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F. Alessio

Publications and source records attributed to F. Alessio.

At least 253 records · Page 14Linked to original sources
arXiv2023

Observation of sizeable $ω$ contribution to $χ_{c1}(3872)\toπ^+π^-J/ψ$ decays

Resonant structures in the dipion mass spectrum from $χ_{c1}(3872)\toπ^+π^- J/ψ$ decays, produced via $B^+\to K^+χ_{c1}(3872)$ decays, are analyzed using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 $fb^{-1}$. A sizeable contribution from the isospin conserving $χ_{c1}(3872)\toωJ/ψ$ decay is established for the first time, $(21.4\pm2.3\pm2.0)\%$, with a significance of more than $7.1σ$. The amplitude of isospin violating decay, $χ_{c1}(3872)\toρ^0 J/ψ$, relative to isospin conserving decay, $χ_{c1}(3872)\toωJ/ψ$, is properly determined, and it is a factor of six larger than expected for a pure charmonium state.

hep-ex↗
arXiv2023

Search for the lepton-flavour violating decays $B^0 \to K^{*0} μ^\pm e^\mp$ and $B_s^0 \to ϕμ^\pm e^\mp$

A search for the lepton-flavour violating decays $B^0 \to K^{*0} μ^\pm e^\mp$ and $B_s^0 \to ϕμ^\pm e^\mp$ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of $9\,\text{fb}^{-1}$. No significant signals are observed and upper limits of \begin{align} {\cal B}( B^0 \to K^{*0} μ^+ e^- ) &< \phantom{1}5.7\times 10^{-9}~(6.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} μ^- e^+ ) &< \phantom{1}6.8\times 10^{-9}~(7.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} μ^\pm e^\mp ) &< 10.1\times 10^{-9}~(11.7\times 10^{-9}),\newline {\cal B}( B_s^0 \to ϕμ^\pm e^\mp ) &< 16.0\times 10^{-9}~(19.8\times 10^{-9}) \end{align} are set at $90\%~(95\%)$ confidence level. These results constitute the world's most stringent limits to date, with the limit on the decay $B_s^0 \to ϕμ^\pm e^\mp$ the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.

hep-ex↗
arXiv2023

Search for the doubly heavy baryon $\itΞ_{bc}^{+}$ decaying to $J/\itψ \itΞ_{c}^{+}$

A first search for the $\itΞ_{bc}^{+}\to J/\itψ\itΞ_{c}^{+}$ decay is performed by the LHCb experiment with a data sample of proton-proton collisions, corresponding to an integrated luminosity of $9\,\mathrm{fb}^{-1}$ recorded at centre-of-mass energies of 7, 8, and $13\mathrm{\,Te\kern -0.1em V}$. Two peaking structures are seen with a local (global) significance of $4.3\,(2.8)$ and $4.1\,(2.4)$ standard deviations at masses of $6571\,\mathrm{Me\kern -0.1em V\!/}c^2$ and $6694\,\mathrm{Me\kern -0.1em V\!/}c^2$, respectively. Upper limits are set on the $\itΞ_{bc}^{+}$ baryon production cross-section times the branching fraction relative to that of the $B_{c}^{+}\to J/\itψ D_{s}^{+}$ decay at centre-of-mass energies of 8 and $13\mathrm{\,Te\kern -0.1em V}$, in the $\itΞ_{bc}^{+}$ and in the $B_{c}^{+}$ rapidity and transverse-momentum ranges from 2.0 to 4.5 and 0 to $20\,\mathrm{Ge\kern -0.1em V\!/}c$, respectively. Upper limits are presented as a function of the $\itΞ_{bc}^{+}$ mass and lifetime.

hep-ex↗
arXiv2023

Amplitude analysis of $B^0 \rightarrow \overline{D}^0 D_s^+ π^-$ and $B^+ \rightarrow D^- D_s^+ π^+$ decays

Resonant contributions in $B^0 \rightarrow \overline{D}^0 D^+_sπ^-$ and $B^+\rightarrow D^- D^+_sπ^+$ decays are determined with an amplitude analysis, which is performed both separately and simultaneously, where in the latter case isospin symmetry between the decays is assumed. The analysis is based on data collected by the LHCb detector in proton-proton collisions at center-of-mass energies of 7, 8 and 13 $\rm{TeV}$. The full data sample corresponds to an integrated luminosity of 9 $\rm fb^{-1}$. A doubly charged spin-0 open-charm tetraquark candidate together with a neutral partner, both with masses near $2.9\,\rm{GeV}$, are observed in the $D_sπ$ decay channel.

