SStudies of b-hadrons at LHCb
Viacheslav Matiunin 𝒂, , ∗ 𝑎 Institute for Theoretical and Experimental Physics, NRC «Kurchatov Institute»,B. Cheremushkinskaya st. 25, Moscow, 117218, Russia
E-mail: [email protected]
A large data set collected at the LHCb experiment in proton-proton collisions during Runs 1and 2 of the Large Hadron Collider has opened a unique possibility to study heavy beautyhadron states and to broaden knowledge of their spectroscopy and production. Recent results onsearches for new excited b-hadron states, and studies of b-hadron production, are reviewed. Inparticular, the observation of new excited Ω − b states, the observation of two new narrow Λ b ( ) and Λ b ( ) states, the observation of new Λ ∗∗ state consistent with the Λ b ( ) prediction, aprecise measurement of the mass and width of Λ b ( ) and Λ b ( ) states, and a measurementof the B − c meson production fraction and B − c − B + c production asymmetry in 7 and 13 TeV of proton-proton collisions are presented. On behalf of the LHCb Collaboration. ∗ Speaker a r X i v : . [ h e p - e x ] F e b tudies of b-hadrons at LHCb Viacheslav Matiunin
1. Introduction
Over the last few years numerous outstanding results have been obtained within the analyses ofb-hadrons. Among those there are observation of pentaquark and tetraquark resonances, observationof series of excited heavy baryons and many others. Still, many of the conventional states remainunobserved, parameters of some of the known hadron states are poorly measured and on top of thatthere are a number of the states which do not fit into the conventional quarkonium spectrum. Hencestudy of b-hadrons spectroscopy is of particular interest in the modern high energy physics.The results described below are based on the data samples collected by the LHCb experimentin proton-proton (pp) collision at the Large Hadron Collider (LHC) with centre-of-mass energies √ s = − (LHC Run 1) and withcentre-of-mass energy √ s =
13 TeV corresponding to a total integrated luminosity of 6 fb − (LHCRun 2).
2. Observation of excited 𝛀 − b states Recently, the new five narrow excited Ω baryons has been observed [1]. Some of the theoreticalmodels describing Ω peaks also predict Ω − b states decaying to Ξ K − final state. Therefore, it isof great interest to search for the analogous excited states with the LHCb experiment data. The Ξ K − mass spectrum is studied to search for narrow resonances close to the kinematic threshold [2].The analysis is done using pp collision data samples corresponding to an integrated luminosity of9 . − (LHC Run 1 and 2). The Ξ baryons are reconstructed using Ξ + c π − decay mode, where Ξ + c baryons are reconstructed with the pK − π + final state. The distributions of right-sign combinationmass difference M ( Ξ K − ) − M ( Ξ ) and wrong-sign combination mass difference M ( Ξ K + ) − M ( Ξ ) are shown in Fig. 1 (a) and (b), respectively. Four narrow peaks are seen in right-sign distribution.Simultaneous fit to wrong-sign and right-sign spectra with common background shape is performed.The peak parameters obtained from the fit results together with the significance of the peaks areshown in Table 1. The mass and width of the new resonances are consistent with expectations forexcited Ω − b states. ) [MeV] X ( M ) - - K X ( M
500 550 600 650 700 N u m be r o f c and i da t e s / M e V LHCb
DataFull fitSignalsBackground (a) ) [MeV] X ( M ) - + K X ( M
500 550 600 650 700 N u m be r o f c and i da t e s / M e V LHCb
DataFull fitBackground (b)
Figure 1:
The distributions of (a) right-sign M ( Ξ K − ) − M ( Ξ ) and (b) wrong-sign M ( Ξ K + ) − M ( Ξ ) massdifferences. Different components employed in the fit are indicated in the legend. tudies of b-hadrons at LHCb Viacheslav Matiunin
Table 1:
The masses, 90% (95%) confidence level upper limits on the natural widths and global (local)significance for the four peaks. The uncertainties are statistical, systematic, and due to the world-averagevalue of the Ξ mass (for the masses). For the Ω b ( ) − peak, the central value of the width is also indicated. Mass [MeV] Width [MeV] Significance [ 𝜎 ] Ω b ( ) − . ± . ± . ± . < . ( . ) Ω b ( ) − . ± . ± . ± . < . ( . ) Ω b ( ) − . ± . ± . ± . < . ( . ) Ω b ( ) − . ± . ± . ± . < . ( . ) . + . − . ± .
