A
bstract
We present a complete description of top quark pair production in association with a hard photon in the dilepton channel. Our calculation is accurate to NLO in QCD. It is based on matrix ...elements for
e
+
ν
e
μ
−
ν
¯
μ
b
b
¯
γ
production and includes all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the
W
gauge bosons. This calculation constitutes the first full computation for top quark pair production with a final state photon in hadronic collisions at NLO in QCD. Numerical results for total and differential cross sections are presented for the LHC at a centre-of-mass energy of
s
=
13
TeV. For a few observables relevant for new physics searches, beyond some kinematic bounds, we observe shape distortions of more than 100%. In addition, we confirm that the size of the top quark off-shell effects for the total cross section is consistent with the expected uncertainties of the narrow width approximation. Results presented here are not only relevant for beyond the Standard Model physics searches but also important for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.
A
bstract
The hadroproduction of a
W
boson in association with a charm quark at the Large Hadron Collider is at the centre of current investigations due to its potential to probe the strangeness ...content of the proton. In this paper we present an implementation of the
W
+
c
production process in the PowHel event generator matched to the PYTHIA8 parton shower approach, allowing to obtain predictions for differential cross-sections with NLO QCD accuracy matched to the accuracy of the Shower Monte Carlo event generator. Effects of non-diagonal CKM matrix elements, finite charm quark mass and off-shell
W
decays including spin correlations are taken into account. We investigate the production of a leptonically decaying
W
boson in association with either a charmed meson (
W
±
+
D
∗∓
) or a charmed jet (
W
±
+
j
c
) and compare our predictions with particle-level measurements by the ATLAS and CMS collaborations at
s
= 7 and 13 TeV. The role of the so-called “opposite sign” and “same sign” contributions to the theoretical cross-sections is presented and discussed, pointing out the importance of including parton shower effects for a reliable estimate of the latter and a faithful comparison with experimental data.
A
bstract
We present a comparative study of various approaches for modelling of the
e
+
v
e
μ
−
v
¯
μ
b
b
¯
γ
final state in
t
t
¯
γ
production at the LHC. Working at the NLO in QCD we compare the ...fully realistic description of the top quark decay chain with the one provided by the narrow-width-approximation. The former approach comprises all double, single and non-resonant diagrams, interferences, and off-shell effects of the top quarks. The latter incorporates only double resonant contributions and restricts the unstable top quarks to on-shell states. We confirm that for the integrated cross sections the finite top quark width effects are small and of the order of
O
Γ
t
/
m
t
. We show, however, that they are strongly enhanced for more exclusive observables. In addition, we investigate fractions of events where the photon is radiated either in the production or in the decay stage. We find that large fraction of isolated photons comes from radiative decays of top quarks. Based on our findings, selection criteria might be developed to reduce such contributions, that constitute a background for the measurement of the anomalous couplings in the
t
t
¯
γ
vertex.
A
bstract
Many theories, from Supersymmetry to models of Strong Electroweak Symmetry Breaking, look at the production of four top quarks as an interesting channel to evidentiate signals of new ...physics beyond the Standard Model. The production of four-top final states requires large partonic energies, above the 4
m
t
threshold, that are available at the CERN Large Hadron Collider and will become more and more accessible with increasing energy and luminosity of the proton beams. A good theoretical control on the Standard Model background is a fundamental prerequisite for a correct interpretation of the possible signals of new physics that may arise in this channel. In this paper we report on the calculation of the next-to-leading order QCD corrections to the Standard Model process
. As it is customary for such studies, we present results for both integrated and differential cross sections. A judicious choice of a dynamical scale allows us to obtain nearly constant
in most distributions.
A
bstract
We extend the Helac-Dipoles package with the implementation of a new subtraction formalism, first introduced by Nagy and Soper in the formulation of an improved parton shower. We discuss a ...systematic, semi-numerical approach for the evaluation of the integrated subtraction terms for both massless and massive partons, which provides the missing ingredient for a complete implementation. In consequence, the new scheme can now be used as part of a complete NLO QCD calculation for processes with arbitrary parton masses and multiplicities. We assess its overall performance through a detailed comparison with results based on Catani-Seymour subtraction. The importance of random polarization and color sampling of the external partons is also examined.
A
bstract
We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, ...focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order
O
α
s
4
α
4
are taken into account, including irreducible backgrounds to
t
t
¯
j
production, interferences and off-shell effects of the top quark and the
W
gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the
t
t
¯
threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.
