| 1. |
- Andersson, Bo, et al.
(författare)
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Bose-Einstein correlations in the Lund model
- 1998
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Ingår i: Nuclear Physics B. - 0550-3213. ; 513:3, s. 627-644
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Tidskriftsartikel (refereegranskat)abstract
- By providing the Lund Model fragmentation process with a quantum-mechanical framework we extend the results of previous work to situations where there are very many identical bosons. We investigate the features of the weight distributions in some detail and in particular exhibit three-particle BE-correlations, the influence on the rho-spectrum and the difference between charged and neutral pion correlations.
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| 2. |
- Andersson, Bo, et al.
(författare)
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Gluon Splitting in the Color Dipole Cascades
- 1990
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Ingår i: Nuclear Physics B. - 0550-3213. ; 339:2, s. 393-405
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Tidskriftsartikel (refereegranskat)abstract
- We present a method to implement the gluon splitting process, g → QQ, into the dipole approximation to QCD cascades. It is known that due to the uncertainties in the interpolation between the pole regions, there are large uncertainties in the results. We show that there are further uncertainties due to the ordering in phase space of the gluon emission and gluon splitting processes. We present two options called the IO scheme (for independent dipoles) and another, the DO scheme (corresponding to an ordering in transverse momentum of the emissions) both of which lead to enhanced (hard) QQ-pair production, as compared to conventional cascades. We compare to present experimental data and give predictions for LEP energies.
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| 3. |
- Bijnens, Johan, et al.
(författare)
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Obtaining K → ππ from off-shell K → π amplitudes
- 1998
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Ingår i: Nuclear Physics B. - 0550-3213. ; 521:1-2, s. 305-333
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Tidskriftsartikel (refereegranskat)abstract
- We properly define off-shell K → π transition amplitudes and use them to extract information for on-shell K → ππ amplitudes within Chiral Perturbation Theory. At order p2 in the chiral expansion all three parameters of weak interaction can be determined. At order p4 we are able to fix eleven additional constants out of thirteen contributing to off-shell K → π transitions, which leaves four undetermined constants in the on-shell K → ππ amplitudes. All script O sign(p4) contributions have been exactly derived with m2 π ≠ 0. We finally discuss the weak mass term issue and find contributions to on-shell ΔS = ±1 kaon decays, in particular to transitions like KL → γγ, KL → μ+μ- and KS → π0γγ at the lowest non-zero order.
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| 4. |
- Bijnens, J., et al.
(författare)
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Pion-pion scattering at low energy
- 1997
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Ingår i: Nuclear Physics B. - 0550-3213. ; 508:1-2, s. 263-310
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Tidskriftsartikel (refereegranskat)abstract
- We present technical details of the evaluation of the elastic ππ scattering amplitude to two loops in chiral perturbation theory. In particular, we elaborate on the renormalization procedure at the two-loop order and on the evaluation of the relevant Feynman diagrams that can all be expressed in terms of elementary functions. For the sake of clarity, we discuss these matters both in the N-component φ4 theory (in its symmetric phase) and in chiral perturbation theory. Estimates for the relevant low-energy constants of O(p6) are presented. Threshold parameters and phase shifts are then calculated for two sets of O(p4) coupling constants and compared with experiment. We comment on the extraction of threshold parameters from phase shift data.
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| 5. |
- Bijnens, J., et al.
(författare)
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Vector meson masses in chiral perturbation theory
- 1997
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Ingår i: Nuclear Physics B. - 0550-3213. ; 501:2, s. 495-517
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the vector meson masses within the context of chiral perturbation theory performing an expansion in terms of the momenta, quark masses and 1/Nc. We extend the previous analysis to include isospin breaking effects and also include up to order p4. We discuss vector meson chiral perturbation theory in some detail and present a derivation from a relativistic Lagrangian. The unknown coefficients are estimated in various ways. We also discuss the relevance of electromagnetic corrections and the implications of the present calculation for the determination of quark masses.
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| 6. |
- Friberg, Christer, et al.
