arXiv subjects

M. Benayoun

Publications and source records attributed to M. Benayoun.

At least 19 recordsLinked to original sources

arXiv2020

The anomalous magnetic moment of the muon in the Standard Model

We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant $α$ and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including $\mathcal{O}(α^5)$ with negligible numerical uncertainty. The electroweak contribution is suppressed by $(m_μ/M_W)^2$ and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at $\mathcal{O}(α^2)$ and is due to hadronic vacuum polarization, whereas at $\mathcal{O}(α^3)$ the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads $a_μ^\text{SM}=116\,591\,810(43)\times 10^{-11}$ and is smaller than the Brookhaven measurement by 3.7$σ$. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future-which are also discussed here-make this quantity one of the most promising places to look for evidence of new physics.

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arXiv2020

Measurement of psi(2S) meson production in pp collisions at sqrt(s)=7 TeV

The differential cross-section for the inclusive production of psi(2S) mesons in pp collisions at sqrt(s)=7 TeV has been measured with the LHCb detector. The data sample corresponds to an integrated luminosity of 36 pb-1. The psi(2S) mesons are reconstructed in the decay channels psi(2S) -> mu+ mu- and psi(2S) -> J/psi pi+ pi-, with the J/psi meson decaying into two muons. Results are presented both for promptly produced psi(2S) mesons and for those originating from b-hadron decays. In the kinematic range pT(psi(2S)) <= 16 GeV/c and 2 < y(psi(2S)) <= 4.5 we measure 1.44 +- 0.01 +- 0.12+0.2-0.4 mub for prompt psi(2S) production and 0.25 +- 0.01 +- 0.02 mub for psi(2S) from b-hadron decays, where the last uncertainty on the prompt cross-section is due to the unknown psi(2S) polarization. Recent QCD calculations are found to be in good agreement with our measurements. Combining the present result with the LHCb J/psi measurements we determine the inclusive branching fraction B(b -> psi(2S) X) = (2.73 +- 0.06 +- 0.16 +- 0.24) x 10^(-3), where the last uncertainty is due to the B(b -> J/psi X), B(J/psi -> mu+ mu-) and B(psi(2S) -> e+ e-) branching fraction uncertainties. All above results are corrected by an erratum included as an appendix.

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arXiv2020

BHLS$_2$, a New Breaking of the HLS Model and its Phenomenology

Previous studies have shown that the Hidden Local Symmetry (HLS) Model, supplied with appropriate symmetry breaking, provides an Effective Lagrangian (BHLS) able to encompass a large number of processes within a unified framework. A global fit provides a simultaneous description of the $e^+ e^-$ annihilation into $π^+π^-$, $π^0γ$, $ηγ$, $π^+π^-π^0$, $K^+K^-$, $K_L K_S$, the dipion spectrum in the $τ$ decay and other light meson partial widths. Additional breaking schemes extend its scope to spacelike processes within a new framework (BHLS$_2$). The phenomenology previously explored with BHLS is revisited in the BHLS$_2$ context with emphasis on the $Φ$ mass region. It is shown that BHLS$_2$ addresses the close spacelike region covered by NA7 and Fermilab data; also,the Lattice QCD pion form factor is accurately predicted by BHLS$_2$ using fits to annihilation data only. These channels contribution to $a_μ$ over the range of validity of BHLS$_2$ is shown to strongly reduce the former BHLS systematics. The uncertainty on $a_μ^{\rm th}(\sqrt{s}< 1.05$ GeV) is much improved compared to standard approaches relying on direct integration methods of measured spectra. Using the BHLS$_2$ results, the leading order HVP contribution to the muon anomalous moment is $a_μ^{HVP-LO}= 686.65 \pm 3.01 +(+1.16,-0.75)_{syst.}$ in units of $10^{-10}$. Using a conservative estimate for the light--by--light contribution, our evaluation is $a_μ^{\rm th}=\left [11\,659\,175.96 \pm 4.17 +(+1.16,-0.75)_{syst.}\right]\times 10^{-10}$. The $ρ^0γ$ mixing in dispersive and LQCD approaches is discussed and may amount to a shift $δa_μ[ππ]_{ργ} = +(3.10\pm0.31)\times 10^{-10}$ at LO+NLO, presently treated as additional systematics. Taking this shift into account, $a_μ^{\rm th}-a_μ^{\rm BNL}$ exhibits a significance not smaller than $3.8 σ$.