Triggered by ongoing dark matter searches in the top quark sector at the Large Hadron Collider we report on the calculation of the next-to-leading order QCD corrections to the Standard Model process ...pp → tt¯\ \overline{t} \ + Z (Z → νℓv¯\ \overline{v} \ℓ). This calculation is based on matrix elements for e+νeμ−ν¯μbb¯ντν¯τ\ {e}^{+}{\nu}_e{\mu}^{-}{\overline{\nu}}_{\mu }b\overline{b}\kern0.33em {\nu}_{\tau }{\overline{\nu}}_{\tau } \ production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Non-resonant and off-shell effects due to the finite W -boson width are also consistently taken into account. As it is common for such studies, we present results for both integrated and differential cross sections for a few renormalisation and factorisation scale choices and three different parton distribution functions. Already with the fairly inclusive cut selection and independently of the scale choice and the parton distribution function non-flat differential K\ \mathcal{K} \ -factors are obtained for pTmiss\ {p}_T^{miss} \, ∆ðoeTMℓℓ, ∆ð'´ℓℓ, cos θℓℓ, HT,HT′\ {H}_T^{\prime } \ observables that are relevant for new physics searches. Good theoretical control over the Standard Model background is a fundamental prerequisite for a correct interpretation of possible signals of new physics that may arise in this channel. Thus, these observables need to be carefully reexamined in the presence of more exclusive cuts before any realistic strategies for the detection of new physics signal can be further developed. Since from the experimental point of view both tt¯\ \overline{t} \ and tt¯\ \overline{t} \ + Z (Z → νℓv¯\ \overline{v} \ℓ) comprise the same final states, we additionally study the impact of the enlarged missing transverse momentum on various differential cross section distributions. To this end normalised differential distributions for pp →e+νeμ−ν¯μbb¯ντν¯τ\ {e}^{+}{\nu}_e{\mu}^{-}{\overline{\nu}}_{\mu }b\overline{b}\kern0.33em {\nu}_{\tau }{\overline{\nu}}_{\tau } \ and pp →e+νeμ−ν¯μbb¯\ {e}^{+}{\nu}_e{\mu}^{-}{\overline{\nu}}_{\mu }b\overline{b} \ are compared.
Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program that is capable of numerically evaluating QCD virtual corrections to scattering amplitudes. A detailed presentation of the ...algorithm is given, along with instructions to run the code and benchmark results. The program is part of the HELAC-NLO framework that allows for a complete evaluation of QCD NLO corrections.
Program title:HELAC-1LOOP
Catalogue identifier: AEOC_v1_0
Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOC_v1_0.html
Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland
Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html
No. of lines in distributed program, including test data, etc.: 290945
No. of bytes in distributed program, including test data, etc.: 3013326
Distribution format: tar.gz
Programming language: Fortran (gfortran(http://gcc.gnu.org/fortran/), lahey95 (http://www.lahey.com), ifort3(http://software.intel.com)).
Computer: Any.
Operating system: Linux, Unix, Mac OS.
Classification: 11.1.
Nature of problem:
The evaluation of virtual one-loop amplitudes for multi-particle scattering is a long-standing problem 1. In recent years the OPP reduction technique 2 opened the road for a fully numerical approach based on the evaluation of the one-loop amplitude for well-defined values of the loop momentum.
Solution method:
By using HELAC 3–5 and CutTools 6, HELAC-1LOOP is capable of evaluating QCD virtual corrections 7. The one-loop n-particle amplitudes are constructed as part of the n+2 tree-order ones, by using the basic recursive algorithm used in HELAC. A Les Houches Event (LHE) file is produced, combining the complete information from tree-order and virtual one-loop contributions. In conjunction with real corrections, obtained with the use of HELAC-DIPOLES 8, the full NLO corrections can be computed. The program has been successfully used in many applications.
Running time:
Depending on the number of particles and generated events from seconds to days.
References:
1R.K. Ellis, Z. Kunszt, K. Melnikov and G. Zanderighi, arXiv:1105.4319hepph.2G. Ossola, C. G. Papadopoulos and R. Pittau, Nucl. Phys. B 763 (2007) 147 arXiv:hep-ph/0609007.3A. Kanaki and C. G. Papadopoulos, Comput. Phys. Commun. 132 (2000) 306 arXiv:hep-ph/0002082.4C. G. Papadopoulos, Comput. Phys. Commun. 137 (2001) 247 arXiv:hepph/ 0007335.5A. Cafarella, C. G. Papadopoulos and M. Worek, Comput. Phys. Commun. 180 (2009) 1941 arXiv:0710.2427 hep-ph.6G. Ossola, C. G. Papadopoulos and R. Pittau, JHEP 0803 (2008) 042 arXiv:0711.3596 hep-ph.7A. van Hameren, C. G. Papadopoulos and R. Pittau, JHEP 0909, 106 (2009) arXiv:0903.4665 hep-ph.8M. Czakon, C. G. Papadopoulos and M. Worek, JHEP 0908, 085 (2009) arXiv:0905.0883 hep-ph.
Women increasingly work beyond age 50+ but their occupational health is under-researched.
To investigate what jobs older contemporary women do, when they exit their jobs and what factors predict job ...exit.
Data came from the Health and Employment After Fifty cohort, which recruited women aged 50-64 at baseline in 2013-14 and has followed them up annually collecting: demographic, lifestyle and work information. Exits from employment were mapped longitudinally over five follow-ups. Time-to-first event Cox regression analyses were used to identify risk factors for job exit.
At baseline, 4436 women participated, 64% of whom were working. The proportions of women working at 50-54, 55-60 and over 60 years were 86%, 79% and 38%, respectively. Amongst all women, after adjustment for age, managing comfortably financially and not coping with the mental demands of the job were associated with exit. Risk factors for job exit differed in the age bands: 50-54; 55-59 and >60 years, reflecting socio-economic status, markers of health (musculoskeletal pain and poor self-rated health) and work factors (under-appreciation, job dissatisfaction, temporary/permanent contracts, coping with work's physical demands).
Factors contributing to exit from work among older women differ by age group, after controlling for perceived financial position, age and mental demands of the job. A number of work characteristics predict job exit and suggest that employers can play an important role in supporting women to continue working until older ages. Identification and treatment of musculoskeletal pain could also enable work amongst older women.
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