PProceedings of the Second Annual LHCPOctober 12, 2018
Charmless B decays at LHCb
Roberta Cardinale
On behalf of the LHCb Experiment,Department of PhysicsUniversity of Genova, Genova, Italy
ABSTRACTThe study of charmless b -hadron decays provides information for testing theCKM picture of CP violation in the Standard Model. In addition, as they canproceed through loop diagrams, they are also sensitive to physics beyond theStandard Model. A review of recent results from LHCb on charmless b -hadrondecays is presented.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 ] S e p Introduction
Charmless b -hadron decays play a central role testing ground for the Standard Model. Recent results usingdata collected in 2011 at the LHCb detector [1] at √ s = 7 TeV, corresponding to an integrated luminosityof ∼ − , are presented. Λ b (Ξ b ) → K s ph − The study of b -baryons decays is almost an unexplored field. Hadronic three-body b -baryons decays tocharmless final states, which have not been observed yet, can provide the possibility to study hadronic decaysand to search for CP violation. In these proceedings are presented the branching fractions measurements ofbeauty baryons decays to the final states K s pπ − and K s pK − , determined relative to the B → K s π + π − decay used as normalisation channel [2]. Each b -hadron decay is reconstructed by combining two chargedtracks with a K s candidate. The K s candidates are reconstructed in the π + π − final states using twodifferent categories. The Long candidates have hits both in the vertex detector and in the tracking stationsdownstream of the dipole magnet while the Downstream candidates have not track segments in the vertexdetector but only in the tracking stations. Events are triggered and selected in a similar way both for thesignal modes and the normalisation channel, exploiting the topology of three-body decays and the b -hadronkinematic properties. Intermediate states containing charmed hadrons are excluded from the signal sampleand studied separately.The decay channel Λ b → K s pπ − is observed for the first time with a significance level of 8 . σ and itsFigure 1: Invariant mass distribution of (top) K s pπ − and (bottom) K s pK − selected candidates for the (left)Downstream and (right) Long K s categories.branching fraction is measured to be B (Λ b → K pπ − ) = (1 . ± . ± . ± . ± . × − , where the first uncertainty is statistical, the second systematic and the third and the fourth related tothe uncertainty on the ratio of fragmentation fraction, f Λ b /f d and on the branching fraction of the B → K π + π − decay respectively. The CP asymmetry integrated over the phase-space of the observed Λ b → s pπ − decay is found to be A CP (Λ b → K s pπ − ) = 0 . ± .
13 (stat) ± .
03 (syst) . No significant signals are seen for the Λ b → K s pK − decay and for the Ξ b decays and upper limits on theirbranching fractions are set to B (Λ b → K pK − ) < . . × − at 90% (95%) CL f Ξ b /f d × B (Ξ b → K pπ − ) < . . × − at 90% (95%) CL f Ξ b /f d × B (Ξ b → K pK − ) < . . × − at 90% (95%) CL B s → K + K − , B → K + π − and B s → π + K − decays The effective lifetime measurement of the B s → K + K − decay, recently measured by LHCb with highprecision, is of great interest as it can constrain contributions from new physical phenomena to the B s system. In addition the B → K + π − and B s → K + π − lifetimes, which contribute to the world average of τ ( B ) and τ ( B s ), are measured [3]. The analysis uses a data driven approach to correct for the decay timeacceptance introduced by the trigger and the final selection. The procedure consists in extracting the per-event acceptance function directly from data. The effective lifetimes are then determined using a factorisedfit to the mass and decay time distributions (see Figure 2). The measured B s → K + K − lifetime isFigure 2: Fit to the KK invariant mass spectrum and to the reconstructed decay times. τ B s → K + K − = 1 . ± .
016 (stat) ± .
007 (syst) pswhich is the world best measurement and is compatible with the SM prediction. The dominant contributionto the systematic uncertainty come from the contamination from misidentified B → h + h (cid:48) − backgroundchannels. The measured lifetimes for B → K + π − and B s → π + K − decays are τ B → K + π − = 1 . ± .
011 (stat) ± .
004 (syst) ps τ B s → π + K − = 1 . ± .
06 (stat) ± .
