Burkert has authored and co-authored over 500 scientific articles and has over 56,000 citations. [9] In his early years of research,he studied nucleon excitations involving high energy polarized electron beams and spin-polarized hydrogen and deuterium targets. [10] At the Bonn University electron accelerator,Burkert developed an electron-spin polarimeter to map the energy and strengths of several depolarizing resonances induced in the electron beam during the acceleration process in the synchrotron. The results enabled designing compensating measures to keep the polarization value high during the acceleration process as required for scientific experiments. [11]
Burkert's research at CERN focused on hard scattering processes employing two colliding proton beams each with beam energies up to 31 GeV. This led to the first direct determination of the gluon structure function of the proton. [12] At Jefferson Lab (CEBAF),Burkert led a research program focused on the experimental study of the structure of protons,neutrons and nuclei using high energy electron and photon beams,polarized hydrogen and deuterium targets and the CLAS detector system,suitably instrumented for high rate operation in intense electron beams. This opened up a high-impact scientific program of exclusive electron scattering measurements,where all particles generated in the interaction are detected and identified in CLAS. The required detector modifications enabled the discovery of the theoretically predicted deeply virtual Compton scattering (DVCS) process, [13] which provided the basis for an extensive program to construct 3D images of the proton's internal quark distribution as well their internal mechanical properties. These modifications were also critical for measurements that clarified the internal structure of a series of excited states of the proton as part of the NSTAR program.
Nuclear and particle physics
Burkert has made major contributions to the design,construction and performance of CLAS. [6] The work on pressure distribution inside the proton provides insights into the strong interaction mechanisms internal to subatomic particles,and the cause of the extremely high pressure observed in protons for the first time. It further introduced a new area of research on the fundamental gravitational and mechanical properties of protons,neutrons and nuclei,which can provide access to the normal and shear stress inside subatomic particles,and their physical radii. [14] The results of this research through an extraction of the Compton Form Factors reveal a tomographic image of the nucleon. [15] This result is based on prior measurements of the differential cross sections and of the beam spin asymmetries in the hard exclusive electroproduction of photons on the proton over a wide kinematic range and with high statistical precision. [16]
Burkert reviewed the experimental findings in a large number of experiments in 2003-2004 had found evidence of the existence of an exotic baryon Ɵ+(1535) consisting of four quarks and one anti-quark,generated in photoproduction processes,e.g. on deuterium ɣd→K-(K+n)p as well as more massive states. Such exotic states would have quantum numbers that cannot be formed from only the 3 quarks present in nucleons or other baryons,but require combinations of four quarks and one anti-quark. He found that the experimental evidence for the Θ+(1535) state had significantly eroded with new precision data,and left only room for a hypothetical baryon state in the (K+n) system with a very narrow intrinsic energy-width of less than 500 KeV,rendering the existence of such a state highly unlikely. [17]
Reviews of the progress in the investigation of the electroexcitation of the excited nucleon resonances,both in experiment and in theory,highlight the transition amplitudes of the four lowest excited states. [18] These results show that the standard quark model of 3 valence quarks,consisting of up-quarks and down-quarks only,cannot explain the resonance transition amplitudes at small photon virtuality. Higher Fock states,including meson-baryon contributions,must be included. These results led the way towards resolving a longstanding controversy about the Roper resonance, [19] the lowest mass radial excitation of the ground state nucleon. Its transition amplitudes strongly deviate from the quark model predictions when probed at large distances that led to invoking more exotic interpretations of the resonance as a hybrid excitation [20] with gluons as structural parts of the wave function,and as a dynamically meson-baryon excitation. [21] This has been discussed in Progress in Nuclear and Particle Physics,and in the Review of Modern Physics. [22] Similar but smaller effects were also found for other excited states,Δ(1232)3/2+,N(1520)3/2−,and N(1535)1/2-,and indicates that this may be a fundamental contribution to resonance excitations in electromagnetic interactions.
