LHCb is a dedicated flavour physics experiment at the LHC. Precision measurements of CP violation and the study of rare decays of hadrons containing beauty and charm quarks constitute powerful searches for New Physics. A selection of recent LHCb results and their implications to physics beyond the Standard Model are discussed

Langenbruch, C.

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DOI 10.1393/ncc/i2014-11732-4Colloquia: LC13IL NUOVO CIMENTO Vol. 37 C, N. 2 Marzo-Aprile 2014LHCb results on flavour physics and implications to BSMC. Langenbruch on behalf of the LHCb CollaborationCERN - Geneva, Switzerlandricevuto il 22 Gennaio 2014Summary. — LHCb is a dedicated flavour physics experiment at the LHC. Pre-cision measurements of CP violation and the study of rare decays of hadrons con-taining beauty and charm quarks constitute powerful searches for New Physics. Aselection of recent LHCb results and their implications to physics beyond the Stan-dard Model are discussed.PACS 13.25.Hw – Decays of bottom mesons.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.1. – IntroductionIn the study of heavy flavour decays the LHCb experiment profits from the largebb¯ production cross section of the LHC of around 300μb at a center of mass energyof√s = 7TeV. The LHCb detector [1] is a single arm forward spectrometer in thepseudo-rapidity range 2 < η < 5. The detector includes a high precision tracking systemwhich provides excellent momentum and impact parameter resolution. In addition, tworing-imaging Cherenkov detectors provide excellent particle identification capabilities forcharged hadrons [2]. Together with the highly efficient trigger system [3], the LHCbexperiment is therefore ideally suited for the study of heavy flavour decays.This article focuses on recent LHCb results relevant to searches for New Physics (NP).Section 2 covers selected precision measurements of CP violation. In sect. 3 results fromrare decay measurements are discussed. Conclusions are given in sect. 4.2. – CP violation2.1. Determination of the CKM angle γ. – CP violation in the Standard Model (SM) iscaused by a single phase in the CKM matrix [4,5]. The unitarity condition of the CKMmatrix results in three unitarity conditions that can be expressed as triangles in thecomplex plane. The least well determined angle in the βd triangle, given in fig. 1, is theangle γ defined as γ = arg(−VudV ∗ubVcdV ∗cb). Combinations of the current direct measurementsc© CERN on behalf of the LHCb CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 2526 C. LANGENBRUCH on behalf of the LHCb COLLABORATIONγααdmΔKεKεsmΔ & dmΔubVβsin 2(excl. at CL > 0.95) < 0βsol. w/ cos 2αβγρ-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0η0.00.10.20.30.40.50.60.7excluded area has CL > 0.95FPCP 13CKMf i t t e r]° [γ1-CL00.20.40.60.810 20 40 60 80 100 120 140 160 18012−+126768.3%95.5%LHCbPreliminaryFig. 1. – Current experimental status of the unitarity triangle [6] (left). Determination of theCKM angle γ combining the GLW/ADS methods [9,10] using 1 fb−1 and the GGSZ method [11]using 3 fb−1 of data [15] (right).give γ = (68.0+8.0−8.5)◦ [6] or γ = (70.8 ± 7.8)◦ [7], depending on the statistical approach.The determination of γ using B− → D0(D¯0)K− tree level decays is free from possibleNP effects. As such it is of major importance since the result can be compared to γmeasurements that could potentially be affected by new particles [8]. The angle γ canbe determined in interference if the D0 and D¯0 decay into a common final state. Thedetermination of γ using the CP eigenstates π+π− or K+K− as common final state iscalled GLW method, named after the proponents in ref. [9]. A second option is to useflavour