arXiv subjects

Wolfgang Ochs

Publications and source records attributed to Wolfgang Ochs.

At least 55 records · Page 3Linked to original sources

arXiv2000

Gluon Fragmentation into Glueballs and Hybrid Mesons

The constituent nature of candidate gluonic mesons can be studied by comparing their production in quark and gluon jets. The production rate for such mesons depends on the colour confinement processes at the end of the perturbative evolution. Whereas we expect enhanced production of hybrids in the fragmentation region of a gluon jet, the rate for glueballs depends on the relative importance of colour triplet and colour octet neutralization. These neutralization processes can be studied independently in events with large rapidity gaps. If octet processes turn out important the recently suggested lightest J^{PC}=0^{++} glueball with mass around 1000 MeV should become visible already in the spectrum of the leading charged particle pairs.

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arXiv1999

Rapidity Gaps in Quark and Gluon Jets - a Perturbative Approach

We derive the probability for rapidity gaps in a parton cascade and investigate the dual connection with hadronic final states. A good description of observations in e+e- annihilations is obtained by perturbative QCD calculations in MLLA using previously determined parameters (QCD scale Λand k_T-cut-off Q_0) and applying the parton hadron duality picture. Further predictions are derived; especially, for gaps between jets at variable resolution we predict a strong variation of gap probabilities for small parameters y_{cut} -> 0 in the transition from jets to hadrons. Large gaps between partons correspond to large spatial separations of colour charges: a colour blanching mechanism by soft processes is suggested.

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arXiv1999

Soft Limits of Multiparticle Observables and Parton Hadron Duality

We discuss observables in multiparticle production for three kinds of limits of decreasing kinematical scales: 1. the transition jet \to hadron (limit y_{cut}\to 0 of the resolution parameter y_{cut}); 2. single particle inclusive distributions normalized at threshold \sqrt s\to 0 and 3. particle densities in the limit of low momentum p,p_T\to 0. The observables show a smooth behaviour in these limits and follow the perturbative QCD predictions, originally designed for large scales, whereby a simple prescription is supplemented to take into account mass effects. A corresponding physical picture is described.

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arXiv1999

Lightest glueball and scalar meson nonet in production and decay

Recent results concerning the evidence and classification of the J^{PC}=0^{++} states, obtained with P. Minkowski, are presented: The isoscalars f_0(980) and f_0(1500) are classified as members of the 0^{++} nonet, while the broad state called f_0(400-1200) and the state f_0(1370) are considered as different components of a single broad resonance, the lowest-lying 0^{++} glueball. Furthermore, we propose the investigation of glueball production in the fragmentation region of gluon jets.

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arXiv1999

The J^PC = 0^++ scalar meson nonet and glueball of lowest mass

The evidence for the low mass J^{PC}=0^{++} states is reconsidered. We suggest classifying the isoscalars f_0(980) and f_0(1500) as members of the 0^{++} nonet, with a mixing rather similar to that of the pseudoscalars η' and η. The broad state called f_0(400-1200) or `sigma'' and the state f_0(1370) are considered as different signals from a single broad resonance, which we take to be the lowest-lying 0^{++} glueball. The main arguments in favor of these hypotheses are presented and compared with theoretical expectations.

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arXiv1999

Identification of the glueballs and the scalar meson nonet of lowest mass

We discuss the theoretical expectations and phenomenological evidence for the lightest glueballs and the members of the meson nonet with quantum numbers J^PC=0^{++}. We reconsider the recent evidence for candidate states with masses below ~1700 MeV, but include also the results from earlier phase-shift analyses. Arguments are presented to classify the scalars f_0(980) and f_0(1500) as members of the 0^{++} nonet, with a mixing rather similar to that of the pseudoscalars eta' and eta. The S-wave states called f_0(400-1200) and f_0(1370) are considered as different signals from a single broad resonance, which we take to be the lowest-lying 0^{++} glueball. This state together with eta(1440) and f_J(1710) with spin J=2 form the basic triplet of binary gluonic bound states. We argue that these hypotheses are consistent with what can be expected theoretically.

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arXiv1999

Soft Particle Emission Accompanying Dijet Photoproduction

The intensity of the soft bremsstrahlung depends on the colour charges of the hard- scattered partons. This intensity is larger in dijet production with resolved photons by the factor C_A/C_F=9/4 in comparison with direct photoproduction for particular configurations. It is investigated how these effects are reflected in Monte Carlo models.

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arXiv1998

Multiparticle Production in the Soft Limit and QCD Coherence

The production of gluons in a jet is considered in limited phase space, either with a cut in transverse momentum with respect to the jet axis $k_\perp<k_\perp^{cut}$ or with a cut in absolute momentum $|\vec{k} | <k^{cut}$. It is shown in the perturbative QCD calculations that in the soft limit $k_\perp^{cut} \to 0$ the multiplicity distribution becomes Poissonian, whereas for $k^{cut} \to 0$ the distribution remains non-Poissonian. The Poissonian limit is a consequence of the soft gluon coherence in the genuine multiparticle correlations. We also investigate how incoherent hadronization processes could possibly modify the predictions for small cut-off parameters.

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arXiv1998

Poissonian limit of soft gluon multiplicity

It is shown that the gluons produced with small transverse momenta $k_\perp$ inside a jet ($k_\perp <k_\perp^{cut}$, $k_\perp^{cut} \to 0$) are independently emitted from the primary parton, as QCD coherence suppresses their showering. Consequently, the low $k_{\perp}$ gluons follow a Poisson distribution, very much like the soft photons radiated by a charged particle in QED. On the contrary, the distribution of gluons with limited absolute momenta $|\vec{k} | <k^{cut}$ remains non-Poissonian even for small $k^{cut}$. It will be interesting to find out to what extent this perturbative prediction for partons survives the hadronization process.

