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تسجيل مستخدم جديدKLEVER: An experiment to measure BR($K_Ltopi^0 ubar{ u}$) at the CERN SPS
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Precise measurements of the branching ratios for the flavor-changing neutral current decays $Ktopi ubar{ u}$ can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. It is important to measure both decay modes, $K^+topi^+ ubar{ u}$ and $K_Ltopi^0 ubar{ u}$, since different new physics models affect the rates for each channel differently. The goal of the NA62 experiment at the CERN SPS is to measure the BR for the charged channel to within 10%. For the neutral channel, the BR has never been measured. We are designing the KLEVER experiment to measure BR($K_Ltopi^0 ubar{ u}$) to $sim$20% using a high-energy neutral beam at the CERN SPS starting in LHC Run 4. The boost from the high-energy beam facilitates the rejection of background channels such as $K_Ltopi^0pi^0$ by detection of the additional photons in the final state. On the other hand, the layout poses particular challenges for the design of the small-angle vetoes, which must reject photons from $K_L$ decays escaping through the beam exit amidst an intense background from soft photons and neutrons in the beam. Background from $Lambda to npi^0$ decays in the beam must also be kept under control. We present findings from our design studies for the beamline and experiment, with an emphasis on the challenges faced and the potential sensitivity for the measurement of BR($K_Ltopi^0 ubar{ u}$).
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Precise measurements of the branching ratios for the $Ktopi ubar{ u}$ decays can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. It is important to measure both decay modes, $K^+topi^+ ubar{ u}$ and $K_Ltopi^0
ubar{ u}$, since different new physics models affect the rates for each channel differently. We are investigating the feasibility of performing a measurement of BR($K_Ltopi^0 ubar{ u}$) using a high-energy secondary neutral beam at the CERN SPS in a successor experiment to NA62. The planned experiment would reuse some of the NA62 infrastructure, including possibly the NA48 liquid-krypton calorimeter. The mean momentum of $K_L$ mesons decaying in the fiducial volume is 70 GeV; the decay products are boosted forward, so that less demanding performance is required from the large-angle photon veto detectors. On the other hand, the layout poses particular challenges for the design of the small-angle vetoes, which must reject photons from $K_L$ decays escaping through the beam pipe amidst an intense background from soft photons and neutrons in the beam. We present some preliminary conclusions from our feasibility studies, summarizing the design challenges faced and the sensitivity obtainable for the measurement of BR($K_Ltopi^0 ubar{ u}$).
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