1,629 research outputs found
Exclusive Radiative Decays of W and Z Bosons in QCD Factorization
We present a detailed theoretical analysis of very rare, exclusive hadronic
decays of the electroweak gauge bosons V=W, Z from first principles of QCD. Our
main focus is on the radiative decays V->M+gamma, in which M is a pseudoscalar
or vector meson. At leading order in an expansion in powers of Lambda_{QCD}/m_V
the decay amplitudes can be factorized into convolutions of calculable
hard-scattering coefficients with the leading-twist light-cone distribution
amplitude of the meson M. Power corrections to the decay rates arise first at
order (Lambda_{QCD}/m_V)^2. They can be estimated in terms of higher-twist
distribution amplitudes and are predicted to be tiny. We include one-loop
O(alpha_s) radiative corrections to the hard-scattering coefficients and
perform the resummation of large logarithms [alpha_s log(m_V^2/mu_0^2)]^n (with
mu_0=1 GeV a typical hadronic scale) to all orders in perturbation theory.
Evolution effects have an important impact both numerically and conceptually,
since they reduce the sensitivity to poorly determined hadronic parameters. We
present detailed numerical predictions and error estimates, which can serve as
benchmarks for future precision measurements. We also present an exploratory
study of the weak radiative decays Z->M+W. Some of the decay modes studied here
have branching ratios large enough to be accessible in the high-luminosity run
of the LHC. Many of them can be measured with high accuracy at a future lepton
collider. This will provide stringent tests of the QCD factorization formalism
and enable novel searches for new physics.Comment: 37 pages (+ appendices and references), 9 figures and 9 tables. v2:
Comparison with recent ATLAS data added, minor revisions + some references
added. v3: decay constant of the phi and omega mesons updated and few typos
fixed; version published in JHE
Dipole operator constraints on composite Higgs models
Flavour- and CP-violating electromagnetic or chromomagnetic dipole operators
in the quark sector are generated in a large class of new physics models and
are strongly constrained by measurements of the neutron electric dipole moment
and observables sensitive to flavour-changing neutral currents, such as the
branching ratio and . After a
model-independent discussion of the relevant constraints, we analyze these
effects in models with partial compositeness, where the quarks get their masses
by mixing with vector-like composite fermions. These scenarios can be seen as
the low-energy limit of composite Higgs or warped extra dimensional models. We
study different choices for the electroweak representations of the composite
fermions motivated by electroweak precision tests as well as different flavour
structures, including flavour anarchy and or flavour
symmetries in the strong sector. In models with "wrong-chirality" Yukawa
couplings, we find a strong bound from the neutron electric dipole moment,
irrespective of the flavour structure. In the case of flavour anarchy, we also
find strong bounds from flavour-violating dipoles, while these constraints are
mild in the flavour-symmetric models.Comment: 30 pages, 2 figures, 11 tables. v3: Misprints in table 8 corrected.
Numerics and conclusions unchange
Exclusive Radiative Decays of Z Bosons in QCD Factorization
We discuss the very rare, exclusive hadronic decays of a Z boson into a meson
and a photon. The QCD factorization approach allows to organize the decay
amplitude as an expansion in powers of , where the
leading terms contain convolutions of perturbatively calculable hard functions
with the leading-twist light-cone distribution amplitudes of the meson. We find
that power corrections to these leading terms are negligible since they are
suppressed by the small ratio .
Renormalization-group effects play a crucial role as they render our
theoretical predictions less sensitive to the hadronic input parameters which
are currently not known very precisely. Thus, measurements of the decays at the LHC or a future lepton collider provide a theoretically very
clean way to test the QCD factorization approach. The special case where
is complicated by the fact that the decay amplitude receives
an additional contribution where the meson is formed from a two-gluon state.
