Measurement of the forward-backward asymmetry of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ production in $p\overline{p}$ collisions

We study $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ production asymmetries in $p\overline{p}\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$, $p\overline{p}\to J/\psi \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$, and $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ events recorded by the D0 detector at the Fermilab Tevatron collider at $\sqrt{s}=1.96\mathrm{TeV}$. We find an excess of $\mathrm{\Lambda }$’s ($\overline{\mathrm{\Lambda }}$’s) produced in the proton (antiproton) direction. This forward-backward asymmetry is measured as a function of rapidity. We confirm that the $\overline{\mathrm{\Lambda }}/\mathrm{\Lambda }$ production ratio, measured by several experiments with various targets and a wide range of energies, is a universal function of “rapidity loss,” i.e., the rapidity difference of the beam proton and the lambda.

Figure 1

Invariant mass distribution of $\mathrm{\Lambda }\to p{\pi }^{-}$ candidates for $0.0<y<1.0$, muon charge $q=+1$, solenoid magnet polarity $-1$, and toroid magnet polarity $-1$, for the $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ data. Other selection requirements are given in the text.

Figure 2

Distributions of (a) generated and (b) reconstructed $\mathrm{\Lambda }$’s (blue circles) and $\overline{\mathrm{\Lambda }}$’s (red triangles), and (c) the corresponding efficiencies, for $p_T>2.0\mathrm{GeV}$, from QCD simulations of inclusive $p\overline{p}$ collisions containing a minimum parton transverse energy $E_T^{\min }>20\mathrm{GeV}$. For details of the simulation see Ref. [12].

Figure 3

Distributions of (a) $p_T$, (b) $p_z$, and (c) $y$ of reconstructed $\mathrm{\Lambda }$’s (blue circles) and $\overline{\mathrm{\Lambda }}$’s (red triangles) with $p_T>2.0\mathrm{GeV}$, for the minimum bias data sample $p\overline{p}\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$.

Figure 4

Asymmetries (a) $A_{FB}=A_{FB}^{\prime }$, (b) $A_{NS}^{\prime }$, and (c) $A_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}^{\prime }$ of reconstructed $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ (blue circles) and $K_S$ (red triangles) with $p_T>2.0\mathrm{GeV}$, as functions of $|y|$, for the minimum bias data samples $p\overline{p}\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ and $p\overline{p}\to K_{S}X$, respectively. Uncertainties are statistical.

Figure 5

Corrected asymmetry $A_{NS}=A_{NS}^{\prime }-A_{NS}^{\prime }(K_S)$ of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ with $p_T>2.0\mathrm{GeV}$, as a function of $|y|$, for the minimum bias data sample $p\overline{p}\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$. Uncertainties are statistical.

Figure 6

$\overline{\mathrm{\Lambda }}/\mathrm{\Lambda }$ production ratio as a function of the rapidity loss $\mathrm{\Delta }y\equiv y_p-y$ for several experiments that study reactions $pZ\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ for targets $Z=p,\overline{p}$, Be, and Pb. The experiments are ALICE [13], ATLAS [14], D0 (this analysis), STAR [15], LHCb [16], ISR R-607 [17], ISR R-603 [18], and the fixed target experiment Fermilab E8 studying $p$-Be and $p$-Pb collisions at a beam energy of 300 GeV [19].

Figure 7

Same as Fig. 6 with logarithmic scale.

Figure 8

Asymmetries (a) $A_{FB}=A_{FB}^{\prime }$ and (b) $A_{NS}=A_{NS}^{\prime }-A_{NS}^{\prime }(K_S)$ of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ with $p_T>2.0\mathrm{GeV}$, as functions of $|y|$, for the data sample $p\overline{p}\to J/\psi \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$. Uncertainties are statistical.

Figure 9

Distributions of rapidity $y$ of reconstructed $\mathrm{\Lambda }$’s (blue circles) and $\overline{\mathrm{\Lambda }}$’s (red triangles) for events with (a) ${\mu }^{+}$ or (b) ${\mu }^{-}$, for $p_T>2.0\mathrm{GeV}$, for events $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$.

Figure 10

Asymmetries (a) $A_{FB}=A_{FB}^{\prime }$, (b) $A_{NS}^{\prime }$, and (c) $A_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}^{\prime }$ of reconstructed $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ (blue circles) and $K_S$ (red triangles) with $p_T>2.0\mathrm{GeV}$, as functions of $|y|$, for events $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ and $p\overline{p}\to {\mu }^{\pm }{K}_{S}X$, respectively. Uncertainties are statistical.

Figure 11

Asymmetry $A_{FB}$ as a function of $|y|$ for events $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ for (a) $2.0<p_T<4.0\mathrm{GeV}$, (b) $4.0<p_T<6.0\mathrm{GeV}$, and (c) $p_T>6.0\mathrm{GeV}$. Uncertainties are statistical.

Figure 12

Asymmetry $A_{FB}$ as a function of $|y|$ for events $p\overline{p}\to \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ (green circles), $p\overline{p}\to J/\psi \mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ (red squares), and $p\overline{p}\to {\mu }^{\pm }\mathrm{\Lambda }(\overline{\mathrm{\Lambda }})X$ (blue triangles) for $p_T>2.0\mathrm{GeV}$. Uncertainties are statistical.