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arXiv2016

A BHLS model based moment analysis of muon g-2, and its use for lattice QCD evaluations of $a_μ^{\rm had}$

We present an up-to-date analysis of muon $g-2$ evaluations in terms of Mellin-Barnes moments as they might be useful for lattice QCD calculations of $a_μ$. The moments up to 4th order are evaluated directly in terms of $e^+e^-$--annihilation data and improved within the Hidden Local Symmetry (HLS) Model, supplied with appropriate symmetry breaking mechanisms. The model provides a reliable Effective Lagrangian (BHLS) estimate of the two-body channels plus the $πππ$ channel up to 1.05~GeV, just including the $ϕ$ resonance. The HLS piece accounts for 80\% of the contribution to $a_μ$. The missing pieces are evaluated in the standard way directly in terms of the data. We find that the moment expansion converges well in terms of a few moments. The two types of moments which show up in the Mellin-Barnes representation are calculated in terms of hadronic cross--section data in the timelike region and in terms of the hadronic vacuum polarization (HVP) function in the spacelike region which is accessible to lattice QCD (LQCD). In the Euclidean the first type of moments are the usual Taylor coefficients of the HVP and we show that the second type of moments may be obtained as integrals over the appropriately Taylor truncated HVP function. Specific results for the isovector part of $a_μ^{\rm had}$ are determined by means of HLS model predictions in close relation to $τ$--decay spectra.

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arXiv2015

Muon g-2 estimates : Can one trust Effective Lagrangians and global fits ?

Previous studies have shown that the Hidden Local Symmetry (HLS) Model, supplied with appropriate symmetry breaking mechanisms, provides an Effective Lagrangian (BHLS) which encompasses a large number of processes within a unified framework; a global fit procedure allows for a simultaneous description of the e+ e- annihilation into the 6 final states $π^+ π^-$, $π^0 γ$, $ηγ$, $π^+ π^- π^0$, $K^+ K^-$, $K_l K_s$ and includes the dipion spectrum in the τ decay and some more light meson decay partial widths. The contribution to the muon anomalous magnetic moment $a_{th}$ of these annihilation channels over the range of validity of the HLS model (up to 1.05 GeV) is found much improved compared to its partner derived from integrating the measured spectra directly. However, most spectra for the process $e^+ e^- \to π^+ π^-$ undergo overall scale uncertainties which dominate the other sources, and one may suspect some bias in the dipion contribution to $a_{th}$. However, an iterated fit algorithm, shown to lead to unbiased results by a Monte Carlo study, is defined and applied succesfully to the $e^+ e^- \to π^+ π^-$ data samples from CMD2, SND, KLOE, BaBar and BESSIII. The iterated fit solution is shown to be further improved and leads to a value for $a_μ$ different from $a_{exp}$ above the 4 $σ$ level. The contribution of the $π^+ π^-$ intermediate state up to 1.05 GeV to $a_{th}$ derived from the iterated fit benefits from an uncertainty about 3 times smaller than the corresponding usual estimate. Therefore, global fit techniques are shown to work and lead to improved unbiased results.

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arXiv2015

Quantum numbers of the $X(3872)$ state and orbital angular momentum in its $ρ^0 Jψ$ decay

Angular correlations in $B^+\to X(3872) K^+$ decays, with $X(3872)\to ρ^0 J/ψ$, $ρ^0\toπ^+π^-$ and $J/ψ\toμ^+μ^-$, are used to measure orbital angular momentum contributions and to determine the $J^{PC}$ value of the $X(3872)$ meson. The data correspond to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions collected with the LHCb detector. This determination, for the first time performed without assuming a value for the orbital angular momentum, confirms the quantum numbers to be $J^{PC}=1^{++}$. The $X(3872)$ is found to decay predominantly through S wave and an upper limit of $4\%$ at $95\%$ C.L. is set on the fraction of D wave.

