arXiv subjects

G. Bassi

Publications and source records attributed to G. Bassi.

At least 109 records · Page 6Linked to original sources

arXiv2024

Study of $B_c^+ \rightarrow χ_c π^+$ decays

A study of $B_c^+ \rightarrow χ_c π^+$ decays is reported using proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9fb$^{-1}$. The decay $B_c^+ \rightarrow χ_{c2} π^+$ is observed for the first time, with a significance exceeding seven standard deviations. The relative branching fraction with respect to the $B_c^+ \rightarrow J/ψπ^+$ decay is measured to be $$ \frac{\mathcal{B}_{B_c^+ \rightarrow χ_{c2} π^+}} {\mathcal{B}_{B_c^+ \rightarrow J/ψπ^+}} = 0.37 \pm 0.06 \pm 0.02 \pm 0.01 , $$ where the first uncertainty is statistical, the second is systematic, and the third is due to the knowledge of the $χ_c \rightarrow J/ψγ$ branching fraction. No significant $B_c^+ \rightarrow χ_{c1} π^+$ signal is observed and an upper limit for the relative branching fraction for the $B_c^+ \rightarrow χ_{c1} π^+$ and $B_c^+ \rightarrow χ_{c2} π^+$ decays of $$ \frac{\mathcal{B}_{B_c^+ \rightarrow χ_{c1} π^+}} {\mathcal{B}_{B_c^+ \rightarrow χ_{c2} π^+}} < 0.49 $$ is set at the 90\% confidence level.

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arXiv2024

A search for rare $B \rightarrow D μ^+ μ^-$ decays

A search for rare $B \rightarrow D μ^+ μ^-$ decays is performed using proton-proton collision data collected by the LHCb experiment, corresponding to an integrated luminosity of 9 fb$^{-1}$. No significant signals are observed in the non-resonant $μ^+μ^-$ modes, and upper limits of $\mathcal{B}(B^0 \rightarrow \overline{D}^0 μ^+ μ^-) < 5.1 \times 10^{-8}$, $\mathcal{B}(B^+ \rightarrow D_s^+ μ^+ μ^-) < 3.2 \times 10^{-8}$, $\mathcal{B}(B_s^0 \rightarrow \overline{D}^0 μ^+ μ^-) < 1.6 \times 10^{-7}$ and $f_c/f_u \cdot \mathcal{B}(B_c^+ \rightarrow D_s^+ μ^+ μ^-) < 9.6 \times 10^{-8}$ are set at the 95\% confidence level, where $f_c$ and $f_u$ are the fragmentation fractions of a $B$ meson with a $c$ and $u$ quark respectively in proton-proton collisions. Each result is either the first such measurement or an improvement by three orders of magnitude on an existing limit. Separate upper limits are calculated when the muon pair originates from a $J/ψ\rightarrow μ^+ μ^-$ decay. The branching fraction of $B_c^+ \rightarrow D_s^+ J/ψ$ multiplied by the fragmentation-fraction ratio is measured to be $f_c/f_u \cdot \mathcal{B}(B_c^+ \rightarrow D_s^+ J/ψ) = (1.63 \pm 0.15 \pm 0.13) \times 10^{-5}$, where the first uncertainty is statistical and the second systematic.

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arXiv2024

Observation of Cabibbo-Suppressed Two-Body Hadronic Decays and Precision Mass Measurement of the $Ω_{c}^{0}$ Baryon

The first observation of the singly Cabibbo-suppressed $Ω_{c}^{0}\toΩ^{-}K^{+}$ and $Ω_{c}^{0}\toΞ^{-}π^{+}$ decays is reported, using proton-proton collision data at a center-of-mass energy of $13\,{\rm TeV}$, corresponding to an integrated luminosity of $5.4\,{\rm fb}^{-1}$, collected with the LHCb detector between 2016 and 2018. The branching fraction ratios are measured to be $\frac{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}K^{+})}{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}π^{+})}=[6.08\pm0.51({\rm stat})\pm0.40({\rm syst})]\%$, $\frac{\mathcal{B}(Ω_{c}^{0}\toΞ^{-}π^{+})}{\mathcal{B}(Ω_{c}^{0}\toΩ^{-}π^{+})}=[15.81\pm0.87({\rm stat})\pm0.44({\rm syst})\pm0.16({\rm ext})]\%$. In addition, using the $Ω_{c}^{0}\toΩ^{-}π^{+}$ decay channel, the $Ω_{c}^{0}$ baryon mass is measured to be $M(Ω_{c}^{0})=2695.28\pm0.07({\rm stat})\pm0.27({\rm syst})\pm0.30({\rm ext})\,{\rm MeV}$, improving the precision of the previous world average by a factor of 4.

