arXiv subjects

Iain W. Stewart

Publications and source records attributed to Iain W. Stewart.

172 records · Page 10Linked to original sources

arXiv1999

Nucleon-Nucleon Effective Field Theory at NNLO: Radiation Pions and 1S0 Phase Shift

Low energy phenomena involving two nucleons can be successfully described using effective field theory. Because of the relatively large expansion parameter, it is only at next-to-next-to-leading order (NNLO) where one can expect to see agreement with experiment at the few percent level. The first part of this talk will focus on radiation pion effects, which first appear at NNLO. The power counting for radiation pions is simple for center of mass momentum p~\sqrt{M m_pi}=Q_r, the threshold for pion production. We explain how graphs calculated with the Q_r power counting scale for p~m_π. The Q_r^3 radiation pion contributions to nucleon-nucleon scattering are suppressed by inverse powers of the S-wave scattering lengths. However, we point out that order Q_r^4 radiation contributions might give a NNLO contribution for p~m_pi. In the second part of the talk, results for the potential pion and contact interaction part of the NNLO 1S0 phase shift are presented. We emphasize the importance of eliminating spurious poles in the expression for the amplitude at each order in the perturbative expansion. Doing this leaves a total of three free parameters at NNLO. We obtain a good fit to the 1S0 phase shift.

nucl-th↗

arXiv1999

Wigner symmetry in the limit of large scattering lengths

We note that in the limit where the NN 1S0 and 3S1 scattering lengths, a^(1S0) and a^(3S1), go to infinity, the leading terms in the effective field theory for strong NN interactions are invariant under Wigner's SU(4) spin-isospin symmetry. This explains why the leading effects of radiation pions on the S-wave NN scattering amplitudes vanish as a^(1S0) and a^(3S1) go to infinity. The implications of Wigner symmetry for NN to NN axion and gamma d to n p are also considered.

hep-ph↗

arXiv1999

Renormalization schemes and the range of two-nucleon effective field theory

The OS and PDS renormalization schemes for the effective field theory with nucleons and pions are investigated. We explain in detail how the renormalization is implemented using local counterterms. Fits to the NN scattering data are performed in the 1S0 and 3S1 channels for different values of mu_R. An error analysis indicates that the range of the theory with perturbative pions is consistent with 500 MeV.

nucl-th↗

arXiv1998

A momentum subtraction scheme for two--nucleon effective field theory

We introduce a momentum subtraction scheme which obeys the power counting of Kaplan, Savage, and Wise (KSW), developed for systems with large scattering lengths, $a$. Unlike the power divergence subtraction scheme, coupling constants in this scheme obey the KSW scaling for all $μ_R > 1/a$. We comment on the low-energy theorems derived by Cohen and Hansen. We conclude that there is no obstruction to using perturbative pions for momenta $p>m_π$.

nucl-th↗

arXiv1998

Extraction of the D^Dπcoupling from D^ decays

The decays D^* -> Dπand D^* -> Dγare well described by heavy meson chiral perturbation theory. With the recent measurement of Br(D^{*+} -> D^+ γ), the D^{*0}, D^{*+}, and D_s^* branching fractions can be used to extract the D^*Dπand D^*Dγcouplings $g$ and $β$. The D^* -> Dγdecays receive important corrections at order $\sqrt{m_q}$ and, from the heavy quark magnetic moment, at order $1/m_c$. Here all the decay rates are computed to one-loop, to first order in $m_q$ and $1/m_c$, including the effect of heavy meson mass splittings, and the counterterms at order $m_q$. A fit to the experimental data gives two possible solutions, g=0.27^{+.04}_{-.02}^{+.05}_{-.02}, β=0.85^{+.2}_{-.1}^{+.3}_{-.1} GeV^{-1} or g=0.76^{+.03}_{-.03}^{+.2}_{-.1}, β=4.90^{+.3}_{-.3}^{+5.0}_{-.7} GeV^{-1}. The first errors are experimental, while the second are estimates of the uncertainty induced by the counterterms. (The experimental limit Γ_{D^{*+}} < 0.13 MeV excludes the $g=0.76$ solution.) Predictions for the D^* and B^* widths are given.

hep-ph↗

arXiv1998

Semileptonic Lambda_b decay to excited Lambda_c baryons at order Lambda_{QCD}/m_Q

