Search Results (30 total)

The NA44 experiment has measured single-particle inclusive spectra for charged pions, kaons, and protons as a function of transverse mass near midrapidity in 158A GeV/c Pb+Pb collisions. From the particle mass dependence of the observed m_T distributions, we are able to deduce a value of about 120 MeV for the temperature at thermal freeze-out. From the observed ratios of the rapidity densities, we find values of the chemical potentials for light and strange quarks and a chemical freeze-out temperature of approximately 140 MeV.

NA44 uses a 512-channel Si pad array covering 1.5<η<3.3 to study charged hadron production in 158A GeV Pb+Pb collisions at the CERN SPS. We apply a multiresolution analysis, based on a discrete wavelet transformation, to probe the texture of particle distributions event by event, allowing a simultaneous localization of features in space and scale. Scanning a broad range of multiplicities, we search for signals of clustering and of critical behavior in the power spectra of local density fluctuations. The data are compared with detailed simulations of detector response, using heavy-ion event generators, and with a reference sample created via event mixing. An upper limit is set on the probability and magnitude of dynamical fluctuations.

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta 〈p_T〉≈0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p_T data, while a two-dimensional analysis was used for the higher p_T data. We find that the source-size parameters are consistent with the m_T scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.

The invariant cross section as a function of transverse momentum for antideuterons produced in 158A GeV/c per nucleon Pb+Pb central collisions has been measured by the NA44 experiment at CERN. This measurement, together with a measurement of antiprotons, allows for the determination of the antideuteron coalescence parameter. The extracted coalescence radius is found to agree with the deuteron coalescence radius and radii determined from two particle correlations.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

Two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.

First results of the m_T dependence of π⁺π⁺ and K⁺K⁺ correlations from S+Pb collisions at 200 GeV /c per nucleon measured by the focusing spectrometer of the NA44 experiment at CERN are presented. Multidimensional fits characterize the pion and kaon emission volume. The pion radius parameter decreases with increasing p_T. Furthermore, the pion and kaon radii show a common 1/sqrt[m_T] dependence. This behavior can be interpreted as a result of a strong momentum-position correlation arising from collective flow.

Collisions of ²⁸Si+Al, Cu, and Pb at E_lab=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ²⁸Si into p+ ²⁷Al at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-²⁷Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ²⁸Si and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ²⁸Si(γ,p) ²⁷Al, and slight differences are observed only at low final-state energy.

We present the rapidity and transverse momentum distributions of protons and neutrons from collisions between 14.6 GeV/nucleon beams of ²⁸Si and targets of Al, Cu, and Pb. The data were measured in the forward spectrometer/target calorimeter detectors of the E814 apparatus. The results indicate the existence of two distinct domains, one of beam rapidity projectilelike nucleons, and the second of participant nucleons. From the former, the in-medium inelastic nucleon-nucleon cross section is deduced. It is found to agree, within 10%, with the ‘‘free’’ value of 30 mb although under present conditions one of the two colliding nucleons has been struck before with a high probability. We compare with the present data the predictions of a fragmentation model as well as of models dealing explicitly with the heavy-ion collision and particle creation and emission.

Electromagnetic dissociation cross sections for one, two, and three nucleon removal were measured for an E_lab/A=14.6 GeV ²⁸Si beam impinging on ²⁷Al, ¹²⁰Sn, and ²⁰⁸Pb targets. The measured cross sections exhibit an approximate Z_T² dependence and are well reproduced by calculations convoluting the virtual photon spectrum with the experimental σ(γ,N) photonuclear cross sections. Comparison with other experimental results and energy dependence of the cross sections are discussed.

Collisions of ²⁸Si+Al, Cu, Pb at E_lab/A=14.6 GeV were studied in a calorimetry-based experiment at the BNL Alternating Gradient Synchrotron. Transverse-energy production was measured for pseudorapidities -0.5<η<0.8. Correlations with the spectra and multiplicity of neutrons and protons emitted into a forward 0.8° cone demonstrate quantitatively the large amount of nuclear stopping observed in these reactions. Calculations in hadronic-fireball or nucleon-nucleon based models underpredict the measured transverse-energy production for Si+Pb and indicate the need to include rescattering of secondaries and /or contributions from target fragmentation.

We have searched for the decay process K⁺→π⁺A⁰, where A⁰ is any particle of mass less than 100 MeV/c² decaying into e⁺e^-. Upper limits for the branching ratio are given as a function of mass and lifetime of the A⁰. For lifetimes shorter than 10^-13 sec, a limit of 4.5×10^-7 at the 90% confidence level is obtained.

Data are presented on the inclusive production of π^±, K^±, p, and p̅ for π⁺, K⁺, and protons incident on nuclear targets at 100 GeV. The results cover the kinematic range 30≤P≤88 GeV/c for P_t=0.3 and 0.5 GeV/c. The observed A dependence of the invariant cross sections exhibits remarkable simplicity, which does not naturally follow from current models of particle production. The results show that the hypothesis of limiting fragmentation can be extended to include collisions with nuclei.

