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Bashir, S.

Publications and source records attributed to Bashir, S..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

First Determination of the Spin-Parity of Ξ c ( 3055 ) + , 0 Baryons

The Ξ b 0 ( − ) → Ξ c ( 3055 ) + ( 0 ) ( → D + ( 0 ) Λ ) π − decay chains are observed, and the spin-parity of Ξ c ( 3055 ) + ( 0 ) baryons is determined for the first time. The measurement is performed using proton-proton collision data at a center-of-mass energy of s = 13 TeV , corresponding to an integrated luminosity of 5.4 fb − 1 , recorded by the LHCb experiment between 2016 and 2018. The spin-parity of the Ξ c ( 3055 ) + ( 0 ) baryons is determined to be 3 / 2 + with a significance of more than 6.5 σ ( 3.5 σ ) compared to all other tested hypotheses. The up-down asymmetries of the Ξ b 0 ( − ) → Ξ c ( 3055 ) + ( 0 ) π − transitions are measured to be − 0.92 ± 0.10 ± 0.05 ( − 0.92 ± 0.16 ± 0.22 ), consistent with maximal parity violation, where the first uncertainty is statistical and the second is systematic. These results support the hypothesis that the Ξ c ( 3055 ) + ( 0 ) baryons correspond to the first D -wave λ -mode excitation of the Ξ c flavor triplet. © 2025 CERN, for the LHCb Collaboration 2025 CERN

Aaij, R.↗
DOE OSTI2024

Observation of muonic Dalitz decays of chib mesons and precise spectroscopy of hidden-beauty states

The decays of the χb1 (1P), χb2 (1P), χb1 (2P) and χb2 (2P) mesons into the Υ(1S)μ + μ − final state are observed with a high significance using proton-proton collision data collected with the LHCb detector and corresponding to an integrated luminosity of 9 fb −1 . The newly observed decays together with the Υ(2S) → Υ(1S)π + π − and Υ(3S) → Υ(2S)π+π− decay modes are used for precision measurements of the mass and mass splittings for the hidden-beauty states.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Constraints on the CKM angle γ from B ± → Dh ± decays using D → h ± h' ∓ 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 ± → Dh ± decays, where h is either a kaon or a pion. The neutral D meson decay is reconstructed in the three-body final states: K ± π ∓ π o ; π + π – π o ; K + K – π o and the suppressed π ± K ∓ π o combination. The mode where a large CP asymmetry is expected, B ± → [π ± K ∓ π o ] D K ± , 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 ± → [K ± π ∓ π o ] D K ± , is R ADS(K) = (1.27 ± 0.16 ± 0.02) × 10 –2 . Evidence for a large CP asymmetry is also seen: A ADS(K) = –0.38 ± 0.12 ± 0.02. Constraints on the CKM angle γ are calculated from the eleven reported observables.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Precision measurement of forward $\mathrm{Z}$ 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 $\mathrm{Z}$ →$μ^+μ^-$ 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 $σ(Z → μ^+μ^-)$ = 196.4 ± 0.2 ± 1.6 ± 3.9 pb, 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.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Study of the doubly charmed tetraquark $\mathrm{T}$ $^+_{cc}$

Quantum chromodynamics, the theory of the strong force, describes interactions of coloured quarks and gluons and the formation of hadronic matter. Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. Particles with an alternative quark content are known as exotic states. Here a study is reported of an exotic narrow state in the D 0 D 0 π + mass spectrum just below the D *+ D 0 mass threshold produced in proton-proton collisions collected with the LHCb detector at the Large Hadron Collider. 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 J 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 consistent with the observed D 0 π+ mass distribution. To analyse the mass of the resonance and its coupling to the D*D system, a dedicated model is developed under the assumption of an isoscalar axial-vector T$^{+}_{cc}$ state decaying to the D*D channel. Using this model, resonance parameters including the pole position, scattering length, effective range and compositeness are determined 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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Observation of an exotic narrow doubly charmed tetraquark

