Helicity Conservation Research Articles

Exclusive [formula omitted] electroproduction on transversely polarized protons

The exclusive electroproduction of ρ0 mesons was studied with the hermes spectrometer at the Desy laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. Spin density matrix elements for this process were determined from the measured production- and decay-angle distributions of the produced ρ0 mesons. These matrix elements embody information on helicity transfer and the validity of s-channel helicity conservation in the case of a transversely polarized target. From the spin density matrix elements, the leading-twist term in the single-spin asymmetry was calculated separately for longitudinally and transversely polarized ρ0 mesons. Neglecting s-channel helicity changing matrix elements, results for the former can be compared to calculations based on generalized parton distributions, which are sensitive to the contribution of the total angular momentum of the quarks to the proton spin.

Electromagnetic properties for arbitrary spin particles: Natural electromagnetic moments from light-cone arguments

We revisit the old-standing problem of the electromagnetic interaction for particles of arbitrary spin. Based on the assumption that light-cone helicity at tree level and ${Q}^{2}=0$ should be conserved nontrivially by the electromagnetic interaction, we are able to derive all the natural electromagnetic moments for a pointlike particle of any spin. We provide here a transparent decomposition of the electromagnetic current in terms of covariant vertex functions. We also define in a general way the electromagnetic multipole form factors, and show their relation with the electromagnetic moments and covariant vertex functions. The light-cone helicity conservation argument determines uniquely the values of all electromagnetic moments, which we refer to as the ``natural'' ones. These specific values are in accordance with the standard model, and the prediction of universal $g=2$ gyromagnetic factor is naturally recovered. We provide a very simple and compact formula for these natural moments. As an application of our results, we generalize the discussion of quark transverse charge densities to particles with arbitrary spin, giving more physical support to the light-cone helicity conservation argument.

Comprehensive calculations on the OZI-forbidden nonleptonic decays of orthoquarkonia [formula omitted

In this work, we calculate the decay rates of the OZI-forbidden processes J / ψ ( ϒ ) → π π , ρ π at the order of the leading-twist distribution amplitude. The process of J / ψ ( ϒ ) → π + π − violates isospin conservation and the amplitude is explicitly proportional to the isospin violation factor m u − m d , our numerical results on their decay rates are consistent with the data. The process J / ψ ( ϒ ) → ρ π violates the hadronic helicity conservation and should be suppressed, as indicated in literature, its decay rate can only be proportional to m q 2 at the order of leading twist. Our theoretical evaluation confirms this statement that the theoretical evaluation on Γ ( J / ψ ( ϒ ) → ρ π ) is almost one order smaller than the data unless the model parameters take certain extreme values. It may imply that the sizable branching ratio of J / ψ ( ϒ ) → ρ π should be explained by either higher twist contributions or other mechanisms.

Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions with STAR

We present a summary of recent photoproduction results in ultra peripheral relativistic heavy ions collisions with STAR. These collisions have impact parameters larger then twice the nuclear radius; the nuclei do not physically collide, but interact via long-range electromagnetic fields. We observe exclusive $\rho^0$ production as well as $AuAu\to Au^*Au^* \rho^0$ with accompanying mutual nuclear excitation at $\sqrt{s_{NN}}=200$ GeV. We report the $\rho^0$ production cross section for both coherent and incoherent coupling accompanied by mutual nuclear excitation. We have studied the cross section as a function of $p_T$, $y_{\rho^0}$ and $M_{\pi\pi}$ and compared it to theoretical models. In addition, we measured the $\rho^0$ helicity matrix elements. They are found to be consistent with s-channel helicity conservation. The ratio of coherent $\rho^0$ and direct $\pi^+\pi^-$ pair photoproduction has been measured and found to be consistent with earlier measurements. The 4-pion final state $AuAu \to \pi^+\pi^-\pi^+\pi^-$ state has also been observed.

The Spin Interaction of a Dirac Particle in an Aharonov-Bohm Potential in First Order Scattering

For a Dirac particle in an Aharonov-Bohm (AB) potential, it is shown that the spin interaction (SI) operator which governs the transitions in the spin sector of the first order S-matrix is related to one of the generators of rotation in the spin space of the particle. This operator, which is given by the projection of the spin operator Σ along the direction of the total momentum of the system, and the two operators constructed from the projections of the Σ operator along the momentum transfer and the z-directions close the SU(2) algebra. It is suggested, then, that these two directions of the total momentum and the momentum transfer form some sort of natural intrinsic directions in terms of which the spin dynamics of the scattering process at first order can be formulated conveniently. A formulation and an interpretation of the conservation of helicity at first order using the spin projection operators along these directions is presented.