3. Excited 𝚲 baryons Beyond the lightest beauty baryon, Λ baryon, a rich spectrum of radially and orbitally excitedstates is expected at higher masses. The excited states has been searched in the Λ π + π − spectrum bythe LHCb experiment with the discovery of two narrow states [3]. Therefore, it is both interesting andpromising to study the Λ π + π − spectrum with the full statistics collected by the LHCb experiment.The Λ π + π − mass spectrum is studied by the LHCb experiment using the full LHC Run 1 and 2data sample of pp collisions, corresponding to an integrated luminosity of 9 fb − [4, 5]. The Λ baryon is reconstructed using two decay modes Λ + c π − and J / ψ pK − . The study is performed inthree Λ π + π − mass intervals: high-, middle- and low-mass. In the high Λ π + π − mass interval a newpeaking structure at approximately 6 .
15 GeV is observed [4]. The peak is above Σ (∗)∓ b 𝜋 ± threshold,hence, spectrum is investigated in three non-overlapping Λ π ± mass regions: two resonant Σ ∓ b and Σ ∗∓ b regions and nonresonant one. Simultaneous fit to the three Λ π + π − mass distributions isperformed. The two-peak hypothesis is favoured with respect to the single-peak hypothesis withmore than 7 𝜎 statistical significance. The Λ π + π − mass distributions are shown in Fig. 2 (a). Mass Λ b (6152) Λ b (6146) backgroundtotalLHCbΣ b regionΣ ∗ b regionNR region C a nd i d a t e s / ( M e V ) m Λ b π + π − [GeV] (a) C a nd i d a t e s / M e V M ( Λ π + π − ) [ GeV ] Λ ∗∗ b Λ b (6146) Λ b (6152) Σ b π Σ ∗ b π comb. backgroundtotal backgroundtotal C a nd i d a t e s / ( M e V ) m Λ b ππ [GeV]Λ b → Λ + c π − Λ b π + π − Λ b π + π + Λ b π − π − LHCb (b) C a nd i d a t e s / . M e V M ( Λ ππ ) [ GeV ] Λ ∗∗ b Λ b (6146) Λ b (6152) Σ b π Σ ∗ b π comb. backgroundtotal backgroundtotal C a nd i d a t e s / ( M e V ) m Λ b ππ [GeV]Λ b → J/ψpK − Λ b π + π − Λ b π + π + Λ b π − π − LHCb (c) C a nd i d a t e s / . M e V M ( Λ ππ ) [ GeV ] Figure 2: (a) Mass distributions of Λ π + π − combination for the three regions in Λ π ± mass: (top) Σ ∓ b ,(middle) Σ ∗∓ b and (bottom) nonresonant region. Mass spectra of (top) Λ π + π − , (middle) Λ π + π + and(bottom) Λ π + π + combinations for the (b) Λ → Λ + c π − and (c) Λ → J / ψ pK − sample. Different componentsemployed in the fit are indicated in the legend. tudies of b-hadrons at LHCb Viacheslav Matiuninand width of the two peaks areM Λ b ( ) = . ± . ± . ± .
16 MeV , Γ Λ b ( ) = . ± . ± . , (1)M Λ b ( ) = . ± . ± . ± .
16 MeV , Γ Λ b ( ) = . ± . ± . , (2)here and throughout the Section the first uncertainty is statistical, the second one is systematicand third one is due to the uncertainty in the nominal Λ baryon mass. The measured masses areconsistent with predictions for Λ b ( ) doublet with J P = + and + [6, 7].In the intermediate Λ π + π − mass interval a new baryon state is observed [5]. The simultaneousfit to the six mass distributions where Λ baryon is reconstructed via two different decay modesand in each case for one right-sign Λ π + π − and two wrong-sign Λ π ± π ± spectra is performed. The Λ π + π − mass distributions are shown in Fig. 2 (b) and (c). Mass and width of the new peak areM Λ ∗∗ = . ± . ± . ± . , Γ Λ ∗∗ = ± ± . (3)The observed peak is consistent with broad excess reported by the CMS experiment [8]. The mea-sured mass and width are in agreement with expectations for the Λ b ( ) state. The contributionsfrom Σ (∗)± b resonances are also studied and the nonresonant component is found to give a dominantcontribution.In the low Λ π + π − mass interval the two states observed earlier [3] are confirmed. Thesimultaneous fit to the six mass distributions where Λ baryon is reconstructed via two differentdecay modes and in each case for one right-sign Λ π + π − and two wrong-sign Λ π ± π ± spectra isperformed. The mass spectra are shown in Fig. 3. The mass and width of the states are measuredto be M Λ b ( ) = . ± . ± . ± .
