Design and operation of a cryogenic charge-integrating preamplifier for the MuSun experiment

The central detector in the MuSun experiment is a pad-plane time projection ionization chamber that operates without gas amplification in deuterium at 31 K; it is used to measure the rate of the muon capture process $\mu^- + d \rightarrow n + n + \nu_\mu$. A new charge-sensitive preamplifier, operated at 140 K, has been developed for this detector. It achieved a resolution of 4.5 keV(D$_2$) or 120 $e^-$ RMS with zero detector capacitance at 1.1 $\mu$s integration time in laboratory tests. In the experimental environment, the electronic resolution is 10 keV(D$_2$) or 250 $e^-$ RMS at a 0.5 $\mu$s integration time. The excellent energy resolution of this amplifier has enabled discrimination between signals from muon-catalyzed fusion and muon capture on chemical impurities, which will precisely determine systematic corrections due to these processes. It is also expected to improve the muon tracking and determination of the stopping location.Comment: 18 pages + title page, 13 figures, to be submitted to JINST; minor corrections, added one reference, updated author lis

The study of the negative pion production in neutron-proton collisions at beam momenta below 1.8 GeV/c

A detailed investigation of the reaction np -> pp\pi^{-} has been carried out using the data obtained with the continuous neutron beam produced by charge exchange scattering of protons off a deuterium target. A partial wave event-by-event based maximum likelihood analysis was applied to determine contributions of different partial waves to the pion production process. The combined analysis of the np -> pp\pi^{-} and pp -> pp\pi^{0} data measured in the same energy region allows us to determine the contribution of isoscalar partial waves (I=0) in the momentum range from 1.1 up to 1.8 GeV/c. The decay of isoscalar partial waves into (^1S_0)_{pp}\pi$ channel provides a good tool for a determination of the pp S-wave scalar scattering length in the final state which was found to be a_{pp}=-7.5\pm 0.3 fm.Comment: 6 pages, 6 figure

Observation of the Cabibbo-suppressed decay Xi_c+ -> p K- pi+

We report the first observation of the Cabibbo-suppressed charm baryon decay Xi_c+ -> p K- pi+. We observe 150 +- 22 events for the signal. The data were accumulated using the SELEX spectrometer during the 1996-1997 fixed target run at Fermilab, chiefly from a 600 GeV/c Sigma- beam. The branching fractions of the decay relative to the Cabibbo-favored Xi_c+ -> Sigma+ K- pi+ and Xi_c+ -> X- pi+ pi+ are measured to be B(Xi_c+ -> p K- pi+)/B(Xi_c+ -> Sigma+ K- pi+) = 0.22 +- 0.06 +- 0.03 and B(Xi_c+ -> p K- pi+)/B(Xi_c+ -> X- pi+ pi+) = 0.20 +- 0.04 +- 0.02, respectively.Comment: 5 pages, RevTeX, 3 figures (postscript), Submitted to Phys. Rev. Let

First observation of a narrow charm-strange meson DsJ(2632) -> Ds eta and D0 K+

We report the first observation of a charm-strange meson DsJ(2632) at a mass of 2632.6+/-1.6 MeV/c^2 in data from SELEX, the charm hadro-production experiment E781 at Fermilab. This state is seen in two decay modes, Ds eta and D0 K+. In the Ds eta decay mode we observe an excess of 49.3 events with a significance of 7.2sigma at a mass of 2635.9+/-2.9 MeV/c^2. There is a corresponding peak of 14 events with a significance of 5.3sigma at 2631.5+/-1.9 MeV/c^2 in the decay mode D0 K+. The decay width of this state is <17 MeV/c^2 at 90% confidence level. The relative branching ratio Gamma(D0K+)/Gamma(Dseta) is 0.16+/-0.06. The mechanism which keeps this state narrow is unclear. Its decay pattern is also unusual, being dominated by the Ds eta decay mode.Comment: 5 pages, 3 included eps figures. v2 as accepted for publication by PR

First Observation of the Doubly Charmed Baryon Xi_cc^+

We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm hadro-production experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1 +/- 0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is (3519 +/- 1) MeV/c^2. The Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its lifetime is less than 33fsec at 90% confidence.Comment: 5 pages, 3 figures, accepted for publication in Physical Review Letter

First Measurement of pi e -> pi e gamma Pion Virtual Compton Scattering

Pion Virtual Compton Scattering (VCS) via the reaction pi e --> pi e gamma was observed in the Fermilab E781 SELEX experiment. SELEX used a 600 GeV/c pi- beam incident on target atomic electrons, detecting the incident pi- and the final state pi-, electron and gamma. Theoretical predictions based on chiral perturbation theory are incorporated into a Monte Carlo simulation of the experiment and are compared to the data. The number of reconstructed events (9) and their distribution with respect to the kinematic variables (for the kinematic region studied) are in reasonable accord with the predictions. The corresponding pi- VCS experimental cross section is sigma=38.8+-13 nb, in agreement with the theoretical expectation sigma=34.7 nb.Comment: 10 pages, 12 figures, 4 tables, 25 references, SELEX home page is http://fn781a.fnal.gov/, revised July 21, 2002 in response to journal referee Comment

Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions

We present measurements of $\pi^-$ and $\pi^+$ elliptic flow, $v_2$, at midrapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}} =$ 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{ch}$, based on data from the STAR experiment at RHIC. We find that $\pi^-$ ($\pi^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $\sqrt{s_{_{\rm NN}}} = \text{27 GeV}$ and higher. At $\sqrt{s_{_{\rm NN}}} = \text{200 GeV}$, the slope of the difference of $v_2$ between $\pi^-$ and $\pi^+$ as a function of $A_{ch}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.Comment: 6 pages, 4 figure