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arXiv2024

A model-independent measurement of the CKM angle $γ$ in partially reconstructed $B^{\pm} \to D^{*} h^{\pm}$ decays with $D \to K_{S}^{0} h^{+}h^{-}$ $(h=π, K)$

A measurement of $C\!P$-violating observables in $B^{\pm} \to D^{*} K^{\pm}$ and $B^{\pm} \to D^{*} π^{\pm}$ decays is made where the photon or neutral pion from the $D^{*} \to Dγ$ or $D^{*} \to Dπ^{0}$ decay is not reconstructed. The $D$ meson is reconstructed in the self-conjugate decay modes, $D \to K_{S}^{0} π^{+} π^{-}$ or $D \to K_{S}^{0} K^{+} K^{-}$. The distribution of signal yields in the $D$ decay phase space is analysed in a model-independent way. The measurement uses a data sample collected in proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of approximately 9 fb$^{-1}$. The $B^{\pm} \to D^{*} K^{\pm}$ and $B^{\pm} \to D^{*} π^{\pm}$ $C\!P$-violating observables are interpreted in terms of hadronic parameters and the CKM angle $γ$, resulting in a measurement of $γ= (92^{+21}_{-17})^{\circ}$. The total uncertainty includes the statistical and systematic uncertainties, and the uncertainty due to external strong-phase inputs.

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arXiv2024

Enhanced production of $Λ_{b}^{0}$ baryons in high-multiplicity $pp$ collisions at $\sqrt{s} = 13$ TeV

The production rate of $Λ_{b}^{0}$ baryons relative to $B^{0}$ mesons in $pp$ collisions at a center-of-mass energy $\sqrt{s} = 13$ TeV is measured by the LHCb experiment. The ratio of $Λ_{b}^{0}$ to $B^{0}$ production cross-sections shows a significant dependence on both the transverse momentum and the measured charged-particle multiplicity. At low multiplicity, the ratio measured at LHCb is consistent with the value measured in $e^{+}e^{-}$ collisions, and increases by a factor of $\sim2$ with increasing multiplicity. At relatively low transverse momentum, the ratio of $Λ_{b}^{0}$ to $B^{0}$ cross-sections is higher than what is measured in $e^{+}e^{-}$ collisions, but converges with the $e^{+}e^{-}$ ratio as the momentum increases. These results imply that the evolution of heavy $b$ quarks into final-state hadrons is influenced by the density of the hadronic environment produced in the collision. Comparisons with several models and implications for the mechanisms enforcing quark confinement are discussed.

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arXiv2024

Improved measurement of $CP$ violation parameters in $B^0_s\to J/ψK^{+}K^{-}$ decays in the vicinity of the $ϕ(1020)$ resonance

The decay-time-dependent $CP$ asymmetry in $B^0_s\to J/ψ(\to μ^{+}μ^{-}) K^{+}K^{-}$ decays is measured using proton-proton collision data, corresponding to an integrated luminosity of 6 $fb^{-1}$, collected with the LHCb detector at a center-of-mass energy of 13 TeV. Using a sample of approximately 349 000 $B^{0}_{s}$ signal decays with an invariant $K^{+}K^{-}$ mass in the vicinity of the $ϕ(1020)$ resonance, the $CP$-violating phase $ϕ_s$ is measured, along with the difference in decay widths of the light and heavy mass eigenstates of the $B^0_s$-$\bar{B}^0_s$ system, $ΔΓ_s$, and the difference of the average $B^0_s$ and $B^0$ meson decay widths, $Γ_s-Γ_d$. The values obtained are $ϕ_s = \ -0.039 \pm 0.022 \pm 0.006$ rad, $ΔΓ_s = 0.0845 \pm 0.0044 \pm 0.0024 $ ps$^{-1}$ and $Γ_s-Γ_d = -0.0056 ^{+ 0.0013}_{-0.0015} \pm 0.0014$ ps$^{-1}$, where the first uncertainty is statistical and the second systematic. These are the most precise single measurements to date and are consistent with expectations based on the Standard Model and with the previous LHCb analyses of this decay. These results are combined with previous independent LHCb measurements. The phase $ϕ_s$ is also measured independently for each polarization state of the $K^{+}K^{-}$ system and shows no evidence for polarization dependence.

