共查询到20条相似文献,搜索用时 0 毫秒
1.
D. Anchishkin 《Acta Physica Hungarica A》2006,27(4):403-411
We revise the μ+μ? and $q\bar q$ annihilation mechanisms of dilepton production during relativistic nuclear collisions. We focus on the modifications caused by the specific features of in-medium pion and quark states rather than by medium modification of the ρ-meson spectral density. The main ingreditent emerging in our approach is a form-factor of the multi-pion (multi-quark) system. Replacing the usual delta-function the form-factor plays the role of distribution which, in some sense, “connects” the total 4-momenta of the annihilating and outgoing particles. The difference between the c.m.s. velocities attributed to annihilating and outgoing particles is a particular consequence of this replacement and results in the appearance of a new factor in the formula for the lepton pair production rate. We obtained that the form-factor of the multi-pion (multi-quark) system causes broadening of the rate which is most pronounced for small invariant masses, in particular, we obtain a growth of the rate for the invariant masses below two masses of the annihilating particles. 相似文献
2.
《Physics letters. [Part B]》1988,207(4):516-518
We discuss the emission of circularly polarized forward photons in high energy e+e− collisions with longitudinally polarized beams. We find substantial asymmetry under inversion of longitudinal electron polarization, thus supporting the proposal that measurement of circular polarization of forward emitted photons may allow for monitoring of longitudinal beam polarization. 相似文献
3.
A. Breakstone H. B. Crawley A. Firestone M. Gorbics J. W. Lamsa W. T. Meyer D. L. Parker D. Drijard F. Fabbri H. G. Fischer H. Frehse P. Hanke P. G. Innocenti O. Ullaland W. Hofmann M. Panter K. Rauschnabel J. Spengler D. Wegener W. Geist M. Heiden E. E. Kluge T. Nakada A. Putzer K. Doroba R. Gokieli R. Sosnowski 《Zeitschrift fur Physik C Particles and Fields》1981,11(3):203-209
We present an analysis of minimum bias events from proton-proton collisions at \(\sqrt s = 7GeV\) in the CERN ISR. We remove the effects of both the leading protons and compare theB=0 mesonic residue of the events to the hadronic events of similar energy produced ine + e ? collisions. This comparison is presented in terms of the standard jet-type analyses involving quantities such as sphericity and aplanarity. We find significant differences between these data and the data frome + e ? annihilations. The data of this experiment are consistent with the predictions of a longitudinal phase space model. 相似文献
4.
Yu-Liang Yan Bao-Guo Dong Dai-Mei Zhou Xiao-Mei Li Hai-Liang Ma Ben-Hao Sa 《The European Physical Journal C - Particles and Fields》2010,67(3-4):405-409
The parton rescattering effect on the charged particle production in ultra-relativistic p+p collisions is studied by the parton and hadron cascade model PACIAE, based on PYTHIA. We have calculated charged particle pseudorapidity density at mid-rapidity and the pseudorapidity distribution in inelastic and non-single diffractive p+p collisions at $\sqrt{s}=200$ , 900, 5500, and 14000 GeV with the PYTHIA and PACIAE models. The calculated results for $\sqrt{s}=900$ GeV are well compared with the ALICE data. Comparing the PYTHIA results (without parton rescattering) with the PACIAE results (with parton rescattering), it turned out that the parton rescattering may play an important role and this effect increases with increasing CMS energy. 相似文献
5.
6.
We report on the measurements of D
0 and D
s
meson production in 200 GeV Cu+Cu collisions at the STAR at RHIC experiment. Results are discussed with reference to pQCD
predictions of the open charm cross-section as well as the statistical hadronization model. 相似文献
7.
《Nuclear Physics B》1998,523(3):423-438
Meson-photon-photon transition form factors for S-, P-, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quark-antiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with Jp = 0−, 0+, 1+, 2+, and 2− in electron-positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for χc1, ηc(2S), and ηb(1S) within reach of LEP.With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e+e− cross sections for 1+ states do not vanish at low Q2, the Landau-Yang suppression factors of the two-photon cross sections being compensated by the photon propagators. 相似文献
8.
I. Koenig E. Berdermann F. Bosch P. Kienle W. Koenig C. Kozhuharov A. Schröter H. Tsertos 《Zeitschrift für Physik A Hadrons and Nuclei》1993,346(2):153-168
We present the results obtained from a series of +–-coincidence measurements in heavy-ion collisions using the double-Orange-spectrometer at GSI. The collision systems U+U, U+Pb, and U+Ta were investigated at bombarding energies close to and slightly above (U+Ta) the Coulomb barrier. For all systems studied, very narrow (FWHM–20 keV) +– lines were observed in the sum-energy spectra, with kinetic energies ranging from 555 keV to 810 keV, superimposed on a continuous distribution mainly due to uncorrelated +– emission. Particularly in the U+Ta system, a pronounced sum-energy line appears at 634 keV, predominantly in deep-inelastic collisions. In some cases (e.g. U+Pb) the line characteristics is consistent with a two-body decay mode of an emitter which moves with the c.m. velocity of the colliding ions. However, other lines, and in particular the 634 keV line (U+Ta), exhibit a rather isotropical opening-angle distribution whereas their energy is unequally shared between positrons and electrons, thus being in clear disagreement with this scenario. In general, the data preclude an emission from the separated (moving) nuclei, and, in the latter cases, provide evidence that the e+e–-pair decay occurs in the vicinity of the Coulomb field of a third heavy (positively charged) partner having only a small transverse velocity (|v|<>Dedicated to Prof. B. Povh on the occasion of his 60th birthday 相似文献
9.
