Beauty meson decays to charmonuim-like states at LHCb
FFebruary 8, 2021
Beauty meson decays tocharmonuim-like states at LHCb
Tatiana Ovsiannikova † on behalf of the LHCb collaboration Institute for Theoretical and Experimental Physics, NRC Kurchatov Institute,B. Cheremushkinskaya 25, Moscow, 117218, Russia.
Abstract
The decays B → J / ψ π + π − K + K − are studied using a data set correspondingto an integrated luminosity of 9 fb − , collected with the LHCb detector in pro-ton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The decaysB → J / ψ K ∗ K ∗ and B → χ c1 (3872)K + K − , where the K + K − pair does not orig-inate from a φ meson, are observed for the first time. Precise measurements ofthe ratios of branching fractions between intermediate χ c1 (3872) φ , J / ψ K ∗ K ∗ , ψ (2S) φ and χ c1 (3872)K + K − states are reported. A structure, denoted as X(4740),is observed in the J / ψ φ mass spectrum with a significance in excess of 5.3 stan-dard deviation. In addition, the most precise single measurement of the mass ofthe B meson is performed. Talk given at 23th International Conference in Quantum Chromodynamics (QCD 20,35th anniversary), 27 - 30 November 2020, Montpellier - FR † E-mail:[email protected] a r X i v : . [ h e p - e x ] F e b i Introduction
Decays of beauty hadrons to final states with charmonia provide a unique laboratoryto study the properties of charmonia and charmonium-like states. A plethora of newcharmonium-like states has been observed in the decays of beauty mesons, such asthe χ c1 (3872) meson [1] and numerous tetraquark candidates [2–11]. The nature of manyexotic charmonium-like candidates remains unclear. A comparison of production rateswith respect to those of conventional charmonium states in decays of beauty hadrons canshed light on their production mechanisms.The reported results are based on the data samples collected by the LHCb experimentin proton-proton (pp) collisions at centre-of-mass energies √ s = 7 , + → J / ψ π + π − K + decays. Candidate B + → J / ψ π + π − K + decays are reconstructed using the J / ψ → µ + µ − decaymode. A loose pre-selection is applied, followed by a multivariate classifier based ona decision tree with gradient boosting [12]. C a nd i d a t e s / ( M e V / c ) C a nd i d a t e s / ( . M e V / c ) m J / ψπ + π − K + [GeV/ c ] m J / ψπ + π − [GeV/ c ] LHCb LHCb B + → X cc K + B + → (cid:0) J / ψπ + π − (cid:1) NR K + comb. X cc K + comb. bkg.total Figure 1: Distributions of the (left) J / ψ π + π − K + and (right) J / ψ π + π − mass for the selectedB + → ψ (3823)K + candidates (points with error bars) [12, 13]. The yields for the B + → J / ψ π + π − K + decays via intermediate χ c1 (3872) → J / ψ π + π − , ψ (3823) → J / ψ π + π − and ψ (2S) → J / ψ π + π − chains are determined using a simultaneousunbinned extended maximum-likelihood fit to the J / ψ π + π − K + mass and the J / ψ π + π − mass distributions. The fit is performed in the three non-overlapping regions around the ψ (3823), χ c1 (3872) and ψ (2S) masses. To improve the resolution on the J / ψ π + π − massand to eliminate a small correlation between m J / ψπ + π − K + and m J / ψπ + π − variables, the m J / ψπ + π − variable is computed using a kinematic fit that constrains the mass of the B + candidate to its known value [14]. The signal yields are determined to be 137 ±
26 eventsfor the B + → ψ (3823)K + decay which correspond to statistical significance above 5.1 σ .The fit to the mass distribution for the signal channel are shown in figure 1. Also thesignificant signal yield is observed for the B + → χ c1 (3872)K + decay 4230 ±
70 events which1llows the precise determination of the parameters of the χ c1 (3872) state, in particularfor the first time the non-zero width for the χ c1 (3872) state is observed with significancemore than 5 standard deviationsΓ χ c1 (3872) = 0 . +0 . − . ± .
