PProceedings of the Second Annual LHCPOctober 11, 2018
Heavy flavour spectroscopy at LHC
Yiming Li
On behalf of the LHCb collaboration,Key Laboratory of Particle & Radiation Imaging (Tsinghua University),Ministry of Education;Center for High Energy Physics, Department of Engineering Physics,Tsinghua University, Beijing 100084, China
ABSTRACTThe pp collision data collected in the LHC Run I provides a great opportunityfor heavy flavour studies. The latest results on exotic states, heavy baryon and B + c mesons are reviewed. PRESENTED ATThe Second Annual Conferenceon Large Hadron Collider PhysicsColumbia University, New York, U.S.AJune 2-7, 2014 a r X i v : . [ h e p - e x ] O c t Introduction
Thanks to the large center-of-mass energy available at the LHC, bb and cc pairs are produced prolifically,which provides great opportunities for studying the production and properties of heavy hadrons. Thisis not only important itself as tests and inputs to QCD models, but also because they have to be wellunderstood as the Standard Model background in the search for new physics. The major LHC detectors arecomplementary to each other in the study of heavy flavour spectroscopy by covering different acceptance andkinematic ranges: general purpose detectors like ATLAS and CMS cover high p T and low rapidity range,while the forward spectrometer LHCb has access to lower p T and higher rapidity region. This proceedingreports recent results from these experiments, on exotic states, heavy baryons and B + c meson. The exotic state that attracts a lot of attention recently is the charged charmonium-like Z (4430) − . Thisstate was first reported as a ψ (2 S ) π − bump in B → ψ (2 S ) π − K + by Belle collaboration in 2008; BaBarcollaboration could explain the enhancement as a reflection of the known K ∗ states, but did not rule outthe existence of Z (4430) − either. With a B signal yield an order of magnitude larger than BaBar or Belledetectors have, LHCb collaboration performs a full amplitude analysis considering known K ∗ states, andobserve the Z (4430) − with a significance larger than 13 . σ , as shown in Figure 1 [1]. The Argand diagramof the Z (4430) − amplitude (Figure 1) shows the resonance behaviour for the first time. The spin-parityis measured to be 1 + , by excluding 0 − , − , − , + hypotheses by at least 9 . σ . For a charged charmoniumstate, Z (4430) − has a minimum quark content of ccdu , which clearly does not fit into traditional quarkmodel.Figure 1: (Left) Distribution of m ψ (2 S ) π − . Black dots are data, the red solid and brown dashed lines representthe total fit with and without the Z (4430) − component. (Right) Fitted values of Z (4430) − amplitude in six m ψ (2 S ) π − bins shown in an Argand diagram. X (3872) is the first charmonium-like exotic state ever observed. Its quantum number is finally pinneddown to 1 ++ in 2013 [2, 3], but the nature is still unclear, drawing a lot of theoretical interests. Usefulinformation can be obtained from its radiative decay since various interpretations predict very differentvalues for the ratio R ψγ ≡ B ( X (3872) → ψ (2 S ) γ ) / B ( X (3872) → J/ψγ ). LHCb lately finds a 4 . σ evidenceof X (3872) → ψ (2 S ) γ decay in B + → X (3872) K + [4] (as shown in Figure 2), and measured its branchingfraction relative to X (3872) → J/ψγ : R ψγ = 2 . ± . ± .
29, where the first uncertainty is statistical andthe second systematic, as followed in the rest of the proceeding. This result is consistent with expectationsof a charmonium cc (2 P ) or a molecule-charmonium mixture interpretation, but does not support a pure DD ∗ molecule interpretation.A search for the bottomonium counterpart of X (3872) is recently performed in X b → Υ(1 S ) π + π − , Υ(1 S ) → µ + µ − decay by CMS [5]. No evidence of X b is observed, and upper limits are set at 95% confidence level1igure 2: Invariant mass distributions of (left) J/ψγ and (right) ψ (2 S ) γ from B decays.on the ratio R X b ≡ σ ( pp → X b → Υ(1 S ) π + π − ) /σ ( pp → Υ(2 S ) → Υ(1 S ) π + π − ) as a function of X b mass(Figure 3). The upper limits are in the range of 0 . − .