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arXiv2014

Measurements of $B_c^+$ production and mass with the $B_c^+ \to J/ψπ^+$ decay

Measurements of $B_c^+$ production and mass are performed with the decay mode $B_c^+ \to J/ψπ^+$ using 0.37 fb$^{-1}$ of data collected in $pp$ collisions at $\sqrt{s}=7$ TeV by the LHCb experiment. The ratio of the production cross-section times branching fraction between the $B_c^+ \to J/ψπ^+$ and the $B^+ \to J/ψK^+$ decays is measured to be $(0.68 \pm 0.10\,({\rm stat.}) \pm 0.03\,({\rm syst.}) \pm 0.05\,({\rm lifetime}))$% for $B_c^+$ and $B^+$ mesons with transverse momenta $p_{\rm T}>4 $GeV/$c$ and pseudorapidities $2.5<η<4.5$. The $B_c^+$ mass is directly measured to be $6273.7 \pm 1.3\,({\rm stat.}) \pm 1.6 \,({\rm syst.})$ MeV/$c^2$, and the measured mass difference with respect to the $B^+$ meson is $M(B_c^+)-M(B^+) = 994.6 \pm 1.3\,({\rm stat.}) \pm 0.6\,({\rm syst.})$ MeV/$c^2$.

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arXiv2014

Observation of double charm production involving open charm in pp collisions at $\sqrt{s}$=7 TeV

The production of $J/ψ$ mesons accompanied by open charm, and of pairs of open charm hadrons are observed in pp collisions at a centre-of-mass energy of 7 TeV using an integrated luminosity of $355pb^{-1}$ collected with the LHCb detector. Model independent measurements of absolute cross-sections are given together with ratios to the measured $J/ψ$ and open charm cross-sections. The properties of these events are studied and compared to theoretical predictions.

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arXiv2013

MesonNet 2013 International Workshop. Mini-proceedings

The mini-proceedings of the MesonNet 2013 International Workshop held in Prague from June 17th to 19th, 2013, are presented. MesonNet is a research network within EU HadronPhysics3 project (1/2012 -- 12/2014). The web page of the conference, which contains all talks, can be found at http://ipnp.mff.cuni.cz/mesonnet13

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arXiv2013

Measurement of $J/ψ$ polarization in $pp$ collisions at $\sqrt{s}=7$ TeV

An angular analysis of the decay $J/ψ\rightarrow μ^{+} μ^{-}$ is performed to measure the polarization of prompt $J/ψ$ mesons produced in $pp$ collisions at $\sqrt{s}=7\; \mathrm{TeV}$. The dataset corresponds to an integrated luminosity of 0.37 fb$^{-1}$ collected with the LHCb detector. The measurement is presented as a function of transverse momentum, $p_\mathrm{T}$, and rapidity, $y$, of the $J/ψ$ meson, in the kinematic region $2 < p_\mathrm{T} < 15\; \mathrm{GeV}/c$ and $2.0<y<4.5$.

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arXiv2013

First observation of the decay $B_s^0 \rightarrow ϕ\bar{K}^{*0}$

The first observation of the decay $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7 TeV$, collected with the LHCb detector. A yield of $30 \pm 6$ $B_s^0 \rightarrow (K^+K^-)(K^-π^+)$ decays is found in the mass windows $1012.5 < M(K^+K^-) < 1026.5 MeV/c^2$ and $746 < M(K^-π^+)< 1046 MeV/c^2$. The signal yield is found to be dominated by $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ decays, and the corresponding branching fraction is measured to be ${\cal B}(B_s^0 \rightarrow ϕ\bar{K}^{*0}) = (1.10 \pm 0.24 (stat) \pm 0.14 (syst) \pm 0.08 (f_d/f_s)) \times 10^{-6}$, where the uncertainties are statistical, systematic and from the ratio of fragmentation fractions $f_d/f_s$ which accounts for the different production rate of $B^0$ and $B_s^0$ mesons. The significance of $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ signal is 6.1 standard deviations. The fraction of longitudinal polarization in $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ decays is found to be $f_0 = 0.51 \pm 0.15 (stat) \pm 0.07 (syst)$.

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arXiv2013

Measurement of B meson production cross-sections in proton-proton collisions at sqrt(s)= 7 TeV

The production cross-sections of B mesons are measured in pp collisions at a centre-of-mass energy of 7 TeV, using data collected with the LHCb detector corresponding to a integrated luminosity of 0.36 fb-1. The B+, B0 and Bs0 mesons are reconstructed in the exclusive decays B+ -> J/psi K+, B0 -> J/psi K*0 and Bs0 -> J/psi phi, with J/psi -> mu+ mu-, K*0 -> K+ pi- and phi -> K+ K-. The differential cross-sections are measured as functions of B meson transverse momentum pT and rapidity y, in the range 0 < pT < 40 GeV/c and 2.0 < y < 4.5. The integrated cross-sections in the same pT and y ranges, including charge-conjugate states, are measured to be sigma(pp -> B+ + X) = 38.9 +- 0.3 (stat.) +- 2.5 (syst.) +- 1.3 (norm.) mub, sigma(pp -> B0 + X) = 38.1 +- 0.6 (stat.) +- 3.7 (syst.) +- 4.7 (norm.) mub, sigma(pp -> Bs0 + X) = 10.5 +- 0.2 (stat.) +- 0.8 (syst.) +- 1.0 (norm.) mub, where the third uncertainty arises from the pre-existing branching fraction measurements.