eigenstates K+π− and K+π−π+π− as final state, the ADS method [10]. Finally,it is possible to determine γ by performing a Dalitz analysis of the three-body final statesK0Sπ+π− and K0SK+K−, which is the GGSZ method [11].LHCb has published measurements of γ using both the GLW and ADS methods [12,13]. Using the GGSZ method a measurement using 1 fb−1 of data was performed [14]. Apreliminary update of the GGSZ method using the full data sample of 3 fb−1 combinedwith the GLW and ADS methods using 1 fb−1 results in γ = (67 ± 11)◦ [15], the mostprecise measurement of γ from a single experiment today, is shown in fig. 1.2.2. The flavour specific CP-violating asymmetry assl. – The flavour specific CP-violating asymmetry asl is defined as(1) asl =Γ(B¯(t)→ f)− Γ(B(t)→ f)Γ(B¯(t)→ f) + Γ(B(t)→ f) .In the SM, asl is predicted to be tiny [16], but it could be enhanced by NP. The LHCbexperiment uses D−s μ+X as final state f , thus determining the flavour specific asymmetryin the B0s system, assl. Production asymmetries can be neglected due to the fast B0smixing frequency. Using a data sample corresponding to 1 fb−1, assl is determined tobe assl = (−0.06 ± 0.50stat. ± 0.36syst.)% [17]. This result is in excellent agreement withthe SM prediction and the most precise measurement of this quantity to date. Figure 2shows the result in comparison with measurements by the D0 collaboration [18-20].2.3. The CP-violating phase φs. – B0s−B¯0s mixing is a flavour changing neutral current(FCNC) process. Since FCNCs in the SM are forbidden at tree level, B0s mixing can onlyoccur via loop-diagrams. New particles beyond the SM can enter the loop and affect themixing phase. The mixing phase is experimentally accessible in the interference betweenthe direct decay of the B0s to the final state J/ψφ and the decay after mixing into a B¯0s .LHCb RESULTS ON FLAVOUR PHYSICS AND IMPLICATIONS TO BSM 27Fig. 2. – Measurements of a(s,d)sl by the LHCb [17] and D0 [18-20] Collaborations (left). Deter-mination of φs and ΔΓs from B0s → J/ψφ decays [21] (right).The resulting time-dependent CP asymmetry depends on the phase φs which, in the SM,is well known to be φs = (−0.0367 ± 0.0014) rad [6]. Since the decay B0s → J/ψφ isa P → V V transition, the final state is not a CP eigenstate. An angular- and decaytime dependent fit is needed to statistically separate the CP-even from the CP-oddcontribution to the final state. Using the decay B0s → J/ψφ, the phase φs and thedecay width difference in the B0s system, ΔΓs, are determined to φs = (0.07± 0.09stat.±0.01syst.) rad and ΔΓs = (0.100±0.016stat.±0.003syst.) ps−1 as shown in fig. 2. Combinedwith the decay mode B0s → J/ψπ+π− the sensitivity to φs increases, resulting in φs =(0.01± 0.07stat.± 0.01syst.) rad and ΔΓs = (0.106± 0.011stat.± 0.007syst.) ps−1 [21]. Thisresult is in excellent agreement with the SM prediction.3. – Rare decaysRare decays are decays that are mediated by FCNCs. Contributions from new parti-cles beyond the SM can affect both the branching fractions of rare decays as well as theangular distributions of the final state particles. Rare b → s transitions can be describedwith the effective Hamiltonian:(2) Heff. = −4GF√2VtdV∗ts∑i[CiOi + C′iO′i] .The quantities Ci and C′i denote generalized couplings, the Wilson coefficients; Oi and O′iare the corresponding local operators. Any observed significant deviation of the Wilsoncoefficients from their SM predictions would not only be a sign of physics beyond the SM,but also give important information on the operator structure of the NP contribution.3.1. The very rare decay B0s → μ+μ−. – The decay B0s → μ+μ− is a b → s FCNCtransition which is not only loop- but, in addition, helicity-suppressed. Since