Conventional, hadronic matter consists of baryons and mesons made of three quarks and a quark–antiquark pair, respectively. Here, we report the observation of a hadronic state containing four quarks in the Large Hadron Collider beauty experiment. This so-called tetraquark contains two charm quarks, a \(\overline{{{{{u}}}}}\) and a \(\overline{{{{{d}}}}}\) quark. This exotic state has a mass of approximately 3,875 MeV and manifests as a narrow peak in the mass spectrum of D 0 D 0 π + mesons just below the D *+ D 0 mass threshold. The near-threshold mass together with the narrow width reveals the resonance nature of the state.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Search for massive long-lived particles decaying semileptonically at ${\sqrt{s}}=13\,\hbox {TeV}$

A search is performed for massive long-lived particles (LLPs) 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\!/c 2 is produced by gluon fusion and decays into two LLPs. The analysis covers LLP mass values from 10\,GeV\!/c 2 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\!/c 2 . 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\,fb -1 . 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.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Observation of $Λ^0_b$ → $D^+ρπ^-π^-$ and $Λ^0_b$ → $D^{*+}ρπ^-π^-$ decays

The multihadron decays $Λ^0_b$ → $D+ρπ-π-$ and $Λ^0_b$ → $D^*+ρπ-π-$ are observed in data corresponding to an integrated luminosity of 3 fb -1 , collected in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV by the LHCb detector. Using the decay $Λ^0_b$ → $Λ^+_cπ^+π^-π^-$ as a normalisation channel, the ratio of branching fractions is measured to be $\frac {\mathcal{B} ( Λ^0_b → D^+ρπ^-π^-)} {\mathcal{B} (Λ^0_b → Λ^0_cπ^+π^-π^-)}$ x $\frac{\mathcal{B} (D^+ → K^-π^+π^+)} {\mathcal{B}(Λ^o_c → ρK^-π^-)}$ = (%.35 ± 0.21 ± 0.16)%, where the first uncertainty is statistical and the second systematic. The ratio of branching fractions for the $Λ^0_b$ → $D^{*+}ρπ^-π^-$ and $Λ^0_b$ → $D^+ρπ^-π^-$ decays is found to be $\frac {\mathcal{B} ( Λ^0_b → D^{*+}ρπ^-π^-)} {\mathcal{B} (Λ^0_b → D^+ρπ^-π^-)}$ X ($\mathcal{B} (D^{*+}→ D^+π^0)$ + $\mathcal{B} (D^{*+}→ D^+γ)$) = (61.3 ± 4.3 ± 4.0)%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Searches for rare ${B}_s^0$ and $B^0$ decays into four muons

Searches for rare ${B}_s^0$ and $B^0$ decays into four muons are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb –1 . Direct decays and decays via light scalar and $J/ψ$ resonances are considered. No evidence for the six decays searched for is found and upper limits at the 95% confidence level on their branching fractions ranging between 1.8 x 10 –10 and 2.6 x 10 –9 are set.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Identification of charm jets at LHCb

The identification of charm jets is achieved at LHCb for data collected in 2015–2018 using a method based on the properties of displaced vertices reconstructed and matched with jets. The performance of this method is determined using a dijet calibration dataset recorded by the LHCb detector and selected such that the jets are unbiased in quantities used in the tagging algorithm. The charm-tagging efficiency is reported as a function of the transverse momentum of the jet. The measured efficiencies are compared to those obtained from simulation and found to be in good agreement.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Measurement of $χ_{c1}$(3872) production in proton-proton collisions at $\sqrt{s}$ = 8 and 13 TeV

The production cross-section of the $χ_{c1}$(3872) state relative to the $ψ(2S)$ meson is measured using proton-proton collision data collected with the LHCb experiment at centre-of-mass energies of $\sqrt{s}$ = 8 and 13 TeV, corresponding to integrated luminosities of 2.0 and 5.4 fb -1 , respectively. The two mesons are reconstructed in the $J/ψπ^+π^-$ final state. The ratios of the prompt and nonprompt $χ_{c1}$(3872) to $ψ(2S)$ production cross-sections are measured as a function of transverse momentum, $ρ_T$, and rapidity, y, of the $χ_{c1}$(3872) and $ψ(2S)$ states, in the kinematic range 4 < $ρ_T$ < 20 GeV/c and 2.0< y < 4.5. The prompt ratio is found to increase with $ρ_T$, independently of y. For the prompt component, the double ratio of the $χ_{c1}$(3872) and $ψ(2S)$ production cross-sections between 13 and 8 TeV is observed to be consistent with unity, independent of $ρ_T$ and centre-of-mass energy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