21 MeV , where the first uncertainty is statistical and the second is systematic. The value ofthe Breit–Wigner width agrees well with the value from the analysis of a large sam-ple of χ c1 (3872) → J / ψ π + π − decays from the inclusive decays of beauty hadrons [15].The improved upper limit for the width of the ψ (3823) meson is found to beΓ ψ (3823) < . .
6) MeV for 90 (95)% C.L. The mass differences between ψ (3823), χ c1 (3872) and ψ (2S) mesons, δ m XY ≡ m X − m Y , are measured to be δ m χ c1 (3872) ψ (3823) = 47 . ± . ± .
13 MeV/ c , δ m ψ (3823) ψ (2S) = 137 . ± . ± .
14 MeV/ c , δ m ψ (3823) ψ (2S) = 185 . ± . ± .
03 MeV/ c , where the first uncertainty is statistical and the second is systematic. Using the mea-sured mass difference the binding energy of the χ c1 (3872) state is calculated to be δE = 0 . ± .
13 MeV. It is consistent with zero within uncertainties, that are currentlydominated by the uncertainty for the charged and neutral kaon mass measurements.The measured yields of the B + → χ c1 (3872)K + , B + → ψ (3823)K + and B + → ψ (2S)K + signal decays allow for a precise determination of the ratios of the branching fractions: B B + → ψ (3823)K + × B ψ (3823) → J / ψπ + π − B B + → χ c1 (3872)K + × B χ c1 (3872) → J / ψπ + π − = (3 . ± . ± . × − , B B + → ψ (3823)K + × B ( ψ (3823) → J / ψπ + π − ) B B + → ψ (2S)K + × B ψ (2S) → J / ψπ + π − = (1 . ± . ± . × − , B B + → χ c1 (3872)K + × B χ c1 (3872) → J / ψπ + π − B B + → ψ (2S)K + × B ψ (2S) → J / ψπ + π − = (3 . ± . ± . × − . → J / ψ π + π − K + K − decays The B → J / ψ π + π − K + K − decays are reconstructed using selection criteria based on kine-matics, particle identification and topology [16]. The yields of B → J / ψ π + π − K + K − decaysvia intermediate ψ (2S) → J / ψ π + π − and χ c1 (3872) → J / ψ π + π − chains are determined us-ing a three-dimensional unbinned extended maximum-likelihood fit to the J / ψ π + π − K + K − ,J / ψ π + π − and K + K − mass distributions. The fit is performed simultaneously in twoseparate regions corresponding to B → χ c1 (3872) φ and B → ψ (2S) φ signals as describedabove.The observed signal yield for the B → χ c1 (3872) φ decays is found to be 154 ± σ deviation. The fitto the mass distribution for the signal channel are shown in figure 2. Using the obtainedsignal yields for B → χ c1 (3872) φ and B → ψ (2S) φ channels and corresponding efficiencyratio the following branching fraction is calculated: B B → χ c1 (3872) φ × B χ c1 (3872) → J / ψπ + π − B B → ψ (2S) φ × B ψ (2S) → J / ψπ + π − = (2 . ± . ± . × − . .3 5.35 5.4 5.45020406080100 3.85 3.86 3.87 3.88 3.89 3.90102030405060 C a nd i d a t e s / ( M e V / c ) C a nd i d a t e s / ( M e V / c ) m J / ψπ + π − K + K + m J / ψπ + π − [GeV/ c ] [GeV/ c ] . < m J / ψπ + π − < .
