arXiv subjects

A. Soni

Publications and source records attributed to A. Soni.

88 records · Page 5Linked to original sources

arXiv1995

Weak Matrix Elements on the Lattice --- Circa 1995

Status of weak matrix elements is reviewed. In particular, $\epspeps$, $\bkastg$, $B_K$, $B_B$ and $B_{B_s}$ are discussed and the overall situation with respect to the lattice effort and some of its phenomenological implications are summarized. For $\epspeps$ the need for the relevant matrix elements is stressed in view of the forthcoming improved experiments. For some of the operators, (e.g.\ $O_6$), even bounds on their matrix elements would be very helpful. On $\bkastg$, a constant behavior of $T_2$ appears disfavored although dependence of $T_2$ could, of course, be milder than a simple pole. Improved data is badly needed to settle this important issue firmly, especially in view of its ramification for extractions of $V_{td}$ from $B\toργ$. On $B_K$, the preliminary result from JLQCD appears to contradict Sharpe \etal\ JLQCD data seems to fit very well to linear $a$ dependence and leads to an appreciably lower value of $B_K$. Four studies of $B_K$ in the ``full'' ($n_f=2$) theory indicate very little quenching effects on $B_K$; the full theory value seems to be just a little less than the quenched result. Based on expectations from HQET, analysis of the $B$-parameter ($B_{h\ell}$) for the heavy-light mesons via $B_{h\ell}={}$constant${}+{}$constants$^\prime/m_{h\ell}$ is suggested. A summary of an illustrative sample of hadron matrix elements is given and constraints on CKM parameters (e.g.\ $V_{td}/V_{ts}$), on the unitarity triangle and on $x_s/x_d$, emerging from the lattice calculations along with experimental results are briefly discussed. In quite a few cases, for the first time, some indication of quenching errors on weak matrix elements are now becoming available.

hep-lat↗

arXiv1995

$f_B$ quenched and unquenched

Results for $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$, and their ratios are presented. High statistics quenched runs at $β=5.7$, $5.85$, $6.0$, and $6.3$, plus a run still in progress at $β=6.52$ make possible a preliminary extrapolation to the continuum. The data allows good control of all systematic errors except for quenching, although not all of the error estimates have been finalized. Results from configurations which include effects of dynamical quarks show a significant deviation from the quenched results and make possible a crude estimate of the quenching error.

hep-lat↗

arXiv1995

Large Tree Level CP Violation in $e^+e^-\to t\bar{t}H^0$ in The Two Higgs Doublet Model

We find a large CP violation effect within the Two-Higgs-Doublet-Model for the reaction $e^+e^-\to t\bar{t}H^0$ at future linear colliders. The CP-asymmetry arises already at the tree level as a result of interference between diagrams with $H^0$ emission from $t$ (and $\bar{t}$) and its emission from a $Z^0$ and can be about 10--20\%. In the best case one needs a few hundred $t\bar{t}H^0$ events to observe CP violation at the 3$σ$ level.

hep-ph↗

arXiv1995

Status and prospect for determining $f_B$, $f_{B_s}$, $f_{B_s} / f_B$ on the lattice

Preliminary results from the MILC collaboration for $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$ and their ratios are presented. We compute in the quenched approximation at $β=6.3$, 6.0 and 5.7 with Wilson light quarks and static and Wilson heavy quarks. We attempt to quantify all systematic errors other than quenching, and have a first indication of the size of quenching errors.

hep-ph↗

arXiv1994

Preliminary heavy-light decay constants from the MILC collaboration

Preliminary results from the MILC collaboration for $f_B$, $f_{B_s}$, $f_D$, $f_{D_s}$ and their ratios are presented. We compute in the quenched approximation at $β=6.3$, 6.0 and 5.7 with Wilson light quarks and static and Wilson heavy quarks. We attempt to quantify systematic errors due to finite volume, finite lattice spacing, large $am$, and fitting and extrapolation uncertainties. The hopping parameter approach of Henty and Kenway is used to treat the heavy quarks; the sources are Coulomb gauge gaussians.