The authors have detected 1203 events of the reaction π^-p→ϕϕn at 22 GeV/c, and have performed a partial-wave analysis of the ϕϕ system. The results are well represented by two resonances, both with quantum numbers I^G=0⁺, J^PC=2⁺⁺. The breakdown of the Okubo-Zweig-Iizuka suppression observed in this reaction is naturally explained in the context of QCD if these states are considered to be glueball candidates.

An experiment using the Fermilab Single Arm Spectrometer (SAS) facility and an associated nonmagnetic vertex detector studied the reactions a+p→c+X, where a and c were π^±, K^±, p, or p̅ . Extensive measurements were made at 100 and 175 GeV/c beam momenta with the outgoing hadrons detected in the SAS covering a kinematic range 0.12<x<1.0 and p_T<1.25 GeV/c. Additional data covering a more restricted range in x were also gathered at 70 GeV/c incident momentum. In this high-statistics experiment, the identification of both the incoming and outgoing charged hadrons were made with a total of eight Čerenkov counters. New and extensive single-particle inclusive data for charged-particle production in low-p_T hadronic fragmentation are presented. The average associated charged-particle multiplicity and pseudorapidity distributions are also given.

We present evidence for the existence of two new I=0 scalar mesons [g_s(1240) and S^*′(1730)] from an amplitude analysis of the reaction π^-p→K_S⁰K_S⁰n at 23 GeV/c. An examination of the mass splittings present shows that neither of these states can be easily assigned to a (near) ideally mixed J^PC=0⁺⁺ nonet. These results are in qualitative agreement with recent theoretical predictions of glucon-glucon bound states.

We have carried out an amplitude analysis of the K_S⁰K_S⁰ system produced in the reaction π^-p→K_S⁰K_S⁰n at 23 GeV/c, based on about 15 000 events in the low-t region (|t-t_min|<0.1 GeV²). Below 1.6 GeV/c², our favored solution is very similar to those from previous analyses. For higher masses, we observe the K_S⁰K_S⁰ decay of the h(2040) meson. In addition, the l=0 partial wave contains a new state, strongly coupled to K_S⁰K_S⁰, with parameters M=1.771_-0.053^+0.077 GeV/c² and Γ=0.200_-0.009^+0.156 GeV/c². Since this state is most probably I=0, we call it the S^*′(1770). We find an f^′ / f production ratio of 0.23_-0.13^+0.14, and branching ratios for f-meson and h(2040)-meson decays into KK̅ of (3.1_-1.7^+0.7)% and (0.67_-0.15^+0.41)%, respectively. We find, in a detailed comparison of our results with those from other experiments, that our solution is compatible with all known features of both charged and neutral KK̅ systems.

The ΛΛ̅ system produced in the reaction π^-p→nΛΛ̅ at 6.0 and 7.0 GeV/c has been studied utilizing the ANL 1.5-m streamer-chamber facility. A 400 000-photograph exposure yielded 284 unweighted nΛΛ̅ events. The cross section for the reaction is 0.51±0.22≥b at 6.0 GeV/c and 1.21±0.23≥b at 7.0 GeV/c. No significant indications of baryonium states decaying into ΛΛ̅ are found.

Quark structure functions have been extracted from low-p_T inclusive hadron production data for the pion and kaon with use of the recombination model. n^π=1.0±0.1 and n^K=2.5±0.6 is obtained, where n is the leading (1-x) power of the nonstrange-valence-quark distribution. Both the pion and kaon nonstrange-sea-quark functions have n≃3.5.

We present high-statistics results on the reactions a+p→c+X where a and c can be any of π^±, K^±, p, or p̅ . The data were taken at 100 and 175 GeV/c incident momenta using the Fermilab Single-Arm Spectrometer operated over the kinematic range 0.2<x<1.0 and p_t<~1.0 GeV/c. Investigating the x dependence of the data, we find agreement with a quark-parton picture, namely the cross sections have a power-law behavior in 1-x independent of p_beam and p_t.

The K_S⁰K_S⁰ system produced in the reaction π^-p→nK_S⁰K_S⁰ at 6.0 and 7 0 GeV/c has been studied utilizing the ANL 1.5-m streamer-chamber facility. A 400 000-photograph exposure yielded 5096 unweighted nK_S⁰K_S⁰ events. The cross section for this reaction was determined to be 9.6 ± 1.3 μb at 6.0 GeV/c and 8.7 ± 1.1 μb at 7.0 GeV/c. The decay angular distributions were parametrized in terms of moments of the spherical harmonics. The 〈Y₄⁰〉 moment was fitted to interfering Breit-Wigner amplitudes for the f and f^′ mesons. Using this fit, the branching ratio R=Γ(f→KK̅ ) / Γ(f→all) was found to be (2.3 ± 0.8)%. An energy-independent production-amplitude analysis revealed an enhancement in the S-wave amplitude near 1300 MeV. The properties of the S-wave enhancement are discussed and compared with those observed in other recent experiments. Extrapolated cross sections for the reaction ππ→K_S⁰K_S⁰ are presented. We find a cross section considerably below the S-wave unitarity limit in the S^* region.