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arXiv2013

Measurement of prompt hadron production ratios in $pp$ collisions at $\sqrt{s} = $ 0.9 and 7 TeV

The charged-particle production ratios $\bar{p}/p$, $K^-/K^+$, $π^-/π^+$, $(p + \bar{p})/(π^+ + π^-)$, $(K^+ + K^-)/(π^+ + π^-)$ and $(p + \bar{p})/(K^+ + K^-)$ are measured with the LHCb detector using $0.3 {\rm nb^{-1}}$ of $pp$ collisions delivered by the LHC at $\sqrt{s} = 0.9$ TeV and $1.8 {\rm nb^{-1}}$ at $\sqrt{s} = 7$ TeV. The measurements are performed as a function of transverse momentum $p_{\rm T}$ and pseudorapidity $η$. The production ratios are compared to the predictions of several Monte Carlo generator settings, none of which are able to describe adequately all observables. The ratio $\bar{p}/p$ is also considered as a function of rapidity loss, $Δy \equiv y_{\rm beam} - y$, and is used to constrain models of baryon transport.

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arXiv2013

Measurements of the branching fractions of $B^{+} \to p \bar p K^{+}$ decays

The branching fractions of the decay $B^{+} \to p \bar p K^{+}$ for different intermediate states are measured using data, corresponding to an integrated luminosity of $1.0 fb^{-1}$, collected by the LHCb experiment. The total branching fraction, its charmless component $(M_{p\bar p} <2.85 {GeV/}c^{2})$ and the branching fractions via the resonant $c\bar c$ states $η_{c}(1S)$ and $ψ(2S)$ relative to the decay via a $J/ψ$ intermediate state are {align*} \frac{{\mathcal B}(B^{+} \to p \bar p K^{+})_{total}}{{\mathcal B}(B^{+} \to J/ψK^{+} \to p \bar p K^{+})}=& \, 4.91 \pm 0.19 \, {(\rm stat)} \pm 0.14 \, {(\rm syst)}, \frac{{\mathcal B}(B^{+} \to p \bar p K^{+})_{M_{p\bar p} <2.85 {GeV/}c^{2}}}{{\mathcal B}(B^{+} \to J/ψK^{+} \to p \bar p K^{+})}=& \, 2.02 \pm 0.10 \, {(\rm stat)}\pm 0.08 \, {(\rm syst)}, \frac{{\mathcal B} (B^{+} \to η_{c}(1S) K^{+} \to p \bar p K^{+})}{{\mathcal B}(B^{+} \to J/ψK^{+} \to p \bar p K^{+})} = & \, 0.578 \pm 0.035 \, {(\rm stat)} \pm 0.027 \, {(\rm syst)}, \frac{{\mathcal B} (B^{+} \to ψ(2S) K^{+} \to p \bar p K^{+})}{{\mathcal B}(B^{+} \to J/ψK^{+} \to p \bar p K^{+})}=& \, 0.080 \pm 0.012 \, {(\rm stat)} \pm 0.009 \, {(\rm syst)}. {align*} Upper limits on the $B^{+}$ branching fractions into the $η_{c}(2S)$ meson and into the charmonium-like states X(3872) and X(3915) are also obtained.

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arXiv2013

Observation of $B^+_c \rightarrow J/ψD_s^+$ and $B^+_c \rightarrow J/ψD_s^{*+}$ decays

The decays $B^+_c \rightarrow J/ψD_s^+$ and $B^+_c \rightarrow J/ψD_s^{*+}$ are observed for the first time using a dataset, corresponding to an integrated luminosity of 3$fb^{-1}$, collected by the LHCb experiment in proton-proton collisions at centre-of-mass energies of $\sqrt{s}$=7 and 8 TeV. The statistical significance for both signals is in excess of 9 standard deviations. The following ratios of branching fractions are measured to be $BR(B^+_c \rightarrow J/ψD_s^+)/BR(B^+_c \rightarrow J/ψπ+) = 2.90 \pm 0.57 \pm 0.24$, $BR(B^+_c \rightarrow J/ψD_s^{*+}) / BR (B^+_c \rightarrow J/ψD_s^+) = 2.37 \pm 0.56 \pm 0.10$, where the first uncertainties are statistical and the second systematic. The mass of the \Bc meson is measured to be $m_{B^+_c} = 6276.28 \pm 1.44 (stat) \pm 0.36(syst) MeV/c^2$, using the $B^+_c \rightarrow J/ψD_s^+$ decay mode.