hep-ex↗
arXiv2023

First observation of a doubly charged tetraquark and its neutral partner

A combined amplitude analysis is performed for the decays $B^0 \rightarrow \overline{D}^0 D^+_sπ^-$ and $B^+\rightarrow D^- D^+_sπ^+$, which are related by isospin symmetry. The analysis is based on data collected by the LHCb detector in proton-proton collisions at center-of-mass energies of 7, 8 and 13$\,\rm{TeV}$. The full data sample corresponds to an integrated luminosity of 9$\,\rm{fb^{-1}}$. Two new resonant states with masses of $2.908\pm0.011\pm0.020\,\rm{GeV}$ and widths of $0.136\pm0.023\pm0.011\,\rm{GeV}$ are observed, which decay to $D^+_sπ^+$ and $D^+_sπ^-$ respectively. The former state indicates the first observation of a doubly charged open-charm tetraquark state with minimal quark content $[c\bar{s}u\bar{d}]$, and the latter state is a neutral tetraquark composed of $[c\bar{s}\bar{u}d]$ quarks. Both states are found to have spin-parity $0^+$, and their resonant parameters are consistent with each other, which suggests that they belong to an isospin triplet.

hep-ex↗
arXiv2023

Amplitude analysis of the $Λ^+_c\to pK^-π^+$ decay and $Λ^+_c$ baryon polarization measurement in semileptonic beauty hadron decays

An amplitude analysis of $Λ^+_c \to pK^-π^+$ decays together with a measurement of the $Λ^+_c$ polarization vector in semileptonic beauty hadron decays is presented. A sample of $400\,000$ candidates is selected from proton-proton collisions recorded by the LHCb detector at a center-of-mass energy of 13 TeV. An amplitude model is developed and the resonance fractions as well as two- and three-body decay parameters are reported. The mass and width of the $Λ(2000)$ state are also determined. A significant $Λ^+_c$ polarization is found. A large sensitivity of the $Λ^+_c \to pK^-π^+$ decay to the polarization is seen, making the amplitude model suitable for $Λ^+_c$ polarization measurements in other systems.

hep-ex↗
arXiv2023

Nuclear modification factor of neutral pions in the forward and backward regions in $p$-Pb collisions

The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of $8.16~{\rm TeV}$ with the LHCb detector. The $π^0$ production cross section is measured differentially in transverse momentum ($p_{\rm T}$) for $1.5<p_{\rm T}<10.0~{\rm GeV}$ and in center-of-mass pseudorapidity ($η_{\rm c.m.}$) regions $2.5<η_{\rm c.m.}<3.5$ (forward) and $-4.0<η_{\rm c.m.}<-3.0$ (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of $π^0$ production, while the backward measurement shows the first evidence of $π^0$ enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.

nucl-ex↗
arXiv2023

Measurement of the ratio of branching fractions $\mathcal{B}(B_c^+ \to B_s^0 π^+)/\mathcal{B}(B_c^+ \to J/ψπ^+)$

The ratio of branching fractions of $B_c^+ \to B_s^0 π^+$ and $B_c^+ \to J/ψπ^+$ decays is measured with proton-proton collision data of a centre-of-mass energy of $13\text{TeV}$. The data were collected with the LHCb experiment during 2016--2018, corresponding to an integrated luminosity of $5.4 \text{fb}^{-1}$. The $B_s^0$ mesons are reconstructed via the decays $B_s^0 \to J/ψϕ$ and $B_s^0 \to D_s^- π^+$. The ratio of branching fractions is measured to be $\mathcal{B}(B_c^+ \to B_s^0 π^+)/\mathcal{B}(B_c^+ \to J/ψπ^+) = 91 \pm 10 \pm 8 \pm 3$ where the first uncertainty is statistical, the second is systematic and the third is due to the knowledge of the branching fractions of the intermediate state decays.

hep-ex↗
arXiv2023

First observation and branching fraction measurement of the $Λ_b^0\to D_s^- p$ decay

The first observation of the $Λ_b^0\to D_s^- p$ decay is presented using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of ${\sqrt{s}=13 \,\textrm{TeV}}$, corresponding to a total integrated luminosity of $6\,\textrm{fb}^{-1}$. Using the $Λ_b^0\toΛ_c^+π^-$ decay as the normalisation mode, the branching fraction of the $Λ_b^0\to D_s^- p$ decay is measured to be ${\mathcal{B}(Λ_b^0\to D_s^- p)=(12.6 \pm 0.5 \pm 0.3 \pm 1.2 )\times 10^{-6}}$, where the first uncertainty is statistical, the second systematic and the third due to uncertainties in the branching fractions of the $Λ_b^0\toΛ_c^+π^-$, $D_s^- \to K^-K^+π^-$ and $Λ_c^+\to p K^- π^+$ decays.

