Gunnar Ingelman

Parton fragmentation and string dynamics

While muchhasbeenlearnedrecentlyaboutquark andgluon interactionsin theframeworkof perturbativeQuantumChromodynamics,the relationbetweencalculatedpartonpropertiesandobservedhadrondensitiesinvolves modelswhere dynamicsandjet empincal ruleshaveto be combined.Thepurposeof this article is to describea presentlysuccessful approachwhich is basedon a cascadejet model usingStringdynamics.It can readily lead to Monte Carlo jet programmesof greatuse when analyzingdata. Production processesin an iterative cascadeapproach,with tunneling in a constantforce field, arereviewed. Expecteddifferencesbetweenquark and gluon jets are discussed.Low transversemomentum phenomenaare alsoreviewed with emphasison hyperon polarization. In so far as this approachusesa fragmentationschemebasedon String dynamics,it wasdeemedappropriateto alsoincludeunderthesamecovera specialreport on theClassicaltheoryof relativisticStrings,seenasthe classicallimit of theDual Resonancemodel. TheEquationsof motion and interactionamongstringsarepresented. Single ordersfor this issue PHYSICSREPOR1’S(Review Sectionof PhysicsLetters)97, Nos.2 & 3 (1983)31—171. Copies of this issue may be obtainedat the price givenbelow. All ordersshouldbe sentdirectly to the Publisher.Ordersmust be accompaniedby check. Single issuepriceDfl. 79.00, postageincluded. 34 B. Andersson et a!., Patton fragmentation and siring dynamics testsof the theory,in particularof the perturbativeQCD structure,containse.g.nonscalingdeviations from the partonmodel. There are at this point already some difficulties becauseit is well-known that any finite energy hadronicdistributionwill containnonscalingcontributions,sometimesto an evenlargerdegreethanthe inherent scale breaking effects of the theory. Further the pencilsharpenergyand momentumdistributions from the single partonsareessentiallydistorted, widenedin transversedirectionsandeven the basicquantumnumberslike chargeandstrangeness etc.seemto havebeentransportedsometimes ratherfar away in longitudinal rapidity space.It has thereforebecomeincreasinglyobvious over the yearsthat in order to compareexperimentto basictheory,it is necessaryto havereliabledescriptionsof the transferfrom the partonicstageto the hadronicone,i.e. to haveconsistentmodelsfor the process of partonfragmentation. Suchmodelsmayon the onehandbe lookedupon solely as phenomenological parametrizationsand rulesof thumb in order to obtain a translationfrom onelanguageto another.As such theyare useful for analysis of experiment as well as for the planning. On the other hand one may as always in connectionwith phenomenologytry to obtain a dynamicalframeworkthat servesas a motivation anda generalizingprinciple for theconstructions. It shouldbekept in mind, however,that thereareno easilyavailablemeasuresof the successof such a venture.As Bacontold us a long time ago,it is actuallyonly possibleto learnthat oneis wrong by a comparisonbetweenmodel calculationsand experimentalfindings. If the predictionagreesthereis no reassurancethat one is evenworking in the right basicdirection (althoughthereis evidentlya possible reasonto feelsomeconfidence!). A modestmeasureof successwould be a demandthat the numberof phenomenological parameters andthe variation in size of their valuesare nonincreasingfunctionsof time as well as the numberof independent experimentalfindings. It is alsoof evidentinterestthat thesamebasicschemeis applicable in different contextssuch as different partonic processesand different parts of phase-space. Several schemeswith a more or less profound theoreticalfoundationhavebeensuggestedbut we will in this review be concernedonly with iterative cascadejet modelsbasedupon string dynamics.The present experiencefrom thesemodelsshowsthat at leastthe above-mentionedcriteriafor successarefulfilled. We will in this reviewmostlydiscussa possibledynamicalframeworkbehindthe modelsandwe will only usecomparisonsto datain order to demonstratemattersof principle.One of the nice featuresof the models is their stochasticstructurewhich readily lends itself to an implementationin terms of computergeneration.Severalsuch MonteCarlojet programsareavailable[21for the interestedreader to makemuchmore detailedcomparisons. Our approachwill primarily be of a semi-classicalnature,i.e. we will at most placesmakeuseof a classicaldynamicalframeworkfor our considerations. We will, however,at all necessaryplacespoint to the basicquantummechanicalconstraints. It is well-known that by meansof a careful choice of dynamicalvariablesit is often possibleto circumvent such constraints.As an example we note that it is in general not possible to give independentvaluesto canonicallyconjugatevariablessuch as momentum(p) andposition(x) dueto Heisenberg’suncertaintyprinciple:

A semiclassical model for the polarization on inclusively produced Λ0-particles at high energies

Abstract We present a semiclassical model for inclusive Λ 0 -production which is able to explain the observed polarization effect in proton-proton collisions. It is well known that hard scattering processes, when calculated in perturbative QCD, do not give rise to large polarization effects. The mechanism producing the polarization in this model is basically a soft process, where quark-antiquark pairs are produced by a tunneling process in the colour field and where perturbatice QCD is not applicable.

