Simulation of many-qubit quantum computation with matrix product states

Matrix product states provide a natural entanglement basis to represent a quantum register and operate quantum gates on it. This scheme can be materialized to simulate a quantum adiabatic algorithm solving hard instances of a NP-Complete problem. Errors inherent to truncations of the exact action of interacting gates are controlled by the size of the matrices in the representation. The property of finding the right solution for an instance and the expected value of the energy are found to be remarkably robust against these errors. As a symbolic example, we simulate the algorithm solving a 100-qubit hard instance, that is, finding the correct product state out of ~ 10^30 possibilities. Accumulated statistics for up to 60 qubits point at a slow growth of the average minimum time to solve hard instances with highly-truncated simulations of adiabatic quantum evolution.Comment: 5 pages, 4 figures, final versio

Higgs couplings in the MSSM at large tan(beta)

We consider tan(beta)-enhanced quantum effects in the minimal supersymmetric standard model (MSSM) including those from the Higgs sector. To this end, we match the MSSM to an effective two-Higgs doublet model (2HDM), assuming that all SUSY particles are heavy, and calculate the coefficients of the operators that vanish or are suppressed in the MSSM at tree-level. Our result clarifies the dependence of the large-tan(beta) resummation on the renormalization convention for tan(beta), and provides analytic expressions for the Yukawa and trilinear Higgs interactions. The numerical effect is analyzed by means of a parameter scan, and we find that the Higgs-sector effects, where present, are typically larger than those from the "wrong-Higgs" Yukawa couplings in the 2HDM.Comment: 27 pages, LaTe

Improving the Hadronization of QCD currents in TAUOLA and PHOKHARA

We present our study of the hadronization structure of both vector and axial-vector currents leading to decays of the tau into two kaons and a pion. The cornerstones of our framework are the large-N_C limit of QCD, the chiral structure exhibited at low energies and the proper asymptotic behaviour, ruled by QCD, that is demanded to the associated form factors. The couplings of the theory are mostly constrained by this procedure and upon the analysis of BaBar data on e^+e^- -> KK pi we are able to predict the hadronic spectra.Comment: 4 pages, 1 figure. To appear in the Proceedings of QCD 08: 14th International QCD Conference, 7-12th July 2008 Montpellier (France). To be published by Elsevier in Nuclear Physics B (Proceedings Supplement

Implementing invariant mass cuts and finite lifetime effects in top-antitop production at threshold

The effects of the finite top quark width in the top pair production cross section close to the threshold are discussed in this talk. We introduce a $t\bar{t}$ cross section with a cut on the invariant masses of the top and antitop that can be calculated theoretically with effective field theory (EFT) methods. The matching procedure to implement the physical phase-space boundaries in the NRQCD framework (``phase-space matching'') is briefly outlined.Comment: 4 pages, 4 figures. Talk given at the 14th International QCD Conference (Montpellier 7-12th July 2008

NNLO tau+tau- production cross section at threshold

The threshold behaviour of the cross section sigma(e+e- -> tau+tau-) is analysed, taking into account the known higher-order corrections. At present, this observable can be determined to next-to-next-to-leading order (NNLO) in a combined expansion in powers of alpha_s and fermion velocities.Comment: 4 pages, 3 figures. To appear in the proceedings of QCD 02: High-Energy Physics International Conference in Quantum Chromodynamics, Montpellier, France, 2-9 Jul 200

The omega rho pi coupling in the VMD model revisited

We determine the value of the \omega -\rho- \pi mesons coupling (g_{\omega\rho\pi}), in the context of the vector meson dominance model, from radiative decays, the \omega \rightarrow 3\pi decay width and the e^+e^- \rightarrow 3\pi cross section. For the last two observables we consider the effect of either a heavier resonance (\rho'(1450)) or a contact term. A weighted average of the results from the set of observables yields g_{\omega\rho\pi} =14.7 \pm 0.1 GeV^{-1} in absence of those contributions, and g_{\omega\rho\pi} =11.9 \pm 0.2 GeV^{-1} or g_{\omega\rho\pi} =11.7 \pm 0.1 GeV^{-1} when including the \rho' or contact term respectively. The inclusion of these additional terms makes the estimates from the different observables to lay in a more reduced range. Improved measurements of these observables and the \rho'(1450) meson parameters are needed to give a definite answer on the pertinence of the inclusion of this last one in the considered processes.Comment: 14 pages, 5 figures. Extended analysis including SND and CMD2 data. References added. Matches published versio

Electroweak non-resonant NLO corrections to e+ e- -> W+ W- b bbar in the t tbar resonance region

We analyse subleading electroweak effects in the top anti-top resonance production region in e+ e- collisions which arise due to the decay of the top and anti-top quarks into the W+ W- b bbar final state. These are NLO corrections adopting the non-relativistic power counting v ~ alpha_s ~ sqrt(alpha_EW). In contrast to the QCD corrections which have been calculated (almost) up to NNNLO, the parametrically larger NLO electroweak contributions have not been completely known so far, but are mandatory for the required accuracy at a future linear collider. The missing parts of these NLO contributions arise from matching coefficients of non-resonant production-decay operators in unstable-particle effective theory which correspond to off-shell top production and decay and other non-resonant irreducible background processes to t tbar production. We consider the total cross section of the e+ e- -> W+ W- b bbar process and additionally implement cuts on the invariant masses of the W+ b and W- bbar pairs.Comment: LaTeX, 33 pages, 6 figure

Form Factors in the radiative pion decay

We perform an analysis of the form factors that rule the structure-dependent amplitude in the radiative pion decay. The resonance contributions to pion -> e nu_e gamma decays are computed through the proper construction of the vector and axial-vector form factors by setting the QCD driven asymptotic properties of the three-point Green functions VVP and VAP, and by demanding the smoothing of the form factors at high transfer of momentum. A comparison between theoretical and experimental determinations of the form factors is also carried out. We also consider and evaluate the role played by a non-standard tensor form factor. We conclude that, at present and due to the hadronic incertitudes, the search for New Physics in this process is not feasible.Comment: 14 pages, no figures. Typos corrected. Accepted for publication in The European Physical Journal