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A. Soni

Publications and source records attributed to A. Soni.

At least 55 records · Page 3Linked to original sources
arXiv2006

Semi-inclusive hadronic B decays as null tests of the Standard Model

We propose a new set of observables that can be used as experimental null tests of the Standard Model in charged and neutral B decays. The CP asymmetries in hadronic decays of charged B mesons into inclusive final states containing at least one of the following mesons: K_{S,L}, eta', c\bar c bound states or neutral K^* or D mesons, for all of which a U-spin rotation is equivalent to a CP conjugation, are CKM suppressed and furthermore vanish in the exact U-spin limit. We show how this reduces the theoretical error by using Soft Collinear Effective Theory to calculate the CP asymmetries for K_{S,L} X_{s+d}, K^* X_{s+d} and eta' X_{s+d} final states in the endpoint region. For these CP asymmetries only the flavor and not the charge of the decaying B meson needs to be tagged up to corrections of NLO in 1/m_b, making the measurements more accessible experimentally.

hep-ph↗
arXiv2006

The Kaon B-parameter from Quenched Domain-Wall QCD

We present numerical results for the kaon B-parameter, B_K, determined in the quenched approximation of lattice QCD. Our simulations are performed using domain-wall fermions and the renormalization group improved, DBW2 gauge action which combine to give quarks with good chiral symmetry at finite lattice spacing. Operators are renormalized non-perturbatively using the RI/MOM scheme. We study scaling by performing the simulation on two different lattices with a^{-1} = 1.982(30) and 2.914(54) GeV. We combine this quenched scaling study with an earlier calculation of B_K using two flavors of dynamical, domain-wall quarks at a single lattice spacing to obtain B_K(MS,NDR,mu=2GeV)=0.563(21)(39)(30), were the first error is statistical, the second systematic (without quenching errors) and the third estimates the error due to quenching.

hep-lat↗
arXiv2006

Systematic effects of the quenched approximation on the strong penguin contribution to epsilon'/epsilon

We discuss the implementation and properties of the quenched approximation in the calculation of the left-right, strong penguin contributions (i.e. Q_6) to epsilon'/epsilon. The coefficient of the new chiral logarithm, discovered by Golterman and Pallante, which appears at leading order in quenched chiral perturbation theory is evaluated using both the method proposed by those authors and by an improved approach which is free of power divergent corrections. The result implies a large quenching artifact in the contribution of Q_6 to epsilon'/epsilon. This failure of the quenched approximation affects only the strong penguin operators and so does not affect the Q_8 contribution to epsilon'/epsilon nor Re A_0, Re A_2 and thus the Delta I=1/2 rule at tree level in chiral perturbation theory.

hep-lat↗
arXiv2005

Calculation of the neutron electric dipole moment with two dynamical flavors of domain wall fermions

We present a study of the neutron electric dipole moment ($\vec d_N$) within the framework of lattice QCD with two flavors of dynamical lig ht quarks. The dipole moment is sensitive to the topological structure of the gaug e fields, and accuracy can only be achieved by using dynamical, or sea quark, calc ulations. However, the topological charge evolves slowly in these calculations, le ading to a relatively large uncertainty in $\vec d_N$. It is shown, using quenched configurations, that a better sampling of the charge d istribution reduces this problem, but because the CP even part of the fermion determinant is absent, both the topological charge dis tribution and $\vec d_N$ are pathological in the chiral limit. We discuss the statistical and systematic uncertainties arising from the topological charge distr ibution and unphysical size of the quark mass in our calculations and prospects fo r eliminating them. Our calculations employ the RBC collaboration two flavor domain wall fermion and DBW2 gauge action lattices with inverse lattice spacing $a^{-1}\approx$ 1.7 GeV, physical volume $V\approx (2$ fm)$^3$, and light quark mass roughly equal to the strange quark mass ($m_{sea}=0.03 $ and 0.04). We determine a value of the electric dipole moment that is zero withi n (statistical) errors, $|\vec d_N| = -0.04(20)$ e-$θ$-fm at the smaller sea quark mass. Satisfactory results for the magnetic and electric form factors of the proton and neutron are also obtained and presented.

hep-lat↗
arXiv2005

K_l3 form factor with two-flavors of dynamical domain-wall quarks

We report on our calculation of K \to πvector form factor by numerical simulations of two-flavor QCD on a 16^3x32x12 lattice at a \simeq 0.12 fm using domain-wall quarks and DBW2 glue. Our preliminary result at a single sea quark mass correponding to m_PS/m_V \simeq 0.53 shows a good agreement with previous estimate in quenched QCD and that from a phenomenological model.

hep-lat↗
arXiv2005

The Discovery Potential of a Super B Factory

The Proceedings of the 2003 SLAC Workshops on flavor physics with a high luminosity asymmetric e+e- collider. The sensitivity of flavor physics to physics beyond the Standard Model is addressed in detail, in the context of the improvement of experimental measurements and theoretical calculations.

