Two-jet Events Research Articles

Using two-jet events to understand hadronization

While the hard phase of the strong interaction is well described by perturbative QCD, the soft hadronization phase is less understood. Benefiting from the high statistics from e+e- experiments at the Z0 resonance, it is possible to impose strong two-jet cuts on the data without loosing the statistical significance. In these events perturbative activity is suppressed and hadronization effects can be more prominent. We show that after proper event cuts a set of observables are sensitive to differences in the hadronization models. These observables can thus be important tools for a more detailed study of the hadronization mechanism.

Using two-jet events to understand hadronization

While the hard phase of the strong interaction is well described by perturbative QCD, the soft hadronization phase is less understood. Benefiting from the high statistics from $e^+e^-$ experiments at the ${\rm Z}^0$ resonance, it is possible to impose strong two-jet cuts on the data without losing the statistical significance. In these events perturbative activity is suppressed and hadronization effects can be more prominent. We show that after proper event cuts a set of observables are sensitive to differences in the hadronization models. These observables can thus be important tools for a more detailed study of the hadronization mechanism.

Time Evolution of the two-jet eventse +−e −→Hadrons in the Dynamical String Model

The time dependent pion emission rate and the mean life time of the pion source in the center of mass frame of the two-jet events have been determined fore +−e −→hadrons at √s=20 – 50 GeV by the Dynamical String Model. It was established that the time needed for the creation of the pion source is 5 fm/c, whereas its life time is τ0=7 – 13 fm/c for energies √s=20 – 50 GeV, respectively.

Observation of hard scattering in photoproduction events with a large rapidity gap at HERA

Events with a large rapidity gap and total transverse energy greater than 5 GeV have been observed in quasi-real photoproduction at HERA with the ZEUS detector. The distribution of these events as a function of the γp centre of mass energy is consistent with diffractive scattering. For total transverse energies above 12 GeV, the hadronic final states show predominantly a two-jet structure with each jet having a transverse energy greater than 4 GeV. For the two-jet events, little energy flow is found outside the jets. This observation is consistent with the hard scattering of a quasi-real photon with a colourless object in the proton.

Energy and particle flow in three-jet and radiative two-jet events from hadronic Z decays

We report on a detailed study of the energy and particle flow in the event plane of three-jet events (q q g) and radiative two-jet events (q q γ) in hadronic Z decays recorded with the L3 detector. We find a significant decrease in particle and energy density in the angular region between quark and antiquark jets for q q g events as compared with q q γ events. Several QCD model predictions are compared with the observed effect.

Measurement of the photon structure function F2γ and jet production at TRISTAN

We have measured the photon structure function F 2 γ in the reaction e + e − → e + e − hadrons for average Q 2 values from 5.1 to 338 GeV 2 by using data collected by the TOPAZ detector at TRISTAN. The data have been corrected for detector effects and are compared with theoretical expectations based on QCD. The structure function F 2 γ increases as ln Q 2, as expected. A sample of events with one or two distinct jets has been identified in the final state. Although two-jet events can be explained solely by the point-like perturbative part, one-jet events require a significant hadron-like part in addition.

Observation of hard scattering in photoproduction at HERA

We report a study of electron proton collisions at very low Q 2, corresponding to virtual photoproduction at centre of mass energies in the range 100–295 GeV. The distribution in transverse energy of the observed hadrons is much harder than can be explained by soft processes. Some of the events show back-to-back two-jet production at the rate and with the characteristics expected from hard two-body scattering. A subset of the two-jet events have energy in the electron direction consistent with that expected from the photon remnant in resolved photon processes.

Monte Carlo study of quantum number retention in hadron jets.

We present a Monte Carlo study in which we used weighted quantum numbers of hadron jets in an attempt to identify the parent parton of these jets. Two-jet events produced by ${e}^{+}{e}^{\ensuremath{-}}$ annihilation were studied using the Lund Monte Carlo program. It was found that the sign of the charge of the leading parton could be determined in a majority of events and that the quark jet could be distinguished from the antiquark jet in a majority of events containing baryons. A careful selection of a subset of the events by making cuts on the value of these weighted quantum numbers increased significantly the accuracy with which both the charge and the baryon number of the leading parton could be determined. Some success was also made in differentiating light-quark from heavy-quark events and in determining the leading quark flavor in the light-quark events. Unfortunately quantum number retention does not differentiate gluon jets from quark jets. The consequences of this for three-jet events and for jet identification in other reactions is discussed.

