An event excess observed in the deeply bound region of the 12C (K−, p) missing-mass spectrum

We have measured, for the first time, the inclusive missing-mass spectrum of the $^{12}$C$(K^-, p)$ reaction at an incident kaon momentum of 1.8 GeV/$c$ at the J-PARC K1.8 beamline. We observed a prominent quasi-elastic peak ($K^-p \rightarrow K^-p$) in this spectrum. In the quasi-elastic peak region, the effect of secondary interaction is apparently observed as a peak shift, and the peak exhibits a tail in the bound region. We compared the spectrum with a theoretical calculation based on the Green’s function method by assuming different values of the parameters for the $\bar{K}$–nucleus optical potential. We found that the spectrum shape in the binding-energy region $-300 \, \text{MeV} < B_{K} < 40$ MeV is best reproduced with the potential depths $V_0 = -80$ MeV (real part) and $W_0 = -40$ MeV (imaginary part). On the other hand, we observed a significant event excess in the deeply bound region around $B_{K} \sim 100$ MeV, where the major decay channel of $K^- NN \to \pi\Sigma N$ is energetically closed, and the non-mesonic decay modes ($K^- NN \to \Lambda N$ and $\Sigma N$) should mainly contribute. The enhancement is fitted well by a Breit–Wigner function with a kaon-binding energy of 90 MeV and width 100 MeV. A possible interpretation is a deeply bound state of a $Y^{*}$-nucleus system.

Feasibility study of the K+d → K0pp reaction for the Θ+ pentaquark

We investigate theoretically the $K^{0} p$ invariant mass spectrum of the $K^{+} d \to K^{0} p p$ reaction and scrutinize how the signal of the “$\Theta ^{+}$” pentaquark, if it exists, emerges in the $K^{0} p$ spectrum. The most prominent advantage of this reaction is that we can clearly assess whether the “$\Theta ^{+}$” exists or not as a direct-formation production without significant backgrounds, in contrast to other reactions such as photoproduction and $\pi$-induced productions. We show that while the impulse or single-step scattering process can cover the “$\Theta ^{+}$” energy region with an initial kaon momentum $k_{\rm lab} \approx 0.40 \text{ GeV} / c$ in the laboratory frame, the contributions from double-step processes may have a potential possibility to reach the “$\Theta ^{+}$” energy region with a higher kaon momentum $k_{\rm lab} \sim 1 \text{ GeV} / c$. Assuming that the full decay width of the “$\Theta ^{+}$” is around $0.5 \text{ MeV}$, we predict that the magnitude of the peak corresponding to the “$\Theta^+$” is around a few hundred $\mu \text{b}$ to $1 \text{ mb}$ with the momentum of the kaon beam $k_{\rm lab} \approx 0.40 \text{ GeV} / c$ while it is around $\lesssim 1 ~\mu \text{b}$ with $k_{\rm lab} \approx 0.85 \text{ GeV} / c$. Thus, the “$\Theta^+$” peak is more likely to be seen at $k_{\mathrm{lab}} \approx 0.40 \text{ GeV} / c$ than at $k_{\mathrm{lab}} \approx 0.85 \text{ GeV} / c$.

THE NA62 EXPERIMENT AT CERN AND THE MEASUREMENT OF THE ULTRA-RARE DECAY K+-π+v¯v

The NA62 experiment at CERN aims at the very challenging task of measuring with 10% relative error the Branching Ratio of the ultra-rare decay of the K⁺ into π⁺ ν¯ν which is expected to occur only in about 8 out of 10¹¹ Kaon decays. This will be achieved by means of an intense hadron beam, an accurate kinematical reconstruction and a redundant veto system for identifying and suppressing all spurious events. Good resolution on the missing mass in the decay is achieved using a high-resolution beam tracker to measure the kaon momentum and with a spectrometer equipped with straw tubes operating in vacuum. Hermetic veto (up to 50 mrad) of the photon from π⁰ decays is achieved with a combination of large angle veto (with a creative reuse of the old OPAL lead glass blocks), the NA48 liquid Krypton calorimeter and two small angle calorimeters to cover the angle down to zero. The identification of the muons and the consequent veto is performed by a fast hodoscope plane (used in the first level of the trigger to reduce the rate) and by a 17 meter, neon-filled RICH counter which is able to separate pions and muons in the momentum interval between 15 and 35 GeV. Particle identification in the beam (K⁺ separation) is achieved with an H2 differential Cherenkov counter. The trigger for the experiment is based on a multilevel structure with a first level implemented in the readout boards and with the subsequent level done in the software. The aim is to reduce the 10MHz level zero rate to a few kHz sent to the CERN computing centre. Studies are underway to use GPU boards in some key point of the trigger system to improve the performance.<p> </p>

INCLUSIVE CHARGED KAON PRODUCTION FROM B MESONS IN ϒ(4S) DECAYS

We have investigated the inclusive production of charged kaons from B mesons in ϒ (4S) decays. We present our results (up to 800 MeV/c) of the measurement of the charged kaon momentum spectrum, using information from the specific ionization (dE/dx) particle identification (PID) device. We fit PID histograms with known samples of primary (pion, kaon, proton) and secondary (electron, muon) control shape histograms, using a χ2 minimization technique. We compare our findings with ARGUS and with MC and comment on the implications for b → s g from the analsis.