hep-ph↗
arXiv2004

Lattice QCD with two dynamical flavors of domain wall quarks

We present results from the first large-scale study of two flavor QCD using domain wall fermions (DWF), a chirally symmetric fermion formulation which has proven to be very effective in the quenched approximation. We work on lattices of size 16^3x32, with a lattice cut-off of a^{-1}\approx 1.7 GeV, and dynamical (or sea) quark masses in the range m_{strange}/2 \simle m_{sea} \simle m_{strange}. After discussing the algorithmic and implementation issues involved in simulating dynamical DWF, we report on the low-lying hadron spectrum, decay constants, static quark potential, and the important kaon weak matrix element describing indirect CP violation in the Standard Model, B_K. In the latter case we include the effect of non-degenerate quark masses (m_s \neq m_u = m_d), finding B_K(MS-bar, 2 GeV) = 0.495(18).

hep-lat↗
arXiv2004

Neutron Electric Dipole Moment with Domain Wall Quarks

We present preliminary results for nucleon dipole moments computed with domain wall fermions. Our main target is the electric dipole moment of the neutron arising from the theta term in the gauge part of the QCD lagrangian. The calculated magnetic dipole moments of the proton and neutron are in rough accord with experimental values.

hep-lat↗
arXiv2004

Scalar meson in dynamical and partially quenched two-flavor QCD: lattice results and chiral loops

This is an exploratory study of the lightest non-singlet scalar $q\bar q$ state on the lattice with two dynamical quarks. Domain Wall fermions are used for both sea and valence quarks on a 16^3*32 lattice with an inverse lattice spacing of 1.7 GeV. We extract the scalar meson mass 1.58(34) GeV from the exponential time-dependence of the dynamical correlators with $m_{val}=m_{sea}$ and N_f=2. Since this statistical error-bar from dynamical correlators is rather large, we analyze also the partially quenched lattice correlators with $m_{val}$ not equal $m_{sea}$. They are positive for $m_{val}>=m_{sea}$ and negative for $m_{val} =m_{sea}$ and negative for $m_{val}<m_{sea}$ at large t. After the subtraction of this unphysical contribution from the partially quenched lattice correlators, the correlators are positive and exponentially falling. The resulting scalar meson mass 1.51(19) GeV from the partially quenched correlators is consistent with the dynamical result and has appreciably smaller error-bar.

hep-lat↗
arXiv2002

Domain wall fermions with improved gauge actions

We study the chiral properties of quenched domain wall fermions with several gauge actions. We demonstrate that the residual chiral symmetry breaking, which is present for a finite number of lattice sites in the fifth dimension ($L_s$), can be substantially suppressed using improved gauge actions. In particular the Symanzik action, the Iwasaki action, and a renormalization group improved gauge action, called doubly blocked Wilson (DBW2), are studied and compared to the Wilson action. All improved gauge actions studied show a reduction in the additive residual quark mass, $\mres$. Remarkably, in the DBW2 case $\mres$ is roughly two orders of magnitude smaller than the Wilson gauge action at $a^{-1}=2$ GeV and $L_s=16$. Significant reduction in $\mres$ is also realized at stronger gauge coupling corresponding to $a^{-1}=1.3$ GeV. As our numerical investigation indicates, this reduction is achieved by reducing the number of topological lattice dislocations present in the gauge field configurations. We also present detailed results for the quenched light hadron spectrum and the pion decay constant using the DBW2 gauge action.

hep-lat↗
arXiv2002

Kaon Matrix Elements and CP-violation from Quenched Lattice QCD: (I) the 3-flavor case

We report the results of a calculation of the K --> pi pi matrix elements relevant for the $\DIhalf$ rule and $\epe$ in quenched lattice QCD using domain wall fermions at a fixed lattice spacing $a^{-1} \sim 2$ GeV. Working in the three-quark effective theory, where only the u, d and s quarks enter and which is known perturbatively to next-to-leading order, we calculate the lattice K --> pi and K --> |0> matrix elements of dimension six, four-fermion operators. Through lowest order chiral perturbation theory these yield K --> pi pi matrix elements, which we then normalize to continuum values through a non-perturbative renormalization technique. For the ratio of isospin amplitudes |A_0|/|A_2| we find a value of $25.3 \pm 1.8$ (statistical error only) compared to the experimental value of 22.2, with individual isospin amplitudes 10-20% below the experimental values. For $\epe$, using known central values for standard model parameters, we calculate $(-4.0 \pm 2.3) \times 10^{-4}$ (statistical error only) compared to the current experimental average of $(17.2 \pm 1.8) \times 10^{-4}$. Because we find a large cancellation between the I = 0 and I = 2 contributions to $\epe$, the result may be very sensitive to the approximations employed. Among these are the use of: quenched QCD, lowest order chiral perturbation theory and continuum perturbation theory below 1.3 GeV. We have also calculated the kaon B parameter, B_K and find $B_{K,\bar{MS}}(2 {\rm GeV}) = 0.532(11)$. Although currently unable to give a reliable systematic error, we have control over statistical errors and more simulations will yield information about the effects of the approximations on this first-principles determination of these important quantities.