hep-lat↗

arXiv1994

Pure Leptonic Radiative Decays $B^\pm, D_s\to\ellνγ$ and the Annihilation Graph

Pure leptonic radiative decays of heavy-light mesons are calculated using a very simple non-relativistic model. Dominant contribution originates from photon emission from light initial quark. We find $BR(B^\pm\to\ellνγ)\sim3.5\times10^{-6}$ and $BR(D_s\to \ellνγ)\sim1.7 \times10^{-4}$. The importance of these reactions to clarify the dynamics of the annihilation graph is emphasized.

hep-ph↗

arXiv1994

Lattice Results for Heavy Light Matrix Elements

Lattice results for heavy light matrix elements are reviewed and some of their implications are very briefly discussed. Despite the fact that in most cases the lattice results for weak matrix elements at the moment have only a modest accuracy of about 20--30\% they already have important phenomenological repercussions; e.g.\ for $V_{td}/V_{ts}$, $x_s/x_d$ and $B\to K^\astγ$. Presented at the XXVII International Conference on High Energy Physics, Glasgow, July 1994.

hep-lat↗

arXiv1994

Feasibility of Extracting V_td from Radiative B(B_s) Decays

We use existing measurements of $D^-\to K^{\ast0}ρ^-$ and $B\to ψ+K^\ast$, coupled with flavor independence of QCD, and with vector meson dominance to show that long distance contributions to $B\to ρ+γ$ are potentially very serious. We note that long distance (LD) contributions can be appreciably different in $B^-\to ρ^-+γ$ and $B^0\to ρ^0 (ω) +γ$. All radiative decays of $B$, $B_S$ are shown to be governed essentially by two LD and two short-distance (SD) hadronic entities. Separate measurements of $B^-\to ρ^-+γ$, $B^0\to ρ^0 (ω) +γ$, along with $B\to K^\ast +γ$ appear necessary for a meaningful extraction of \vtd. Measurements of $B_S\to ϕ+γ$ and $K^{\ast0}+γ$ could also provide very useful consistency checks.

hep-ph↗

arXiv1994

CP Violation in $B^\pm\to γπ^\pmπ^+π^-$

We consider CP violating effects in decays of the type $B^\pm\to γa^\pm_{1,2}$ where $a_{1,2}$ are the $J^P=1^+$ and $2^+$ resonances each decaying to the common final state via $a^\pm_{1,2} \toπ^\pmρ^0$. The resonances enhance the CP asymmetries and also knowledge of their masses and widths facilitates calculations of the effects. Several types of CP asymmetries are sizable ($\sim10$--30\%) requiring about (3--10)${}\times10^8$ $B^\pm$ mesons for detection at the $3σ$ level thereby providing a method for measuring the angle $α$ in the unitarity triangle. \bigskip

hep-ph↗

arXiv1994

$B \to K^* + γ$ and $B_s \to ϕ+ γ$ on the Lattice

A lattice calculation of the form factors that determine the ``hadronization ratios'', such as $R_{K^*}$ and $R_ϕ$, where $R_{K^*} \equiv [Γ($B \to K^* γ$)/Γ(b \to s γ)]$, is presented in the quenched approximation. Lattice data shows strong evidence for the scaling law suggested by heavy quark symmetry for one of the form factors ({\it i.e.}\ $T_2$). The data also gives strong support for the simple pole ansatz for the $q^2$ dependence of $T_2$ in the range of available $q^2$. We thus find $T_2(0) = .10 \pm.01\pm.03$ yielding $R_{K^*} =(6.0\pm1.2\pm3.4)\%$; we also find $R_ϕ=(6.6\pm1.3\pm3.7)\%$.

hep-lat↗

arXiv1994

Influence of Kaonic Resonances on the CP Violation in $B\to K^*γ$Like Processes

We consider CP violating effects in decays of the type $B\rightarrow k_iγ\rightarrow Kπγ$, $K^*πγ$ and $Kργ$, where $k_i$ represents a strange meson resonance. We include in our calculations five of the low-lying resonances with quantum numbers ($J^P$) $1^-$, $1^+$ and $2^+$. At the quark level these decays are driven by the penguin graph as well as tree graphs. CP violation arises due to the difference in the CKM phase between these graphs. We model the final state interaction of the hadronic system using the low lying $k_i$ resonances. We find that radiative decays of B mesons give rise to four of the five kaonic resonances at about $1$ to $7 \%$ of the inclusive $b \rightarrow s γ$ rate. We find that the probability of formation of the other three higher resonances is roughly the same as that of $K^*(892)$, which was recently seen at CLEO. In addition to the partial rate asymmetry which arises due to interference between resonances of the same quantum numbers, we show how interference between resonances of the same parity, and also between resonances of the opposite parity, result in two different types of energy asymmetries. CP differential asymmetries at the level of a few percent seem possible. We thus obtain CP violating distributions which may be observed in a sample of about $10^9$\relax $B^\pm$ mesons.

