2,060 research outputs found

### Positivity bounds on gluon TMDs for hadrons of spin $\le$ 1

We consider the transverse momentum dependent gluon distribution functions
(called gluon TMDs) by studying the light-front gluon-gluon correlator,
extending the results for unpolarized and vector polarized targets to also
include tensor polarized targets -- the latter type of polarization is relevant
for targets of spin $\ge1$. The light-front correlator includes
process-dependent gauge links to guarantee color gauge invariance. As from the
experimental side the gluon TMDs are largely unknown, we present positivity
bounds for combinations of leading-twist gluon distributions that may be used
to estimate their maximal contribution to observables. Since the gluonic
content of hadrons is particularly relevant in the small-$x$ kinematic region,
we also study these bounds in the small-$x$ limit for the dipole-type gauge
link structure using matrix elements of a single Wilson loop.Comment: 10 page

### Polarization effects in double open-charm production at LHCb

Double open-charm production is one of the most promising channels to
disentangle single from double parton scattering (DPS) and study different
properties of DPS. Several studies of the DPS contributions have been made. A
missing ingredient so far has been the study of polarization effects, arising
from spin correlations between the two partons inside an unpolarized proton. We
investigate the impact polarization has on the double open-charm cross section.
We show that the longitudinally polarized gluons can give significant
contributions to the cross section, but for most of the considered kinematic
region only have a moderate effect on the shape. We compare our findings to the
LHCb data in the D0D0 final state, identify observables where polarization does
have an impact on the distribution of the final state particles, and suggest
measurements which could lead to first experimental indications of, or limits
on, polarization in DPS.Comment: 16 pages, 13 figure

### Suppression of maximal linear gluon polarization in angular asymmetries

We perform a phenomenological analysis of the $\cos 2 \phi$ azimuthal
asymmetry in virtual photon plus jet production induced by the linear
polarization of gluons in unpolarized $pA$ collisions. Although the linearly
polarized gluon distribution becomes maximal at small $x$, TMD evolutionleads
to a Sudakov suppression of the asymmetry with increasing invariant mass of the
$\gamma^*$-jet pair. Employing a small-$x$ model input distribution, the
asymmetry is found to be strongly suppressed under TMD evolution, but still
remains sufficiently large to be measurable in the typical kinematical region
accessible at RHIC or LHC at moderate photon virtuality, whereas it is expected
to be negligible in $Z/W$-jet pair production at LHC. We point out the optimal
kinematics for RHIC and LHC studies, in order to expedite the first
experimental studies of the linearly polarized gluon distribution through this
process. We further argue that this is a particularly clean process to test the
$k_t$-resummation formalism in the small-$x$ regime.Comment: 11 pages, 6 figure

### Directed flow from C-odd gluon correlations at small $x$

It is shown that odd harmonic azimuthal correlations, including the directed
flow $v_1$, in forward two-particle production in peripheral proton-nucleus
($pA$) collisions can arise simply from the radial nuclear profile of a large
nucleus. This requires consideration of the C-odd part of the gluonic
generalized transverse momentum dependent (GTMD) correlator of nucleons in the
nucleus. The gluonic GTMD correlator is the Fourier transform of an off-forward
hadronic matrix element containing gluonic field strength tensors that are
connected by gauge links. It is parametrized in terms of various gluon GTMD
distribution functions (GTMDs). We show (in a gauge invariant way) that for the
relevant dipole-type gauge link structure in the small-$x$ limit the GTMD
correlator reduces to a generalized Wilson loop correlator. The Wilson loop
correlator is parametrized in terms of a single function, implying that in the
region of small $x$ there is only one independent dipole-type GTMD, which can
have a C-odd part. We show that the odderon Wigner distribution, which is
related to this C-odd dipole GTMD by a Fourier transform, generates odd
harmonics in the two-particle azimuthal correlations in peripheral $pA$
collisions. We calculate the first odd harmonic $v_1$ for forward production
within the color glass condensate framework in the limit of a large number of
colors. We find that nonzero odd harmonics are present without breaking the
rotational symmetry of the nucleus, arising just from its inhomogeneity in the
radial direction. Using a CGC model with a cubic action, we illustrate that
percent level $v_1$ can arise from this C-odd mechanism. In contrast, we show
that only even harmonics arise in diffractive dijet production in
ultra-peripheral $pA$ collisions where this gluon dipole GTMD also appears.Comment: 23 pages, 3 figures. v2: References added; Added plot for A=63 in
Fig.3 and a discussion on the A dependence. Version accepted for publication
in JHE

