Measurement of the forward-backward asymmetry in ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ baryon production in $p\overline{p}$ collisions at $\sqrt{s}=1.96$ TeV

We measure the forward-backward asymmetry in the production of ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ baryons as a function of rapidity in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ using $10.4{\mathrm{fb}}^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron collider. The asymmetry is determined by the preference of ${\mathrm{\Lambda }}_b^0$ or ${\overline{\mathrm{\Lambda }}}_b^0$ particles to be produced in the direction of the beam protons or antiprotons, respectively. The measured asymmetry integrated over rapidity $y$ in the range $0.1<|y|<2.0$ is $A=0.04\pm 0.07(\text{stat})\pm 0.02(\text{syst})$.

Figure 1

Invariant mass distribution of ${\mathrm{\Lambda }}_b^0\to J/\psi \mathrm{\Lambda }$ and ${\overline{\mathrm{\Lambda }}}_b^0\to J/\psi \overline{\mathrm{\Lambda }}$ candidates in the rapidity range $0.5<|y|<1.0$ in the (a) forward and (b) backward categories. The fit of a Gaussian signal function with a second-order Chebyshev polynomial background function is superimposed. The vertical lines define the signal region.

Figure 2

Measured forward-backward asymmetry $A$ versus rapidity $|y|$ compared to predictions of the Heavy Quark Recombination model [15] and a simulated effect of the longitudinal momentum shift due to beam drag (see Ref. [2] and text). The background asymmetry is obtained from $J/\psi \mathrm{\Lambda }$ candidates in the ${\mathrm{\Lambda }}_b^0$ mass sidebands (uncertainties are small compared to the symbol size). Measurements are placed at the centers of the rapidity intervals defined in Table 1.

Figure 3

Measured ratio of the backward to forward production cross sections versus rapidity loss compared to the ${\overline{\mathrm{\Lambda }}}_b^0$ to ${\mathrm{\Lambda }}_b^0$ production cross-section ratio at CMS taken from Table II of Ref. [16]. Measurements are placed at the centers of their rapidity loss ranges.