An overview of the latest LHCb’s measurements in the charm physics sector is presented. This includes searches for rare decays, measurements of direct and indirect CP-violating observables and precise determination of mixing parameters using “wrong-sign” D0 → K+π− decays

Di Canto, A.

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DOI 10.1393/ncc/i2015-15015-4Colloquia: IFAE 2014IL NUOVO CIMENTO 38 C (2015) 15Charm physics at LHCbA. Di CantoPhysics Department, CERN – CH-1211 Geneva 23, Switzerlandreceived 7 January 2015Summary. — An overview of the latest LHCb’s measurements in the charm physicssector is presented. This includes searches for rare decays, measurements of directand indirect CP -violating observables and precise determination of mixing parame-ters using “wrong-sign” D0 → K+π− decays.PACS 13.20.Fc – Decays of charmed mesons.PACS 13.25.Ft – Decays of charmed mesons.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.1. – IntroductionProcesses involving K and B(s) mesons have always been regarded as the most inter-esting probe of flavor and CP violation. Indeed, within the standard model the largestflavor- and CP -violating effects reside in systems involving down-type quarks, wherecharged-current loops dominated by the heavy top quark communicate the symmetryviolation. However, while these properties hold in the standard model, there is no goodreason for them to be true if new physics is present at the electroweak scale. In par-ticular, it is quite plausible that new-physics contributions affect mostly the up-typesector, possibly in association with the mechanism responsible for the large top mass [1].Hence, decays of D mesons represent a unique probe of new-physics flavor effects, quitecomplementary to tests in K and B systems.Since flavor- and CP -violating processes in the charm sector are much more sup-pressed than in K and B(s) sectors, huge and clean samples of D decays are needed tosearch for possible new-physics contributions. These can currently be accessed only athadron-collider experiments. LHCb has therefore a broad programme of charm physics,which includes searches for rare decays, studies of CP violation and mixing, as well asspectroscopy and measurements of production cross-sections.In the following an overview of the most recent results is presented.c© CERN on behalf of the LHCb CollaborationCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. DI CANTO2. – Rare decaysThe LHCb collaboration has recently published several searches for rare charm de-cays using the sample of pp collisions, corresponding to 1 fb−1 of integrated luminosity,collected during 2011 [2]:B(D0 → μ+μ−) < 6.8× 10−9 at 95% CL,B(D0 → π+π−μ+μ−) < 6.7× 10−7 at 95% CL,B(D+ → π+μ+μ−) < 8.3× 10−8 at 95% CL,B(D+s → π+μ+μ−) < 4.1× 10−7 at 95% CL.In all cases the measurements are several orders of magnitude above the standard-modelpredictions, but, improving existing limits from the B-factories by about a factor of fifty,provide more stringent constraint on possible new-physics models [3].3. – Direct CP violationDirect CP violation occurs when the magnitudes of a given decay amplitude is differentfrom the magnitude of the amplitude for the corresponding CP -conjugated decay. Theearly evidence for large direct CP violation in singly-Cabibbo-suppressed D0 decaysto two charged hadrons [4] has not been confirmed by later results based on the datasample collected during 2011 [5, 6]. Recently, LHCb reported an updated measurementof the difference between time-integrated CP asymmetries in D0 → K+K− and D0 →π+π− decays, ΔACP = ACP (K+K−) − ACP (π+π−), using the full Run I data set,corresponding to 3 fb−1 of integrated luminosity [7]. In this analysis, neutral D mesonsare reconstructed from a semileptonic b-hadron decay, so that the flavor at productiontime can be determined by the charge of the accompanying charged lepton. Togetherwith ΔACP , the analysis also measures the individual CP asymmetries, ACP (K+K−) andACP (π+π−), by subtracting spurious detector and production asymmetries with controlsamples of Cabibbo-favored D decays where CP violation can be neglected. The results,ΔACP = [+0.14± 0.16(stat.)