Leptonic and Charged Kaon Decay Modes of the $\varphi$ Meson Measured in Heavy-Ion Collisions at the CERN Super Proton Synchrotron

We report on results of a measurement of $\varphi$ meson production in central Pb-Au collisions at $E_{\mathrm{lab}}=158A\mathrm{GeV}$. For the first time in the history of high energy heavy-ion collisions, $\varphi$ mesons were reconstructed both in the $K^{+}{K}^{-}$ and the dilepton decay channels in the same experiment. This measurement yields rapidity densities near midrapidity, from the two decay channels, of $2.05\pm 0.14(\mathrm{stat})\pm 0.25(\mathrm{syst})$ and $2.04\pm 0.49(\mathrm{stat})\pm 0.32(\mathrm{syst})$, respectively. The shape of the measured transverse momentum spectrum is also in close agreement in both decay channels. The data rule out a possible enhancement of the $\varphi$ yield in the leptonic over the hadronic decay channel of a factor 1.6 or larger at the 95% C.L. This rules out the discrepancy reported in the literature between measurements of the hadronic and dimuon decay channels by two different experiments.

Figure 1

Invariant-mass spectra of $K^{+}{K}^{-}$ pairs after background subtraction for the ($p_t^{\varphi },y^{\varphi }$) bin $1.5\mathrm{GeV}/c<p_t^{\varphi }<1.75\mathrm{GeV}/c$ and $2.2<y^{\varphi }<2.4$.

Figure 2

Invariant-mass spectrum of $e^{+}{e}^{-}$ pairs per charged particle compared to the hadron decay cocktail (thick solid line). Also plotted is a model calculation assuming in-medium spread $\rho$ width plus the dilepton yield from the QGP phase (dashed line) used to extract the physics background in the $\varphi$ mass region.

Figure 3

Transverse momentum spectrum of $\varphi$ mesons corrected for acceptance and efficiency reconstructed in the $e^{+}{e}^{-}$ decay mode (circles) and in the $K^{+}{K}^{-}$ decay channels (triangles). The spectrum in the $e^{+}{e}^{-}$ decay mode has been scaled by 0.93 before comparing to the $K^{+}{K}^{-}$ decay channels to account for the different rapidity intervals covered in the two analyses. Systematic errors in the kaon decay channel are shown as boxes.

Figure 4

Transverse mass distribution of $\varphi$ mesons measured in the charged kaon (triangles) and in the dilepton (circles) decay mode after scaling (see text) compared to the results from NA49 (squares) and NA50 (diamonds). Error bars are statistical and systematic errors in the kaon channel are shown as boxes.