19 research outputs found
Positivity bounds on gluon TMDs for hadrons of spin 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 . 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- kinematic region,
we also study these bounds in the small- limit for the dipole-type gauge
link structure using matrix elements of a single Wilson loop.Comment: 10 page
Renormalization group invariants in supersymmetric theories: one- and two-loop results
We stress the potential usefulness of renormalization group invariants.
Especially particular combinations thereof could for instance be used as probes
into patterns of supersymmetry breaking in the MSSM at inaccessibly high
energies. We search for these renormalization group invariants in two
systematic ways: on the one hand by making use of symmetry arguments and on the
other by means of a completely automated exhaustive search through a large
class of candidate invariants. At the one-loop level, we find all known
invariants for the MSSM and in fact several more, and extend our results to the
more constrained pMSSM and dMSSM, leading to even more invariants. Extending
our search to the two-loop level we find that the number of invariants is
considerably reduced
Directed flow from C-odd gluon correlations at small
It is shown that odd harmonic azimuthal correlations, including the directed
flow , in forward two-particle production in peripheral proton-nucleus
() 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- 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 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
collisions. We calculate the first odd harmonic 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 can arise from this C-odd mechanism. In contrast, we show
that only even harmonics arise in diffractive dijet production in
ultra-peripheral 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
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 , 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- 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
Mapping the internal structure of hadrons through color and spin effects
Originally it was thought that the proton is not divisible into smaller particles. However, from collision experiments it follows that it consists of a ‘soup’ of countless ‘colored’ elementary particles such as quarks and gluons that are kept together by the strong nuclear force. The particles constituting the proton are also referred to as partons and their distributions are described by parton distribution functions (PDFs). As these functions cannot be calculated, they must be extracted from experiment. More knowledge on PDFs teaches us more about the inner workings of protons and enables us to more accurately describe collisions between protons at the Large Hadron Collider (LHC). In this thesis we have studied quantities that depend on various types of PDFs. We have investigated one particular PDF called the Boer-Mulders function that describes correlations between the spin and transverse momentum of quarks inside the proton. This function gives rise to a very specific asymmetry in the directions of produced particles. In contrast to earlier expectations, we have shown that this PDF does not suffer from quantum effects related to color entanglement. Furthermore, we have introduced various new gluon PDFs that contain interesting information on the behavior of gluons. As it turns out, the description of the gluon content of particles such as protons simplifies tremendously in certain collisions at very high energies. Finally, we have derived how certain asymmetries that have been observed in collisions between protons and lead nuclei at the LHC can be related to quantum correlations between gluons