Photon polarisation in b → sγ using B → K∗e+e− at LHCb MichelleNicol(onbehalfoftheLHCbcollaboration)1,a Laboratoiredel’Acce´le´rateurLine´aire,Orsay,France Abstract. Theb → sγtransitionproceedsthroughflavourchangingneutralcurrents,andthusisparticularly sensitive to the effects of new physics. An overview of the method to measure the photon polarisation at the LHCbexperimentviaanangularanalysisofB→K∗e+e−atlowq2ispresented.ThestatusoftheB→K∗µ+µ− √ 2 analysiswith309pb−1ofppcollisionsat s=7TeVatLHCbisalsogiven. 1 0 2 1 Introduction n a J Althoughthebranchingratioofb→ sγhasbeenmeasured 4 to be consistent with Standard Model (SM) predictions, 2 new physics could still be present and detectable through the analysis of details of the decay process. In particular, ] thephotonfromthebispredominantlylefthandedinthe x SM,whereasadditionalrighthandedcurrentscanarisein e - certain new physics models, such as the Left-Right sym- p metricmodels,orinsomesupersymmetricmodels[1].Ac- e cesstothepolarisationinformationisavailableviaanan- h gularanalysisofB→ K∗e+e−. [ Hadronicformfactorsrendertheoreticalpredictionover 1 the whole q2 (the dilepton invariant mass squared) range v difficult.However,ithasbeenshownthattheseuncertain- 0 tiesarecontrollableatlowq2,wherethephotontermdom- 0 0 inates, and certain asymmetries providing information on 5 thephotonpolarisationcanbeformed.[2] . 1 0 2 2 B → K∗µ+µ− status at LHCb 1 Fig. 1. A and F as a function of q2, as measured at LHCb FB L : withB→K∗µ+µ−[3].TheSMpredictionsaregivenbythecyan v √ i With 309 pb−1 of pp collisions at s=7 TeV, collected (light)band,andthispredictionintegratedintheq2 binsisindi- X in three months during the first half of 2011, the forward catedbythepurple(dark)regions. r backwardasymmetryofthedileptonsystem,A hasbeen a measured [3] using B → K∗µ+µ− events, (aFsBis shown inFig.2),alongwith F ,theK∗ longitudinalpolarisation L (Fig.2);aninputrequiredforthephotonpolarisationmea- surement.Theseobservableshavebeenmeasuredasbeing in good agreement with SM predictions, [4], implying a SM like Wilson Coefficient C , but still allowing for the 7 existenceofC(cid:48) (righthandedcurrents).Asstressedabove, 7 themeasurementismostsensitiveatlowq2.Itwouldthere- forebepreferabletoperformtheanalysisusingelectrons. However,experimentallyitismorechallengingtoobserve electronsthanmuons,primarilyduetothefactthatmuons provideaverycleansignaturetotriggeron.With309pb−1 ofLHCbdata, B → K∗µ+µ− intheq2 range0-2GeVhas beenobserved,asisshown,alongwithotherq2 ranges,in Fig. 2. The mass distributions of B → K∗µ+µ− in six q2 bins. Fig.2.Withtherestofthe2011data,onecanexpecttosee Thesolidlineshowsafitwithadouble-Gaussiansignalcompo- aB→ K∗e+e−signal. nent(thin-greenline)andanexponentialbackgroundcomponent (dashed-redline). a e-mail:[email protected] EPJWebofConferences When expressed in terms of the helicity amplitudes, for smallrealvaluesof AR,oneobtainsA(2) ≈−2Aright. AL T Aleft 4 B → K∗e+e− Monte Carlo studies Although work is ongoing on the analysis of the B → K∗e+e−data,andyieldpredictionsfromMonteCarlo(MC) havebeenvalidatedusingthecontrolchannelB→K∗J/Ψ withJ/Ψ →(e+e−),thereisnotyet,atthetimeofthiscon- ference,enoughdatatoperformtheanalysisortestthefit- tingprocedure.ToyMCstudieshavethereforebeencarried Fig. 3. Definition of the angles φ, θK and θL in the decay B → out for this purpose [6]. 