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Register a new userKLEVER: 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($K_Ltopi^0 ubar{ u}$).
Recently, the ATOMKI experiment has reported new evidence for the excess of $e^+ e^-$ events with a mass $sim$17 MeV in the nuclear transitions of $^4$He, that they previously observed in measurements with $^8$Be. These observations could be explained by the existence of a new vector $X17$ boson. So far, the search for the decay $X17 rightarrow e^+ e^-$ with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining $X17$ parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the $X17$ decay with the proposed method. To reach this goal an optimization of the $X17$ production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [Phys. Rev. D101, 071101 (2020)], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate shows that the goal of the proposed search is feasible.
The DsTau project proposes to study tau-neutrino production in high-energy proton interactions. The outcome of this experiment are prerequisite for measuring the $ u_tau$ charged-current cross section that has never been well measured. Precisely measuring the cross section would enable testing of lepton universality in $ u_tau$ scattering and it also has practical implications for neutrino oscillation experiments and high-energy astrophysical $ u_tau$ observations. $D_s$ mesons, the source of tau neutrinos, following high-energy proton interactions will be studied by a novel approach to detect the double-kink topology of the decays $D_s rightarrow tau u_tau$ and $taurightarrow u_tau X$. Directly measuring $D_srightarrow tau$ decays will provide an inclusive measurement of the $D_s$ production rate and decay branching ratio to $tau$. The momentum reconstruction of $D_s$ will be performed by combining topological variables. This project aims to detect 1,000 $D_s rightarrow tau$ decays in $2.3 times 10^8$ proton interactions in tungsten target to study the differential production cross section of $D_s$ mesons. To achieve this, state-of-the-art emulsion detectors with a nanometric-precision readout will be used. The data generated by this project will enable the $ u_tau$ cross section from DONUT to be re-evaluated, and this should significantly reduce the total systematic uncertainty. Furthermore, these results will provide essential data for future $ u_tau$ experiments such as the $ u_tau$ program in the SHiP project at CERN. In addition, the analysis of $2.3 times 10^8$ proton interactions, combined with the expected high yield of $10^5$ charmed decays as by-products, will enable the extraction of additional physical quantities.
A New QCD facility at the M2 beam line of the CERN SPS COMPASS++/AMBER
A new heavy-ion experiment on fixed target, NA60+, has been proposed at the CERN SPS for data taking in the next years. Its main goals will be focused on precision studies of thermal dimuons, heavy quark and strangeness production in Pb-Pb collisions at center-of-mass energies ranging from 5 to 17 GeV, which will provide a unique opportunity to investigate the region of the QCD phase diagram at high baryochemical potential ($mu_B sim 200-400$~MeV). The key points of the NA60+ very broad and ambitious physics program will be described.
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