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Register a new userIntrinsic Transverse Momentum and Evolution in Weighted Spin Asymmetries
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The transverse momentum dependent (TMD) and collinear higher twist theoretical factorization frameworks are the most frequently used approaches to describing spin dependent hard cross sections weighted by and integrated over transverse momentum. Of particular interest is the contribution from small transverse momentum associated with the target bound state. In phenomenological applications, this contribution is often investigated using transverse momentum weighted integrals that sharply regulate the large transverse momentum contribution, for example with Gaussian parametrizations. Since the result is a kind of hybrid of TMD and collinear (inclusive) treatments, it is important to establish if and how the formalisms are related in applications to weighted integral observables. The suppression of a large transverse momentum tail, for example, can potentially affect the type of evolution that is applicable. We find that a naive version of a widely used identity relating the $k_T^2$-weighted and integrated Sivers TMD function to a renormalized twist-3 function has strongly ambiguous ultraviolet contributions, and that corrections to it are not necessarily perturbatively suppressed. We discuss the implications for applications, arguing in particular that the relevant evolution for transverse momentum weighted and integrated cross sections with sharp effective large transverse momentum cutoffs is of the TMD form rather than the standard renormalization group evolution of collinear correlation functions.
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The transverse momentum distributions of charged particles in p-Pb collisions as sqrt{s_{NN}} = 5.02 TeV measured by the ALICE collaboration are fitted using Tsallis statistics. The use of a thermodynamically consistent form of this distribution leads to an excellent description of the transverse momentum distributions for all rapidity intervals. The values of the Tsallis parameter q, the temperature T and the radius R of the system do not change within the measured pseudorapidity intervall.
In the TMD approach, the average transverse momentum of the unpolarised TMD PDFs and FFs is crucial not only to reproduce unpolarised cross sections and hadron multiplicities, but also for the understanding of azimuthal and spin asymmetries. Information on these transverse momenta is nowadays obtained mainly by fitting multiplicities data for SIDIS, where the intrinsic motion in the initial parton distributions and in the hadronisation process are strongly correlated and difficult to estimate separately without ambiguities. In this contribution we discuss the consequences of this correlation effects on the predictions for the Sivers and Collins asymmetries measured in SIDIS and $e^+e^-$ annihilations, and under active investigation for Drell-Yan processes at RHIC and at CERN by the COMPASS experiment. We show that these effects may be relevant and can sensibly modify the size of the predicted asymmetries. Therefore, they must be taken into careful account when investigating other aspects of TMDs, like the evolution properties of the Sivers and Collins functions and the expected process dependence of the Sivers function.
Exclusive production of $rho^0$ mesons was studied at the COMPASS experiment by scattering 160 GeV/$c$ muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured as a function of $Q^2$, $x_{Bj}$, or $p_{T}^{2}$. The $sin phi_S$ asymmetry is found to be $-0.019 pm 0.008(stat.) pm 0.003(syst.)$. All other asymmetries are also found to be of small magnitude and consistent with zero within experimental uncertainties. Very recent calculations using a GPD-based model agree well with the present results. The data is interpreted as evidence for the existence of chiral-odd, transverse generalized parton distributions.
Elastic lepton scattering off of a nucleon has proved to be an efficient tool to study the structure of the hadron. Modern cross section and asymmetry measurements at Jefferson Lab require effects beyond the leading order Born approximation to be taken into account. Availability of unpolarized beams of both electrons and positrons in respective experiments would enable to reduce systematic uncertainties due to higher-order charge-odd contributions. In addition, information on an unpolarized electron-to-positron cross section ratio could serve as a test for theoretical models that provide predictions for charge-dependent radiative corrections to elastic lepton-nucleon scattering. Availability of polarized beams of leptons would allow for even more comprehensive study of higher-order effects as some of them are dominant in polarized lepton-nucleon scattering asymmetries. We present a brief overview of effects due to the leptons charge and targets polarization on elastic lepton-nucleon scattering measurements.
An overview is presented of transverse momentum distributions of particles at the LHC using the Tsallis distribution. The use of a thermodynamically consistent form of this distribution leads to an excellent description of charged and identified particles. The values of the Tsallis parameter q are truly remarkably consistent.
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