hep-ex↗
arXiv2023

Observation of the $B^0_s\!\to D^{*+}D^{*-}$ decay

The first observation of the $B^0_s\!\to D^{*+}D^{*-}$ decay and the measurement of its branching ratio relative to the $B^0\!\to D^{*+}D^{*-}$ decay are presented. The data sample used corresponds to an integrated luminosity of $9\,\text{fb}^{-1}$ of proton-proton collisions recorded by the LHCb experiment at centre-of-mass energies of 7, 8 and $13\,\text{TeV}$ between 2011 and 2018. The decay is observed with more than $10$ standard deviations and the time-integrated ratio of branching fractions is determined to be \begin{align*} \frac{\mathcal{B}(B^0_s\!\to D^{*+}D^{*-})}{\mathcal{B}(B^0\!\to D^{*+}D^{*-})} = 0.269 \pm 0.032 \pm 0.011 \pm 0.008\, , \end{align*} where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the fragmentation fraction ratio $f_s/f_d$. The $B^0_s\!\to D^{*+}D^{*-}$ branching fraction is calculated to be \begin{align*} \mathcal{B}(B^0_s\!\to D^{*+}D^{*-}) = (2.15 \pm 0.26 \pm 0.09 \pm 0.06 \pm 0.16)\times 10^{-4} \,, \end{align*} where the fourth uncertainty is due to the $B^0\!\to D^{*+}D^{*-}$branching fraction. These results are calculated using the average $B^0_s$ meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light $B^0_s$ eigenstate is considered.

hep-ex↗
arXiv2023

Observation of the decay $ Λ_b^0\rightarrow Λ_c^+τ^-\overlineν_τ$

The first observation of the semileptonic $b$-baryon decay $ Λ_b^0 \rightarrow Λ_c^+ τ^-\overlineν_τ$, with a significance of $6.1\,σ$, is reported using a data sample corresponding to 3 fb$^{-1}$ of integrated luminosity, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV at the LHC. The $τ^-$ lepton is reconstructed in the hadronic decay to three charged pions. The branching fraction ${\mathcal{B}}(Λ_b^0 \rightarrow Λ_c^+τ^-\overlineν_τ) = (1.50 \pm 0.16\pm 0.25\pm 0.23)\%$ is obtained, where uncertainties are statistical, systematic and from the external branching fraction of the normalisation channel $Λ_b^0\rightarrow Λ_c^+π^-π^+π^-$. The ratio of semileptonic branching fractions ${\mathcal{R}}( Λ_c^+)\equiv {\mathcal{B}}( Λ_b^0 \rightarrow Λ_c^+ τ^-\overlineν_τ)/{\mathcal{B}}( Λ_b^0 \rightarrow Λ_c^+ μ^-\overlineν_μ)$ is derived to be $0.242 \pm 0.026 \pm 0.040\pm 0.059$, where the external branching fraction uncertainty from the channel $Λ_b^0\rightarrow Λ_c^+μ^-\overlineν_μ$ contributes to the last term. This result is in agreement with the Standard Model prediction.

hep-ex↗
arXiv2022

Test of lepton universality in beauty-quark decays

The Standard Model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the Standard Model, such as a new fundamental interaction between quarks and leptons.

hep-ex↗
arXiv2022

Search for massive long-lived particles decaying semileptonically at sqrt(s)=13 TeV

A search is performed for massive long-lived particles (LLP) decaying semileptonically into a muon and two quarks. Two kinds of LLP production processes were considered. In the first, a Higgs-like boson with mass from 30 to 200 GeV is produced by gluon fusion and decays into two LLPs. The analysis covers LLP mass values from 10 GeV up to about one half the Higgs-like boson mass. The second LLP production mode is directly from quark interactions, with LLP masses from 10 to 90 GeV. The LLP lifetimes considered range from 5 to 200 ps. This study uses LHCb data collected from proton-proton collisions at sqrt(s)=13 TeV, corresponding to an integrated luminosity of 5.4 1/fb. No evidence of these long-lived states has been observed, and upper limits on the production cross-section times branching ratio have been set for each model considered.

hep-ex↗
arXiv2022

First measurement of the $Z\rightarrow μ^+ μ^-$ angular coefficients in the forward region of $pp$ collisions at $\sqrt{s}=13$ TeV