《Nuclear Physics B》1988,309(2):221-258
We examine one-loop radiative corrections to e+e−→W+W− in the standard model with one Higgs doublet, concentrating on the effects of very heavy fermions. These disturb the delicate unitarity cancellation between s- and t-channel diagrams, raising the cross section even well below the fermion threshold and giving a clear experimental signature for the heavy sector. 相似文献
10.
《Physics letters. [Part B]》1987,194(1):141-146
We demonstrate that vibrational perturbations of supercritical “quasimolecules” which may occur in collisions of heavy ions can, within the context of first-order perturbation,theory, result in the production of coincident narrow e+e− peaks, in place of and with a larger yield than that predicted for spontaneous positron emission from static supercritical systems. Certain features of this phenomenon coincide with those of the e+e− peaks observed by the EPOS collaboration although some important aspects of the observations cannot be reproduced within our approximation. A detailed attempt to reconcile this theoretical model with the EPOS experiment must go beyond perturbation theory. 相似文献
11.
《Physics letters. [Part B]》1986,180(4):353-357
12.
Polarized and unpolarizede + e ? annihilation is considered in lowest order at energies 30–150 GeV taking γ andZ 0 exchange in the standardSU(2)×U(1) model into account. The fragmentation of polarized quarks into polarized baryons is described by two kinds of fragmentation functions; the first is based onSU(6) weights and empirical expressions for favoured and disfavoured fragmentation; the second is based upon a Monte Carlo model of the chain decay including helicities of quarks and hadrons. From the cross sections for \(e^ + e^ - \to q\bar q\) and the fragmentation functions we obtain predictions for the baryon polarization as function of the collision energy and of kinematical variables. 相似文献
13.
《中国物理C(英文版)》2017,(2)
The φ-charmonium dissociation reactions in hadronic matter are studied.Unpolarised cross sections forφJ/ψ→D_s~-D_s~+,φJ/ψ→D_s~(*-)D_s~+ or D_s~-D_s~(*+),φJ/ψ→D_s~(*-)D_s~(*+),φψ'→D_s~-D_s~+,φψ'→D_s~(*-)D_s~+ or D_s~-D_s~(*+),φψ'→D_s~(*-)D_s~(*+),φχ_c→D_s~-D_s~+,φχ_c→D_s~(*-)D_s~+ or D_s~-D_s~(*+) and φχ_c→D_s~(*-)D_s~(*+) are calculated in the Born approximation,in the quark-interchange mechanism and with a temperature-dependent quark potential.The potential leads to remarkable temperature dependence of the cross sections.With the cross sections and the φ distribution function we calculate the dissociation rates of the charmonia in interactions with the φ meson in hadronic matter.The dependence of the rates on temperature and charmonium momentum is relevant to the influence of φ mesons on charmonium suppression. 相似文献
14.
Pistor P Kötschau I Grimm A Jung C Lauermann I Lux-Steiner MCh Fischer CH 《Journal of synchrotron radiation》2010,17(6):791-798
Soft X‐ray emission spectroscopy (SXES) in the energy range between 150 eV and 1500 eV has typical attenuation lengths between tens and a few hundred nanometres. In this work the transmission of soft X‐rays in synchrotron‐based SXES has been quantitatively analysed using specially prepared layer samples. The possibility of extending the standard qualitative analysis of SXES by exploiting the information underlying the emission intensity was examined for thin layer structures. Three different experiment series were accomplished with model layer systems based on different sulfur‐containing substrates: (i) MoS2, (ii) CuInS2, (iii) Cu(In,Ga)(S,Se)2. The absorption of the S L2,3 emission line by ZnO cover layers of up to 80 nm thickness was monitored and compared with theoretical expectations. By comparison with a reference intensity recorded from a bare substrate, the attenuation of the S L2,3 emission could be used to accurately determine the ZnO overlayer thickness up to a critical thickness, depending on the set‐up and the net S L2,3 emission intensity. The results from these local energy‐resolved spot measurements were compared with spatially resolved scans of the integral S L2,3 emission intensity over areas of several mm2. In the scan images the attenuation of the S L2,3 emission intensity clearly reflects the local ZnO layer thickness. From the attenuation the ZnO layer thicknesses were calculated and compared with ellipsometric measurements and were found to be in excellent agreement. These results demonstrate the benefits of a quantitative analysis of SXES, making it an even more powerful tool for examining buried interfaces and for monitoring lateral inhomogeneities. 相似文献
15.
