Abstract. We have measured cross sections for forward neutron production from a variety of targets using proton beams from the Fermilab Main Injector. Measurements were performed for proton beam momenta of 58 GeV/c, 84 GeV/c, and 120 GeV/c. The cross section dependence on the atomic weight (A) of the targets was found to vary as A α where α is 0.46 ± 0.06 for a beam momentum of 58 GeV/c and 0.54 ± 0.05 for 120 GeV/c. The cross sections show reasonable agreement with FLUKA and DPMJET Monte Carlos.Comparisons have also been made with the LAQGSM Monte Carlo.

Using data collected with the HyperCP (E871) spectrometer during the 1997 fixed-target run at Fermilab, we report the first observation of the decay K − → π − µ + µ − and new measurements of the branching ratios for K ± → π ± µ + µ − . By combining the branching ratios for the decaysThe CP asymmetry between the rates of the two decay modes is [Γ(PACS numbers: 13.20. Eb, 14.40.Aq, 11.30.Er The rare decay of charged K mesons to a pion and a lepton pair (K ± πll , where l = e or µ) can be used to study flavor-changing neutral currents as a higher-order process in the standard model and to explore new physics. However, to realize these goals, it is necessary to understand the dominant K ± → π ± γ * radiative transition which involves long-distance hadronic effects. Since it is difficult to calculate such effects, chiral perturbation theory (ChPT) including electroweak interactions has been applied to K + πll [1]. In a recent model-independent analysis with ChPT at O(p 6 ) [2], the K + πll decay rate and the form factor characterizing the dilepton invariant-mass spectrum are calculated in terms of two parameters a + and b + . Determining these parameters from the measured branching ratio and the dilepton mass spectrum of the K + πee decay, this analysis predicts the ratio R = B(K + πµµ )/B(K + πee ) to be greater than 0.23 and an increase in the CP asymmetry between the decay rates of K + πll and K − πll compared to the leading-order estimation in the chiral expansion [3]. The resulting expected CP asymmetry is ∼ 10 −5 [2, 3].The K + πee decay has been studied by several experiments [4,5,6], and the Particle Data Group (PDG) has compiled a mean branching ratio B(K + πee ) = (2.88 ± 0.13) × 10 −7 [7]. The K + πµµ decay was first observed by the E787 Collaboration at the Brookhaven National Laboratory (BNL). With 13 fully reconstructed 3-track and 196 partially reconstructed 2-track events, they determined B(K + πµµ ) = [5.0 ± 0.4(stat) ± 0.7(syst) ± 0.6(theor)] ×10 −8 [8]. The E865 Collaboration at BNL has subsequently observed 430 fully reconstructed K + πµµ events and measured B(K + πµµ ) = [9.22 ± 0.60(stat) ± 0.49(syst)] ×10 −8 [9]. The discrepancy between these two experimental results is more than three standard deviations and is not understood. In addition the value of R obtained with the E787 result and the average value of B(K + πee ) is inconsistent with the prediction and is difficult to accommodate within the standard model [2]. It is thus important to resolve the discrepancy in the B(K + πµµ ) measurements.

Abstract. The purpose of the MIPP experiment is to study the inclusive production of photons, pions, kaons, and nucleons produced in π, K, and p interactions on various targets using beams from the Main Injector at Fermilab. The purpose of the calorimeters is to measure the production of forward-going photons and neutrons. The electromagnetic calorimeter consists of 10 lead plates interspersed with proportional chambers followed by the hadron calorimeter with 64 steel plates interspersed with scintillator. We collected data with a variety of targets with beam energies from 5 GeV/c up to 120 GeV/c. The energy calibration of both calorimeters with electrons, pions, kaons and protons is discussed. The performance of the calorimeters was tested on a neutron sample.

The two most recent and precise measurements of the charged kaon mass use X-rays from kaonic atoms and report uncertainties of 14 ppm and 22 ppm yet differ from each other by 122 ppm. We describe the possibility of an independent mass measurement using the measurement of Cherenkov light from a narrow-band beam of kaons, pions, and protons. This technique was demonstrated using data taken opportunistically by the Main Injector Particle Production experiment at Fermi National Accelerator Laboratory which recorded beams of protons, kaons, and pions ranging in momentum from +37 GeV/c to +63 GeV/c. The measured value is 491.3 ± 1.7 MeV/c 2 , which is within 1.4σ of the world average. An improvement of two orders of magnitude in precision would make this technique useful for resolving the ambiguity in the X-ray data and may be achievable in a dedicated experiment.

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