The first study of the angular distribution of $μ^+ μ^-$ pairs produced in the forward rapidity region via the Drell-Yan reaction $pp \rightarrow γ^{*}/Z +X \rightarrow l^+ l^- + X$ is presented, using data collected with the LHCb detector at a centre-of-mass energy of 13TeV, corresponding to an integrated luminosity of 5.1 $\rm{fb}^{-1}$. The coefficients of the five leading terms in the angular distribution are determined as a function of the dimuon transverse momentum and rapidity. The results are compared to various theoretical predictions of the $Z$-boson production mechanism and can also be used to probe transverse-momentum-dependent parton distributions within the proton.

hep-ex↗
arXiv2022

Constraints on the CKM angle $γ$ from $B^\pm\to Dh^\pm$ decays using $D\rightarrow h^\pm h^{\prime\mp}π^0$ final states

A data sample collected with the LHCb detector corresponding to an integrated luminosity of 9 fb$^{-1}$ is used to measure eleven $CP$ violation observables in $B^\pm\to Dh^\pm$ decays, where $h$ is either a kaon or a pion. The neutral $D$ meson decay is reconstructed in the three-body final states: $K^\pmπ^\mpπ^0$}; $π^+π^-π^0$; $K^+K^-π^0$ and the suppressed $π^\pm K^\mpπ^0$ combination. The mode where a large $CP$ asymmetry is expected, $B^\pm\to [π^\pm K^\mpπ^0]_DK^\pm$, is observed with a significance greater than seven standard deviations. The ratio of the partial width of this mode relative to that of the favoured mode, $B^\pm\to [K^\pmπ^\mpπ^0]_D K^\pm$, is $R_{{\rm ADS}(K)} = (1.27\pm0.16\pm0.02)\times 10^{-2}$. Evidence for a large $CP$ asymmetry is also seen: $A_{{\rm ADS}(K)} = -0.38\pm0.12\pm0.02$. Constraints on the CKM angle $γ$ are calculated from the eleven reported observables.

hep-ex↗
arXiv2022

Precision measurement of forward $Z$ boson production in proton-proton collisions at $\sqrt{s} = 13$ TeV

A precision measurement of the $Z$ boson production cross-section at $\sqrt{s} = 13$ TeV in the forward region is presented, using $pp$ collision data collected by the LHCb detector, corresponding to an integrated luminosity of 5.1 fb$^{-1}$. The production cross-section is measured using $Z\rightarrowμ^+μ^-$ events within the fiducial region defined as pseudorapidity $2.0<η<4.5$ and transverse momentum $p_{T}>20$ GeV/$c$ for both muons and dimuon invariant mass $60<M_{μμ}<120$ GeV/$c^2$. The integrated cross-section is determined to be \begin{equation*} σ(Z\rightarrowμ^+μ^-) = 196.4 \pm 0.2 \pm 1.6 \pm 3.9~pb, \end{equation*} where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measured results are in agreement with theoretical predictions within uncertainties.

hep-ex↗
arXiv2022

Measurement of the nuclear modification factor and prompt charged particle production in $p\mathrm{Pb}$ and $pp$ collisions at $\sqrt{s_{\scriptscriptstyle\mathrm{NN}}}=5\,\mathrm{TeV}$

The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon centre-of-mass energy ${\sqrt{s_{\scriptscriptstyle\mathrm{NN}}}=5\,\mathrm{TeV}}$ is studied at LHCb as a function of pseudorapidity ($η$) and transverse momentum ($p_{\mathrm{T}}$) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of $η$ between ${-4.8<η<-2.5}$ (backward region) and ${2.0<η<4.8}$ (forward region), and $p_{\mathrm{T}}$ between ${0.2<p_{\mathrm{T}}<8.0\,\mathrm{GeV}/c}$. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for $p_{\mathrm{T}}$ larger than $1.5\,\mathrm{GeV}/c$. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to $10^{-6}$.

hep-ex↗
arXiv2022

Study of the doubly charmed tetraquark $T_{cc}^+$

An exotic narrow state in the $D^0D^0π^+$ mass spectrum just below the $D^{*+}D^0$ mass threshold is studied using a data set corresponding to an integrated luminosity of 9 fb$^{-1}$ acquired with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The state is consistent with the ground isoscalar $T^+_{cc}$ tetraquark with a quark content of $cc\bar{u}\bar{d}$ and spin-parity quantum numbers $\mathrm{J}^{\mathrm{P}}=1^+$. Study of the $DD$ mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell $D^{*+}$ mesons is confirmed by the $D^0π^+$ mass distribution. The mass of the resonance and its coupling to the $D^{*}D$ system are analysed. Resonance parameters including the pole position, scattering length, effective range and compositeness are measured to reveal important information about the nature of the $T^+_{cc}$ state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.

hep-ex↗