Electroproduction ofϕ(1020)mesons at1.4⩽Q2⩽3.8GeV2measured with the CLAS spectrometer

Electroproduction of exclusive $\phi$ vector mesons has been studied with the CLAS detector in the kinematical range $1.4\leq Q^2\leq 3.8$ GeV$^{2}$, $0.0\leq t^{\prime}\leq 3.6$ GeV$^{2}$, and $2.0\leq W\leq 3.0$ GeV. The scaling exponent for the total cross section as $1/(Q^2+M_{\phi}^2)^n$ was determined to be $n=2.49\pm 0.33$. The slope of the four-momentum transfer $t'$ distribution is $b_{\phi}=0.98 \pm 0.17$ GeV$^{-2}$. Under the assumption of s-channel helicity conservation (SCHC), we determine the ratio of longitudinal to transverse cross sections to be $R=0.86 \pm 0.24$. A 2-gluon exchange model is able to reproduce the main features of the data.

Highlights and perspectives of the JLab spin physics program

Nucleon spin structure has been an active, exciting and intriguing subject of interest for the last three decades. Recent precision spin-structure data from Jefferson Lab have significantly advanced our knowledge of nucleon structure in the valence quark (high-x) region and improved our understanding of higher-twist effects, spin sum rules and quark-hadron duality. First, results of spin sum rules and polarizabilities in the low to intermediate Q 2 region are presented. Comparison with theoretical calculations, in particular with Chiral Perturbation Theory (ChPT) calculations, are discussed. Surprising disagreements of ChPT calculations with experimental results on the generalized spin polarizability, δ LT, were found. Then, precision measurements of the spin asymmetry, A 1, in the high-x region are presented. They provide crucial input for global fits to world data to extract polarized parton distribution functions. The up and down quark spin distributions in the nucleon were extracted. The results for Δd/d disagree with the leading-order pQCD prediction assuming hadron helicity conservation. Results of precision measurements of the g 2 structure function to study higher-twist effects are presented. The data indicate a significant higher-twist (twist-3 or higher) effect. The second moment of the spin structure functions and the twist-3 matrix element d 2 results were extracted. The high Q 2 result was compared with a Lattice QCD calculation. Results on the resonance spin-structure functions in the intermediate Q 2 range are presented, which, in combination with DIS data, enable a detailed study of quark-hadron duality in spin-structure functions. Finally, an experiment to study neutron transversity and transverse spin asymmetries is discussed. A future plan with the 12 GeV energy upgrade at JLab is briefly outlined.

Genuine SUSY signatures fromug→d˜χ˜i+andug→dW+, at high energies

We analyze the quark-gluon induced process $ug\ensuremath{\rightarrow}\stackrel{\texttildelow{}}{d}{\stackrel{\texttildelow{}}{\ensuremath{\chi}}}_{i}^{+}$, including the one-loop electroweak effects in the minimal supersymmetric model. This process is dominated by ${\stackrel{\texttildelow{}}{d}}_{L}$-production and is determined by four helicity amplitudes, three of which are violating helicity conservation, and another one which respects it and is logarithmically enhanced at high energy. Combining this $ug\ensuremath{\rightarrow}{\stackrel{\texttildelow{}}{d}}_{L}{\stackrel{\texttildelow{}}{\ensuremath{\chi}}}_{i}^{+}$ analysis with a corresponding one for $ug\ensuremath{\rightarrow}d{W}^{+}$, we obtain simple approximate relations between the two processes, testing the minimal supersymmetric model structure at the amplitude and the cross section levels. These relations become exact at asymptotic energies and, provided the supersymmetry (SUSY) scale is not too heavy, they may be approximately correct at LHC energies also. In addition to these, we study the one-loop electroweak corrections to the inclusive $\stackrel{\texttildelow{}}{d}{\stackrel{\texttildelow{}}{\ensuremath{\chi}}}_{i}^{+}$ production at LHC, presenting as examples, the ${p}_{T}$ and angular distributions. Comparing these to the corresponding predictions for $W+\mathrm{\text{jet}}$ production derived earlier, provides an accurate test of the supersymmetric assignments.

