Search results for "Polarization"

Longitudinal Spin Physics at COMPASS

2012

Abstract The spin structure of the nucleon is studied by the COMPASS collaboration using scattering of muons off polarised protons and deuterons. From the resulting asymmetries spin structure functions are extracted and the flavour decomposition of the quark contribution to the nucleon spin is studied. The gluon polarisation is investigated using the photon–gluon fusion process. Here, two main channels are investigated, open charm production and high p T hadron pair production. Also first measurements of unpolarised cross sections are reported.

Polarized structure functions in a constituent quark scenario

1998

Using a simple picture of the constituent quark as a composite system of point-like partons, we construct the polarized parton distributions by a convolution between constituent quark momentum distributions and constituent quark structure functions. Using unpolarized data to fix the parameters we achieve good agreement with the polarization experiments for the proton, while not so for the neutron. By relaxing our assumptions for the sea distributions, we define new quark functions for the polarized case, which reproduce well the proton data and are in better agreement with the neutron data. When our results are compared with similar calculations using non-composite constituent quarks the ac…

Gluon polarization ine+e−→tt¯G

1997

We calculate the linear polarization of gluons radiated off top quarks produced in e{sup +}e{sup {minus}} annihilations. For typical top quark pair production energies at the Next Linear Collider the degree of linear polarization remains close to its soft gluon value of 100{percent} over almost the whole energy spectrum of the gluon. The massive quark results are compared with the corresponding results for the massless quark case. {copyright} {ital 1997} {ital The American Physical Society}

O(αs) longitudinal spin polarization in heavy-quark production

1995

We present the massive one-loop QCD corrections to the production cross sections of polarized quarks in the annihilation process ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}qq\ifmmode\bar\else\textasciimacron\fi{}(g) for bottom, top, and charm quarks. From the full analytical expressions for the production cross sections, Schwinger-type interpolation formulas for all parity-parity combinations (VV, VA, AA) are derived. The parity-odd interpolation formula contains the correct limit for vanishing quark masses taking into account a residual coupling of left- and right-chiral states in the massless theory. Numerical results for the total cross section and …

2019

Abstract The transverse spin asymmetries measured in semi-inclusive leptoproduction of hadrons, when weighted with the hadron transverse momentum P T , allow for the extraction of important transverse-momentum-dependent distribution functions. In particular, the weighted Sivers asymmetries provide direct information on the Sivers function, which is a leading-twist distribution that arises from a correlation between the transverse momentum of an unpolarised quark in a transversely polarised nucleon and the spin of the nucleon. Using the high-statistics data collected by the COMPASS Collaboration in 2010 with a transversely polarised proton target, we have evaluated two types of P T -weighted…

Hindered M1 radiative decay ofϒ(2S)from lattice NRQCD

2015

We present a calculation of the hindered M1 $\mathrm{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}{\ensuremath{\eta}}_{b}(1S)\ensuremath{\gamma}$ decay rate using lattice nonrelativistic quantum chromodynamics. The calculation includes spin-dependent relativistic corrections to the NRQCD action through $\mathcal{O}({v}^{6})$ in the quark's relative velocity, relativistic corrections to the leading order current which mediates the transition through the quark's magnetic moment, radiative corrections to the leading spin-magnetic coupling and for the first time a full error budget. We also use gluon field ensembles at multiple lattice spacing values, all of which include $u$, $d$, $s$ and…

Longitudinal contribution to the alignment polarization of quarks produced in e + e - -annihilation: an O (α s ) effect

1996

We calculate the longitudinal contribution to the alignment polarization Pl of quarks produced in e+e− annihilation. In the Standard Model, the longitudinal alignment polarization vanishes at the Born term level and thus receives its first non-zero contribution from the O(αs) tree graph process. We provide analytical and numerical results for the longitudinal alignment polarization of massless and massive quarks, in particular for the recently discovered top quark.

Polar angle dependence of the longitudinal polarization of quarks produced in e + e - -annihilation

1997

We calculate one-loop radiative QCD corrections to the three polarized and unpolarized structure functions that determine the beam-quark polar angle dependence of the longitudinal polarization of light and heavy quarks produced in e+e−-annihilations. We present analytical and numerical results for the longitudinal polarization and its polar angle dependence. We discuss in some detail the zero-mass limit of our results and the role of the anomalous spin-flip contributions to the polarization observables in the zero-mass limit. Our discussion includes transverse and longitudinal beam polarization effects.

Beam-Recoil Polarization Measurement of π0 Electroproduction on the Proton in the Region of the Roper Resonance

2017

The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(e[over →],e^{'}p[over →])π^{0} reaction at four-momentum transfer Q^{2}≃0.1  GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440  MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in t…

Quantum light depolarization: the phase-space perspective

2008

Quantum light depolarization is handled through a master equation obtained by coupling dispersively the field to a randomly distributed atomic reservoir. This master equation is solved by transforming it into a quasiprobability distribution in phase space and the quasiclassical limit is investigated.