Unpolarized Case Research Articles

On the choice of beam polarization in e^+e^-rightarrow ZZ/Zgamma and anomalous triple gauge-boson couplings

The anomalous trilinear gauge couplings of Z and gamma are studied in e^+e^-rightarrow ZZ/Zgamma with longitudinal beam polarizations using a complete set of polarization asymmetries for the Z boson. We quantify the goodness of the beam polarization in terms of the likelihood and find the best choice of e^- and e^+ polarizations to be (+0.16, -0.16), (+0.09, -0.10) and (+0.12, -0.12) for ZZ, Zgamma and combined processes, respectively. Simultaneous limits on anomalous couplings are obtained for these choices of beam polarizations using Markov-Chain–Monte-Carlo (MCMC) for an e^+e^- collider running at sqrt{s}=500 GeV and mathscr {L}=100 fb^{-1}. We find the simultaneous limits for these beam polarizations to be comparable with each other and also comparable with the unpolarized beam case.

Nucleon helicity and transversity parton distributions from lattice QCD

We present the first lattice-QCD calculation of the isovector polarized parton distribution functions (both helicity and transversity) using the large-momentum effective field theory (LaMET) approach for direct Bjorken-x dependence. We first review the detailed steps of the procedure in the unpolarized case, then generalize to the helicity and transversity cases. We also derive a new mass-correction formulation for all three cases. We then compare the effects of each finite-momentum correction using lattice data calculated at Mπ≈310 MeV. Finally, we discuss the implications of these results for the poorly known antiquark structure and predict the sea-flavor asymmetry in the transversely polarized nucleon.

Robust band gap and half-metallicity in graphene with triangular perforations

Ideal graphene antidot lattices are predicted to show promising band gap behavior (i.e., ${E}_{G}\ensuremath{\simeq}500$ meV) under carefully specified conditions. However, for the structures studied so far this behavior is critically dependent on superlattice geometry and is not robust against experimentally realistic disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag-edged antidots give rise to large band gaps compared to armchair-edged antidots, irrespective of the rules which govern the existence of gaps in armchair-edged antidot lattices. In addition the zigzag-edged antidots appear more robust than armchair-edged antidots in the presence of geometrical disorder. The inclusion of spin polarization within a mean-field Hubbard approach gives rise to a large overall magnetic moment at each antidot due to the sublattice imbalance imposed by the triangular geometry. Half-metallic behavior arises from the formation of spin-split dispersive states near the Fermi energy, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder, and furthermore, of exploiting the strong spin dependence of the system for spintronic applications.

Ab initioquantum Monte Carlo simulations of the uniform electron gas without fixed nodes: The unpolarized case

In a recent publication [S. Groth \textit{et al.}, PRB (2016)], we have shown that the combination of two novel complementary quantum Monte Carlo approaches, namely configuration path integral Monte Carlo (CPIMC) [T. Schoof \textit{et al.}, PRL \textbf{115}, 130402 (2015)] and permutation blocking path integral Monte Carlo (PB-PIMC) [T. Dornheim \textit{et al.}, NJP \textbf{17}, 073017 (2015)], allows for the accurate computation of thermodynamic properties of the spin-polarized uniform electron gas (UEG) over a wide range of temperatures and densities without the fixed-node approximation. In the present work, we extend this concept to the unpolarized case, which requires non-trivial enhancements that we describe in detail. We compare our new simulation results with recent restricted path integral Monte Carlo data [E. Brown \textit{et al}., PRL \textbf{110}, 146405 (2013)] for different energy contributions and pair distribution functions and find, for the exchange correlation energy, overall better agreement than for the spin-polarized case, while the separate kinetic and potential contributions substantially deviate.

Anomalous WWγ couplings with beam polarization at the Compact Linear Collider

We study the anomalous WWγ couplings at the Compact Linear Collider through the processes e+e−→W+W−, e−e+→e−γ⁎e+→e+νeW− and e−e+→e−γ⁎γ⁎e+→e−W+W−e+ (γ⁎ is the Weizsacker–Williams photon). We give the 95% confidence level limits for unpolarized and polarized electron (positron) beam on the anomalous couplings for various values of the integrated luminosities and center-of-mass energies. We show that the obtained limits on the anomalous couplings through these processes can highly improve the current experimental limits. In addition, our limits with beam polarization are approximately two times better than the unpolarized case.

Pressure, compressibility, and contact of the two-dimensional attractive fermi gas.

Using abinitio lattice methods, we calculate the finite temperature thermodynamics of homogeneous two-dimensional spin-1/2 fermions with attractive short-range interactions. We present results for the density, pressure, compressibility, and quantum anomaly (i.e., Tan's contact) for a wide range of temperatures (mostly above the superfluid phase, including the pseudogap regime) and coupling strengths, focusing on the unpolarized case. Within our statistical and systematic uncertainties, our prediction for the density equation of state differs quantitatively from the prediction by Luttinger-Ward theory in the strongly coupled region of parameter space, but otherwise agrees well with it. We also compare our calculations with the second- and third-order virial expansion, with which they are in excellent agreement in the low-fugacity regime.

