Publications (27)76.93 Total impact
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ABSTRACT: The exclusive semileptonic decay B→πℓν is a key process for the determination of the CabibboKobayashiMaskawa matrix element Vub from the comparison of experimental rates as a function of q2 with theoretically determined form factors. The sensitivity of the form factors to the u/d quark mass has meant significant systematic uncertainties in lattice QCD calculations at unphysically heavy pion masses. Here, we give the first lattice QCD calculations of this process for u/d quark masses going down to their physical values, calculating the f0 form factor at zero recoil to 3%. We are able to resolve a longstanding controversy by showing that the softpion theorem result f0(qmax2)=fB/fπ does hold as mπ→0. We use the highly improved staggered quark formalism for the light quarks and show that staggered chiral perturbation theory for the mπ dependence is almost identical to continuum chiral perturbation theory for f0, fB, and fπ. We also give results for other processes such as Bs→Kℓν.  [Show abstract] [Hide abstract]
ABSTRACT: We present the first calculation of the electromagnetic form factor of the $\pi$ meson at physical light quark masses. We use configurations generated by the MILC collaboration including the effect of $u$, $d$, $s$ and $c$ sea quarks with the Highly Improved Staggered Quark formalism. We work at three values of the lattice spacing on large volumes and with $u$/$d$ quark masses going down to the physical value. We study scalar and vector form factors for a range in spacelike $q^2$ from 0.0 to 0.1 $\mathrm{GeV}^2$ and from their shape we extract mean square radii. Our vector form factor agrees well with experiment and we find $\langle r^2 \rangle_V = 0.403(18)(6) \,\mathrm{fm}^2$. For the scalar form factor we include quarkline disconnected contributions which have a significant impact on the radius. We give the first results for SU(3) flavoursinglet and octet scalar mean square radii, obtaining: $\langle r^2 \rangle_S^{\mathrm{singlet}} = 0.506(38)(53) \mathrm{fm}^2$ and $\langle r^2 \rangle_S^{\mathrm{octet}} = 0.431(38)(46) \mathrm{fm}^2$. We discuss the comparison with expectations from chiral perturbation theory. 
Conference Paper: Phenomenology with Lattice NRQCD b Quarks
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ABSTRACT: The HPQCD collaboration has used radiativelyimproved NonRelativistic QCD (NRQCD) for $b$ quarks in bottomonium to determine the decay rate of $\Upsilon$ and $\Upsilon^\prime$ mesons to leptons in lattice QCD. Using timemoments of vector bottomonium currentcurrent correlators, we are also able to determine the $b$ quark mass in the $\overline{\mathrm{MS}}$ scheme. We use the same NRQCD $b$ quarks and Highly Improved Staggered Quark (HISQ) light quarks  with masses down to their physical values  to give a complete picture of heavylight meson decay constants including those for vector mesons. We also study the semileptonic $B\rightarrow\pi\ell\nu$ decay at zero recoil to show that lattice QCD is consistent with the soft pion theorem for this decay: $f_0(q^2_{\mathrm{max}})=f_B/f_\pi$ in the massless pion limit. Finally, we present preliminary results for the $B_c \rightarrow \eta_c \ell \nu$ semileptonic decay form factors. This is a showcase for the comparison of results for NRQCD $b$ quarks with those from HISQ $b$ quarks (both with HISQ $c$ quarks). We give the first 3point results from our `heavy HISQ' programme, which will allow us to improve the normalisation of NRQCDHISQ currents for other calculations.  [Show abstract] [Hide abstract]
ABSTRACT: The exclusive semileptonic decay $B \rightarrow \pi \ell \nu$ is a key process for the determination of the CabibboKobayashiMaskawa matrix element $V_{ub}$ from the comparison of experimental rates as a function of $q^2$ with theoretically determined form factors. The sensitivity of the form factors to the $u/d$ quark mass has meant significant systematic uncertainties in lattice QCD calculations at unphysically heavy pion masses. Here we give the first lattice QCD calculations of this process for u/d quark masses going down to their physical values, calculating the $f_0$ form factor at zero recoil to 3\%. We are able to resolve a longstanding controversy by showing that the softpion theorem result $f_0(q^2_{max}) = f_B/f_{\pi}$ does hold as $m_{\pi} \rightarrow 0$. We use the Highly Improved Staggered Quark formalism for the light quarks and show that staggered chiral perturbation theory for the $m_{\pi}$ dependence is almost identical to continuum chiral perturbation theory for $f_0$, $f_B$ and $f_{\pi}$. We also give results for other processes such as $B_s \rightarrow K \ell \nu$. 