ρ0photoproduction in ultraperipheral relativistic heavy ion collisions atsNN=200GeV

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of rho(0) and direct pi(+)pi(-) photoproduction in ultraperipheral relativistic heavy ion collisions at root s(NN) = 200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of sigma(AuAu -> Au*Au*rho(0)) = 530 +/- 19(stat.) +/- 57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The rho 0 transverse momentum spectrum (p(T)(2)) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find sigma(inc)/sigma(coh) = 0.29 +/- 0.03 (stat.) +/- 0.08 (syst.). The ratio of direct pi(+)pi(-) to rho(0) production is comparable to that observed in gamma(p) collisions at HERA and appears to be independent of photon energy. Finally, the measured rho(0) spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

On the magnetohydrodynamic load and the magnetohydrodynamic metage

In analogy with the load and the metage in hydrodynamics, this paper defines magnetohydrodynamic load and magnetohydrodynamic metage in the case of magnetofluids. They can be used to write the magnetic field in MHD in Clebsch’s form. It has been shown herein how these two concepts can be utilized to derive the magnetic analog of the Ertel’s theorem and also, how in the presence of nontrivial topology of the magnetic field in the magnetofluid one may associate the linking number of the magnetic field lines with the invariant MHD loads. The paper illustrates that the symmetry translation of the MHD metage in the corresponding label space generates the conservation of cross helicity.

Theory and simulations of principle of minimum dissipation rate

We perform a self-consistent, time-dependent numerical simulations of dissipative turbulent plasmas at a higher Lundquist number, typically up to O(106), using full three-dimensional compressible magnetohydrodynamics code with a numerical resolution of 1283. Our simulations follow the time variation of global helicity, magnetic energy, and the dissipation rate and show that the global helicity remains approximately constant, while magnetic energy is decaying faster and the dissipation rate is decaying even faster than the magnetic energy. This establishes that the principle of minimum dissipation rate under the constraint of (approximate) conservation of global helicity is a viable approach for plasma relaxation.

Magnetic helicity and the relaxation of fossil fields

In the absence of an active dynamo, purely poloidal magnetic field configurations are unstable to large-scale dynamical perturbations, and decay via reconnection on an Alfvenic time-scale. Nevertheless, a number of classes of dynamo-free stars do exhibit significant, long-lived, surface magnetic fields. Numerical simulations suggest that the large-scale poloidal field in these systems is stabilized by a toroidal component of the field in the stellar interior. Using the principle of conservation of total helicity, we develop a variational principle for computing the structure of the magnetic field inside a conducting sphere surrounded by an insulating vacuum. We show that, for a fixed total helicity, the minimum energy state corresponds to a force-free configuration. We find a simple class of axisymmetric solutions, parametrized by angular and radial quantum numbers. However, these solutions have a discontinuity in the toroidal magnetic field at the stellar surface which will exert a toroidal stress on the surface of the star. We then describe two other classes of solutions, the standard spheromak solutions and ones with fixed surface magnetic fields, the latter being relevant for neutron stars with rigid crusts. We discuss the implications of our results for the structure of neutron star magnetic fields, the decay of fields, and the origin of variability and outbursts in magnetars.

Study of spin density matrix in exclusive diffractive [formula omitted] meson production at HERMES

Spin Density Matrix Elements (SDMEs) are determined from exclusive diffractive ρ 0 electroproduction in the HERMES experiment at 27.5 GeV beam energy. A set of 23 SDMEs is extracted from the data on hydrogen and deuterium targets in the kinematic region 1 Q 2 5 GeV 2 , 3 W 6.3 GeV , and 0 | t ′ | 0.4 GeV 2 using the maximum likelihood method. A statistically significant violation of s -channel helicity conservation is established. A signal of unnatural parity exchange is observed at the 2 σ level.

Measurement of coherent ϕ-meson photoproduction from the deuteron at low energies

The cross section and decay angular distributions for the coherent \phi meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t =(p_{\gamma}-p_{\phi})^2 =-2 GeV^2/c^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total \phi-N cross section \sigma_{\phi N} at about 10 mb. If vector meson dominance is violated, a larger \sigma_{\phi N} is possible by introducing larger t-slope for the \phi N \to \phi N process than that for the \gamma N \to \phi N process. The decay angular distributions of the \phi are consistent with helicity conservation.