4% for X b mass between 10 and 11 GeV.Figure 3: Upper limit at the 95% confidence level on R X b (see definition in text) as a function of X b mass. According to the Heavy Quark Expansion (HQE) theory, the b baryon lifetime is expected to be close tothat of the B mesons, and τ (Λ b ) /τ ( B ) should differ from unity by no more than a few percent. HoweverLEP results indicate this ratio is much smaller, which became a puzzle for the last decade. With pp collisiondata at √ s = 7 TeV, ATLAS [6] and CMS [7] determined Λ b lifetime, resulting in a ratio over B lifetimemuch closer to one with large uncertainties. LHCb precisely measures τ (Λ b ) /τ ( B ) with 1 fb − data [8]and the result is consistent with HQE prediction. Lately LHCb updated this measurement with 3 fb − fulldata from Run I [9]. Figure 4 shows the reconstructed Λ b and B signals using similar decay final statesΛ b → J/ψpK − and B → J/ψK (892) ∗ ( K (892) ∗ → π + K − ), as well as their yield ratio as function ofdecay time. The lifetime ratio is measured to be τ (Λ b ) /τ ( B ) = 0 . ± . ± . b lifetime measurement using τ ( B )world average. In another measurement of τ (Λ b ), LHCb uses a different decay channel of Λ b → J/ψ
Λ with1 fb − [10]. Combined result gives τ (Λ b ) = 1 . ± . ± .
008 ps. The latter analysis also gives the mostprecise single measurement of B + , B and B s (effective) lifetime, as listed in Table 1.Unlike the Λ b state, the bottom strange baryons such as Ξ b and Ω b are less abundantly produced, henceless studied. LHCb lately measured their lifetime, using Ξ − b → J/ψ Ξ − and Ω − b → J/ψ Ω − channels, withsubsequent decays of Ξ − → Λ π − , Ω − → Λ K − , Λ → pπ − and J/ψ → µ + µ − [11]. The reconstructed mass anddecay time of Ξ − b and Ω − b are shown in Figure 5, their lifetime are measured to be τ (Ξ − b ) = 1 . +0 . − . ± .
03 ps2igure 4: Fits to invariant mass distribution of (left)
J/ψpK − and (middle) J/ψπ + K − combinations. Thesolid purple lines represents the fitted Λ b and B signals. (Right) The yield ratio of N (Λ b ) /N ( B ) as afunction of decay time. Lifetime Value (ps) τ B + → J/ψK + ± ± τ B → J/ψK ∗ ± ± τ B → J/ψK ± ± τ Λ b → J/ψ Λ ± ± τ B s → J/ψφ ± ± B + , B , Λ b and B s (effective) lifetime as in Ref. [10].and τ (Ω − b ) = 1 . +0 . − . ± .