190k signal events were gener- K∗e+e−. atedusingEvtGen,andseparatedintofilescontaining250 events:thepredictedyieldsfromMCstudieswith2fb−1at 3 Analysis formalism acentreofmassenergyof14TeV,excludingeffectsfrom LHCb’s high level trigger. By performing the fit on each B → K∗e+e− canbeuniquelydescribedbyfourvariables: file,itisshownthatwith200-250signaleventsandasig- q2andthreeangularvariables,θ ,θ andφ,(thedefinitions naltobackgroundratiooftheorderof1,aprecisionof0.2 L K of which can be seen in Fig. 3). Following the formalism isattainableonA2,equivalenttoanaccuracyonthefrac- T as described in [5], the differential decay distribution can tion of wrongly polarised photons of 0.1. An example of bewrittenintermsofthesevariablesas: oneofthefitsforonetoyMCstudycanbeseeninFig.4. Theanalysisalsodemonstratesthatthemeasurementsare dΓ = not sensitive to the knowledge of the angular acceptance, dq2dcosΘdcosΘ dφ l K andhenceshallnotbesystematicslimited. 9 [I (cosΘ )+I (cosΘ )cos2Θ+ 32π 1 K 2 K l I (cosΘ )sin2Θ cos2φ+I (cosΘ )sin2Θ cosφ+ 3 K l 4 K l I (cosΘ )sinΘ cosφ+I (cosΘ )cosΘ+ 5 K l 6 K l I (cosΘ )sinΘ sinφ+I (cosΘ )sin2Θ sinφ+ 7 K l 8 K l I (cosΘ )sin2Θ sin2φ] (1) 9 K l WhenmeasuringthisrateatLHCb,the3Dangularaccep- tance,ε(cosΘ,cosΘ ,φ)mustalsobetakenintoaccount. l K It is assumed to be factorisable as the products of ε , the 1 acceptanceasafunctionofφ,andε ,theacceptanceasa D functionofcosΘ andcosΘ .Furthermore,assumingthat K L ε isanevenfunction,Equation1canbesimplifiedbyper- 1 formingtheφtransformationthatifφ>0,thenφ=φ+π.A similartransformationcanbeperformedforcosΘ .Equa- L tion1canthenbewrittenas: Fig.4.ExampleofthefitofcosΘ ,cosΘ andφforonetoyMC L K studycontaining250signaleventsandnobackgroundevents. dΓ = dq2dcosΘdcosΘ dφ l K 9 [I (cosΘ )+I (cosΘ )cos2Θ+ 32π 1 K 2 K l I (cosΘ )sin2Θ cos2φ+I (cosΘ )sin2Θ sin2φ] References 3 K l 9 K l ×ε (cosΘ,cosΘ ) (2) D l K 1. for example E. Lunghi and J. Matias, J. High Enerfy In order to minimize theoretical uncertainties, it is desir- Physics,04,(2007)058 able to measure ratios of the amplitudes. Neglecting the 2. Y.GrossmanandD.Pirjol,J.HighEnerfyPhysics,06, lepton mass, the remaining I terms in equation 2 can be (2009)029 writtenintermsofthreesuchparameters,F ,A(2),A : 3. LHCB-CONF-2011-039 L T Im 4. C. Bobeth, G. Hiller and D. van Dyk, J. High Enerfy |A |2 Physics,07,(2010)098 F = 0 L |A0|2+|A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2 5.0C94F0.0K9ruger and J. Matias, Physics Rev, D71, (2005) (cid:12) (cid:12) A(2) = |A⊥|2−(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)2 (3) 6. J.LeFranc¸ois,M.H.Schune,LHCb-2009-008 T |A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2 (cid:61)(A∗ A )−(cid:61)(A∗ A ) A = ⊥L ⊥L ⊥R ⊥R Im |A0|2+|A⊥|2+(cid:12)(cid:12)(cid:12)A(cid:107)(cid:12)(cid:12)(cid:12)2