880 GeV/ c . < m K + K − < .
03 GeV/ c . < m J / ψπ + π − K + K − < .
384 GeV/ c . < m K + K − < .
03 GeV/ c LHCb LHCbFigure 2: Distributions of the (left) J / ψ π + π − K + K − and (right) J / ψ π + π − mass for selectedB → χ c1 (3872) φ candidates (points with error bars) [16]. The red filled area corresponds to theB → χ c1 (3872) φ signal. The orange line is the total fit. The obtained value is found to be in a good agreement with the recent result by the CMScollaboration [17] but is more precise.The decay B → χ c1 (3872)K + K − where the K + K − pair does not originate from a φ meson, is studied using a sample of selected B → J / ψ π + π − K + K − signal decays. A two-dimensional unbinned extended maximum-likelihood fit is performed to the J / ψ π + π − andJ / ψ π + π − K + K − mass distributions. The yield of the B → χ c1 (3872)K + K − signal decays is378 ±
33, that is significantly large than the yield of the B → χ c1 (3872) φ decays, indicatinga large B → χ c1 (3872)K + K − contribution. The background-subtracted and efficiency-corrected K + K − mass distribution of the B → χ c1 (3872)K + K − candidates is shown infigure 3. The K + K − mass distribution for m K + K − > . c region cannot be describedby phase-space shape, and possibly contains contributions from the f (980), f (1270), f (1370) and f (cid:48) (1520) resonances decaying to a pair of kaons, as has been observed inB → J / ψ K + K − decays [18, 19]. Therefore a component that accounts for non-resonantB → χ c1 (3872)K + K − decays and decays via broad high-mass K + K − intermediate states,modelled by a product of a phase-space function for three-body B → χ c1 (3872)K + K − de-cays and a third-order polynomial function. An amplitude analysis of a larger datasample would be required to properly disentangle individual contributions. However,a narrow φ component can be separated from the non- φ components using an unbinnedmaximum-likelihood fit to the background-subtracted and efficiency-corrected K + K − massdistribution. The fraction of the B → χ c1 (3872)K + K − signal component is found to be(38 . ± . B B → χ c1 (3872)(K + K − ) non- φ B B → χ c1 (3872) φ × B φ → K + K − = 1 . ± . ± . . The yield of B → J / ψ K ∗ K ∗ decays is determined using a three-dimensional unbinnedextended maximum-likelihood fit to the J / ψ π + π − K + K − , K − π + and K + π − mass distribu-tions. Using the obtained signal yields and the corresponding efficiency ratio the branching3 - Y i e l d s / ( M e V / c ) m K + K + (cid:2) GeV/ c (cid:3) B → χ c1 (3872) φ B → χ c1 (3872)K + K − total LHCb
Figure 3: Background-subtracted K + K − mass distribution for selected B → χ c1 (3872)K + K − can-didates (points with error bars) [16]. A fit, described in the text, is overlaid. The red filledarea corresponds to the B → χ c1 (3872) φ signal. The blue dashed line corresponds to theB → χ c1 (3872)K + K − component. The orange line is the total fit. The expectation for phase-space simulated decays is shown as a green solid line. fraction ratio is calculated: B B → J / ψ K ∗ K ∗ × B ∗ → K + π − B B → ψ (2S) φ × B ψ (2S) → J / ψπ + π − × B φ → K + K − = 1 . ± . ± . . The J / ψ φ spectrum is studied using the B → J / ψ π + π − φ decays.The B → J / ψ π + π − φ candidates are determined with two-dimensional unbinnedextended maximum-likelihood fit to the J / ψ π + π − K − K + and K + K − mass distribu-tions. The background-subtracted J / ψ φ mass spectrum of selected B → J / ψ π + π − φ decays shown in figure 4. It shows a prominent structure at a mass around4 .