01 (syst) ps B ± → K + K − π ± and B ± → π + π − π ± Charmless decays of B mesons to three hadrons are dominated by quasi-two body processes involving in-termediate resonant states. The rich interference pattern makes them favorable for the investigations of CP2symmetries that are localized in the phase space. Interference between intermediate states of the decaycan introduce large strong phase differences which can explain local asymmetries in the phase space [4, 5].Another explanation focuses on final-state KK ↔ ππ rescattering, which can occur between decay channelswith the same flavour quantum numbers [5, 6]. CP violation in the phase space of B + → K + K − π + and B + → π + π − π + is measured [7].Events are selected requiring that the three charged tracks satisfy selection criteria related to their transversemomenta, vertex and track quality. Final state kaons and pions are further selected.Raw asymmetries are extracted from an unbinned maximum likelihood fit to the mass spectra of the selectedcandidates and then corrected for detector induced effects and for the B ± meson production asymmetry A CP = A raw − A D ( π ± ) − A P ( B ± )The π ± detection asymmetry, A D ( π ± ), is calculated using the ratio of full to partially reconstructed D ∗ + → π + D decays [8], while the production asymmetry, A P ( B ± ), is evaluated using B ± → J/ψK ± decay ascontrol channel. The CP asymmetries are found to be A CP ( B ± → π ± K + K − ) = − . ± .
040 (stat) ± . ± .
007 ( A CP ( J/ψK )) A CP ( B ± → π ± π + π − ) = 0 . ± .
021 (stat) ± .
009 (syst) ± .
007 ( A CP ( J/ψK ))where the first uncertainty is statistical, the second is the systematic uncertainty and the third is due to theuncertainty on the measurement of the CP asymmetry of the B ± → J/ψK ± decay. These measurementsrepresent the first evidence of inclusive CP asymmetries of the B ± → K + K − π ± and B ± → π + π − π ± decayswith significances of 3 . σ and 4 . σ respectively.Asymmetry distributions over the phase space have been studied, as reported in Figure 3, where the rawasymmetries in each bin of the Dalitz plot are shown.For the B ± → π ± K + K − decays a large negative charge asymmetry is observed in the low m K + K − < . /c Figure 3: Asymmetries of the number of events in bin of the Dalitz plot for (a) B ± → π ± π − π + and (b) B ± → π ± K + K − . The inset figures show the projections of the number of events in bins of (a) m π + π − low variable for m π + π − high >
15 GeV /c and (b) the m K + K − variable.where no resonant contribution is expected. For B ± → π ± π − π + decays, a large positive charge asymmetry ismeasured in the low m π + π − low < . /c and in the high m π + π − high >
15 GeV /c , not clearly associatedto a resonant state. Unbinned extended maximum likelihood fits are performed to the mass spectra of thecandidates in the regions where large raw asymmetries are found. The local charge asymmetries for the tworegions are measured to be A regCP ( B ± → K + K − π ± ) = − . ± .
070 (stat) ± .
013 (syst) ± .
007 ( A CP ( J/ψK )) A regCP ( B ± → π + π − π ± ) = − . ± .
082 (stat) ± .
027 (syst) ± .
007 ( A CP ( J/ψK ))3here the first uncertainty is statistical, the second is the systematic uncertainty and the third is due tothe uncertainty on the measurement of the CP asymmetry of the B ± → J/ψK ± decay. Those results alongwith recent theoretical developments, may indicate new mechanisms for CP asymmetries [4, 5, 6, 9]. B → φK ∗ decays In the Standard Model the B → φK ∗ decay is expected to proceed mainly via a gluonic penguin diagram.For this reason the measurement of CP violation in this decay is sensitive to possible physics beyond theStandard Model, arising in the penguin loop. Since this decay involves a spin-0 B meson decaying into twospin-1 vector mesons, due to angular momentum conservation, there are only three independent configura-tions of the final state spin vectors. They can be written in term of a longitudinal polarization, A , and twotransverse components with collinear, A || , and orthogonal, A ⊥ , polarizations.Angular analyses have shown that the longitudinal and transverse components in this decay have roughlyequal amplitudes. Similar results have been observed also in other B → V V transitions in contrast to tree-level decays [10, 11, 12, 13]. The different behaviour of tree and penguin decays has attracted much theoreticalattention [14, 15]. In addition to the P-wave amplitudes, there are also contributions where K + K − or K + π − are produced in a spin-0 (S-wave) state, ( A K + K − s and A K + π − s ).Polarization