Using neural networks with jet shapes to identify b jets in e +e − interactions

A feed-forward neural network trained using backpropagation was used to discriminate between b and light quark jets in e + e − → Z 0 → q q events. The information presented to the network consisted of 25 jet shape variables. Th network successfully identified b jets in two- and three-jet events modeled using a detector simulation. The jet identification efficiency for two-jet events was 61% and the probability to call a light quark jet a b jet equal to 20%.

The polarized gluon distribution and large transverse momentum jets

The Carlitz-Collins-Mueller proposal to measure the polarized gluon distribution in the proton by measuring two-jet events is examined in greater detail. It is shown that an experiment with a proton target at rest in the laboratory cannot be used to measure the polarized gluon distribution. However, one can measure the polarized gluon if a relativistic proton beam is used instead. The resulting jet cross-section is calculable; the gluon contribution to the total cross-section is not − α s /2 π.

Coplanar jets.

We argue that the observation of multiple cores in very-high-energy cosmic-ray collisions is expected in models where jet production becomes a feature of all interactions above ${10}^{4}$ TeV. The most striking feature of such events is the collinearity of cores in emulsion experiments. We argue that the observed alignment of three cores is a consequence of transverse-momentum conservation about the beam in a boosted two-jet event. We predict a certain degree of alignment in events with four cores which reflects a dynamical correlation in three-jet events given by QCD. Such correlations have already been observed in collider experiments. Additional four-core events must be observed in emulsion experiments before the alignment hypothesis is tested for cosmic-ray interactions.

Expectations for two- and three-jet events at HERA

We calculate cross-sections for two- and three-jet events ine − p collisions at $$\sqrt s $$ =314 GeV. We include contributions from point-like photon interactions, as well as contributions from when the photon is resolved into constituent quarks and gluons. It is shown that measurements of the angular distribution of the dijet system in two-jet events as a function of the dijet mass will assist in the separation of these two components. Aside from being a direct test of QCD photoproduction subprocesses, measurement of these events will yield information on the hadronic structure of the photon.

Importance of initial-state radiation to the search for the top quark at hadron colliders.

A study was performed on initial-state radiation in the process of heavy-top-quark production, at hadron colliders (both the Fermilab Tevatron and the Superconducting Super Collider). In particular, we studied events with the hard subprocesses gg\ensuremath{\rightarrow}tt\ifmmode\bar\else\textasciimacron\fi{} and qq\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}tt\ifmmode\bar\else\textasciimacron\fi{}, with the subsequent semileptonic decay tt\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}bb\ifmmode\bar\else\textasciimacron\fi{}+${l}^{+}$${l}^{\ensuremath{-}+}$missing ${p}_{T}$. Using the Webber-Marchesini Monte Carlo program herwig, we produced 5000 events of this process, using top-quark masses of 100, 150, and 200 GeV. We find that in the process of tt\ifmmode\bar\else\textasciimacron\fi{} production, when the top quark is heavier than the W boson, the initial-state radiation produces jets that can obscure the nature of the tt\ifmmode\bar\else\textasciimacron\fi{} decay. These jets, which are mainly gluon jets, pass all the widely used cuts that are set for top-quark detection. The events therefore look like multijet events, rather than pure two-jet events. We find that there are many pure two-jet events which consist of b or b\ifmmode\bar\else\textasciimacron\fi{} jet plus an extra gluon jet. The effect is most prominent for top-quark mass near the W mass, 100 GeV in our case. For heavier top quark, 150 and 200 GeV, the cuts that are put on the leptons and the missing energy, which come off the top-quark decay, basically eliminate this effect. The significance of this effect will extend beyond the pure tt\ifmmode\bar\else\textasciimacron\fi{} production channel, since it is resident in every subprocess at hadron colliders.

Comparison of charged particle multiplicities in three-jet and two-jet events from e+e- annihilation at high energies

The charged particle multiplicity < n3j > in the three-jet events in e+e- annihilation is considered. The comparison of < n3j > with < n2j > is presented. The prediction of the multiplicity agrees well with experiments. Moreover, some predictions for high energy range are made.

Bose-Einstein correlations in e+e- collisions.