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arXiv1997

QCD connection between hadron and jet multiplicities

The perturbative QCD provides a good overall description of both the jet and hadron multiplicities in the $e^+e^-$ annihilation reaction. In this description the hadrons are considered as dual to partons at a small resolution parameter $Q_0$ characteristic of a hadronic scale of few hundred MeV.

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arXiv1997

Perturbative Universality in Soft Particle Production

The spectrum of partons in a QCD jet becomes independent of the primary energy in the low momentum limit. This follows within the perturbative QCD from the colour coherence in soft gluon branching. Remarkably, the hadrons follow such behaviour closely, suggesting the parton hadron duality picture to be appropriate also for the low momentum particles. More generally, this scaling property holds for particles of low transverse and arbitrary longitudinal momentum, which explains an old experimental observation (``fan invariance''). Further tests of the perturbatively based picture for soft particle production are proposed for three-jet events in e+e- annihilation and di-jet production events in gamma p, gamma-gamma and p\bar p collisions. They are based upon the difference in the intensity of the soft radiation from primary q\bar q and gg antennae.

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arXiv1997

Perturbative QCD Description of Mean Jet and Particle Multiplicities in e+e- annihilation

A complete numerical solution of the evolution equation for parton multiplicities in quark and gluon jets with initial conditions at threshold is presented. Data on both hadron and jet multiplicities in e+e- annihilation are well described with a common normalization, giving further support to the picture of Local Parton Hadron Duality. Predictions for LEP-II energies are presented. Furthermore we study the sensitivity to the cutoff parameter Q_0 and the scale of alpha_s.

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arXiv1997

Recent tests of parton hadron duality in multiparticle final states

A large variety of jet properties can be described perturbatively by evolving the parton cascade down to small scales of order of a few 100 MeV. We discuss two recent applications of this approach: 1. the soft limit of the particle spectrum which is nearly energy independent as expected for the soft gluon bremsstrahlung; further tests of this interpretation in $e^+e^-$ and DIS are presented; 2. the steep decrease of the distribution of rapidity gaps in $e^+e^-$ annihilation can be explained by the Sudakov formfactor for quark jets.

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arXiv1997

Hadron multiplicity as the limit of jet multiplicity at high resolution

Recently exact numerical results from the evolution equation for parton multiplicities in QCD jets have been obtained. A comparison with various approximate results is presented. A good description is obtained not only of the jet multiplicities measured at LEP-1 but also of the hadron multiplicities for $cms$ energies above 1.6 GeV in \epem annihilation. The solution suggests that a final state hadron can be represented by a jet in the limit of small (nonperturbative) $k_\perp$ cut-off $Q_0$. In this description using as adjustable parameters only the QCD scale $Λ$ and the cut-off $Q_0$, the coupling $α_s$ can be seen to rise towards large values above unity at low energies.

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arXiv1997

Unified QCD Description of Hadron and Jet Multiplicities

The evolution equation for parton multiplicities in quark and gluon jets which takes into account the soft gluon interference is solved numerically using the initial conditions at threshold. If the k_T-cutoff Q_c is lowered towards the hadronic scale Q_0 of a few hundred MeV, the jets are fully resolved into hadrons. Both hadron and jet multiplicities in e+e- annihilation are well described with a common normalization. Evidence is presented within this perturbative approach that the coupling α_s(k_T) rises by an order of magnitude when approaching the low energy region. The ratio of hadron multiplicities in gluon and quark jets is found smaller than in previous approximate solutions of the evolution equation.

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arXiv1997

Low and High Energy Limits of Particle Spectra in QCD Jets

Charged particle energy spectra in e+e- annihilation are compared with the analytical predictions from the QCD evolution equation in the Modified Leading Log Approximation. With the nonperturbative initial condition shifted down to threshold as suggested by the Local Parton Hadron Duality picture a good description of the data from the lowest up to highest available energies results. The two essential parameters in this approach are determined from a moment analysis. The sensitivity of the fit to the running of alpha_s and to the number of active flavours (including a light gluino) is demonstrated. For very high energies the theory predicts a scaling behaviour in certain rescaled variables (``zeta-scaling''). The data show an approximate behaviour of this type in the present energy range and come close to the predicted asymptotic scaling function for the small particle energies.

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arXiv1996

Scaling and scale breaking phenomena in QCD jets

The perturbative QCD approach to multiparticle production assuming Local Parton Hadron Duality (LPHD) and some recent results are discussed. Finite asymptotic scaling limits are obtained for various observables, after an appropriate rescaling, in the Double Logarithmic Approximation (DLA). Non-asymptotic corrections are also known in some cases. The DLA applies also to very soft particle production where energy conservation constraints can be neglected. In this region the particle density follows rather well a scaling behaviour over the full energy range explored so far in $e^+e^-$ annihilation.

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arXiv1996

QCD Description of Particle Spectra up to LEP-1.5 Energies and the Running of $α_s$

Recent results on the energy spectra in QCD jets are reported. Within the Modified Leading Logarithmic Approximation (MLLA) and the Local Parton Hadron Duality (LPHD) model one finds a very good description of the $e^+e^-$ data from the lowest up to the LEP 1.5 energies. A model with fixed $α_s$ can be excluded, already in certain finite energy intervals. The fits also extrapolate smoothly into the region of small particle energies, in particular, the data follow a scaling prediction for the low energy limit derived from the colour coherence of the soft gluon emission.

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