Strange particle production in $p+p$ collisions at $\sqrt{s}=200$ GeV

We present strange particle spectra and yields measured at midrapidity in $\sqrt{s}=200$ GeV proton-proton ($p+p$) collisions at the BNL Relativistic Heavy Ion Collider (RHIC). We find that the previously observed universal transverse mass ($m_T\equiv \sqrt{{p_T}^2+m^2}$) scaling of hadron production in $p+p$ collisions seems to break down at higher $m_T$ and that there is a difference in the shape of the $m_T$ spectrum between baryons and mesons. We observe midrapidity antibaryon to baryon ratios near unity for Λ and Ξ baryons and no dependence of the ratio on transverse momentum, indicating that our data do not yet reach the quark-jet dominated region. We show the dependence of the mean transverse momentum $\langle p_T\rangle$ on measured charged particle multiplicity and on particle mass and infer that these trends are consistent with gluon-jet dominated particle production. The data are compared with previous measurements made at the CERN Super Proton Synchrotron and Intersecting Storage Rings and in Fermilab experiments and with leading-order and next-to-leading-order string fragmentation model predictions. We infer from these comparisons that the spectral shapes and particle yields from $p+p$ collisions at RHIC energies have large contributions from gluon jets rather than from quark jets.

Figure 1

$\mathit{dE}/\mathit{dx}$ vs momentum for STAR $p+p$ collisions at $\sqrt{s}=200$ GeV. The curves are Bichsel parametri- zations [8].

Figure 2

Distribution of $\Delta (z)$. Unshaded region is the accepted range of good reconstructed event vertices.

Figure 3

Primary vertex finding efficiency vs measured primary track multiplicity. Horizontal line at unity is only a guide for the eye.

Figure 4

Invariant mass distribution of ${\mathrm{K}}_{\mathrm{S}}^0,\Lambda ,\overline{\Lambda },{\Xi }^{-},{\overline{\Xi }}^{+}$, and ${\Omega }^{-}+{\overline{\Omega }}^{+}$ after applying the geometrical cuts outlined in Tables 1 and 2.

Figure 5

Kink angle cut regions for $K^{+}$ and $K^{-}$ identified via the kink method. Particles falling between the two lines are selected.

Figure 6

Total correction factors (efficiency × acceptance) after cuts.

Figure 7

Efficiency times acceptance for primary Λ and secondary Λ from Ξ decays.

Figure 8

Corrected midrapidity ($\left|y\right|<0.5$) $p_T$ spectra for $K^{+},K^{-},K_S^0,\Lambda ,\Xi$, and Ω. Λ spectra corrected for feed-down are shown as open symbols in the Λ panel. Dashed lines are fits using Eq. (11) except for the $\Omega +\overline{\Omega }$ where the fit uses Eq. (9). Error bars include systematic errors, while the fits were done using only statistical errors for all species except the charged kaons.

Figure 9

Comparison of (a) unscaled and (b) scaled transverse mass midrapidity ($\left|y\right|<0.5$) spectra for $\pi ,K^{+}$, $K_S^0,p,\Lambda$, and Ξ from $p+p$ collisions in STAR and PHENIX studies at $\sqrt{s}=200$ GeV. STAR $\pi ,K$, and $p$ spectra are from Refs. [26, 27, 28]; PHENIX ${\pi }^0$ spectrum is from Ref. [17]. Ratios of data to power-law fits for each data point in (b) are given in (c) for meson fits and (d) for baryon fits. Error bars include systematics.

Figure 10

Panel (a) shows the $p_T$ averaged ratios vs strangeness content for our measured species compared with measurements from Au+Au; dashed line is at unity for reference. Panels (b) and (c) show midrapidity ($\left|y\right|<0.5$) ratios of $\overline{\Lambda }$ to Λ and ${\overline{\Xi }}^{+}$ to ${\Xi }^{-}$ vs $p_T$; dashed lines in (b) and (c) are the error-weighted means over the measured $p_T$ range, $0.882\pm 0.017$ for $\overline{\Lambda }/\Lambda ,0.921\pm 0.062$ for $\overline{\Xi }/\Xi$. Error bars are statistical only.

Figure 11

$\langle p_T\rangle$ vs particle mass for different particles measured by STAR. Error bars include systematic errors. The ISR parametrization is given in Ref. [32].

Figure 12

$\langle p_T\rangle$ vs charged multiplicity for $K^{+},K^{-},K_S^0,\Lambda +\overline{\Lambda }$, and $\Xi +\overline{\Xi }$. The points for $\Xi +\overline{\Xi }$ have been determined using only the measured region. Error bars are statistical only. See text for more details.

Figure 13

Ratios of multiplicity binned spectra to minimum bias spectra $R_{\mathit{pp}}$ for (a) $K_S^0$ and (b) Λ; and (c) ratio of the Λ spectrum to the $K_S^0$ spectrum in each multiplicity bin. See text for further details.

Figure 14

(a) $K_S^0$ , (b) Λ, and (c) ${\Xi }^{-}$ $p_T$ spectra compared with PYTHIA(v6.2 MSEL = 1, and v6.3) with the default $K$ factor of 1 (solid and dashed curves, respectively), and $K$ factor of 3 (dot-dashed curve).

Figure 15

$K_S^0$ and Λ multiplicity-binned $\langle p_T\rangle$ compared to PYTHIA v6.2 default (solid curve) and v6.3 default (dashed curve) and v6.3 with $K$ factor of 3 (dot-dashed curve).

Figure 16

$m_T$ scaling results from PYTHIA v6.3 with default settings. Quark- or gluon-jet selections are based on the final state partons being $\mathit{qq}$ or $\mathit{gg}$, respectively. Events with (a) only quark-jet final states ($\mathit{qq}$), (b) gluon-jet ($\mathit{gg}$) and mixed final states ($\mathit{qg}$), and (c) all three final states. Spectra have been scaled by the factors listed in the legends. See text for more details.

Figure 17

$\Lambda /K_S^0$ as a function of $p_T$ compared with PYTHIA.

Figure 18

$K_S^0$ and Λ particle spectra (circles) compared with NLO calculations by DSV and AKK based on specific (a) $K_S^0$ [43] and (b) Λ [44] fragmentation functions. Dashed lines illustrate the uncertainty due to the choice of factorization scale μ. Upper dashed curve for each model is from $\mu =2p_T$, solid curve from $\mu =p_T$, and lower dashed curve from $\mu =p_T/2$.

Figure 19

Comparison of $K_S^0$, Λ, and ${\Xi }^{-},{\overline{\Xi }}^{+}$ spectra with calculations from EPOS v1.02.

Figure 20

Parameters of ratio data to statistical model fit using THERMUS. Filled circles are ratios from $\sqrt{s}=200$ GeV collisions in STAR. Solid lines are the results from the statistical model fit. All ratios to the left of the vertical line were used in the fit. The $({\Omega }^{-}+{\overline{\Omega }}^{+}/2)/{\Xi }^{-}$ ratio was then predicted from the fit results. Dashed lines in the lower panel are guides for the eye at $1\sigma$.