hep-ph↗

arXiv1993

A Lattice Calculation of the Isgur-Wise Function

We calculate the Isgur-Wise function ($ξ_0$) by measuring the heavy-heavy meson transition matrix element on the lattice in the quenched approximation. The standard Wilson action is used for both the heavy and the light quarks. Our numerical results are compared with various model calculations and experimental data. In particular, using a linear fit, we find $ρ^2 = 1.24\pm .26 \pm .26 $. (The slope of $ξ_0$ at the zero-recoil point is $-ρ^2$.) Using instead the parametrization of $ξ_0$ suggested by Neubert and Rieckert and fitting the lattice data up to $v\cdot v' \sim 1.2$, we find $ρ^2_{NR}=1.41\pm.19\pm.19$

hep-lat↗

arXiv1993

A Lattice Study of the Gluon Propagator in Momentum Space

We consider pure glue QCD at beta=5.7, beta=6.0 and beta=6.3. We evaluate the gluon propagator both in time at zero 3-momentum and in momentum space. From the former quantity we obtain evidence for a dynamically generated effective mass, which at beta=6.0 and beta=6.3 increases with the time separation of the sources, in agreement with earlier results. The momentum space propagator G(k) provides further evidence for mass generation. In particular, at beta=6.0, for k less than 1 GeV, the propagator G(k) can be fit to a continuum formula proposed by Gribov and others, which contains a mass scale b, presumably related to the hadronization mass scale. For higher momenta Gribov's model no longer provides a good fit, as G(k) tends rather to follow an inverse power law. The results at beta=6.3 are consistent with those at beta=6.0, but only the high momentum region is accessible on this lattice. We find b in the range of three to four hundred MeV and the exponent of the inverse power law about 2.7. On the other hand, at beta=5.7 (where we can only study momenta up to 1 GeV) G(k) is best fit to a simple massive boson propagator with mass m. We argue that such a discrepancy may be related to a lack of scaling for low momenta at beta=5.7. {}From our results, the study of correlation functions in momentum space looks promising, especially because the data points in Fourier space turn out to be much less correlated than in real space.

hep-lat↗

arXiv1993

Probing CP Violation Via Higgs Decays to Four Leptons

Since decays to four leptons is widely considered a promising way to search for the Higgs particle, we show how the same final state can also be used to search for signals of CP nonconservation. Energy asymmetries and triple correlations are related to parameters in the underlying CP violating effective interaction at the $H^0$-$W$-$W$ and $H^0$-$Z$-$Z$ vertex. Expected size of the effects are shown to be small for both the Sandard Model and its extension with an extra Higgs doublet.

hep-ph↗

arXiv1992

The Gluon Propagator in Momentum Space

We give preliminary numerical results for the gluon propagator evaluated both in coordinate and momentum space on a 16^3X40 quenched lattice at beta=6.0. Our findings are compared with earlier results in the literature at zero momentum. In addition, by considering nonzero momenta we attempt to extract the form of the propagator and compare it to continuum predictions formulated by Gribov and others. latex, file espcrc2.sty needed (appended at the end: search for espcrc2.sty).

hep-lat↗

arXiv1992

A General Lower Bound for Radiative B Decay

In a wide class of models -- including the standard model -- flavor-changing neutral currents are suppressed by an approximate chiral symmetry of the low-energy lagrangian. This symmetry allows the derivation of general relations among low-energy flavour-changing processes. We derive one such: the relative sizes of the decay rates b --> s gamma, b --> d gamma and s --> d gamma. Together with a unitarity-related lower bound on the rate for the strangeness-changing hyperon decay Omega^- --> Xi^- γwe obtain a reasonably model-independent lower bound for the inclusive b --> s gamma rate.

hep-ph↗