Exclusive semileptonic Lambda_b decays to excited charmed Lambda_c baryons are investigated at order Lambda_{QCD}/m_Q in the heavy quark effective theory. The differential decay rates are analyzed for the J^π=1/2^- Lambda_c(2593) and the J^π=3/2^- Λ_c(2625). They receive 1/m_{c,b} corrections at zero recoil that are determined by mass splittings and the leading order Isgur-Wise function. With some assumptions, we find that the branching fraction for Lambda_b decays to these states is 2.5-3.3%. The decay rate to the helicity \pm 3/2 states, which vanishes for m_Q \to \infty, remains small at order Lambda_{QCD}/m_Q since 1/m_c corrections do not contribute. Matrix elements of weak currents between a Lambda_b and other excited Lambda_c states are analyzed at zero-recoil to order Lambda_{QCD}/m_Q. Applications to baryonic heavy quark sum-rules are explored.

hep-ph↗

arXiv1998

Comment on $V_{ub}$ from Exclusive Semileptonic B and D Decays

The prospects for determining $|V_{ub}|$ from exclusive B semileptonic decay are discussed. The double ratio of form factors $(f^{B\toρ}/f^{B\to K^*})/ (f^{D\toρ}/f^{D\to K^*})$ is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from (B->rho l nu), (B->K* l l) and (D->rho l nu) can lead to a model independent determination of $|V_{ub}|$ with an uncertainty from theory of about 10%.

hep-ph↗

arXiv1997

Semileptonic B decays to excited charmed mesons

Exclusive semileptonic B decays into excited charmed mesons are investigated at order $Λ_{QCD}/m_Q$ in the heavy quark effective theory. Differential decay rates for each helicity state of the four lightest excited $D$ mesons ($D_1$, $D_2^*$, $D_0^*$, and $D_1^*$) are examined. At zero recoil, $Λ_{QCD}/m_Q$ corrections to the matrix elements of the weak currents can be written in terms of the leading Isgur-Wise functions for the corresponding transition and meson mass splittings. A model independent prediction is found for the slope parameter of the decay rate into helicity zero $D_1$ at zero recoil. The differential decay rates are predicted, including $Λ_{QCD}/m_Q$ corrections with some model dependence away from zero recoil and including order $α_s$ corrections. Ratios of various exclusive branching ratios are computed. Matrix elements of the weak currents between $B$ mesons and other excited charmed mesons are discussed at zero recoil to order $Λ_{QCD}/m_Q$. These amplitudes vanish at leading order, and can be written at order $Λ_{QCD}/m_Q$ in terms of local matrix elements. Applications to $B$ decay sum rules and factorization are presented.

hep-ph↗

arXiv1997

Model independent results for $B\to D_1(2420)\ell\barν$ and $B\to D_2^*(2460)\ell\barν$ at order $Λ_{QCD}/m_{c,b}$

Exclusive semileptonic B decays into $D_1$ and $D_2^*$ mesons are investigated including order $Λ_{QCD}/m_{c,b}$ corrections using the heavy quark effective theory. At zero recoil, the $Λ_{QCD}/m_{c,b}$ corrections can be written in terms of the leading Isgur-Wise function for these transitions, $τ$, and known meson mass splittings. We obtain an almost model independent prediction for the shape of the spectrum near zero recoil, including order $Λ_{QCD}/m_{c,b}$ corrections. We determine $τ(1)$ from the measured $B\to D_1\ell\barν$ branching ratio. Implications for B decay sum rules are discussed.

hep-ph↗

arXiv1996

Quantum solitons at strong coupling

We examine the effect of one loop quantum corrections on the formation of nontopological solitons in a strongly coupled scalar-fermionic Yukawa theory. The exact one fermion loop contribution is incorporated by using a nonlocal method to correct the local derivative expansion approximation (DE) of the effective action. As the Yukawa coupling is increased we find that the nonlocal corrections play an increasingly important role. The corrections cause the scalar field to increase in depth while maintaining its size. This increases the energy of the bag configuration, but this is compensated for by more tightly bound fermionic states with lower energy. In contrast to the semi-classical picture without quantum corrections, the binding energy is small, and the total energy scales directly with the Yukawa coupling. This confirms the qualitative behavior found in earlier work using the second order DE, although the quantitative solutions differ.

hep-ph↗