Mass and energy dependence of the transverse momentum densities in covariant parton model

The parton densities which are dependent on transverse momentum, open a way to understand better the structure of quarks and gluons in a more complete way. We are investigating a method based on the covariant quark model which enables us to extract the transverse momentum dependent (TMD) densities from the usual parton densities which are just dependent on the longitudinal momentum. In continuation, we obtain the dependence of the TMDs on binding energy and the mass of quarks. We do some calculations to obtain the TMDs in the unpolarized case while the mass and binding energy of partons are varying. Considering these effects, the results for TMDs are in good agreement with the results of the recent related models.

The three-loop splitting functions in QCD: The helicity-dependent case

We present the next-to-next-to-leading order (NNLO) contributions to the main splitting functions for the evolution of longitudinally polarized parton densities of hadrons in perturbative QCD. The quark–quark and gluon–quark splitting functions have been obtained by extending our previous all Mellin-N calculations to the structure function g1 in electromagnetic deep-inelastic scattering (DIS). Their quark–gluon and gluon–gluon counterparts have been derived using third-order fixed-N calculations of structure functions in graviton-exchange DIS, relations to the unpolarized case and mathematical tools for systems of Diophantine equations. The NNLO corrections to the splitting functions are small outside the region of small momentum fractions x where they exhibit a large double-logarithmic enhancement, yet the corrections to the evolution of the parton densities can be unproblematic down to at least x≈10−4.

Effect of three-body clusters in the ground-state properties of spin-polarized liquid 3He

The ground-state energy of polarized and unpolarized liquid 3He is calculated using the variational theory. A variational wave function is constrained to be normalized appropriately by including the three-body terms in the cluster expansion of the two-body radial distribution function. The higher-order terms have been found to be important to obtain an equation of state which is in agreement with experimental data. The saturation density of unpolarized liquid 3He was found to be 0.267σ−3, which decreases by enhancing the polarization. For all relevant densities, the ground-state energy of the spin-polarized system is higher than that in the unpolarized case.

Charge symmetry breaking from a chiral extrapolation of moments of quark distribution functions

We present a determination, from lattice QCD, of charge symmetry violation in the spin-independent and spin-dependent parton distribution functions of the nucleon. This is done by chirally extrapolating recent QCDSF/UKQCD Collaboration lattice simulations of the first several Mellin moments of the parton distribution functions of octet baryons to the physical point. We find small chiral corrections for the polarized moments, while the corrections are quantitatively significant in the unpolarized case.

TWO-PION PRODUCTION IN ELECTRON-POLARIZED PROTON SCATTERING

The process of two-pion production in the electron-polarized proton scattering is investigated. In the Weizsäcker–Williams approximation the differential spectral distributions and the spin-momentum correlations are considered. The spin correlation effects caused by ρ-meson widths are estimated to be of an order of several per cent. Both channels of the π+π- and π+π0 creation are considered. The effects of intermediate excited baryons are not considered. The spectral distributions on pion energy fractions in polarized and unpolarized cases are presented analytically and numerically.

The gap equation for spin-polarized fermions

We study the BCS gap equation for a Fermi gas with unequal population of spin-up and spin-down states. For cosh (δμ/T) ⩽ 2, with T the temperature and δμ the chemical potential difference, the question of existence of non-trivial solutions can be reduced to spectral properties of a linear operator, similar to the unpolarized case studied previously in [Frank, R. L., Hainzl, C., Naboko, S., and Seiringer, R., J., Geom. Anal. 17, 559–567 (2007)10.1007/BF02937429; Hainzl, C., Hamza, E., Seiringer, R., and Solovej, J. P., Commun., Math. Phys. 281, 349–367 (2008)10.1007/s00220-008-0489-2; and Hainzl, C. and Seiringer, R., Phys. Rev. B 77, 184517–110 435 (2008)]10.1103/PhysRevB.77.184517. For cosh (δμ/T) > 2 the phase diagram is more complicated, however. We derive upper and lower bounds for the critical temperature, and study their behavior in the small coupling limit.

An alternative for polarized antiproton beams

Presently the most popular way to prepare high quality polarized antiproton beams is the so called spin filter method. The feasibility of the method has been proven for a proton beam and measurements of the spin dependent interaction of antiprotons have been proposed by the PAX collaboration. Another well known source for polarized antiprotons is the \(\bar{\Lambda}\) decay which was used at FERMILAB in the only experiment performed so far with polarized antiprotons. An alternative approach for polarized antiproton beams may be the production process itself. If the produced antiprotons show polarization it would be rather simple to handle a polarized antiproton beam in the existing antiproton collector and cooler at CERN just like in the unpolarized case.

Computation of the structure of a magnetized strange quark star

We have calculated some properties of spin polarized strange quark matter (SQM) in a strong magnetic field at zero temperature using the MIT bag model. We showed that the equation of state of spin polarized SQM is stiffer than that for un-polarized cases. We have also computed the structural properties of a spin polarized strange quark star (SQS) and found that the presence of a magnetic field leads to a more stable SQS when compared to the structural properties of an unpolarized SQS.