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ABSTRACT: We present a calculation of the hindered M$1$ $\Upsilon(2S) \to \eta_b(1S) \gamma$ decay rate using lattice nonrelativistic QCD. The calculation includes spindependent relativistic corrections to the NRQCD action through $\mathcal{O}(v^6)$ in the quark's relative velocity, relativistic corrections to the leading order current which mediates the transition through the quark's magnetic moment, radiative corrections to the leading spinmagnetic coupling and for the first time a full error budget. We also use gluon field ensembles at multiple lattice spacing values, all of which include $u$, $d$, $s$ and $c$ quark vacuum polarisation. Our result for the branching fraction is $\mathcal{B}(\Upsilon(2S)\to\eta_b(1S)\gamma) = 5.4(1.8)\times 10^{4} $, which agrees with the current experimental value.  [Show abstract] [Hide abstract]
ABSTRACT: We extend the picture of $B$meson decay constants obtained in lattice QCD beyond those of the $B$, $B_s$ and $B_c$ to give the first full lattice QCD results for the $B^*$, $B^*_s$ and $B^*_c$. We use improved NonRelativistic QCD for the valence $b$ quark and the Highly Improved Staggered Quark (HISQ) action for the lighter quarks on gluon field configurations that include the effect of $u/d$, $s$ and $c$ quarks in the sea with $u/d$ quark masses going down to physical values. For the ratio of vector to pseudoscalar decay constants, we find $f_{B^*}/f_B$ = 0.941(26), $f_{B^*_s}/f_{B_s}$ = 0.953(23) (both $2\sigma$ less than 1.0) and $f_{B^*_c}/f_{B_c}$ = 0.988(27). Taking correlated uncertainties into account we see clear indications that the ratio increases as the mass of the lighter quark increases. We compare our results to those using the HISQ formalism for all quarks and find good agreement both on decay constant values when the heaviest quark is a $b$ and on the dependence on the mass of the heaviest quark in the region of the $b$. Finally, we give an overview plot of decay constants for goldplated mesons, the most complete picture of these hadronic parameters to date.  [Show abstract] [Hide abstract]
ABSTRACT: We present the first lattice QCD calculation of the $B_s$ and $B_d$ mixing parameters with physical light quark masses. We use MILC gluon field configurations that include $u$, $d$, $s$ and $c$ sea quarks at 3 values of the lattice spacing and with 3 values of the $u/d$ quark mass going down to the physical value. We use improved NRQCD for the valence $b$ quarks. Preliminary results show significant improvements over earlier values.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a new technique (presented in arXiv:1403.1778) to determine the contribution to the anomalous magnetic moment (g2) of the muon coming from the hadronic vacuum polarisation using lattice QCD. Our method uses Pad\'{e} approximants to reconstruct the Adler function from its derivatives at $q^2=0$. These are obtained simply and accurately from timemoments of the vector currentcurrent correlator at zero spatial momentum. We test the method using strange quark correlators calculated on MILC Collaboration's $n_f$ = 2+1+1 HISQ ensembles at multiple values of the lattice spacing, multiple volumes and multiple light sea quark masses (including physical pion mass configurations). 
Article: The $\Upsilon$ and $\Upsilon^{\prime}$ Leptonic Widths, $a_{\mu}^b$ and $m_b$ from full lattice QCD
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ABSTRACT: We determine the decay rate to leptons of the groundstate $\Upsilon$ meson and its first radial excitation in lattice QCD for the first time. We use radiativelyimproved NRQCD for the $b$ quarks and include $u$, $d$, $s$ and $c$ quarks in the sea with $u/d$ masses down to their physical values. We find $\Gamma(\Upsilon \rightarrow e^+e^)$ = 1.19(11) keV and $\Gamma(\Upsilon^{\prime} \rightarrow e^+e^)$ = 0.69(9) keV, both in good agreement with experiment. The decay constants we obtain are included in a summary plot of meson decay constants from lattice QCD given in the Conclusions. We also test timemoments of the vector currentcurrent correlator against values determined from the $b$ quark contribution to $\sigma(e^+e^ \rightarrow \mathrm{hadrons})$ and calculate the $b$quark piece of the hadronic vacuum polarisation contribution to the anomalous magnetic moment of the muon, $a_{\mu}^b = 0.271(37) \times 10^{10}$. Finally we determine the $b$quark mass, obtaining in the $\overline{MS}$ scheme, $\overline{m}_b(\overline{m}_b, n_f=5)$ = 4.196(23) GeV, the most accurate result from lattice QCD to date.  [Show abstract] [Hide abstract]