hep-lat↗
arXiv2001

Non-perturbative Renormalisation of Domain Wall Fermions: Quark Bilinears

We find the renormalisation coefficients of the quark field and the flavour non-singlet fermion bilinear operators for the domain wall fermion action, in the regularisation independent (RI) renormalisation scheme. Our results are from a quenched simulation, on a 16^3x32 lattice, with beta=6.0 and an extent in the fifth dimension of 16. We also discuss the expected effects of the residual chiral symmetry breaking inherent in a domain wall fermion simulation with a finite fifth dimension, and study the evidence for both explicit and spontaneous chiral symmetry breaking effects in our numerical results. We find that the relations between different renormalisation factors predicted by chiral symmetry are, to a good approximation, satisfied by our results and that systematic effects due to the (low energy) spontaneous chiral symmetry breaking and zero-modes can be controlled. Our results are compared against the perturbative predictions for both their absolute value and renormalisation scale dependence.

hep-lat↗
arXiv2000

Pathways to Rare Baryonic B Decays

We point out new ways to search for charmless baryonic B decays: baryon pair production in association with $η^\prime$ is very likely as large as or even a bit larger than two body $Kπ/ππ$ modes. We extend our argument, in weaker form, to $B\to γ+ X_s$ and $\ellν+X$. Although calculations are not reliable, estimates give branching ratios of order $10^{-5}$--$10^{-6}$, where confidence is gained from recent experimental finding that $B \to D^{*} p \bar n$, $D^{*} p \bar p π$ are not far below $D^*π$ and $D^*ρ$ rates. Observation of charmless baryon modes would help clarify the dynamics of weak decays to baryonic final states, while the self-analyzing prowess of the $Λ$-baryon can be helpful in CP- and T-violation studies.

hep-ph↗
arXiv2000

Quenched Lattice QCD with Domain Wall Fermions and the Chiral Limit

Quenched QCD simulations on three volumes, $8^3 \times$, $12^3 \times$ and $16^3 \times 32$ and three couplings, $β=5.7$, 5.85 and 6.0 using domain wall fermions provide a consistent picture of quenched QCD. We demonstrate that the small induced effects of chiral symmetry breaking inherent in this formulation can be described by a residual mass ($\mres$) whose size decreases as the separation between the domain walls ($L_s$) is increased. However, at stronger couplings much larger values of $L_s$ are required to achieve a given physical value of $\mres$. For $β=6.0$ and $L_s=16$, we find $\mres/m_s=0.033(3)$, while for $β=5.7$, and $L_s=48$, $\mres/m_s=0.074(5)$, where $m_s$ is the strange quark mass. These values are significantly smaller than those obtained from a more naive determination in our earlier studies. Important effects of topological near zero modes which should afflict an accurate quenched calculation are easily visible in both the chiral condensate and the pion propagator. These effects can be controlled by working at an appropriately large volume. A non-linear behavior of $m_π^2$ in the limit of small quark mass suggests the presence of additional infrared subtlety in the quenched approximation. Good scaling is seen both in masses and in $f_π$ over our entire range, with inverse lattice spacing varying between 1 and 2 GeV.

hep-lat↗
arXiv1998

R-parity violation, sneutrino mixing phenomena and CP-violation in p \bar p -> sneutrino -> l^+ l^- + X

The sneutrino resonance reactions p \bar p -> sneutrino -> l^+ l^- +X, for l=e,μ,τ, in the MSSM without R-parity are considered. We perform a cross-section analysis and show that present limits on some products of R-parity violating couplings in the sneutrino sector can be significantly improved in future upgraded Fermilab Tevatron runs. Furthermore, we introduce CP-violating and CP-conserving τ-spin asymmetries which are generated already at the TREE-LEVEL in the reaction p \bar p -> sneutrino -> τ^+ τ^- +X if there is muon-sneutrino - \bar muon-sneutrino mixing and that vanish in the SM. We find, for example, that for muon-sneutrino masses in the range 150 GeV < m_{muon-sneutrino} < 450 GeV, these spin asymmetries reach ~ 20 - 30% and ~ 10% for mass splitting between the muon-sneutrino CP-odd and CP-even states at the level of Δm ~ Γand Δm ~ Γ/4, respectively, where Γis the muon-sneutrino width. Both the CP-violating and the CP-conserving spin asymmetries should be detectable in future Tevatron runs even for a heavy sneutrino with a mass < 500 GeV. If detected, such asymmetries - being proportional to the mass splitting between the CP-even and CP-odd sneutrino states - may serve as a strong indication for the existence of the sneutrino mixing phenomenon.

hep-ph↗