The corresponding branching ratios are very sensitive to the hadronic
parameters describing the system. Future measurements of these
decays could yield interesting information about these parameters and the gluon
distribution amplitude.Comment: 6 pages, 3 figures, 1 table, contribution to the proceedings of the
38th International Conference on High Energy Physics, 3-10 August 2016,
Chicago, US
Virtual Borders: Accurate Definition of a Mobile Robot's Workspace Using Augmented Reality
We address the problem of interactively controlling the workspace of a mobile
robot to ensure a human-aware navigation. This is especially of relevance for
non-expert users living in human-robot shared spaces, e.g. home environments,
since they want to keep the control of their mobile robots, such as vacuum
cleaning or companion robots. Therefore, we introduce virtual borders that are
respected by a robot while performing its tasks. For this purpose, we employ a
RGB-D Google Tango tablet as human-robot interface in combination with an
augmented reality application to flexibly define virtual borders. We evaluated
our system with 15 non-expert users concerning accuracy, teaching time and
correctness and compared the results with other baseline methods based on
visual markers and a laser pointer. The experimental results show that our
method features an equally high accuracy while reducing the teaching time
significantly compared to the baseline methods. This holds for different border
lengths, shapes and variations in the teaching process. Finally, we
demonstrated the correctness of the approach, i.e. the mobile robot changes its
navigational behavior according to the user-defined virtual borders.Comment: Accepted on 2018 IEEE/RSJ International Conference on Intelligent
Robots and Systems (IROS), supplementary video: https://youtu.be/oQO8sQ0JBR
Consistent Searches for SMEFT Effects in Non-Resonant Dijet Events
We investigate the bounds which can be placed on generic new-physics
contributions to dijet production at the LHC using the framework of the
Standard Model Effective Field Theory, deriving the first consistently-treated
EFT bounds from non-resonant high-energy data. We recast an analysis searching
for quark compositeness, equivalent to treating the SM with one
higher-dimensional operator as a complete UV model. In order to reach
consistent, model-independent EFT conclusions, it is necessary to truncate the
EFT effects consistently at order and to include the possibility
of multiple operators simultaneously contributing to the observables, neither
of which has been done in previous searches of this nature. Furthermore, it is
important to give consistent error estimates for the theoretical predictions of
the signal model, particularly in the region of phase space where the probed
energy is approaching the cutoff scale of the EFT. There are two linear
combinations of operators which contribute to dijet production in the SMEFT
with distinct angular behavior; we identify those linear combinations and
determine the ability of LHC searches to constrain them simultaneously.
Consistently treating the EFT generically leads to weakened bounds on
new-physics parameters. These constraints will be a useful input to future
global analyses in the SMEFT framework, and the techniques used here to
consistently search for EFT effects are directly applicable to other
off-resonance signals.Comment: v1: 23 pages, 9 figures, 3 tables; v2: references added, typos
corrected, matches version published in JHE
This Far, No Further: Introducing Virtual Borders to Mobile Robots Using a Laser Pointer
We address the problem of controlling the workspace of a 3-DoF mobile robot.
In a human-robot shared space, robots should navigate in a human-acceptable way
according to the users' demands. For this purpose, we employ virtual borders,
that are non-physical borders, to allow a user the restriction of the robot's
workspace. To this end, we propose an interaction method based on a laser
pointer to intuitively define virtual borders. This interaction method uses a
previously developed framework based on robot guidance to change the robot's
navigational behavior. Furthermore, we extend this framework to increase the
flexibility by considering different types of virtual borders, i.e. polygons
and curves separating an area. We evaluated our method with 15 non-expert users
concerning correctness, accuracy and teaching time. The experimental results
revealed a high accuracy and linear teaching time with respect to the border
length while correctly incorporating the borders into the robot's navigational
map. Finally, our user study showed that non-expert users can employ our
interaction method.Comment: Accepted at 2019 Third IEEE International Conference on Robotic
Computing (IRC), supplementary video: https://youtu.be/lKsGp8xtyI
- …