LHCb Detector Research Articles

Evidence of a J/ψK_{S}^{0} Structure in B^{0}→J/ψϕK_{S}^{0} Decays.

An amplitude analysis of B^{0}→J/ψϕK_{S}^{0} decays is performed using proton-proton collision data, corresponding to an integrated luminosity of 9 fb^{-1}, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13TeV. Evidence with a significance of 4.0 standard deviations of a structure in the J/ψK_{S}^{0} system, named T_{ψs1}^{θ}(4000)^{0}, is seen, with its mass and width measured to be 3991_{-10}^{+12} _{-17}^{+9} MeV/c^{2} and 105_{-25}^{+29} _{-23}^{+17} MeV, respectively, where the first uncertainty is statistical and the second systematic. The T_{ψs1}^{θ}(4000)^{0} state is likely to be the isospin partner of the T_{ψs1}^{θ}(4000)^{+} state, previously observed in the J/ψK^{+} system of the B^{+}→J/ψϕK^{+} decay. When isospin symmetry for the charged and neutral T_{ψs1}^{θ}(4000) states is assumed, the signal significance increases to 5.4 standard deviations.

Spectroscopy at LHCb: experimental overview

The large dataset collected in proton-proton collisions at the LHC allows precision studies and discovery of new states in the field of conventional and exotic heavy-hadron spectroscopy; both are crucial to understand QCD dynamics and binding rules. This is possible due to high luminosity, high b/c-hadron production cross-section and the unique, dedicated design of the LHCb detector, optimised to reconstruct b/c-hadron decays. The latest results on conventional and exotic spectroscopy from LHCb are presented.

Measurement of the Time-Integrated CP Asymmetry in D^{0}→K^{-}K^{+} Decays.

The time-integrated CP asymmetry in the Cabibbo-suppressed decay D^{0}→K^{-}K^{+} is measured using proton-proton collision data, corresponding to an integrated luminosity of 5.7 fb^{-1} collected at a center-of-mass energy of 13TeV with the LHCb detector. The D^{0} mesons are required to originate from promptly produced D^{*+}→D^{0}π^{+} decays, and the charge of the companion pion is used to determine the flavor of the charm meson at production. The time-integrated CP asymmetry is measured to be A_{CP}(K^{-}K^{+})=[6.8±5.4±1.6]×10^{-4} where the first uncertainty is statistical and the second systematic. The direct CP asymmetries in D^{0}→K^{-}K^{+} and D^{0}→π^{-}π^{+} decays, a_{K^{-}K^{+}}^{d} and a_{π^{-}π^{+}}^{d}, are derived by combining A_{CP}(K^{-}K^{+}) with the time-integrated CP asymmetry difference, ΔA_{CP}=A_{CP}(K^{-}K^{+})-A_{CP}(π^{-}π^{+}), and other inputs, giving a_{K^{-}K^{+}}^{d}=(7.7±5.7)×10^{-4},a_{π^{-}π^{+}}^{d}=(23.2±6.1)×10^{-4},with a correlation coefficient corresponding to ρ=0.88. The compatibility of these results with CP symmetry is 1.4 and 3.8 standard deviations for D^{0}→K^{-}K^{+} and D^{0}→π^{-}π^{+} decays, respectively. This is the first evidence for direct CP violation in a specific D^{0} decay.

Observation of a Resonant Structure near the D_{s}^{+}D_{s}^{-} Threshold in the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} Decay.

An amplitude analysis of the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} decay is carried out to study for the first time its intermediate resonant contributions, using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13TeV. A near-threshold peaking structure, referred to as X(3960), is observed in the D_{s}^{+}D_{s}^{-} invariant-mass spectrum with significance greater than 12 standard deviations. The mass, width, and the quantum numbers of the structure are measured to be 3956±5±10 MeV, 43±13±8 MeV, and J^{PC}=0^{++}, respectively, where the first uncertainties are statistical and the second systematic. The properties of the new structure are consistent with recent theoretical predictions for a state composed of cc[over ¯]ss[over ¯] quarks. Evidence for an additional structure is found around 4140MeV in the D_{s}^{+}D_{s}^{-} invariant mass, which might be caused either by a new resonance with the 0^{++} assignment or by a J/ψϕ↔D_{s}^{+}D_{s}^{-} coupled-channel effect.

Test of Lepton Universality in b→sℓ^{+}ℓ^{-} Decays.

The first simultaneous test of muon-electron universality using B^{+}→K^{+}ℓ^{+}ℓ^{-} and B^{0}→K^{*0}ℓ^{+}ℓ^{-} 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 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.

First Observation of a Doubly Charged Tetraquark and Its Neutral Partner.

