Cross sections and density-matrix elements for π−p→ϕn have been measured for - −t≲1.5 GeV2 at 3, 4, 5, and 6 GeV/c, using the Argonne effective-mass spectrometer to observe the decay ϕ(1019)→K+K−. This is the first observation of the reaction in this energy range. The remarkably flat differential cross section at 4 GeV/c and the strong energy dependence suggest a production mechanism not normally seen at these energies. Data on K−p→ϕΛ and K−p→ϕΣ0 from the same experiment are also presented.
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We report on a search for metastable positively and negatively charged states of strange quark matter in Au+Pb reactions at 11.6 A GeV/c in experiment E864. We have sampled approximately six billion 10% most central Au+Pb interactions and have observed no strangelet states (baryon number A < 100 droplets of strange quark matter). We thus set upper limits on the production of these exotic states at the level of 1-6 x 10^{-8} per central collision. These limits are the best and most model independent for this colliding system. We discuss the implications of our results on strangelet production mechanisms, and also on the stability question of strange quark matter.
ABOUT SIX BILLION 10% MOST CENTRAL INTERACTIONS.
Measurements of π±p, K±p, pp, and p¯p elastic scattering are presented for incident momenta of 3, 3.65, 5, and 6 GeVc and momentum transfers typically 0.03 to 1.8 GeV2. The angle and momentum of the scattered particle were measured with the Argonne Effective Mass Spectrometer for 300 000 events, yielding 930 cross-section values with an uncertainty in absolute normalization of ±4%. Only the K+ and proton data show any significant change in slope of the forward diffraction peak with incident momentum. The particle-antiparticle crossover positions are consistent with no energy dependence, average values being 0.14 ± 0.03, 0.190 ± 0.006, and 0.162 ± 0.004 GeV2 for π' s, K' s, and protons, respectively; these errors reflect both statistics and the ±1.5% uncertainty in particle-antiparticle relative normalization. Differences between particle and antiparticle cross sections isolate interference terms between amplitudes of opposite C parity in the t channel; these differences indicate that the imaginary part of the odd-C nonflip-helicity amplitude has a J0(r(−t)12) structure for −t<0.8 GeV2, as predicted by strong absorption models. The cross-section differences for K± and proton-antiproton are in qualitative agreement with the predictions of ω universality, the agreement improving with increasing energy. The corresponding quark-model predictions relating the π± and K± differences failed by more than a factor of 2. We have combined our π± cross sections with other data to better determine the πN amplitudes in a model-independent way; results of this analysis are presented.
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A sample of 1200 ϕ mesons produced in the Zweig-rule-forbidden reactions π±N→ϕN shows a flat t distribution (slope=1.7 ± 0.2 GeV−2) and a cross section suppressed by 0.0032±0.0004 relative to π−p→ωn. Natural-parity-exchange ω and ϕ production have similar t dependences, while ω and ϕ production by unnatural-parity exchange differ markedly. Interference between the ϕ and the underlying K¯K S wave indicates substantial nucleon-spin incoherence between the amplitudes for ϕ and S-wave production.
Axis error includes +- 10/10 contribution (RES-DEF(RES=PHI,BACK=CORRECTED)).
Axis error includes +- 10/10 contribution (RES-DEF(RES=PHI,BACK=CORRECTED)).
STATISTICAL ERRORS ONLY.
Differential cross sections for K−p→K¯0n and K+n→K0p have been measured at 3, 4, and 6 GeV/c using a data sample of 6000 events. Contrary to simple exchange-degenerate models, the ratio of K+ to K− cross sections was found to be approximately 1.35, with little dependence on either s or t. Both reactions show a shallow dip near the forward direction, suggesting the importance of spin-flip amplitudes.
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LARGE -T CONTRIBUTIONS (TYPICALLY 12 PCT) ESTIMATED FROM BUBBLE-CHAMBER DATA.
Differential cross sections have been measured for the reaction π−p→p¯d. At 4 GeV/c the total cross section is 0.58 ± 0.08 μb with an angular distribution consistent with isotropy. At 5 GeV/c a pronounced forward dip occurs. In contradiction to simple baryon-exchange models, the cross section is found to be an order of magnitude smaller than that for the line-reversed reaction pp→π+d. Upper limits for π−d→p¯t were also found.
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We have obtained 1400 events of the baryon-exchange reaction pp→ΛpK+ using the ANL Zero Gradient Synchrotron 11.75-GeV/c polarized proton beam and the effective-mass spectrometer. A search for enhancements in the Λp mass spectrum has been made. No enhancements are observed; in particular, the narrow Λp enhancement at 2.13 GeV seen in K−d→Λpπ− is not observed. We establish limits on the production of narrow states of ∼ 10 nb. Also, 100000 events of the forward reaction pp→ΛK+p were examined and no narrow Λp enhancements are seen.
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The differential cross sections for the elastic scattering of π+, π−, K+, K−, p, and p¯ on protons have been measured in the t interval -0.04 to -0.75 GeV2 at five momenta: 50, 70, 100, 140, and 175 GeV/c. The t distributions have been parametrized by the quadratic exponential form dσdt=Aexp(B|t|+C|t|2) and the energy dependence has been described in terms of a single-pole Regge model. The pp and K+p diffraction peaks are found to shrink with α′∼0.20 and ∼0.15 GeV−2, respectively. The p¯p diffraction peak is antishrinking while π±p and K−p are relatively energy-independent. Total elastic cross sections are calculated by integrating the differential cross sections. The rapid decline in σel observed at low energies has stopped and all six reactions approach relatively constant values of σel. The ratio of σelσtot approaches a constant value for all six reactions by 100 GeV, consistent with the predictions of the geometric-scaling hypothesis. This ratio is ∼0.18 for pp and p¯p, and ∼0.12-0.14 for π±p and K±p. A crossover is observed between K+p and K−p scattering at |t|∼0.19 GeV2, and between pp and p¯p at |t|∼0.11 GeV2. Inversion of the cross sections into impact-parameter space shows that protons are quite transparent to mesons even in head-on collisions. The probability for a meson to pass through a proton head-on without interaction inelastically is ∼20% while it is only ∼6% for an incident proton or antiproton. Finally, the results are compared with various quark-model predictions.
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We present the final results from Experiment 864 of a search for charged and neutral strange quark matter produced in interactions of 11.5 GeV/c per nucleon Au beams with Pt or Pb targets. Searches were made for strange quark matter with A>4. Approximately 30 billion 10% most central collisions were sampled and no strangelet states with A<100 were observed. We find 90% confidence level upper limits of approximately 10^{-8} per central collision for both charged and neutral strangelets. These limits are for strangelets with proper lifetimes greater than 50 ns. Also limits for H^{0}-d and pineut production are given. The above limits are compared with the predictions of various models. The yields of light nuclei from coalescence are measured and a penalty factor for the addition of one nucleon to the coalescing nucleus is determined. This is useful in gauging the significance of our upper limits and also in planning future searches for strange quark matter.
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A strangelet search in Si+Pt and Au+Pt collisions at alternating-gradient synchrotron (AGS) energies, using a focusing spectrometer, sensitive to mass per charge of 3-14 GeV/c2 was conducted during the 1992 and 1993 heavy ion runs at the AGS. The null results thereof are presented as upper limits on the invariant production cross section, in the range of 10−5-10−4 mb c3/GeV2, and model dependent sensitivity limits in the range of 10−7-10−5 per collision. Measurements of the production cross sections of several nonstrange nuclear systems, from p to Be7 and Li8, the background of the strangelet search, are also presented.
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