the finalstate of the decay is purely leptonic, the decay is both theoretically and experimentallyextremely clean. The branching fraction of the decay is particularly sensitive to scalarand pseudoscalar NP contributions. The SM prediction for this decay mode is extremelysmall, B(B0s → μ+μ−)SM = (3.23 ± 0.27) × 10−9 [22]. The time integrated branchingfraction, which corresponds to the experimentally measured branching fraction in the B0ssystem, needs to account for the sizeable ΔΓs, resulting in B(B0s → μ+μ−)SMΔΓs = (3.56±0.18)× 10−9 [23]. In the SM, the decay B0 → μ+μ− is further suppressed by |Vtd/Vts|228 C. LANGENBRUCH on behalf of the LHCb COLLABORATION]2c [MeV/−μ+μm5000 5500)2 cCandidates / (44 MeV/0246810121416LHCbBDT>0.7-13 fb]10−) [10−μ +μ → 0BB(1 2 3 4 5 6 7 8 9 10sCL-210-11011−3fbLHCbFig. 3. – Signal-like B0q → μ+μ− candidates (left) and limit of the B0 → μ+μ− branchingfraction (right).with respect to the decay B0s → μ+μ−. The LHCb experiment has performed an analysisof the full data sample taken in 2011 and 2012 corresponding to an integrated luminosityof 3 fb−1. Figure 3 shows signal candidates that are classified to be signal-like by amultivariate classifier. The significance of the B0s → μ+μ− signal corresponds to 4σ. Forthe decay B0 → μ+μ− an upper limit of B(B0 → μ+μ−) < 7.4×10−10 is set at 95% CL. Afit of the branching fractions results in B(B0s → μ+μ−) = (2.9+1.1−1.0(stat)+0.3−0.1(syst))×10−9and B(B0 → μ+μ−) = (3.7+2.4−2.1(stat)+0.6−0.4(syst)) × 10−10 [24]. Combination with theCMS experiment, which sees the decay with a significance of 4.3σ [25], results in thefirst observation of the decay Bs → μ+μ− [26]. The result is in agreement with the SMprediction. No large contributions from NP are observed.3.2. The rare decay B+ → K+μ+μ−. – The final state of the decay B+ → K+μ+μ−is fully described by one angle θ and the invariant mass of the dimuon system squared,q2 = m(μ+μ−)2. The angular distribution of this decay is given by(3)1ΓdΓd cos θ=34(1− FH)(1− cos2 θ) + 12FH + AFB cos θ,where FH denotes the flat parameter and AFB the forward-backward asymmetry. Inref. [27] these angular observables are determined together with the differential branch-ing fraction dΓ/dq2 in bins of q2. Good agreement with the SM prediction is observed.In addition, the CP asymmetry of the decay, ACP = Γ(B−→K−μ+μ−)−Γ(B+→K+μ+μ−)Γ(B−→K−μ+μ−)+Γ(B+→K+μ+μ−) , isdetermined, using the decay B+ → J/ψK+ to control production and detection asym-metries. The resulting CP asymmetry is measured in bins of q2 and is in good agreementwith the SM prediction [28]. Furthermore, an interesting resonance structure is foundat high q2 in the K+μ+μ− final state [29]. The mass of the state is determined to4191+9−8 MeV, its width to 65+22−16 MeV, compatible with the known Ψ(4160).3.3. The rare decay B0 → K∗0μ+μ−. – The decay B0 → K∗0μ+μ− has a particu-larly rich phenomenology due to the many observables that are accessible in this mode.The final state is fully described by the three decay angles θ, θK and Φ and q2. TheLHCb RESULTS ON FLAVOUR PHYSICS AND IMPLICATIONS TO BSM 29]4c/2 [GeV2q0 5 10 15 20'5P-1-0.8-0.6-0.4-0.200.20.40.60.81SM PredictionsDataLHCb  	 		  Fig. 4. – The angular observable P ′5 determined in the decay B0 → K∗0μ+μ− [31], togetherwith the SM predictions from [32] (left), and the result of a fit of the Wilson coefficients C7 andC9 [33] (right).differential decay rate is given by1Γd3(Γ + Γ¯)d cos θd cos θKdΦ=932π[34(1− FL) sin2 θK + FL cos2 θK(4)+14(1− FL) sin2 θK cos 2θ−FL cos2 θK cos 2θ + S3 sin2 θK sin2 θ cos 2Φ+S4 sin 2θK sin 2θ cosΦ + S5 sin 2θK sin θ cosΦ+43AFB sin2 θK cos θ + S7 sin 2θK sin θ sinΦ+S8 sin 2θK sin 2θ sinΦ + S9 sin2 θK sin2 θ sin 2Φ],where