05 ps. These are the most precise measurements to date, consistent with CDFresults [12, 13] and with theoretical predictions. B + c physics The B + c state is the ground state of a family of unique mesons that consist of two different heavy flavourquarks. Its production cross-section at the LHC is expected to be an order of magnitude larger than it wasat Tevatron (where it was discovered), and this allows more detailed study on its properties. Using the finalstates of B + c → J/ψπ + , the ratio R σ = σ ( B + c ) B ( B + c → J/ψπ + ) /σ ( B + ) B ( B + → J/ψK + ) is measured byLHCb [14] and CMS [19] in different kinematic regions as shown in Table 2. The dominating systematicuncertainties for R σ measurements come from the uncertainty of B + c lifetime which was measured only atTevatron. However this situation has been changed lately, since LHCb measured the lifetime with betterprecision: τ ( B + c ) = 509 ± ±
12 fs [15] using semileptonic decay B + c → J/ψµ + ν . Figure 6 shows the results ofsimultaneous fits to the distributions of reconstructed B + c pseudo decaytime and invariant mass of the J/ψµ + combinations. The pseudo decaytime is defined as p · ( v − x ) M µ / | p | , where p is the three-momentumof the J/ψµ system in the laboratory frame, and v and x are the measured position of the B + c decay andproduction vertices. This improvement will benefit many other B + c measurements such as its mass and decaybranching ratios.The B + c mesons are expected to decay via many modes: either c or b quark decays weakly with theother as spectator, or they can annihilate. Only very few channels are experimentally observed before theoperation of LHC due to the small production cross-section available. Nowadays the list of observed decayshas been expanded, mostly by LHCb, including observation of the very first c quark decay B + c → B s π + [16],as shown in Figure 7. For b quark decay the list of new channels is longer, the latest being observation of B + c → J/ψK + K − π + [17] and a 4.5 σ evidence of B + c → J/ψ π + π − decay [18] (as shown in Figure 8). CMSrecently observed B + c → J/ψπ + π − π + [19] and measured its branching fraction relative to B + c → J/ψπ + ,the result consistent with LHCb result [20]. 3igure 5: (Top) Invariant mass of (left) J/ψ Ξ − and (right) J/ψ Ω − combinations. (Bottom) Decay time ofreconstructed (left) Ξ − b and (right) Ω − b candidates.Experiment R σ = σ ( B + c ) B ( B + c → J/ψπ + ) σ ( B + ) B ( B + → J/ψK + ) Kinematic rangeLHCb [14] (0 . ± . ± . ± . p T > . < η < . . ± . ± . +0 . − . )% p T >
15 GeV, | y | < . R σ measured at different kinematic ranges, where the uncertainties are statistical, systematic andthat caused by uncertainty on B + c lifetime using world average. The LHC experiments have been fruitful at heavy flavour spectroscopy studies. This proceeding reviewssome latest highlights, including observation of a charged charmonium-like state Z (4430) − , study of the X (3872) radiative decays, more precise determination of b -baryon lifetimes, and a better understanding ofthe B + c properties. As the analysis of Run I data still ongoing and Run II at higher center-of-mass energy isstarting soon, more interesting results will keep coming out to gain us more knowledge on the heavy hadronspectroscopy. References [1] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 222002 (2014) [arXiv:1404.1903][2] A. Abulencia et al. [CDF Collaboration], Phys. Rev. Lett. , 132002 (2007) [hep-ex/0612053].[3] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 222001 (2013) [arXiv:1302.6269][4] R. Aaij et al. [LHCb Collaboration], Nucl. Phys. B , 665 (2014) [arXiv:1404.0275][5] S. Chatrchyan et al. [CMS Collaboration], Phys. Lett. B , 57 (2013) [arXiv:1309.0250][6] G. Aad et al. [ATLAS Collaboration], Phys. Rev. D , 032002 (2013) [arXiv:1207.2284]4igure 6: (Left) Pseudo decaytime of reconstructed B + c candidates; (right) invariant mass distributions of J/ψµ combinations.Figure 7: Invariant mass of B s π + combination, using (left) B s → D − s π + and (right) B s → J/ψφ final statesrespectively.