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arXiv2013

Search for CP violation in D+ -> phi pi+ and Ds+ -> Ks pi+ decays

A search for CP violation in D+ -> phi pi+ decays is performed using data collected in 2011 by the LHCb experiment corresponding to an integrated luminosity of 1.0 fb^{-1} at a centre of mass energy of 7 TeV. The CP-violating asymmetry is measured to be (-0.04+/-0.14+/-0.14)% for candidates with K-K+ mass within 20 MeV/c^{2} of the phi meson mass. A search for a CP-violating asymmetry that varies across the phi mass region of the D+ -> K-K+pi+ Dalitz plot is also performed, and no evidence for CP violation is found. In addition, the CP asymmetry in the Ds+ -> Ks pi+ decay is measured to be (0.61+/-0.83+/-0.14)%.

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arXiv2013

Precision measurement of D meson mass differences

Using three- and four-body decays of $D$ mesons produced in semileptonic $b$-hadron decays, precision measurements of $D$ meson mass differences are made together with a measurement of the $D^{0}$ mass. The measurements are based on a dataset corresponding to an integrated luminosity of $1.0 fb^{-1}$ collected in $pp$ collisions at 7\,TeV. Using the decay $D^0 \rightarrow K^{+} K^{-} K^{-} π^{+}$, the $D^0$ mass is measured to be $M(D^0) &=& 1864.75 \pm 0.15 \,({\rm stat}) \pm 0.11 \,({\rm syst}) \, \textrm{MeV/c^2}$. The mass differences $M(D^{+}) - M(D^{0}) = 4.76 \pm 0.12 \,({\rm stat}) \pm 0.07 \,({\rm syst}) \, \textrm{MeV/c^2}$ and $M(D^{+}_s) - M(D^{+}) = 98.68 \pm 0.03 \,({\rm stat}) \pm 0.04 \,({\rm syst}) \, \textrm{MeV/c^2}$ are measured using the $D^0 \rightarrow K^{+} K^{-} π^{+} π^{-}$ and $D^{+}_{(s)} \rightarrow K^{+}K^{-} π^{+}$ modes.

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arXiv2013

Measurement of CP violation and the B_s^0 meson decay width difference with B_s^0\to J/ψK^+ K^- and B_s^0\to J/ψπ^+π^- decays

The time-dependent CP asymmetry in B_s^0\to J/ψK^+K^- decays is measured using $pp$ collision data at \sqrt{s}=7TeV, corresponding to an integrated luminosity of 1.0fb^-1, collected with the LHCb detector. The decay time distribution is characterised by the decay widths Γ_L and Γ_H of the light and heavy mass eigenstates of the B_s^0--\bar{B}_s^0 system and by a CP-violating phase ϕ_s. In a sample of 27\,617 B_s^0\to J/ψK^+K^- decays, where the dominant contribution comes from B_s^0\to J/ψϕ decays, these parameters are measured to be ϕ_s = 0.07 \pm 0.09 (stat) \pm 0.01 (syst) rad, Γ_s \equiv (Γ_L+Γ_H)/2 = 0.663 \pm 0.005 (stat) \pm 0.006 (syst) ps^-1, ΔΓ_s \equiv Γ_L -Γ_H = 0.100 \pm 0.016 (stat) \pm 0.003 (syst) & ps^-1, corresponding to the single most precise determination of ϕ_s, ΔΓ_s and Γ_s. The result of performing a combined analysis with B_s^{0} \to J/ψπ^+π^- decays gives ϕ_s = 0.01 \pm 0.07 (stat) \pm 0.01 (syst) rad, Γ_s = 0.661 \pm 0.004 (stat) \pm 0.006 (syst) ps^-1, ΔΓ_s = 0.106 \pm 0.011 (stat) \pm 0.007 (syst) & ps^-1. All measurements are in agreement with the Standard Model predictions.

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