A combined amplitude analysis is performed for the decays B^{0}→D[over ¯]^{0}D_{s}^{+}π^{-} and B^{+}→D^{-}D_{s}^{+}π^{+}, 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 13TeV. The full data sample corresponds to an integrated luminosity of 9 fb^{-1}. Two new resonant states with masses of 2.908±0.011±0.020 GeV and widths of 0.136±0.023±0.013 GeV are observed, which decay to D_{s}^{+}π^{+} and D_{s}^{+}π^{-} respectively. The former state indicates the first observation of a doubly charged open-charm tetraquark state with minimal quark content [cs[over ¯]ud[over ¯]], and the latter state is a neutral tetraquark composed of [cs[over ¯]u[over ¯]d] quarks. Both states are found to have spin-parity of 0^{+}, and their resonant parameters are consistent with each other, which suggests that they belong to an isospin triplet.

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^{+} → J/ψK+K−π+π+π− is observed for the first time, and evidence for the {B}_c^{+} → J/ψ4π+3π− decay is found. The decay {B}_c^{+} → J/ψ3π+2π− is observed and the previous observation of the {B}_c^{+} → ψ(2S)π+π+π− decay is confirmed using the ψ(2S) → J/ψπ+π− decay mode. Ratios of the branching fractions of these four {B}_c^{+} decay channels are measured.

Search for D^{*}(2007)^{0} rightarrow mu ^{+} mu ^{-} in B^{-}rightarrow pi ^{-} mu ^{+} mu ^{-} decays

The very rare {{D} ^*} (2007)^0!rightarrow {mu ^+mu ^-} decay is searched for by analysing {{{B} ^-}} !rightarrow {{pi } ^-} {mu ^+mu ^-} decays. The analysis uses a sample of beauty mesons produced in proton–proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9text {,fb} ^{-1}. The signal signature corresponds to simultaneous peaks in the {mu ^+mu ^-} and {{pi } ^-} {mu ^+mu ^-} invariant masses. No evidence for an excess of events over background is observed and an upper limit is set on the branching fraction of the decay at mathcal{B}({{D} ^*} (2007)^0!rightarrow {mu ^+mu ^-} ) < 2.6times 10^{-8} at 90% confidence level. This is the first limit on the branching fraction of {{D} ^*} (2007)^0!rightarrow {mu ^+mu ^-} decays and the most stringent limit on {{D} ^*} (2007)^0 decays to leptonic final states. The analysis is the first search for a rare charm-meson decay exploiting production via beauty decays.

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 8.16TeV with the LHCb detector. The π^{0} production cross section is measured differentially in transverse momentum (p_{T}) for 1.5<p_{T}<10.0 GeV and in center-of-mass pseudorapidity (η_{c.m.}) regions 2.5<η_{c.m.}<3.5 (forward) and -4.0<η_{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.

Observation of a J/ψΛ Resonance Consistent with a Strange Pentaquark Candidate in B^{-}→J/ψΛp[over ¯] Decays.

An amplitude analysis of B^{-}→J/ψΛp[over ¯] decays is performed using 4400 signal candidates selected on a data sample of pp collisions recorded at center-of-mass energies of 7, 8, and 13TeV with the LHCb detector, corresponding to an integrated luminosity of 9 fb^{-1}. A narrow resonance in the J/ψΛ system, consistent with a pentaquark candidate with strangeness, is observed with high significance. The mass and the width of this new state are measured to be 4338.2±0.7±0.4 MeV and 7.0±1.2±1.3 MeV, where the first uncertainty is statistical and the second systematic. The spin is determined to be 1/2 and negative parity is preferred. Because of the small Q-value of the reaction, the most precise single measurement of the B^{-} mass to date, 5279.44±0.05±0.07 MeV, is obtained.

Measurement of tau _text {L} using the {B} _s^0 rightarrow J/psi eta decay mode

Using a proton–proton collision data sample collected by the LHCb detector and corresponding to an integrated luminosity of 5.7~text {fb}^{-1}, the lifetime of the light {{B} ^0_{s}} mass eigenstate, tau _{L}, is measured using the B^0_s rightarrow J/psi eta decay mode to be τL=1.445±0.016(stat)±0.008(syst)ps.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} \ au _{\ ext {L}} = 1.445 \\pm 0.016 \ ext {(stat)} \\pm 0.008 \ ext {(syst)} \\,\ ext {ps}. \\end{aligned}$$\\end{document}A combination of this result with a previous LHCb analysis using an independent dataset corresponding to 3 fb^{-1} of integrated luminosity gives τL=1.452±0.014±0.007±0.002ps,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} \ au _{\ ext {L}} = 1.452 \\pm 0.014 \\pm 0.007 \\pm 0.002 \\,\ ext {ps}, \\end{aligned}$$\\end{document}where the first uncertainty is statistical, the second due to the uncorrelated part of the systematic uncertainty and the third due to the correlated part of the systematic uncertainty.

