$K^{*}{\left(892\right)}^{+}$ production in proton-proton collisions at $E_{\mathrm{beam}}=3.5$ GeV

We present results on the $K^{*}{\left(892\right)}^{+}$ production in proton-proton collisions at a beam energy of $E=3.5$ GeV, which is hitherto the lowest energy at which this mesonic resonance has been observed in nucleon-nucleon reactions. The data are interpreted within a two-channel model that includes the three-body production of $K^{*}{\left(892\right)}^{+}$ associated with the $\mathrm{\Lambda }$ or $\mathrm{\Sigma }$ hyperon. The relative contributions of both channels are estimated. Besides the total cross section $\sigma (p+p\to K^{*}{\left(892\right)}^{+}+X)=9.5\pm 0.9_{-0.9}^{+1.1}\pm 0.7\mu \mathrm{b}$, which adds a new data point to the excitation function of the $K^{*}{\left(892\right)}^{+}$ production in the region of low excess energy, transverse momenta and angular spectra are extracted and compared with the predictions of the two-channel model. The spin characteristics of $K^{*}{\left(892\right)}^{+}$ are discussed as well in terms of the spin-alignment.

Figure 1

Specific energy loss of charged particles in MDC chambers as a function of the momentum times polarity. Solid curves show graphical cuts for the ${\pi }^{\pm }$ selection. The Bethe-Bloch curves are shown by dashed curves.

Figure 2

${\pi }^{+}{\pi }^{-}$ invariant mass distribution from events with at least three pions used for the $K_S^0$ reconstruction. The legend delivers information on the fit $p$ value, the number of $K_S^0$ over background, extracted mass, width, and signal to background ratio.

Figure 3

Invariant mass spectra of $K_S^0\text{−}{\pi }^{+}$ pairs (symbols) for the total sample (upper left) and in five transverse momentum bins. Solid curves are for the fits with (2), dashed curves depict the background, separately. The boxes deliver information on the fit $p$ value, the number of $K^{*}$ over background, mean mass, and signal-to-background ratio.

Figure 4

Raw $p_t$ spectrum of $K^{*}$'s produced in proton-proton reactions. Markers—experimental data with statistical uncertainties, curves—expectations from the channel Eqs. (3), (4), and their mixture “$0.6\times \mathrm{\Lambda }+0.4\times \mathrm{\Sigma }$.”

Figure 5

$K^{*}$ transverse momentum spectrum. Markers—experimental data with statistical uncertainties, empty boxes—systematical uncertainties. The two-channel model (“$0.6\times \mathrm{\Lambda }+0.4\times \mathrm{\Sigma }$”) final-state phase-space distribution is shown by the solid curve.

Figure 6

Same as described in the caption of Fig. 5 for the momentum spectrum in the pp center-of-mass reference frame.

Figure 7

Same as described in the caption of Fig. 5 for the angular spectrum in the pp center-of-mass reference frame. The dashed curve correspond to the fit with Legendre polynomials (only 0th and 2nd order are used due to the symmetry arguments, resulting coefficients are shown in the inset).

Figure 8

Energy ($\sqrt{s}-\sqrt{s_{\text{thr}}}$) dependence of the total cross-section for the processes: (i) $\text{pp}\to K^{*}{\left(892\right)}^{+}X$ (red squares, world data [1, 2, 4, 5, 6, 7, 8]; red triangle, present work), (ii) $\text{pp}\to K^{*}{\left(892\right)}^{-}X$ (empty green crosses) [2, 4, 5, 6, 7, 8], and (iii) $\text{pp}\to K^{+}X$ (empty circles) ([9, 10, 21] and references therein). The solid (dashed) line is a fit to the $K^{*}{\left(892\right)}^{+}\left(K^{+}\right)$ data with $f\left(x\right)=C{\left[(1-{\left(D/x\right)}^{\mu }\right]}^{\nu }$, where $x=\sqrt{s}$. The numerical values are $C=3.22\times {10}^6(1.04\times {10}^5)$, $D=2.89\left(2.55\right)$ GeV, $\mu =1.19\times {10}^{-2}(1.16\times {10}^{-1})$, $\nu =1.86\left(1.67\right)$.

Figure 9

Angular distribution of $K_S^0$ in the $K^{*}$ rest frame. Markers, experimental data with statistical uncertainties; empty boxes, systematical uncertainties. The two-channel model (“$0.6\times \mathrm{\Lambda }+0.4\times \mathrm{\Sigma }$”) final-state phase-space distribution as simulated with pluto without any spin alignment is shown by the solid curve. The dashed curve is the fit of Eq. (8) to the data with ${\rho }_{00}=0.39$.