In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD). These partons subsequently emit further partons in a process that can be described as a parton shower which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass $m_{\rm{Q}}$ and energy $E$, within a cone of angular size $m_{\rm{Q}}$/$E$ around the emitter. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.
The cross section of the process e+e−→π+π−π0 has been measured in the c.m. energy range 984–1060 MeV with the CMD-2 detector at the VEPP-2M collider. The obtained value of Br(ϕ→e+e−)Br(ϕ→π+π−π0)=(4.51±0.16±0.11)×10−5 is in good agreement with the previous measurements and has the best accuracy. Analysis of the Dalitz plot was performed. The contributions of the dominant ϕ→ρπ mechanism as well as of a small direct ϕ→3π amplitude were determined.
Radiative decays of the $\phi$ meson have been studied using a data sample of about 19 million $\phi$ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From selected $e^+e^-\to\pi^{0}\pi^{0}\gamma$ and $e^+e^-\to\eta\pi^{0}\gamma$ events the following model independent results have been obtained: \par $Br(\phi\to\pi^{0}\pi^{0}\gamma) = (0.92\pm 0.08\pm0.06)\times10^{-4}$ for $M_{\pi^{0}\pi^{0}}>700$ MeV, \par $Br(\phi\to\eta\pi^{0}\gamma) = (0.90\pm 0.24\pm 0.10)\times10^{-4}$. It is shown that the intermediate mechanism $f_{0}(980)\gamma$ dominates in the $\phi\to\pi^{0}\pi^{0}\gamma$ decay and the corresponding branching ratio is \par $Br(\phi\to f_{0}(980)\gamma)=(2.90\pm 0.21\pm1.54)\times10^{-4}$. The systematic error is dominated by the possible model uncertainty. \par Using the same data sample the upper limit has been obtained for the P- and CP-violating decay of $\eta$ at 90% CL: \par $Br(\eta\to\pi^{0}\pi^{0}) < 4.3\times10^{-4}$ >.
Using 11.6 pb^{-1} of data collected in the energy range 0.984--1.06 GeV by CMD-2 at VEPP-2M, the cross section of the reaction e+e- to pi+pi-pi+pi- has been studied. For the first time an interference pattern was observed in the energy dependence of the cross section near the phi meson. The branching ratio of the phi to pi+pi-pi+pi- decay double suppressed by the G-parity and OZI rule is measured Br(phi to pi+pi-pi+pi-) = (3.93 +- 1.74 +- 2.14) \cdot 10^{-6}. The upper limits have been placed for the decays phi to pi+pi-pi+pi-pi0 and phi to eta pi+pi- Br(phi to pi+pi-pi+pi-pi0) < 4.6 \cdot 10^{-6} 90% CL, Br(phi to eta pi+pi-) < 1.8 \cdot 10^{-5} 90% CL.
A partial wave analysis of the centrally produced eta pi0 and eta pi- channels has been performed in pp collisions using an incident beam momentum of 450 GeV/c. Clear a0(980) and a2(1320) signals have been observed in S and D+ waves respectively. The dPT, phi and |t| distributions of these resonances are presented.
The πpi-system produced in the charge exchange π−p-reaction at 100 GeV/c has been studied. The experiment was performed at the CERN SPS accelerator with the multiphoton hodoscope spectrometer GAMS-400
Using 3.07 ${pb}^{-1}$ of data collected in the energy range 0.60-0.97 GeV by CMD-2, about 150 events of the process $\epm \to \pch$ have been selected. The energy dependence of the cross section agrees with the assumption of the $a_1(1260) \pi$ intermediate state which is dominant above 1 GeV. For the first time \fourpi events are observed at the $\rho$ meson energy. Under the assumption that all these events come from the $\rho$ meson decay, the value of the cross section at the $\rho$ meson peak corresponds to the following decay width: \Gamma(\rho^0 \to \fourpi) = (2.8 \pm 1.4 \pm 0.5) {keV} or to the branching ratio B(\rho^0 \to \fourpi) = (1.8 \pm 0.9 \pm 0.3) \cdot 10 ^{-5}.
First results of the study of the process e+e- \to 4\pi by the CMD-2 collaboration at VEPP-2M are presented for the energy range 1.05--1.38 GeV. Using an integrated luminosity of 5.8 pb^{-1}, energy dependence of the processes e+e- \to \pi^+\pi^- 2\pi^0 and e+e- \to 2\pi^+ 2\pi^- has been measured. Analysis of the differential distributions demonstrates the dominance of the a_1\pi and \omega\pi intermediate states. Upper limits for the contributions of other alternative mechanisms are also placed.
The cross section of the process e+e- -> pi+pi- has been measured using about 114000 events collected by the CMD-2 detector at the VEPP-2M e+e- collider in the center-of-mass energy range from 0.61 to 0.96 GeV. Results of the pion form factor determination with a 0.6% systematic uncertainty are presented. Implications for the hadronic contribution to the muon anomalous magnetic moment are discussed.
We present a measurement of the pion form factor based on e+e- annihilation data from the CMD-2 detector in the energy range 0.6<sqrt(s)<1.0 GeV with a systematic uncertainty of 0.8%. A data sample is five times larger than that used in our previous measurement.
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