Table_8.pdf [38 KiB]
HiDef png [180 KiB]
Thumbnail [27 KiB]
tex code
Table_8.pdf

Fit fractions (%) of contributing components for the best model with adding one additional resonance.

Table_9.pdf [52 KiB]
HiDef png [97 KiB]
Thumbnail [16 KiB]
tex code
Table_9.pdf

Branching fractions for each channel. The upper limit at 90% CL is also quoted for the $f_0(980)$ resonance which has a significance smaller than 3$\sigma$. The first uncertainty is statistical and the second the total systematic.

Table_10.pdf [55 KiB]
HiDef png [73 KiB]
Thumbnail [12 KiB]
tex code
Table_10.pdf

Relative systematic uncertainties on branching fractions (%).

Table_11.pdf [49 KiB]
HiDef png [98 KiB]
Thumbnail [15 KiB]
tex code
Table_11.pdf

Absolute systematic uncertainties on the results of the Dalitz analysis.

Table_12.pdf [56 KiB]
HiDef png [96 KiB]
Thumbnail [15 KiB]
tex code
Table_12.pdf

Masses of light vector and scalar resonances. All values are taken from [31], except for the $f_0(500)$ [43].

Table_13.pdf [38 KiB]
HiDef png [25 KiB]
Thumbnail [3 KiB]
tex code
Table_13.pdf

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.