Measurement of Υ production in pp collisions at sqrt{s} = 5 TeV

The production cross-sections of Υ mesons, namely Υ(1S), Υ(2S) and Υ(3S), in pp collisions at sqrt{s} = 5 TeV are measured with a data sample corresponding to an integrated luminosity of 9.13 ± 0.18 pb−1 collected by the LHCb detector. The Υ mesons are reconstructed in the decay mode Υ → μ+μ−. Double differential cross-sections times branching fractions, as functions of the transverse momentum pT and the rapidity y of the Υ mesons, are measured in the range pT< 20 GeV/c and 2.0 < y < 4.5. The results integrated over these pT and y ranges areσΥ1S×BΥ1S→μ+μ−=2101±33±83pb,σΥ2S×BΥ2S→μ+μ−=526±20±21pb,σΥ3S×BΥ3S→μ+μ−=242±16±10pb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sigma \\left(\ extrm{Y}(1S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(1S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=2101\\pm 33\\pm 83\\ \ extrm{pb}, $$$$ \\sigma \\left(\ extrm{Y}(2S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(2S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=526\\pm 20\\pm 21\\ \ extrm{pb}, $$$$ \\sigma \\left(\ extrm{Y}(3S)\\right)\ imes \\mathcal{B}\\left(\ extrm{Y}(3S)\ o {\\mu}^{+}{\\mu}^{-}\\right)=242\\pm 16\\pm 10\\ \ extrm{pb}, $$\\end{document}where the first uncertainties are statistical and the second are systematic. The ratios of cross-sections between measurements of two different Υ states and between measurements at different centre-of-mass energies are determined. The nuclear modification factor of Υ(1S) at sqrt{s} = 5 TeV is updated as well using the directly measured cross-section results from this analysis.

Search for the lepton-flavour violating decays B0→ K*0τ±μ∓

A first search for the lepton-flavour violating decays B0 → K*0τ±μ∓ is presented. The analysis is performed using a sample of proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 TeV between 2011 and 2018, corresponding to an integrated luminosity of 9 fb−1. No significant signal is observed, and upper limits on the branching fractions are determined to be mathcal{B}left({B}^0to {K}^{ast 0}{tau}^{+}{mu}^{-}right)<1.0(1.2)times {10}^{-5} and mathcal{B}left({B}^0to {K}^{ast 0}{tau}^{-}{mu}^{+}right)<8.2(9.8)times {10}^{-6} at the 90% (95%) confidence level.

Measurement of the {Lambda}_c^{+} to D0 production ratio in peripheral PbPb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

We report on a measurement of the {Lambda}_c^{+} to D0 production ratio in peripheral PbPb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV with the LHCb detector in the forward rapidity region 2 < y < 4.5. The {Lambda}_c^{+} (D0) hadrons are reconstructed via the decay channel {Lambda}_c^{+} → pK−π+ (D0→ K−π+) for 2 < pT< 8 GeV/c and in the centrality range of about 65–90%. The results show no significant dependence on pT, y or the mean number of participating nucleons. They are also consistent with similar measurements obtained by the LHCb collaboration in pPb and Pbp collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV. The data agree well with predictions from PYTHIA in pp collisions at sqrt{s} = 5 TeV but are in tension with predictions of the Statistical Hadronization model.

TORCH pattern recognition and particle identification performance

The TORCH detector aims to provide K/π (K/p) separation up to a momentum of about 10 (15) ▪ by measuring their time-of-flight at the LHCb detector. Prompt Cherenkov photons are produced in a quartz radiator bar of 10 mm thickness, and propagated via total internal reflection to the periphery of the detector, where they are focused onto an array of microchannel plate photomultipliers that measure the photon arrival time and position. Pattern recognition techniques are used to compare the likelihood that the detector image is due to a given particle hypothesis. Good performance is obtained even for very high detector occupancies.