21 MeV , Γ Λ b ( ) < . ( . ) MeV , (4)M Λ b ( ) = . ± . ± . ± .
21 MeV , Γ Λ b ( ) < . ( . ) MeV . (5)For the natural widths the upper limits at 90% ( ) confidence level is specified. The parametersare measured with about four times higher precision with respect to those reported in Ref. [3]. Λ b (5912) Λ b (5920) backgroundtotal C a nd i d a t e s / ( . M e V ) m Λ b ππ [GeV]Λ b → Λ + c π − Λ b π + π − Λ b π + π + Λ b π − π − LHCb (a) C a nd i d a t e s / . M e V M ( Λ ππ ) [ GeV ] Λ b (5912) Λ b (5920) backgroundtotal C a nd i d a t e s / ( . M e V ) m Λ b ππ [GeV]Λ b → J/ψpK − Λ b π + π − Λ b π + π + Λ b π − π − LHCb (b) C a nd i d a t e s / . M e V M ( Λ ππ ) [ GeV ] Figure 3:
Mass spectra of (top) Λ π + π − , (middle) Λ π + π + and (bottom) Λ π + π + combinations for the(a) Λ → Λ + c π − and (b) Λ → J / ψ pK − sample. Different components employed in the fit are indicated inthe legend. tudies of b-hadrons at LHCb Viacheslav Matiunin
4. Measurement of the B − c meson production fraction and asymmetry inpp collisions at 7 and 13 TeV The b-hadrons cross-sections as functions of transverse momentum ( 𝑝 T ) and pseudorapidity ( 𝜂 )are predicted using non-relativistic quantum chromodynamics along with fragmentation functions.The corresponding literature is systematically reviewed in the Ref. [9]. The measurement of theB − c production fraction would allow to further probe quantum chromodynamics.The production fraction ratio f c f u + f d and the B − c − B + c production asymmetry are measured bythe LHCb experiment [10]. The analysis is done using data samples of 7 and 13 TeV pp collisionscorresponding to an integrated luminosity of 1 . . − , respectively. The B mesons inthe analysis are reconstructed using the B − c → J / ψ µ − ν µ , B → D + X µ − ν µ and B − → D X µ − ν µ decay modes. The X symbol is used here and throughout to refer to any additional undetectedparticles. The production fractions are obtained as a function of 𝑝 T and 𝜂 of the B mesons. Themeasured production fraction for the 7 TeV data sample is shown in Fig. 4 (a) and (b). There is asmall dependence on the transverse momentum, but no dependence on 𝜂 is observed. The ratio ofproduction fractions averaged over 𝑝 T and 𝜂 of the B mesons are measured to bef c f u + f d = ( . ± . ± . ± . ) × − for 7 TeV , (6)f c f u + f d = ( . ± . ± . ± . ) × − for 13 TeV , (7)where the first uncertainty is statistical, the second one is systematic and third one is due to thespread of theoretical predictions of the B − c → J / ψ µ − ν µ branching fraction available in literature(see references in Ref. [10]). Recently the HPQCD collaboration provided the first lattice predictionof the B − c → J / ψ µ − ν µ decay width, which has an uncertainty of only 10% [11].The B − c − B + c production asymmetry is measured in different intervals of 𝑝 T and 𝜂 of theB mesons, no significant asymmetry is observed. The B − c meson production asymmetries, averagedover 𝑝 T and 𝜂 , are measured to be (− . ± . ± . ) % and (− . ± . ± . ) % for the 7 and13 TeV data samples, respectively. ) [GeV] b H ( T p ) - × ( d f + u f c f (a) LHCb 7 TeVDataFitAverage (a) η ) - × ( d f + u f c f (c) LHCb 7 TeVDataAverage (b) Figure 4:
Ratio of production fractions as a function of (a) 𝑝 T and (b) 𝜂 of B mesons for the 7 TeV datasample. The statistical uncertainties are shown with smaller error bars, whereas larger bars include bothstatistical and systematic uncertainties. tudies of b-hadrons at LHCb Viacheslav Matiunin
5. Conclusion
A significant contribution to the knowledge of beauty hadron spectroscopy is provided by theLHCb experiment. In particular, the new excited Ω − b and Λ states are observed, mass and width ofnewly observed states are measured. The production fraction of B − c meson with respect to B + andB mesons and B − c − B + c production asymmetry are measured in pp collisions at centre-of massenergies of 7 and 13 TeV.
6. Acknowledgements
This work is supported by the Russian Foundation for Basic Research under grant № References [1] LHCb collaboration,
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