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arXiv2024

Open charm production and asymmetry in $p$Ne collisions at $\sqrt{s_{\scriptscriptstyle\rm NN}} =$ 68.5 GeV

A measurement of $D^0$ meson production by the LHCb experiment in its fixed-target configuration is presented. The production of $D^0$ mesons is studied with a beam of 2.5 TeV protons colliding on a gaseous neon target at rest, corresponding to a nucleon-nucleon centre-of-mass energy of $\sqrt{s_{\rm NN}}$ = 68.5 GeV. The sum of the $D^0$ and ${\overline D^0}$ production cross-section in $p$Ne collisions in the centre-of-mass rapidity range $y^{\star}\in [-2.29, 0]$ is found to be $σ_{D^{0}}^{y^\star \in [-2.29, 0]} = 48.2 \pm 0.3 \pm 4.5 \,μ\textrm{b/nucleon}$ where the first uncertainty is statistical and the second is systematic. The $D^0-{\overline D^0}$ production asymmetry is also evaluated and suggests a trend towards negative values at large negative $y^{\star}$. The considered models do not account precisely for all the features observed in the LHCb data, but theoretical predictions including 1$\%$ intrinsic charm and 10$\%$ recombination contributions better describe the data than the other models considered.

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arXiv2024

Momentum scale calibration of the LHCb spectrometer

For accurate determination of particle masses accurate knowledge of the momentum scale of the detectors is crucial. The procedure used to calibrate the momentum scale of the LHCb spectrometer is described and illustrated using the performance obtained with an integrated luminosity of $1.6~ fb^{-1}$ collected during 2016 in $pp$ running. The procedure uses large samples of $J/ψ\rightarrow μ^+ μ^-$ and $B^+ \rightarrow J/ψK^+$ decays and leads to a relative accuracy of $3 \times 10^{-4}$ on the momentum scale.

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arXiv2024

Helium identification with LHCb

The identification of helium nuclei at LHCb is achieved using a method based on measurements of ionisation losses in the silicon sensors and timing measurements in the Outer Tracker drift tubes. The background from photon conversions is reduced using the RICH detectors and an isolation requirement. The method is developed using $pp$ collision data at $\sqrt{s}=13\,{\rm TeV}$ recorded by the LHCb experiment in the years 2016 to 2018, corresponding to an integrated luminosity of $5.5\,{\rm fb}^{-1}$. A total of around $10^5$ helium and antihelium candidates are identified with negligible background contamination. The helium identification efficiency is estimated to be approximately $50\%$ with a corresponding background rejection rate of up to $\mathcal O(10^{12})$. These results demonstrate the feasibility of a rich programme of measurements of QCD and astrophysics interest involving light nuclei.

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arXiv2024

Measurement of prompt $D^+$ and $D^+_{s}$ production in $p\mathrm{Pb}$ collisions at $\sqrt {s_{\mathrm{NN}}}=5.02\,$TeV

The production of prompt $D^+$ and $D^+_{s}$ mesons is studied in proton-lead collisions at a centre-of-mass energy of $\sqrt {s_{\mathrm{NN}}}=5.02\,$TeV. The data sample corresponding to an integrated luminosity of $(1.58\pm0.02)\mathrm{nb}^{-1}$ is collected by the LHCb experiment at the LHC. The differential production cross-sections are measured using $D^+$ and $D^+_{s}$ candidates with transverse momentum in the range of $0<p_{\mathrm{T}} <14\,\mathrm{GeV}/c$ and rapidities in the ranges of $1.5<y^*<4.0$ and $-5.0<y^*<-2.5$ in the nucleon-nucleon centre-of-mass system. For both particles, the nuclear modification factor and the forward-backward production ratio are determined. These results are compared with theoretical models that include initial-state nuclear effects. In addition, measurements of the cross-section ratios between $D^+$, $D^+_{s}$ and $D^0$ mesons are presented, providing a baseline for studying the charm hadronization in lead-lead collisions at LHC energies.