Transient Coronal Sigmoids and Rotating Erupting Flux Ropes

To determine the relationship between transient coronal (soft X-ray or EUV) sigmoids and erupting flux ropes, we analyse four events in which a transient sigmoid could be associated with a filament whose apex rotates upon eruption and two further events in which the two phenomena were spatially but not temporally coincident. We find the helicity sign of the erupting field and the direction of filament rotation to be consistent with the conversion of twist into writhe under the ideal MHD constraint of helicity conservation, thus supporting our assumption of flux rope topology for the rising filament. For positive (negative) helicity the filament apex rotates clockwise (counterclockwise), consistent with the flux rope taking on a reverse (forward) S shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one identifying sigmoids with upward arching kink-unstable flux ropes and one identifying sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and Demoulin (Astron. Astrophys.351, 707, 1999), which identifies transient sigmoids with steepened current layers below rising flux ropes.

Model independent analysis of polarization effects in elastic electron–deuteron scattering in presence of two-photon exchange

The general spin structure of the matrix element, for the elastic electron (positron)–deuteron scattering is parametrized using general symmetry properties of the hadron electromagnetic interaction, such as P-, C- and T-invariances as well as lepton helicity conservation in QED at high energy, taking into account the two-photon exchange contribution. The expressions for the differential cross section and for all polarization observables are given in terms of fifteen real functions. In presence of two photon exchange, deuteron electromagnetic form factors can still be extracted from electron and positron elastic scattering in the same kinematical conditions.

Remarks on the magnetic helicity and energy conservation for ideal magneto-hydrodynamics

In Caflisch et al (1997 Commun. Math. Phys. 184 443–55), it was shown that magnetic helicity for ideal magneto-hydrodynamics is conserved for the velocities and magnetic fields in some adequate Besov spaces. We show that magnetic helicity is conserved for the velocities and magnetic fields in L3. We also obtain the necessary condition for the energy and cross-helicity conservation following the methods developed in Cheskidov et al (2007 Preprint 0704.0759 [math.AP]).

Shell models for Hall effect induced magnetic turbulence

The Hall effect occurs in strongly magnetized conductive media and results in non-dissipative currents perpendicular to the electric field. We discuss its influence on the magnetic field dynamics ignoring fluid motion and ambipolar diffusion. The magnetic field evolution can then be basically similar to that of the velocity field in hydrodynamic turbulence resulting in a magnetic turbulence. Shell models for the induction equation with Hall effect are constructed on the basis of the conservation of magnetic energy and helicity in the dissipation-free limit. Numerical simulations of these models indicate that a magnetic energy cascade does occur, but the time behaviour and spatial spectrum of the magnetic field are very different from those of the velocity in shell models of hydrodynamic turbulence.

Angular Distributions for Heavy Quarkonium Decays intoBaryon–Antibaryon Pairs

Based on the naive quark model, we calculate angular distributions forheavy quarkonium decays into pN̄*(1440), decuplet baryon pairs,and octet–decuplet baryon pairs. The general formulae of angulardistributions are given explicitly. Quark mass effects on the violationof the helicity conservation and the prospect to look for them inJ/ψ hadronic decays are discussed.

Generation of solar magnetic fields by the αδΘ dynamo

We consider a solar dynamo mechanism that generates large-scale magnetic fields due to the combined action of cyclonic flows (the α effect), differential rotation (the Θ effect), and the non-uniformity of large-scale magnetic fields (the Θ × J effect). Our results are based on numerical model which takes into account currently available data on the differential rotation of the convection zone and the intensity of convective flows in the solar interior. A reasonable choice of parameters characterizing the intensity of magnetic-field generation by the α and Θ × J mechanisms can account for an oscillatory dynamo regime with properties similar to the 22-year magnetic-activity cycle of the Sun. We analyze the nonlinear saturation of the generation effects in the large-scale magnetic field, due to either magnetic stresses or the conservation of magnetic helicity. Allowance for the helicity of the small-scale magnetic fields is of crucial importance in limiting the energy of the generated large-scale magnetic field.