《Nuclear Physics A》1988,489(4):751-762
We discuss the speculations that the e+e− peaks seen in large-Z heavy-ion collisions are evidence for a confined phase of an effective theory similar to strongly-coupled QED. It has been suggested that the observed radiation arises from the decay of nontopological solitons in this confined phase. (In this work we develop a scenario in which a photon condensate is formed in a finite region of space. We show that the existence of such a condensate will lead to the confinement of electrons and positrons.) However, as has been recently pointed out by Peccei, Solà and Wetterich, a new phase of QED requires that the nonlinear effects in the effective Lagrangian for the electromagnetic field become large. We describe how this might come about, within the context of electroweak theory with two Higgs doublets. 相似文献
16.
《Physics letters. [Part B]》1999,456(1):60-67
It has been proposed recently that the scale of quantum gravity (“the string scale”) can be MS∼few TeV with n≥2 extra dimensions of size R≲mm so that, at distances greater than R, Newtonian gravity with MPl∼1018 GeV is reproduced if MPl2∼RnMSn+2. Exchange of virtual gravitons in this theory generates higher-dimensional operators involving SM fields, suppressed by powers of MS. We discuss constraints on this scenario from the contribution of these operators to the processes e+e−→W+W−, ZZ, γγ. We find that LEP2 can place a limit MS≈1 TeV from e+e−→W+W−, ZZ, γγ. 相似文献
17.
A. Brandenburg M. Maniatis M. M. Weber P. M. Zerwas 《The European Physical Journal C - Particles and Fields》2008,58(2):291-300
Supersymmetry predicts the identity of Yukawa and gauge couplings in the QCD sector:
. We examine whether the
Yukawa coupling can be determined, by methods complementary to LHC, by analyzing squark–gluino final states at a TeV e
+
e
− collider. 相似文献
18.
L. Campajola A. De Rosa A. D'Onofrio L. Gialanella G. Inglima M. La Commara A. Ordine D. Pierroutsakou V. Roca M. Romano M. Romoli M. Sandoli F. Terrasi G. Cardella A. Di Pietro A. Musumarra M. Papa G. Pappalardo F. Rizzo N. Alamanos F. Auger A. Gillibert A. Incicchitti D. Prosperi G. De Angelis E. Fioretto R. Bernabei 《Zeitschrift für Physik A Hadrons and Nuclei》1995,352(4):421-425
-rays are measured in coincidence with the dissipative complex fragments from the35Cl +64Ni reaction at 7.7 A MeV. Theoretical -ray coincidence spectra are determined through a fitting procedure using the contribution of the excited fragment statistical -decay calculated by means of the CASCADE code. Whereas the expected statistical spectra fit very well the data in the energy regionE
=2 to 8 MeV, above this energy another emission mechanism in addition to the statistical decay has to be considered in order to reproduce the experimental spectra. Simulations based on the Vlasov equation suggest that this emission could come from the decay of dipole strength excited in the intermediate dinuclear system.The authors are greatly indebted with A. Boiano, A. Buccheri, M. Cipriano, F. Pagano and R. Rocco for their assistance during the set up of the experimental apparatus. Dr. J. Girard and the technical staff of the CEA/DAPNIA-DPhN are acknowledged for the valuable operation of the accelerator. 相似文献
19.
Raimo Vuopionperä 《Zeitschrift fur Physik C Particles and Fields》1995,65(2):311-325
The production of heavy Dirac and Majorana neutrinos ine + e ? collisions is investigated. The heavy Dirac and/or Majorana neutrinos can be produced in charged and neutral current processes $(e^ + e^ - \to N_1 \bar N_2 )$ . The production of a single heavy neutrino is possible if it mixes with the light neutrino species. The production of heavy neutrinos in Higgs channels is also studied, since in some specific models the Yukawa couplings could be large enough to make the production of heavy neutrinos through Higgs boson exchanges sufficiently large for detection. The most general left-right symmetric model with possibly complexV orA couplings is used in the analytic calculations of the production cross sections, but the numerical examples are given using simplified left-right symmetric model. The interference terms between different production channels have been studied in great detail and in some cases the interference terms are found to be non-negligible in wide range of production spectrum. The pair production cross section is larger in the Dirac case than it is in the Majorana case, but the single heavy Majorana neutrino production cross section is roughly twice as large as that of a Dirac neutrino. 相似文献
20.
《Nuclear Physics A》1986,456(1):159-172
We calculate double-differential cross sections for energetic photon production in intermediate energy nucleus-nucleus collisions. The production mechanism is assumed to be either that of collective bremsstrahlung or that of electromagnetic transitions between time-dependent single-particle states as emerging from a self-consistent treatment of the heavy-ion dynamics in the one-body limit (TDHF). Effects from residual nucleon-nucleon collisions on the collective current are further taken into account via a relaxation ansatz for the single-particle occupation numbers. The total yields for energetic photon production above 50 MeV in the simple limit considered indicate that up to 10% of the experimental cross sections can be attributed to collective bremsstrahlung and electromagnetic transitions. 相似文献