Transverse momentum effects and angular energy flow in leptoproduction

We show that with the inclusion of soft gluon emission the experimental pt spectra of leptoproduced hadrons are well reproduced without having an unphysically large primordial transverse momentum. A way to distinguish experimentally between primordial and soft gluon effects is given. We further show that the forward-backward asymmetry in the hadronic energy flow predicted by perturbative QCD is, at presently available energies, difficult to observe in the final state of observable particles due to the fragmentation of the jets. The decay of unstable particles produces an energy flow at large angles which washes out the QCD parton level asymmetry. Kinematical effects from the production of a baryon in the target fragmentation jet will, however, give rise to a forward-backward asymmetry that is larger than the QCD asymmetry at present energies.

ON HIGH-ENERGY LEPTOPRODUCTION

The particle distributions and the event structures in high energy leptoproduction are considered in a model where perturbative QCD is used to compute the cross sections to order αs and the Lund jet model is used for the soft hadronization process. Since complete events are generated by a Monte Carlo program not only single-particle spectra can be studied but also various correlations, thus making a more detailed comparison between theory and experiment possible. The model is found to be in agreement with data from the European Muon Collaboration. We indicate how a one-particle trigger can be used to study an enriched sample of two forward jet events, and exhibit a useful experimental test for the existence of such events with the properties predicted by perturbative QCD. We also indicate how to gain further information on the confinement and soft fragmentation mechanisms.

Baryon production in lepton-nucleon scattering and diquark fragmentation

We study baryon production in deep inelastic scattering using an extended version of the Lund jet model. There are two contributing sources. The first is baryon production in the target fragmentation. In a scheme related to our earlier work on lowphu baryon fragmentation we present some details of the fragmentation of a diquark into baryons and mesons. A non-negligible baryon-antibaryon production is observed ine+e− annihilation. In a previous paper we developed a model for this production, and the same mechanism should also give fast baryons in leptoproduction. In this paper we discuss those features of baryon production which can be more easily studied in a leptoproduction experiment.

String model description of polarisation and angular distributions in pp-bar -gLambda-bar Lambda at low energies

Abstract:The observed polarisation of Λ hyperons from the inclusive pA → ΛX reaction at high energies has previously been well described within the Lund string model through polarised s¯s quark pair production in the string breaking hadronisation process. This model is here applied to the exclusive ¯pp → ¯ΛΛ reaction at low energies and compared to available data sets down to an incident-beam momentum of 1.835 GeV/c. This required an extension of the diquark scattering model to involve three components: an isotropic part relevant close to the threshold, a spectator part and a forward-scattering part as in pA → ΛX at high energies. The observed angular distributions are then reproduced and, for momentum transfers above | t| = 0.2 GeV2, agreement with the measured polarisation is also obtained.

Detection of short-lived particles in nuclear emulsions

Abstract We present a method for detecting short-lived particles in nuclear emulsions. Production and decay vertices as close as 0.5 μm can be resolved. This corresponds to a lifetime of the order of 10 −15 s. The principle is to fit straight lines to the tracks and make a chi-square test of the hypothesis of a single vertex. This technique may be useful for detecting decays of heavy quarks.

String model description of polarization and angular distributions in anti-p p ---> anti-Lambda Lambda at low-energies

Abstract:The observed polarisation of Λ hyperons from the inclusive pA → ΛX reaction at high energies has previously been well described within the Lund string model through polarised s¯s quark pair production in the string breaking hadronisation process. This model is here applied to the exclusive ¯pp → ¯ΛΛ reaction at low energies and compared to available data sets down to an incident-beam momentum of 1.835 GeV/c. This required an extension of the diquark scattering model to involve three components: an isotropic part relevant close to the threshold, a spectator part and a forward-scattering part as in pA → ΛX at high energies. The observed angular distributions are then reproduced and, for momentum transfers above | t| = 0.2 GeV2, agreement with the measured polarisation is also obtained.

On the production of thick pellicles of kodak NTB-3 nuclear track emulsion

Abstract We describe procedures for making thick pellicles of Kodak NTB-3 nuclear emulsion. The technique has successfully been applied for the production of a 15 liter emulsion target, consisting of 600 μm thick pellicles. Melting and pouring of the gel is discussed as well as the conditions during the drying and processing.