(författare)
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Colour Connections in e+e- annihilation
- 1997
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Ingår i: Nuclear Physics, Section B. - 0550-3213. ; B490, s. 289-305
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Tidskriftsartikel (refereegranskat)abstract
- We have a very limited knowledge about how the confinement mechanism works in processes like $\epea\epe\ra\epq\epqa\ggn\ldots\ggn$, when there are identical colour charges. In this case the partons can be colour connected by a string or a cluster chain in several different ways. We do not know if in such a situation Nature chooses a particular configuration at random, or if some configuration is dynamically favoured. Also in the perturbative parton cascade we have no well founded recipe for describing the interference effects, which correspond to non planar diagrams and are caused by identical colour charges. We have studied two different models, and are in particular interested in the possibility that a colour singlet gluon system hadronizes isolated from the remainder of the state. Using double tagged events with heavy $\epc\epca$ or $\epb\epba$ quarks, it appears to be possible to find a significant signal, if such events appear in Nature. If this type of (re)connected states appear in Z decays it may also be an indication that reconnection might appear between the decay products of two W's in the reaction $\epea\epe\ra\gwp\gwm\ra\epq_1\epqa_2\epQ_1\epQa_2$ at LEP2. This would be important e.g. for a precision measurement of the W mass.
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| 7. |
- Gupta, Sourendu, et al.
(författare)
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Finite-size scaling at phase coexistence
- 1993
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Ingår i: Nuclear Physics, Section B. - 0550-3213. ; 409:3, s. 663-683
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Tidskriftsartikel (refereegranskat)abstract
- From a finite-size scaling (FSS) theory of cumulants of the order parameter at phase-coexistence points, we reconstruct the scaling of the moments. Assuming that the cumulants allow a reconstruction of the free energy density no better than as an asymptotic expansion, we find that FSS for moments of low order is still complete. We suggest ways of using this theory for the analysis of numerical simulations. We test these methods numerically through the scaling of cumulants and moments of the magnetisation in the low-temperature phase of the two-dimensional Ising model.
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| 8. |
- Lönnblad, Leif, et al.
(författare)
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Using neural networks to identify jets
- 1991
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Ingår i: Nuclear Physics, Section B. - 0550-3213. ; 349:3, s. 675-702
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Tidskriftsartikel (refereegranskat)abstract
- A neural network method for identifying the ancestor of a hadron jet is presented. The idea is to find an efficient mapping between certain observed hadronic kinematical variables and the quark-gluon identity. This is done with a neuronic expansion in terms of a network of sigmoidal functions using a gradient descent procedure, where the errors are back-propagated through the network. With this method we are able to separate gluon from quark jets originating from Monte Carlo generated e+e- events with ≈85% approach. The result is independent of the MC model used. This approach for isolating the gluon jet is then used to study the so-called string effect. In addition, heavy quarks (b and c) in e+e- reactions can be identified on the 50% level by just observing the hadrons. In particular we are able to separate b-quarks with an efficiency and purity, which is comparable with what is expected from vertex detectors. We also speculate on how the neutral network method can be used to disentangle different hadronization schemes by compressing the dimensionality of the state space of hadrons.
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| 9. |
- Amaudruz, P, et al.
(författare)
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A re-evaluation of the nuclear structure function ratios for D, He, 6Li, C and Ca
- 1995
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Ingår i: Nuclear Physics B. - : Elsevier. - 0550-3213 .- 1873-1562. ; 441:1-2, s. 3-11
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Tidskriftsartikel (refereegranskat)abstract
- We present a re-evaluation of the structure function ratios F-2(He)/F-2(D), F-2(C)/F-2(D) and F-2(Ca)/F-2(D) measured in deep inelastic muon-nucleus scattering at an incident muon momentum of 200 GeV. We also present the ratios F-2(C)/F-2(Li), F-2(Ca)/F-2(Li) and F-2(Ca)/F-2(C) measured at 90 GeV. The results are based on data already published by NMC; the main difference in the analysis is a correction for the masses of the deuterium targets and an improvement in the radiative corrections. The kinematic range covered is 0.0035 < x < 0.65, 0.5 < Q(2) < 90 GeV2 for the He/D, C/D and Ca/D data and 0.0085 < x < 0.6, 0.84 < Q(2) < 17 GeV2 for the Li/C/Ca ones.
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| 10. |
- Amaudruz, P, et al.
(författare)
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The ratio F2 n/F2 p in deep inelastic muon scattering
- 1992
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Ingår i: Nuclear Physics B. - : Elsevier. - 0550-3213 .- 1873-1562. ; 371:1-2, s. 3-31
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented on the ratio of neutron and proton structure functions, F2n/F2p, deduced from deep inelastic scattering of muons from hydrogen and deuterium. The data, which were obtained at the CERN muon beam at 90 and 280 GeV incident energy, cover the kinematic range x = 0.002-0.80 and Q2 = 0.1-190 GeV2. The measured structure function ratios have small statistical and systematic errors, particularly at small and intermediate x. The observed Q2 dependence in the range x = 0.1-0.4 is stronger than predicted by perturbative QCD. From the present data together with results from other experiments it is suggested that the twist-four coefficient for the proton is smaller than that for the neutron for x larger than 0.2.
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