± 0.08(stat.)]%,ACP (K+K−) = [−0.06± 0.15(stat.)± 0.10(stat.)]%,ACP (π+π−) = [−0.20± 0.19(stat.)± 0.10(stat.)]%,further increase the consistency of the available data with the hypothesis of CP symmetryin two-body charm decays [8], as also shown in fig. 1.In addition to two-body final states, LHCb also exploits multi-body decays to searchfor CP violation. These are particularly interesting as they offer the opportunity tostudy effects localized in phase space, providing sensitivity to phenomena that mightbe “washed out” in global decay rate asymmetries. LHCb performed searches for localCP asymmetries in Cabibbo-suppressed three-body D+ → π+π−π+ [10] and four-bodyD0 → π+π−π+π− and D0 → K+K−π+π− decays [11], using the data collected during2011. Neutral D mesons are here reconstructed using the D∗+ → D0π+ decay, so thatthe production flavor is determined by the charge of the accompanying pion. In allcases, local CP violation is searched using a model-independent binned method, whilefor the D+ case also an unbinned technique has been developed, providing consistentCHARM PHYSICS AT LHCb 3) [%]+π−π(CPA-1 -0.5 0 0.5 1) [%]+K−K(CPA-1-0.8-0.6-0.4-0.200.20.40.6(muon tagged)LHCbCDFBaBarprelim.BelleWA(pion tagged)prelim.LHCbFig. 1. – Overview of 68% CL contours from measurements of CP -violating asymmetries in D0decays to two charged hadrons [5,7,9]. The world average (WA) neglects any effect from indirectCP violation.results [10]. The binned method divides the multi-body phase space into n independentvolumes (“bins”) and defines an asymmetry significance for each asSiCP =N i(D)− αN i(D)√N i(D) + α2N i(D),where N i is the number of candidates in the bin i and α =∑i Ni(D)/∑i Ni(D) is usedto remove any global asymmetry. This procedure makes the search insensitive to globalCP asymmetries, but also to any unwanted production and/or detection asymmetrythat does not vary across the phase space. In absence of local CP violation, SiCP wouldbe distributed as a normal distribution. The no-CP -violation hypothesis is tested bycomputing a p-value from χ2 =∑i(SiCP)2 with n − 1 degrees of freedom. Differentbinning schemes are tested and p-values are found to exceed 50%, 40% and 9% in theD+ → π+π−π+, D0 → π+π−π+π− and D0 → K+K−π+π− cases, respectively.4. – Mixing and indirect CP violationBuilding upon the previous measurement of charm mixing parameters in D0 → K+π+decays [12], LHCb performed an updated measurement of charm mixing and searched fortime-dependent CP violation using the full data sample consisting of 3 fb−1 of integratedluminosity collected during Run I [13]. CP violation in D0 −D 0 mixing is searched bycomparing the decay-time-dependent ratio of “wrong-sign” D∗+ → D0(→ K+π−)π+ to“right-sign” D∗+ → D0(→ K−π+)π+ rates with the corresponding ratio for the charge-conjugate processes. In the limit of |x|, |y|  1, and assuming negligible CP violation,the decay-time dependence of this ratio is approximated byR(t) ≈ RD +√RD y′ tτ+x′2 + y′24(tτ)2,4 A. DI CANTOτ/t0 2 4 6 20]-3 [10− R −+ R -0.200.2(c)]-3 [10− R456CPV allowedNo direct CPVNo CPV(b)]-3 [10+ R456LHCb(a)Fig. 2. – Ratios of wrong-sign to right-sign yields for (a) D0 decays, (b) D 0 decays, and (c) theirdifferences as functions of decay time in units of D0 lifetime. Projections of fits allowing for(dashed line) no CP violation, (dotted line) no direct CP violation, and (solid line) full CPviolation are overlaid. The abscissa of the data points corresponds to the average decay timeover the bin; the error bars indicate the statistical uncertainties.where τ is the average D0 lifetime. The parameters x′ and y′ depend linearly onthe mixing parameters as x′ ≡ x cos δ + y sin δ and y′ ≡ y cos δ − x sin δ. The pa-rameter RD and the strong phase δ are related to the decay amplitudes as A(D0 →K+π−)/A(D 0 → K+π−) = −√RDe−iδ. Allowing for CP violation, the rates R+(t)and R−(t) of initially produced D0 and D 0 mesons are functions of independentsets of mixing parameters (R±D, x′2±, y′±), where x′± = |q/p|±1(x′ cosφ ± y′ sinφ)and y′± = |q/p|±1(y′ cosφ ∓ x′ sinφ) and φ is