1,722,670 research outputs found
LHCb searches for the strong CP-violating decays η → ππ and η′ → ππ
No full textA search for the CP-violating strong decays η → π(+)π(−) and η′(958) → π(+)π(−) has been performed using approximately 2.5 × 10(7) events of each of the decays D(+) → π(+)π(+)π(−) and , recorded by the LHCb experiment. The data set corresponds to an integrated luminosity of 3.0 fb(−)(1) of pp collision data recorded during LHC Run 1 and 0.3 fb(−)(1) recorded in Run 2. No evidence is seen for with η (()(′)()) → π(+)π(−) and upper limits at 90% confidence level are set on the branching fractions, and . The limit for the η decay is comparable with the existing one, while that for the η′ is a factor of three smaller than the previous limit
Spontaneous plaquette dimerization in the J(1)-J(2) Heisenberg model
No full textWe investigate the nonmagnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected "plaquette resonating valence bond," with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from our numerical simulations as the most plausible ground state for J(2)/J(1) similar or equal to 0.5
Green function Monte Carlo with stochastic reconfiguration: An effective remedy for the sign problem
No full textA recent technique, proposed to alleviate the "sign problem disease," is discussed in detail. As is well known, the ground state of a given Hamiltonian H can be obtained by applying the propagator e(-H tau) to a trial wave function psi(T) and sampling statistically the state psi(tau)=e(-H tau)psi(T) for large imaginary time tau. However, the sign problem may appear in the simulation and such statistical propagation would be practically impossible without employing some approximation such as the "fixed node" (FN) one. The present method allows the improvement of the FN dynamics with a systematic correction scheme. This is possible by the simple requirement that, after a short imaginary time propagation via the FN Hamiltonian, a number p of correlation functions can be further constrained to be exact by small perturbations of the FN state, which is free from the sign problem. By iterating this procedure, the Monte Carlo average sign, which is almost zero when there is a sign problem, remains stable and finite even for large tau. The proposed algorithm is tested against exact diagonalization data available on finite lattices. It is also shown, in some test cases, that the dependence of the results upon the few parameters entering the stochastic technique can be very easily controlled, unless for exceptional cases
Algorithmic differentiation and the calculation of forces by quantum Monte Carlo
Full text linkWe describe an efficient algorithm to compute forces in quantum Monte Carlo using adjoint algorithmic differentiation. This allows us to apply the space warp coordinate transformation in differential form, and compute all the 3M force components of a system with M atoms with a computational effort comparable with the one to obtain the total energy. Few examples illustrating the method for an electronic system containing several water molecules are presented. With the present technique, the calculation of finite-temperature thermodynamic properties of materials with quantum Monte Carlo will be feasible in the near future
Finite-size spin-wave theory of the triangular Heisenberg model
No full textWe present a finite-size spin-wave calculation on the Heisenberg antiferromagnet on the triangular lattice focusing in particular on the low-energy part of the excitation spectrum. For s=1/2 the good agreement with the exact diagonalization and quantum Monte Carlo results supports the reliability of the spin-wave expansion to describe the low-energy spin excitations of the Heisenberg model even in presence of frustration. This indicates that the spin susceptibility of the triangular antiferromagnet is very close to the linear spin-wave result
Long-range Neel order in the triangular Heisenberg model
No full textWe have studied the Heisenberg model on the triangular lattice using quantum Monte Carlo techniques (up to 144 sites) and exact diagonalization (up to 36 sites). By studying the spin gap as a function of the system size we have obtained robust evidence for a gapless spectrum, confirming the existence of long-range Neel order. Our best estimate is that in the thermodynamic limit the order parameter m dagger = 0.41 +/- 0.02 is reduced by about 59% from its classical value and the ground state energy per site is e(o) = -0.5458 +/- 0.0001 in units of the exchange coupling. We have identified the ground state correlations that are important at shea distances
Chiral spin liquid wave function and the Lieb-Schultz-Mattis theorem
No full textWe study a chiral spin liquid wave function defined as a Gutzwiller projected BCS state with a complex pairing function. After projection, spontaneous dimerization is found for any odd but finite number of chains, thus satisfying the Lieb-Schultz-Mattis theorem, whereas for an even number of chains there is no dimerization. The two-dimensional thermodynamic limit is consistently reached for a large number of chains since the dimer order parameter vanishes in this limit. This property clearly supports the possibility of a spin liquid ground state in two dimensions with a gap to all physical excitations and with no broken translation symmetry
Quantum phase transition in coupled spin ladders
No full textThe ground state of an array of coupled, spin-1/2 antiferromagnetic ladders is studied using spin-wave theory, exact diagonalization (up to 36 sites), and quantum Monte Carlo techniques (up to 256 sites). Our results clearly indicate the occurrence of a zero-temperature phase transition between a Neel ordered and a nonmagnetic phase at a finite value of the interladder coupling (a(c)similar or equal to0.3). This transition is marked by remarkable changes in the structure of the excitation spectrum
Flavour anomalies in rare decays at LHCb
No full textRare decays are powerful probes for Physics beyond the Standard Model (SM), as new particles can have a large impact on physics observables. Recent results on lepton universality tests and measurements of branching fractions and angular distributions of rare decays have shown tensions with the SM predictions. The LHCb experiment is ideally suited for the study of the these flavour anomalies, due to its large acceptance, precise vertexing and powerful particle identification capabilities. The latest results from LHCb on the flavour anomalies will be presented and their interpretation will be discussed
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