Measurement of the transverse polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons produced in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ using the ATLAS detector

The transverse polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons produced in proton-proton collisions at a center-of-mass energy of 7 TeV is measured. The analysis uses $760\mu {\mathrm{b}}^{-1}$ of minimum bias data collected by the ATLAS detector at the LHC in the year 2010. The measured transverse polarization averaged over Feynman $x_{\mathrm{F}}$ from $5\times 10^{-5}$ to 0.01 and transverse momentum $p_{\mathrm{T}}$ from 0.8 to 15 GeV is $-0.010\pm 0.005(\text{stat})\pm 0.004(\text{syst})$ for $\mathrm{\Lambda }$ and $0.002\pm 0.006(\text{stat})\pm 0.004(\text{syst})$ for $\overline{\mathrm{\Lambda }}$. It is also measured as a function of $x_{\mathrm{F}}$ and $p_{\mathrm{T}}$, but no significant dependence on these variables is observed. Prior to this measurement, the polarization was measured at fixed-target experiments with center-of-mass energies up to about 40 GeV. The ATLAS results are compatible with the extrapolation of a fit from previous measurements to the $x_{\mathrm{F}}$ range covered by this measurement.

Figure 1

Fit to the invariant mass distribution for $\mathrm{\Lambda }$ candidates used to extract the signal fractions in the signal region and sidebands. The vertical dashed lines mark the boundaries between the mass regions. The fit for $\overline{\mathrm{\Lambda }}$ candidates yields similar results, which are listed in Table 1.

Figure 2

The signal fraction in the left sideband, the signal region, and the right sideband. Displayed statistical uncertainties are obtained from the mass fits.

Figure 3

The first moment of the (a) $\mathrm{\Lambda }$ and (b) $\overline{\mathrm{\Lambda }}$ decay angle distribution, $E$, and the corresponding least-squares fit. The signal component corresponds to the product $f_i^{\text{sig}}\{E_i^{\mathrm{MC}}(0)+[E_i^{\mathrm{MC}}(1)-E_i^{\mathrm{MC}}(0)]P\}$, the background component to $(1-f_i^{\text{sig}})E_{\text{bkg}}$, and the fit is the sum of the two [see Eq. (5)].

Figure 4

First moments of the angular distributions of MC signal events with known polarization.

Figure 5

Comparison of the measured decay angle distribution in data and MC samples for $\mathrm{\Lambda }$ events in the signal region. The MC events are reweighted using the extracted value of the polarization. The background contribution is estimated from the sideband regions. The probability of the ${\chi }^2$ test, $P_{{\chi }^2}$, is 0.91. For comparison, the simulated distribution for $P=-0.30$ is also shown.

Figure 6

Transverse polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons as a function of (a) $x_{\mathrm{F}}$ and (b) $p_{\mathrm{T}}$. The thick error bars represent statistical uncertainty of the measurements; the narrow error bars are total uncertainties. Data points for $\overline{\mathrm{\Lambda }}$ are displaced horizontally for better legibility of the results.

Figure 7

The $\mathrm{\Lambda }$ reconstruction and selection efficiency as a function of $p_{\mathrm{T}}$ and $x_{\mathrm{F}}$. A drop in efficiency for events with $|\eta |\approx 1.8$ is caused by the geometry of the ID. Absolute statistical uncertainties are less than 0.6% in all bins.

Figure 8

The $\mathrm{\Lambda }$ transverse polarization measured by ATLAS compared to measurements from lower center-of-mass energy experiments. HERA-B data are taken from Ref. [5], NA48 from Ref. [4], E799 data from Ref. [3], and M2 from Ref. [2]. The HERA-B results are transformed to positive values of $x_{\mathrm{F}}$ using Eq. (1).