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arXiv2024

Measurement of CP violation in $B^0\toψ(\to\ell^+\ell^-)K^0_S(\toπ^+π^-)$ decays

A measurement of time-dependent CP violation in the decays of $B^0$ and $\overline{B}^0$ mesons to the final states $J/ψ(\toμ^+μ^-)K^0_S$, $ψ(2S)(\toμ^+μ^-)K^0_S$ and $J/ψ(\to e^+e^-)K^0_S$ with $K^0_S\toπ^+π^-$ is presented. The data correspond to an integrated luminosity of 6 fb${}^{-1}$ collected at a center-of-mass energy of $\sqrt{s}=13$ TeV with the LHCb detector. The CP-violation parameters are measured to be \begin{align*} S_{ψK^0_S} &= 0.717 \pm 0.013 (\text{stat}) \pm 0.008 (\text{syst}), \\ C_{ψK^0_S} &= 0.008 \pm 0.012 (\text{stat}) \pm 0.003 (\text{syst}). \end{align*} This measurement of $S_{ψK^0_S}$ represents the most precise single measurement of the CKM angle $β$ to date and is more precise than the current world average. In addition, measurements of the CP-violation parameters of the individual channels are reported and a combination with the LHCb Run 1 measurements is performed.

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arXiv2023

Observation of the $B^0_s\rightarrow χ_{c1}(3872)π^+π^-$ decay

The first observation of the $B^0_s \rightarrow \left( χ_{c1}(3872) \rightarrow J/ψπ^+π^-\right) π^+ π^-$ decay is reported using proton-proton collision data, corresponding to integrated luminosities of 1, 2 and 6fb$^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13TeV, respectively. The ratio of branching fractions relative to the $B^0_s \rightarrow \left( ψ(2S) \rightarrow Jψπ^+π^- \right) π^+ π^-$ decay is measured to be $$ \frac{ \mathcal{B} \left( B^0_s \rightarrow χ_{c1}(3872) π^+π^-\right) \times \mathcal{B} \left( χ_{c1}(3872) \rightarrow Jψπ^+π^-\right)} { \mathcal{B} \left( B^0_s \rightarrow ψ(2S) π^+ π^- \right) \times \mathcal{B} \left( ψ(2S) \rightarrow Jψπ^+π^-\right) } = \left( 6.8 \pm 1.1 \pm 0.2 \right) \times 10^{-2} , $$ where the first uncertainty is statistical and the second systematic. The mass spectrum of the $π^+π^-$ system recoiling against the $χ_{c1}(3872)$ meson exhibits a large contribution from $B^0_s \rightarrow χ_{c1}(3872) \left( f_0(980) \rightarrow π^+ π^-\right)$ decays.

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arXiv2023

Study of $B_c^+$ meson decays to charmonia plus multihadron final states

Four decay modes of the $B_c^+$ meson into a $J/ψ$ meson and multiple charged kaons or pions are studied using proton-proton collision data, collected with the~LHCb detector at centre-of-mass energies of 7, 8, and 13~TeV and corresponding to an integrated luminosity of $9$~fb$^{-1}$. The decay $B_c^+\to J/ψK^+ K^- π^+ π^+ π^-$ is observed for the first time, and evidence for the $B_c^+\to J/ψ4π^+ 3π^-$ decay is found. The decay $B_c^+\to J/ψ3π^+ 2π^-$ is observedand and the previous observation of the $B_c^+\toψ(2S) π^+ π^+ π^-$ decay is confirmed using the $ψ(2S) \to J/ψπ^+ π^-$ decay mode. Ratios of the branching fractions of these four $B_c^+$ decay channels are measured.

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arXiv2023

Observation of the $B^+ \rightarrow J/ψη^{\prime} K^+$ decay

The $B^+ \rightarrow J/ψη^{\prime} K^+$ decay is observed for the first time using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13TeV, corresponding to a total integrated luminosity of 9fb$^{-1}$. The branching fraction of this decay is measured relative to the known branching fraction of the $B^+ \rightarrow ψ(2S) K^+$ decays and found to be $$ \frac{\mathcal{B}( B^+ \rightarrow J/ψη^{\prime}K^+)}{\mathcal{B}( B^+ \rightarrow ψ(2S)K^+)} = \left(4.91\pm 0.47\pm0.29\pm0.07\right)\times10^{-2}, $$ where the first uncertainty is statistical, the second is systematic and the third is related to external branching fractions. A first look at the $J/ψη^{\prime}$ mass distribution is performed and no signal of intermediate resonances is observed.