The MARK II detector is used to study the Bose-Einstein correlation between pairs and triplets of charged pions produced in hadronic decays of the J)psi), the ..sqrt..s = 4 to 7 GeV continuum above the J)psi), two photon events at ..sqrt..s = 29 GeV, and e/sup )plus/)e/sup )minus/) annihilation events at ..sqrt..s = 29 GeV as a function of Q/sup 2/, the four-momentum transfer squared. After corrections for Coulomb effects and pion misidentification, we find a nearly full Bose-Einstein enhancement ..cap alpha.. in the J)psi) and the two photon data and about half the maximum value in the other two data sets. The radius parameter )tau)(an average over space and time) given by pion pair analyses lies within a band of +-0.10 fm around 0.73 fm and is the same, within errors, for all four data sets. Pion triplet analyses also give a consistent radius of approx. 0.54 fm. fits to two-dimensional distributions R(q/sub T//sup 2/, q/sub C//sup 2/) of invariant components of Q/sup 2/ = q/sub T//sup 2/ )plus) q/sub C//sup 2/ give )tau)/sub T/ approx. )tau)C approx. )tau), where q/sub T/ is the transverse three-momentum difference calculated with respect to the net pair three-momentum, and q/sub C/ ismore » in effect the longitudinal three-momentum difference in the pion pair rest frame. When q/sub T/ is calculated with respect to the jet axis for two-jet events in the e/sup )plus/)e/sup )minus/) annihilation data at ..sqrt..s = 29 GeV, a fit to R(q/sub T//sup 2/, q/sub C//sup 2/) also gives )tau)/sub T/ approx. )tau)/sub C/ approx. )tau). Noting that q/sub L/ and q/sub 0/ are not invariant, we make fits to R(/sub T//sup T/, q/sub L//sup 2/) and to R(q/sub T//sup 2/, q/sub 0//sup 2/) (Kopylov formulation), and we find )tau)/sub 0/ approx. )tau)/sub L/ approx. )23))tau)/sub T/ to )12))tau)/sub T/. 44 refs., 43 figs., 15 tabs« less

An infrared-stable multiplicity measure on QCD parton states

We suggest a measure of multiplicity in e +e − annihilation events which is based upon a generalization of the available phase phase in two-jet events to multi-gluon events. This measure exhibits infrared stability, the anomalous dimensions found in perturbative QCD, together with an intermittent behaviour. Well defined expressions can be obtained not only asymptotically but for all energies.

Angular distribution of hadrons in two-jet events

Well-known simple arguments suggest that the distribution of hadrons in an e +e − two-jet event should be approximately invariant under boosts along the jet axis. One consequence of this is the rapidity plateau. In this paper, stronger predictions based on this invariance are made for the angular distribution of hadrons, and its relationship to fan invariance is discussed. Comparison of these predictions with experiment is fair.

Hadronic production of gluinonium in three-jet events

The third-order supersymmetric QCD contribution to the high-energy hadronic production of gluino-gluino bound states is calculated. Several experimental cuts are proposed in order to increase the signal-to-background ratio. This ratio is shown to be 2 to 3 times larger in three-jet events than in two-jet events, reaching 5–7%. This gain in efficiency could be crucial in the case of limited statistics and an experimental two-jet mass resolution of a few GeV.

Forward-backward asymmetry in two-jet events: Signature of axigluons in proton-antiproton collisions

Models of chiral colour suggest the existence of massive axigluons with strong couplings to quarks. Interference of gluon- and axigluon-induced amplitudes will give rise to a forward-backward asymmetry in q q →Q Q which will be reflected in two-jet events containing a prompt lepton. We estimate this asymmetry for the CERN and Fermilab colliders.

Charged particle multiplicity distributions ine + e − annihilation at 29 GeV: a comparison with hadronic data

The charged particle multiplicity distributions for two-jet events ine + e − annihilation at 29 GeV have been measured using the High Resolution Spectrometer at PEP. A Poisson distribution describes the data for both the complete event and for the single jets. In addition, no correlation is observed between the multiplicities in the two jets of an event. For fixed values of the prong number of the complete event, the multiplicity sharing between the two jets is in good agreement with a binomial distribution. The rapidity gap distribution is exponential with a slope equal to the mean rapidity density. These observations, which are consistent with a picture of independent emission of single particles, are contrasted to the results from soft hadronic collisions and conclusions are drawn about the nature of clusters.