amplitudes and phases are measured by LHCb performing the studies of the angular dis-tributions of the decay products [16]. Candidates are selected from charged tracks with high transversemomentum and impact parameter. Pions and kaons are then selected using particle identification infor-mation provided by the RICH detectors. The resulting charged tracks are combined to form φ and K ∗ meson candidates requiring the invariant mass to be close to the known mass. Kinematic and topologicalvariables are then used in a geometric likelihood method to further suppress background, obtaining about1800 candidates. A simultaneous fit to the invariant masses and angular observables distributions is per-formed. The angular analysis results are reported in Table 1. The P-wave parameters are consistent with,Parameter Definition Fitted value f L . | A | /F P + | A | /F P ) 0 . ± . ± . f ⊥ . | A ⊥ | /F P + | A ⊥ | /F P ) 0 . ± . ± . f s ( Kπ ) 0 . | A Kπs | + | A Kπs | ) 0 . ± . ± . f s ( KK ) 0 . | A KKs | + | A KKs | ) 0 . ± . ± . δ ⊥ . ⊥ + arg A ⊥ ) 2 . ± . ± . δ (cid:107) . || + arg A || ) 2 . ± . ± . δ s ( Kπ ) 0 . K π s + arg A K π s ) 2 . ± . ± . δ s ( KK ) 0 . KKs + arg A
KKs ) 2 . ± . ± . A CP0 ( | A | /F P − | A | /F P ) / ( | A | /F P + | A | /F P ) − . ± . ± . A CP ⊥ ( | A ⊥ | /F P − | A ⊥ | /F P ) / ( | A ⊥ | /F P + | A ⊥ | /F P ) +0 . ± . ± . A s ( Kπ ) CP ( | A Kπs | − | A Kπs | ) / ( | A Kπs | + | A Kπs | ) +0 . ± . ± . A s ( KK ) CP ( | A KKs | − | A KKs | ) / ( | A KKs | + | A KKs | ) − . ± . ± . δ CP ⊥ . ⊥ − arg A ⊥ ) +0 . ± . ± . δ CP (cid:107) . || − arg A || ) +0 . ± . ± . δ s ( Kπ ) CP . K π s − arg A K π s ) 0 . ± . ± . δ s ( KK ) CP . KKs − arg A KKs ) 0 . ± . ± . F P = | A | + | A || | + | A ⊥ | , F P = | A Kπs | + | A KKs | , F P + F s = 1. 4ut more precise than previous measurements and the value of f L indicates that longitudinal and transversepolarizations have similar size [17, 18]. Significant S-wave contributions, A K + K − s and A K + π − s , are foundin both the K + π − and K + K − systems. The CP asymmetries in both the amplitudes and the phases areconsistent with zero. The largest systematic uncertainty on the angular analysis is due to the understandingof the detector acceptance which is determined from simulated events. An overview of the latest LHCb results on charmless b -hadron decays has been given. First observation of b -baryons decays to hadronic three-body charmless final states has been obtained. The measured effectivelifetime in the B s → K + K − decay has been found compatible with the SM expectation. In the B ± → K + K − π ± and B ± → π + π − π ± decays, a large CP asymmetry has been found in regions of the Dalitz whichdo not correspond to resonant contributions. This may indicate new mechanisms for CP asymmetries. Moreinteresting results are expected using the complete 2011 and 2012 available data samples which correspondto an integrated luminosity of ∼ − . References [1] A. A. Alves, Jr. et al. [LHCb Collaboration], JINST (2008) S08005.[2] R. Aaij et al. [LHCb Collaboration], JHEP (2014) 087 [arXiv:1402.0770 [hep-ex]].[3] R. Aaij et al. [LHCb Collaboration], arXiv:1406.7204 [hep-ex].[4] Z. H. Zhang, X. H. Guo and Y. D. Yang, Phys. Rev. D (2013) 7, 076007 [arXiv:1303.3676 [hep-ph]].[5] B. Bhattacharya, M. Gronau and J. L. Rosner, Phys. Lett. B (2013) 337 [arXiv:1306.2625 [hep-ph]].[6] I. Bediaga, T. Frederico and O. Loureno, Phys. Rev. D (2014) 094013 [arXiv:1307.8164 [hep-ph]].[7] R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. (2014) 1, 011801 [arXiv:1310.4740 [hep-ex]].[8] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B (2012) 186 [arXiv:1205.0897 [hep-ex]].[9] D. Xu, G. N. Li and X. G. He, Int. J. Mod. Phys. A (2014) 1450011 [arXiv:1307.7186 [hep-ph]].[10] P. del Amo Sanchez et al. [BaBar Collaboration], Phys. Rev. D (2011) 051101 [arXiv:1012.4044[hep-ex]].[11] J. Zhang et al. [BELLE- Collaboration], Phys. Rev. Lett. (2005) 141801 [hep-ex/0408102].[12] B. Aubert et al. [BaBar Collaboration], Phys. Rev. Lett. (2006) 201801 [hep-ex/0607057].[13] R. Aaij et al. [LHCb Collaboration], Phys. Lett. B (2012) 50 [arXiv:1111.4183 [hep-ex]].[14] A. L. Kagan, Phys. Lett. B (2004) 151 [hep-ph/0405134].[15] A. Datta, A. V. Gritsan, D. London, M. Nagashima and A. Szynkman, Phys. Rev. D (2007) 034015[arXiv:0705.3915 [hep-ph]].[16] R. Aaij et al. [LHCb Collaboration], JHEP (2014) 069 [arXiv:1403.2888 [hep-ex]].[17] B. Aubert et al. [BaBar Collaboration], Phys. Rev. D (2008) 092008 [arXiv:0808.3586 [hep-ex]].[18] M. Prim et al. [Belle Collaboration], Phys. Rev. D88