### No Generalized TMD-Factorization in the Hadro-Production of High Transverse Momentum Hadrons

It has by now been established that standard QCD factorization using
transverse momentum dependent parton distribution functions fails in
hadro-production of nearly back-to-back hadrons with high transverse momentum.
The essential problem is that gauge invariant transverse momentum dependent
parton distribution functions cannot be defined with process-independent Wilson
line operators, thus implying a breakdown of universality. This has led
naturally to proposals that a correct approach is to instead use a type of
"generalized" transverse momentum dependent factorization in which the basic
factorized structure is assumed to remain valid, but with transverse momentum
dependent parton distribution functions that contain non-standard, process
dependent Wilson line structures. In other words, to recover a factorization
formula, it has become common to assume that it is sufficient to simply modify
the Wilson lines in the parton correlation functions for each separate hadron.
In this paper, we will illustrate by direct counter-example that this is not
possible in a non-Abelian gauge theory. Since a proof of generalized transverse
momentum dependent factorization should apply generally to any hard
hadro-production process, a single counter-example suffices to show that a
general proof does not exist. Therefore, to make the counter-argument clear and
explicit, we illustrate with a specific calculation for a double spin asymmetry
in a spectator model with a non-Abelian gauge field. The observed breakdown of
generalized transverse momentum dependent factorization challenges the notion
that the role of parton transverse momentum in such processes can be described
using separate correlation functions for each external hadron.Comment: 19 pages, 11 figures, typos fixed and minor explanations added,
version to appear in Physical Review

### Dijet imbalance in hadronic collisions

The imbalance of dijets produced in hadronic collisions has been used to
extract the average transverse momentum of partons inside the hadrons. In this
paper we discuss new contributions to the dijet imbalance that could complicate
or even hamper this extraction. They are due to polarization of initial state
partons inside unpolarized hadrons that can arise in the presence of nonzero
parton transverse momentum. Transversely polarized quarks and linearly
polarized gluons produce specific azimuthal dependences of the two jets that in
principle are not suppressed. Their effects cannot be isolated just by looking
at the angular deviation from the back-to-back situation, rather they enter jet
broadening observables. In this way they directly affect the extraction of the
average transverse momentum of unpolarized partons that is thought to be
extracted. We discuss appropriately weighted cross sections to isolate the
additional contributions.Comment: 15 pages, 2 figures; revised version, published in Phys. Rev.

### Colour unwound - disentangling colours for azimuthal asymmetries in Drell-Yan scattering

It has been suggested that a colour-entanglement effect exists in the
Drell-Yan cross section for the 'double T-odd' contributions at low transverse
momentum $Q_T$, rendering the colour structure different from that predicted by
the usual factorisation formula [1]. These T-odd contributions can come from
the Boer-Mulders or Sivers transverse momentum dependent distribution
functions. The different colour structure should be visible already at the
lowest possible order that gives a contribution to the double Boer-Mulders
(dBM) or double Sivers (dS) effect, that is at the level of two gluon
exchanges. To discriminate between the different predictions, we compute the
leading-power contribution to the low-$Q_T$ dBM cross section at the two-gluon
exchange order in the context of a spectator model. The computation is
performed using a method of regions analysis with Collins subtraction terms
implemented. The results conform with the predictions of the factorisation
formula. In the cancellation of the colour entanglement, diagrams containing
the three-gluon vertex are essential. Furthermore, the Glauber region turns out
to play an important role - in fact, it is possible to assign the full
contribution to the dBM cross section at the given order to the region in which
the two gluons have Glauber scaling. A similar disentanglement of colour is
found for the dS effect.Comment: 36 pages, 11 figures; v2: typos corrected/ reference added, v3: minor
corrections/ small explanations added/ references added, v4: very minor
correction/ small explanations added/ references added (this version has been
accepted for publication in SciPost

### Effects of TMD evolution and partonic flavor on $e^+e^-$ annihilation into hadrons

We calculate the transverse momentum dependence in the production of two
back-to-back hadrons in electron-positron annihilations at the medium/large
energy scales of BES-III and BELLE experiments. We use the parameters of the
transverse-momentum-dependent (TMD) fragmentation functions that were recently
extracted from the semi-inclusive deep-inelastic-scattering multiplicities at
low energy from HERMES. TMD evolution is applied according to different
approaches and using different parameters for the nonperturbative part of the
evolution kernel, thus exploring the sensitivity of our results to these
different choices and to the flavor dependence of parton fragmentation
functions. We discuss how experimental measurements could discriminate among
the various scenarios.Comment: 33 pages, 10 composite figures, JHEP style fil

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