P. Schlein

Evidence for Transverse Jets in High Mass Diffraction

Abstract Localized energy clusters (jets) with transverse energy between 5 and 13 GeV are observed semi-inclusively in proton-antiproton interactions with√s = 630GeV which also contain a final state proton with more than 90% of the beam protons momentum. The system which recoils against the proton is studied for invariant masses in the range 105–190 GeV. The production rate, angular and energy distributions and profiles of the clusters are in good agreement with predictions of a QCD model of diffractive scattering, in which the clusters result from the hard scattering of gluons in an exchange pomeron with the partons in an incident antiproton. A soft gluon structure function of the pomeron, such as(1 − x) 5 , is preferred.

Evidence for a broad scalar Kπ resonance near 1.1 GeV

Abstract A one-pion-exchange pole-extrapolation analysis of K + p→K πΔ ++ at 7.3 GeV/ c provides evidence for the existence of a broad scalar T = 1 2 K π resonance near 1.1GeV. The angular distribution in the Kπ center-of-mass is consistent with this result.

Hyperon production in pp interactions at √s = 53 and 62 GeV

Abstract Inclusive measurements of Λ 0 , Λ 0 , Ξ − , Σ(1385) ± ) production in the forward direction at the CERN intersecting storage rings are presented. A signal for simulataneous Λ 0 Λ 0 production is also observed with total x > 0.6, 2.3 Λ Λ GeV and with a cross section of (1.7 ± 0.2) μb.

Evidence for Λc+ in inclusive pp → (Λ0π+π+π−) + X and pp → (K−π+p) + X at √s = 53 and 62 GeV

Abstract A search for the charmed baryon, Λ c + , has been carried out in inclusively, measured Λ 0 π ± π + π − and K − π ± p final states with longitudinal x > 0.75 in an experiment at the CERN intersecting storage rings. An effect with invariant mass near 2.3 GeV is observed in Λ 0 π + π + π − with cross section times branching ratio Bσ = (2.8±1.0) μ b; similar effect is observed in K − π + p with Bσ = (2.3 ± 0.3) μ b. The effect is seen only in the positive charge state, in agreement with an interpretation as Λ c + . However, it is pointed out that in the same experiment, the non-charmed hyperon state Σ(1385) + also has a cross section much larger than its corresponding negative charge state Σ(1385) − .

Λ0 polarization in proton-proton interactions at √s = 53 and 62 GeV

Abstract Λ 0 s produced in the inclusive reaction pp → Λ° + X with √s = 53 and 62 GeV at the CERN Intersecting Storage Rings are observed to be polarized along the normal to the production plane. In the ranges of longitudinal and transverse momenta, 15–24 and 0.6-1.4 GeV/ c , respectively, the mean polarization is found to be −(0.357±0.055).

Mesic decays of hypernuclei from K−-capture: II. Branching ratios in the charged mesic decay modes of3HΛ,4HΛ,4HeΛ and5HΛ

SummaryBranching ratios in the charged mesic decay modes of hyperfragments with chargeZ⩽2 and mass numberA⩽5 are presented here, based on a total of 162 events. Such relative frequencies are relevant in discussing the consequences of charge independence, the ΔT=1/2 rule as applied to hypernuclei, and the spin dependence of the Λ-nucleon interaction. In particular, the ratioR4 of (π−-recoil) with respect to all π− decays for4HΛ was found to beR4=0.67−0.05+0.06, where the errors represent statistical uncertainties only. Comparing this with the curves ofR4vs. (p/s) calculated byDalitz andLiu (6) on the two assumptions (J=0, 1) regarding the spin of4HΛ, we conclude that the valueJ=0 is more probable, as already suggested (3,6) on the basis of our preliminary data. If this spin assignment is accepted, following the above authors (6), one may infer that for the ratio ofp- tos-wave amplitude in the charged decay of the free Λ, 0.45⩽p/s⩽1.4. Finally, although they should have the same general configuration as the π− events, no examples of decays involving either the emission of a π+ or a charged lepton have been identified in the present sample.RiassuntoSi presentano in questo lavoro i rapporti di ripartizione tra i vari modi di decadimento con emissione di pioni carichi, per gli iperframmenti di caricaZ⩽2 e numero di massaA⩽5, su un totale di 162 eventi. Tali frequenze relative sono importanti nel discutere le conseguenze dell’indipendenza di carica, della regola di selezione ΔT=1/2, applicate agli ipernuclei, e della dipendenza di spin nell’interazione Λ-nucleone. In particolare, il rapportoR4 dei decadimenti (π−-rinculo) rispetto al totale dei decadimenti con emissione di π− per l’HΛ risultaR4=0.67−0.05+0.06 dove i limiti d’errore rappresentano soltanto incertezze statistiche. Dal confronto con le curve che leganoR4 al valore del rapportop/s, calcolate daDalitz eLiu (6) per le due ipotesi (J=0, 1) al riguardo dello spin dell’4HΛ, si conclude che il valoreJ=0 è il più probabile, come già suggerito (3,6) in base ai nostri risultati preliminari. Se si accetta questo valore dello spin, seguendo gli autori citati (6), è possibile assegnare i seguenti limiti per il rapporto delle ampiezze d’ondap eds nel decadimento della Λ libera in particelle cariche: 0.45⩽p/s⩽1.4. Infine, nel presente gruppo di eventi, non si sono trovati esempi di decadimento con emissione di π+ o di leptoni carichi, sebbene tali decadimenti non dovrebbero differire nelle loro caratteristiche esteriori da quelli in cui viene emesso un π−.