the relative week phase between q/pand A(D0 → K+π−)/A(D 0 → K+π−). Direct CP violation would produce a differ-ence between R+D and R−D, which can be quantified by the direct CP asymmetryAD = (R+D−R−D)/(R+D +R−D). Violation of CP symmetry either in mixing (|q/p| = 1) orin the interference between mixing and decay amplitudes (φ = 0) are usually referred to asindirect CP violation and would result in differences between (x′2+, y′+) and (x′2−, y′−).The measured RS-to-WS ratios as a function of time, shown in fig. 2, are consistent withCP conservation as no difference between R+(t) and R−(t) is found. This translates intothe world’s most precise bounds on |q/p| and AD from a single experiment:0.75 < |q/p| < 1.24 at 68.3% CL, AD = (−0.7± 1.9)%.Assuming CP conservation, the mixing parameters are measured to be x′2 = (5.5 ±4.9) × 10−5, y′ = (4.8 ± 1.0) × 10−3, and RD = (3.568 ± 0.066) × 10−3. The reportedinterval and uncertainties include both statistical and systematic components.Indirect CP violation is also searched by measuring the asymmetry between the effec-tive lifetimes of D0 and D 0 mesons decaying into CP eigenstates [14], such as D0 → h+h−CHARM PHYSICS AT LHCb 5decays with h = K or π:AΓ =τˆ(D 0 → h+h−)− τˆ(D0 → h+h−)τˆ(D 0 → h+h−) + τˆ(D0 → h+h−) ≈y2(∣∣∣∣qp∣∣∣∣−∣∣∣∣pq∣∣∣∣)cosφ− x2(∣∣∣∣qp∣∣∣∣+∣∣∣∣pq∣∣∣∣)sinφ.The sample of neutral D mesons from a D∗+ → D0π+ decays collected during 2011 isfit with a multi-dimensional unbinned likelihood to separate the signal from the relevantbackgrounds and determine the value of AΓ. The measured values, separately for thetwo final states, are consistent with zero and thus with CP symmetry:AΓ(D0 → K+K−) = [−0.35± 0.62(stat.)± 0.12(stat.)]× 10−3,AΓ(D0 → π+π−) = [+0.33± 1.06(stat.)± 0.14(stat.)]× 10−3.These are the world’s most precise measurement of this quantity to date. No differencebetween the two final states is observed, as expected from Standard Model predictions.5. – ConclusionsThanks to the world’s largest, high-purity samples of charm decays collected duringRun I in both hadronic and (semi)leptonic final states, LHCb has already shown itspotential to substantially improve the precision on all the key observables of the charmphysics sector in the next years. Major advances will be made in the coming monthssince the full Run I data sample has not yet been fully analyzed and additional data willsoon be collected in the forthcoming LHC runs.REFERENCES[1] Nir Y. and Seiberg N., Phys. Lett. B, 309 (1993) 337; Leurer M., Nir Y.and Seiberg N., Nucl. Phys. B, 420 (1994) 468; Giudice G. F., Gripaios B. andSundrum R., JHEP, 08 (2011) 055; Giudice G. F., Isidori G. and Paradisi P.,JHEP, 04 (2012) 060; Keren-Zur B. et al., Nucl. Phys. B, 867 (2013) 429; Calibbi L.,Paradisi P. and Ziegler R., JHEP, 06 (2013) 052.[2] Aaij R. et al., Phys. Lett. B, 724 (2013) 203; 725 (2013) 15; 728 (2014) 234.[3] Burdman G. et al., Phys. Rev. D, 66 (2002) 014009.[4] Aaij R. et al., Phys. Rev. Lett., 108 (2012) 111602.[5] Aaij R. et al., LHCb-CONF-2013-003.[6] Aaij R. et al., Phys. Lett. B, 723 (2013) 33.[7] Aaij R. et al., JHEP, 07 (2014) 041.[8] Amhis Y. et al., arXiv:1207.1158 and updated results at http://www.slac.stanford.edu/xorg/hfag/.[9] Aubert B. et al., Phys. Rev. Lett., 100 (2008) 061803; Aaltonen T. et al., Phys. Rev.Lett., 109 (2012) 111801; Di Canto A., Il Nuovo Cimento C, 36 (2013) 26; Ko B., PoS,ICHEP2012 (2013) 353.[10] Aaij R. et al., Phys. Lett. B, 728 (2014) 585.[11] Aaij R. et al., Phys. Lett. B, 726 (2013) 623.[12] Aaij R. et al., Phys. Rev. Lett., 110 (2013) 101802.[13] Aaij R. et al., Phys. Rev. Lett., 111 (2013) 251801.[14] Aaij R. et al., Phys. Rev. Lett., 112 (2014) 041801.

Charm physics at LHCb

An overview of the latest LHCb’s measurements in the charm physics sector is presented. This includes searches for rare decays, measurements of direct and indirect CP -violating observables and precise determination of mixing parame- ters using “wrong-sign” D 0 → K + π − decay

Di Canto, A

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Charm physics at LHCb

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