ABSTRACT: We present a new lattice QCD analysis of heavyquark pseudoscalarpseudoscalar correlators, using gluon configurations from the MILC collaboration that include vacuum polarization from $u$, $d$, $s$ and $c$ quarks ($n_f=4$). We extract new values for the QCD coupling and for the $c$ quark's $\mathrm{\overline{MS}}$ mass: $\alpha_\mathrm{\overline{MS}}(M_Z,n_f=5) = 0.11881(86)$ and $m_c(3\,\mathrm{GeV}, n_f=4) = 0.9896(69)$GeV. These agree well with our earlier simulations using $n_f=3$ sea quarks, vindicating the perturbative treatment of $c$ quarks in that analysis. A joint $n_f=3$, $n_f=4$ analysis gives improved values for the coupling and heavyquark masses: $\alpha_\mathrm{\overline{MS}}(M_Z,n_f=5) = 0.11856(53)$, $m_c(3\,\mathrm{GeV}, n_f=4) = 0.9864(41)$GeV, $m_b(10\,\mathrm{GeV}, n_f=5) = 3.625(25)$GeV, and $m_b/m_c=4.54(3)$. Finally we obtain a new nonperturbative result for the ratio of $c$ and $s$ quark masses: $m_c/m_s=11.652(65)$. This ratio implies $m_s(2\,\mathrm{GeV}, n_f=3)=94.0(6)$MeV when it is combined with our best $c$ mass. Combining $m_c/m_s$ with our new $m_b/m_c$ gives $m_b/m_s=52.90(44)$, which is several standard deviations away from the GeorgiJarlskop prediction from certain GUTs.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a new technique to determine the contribution to the anomalous magnetic moment of the muon coming from the hadronic vacuum polarization using lattice QCD. Our method reconstructs the Adler function, using Pad\'{e} approximants, from its derivatives at $q^2=0$ obtained simply and accurately from timemoments of the vector currentcurrent correlator at zero spatial momentum. We test the method using strange quark correlators on largevolume gluon field configurations that include the effect of up and down (at physical masses), strange and charm quarks in the sea at multiple values of the lattice spacing and multiple volumes and show that 1% accuracy is achievable. For the charm quark contributions we use our previously determined moments with up, down and strange quarks in the sea on very fine lattices. We find the (connected) contribution to the anomalous moment from the strange quark vacuum polarization to be $a_\mu^s = 53.41(59) \times 10^{10}$, and from charm to be $a_\mu^c = 14.42(39)\times 10^{10}$. These are in good agreement with flavourseparated results from nonlattice methods, given caveats about the comparison. The extension of our method to the light quark contribution and to that from the quarkline disconnected diagram is straightforward. 
Article: Nonperturbative tests of the renormalization of mixed cloverstaggered currents in lattice QCD
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ABSTRACT: The Fermilab Lattice and MILC collaborations have shown in oneloop lattice QCD perturbation theory that the renormalization constants of vector and axialvector mixed cloverasqtad currents are closely related to the product of those for cloverclover and asqtadasqtad (local) vector currents. To be useful for future higher precision calculations this relationship must be valid beyond oneloop and very general. We test its validity nonperturbatively using clover and Highly Improved Staggered (HISQ) strange quarks, utilising the absolute normalization of the HISQ temporal axial current. We find that the renormalization of the mixed current differs from the square root of the product of the pure HISQ and pure clover currents by $23\%$. We also compare discretization errors between the clover and HISQ formalisms. 
Conference Paper: Bottomonium and B results from full lattice QCD
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ABSTRACT: We have developed two methods for handling $b$ quarks in lattice QCD. One uses NRQCD (now improved to include radiative corrections) and the other uses Highly Improved Staggered Quarks (HISQ), extrapolating to the $b$ quark from lighter masses and using multiple lattice spacings to control discretisation errors. Comparison of results for the two different methods gives confidence in estimates of lattice QCD systematic errors, since they are very different in these two cases. Here we show results for heavyonium hyperfine splittings and vector currentcurrent correlator moments using HISQ quarks, to add to earlier results testing the heavy HISQ method with pseudoscalar mesons. We also show the form factor for $B \rightarrow \pi l \nu$ decay at zero recoil using NRQCD $b$ quarks and $u/d$ quarks with physical masses. This allows us to test the soft pion theorem relation ($f_0(q^2_{max})=f_B/f_{\pi}$) accurately and we find good agreement as $M_{\pi} \rightarrow 0$. }  [Show abstract] [Hide abstract]
ABSTRACT: We present preliminary results from the first calculation of the pion electromagnetic form factor at physical light quark masses. This form factor parameterises the deviations from the behaviour of a pointlike particle when a photon hits the pion. These deviations result from the internal structure of the pion and can thus be calculated in QCD. We use three sets (different lattice spacings) of n_f=2+1+1 lattice configurations generated by the MILC collaboration. The Highly Improved Staggered Quark formalism (HISQ) is used for all of the sea and valence quarks. Using lattice configurations with u/d quark masses very close to the physical value is an advantage, as we avoid the chiral extrapolation. We study the shape of the vector (f_+) form factor in the q^2 range from 0 to 0.12 GeV^2 and extract the mean square radius, <r^2_v>. The shape of the vector form factor and the resulting radius is compared with experiment. 