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arXiv2023

Measurement of lepton universality parameters in $B^+\to K^+\ell^+\ell^-$ and $B^0\to K^{*0}\ell^+\ell^-$ decays

A simultaneous analysis of the $B^+\to K^+\ell^+\ell^-$ and $B^0\to K^{*0}\ell^+\ell^-$ decays is performed to test muon-electron universality in two ranges of the square of the dilepton invariant mass, $q^2$. The measurement uses a sample of beauty meson decays produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of $9$ $\text{fb}^{-1}$. A sequence of multivariate selections and strict particle identification requirements produce a higher signal purity and a better statistical sensitivity per unit luminosity than previous LHCb lepton universality tests using the same decay modes. Residual backgrounds due to misidentified hadronic decays are studied using data and included in the fit model. Each of the four lepton universality measurements reported is either the first in the given $q^2$ interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.

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arXiv2023

First observation of the $B^+ \rightarrow D_s^+ D_s^- K^+$ decay

The $B^+ \rightarrow D_s^+ D_s^- K^+$ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at centre-of-mass energies of $7$, $8$ and $13\, \text{TeV}$, corresponding to an integrated luminosity of $9\,\text{fb}^{-1}$. Its branching fraction relative to that of the $B^{+} \rightarrow D^{+} D^{-} K^{+}$ decay is measured to be $$\frac{B\left(B^{+} \rightarrow D_s^{+} D_s^{-} K^{+}\right)}{B\left(B^{+} \rightarrow D^{+} D^{-} K^{+}\right)}=0.525 \pm 0.033 \pm 0.027 \pm 0.034,$$ where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the $D_s^{\pm} \rightarrow K^{\mp} K^{\pm} π^{\pm}$ and $D^{\pm} \rightarrow K^{\mp} π^{\pm} π^{\pm}$ decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo$-$favoured $\bar{b} \rightarrow \bar{c} c \bar{s}$ transitions and opens the path for unique studies of spectroscopy in future.

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arXiv2023

Test of lepton universality in $b \rightarrow s \ell^+ \ell^-$ decays

The first simultaneous test of muon-electron universality using $B^{+}\rightarrow K^{+}\ell^{+}\ell^{-}$ and $B^{0}\rightarrow K^{*0}\ell^{+}\ell^{-}$ decays is performed, in two ranges of the dilepton invariant-mass squared, $q^{2}$. The analysis uses beauty mesons produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 $\mathrm{fb}^{-1}$. Each of the four lepton universality measurements reported is either the first in the given $q^{2}$ interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.

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arXiv2023

Fraction of $χ_c$ decays in prompt $J/ψ$ production measured in pPb collisions at $\sqrt{s_{NN}}=8.16$ TeV

The fraction of $χ_{c1}$ and $χ_{c2}$ decays in the prompt $J/ψ$ yield, $F_{χc}=σ_{χ_c \to J/ψ}/σ_{J/ψ}$, is measured by the LHCb detector in pPb collisions at $\sqrt{s_{NN}}=8.16$ TeV. The study covers the forward ($1.5<y^*<4.0$) and backward ($-5.0<y^*<-2.5$) rapidity regions, where $y^*$ is the $J/ψ$ rapidity in the nucleon-nucleon center-of-mass system. Forward and backward rapidity samples correspond to integrated luminosities of 13.6 $\pm$ 0.3 nb$^{-1}$ and 20.8 $\pm$ 0.5 nb$^{-1}$, respectively. The result is presented as a function of the $J/ψ$ transverse momentum $p_{T,J/ψ}$ in the range 1$<p_{T, J/ψ}<20$ GeV/$c$. The $F_{χc}$ fraction at forward rapidity is compatible with the LHCb measurement performed in $pp$ collisions at $\sqrt{s}=7$ TeV, whereas the result at backward rapidity is 2.4 $σ$ larger than in the forward region for $1<p_{T, J/ψ}<3$ GeV/$c$. The increase of $F_{χc}$ at low $p_{T, J/ψ}$ at backward rapidity is compatible with the suppression of the $ψ$(2S) contribution to the prompt $J/ψ$ yield. The lack of in-medium dissociation of $χ_c$ states observed in this study sets an upper limit of 180 MeV on the free energy available in these pPb collisions to dissociate or inhibit charmonium state formation.

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