74 GeV/ c . No such structure is seen if the K + K − mass is restricted to the region of1 . < m K + K − < .
15 GeV/ c . This structure cannot be explained by B → χ c1 (3872) φ and B → ψ (2S) φ decays via a narrow intermediate ψ (2S) and χ c1 (3872) resonance sincecontributions from these decays are explicitly vetoed. No sizeable contributions fromdecays via other narrow charmonium states are observed in the background-subtractedJ / ψ π + π − mass spectrum. The φπ + π − spectrum exhibits significant deviations fromthe phase-space distribution, indicating possible presence of excited φ states, referred4 .5 4.6 4.7 4.8 4.9020406080100120 m J / ψ φ (cid:2) GeV/ c (cid:3) Y i e l d s / ( M e V / c ) LHCb B → X(4740) π + π − B → J / ψ π + π − φ total Figure 4: Background-subtracted J / ψ φ mass distribution for selected B → J / ψ π + π − φ candi-dates (points with error bars) [16]. The red filled area corresponds to the B → X (4740) π + π − signal. The orange line is the total fit. to as φ ∗ states hereafter. The decays B → J / ψ φ ∗ via intermediate φ (1680), φ (1850)or φ (2170) states [14] are studied using simulated samples and no peaking structuresare observed. Under the assumption that the observed structure, referred to as X(4740)hereafter, has a resonant nature, its mass and width are determined through an unbinnedextended maximum-likelihood fit to the background-subtracted J / ψ φ mass distributionin the range 4 . < m J / ψφ < .
90 GeV/ c . The fit result is superimposed in figure 4. Theobtained signal yield is 175 ±
39 events and corresponds to a statistical significance abovethe 5.3 σ . The mass and width for the X(4740) state are found to be m X (4740) = 4741 ± ± c , Γ X (4740) = 53 ± ±
11 MeV . The observed parameters qualitatively agree with those of the χ c1 (4700) state observedby the LHCb collaboration in an amplitude analysis of B + → J / ψ φ K + decays [8, 9].The obtained mass also agrees with the one expected for the 2 ++ cscs tetraquark state [20].The B decays to the J / ψ π + π − K + K − final states characterize the relatively small energyrelease allowing precise measurement of the B meson mass, The mass of the B mesonis determined from an unbinned extended maximum-likelihood fit to the ψ (2S)K + K − mass distribution for a sample of B → J / ψ π + π − K + K − decays with m K + K − < .
06 GeV/ c / ψ π + π − mass within a narrow region around the known mass of the ψ (2S) meson. The improvement in the B mass resolution and significant decrease of thesystematic uncertainties is achieved by imposing a constraint on the reconstructed massof the J / ψ π + π − system to a known mass of the ψ (2S) meson [14, 21]. The measured valueof the B meson mass is found to be m B = 5366 . ± . ± .
13 MeV/ c , that is the most precise single measurement of this quantity. This result is combinedwith other precise measurements by the LHCb collaboration using B → J / ψ φ [22],B → J / ψ φφ [23], B → χ c2 K + K − [24] and B → J / ψ pp [25] decays. The combined massis calculated accounting for correlations of systematic uncertainties between the measure-ments. The LHCb average for the mass of the B meson is found to be m LHCbB = 5366 . ± . ± .
09 MeV/ c . The comparison with previous measurements is presented in figure 5.
A study of B-meson decays B + → J / ψ π + π − K + and B → J / ψ π + π − K + K − is made usingpp collision data corresponding to an integrated luminosity of 1, 2 and 6 fb − , col-lected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 TeV, respec-tively [12, 16]. The reported results include the first observation of the non-zero widthof the χ c1 (3872) state; the most precise measurement of the masses of the χ c1 (3872) and ψ (3823) states; the first observation of the ψ (3823) → J / ψ π + π − , B + → ψ (3823)K + ,B → χ c1 (3872) (K + K − ) non- φ and B → J / ψ K ∗ K ∗ decays; the most precise measurementof the ratios of branching fractions of the B + and B mesons into the final states with χ c1 (3823) and ψ (3823) particles; the most precise single measurement of the B mesonmass and an observation of a new structure, denoted as a X(4740) state, in the J / ψ φ massspectrum. Acknowledgments
I would like to express my gratitude to the QCD20 organizers for the great conference.Also, I’m thankful for my colleagues from LHCb collaboration who helped with preparationof this talk.
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