Higher Twist Contributions to the Azimuthal Asymmetries in SIDIS

This is intended to be a summary of a series of publications [1, 2, 3] carried out by our group. We showed that collinear expansion can be extended to the semi-inclusive deep inelastic scattering process e + p → e + q + X as a systematic way of studying higher twist effects. We calculated the cross section up to twist-3 level for different polarized and unpolarized cases, and obtained the azimuthal asymmetry ⟨cos ϕ⟩ in terms of gauge invariant twist-3 TMD parton correlation functions [1]. We further calculated the complete cross section up to twist-4 level for the unpolarized case, and obtained the azimuthal asymmetry ⟨cos 2ϕ⟩ expressed in terms of gauge invariant twist-4 TMD parton correlation functions [3]. We also showed that the nuclear dependence of the TMD distributions can be obtained from the gauge link and studied the nuclear dependence of both ⟨cos ϕ⟩ and ⟨cos 2ϕ⟩ asymmetries with a Gaussian ansatz for transverse momentum dependence of parton correlation functions [2, 3].

Azimuthal asymmetries for hadron distributions inside a jet in hadronic collisions

Using a generalized parton model approach including spin and intrinsic parton motion effects, and assuming the validity of factorization for large pT jet production in hadronic collisions, we study the azimuthal distribution around the jet axis of leading pions, produced in the jet fragmentation process. We identify the observable leading-twist azimuthal asymmetries for the unpolarized and single-polarized case related to both quark and gluon-originated jets. We account for all physically allowed combinations of the transverse momentum dependent (TMD) parton distribution and fragmentation functions, with special attention to the Sivers, Boer-Mulders, and transversity quark distributions, and to the Collins fragmentation function for quarks (and to the analogous functions for gluon partons).

S1×S2Gowdy supersymmetric constraint

We obtain the supersymmetric constraint for S{sup 1}xS{sup 2} Gowdy spacetime in the N=1 supergravity formalism of quantum cosmology in four dimensions. The physical states of the model for both polarized and unpolarized cases are presented.

Azimuthal asymmetries for hadron distributions inside a jet in hadronic collisions

Using a generalized parton model approach including spin and intrinsic parton motion effects, and assuming the validity of factorization for large-pT jet production in hadronic collisions, we study the azimuthal distribution around the jet axis of leading unpolarized or (pseudo)scalar hadrons, namely pions, produced in the jet fragmentation process. We identify the observable leading-twist azimuthal asymmetries for the unpolarized and single-polarized case related to quark and gluon-originated jets. We account for all physically allowed combinations of the transverse momentum dependent (TMD) parton distribution and fragmentation functions, with special attention to the Sivers, Boer-Mulders, and transversity quark distributions, and to the Collins fragmentation function for quarks (and to the analogous functions for gluons). For each of these effects we evaluate, at central and forward rapidities and for kinematical configurations accessible at BNL-RHIC, the corresponding potentially maximized asymmetry (for pi+ production), obtained by saturating natural positivity bounds (and the Soffer bound for transversity) for the distribution and fragmentation functions involved and summing additively all partonic contributions. We then estimate, for both neutral and charged pions, the asymmetries involving TMD functions for which parameterizations are available. We also study the role of the different mechanisms, and the corresponding transverse single spin asymmetries, for large-pT inclusive jet production.

TMD FACTORIZATION AND EVOLUTION FOR TMD CORRELATION FUNCTIONS

We discuss the application of transverse momentum dependent (TMD) factorization theorems to phenomenology. Our treatment relies on recent extensions of the Collins-Soper-Sterman (CSS) formalism. Emphasis is placed on the importance of using well-defined TMD parton distribution functions (PDFs) and fragmentation functions (FFs) in calculating the evolution of these objects. We explain how parametrizations of unpolarized TMDs can be obtained from currently existing fixed-scale Gaussian fits and previous implementations of the CSS formalism in the Drell-Yan process, and provide some examples. We also emphasize the importance of agreed-upon definitions for having an unambiguous prescription for calculating higher orders in the hard part, and provide examples of higher order calculations. We end with a discussion of strategies for extending the phenomenological applications of TMD factorization to situations beyond the unpolarized case.

Suppression of Penning ionization in a spin-polarized mixture of rubidium and He*

This paper presents the first study of the collision dynamics of an ultra-cold spin-polarized mixture of rubidium and metastable helium (He*) atoms. Our experiment monitors ion production from the mixture for both magnetically polarized and unpolarized cases. In the unpolarized case, we observe an increase in our background ion rate. However, in the completely polarized sample the ion production is below the sensitivity of our experiment. Nonetheless, we determine an upper limit of 5×10−12 cm3 s−1 for the polarized rate constant (βRb-He*), which is two orders of magnitude below the unpolarized rate constant. Such a suppression of the He*–87Rb polarized rate was not apparent a priori and opens the intriguing possibility of creating a dual Bose–Einstein condensate comprising an alkali ground-state atom and an excited-state noble-gas atom.