General operator form of the non-local three-nucleon force

This paper describes a procedure to obtain the general form of the three-nucleon force. The result is an operator form where the momentum space matrix element of the three-nucleon potential is written as a linear combination of 320 isospin-spin-momentum operators and scalar functions of momenta. Any spatial and isospin rotation invariant three-nucleon force can be written in this way and in order for the potential to be Hermitian, symmetric under parity inversion, time reversal and particle exchange, the scalar functions must have definite transformation properties under these discrete operations. A complete list of the isospin-spin-momentum operators and scalar function transformation properties is given

Few-nucleon reactions in three dimensional formalism

In this contribution, we present few-nucleon calculations performed in a three-dimensional framework. References are given to our treatment of two- and three-nucleon bound states as well as for the transition operator in the positive energy range. New results for the transition operator in the negative energy range are shown. Different features of the standard partial wave and three dimensional calculations are presented

Calculations of three-nucleon reactions with N3LO chiral forces: achievements and challenges

We discuss the application of the chiral N3LO forces to three-nucleon reactions and point to the challenges which will have to be addressed. Present approaches to solve three-nucleon Faddeev equations are based on a partial-wave decomposition. A rapid increase of the number of terms contributing to the chiral three-nucleon force when increasing the order of the chiral expansion from N2LO to N3LO forced us to develop a fast and effective method of automatized partial wave decomposition. At low energies of the incoming nucleon below about 20MeV, where only a limited number of partial waves is required, this method allowed us to perform calculations of reactions in the three-nucleon continuum using N3LO two- and three-nucleon forces. It turns out that inclusion of consistent chiral interactions, with relativistic 1/m corrections and short-range 2pi-contact term omitted in the N3LO three-nucleon force, does not explain the long standing low energy Ay-puzzle. We discuss problems arising when chiral forces are applied at higher energies, where large three-nucleon force effects are expected. It seems plausible that at higher energies, due to a rapid increase of a number of partial waves required to reach convergent results, a three-dimensional formulation of the Faddeev equations which avoids partial-wave decomposition is desirable.Comment: 31 pages, 13 figure

Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at s\sqrt{s} = 13 TeV with the ATLAS experiment

A measurement of the polarisation of W bosons produced in top-quark decays is presented, using proton–proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb$^{-1}$. The measurement is performed selecting $t\bar{t}$ events decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the $cos\theta$* variable, where $cos\theta$* is the angle between the momentum direction of the charged lepton from the W boson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the W boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the $cos\theta$* angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f$_{0}$ = 0.684 $\pm$ 0.005 (stat. ) $\pm$ 0.014 (syst. ), f$_{L}$ = 0.318 $\pm$ 0.003 (stat. ) $\pm$ 0.008 (syst. ) and f$_{R}$ = - 0.002 $\pm$ 0.002 (stat. ) $\pm$ 0.014 (syst. ), in agreement with the Standard Model prediction

Calibration of the light-flavour jet mistagging efficiency of the b-tagging algorithms with Z+jets events using 139 fb−1^{-1} of ATLAS proton–proton collision data at s\sqrt{s} = 13 TeV

The identification of b-jets, referred to as b-tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data sample of proton–proton collision events at $\sqrt{s}$ = 13 TeV corresponding to an integrated luminosity of 139 fb$^{-1}$. The calibration is performed in a sample of Z bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the b-tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract a scale factor from data, to correct the light-flavour jet mistagging efficiency in Monte Carlo simulations, while simultaneously correcting the b-jet efficiency. With this procedure, uncertainties coming from the modeling of jets from heavy-flavour hadrons are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties

Search for dark photons from Higgs boson decays via ZH production with a photon plus missing transverse momentum signature from pp collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

This paper describes a search for dark photons (γd) in proton-proton collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider (LHC). The dark photons are searched for in the decay of Higgs bosons (H → γγ$_{d}$) produced through the ZH production mode. The transverse mass of the system, made of the photon and the missing transverse momentum from the non-interacting γd, presents a distinctive signature as it peaks near the Higgs boson mass. The results presented use the total Run-2 integrated luminosity of 139 fb$^{−1}$ recorded by the ATLAS detector at the LHC. The dominant reducible background processes are estimated using data-driven techniques. A Boosted Decision Tree technique is adopted to enhance the sensitivity of the search. As no excess is observed with respect to the Standard Model prediction, an observed (expected) upper limit on the branching ratio BR(H →γγ$_{d}$) of 2.28% (2.82$^{+1.33}_{-0.84}$%) is set at 95% CL for massless γ$_{d}$. For massive dark photons up to 40 GeV, the observed (expected) upper limits on BR(H → γγ$_{d}$) at 95% confidence level is found within the [2.19,2.52]% ([2.71,3.11]%) range

Orthogonal polynomial approach to calculate the two-nucleon transition operator in three dimensions

We give a short report on the possibility to use orthogonal polynomials (OP) in calculations that involve the two-nucleon (2N) transition operator. The presented work adds another approach to the set of previously developed methods (described in Phys. Rev. C 81, 034006 (2010); Few-Body Syst. 53, 237 (2012); K. Topolnicki, PhD thesis, Jagiellonian University (2014)) and is applied to the transition operator calculated at laboratory kinetic energy 300MeV. The new results for neutron-neutron and neutron-proton scattering observables converge to the results presented in Few-Body Syst. 53, 237 (2012) and to results obtained using the Arnoldi algorithm (Y. Saad, Iterative methods for sparse linear systems (SIAM Philadelphia, PA, USA 2003)). The numerical cost of the calculations performed using the new scheme is large and the new method can serve only as a backup to cross-check the previously used calculation schemes

Achievements and challenges in understanding nucleon-deuteron reactions

Results on three-nucleon (3N) elastic scattering below the pion production threshold are discussed with an emphasis on a need for a three-nucleon force (3NF). The large discrepancies found between a theory based on numerical solutions of 3N Faddeev equations with (semi) phenomenological nucleon-nucleon (NN) potentials only and data point to the need for 3NF’s. This notion is supported by the fact that another possible reason for the discrepancies in elastic nucleondeuteron (Nd) scattering, relativistic effects, turned out to be small. Results based on a new generation of chiral NN forces (up to N4LO) alone or combined with N2LO 3NF support predictions found with standard interactions. To resolve higher energy discrepancies found in nucleon-deuteron (Nd) reactions requires application of a chiral 3NF up to at least N3LO order of chiral expansion

The general operator form for the total-momentum-dependent nucleon-nucleon potential

In this paper we describe a procedure to obtain the general operator form of two-nucleon (2N) potentials and apply it to the case of the 2N potential that has an additional dependence on the total momentum of the system. This violates Galilean invariance but terms including the total momentum appear in some relativistic approaches. In operator form, the potential is expressed as a linear combination of a fixed number of known spin-momentum operators and scalar functions of momenta. Since the scalar functions effectively define the potentials, using the operator form significantly reduces the number of parameters that are needed in numerical implementations. The proposed operator form explicitly obeys the usual symmetries of rotational invariance, particle exchange, time reflection and parity