The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.

A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the g, w , 2!(958), and @mesons near their respective thresholds in the reaction a-+ p -missing mass + n . The incident momentum is varied in small steps through the threshold while neutrons of a given momentum are detected near zero degrees. The lower limit of the c.m. momentum P* at which measurements have been made ranges from about 50 MeV/c at the @ t o about 30 MeV/c at the 7. A somewhat low value for the w mass, 782.3 1-0.6 MeV, is found. The width of the 2! is < 1.9 MeV (95% confidence level). All four mesons show evidence of S-wave production, with values of u/P* of 21.2 1-1.8, 0.35 i0.03, and 0.2910.06 pb/(MeV/c) for the q, A ! , and @, respectively. A rapid rise in the w cross section appears to be modified by a final-state interaction. The effect of this rise can probably be seen in some Sli pion-nucleon phase-shift solutions. Evidence is also presented of a sudden drop in the s i n -mass spectrum just above the threshold for the production of a KiKpair. The paper includes a comprehensive discussion of the method and of the details of the spectrometer.-

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Measurements are reported of the differential cross section for the reaction T -p -7 -p , Ton, and ~n at three angles close to 180" and for incident momenta in the range 0.6 to 1.0 GeV/c. The three measurements were made simultaneously at 1% intervals of beam momentum. The data on elaqtic scattering resolve a discrepancy between two earlier experiments. They also show clearly the effect of the opening of the t) n channel. The charge-exchange data show that I-spin bounds are not violated in the kinematic region covered. The 7 ) n data can be adequately described with known s-channel resonances. No evidence for narrow N*'s is seen in any channel. I. EXPERIMENTWe present the results of an experiment performed at the Rutherford High Energy Laboratory to measure the differential cross sections of the reactions at three angles close to 180" in the center-of-mass system, for pion momenta in the range 0.6-1.0 G e~/ c . Figure 1 shows a plan of the apparatus, much of which was previously used for a study of meson thresholds.' A beam of negative pions was incident upon a liquid-hydrogen target 29.4 cm long, and the recoiling nucleons were detected in an array of ten "neutron" counters situated 7 m downstream of the target. A set of counters, A,-,,, covering the front faces of the neutron counters, determined the charge state of the nucleon. Reactions were further identified by a measurement of the recoil particle's time of flight, and by the detection of reaction products in an a r r a y of y detectors and charged-particle detectors surrounding the hydrogen target. The direct detection of the nucleon enabled a precise determination of the scattering angle to be made and also allowed all three reactions to be observed simultaneously. This feature was combined with a fine resolution on the incident momentum with measurements at closely spaced intervals.The beam of negative pions was produced by collisions of the circulating proton beam in Nimrod on an internal copper target, and was transported to the hydrogen target by a two-stage beam line. The first stage accepted a 3% bite of momenta and formed an image at an intermediate focus. The particles were momentum analyzed in the second stage and brought to a second focus in the hydrogen target. The spectrometer consisted of a bending magnet and quadrupole doublet with hodoscopes at conjugate foci of the doublet. With this system, beam particles were assigned to momentum bins of 1% separation; the momentum distribution in each bin being approximately triangular upon a base of width 2%. Three adjacent bins were accepted for each setting of the spectrometer central momentum.Apart from some changes to the hodoscope counters, the techniques used were similar to those described in Ref. 2. Electrons in the beam were vetoed using a CO, gas cerenkov counter, situated upstream of the spectrometer, and contamination by muons was measured in a separate exp_eriment using a high-pressure Freon-13 gas Cerenkov counter. Typical numbers of useful beam particles accepted during the Nimrod spill ...

We have investigated o production in the reaction .rr-p -o n very close to threshold. The dependence of the mass, width, branching ratio, and cross section upon the final-state c.m, momentum, P*, were studied. The mass and width were independent of P* with values of 782.4 * 0.5 and 10.22 0.43 MeV, respectively. The branching ratio r ( w i . r r o y ) / r ( w i . r r + . r r -. r r O ) was also constant, having a value of 0.084 + 0.013. An upper limit of 0.18 was set on the branching ratio ~( o -+ . r r~. r r~~) /~( w -~~y ) .We observed a rapid fall in the cross section below P* = 100 MeV/c. This could not be explained in terms of S-wave production alone, but could be fitted by a resonant P wave plus a noninterfering S wave.