[7] S. Chatrchyan et al. [CMS Collaboration], JHEP , 163 (2013) [arXiv:1304.7495][8] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 102003 (2013) [arXiv:1307.2476][9] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B , 122 (2014) [arXiv:1402.6242][10] R. Aaij et al. [LHCb Collaboration], JHEP , 114 (2014) [arXiv:1402.2554][11] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B , 154 (2014) [arXiv:1405.1543][12] T. Aaltonen et al. [CDF Collaboration], Phys. Rev. D , 072003 (2009) [arXiv:0905.3123][13] T. A. Aaltonen et al. [CDF Collaboration], Phys. Rev. D , 072014 (2014) [arXiv:1403.8126][14] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 232001 (2012) [arXiv:1209.5634][15] R. Aaij et al. [LHCb Collaboration], Eur. Phys. J. C , 2839 (2014) [arXiv:1401.6932][16] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 181801 (2013) [arXiv:1308.4544][17] R. Aaij et al. [LHCb Collaboration], JHEP , 094 (2013) [arXiv:1309.0587][18] R. Aaij et al. [LHCb Collaboration], JHEP , 148 (2014) [arXiv:1404.0287][19] CMS Collaboration,CMS-PAS-BPH-12-011.[20] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 251802 (2012) [arXiv:1204.0079]5igure 8: Mass distributions of the B + c candidates reconstructed from (left) J/ψπ + π − π + , (middle) J/ψK + K − π + and (right) J/ψπ + π + π + π −−
15 GeV, | y | < . R σ measured at different kinematic ranges, where the uncertainties are statistical, systematic andthat caused by uncertainty on B + c lifetime using world average. The LHC experiments have been fruitful at heavy flavour spectroscopy studies. This proceeding reviewssome latest highlights, including observation of a charged charmonium-like state Z (4430) − , study of the X (3872) radiative decays, more precise determination of b -baryon lifetimes, and a better understanding ofthe B + c properties. As the analysis of Run I data still ongoing and Run II at higher center-of-mass energy isstarting soon, more interesting results will keep coming out to gain us more knowledge on the heavy hadronspectroscopy. References [1] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 222002 (2014) [arXiv:1404.1903][2] A. Abulencia et al. [CDF Collaboration], Phys. Rev. Lett. , 132002 (2007) [hep-ex/0612053].[3] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 222001 (2013) [arXiv:1302.6269][4] R. Aaij et al. [LHCb Collaboration], Nucl. Phys. B , 665 (2014) [arXiv:1404.0275][5] S. Chatrchyan et al. [CMS Collaboration], Phys. Lett. B , 57 (2013) [arXiv:1309.0250][6] G. Aad et al. [ATLAS Collaboration], Phys. Rev. D , 032002 (2013) [arXiv:1207.2284]4igure 6: (Left) Pseudo decaytime of reconstructed B + c candidates; (right) invariant mass distributions of J/ψµ combinations.Figure 7: Invariant mass of B s π + combination, using (left) B s → D − s π + and (right) B s → J/ψφ final statesrespectively.[7] S. Chatrchyan et al. [CMS Collaboration], JHEP , 163 (2013) [arXiv:1304.7495][8] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 102003 (2013) [arXiv:1307.2476][9] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B , 122 (2014) [arXiv:1402.6242][10] R. Aaij et al. [LHCb Collaboration], JHEP , 114 (2014) [arXiv:1402.2554][11] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B , 154 (2014) [arXiv:1405.1543][12] T. Aaltonen et al. [CDF Collaboration], Phys. Rev. D , 072003 (2009) [arXiv:0905.3123][13] T. A. Aaltonen et al. [CDF Collaboration], Phys. Rev. D , 072014 (2014) [arXiv:1403.8126][14] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 232001 (2012) [arXiv:1209.5634][15] R. Aaij et al. [LHCb Collaboration], Eur. Phys. J. C , 2839 (2014) [arXiv:1401.6932][16] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 181801 (2013) [arXiv:1308.4544][17] R. Aaij et al. [LHCb Collaboration], JHEP , 094 (2013) [arXiv:1309.0587][18] R. Aaij et al. [LHCb Collaboration], JHEP , 148 (2014) [arXiv:1404.0287][19] CMS Collaboration,CMS-PAS-BPH-12-011.[20] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. , 251802 (2012) [arXiv:1204.0079]5igure 8: Mass distributions of the B + c candidates reconstructed from (left) J/ψπ + π − π + , (middle) J/ψK + K − π + and (right) J/ψπ + π + π + π −− π −−