Article: Bottomonium hyperfine splittings from lattice NRQCD including radiative and relativistic corrections
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ABSTRACT: We present a calculation of the hyperfine splittings in bottomonium using lattice Nonrelativistic QCD. The calculation includes spindependent relativistic corrections through O(v^6), radiative corrections to the leading spinmagnetic coupling and, for the first time, nonperturbative 4quark interactions which enter at alpha_s^2 v^3. We also include the effect of u,d,s and c quark vacuum polarisation. Our result for the 1S hyperfine splitting is M(Upsilon,1S)  M(eta_b,1S)= 62.8(6.7) MeV. We find the ratio of 2S to 1S hyperfine splittings (M(Upsilon,2S)  M(eta_b,2S))/ (M(Upsilon,1S)  M(eta_b,1S)) = 0.425(25).  [Show abstract] [Hide abstract]
ABSTRACT: Calculations of pseudoscalar decay constants of B, Bs, K and pi mesons with physical light quarks are presented. We use HISQ ensembles that include u,d,s and c sea quarks at three lattice spacings. HISQ is used for the valence light quarks and a radiatively improved NRQCD action for the heavy quarks. The key results are f_{B^+}=0.184(4)$ GeV, f_{B_s}=0.224(4) GeV, f_{B_s}/f_{B^+}=1.217(8), f_{K^+}/f_{pi^+}=1.1916(21), f_{K^+}=155.37(34) MeV, giving a significant improvement over previous results that required chiral extrapolation. We also calculate the Wilson flow scale w_0, finding w_0=0.1715(9) fm.  [Show abstract] [Hide abstract]
ABSTRACT: We present the first lattice QCD calculation of the decay constants fB and fBs with physical light quark masses. We use configurations generated by the MILC Collaboration including the effect of u, d, s, and c highly improved staggered quarks in the sea at three lattice spacings and with three u/d quark mass values going down to the physical value. We use improved nonrelativistic QCD (NRQCD) for the valence b quarks. Our results are fB=0.186(4) GeV, fBs=0.224(4) GeV, fBs/fB=1.205(7), and MBsMB=85(2) MeV, superseding earlier results with NRQCD b quarks. We discuss the implications of our results for the standard model rates for B(s)→μ+μ and B→τν. 
Article: V u s from π and K decay constants in full lattice QCD with physical u , d , s , and c quarks
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ABSTRACT: We determine the decay constants of the pi and K mesons on gluon field configurations from the MILC collaboration including u, d, s and c quarks. We use three values of the lattice spacing and u/d quark masses going down to the physical value. We use the w_0 parameter to fix the relative lattice spacing and f_pi to fix the overall scale. This allows us to obtain a value for f{K^+}/f{pi^+} = 1.1916(21). Comparing to the ratio of experimental leptonic decay rates gives Vus = 0.22564(28){Br(K^+)}(20){EM}(40){latt}(5){Vud} and the test of unitarity of the first row of the CabibboKobayashiMaskawa matrix: Vud^2+Vus^2+Vub^2  1 = 0.00009(51).  [Show abstract] [Hide abstract]
ABSTRACT: We present a determination of the bquark mass accurate through O(\alpha_s^2) in perturbation theory and including partial contributions at O(\alpha_s^3). Nonperturbative input comes from the calculation of the Upsilon and B_s energies in lattice QCD including the effect of u, d and s sea quarks. We use an improved NRQCD action for the bquark. This is combined with the heavy quark energy shift in NRQCD determined using a mixed approach of highbeta simulation and automated lattice perturbation theory. Comparison with experiment enables the quark mass to be extracted: in the MS bar scheme we find m_b(m_b) = 4.166(43) GeV.
Publication Stats
254  Citations  
76.93  Total Impact Points  
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Institutions

20132015

University of Cambridge
 Department of Applied Mathematics and Theoretical Physics
Cambridge, England, United Kingdom


20112012

University of Glasgow
 School of Physics and Astronomy
Glasgow, Scotland, United Kingdom