FL denotes the longitudinal polarisation fraction of the K∗0, AFB the forward-backward asymmetry and Si the remaining CP-averaged angular observables. Due to thelimited size of the analyzed data sample corresponding to only 1 fb−1, angular foldings areemployed to reduce the number of observables to be determined in a fit of the angular dis-tributions. Good agreement with the SM predictions is seen for the angular observablesFL, AFB and S3,9 [30]. Applying different angular foldings, the less form-factor depen-dent angular observables P ′4,5,6,8 = S4,5,7,8/√FL(1− FL) are determined [31]. Goodagreement with the SM predictions is seen for all angular observables except for P ′5,given in fig. 4. Here, a local deviation corresponding to 3.7σ is observed in one q2 bin.Assuming that all observables are statistically independent, and taking into account thatfour observables in six bins of q2 are determined in ref. [31], this deviation correspondsto a p-value of 0.5%. Several theory groups have performed global fits to determine theWilson coefficients from the angular observables in B0 → K∗0μ+μ− and other b → stransitions. The authors of ref. [33] have found improved agreement of the predictionwith the data for a negative shift of the coefficient C9 as shown in fig. 4. An updatedanalysis of the full LHCb data sample corresponding to 3 fb−1 is currently ongoing toclarify wether the observed deviation is a hint of NP or just a statistical fluctuation.30 C. LANGENBRUCH on behalf of the LHCb COLLABORATION3.4. The radiative decay B+ → K+π+π−γ. – The radiative decay B+ → K+π+π−γ issensitive to NP contributions to the photonic penguin. In the SM the photon emitted inthe decay is predominantly left-handed. NP can however cause significant right-handedcontributions.Using the three final state hadrons a plane is defined in the K+π+π− system. Theasymmetry in the number of events with the photon going upwards and downwards withrespect to this plane carries information on the photon polarisation [34]. A preliminarymeasurement of the up-down asymmetry Aud results in Aud = −0.085 ± 0.019stat. ±0.003syst. [35], which corresponds to a 4.6σ evidence for photon polarization. In addition,the first measurement of CP violation in this mode is performed resulting in ACP =−0.007± 0.015stat. ± 0.008syst..4. – ConclusionsThe LHCb experiment has performed many studies of CP violation and rare decayswith excellent sensitivity to physics beyond the SM, yet clear signs of NP are still ab-sent. There are however some interesting hints, for example a deviation in the angularobservable P ′5 in the decay B0 → K∗0μ+μ−. An update of the analysis of this decayusing the full LHCb data sample is currently ongoing.REFERENCES[1] Alves A. A., jr. et al. (LHCb Collaboration), JINST, 3 (2008) S08005.[2] Adinolfi M. et al. (LHCb Collaboration), Eur. Phys. J. C, 73 (2013) 2431.[3] Aaij R. et al., JINST, 8 (2013) P04022.[4] Cabibbo N., Phys. Rev. Lett., 10 (1963) 531.[5] Kobayashi M. and Maskawa T., Prog. Theor. Phys., 49 (1973) 652.[6] CKMfitter Group (J. Charles et al.), Eur. Phys. J. 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LHCb results on flavour physics and implications to BSM

LHCb is a dedicated flavour physics experiment at the LHC.  Precision measurements of CP violation and the study of rare decays of hadrons containing beauty and charm quarks constitute powerful searches for New Physics.  A selection of recent LHCb results and their implications to physics beyond the Standard Model are discussed

Langenbruch, C

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LHCb results on flavour physics and implications to BSM

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