Search for the lepton-flavour violating decays B0→ K*0μ±e∓ and {B}_s^0 → ϕμ±e∓

A search for the lepton-flavour violating decays B0 → K*0μ±e∓ and {B}_s^0 → ϕμ±e∓ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of 9 fb−1. No significant signals are observed and upper limits ofBB0→K∗0μ+e−<5.7×10−96.9×10−9,BB0→K∗0μ−e+<6.8×10−97.9×10−9,BB0→K∗0μ±e∓<10.1×10−911.7×10−9,BBs0→ϕμ±e∓<16.0×10−919.8×10−9\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\displaystyle \\begin{array}{c}\\mathcal{B}\\left({B}^0\ o {K}^{\\ast 0}{\\mu}^{+}{e}^{-}\\right)<5.7\ imes {10}^{-9}\\left(6.9\ imes {10}^{-9}\\right),\\\\ {}\\mathcal{B}\\left({B}^0\ o {K}^{\\ast 0}{\\mu}^{-}{e}^{+}\\right)<6.8\ imes {10}^{-9}\\left(7.9\ imes {10}^{-9}\\right),\\\\ {}\\mathcal{B}\\left({B}^0\ o {K}^{\\ast 0}{\\mu}^{\\pm }{e}^{\\mp}\\right)<10.1\ imes {10}^{-9}\\left(11.7\ imes {10}^{-9}\\right),\\\\ {}\\mathcal{B}\\left({B}_s^0\ o \\phi {\\mu}^{\\pm }{e}^{\\mp}\\right)<16.0\ imes {10}^{-9}\\left(19.8\ imes {10}^{-9}\\right)\\end{array}} $$\\end{document}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 → ϕμ±e∓ the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.

Amplitude analysis of the D+→ π−π+π+ decay and measurement of the π−π+ S-wave amplitude

An amplitude analysis of the D+→ π−π+π+ decay is performed with a sample corresponding to 1.5 fb−1 of integrated luminosity of pp collisions at a centre-of-mass energy sqrt{s} = 8 TeV collected by the LHCb detector in 2012. The sample contains approximately six hundred thousand candidates with a signal purity of 95%. The resonant structure is studied through a fit to the Dalitz plot where the π−π+ S-wave amplitude is extracted as a function of π−π+ mass, and spin-1 and spin-2 resonances are included coherently through an isobar model. The S-wave component is found to be dominant, followed by the ρ(770)0π+ and f2(1270)π+ components. A small contribution from the ω(782) → π−π+ decay is seen for the first time in the D+→ π−π+π+ decay.

Measurement of the Z boson production cross-section in proton-lead collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV

This article presents the first measurement of the differential Z-boson production cross-section in the forward region using proton-lead collisions with the LHCb detector. The dataset was collected at a nucleon-nucleon centre-of-mass energy of sqrt{s_{textrm{NN}}} = 8.16 TeV in 2016, corresponding to an integrated luminosity of 30.8 nb−1. The forward-backward ratio and the nuclear modification factors are measured together with the differential cross-section as functions of the Z boson rapidity in the centre-of-mass frame, the transverse momentum of the Z boson and a geometric variable ϕ*. The results are in good agreement with the predictions from nuclear parton distribution functions, providing strong constraining power at small Bjorken-x.

Exclusive ρ and J/Ψ photoproduction in ultraperipheral pO and OO collisions at energies available at the CERN Large Hadron Collider

Collisions with Oxygen ions at the LHC opens the possibility of investigating the description of the QCD dynamics in an unexplored regime, complementary to those performed in $pp$, $pPb$ and $PbPb$ collisions. In this paper we estimate the exclusive $\rho$ and $J/\Psi$ photoproduction in ultraperipheral $pO$ and $OO$ collisions at the Large Hadron Collider using the color dipole formalism and considering distinct models for the dipole - target scattering amplitude and different approaches for the modelling of the vector meson wave function. We present our predictions for the rapidity and transverse momentum distributions, as well as for the total cross sections considering the rapidity ranges covered by the ALICE and LHCb detectors. Such results indicate that a future experimental analysis of these final states is feasible and that its study can be useful to improve our understanding of the QCD dynamics at high energies.

Associated $$\phi $$ and $$J/\Psi $$ photoproduction in ultraperipheral PbPb collisions at the Large Hadron Collider and Future Circular Collider

In this paper we analyze the associated $\phi$ and $J/\Psi$ photoproduction in ultraperipheral $PbPb$ collisions through the double scattering mechanism for the energies of the Large Hadron Collider (LHC) and Future Circular Collider (FCC). Our results complement a previous analysis for the $\rho\rho$, $J/\Psi J/\Psi$ and $\rho J/\Psi$ production. We present our predictions for the total cross sections and rapidity distributions considering the rapidity ranges covered by the ALICE and LHCb detectors, which indicate that a future experimental analysis of $\phi J/\Psi$ final state is feasible. These results point out that the study of the double vector meson photoproduction in heavy ion collisions can be useful to constrain the double scattering mechanism and improve our understanding of the QCD dynamics at high energies.