Production of jets and single particles at high p⊥ in 200 GeV hadron-beryllium collisions

Abstract We present results from an experiment studying the production of single particles and jets (groups of particles) with high p ⊥ (transverse momentum) in 200 GeV/ c interactions on a beryllium target. We give a detailed discussion of the ambiguities in the jet definition. The jet and single-particle cross sections have a similar shape but the jet cross section is over two orders of magnitude larger. The events show evidence for the coplanar structure suggested by constituent models, and the momentum distributions of charged particles give strong support to a simple quark-quark scattering model.

Pseudorapidity distribution of charged particles in collisions at GeV

Using a silicon vertex detector, we measure the charged particle pseudorapidity distribution over the range 1.5 to 5.5 using data collected from PbarP collisions at root s = 630 GeV. With a data sample of 3 million events, we deduce a result with an overall normalization uncertainty of 5%, and typical bin to bin errors of a few percent. We compare our result to the measurement of UA5, and the distribution generated by the Lund Monte Carlo with default settings. This is only the second measurement at this level of precision, and only the second measurement for pseudorapidity greater than 3.

Measurements of single diffraction at √s = 630 GeV; Evidence for a non-linear α(t) of the pomeron

Abstract We report measurements of the inclusive differential cross section for the single-diffractive reactions p i + p − → p ƒ + X and p + p − i → X + p − ƒ at √s = 630 GeV , in the momentum transfer range 0.8 0.90. Based on the assumption of factorization, several new features of the pomeron flux factor are determined from simultaneous fits to our UA8 data and lower energy data from the CHLM collaboration at the CERN Intersecting Storage Rings. Prominent among these is that the effective pomeron Regge trajectory requires a term quadratic in τ, with coefficient α″ = 0.079 ± 0.012 GeV−4. We also show that the data require a pomeron-proton cross section that first decreases with increasing diffractive mass (corresponding to the PPR term in the triple-Regge expansion) and then increases at larger mass (the PPP term), similar to real particle total cross sections. We measure the product Kσ0 = 0.72 ± 0.10 mb GeV−2, where K is the normalization constant of the pomeron flux factor in the proton and σ0 is the scale constant in the pomeron-proton total cross section. Finally, we report the occurrence of “beam jets” in the pomeron direction in the rest frame of the diffractive system.

K+p elastic scattering at 7.3 GeV/c

Abstract A study of K + p elastic scattering at 7.3 GeV/ c has yielded a cross section of 3.86 ± 0.38 mb. For events produced in the forward direction, a diffractive peak is observed which fits the distribution (d σ /d t ) = (d σ /d t ) 0 exp ( αt + βt 2 ) with (d σ /d t ) 0 = (20.6 + 2.6) mb/(GeV/ c 2 , α = (5.46 ±_0.29)(GeV/ c ) -2 an β = (0.38 ± 0.25)(GeV/ c ) -4 . The ratio |Re ƒ(0)/Im ƒ (0)| for the forward scattering amplitude is 0.60 ± 0.14. One elastic scatter was found in the backward direction.