Table Of Content

b g BABAR: sin(2 + ) 6 0 0 2 n a J 0 1 Cecilia Voena† ∗ Università“LaSapienza”diRomaandINFNRoma,Italy 1 E-mail: [email protected] v 8 1 0 Thetime-dependentCPasymmetriesinfullyreconstructedB0 D( ) p /r decays(newpre- ∗± ∓ ∓ 1 → liminary result), and in partially reconstructed B0 D( ) p decays, are measured with the 0 ∗± ∓ → 6 BABARdetector at the PEP-II asymmetric B factory at SLAC, using 232 million¡ (4S) BB 0 → decays. We combine the above results and, using other measurementsand theoretical assump- / x tions, we interpretthem in terms of the anglesof the unitarity triangle describingthe Cabibbo- e - Kobayashi-Maskawamatrix. We find sin(2b +g ) >0.64(0.42)at68%(90%)confidencelevel p | | e usingafrequentisticapproachand 2b +g =(90 43)ousingaBayesianapproach. h | | ± : v i X r a InternationalEurophysicsConferenceonHighEnergyPhysics July21st-27th2005 Lisboa,Portugal Speaker. ∗ †OnbehalfoftheBABARcollaboration BABAR: sin(2b +g ) CeciliaVoena 1. Introduction Inthe Standard Model,CP violation inthe weak interactions between quarks manifests itself as a non-zero area of the Cabibbo-Kobayashi-Maskawa (CKM) unitarity triangle [1]. Constraints on the quantity sin(2b +g )canbe obtained from the study of thetime evolution ofB0 D( ) p ∗ ± ∓ → andB0 D r decays[2]thatissensitivetog duetointerferencebetweentheCKM-favoredb c ± ∓ → → andtheCKM-suppressed b utransitions throughB0B0 mixing. → The BABARdetector [3] collects data at the PEP-II asymmetric e+e collider operated at or − near the ¡ (4S) resonance. BB pairs from the ¡ (4S) decay move along the high-energy beam direction with a Lorentz boost bg =0.56. The time dependent distribution for B0 decays to a final state m (m =Dp ,D p andDr ),isgivenby(neglecting thedecaywidthdifference): ∗ e D t/t f ,m (h ,D t) = −| | [1 (am h b h cm ) ± 4t × ∓ ∓ − sin(D m D t) h cos(D m D t)], (1.1) d d ∓ where t is the B0 lifetime, D m is the B0B0 mixing frequency and D t =t t is the time of d rec tag − the B0 D( ) p or B0 D r decay (B ) relative to the decay of the other B (B ) from the ∗ ± ∓ ± ∓ rec tag → → ¡ (4S) BB decay. D t is calculated from the measured separation along the beam collision axis → (z), between the B and B decay vertices: D z=bg cD t. In equation 1.1 the upper (lower) sign rec tag refers to the flavor of B as B0 (B0), while h =+1 ( 1) for the final state with a D( ) (D( )+). tag ∗ − ∗ − IntheStandardModel: am = 2rm sin(2b +g )cosd m , b = 2r sin(2b +g )cosd , ′ ′ cm = 2cos(2b +g )(rm sind m r sind ). (1.2) ′ ′ − The parameters r and d accounts for non-negligible CP violating effects on the B side that ′ ′ tag m depend on the type of events used to determine the B flavor. We use only the c parameters of tag events tagged with leptons (cm ) and the am parameters in the determination of sin(2b +g ) since lep theyareindependent ofr andd (semileptonic Bdecayshaver =0). ′ ′ l′ep In the following the analysis based on fully reconstructed B0 D( ) p /r decays (Sec- ∗ ± ∓ ∓ → tion2),andwithpartiallyreconstructed B0 D( ) p decays(Section3)arediscussed. Section4 ∗ ± ∓ → presents theinterpretation oftheresultsintermsofsin(2b +g ). 2. Fully reconstructed B0 D( ) p /r decays ∗ ± ∓ ∓ → The analysis is based on a sample of 15635 B0 D p , 14554 B0 D p and 8736 ± ∓ ∗± ∓ → → B0 D r decays selected from232million¡ (4S) BBdecays[4]. Thepurity ofthesamples ± ∓ → → is82% 93%dependingonthedecaymode. WereconstructtheB byfullyreconstructingallthe rec − particlesinthefinalstate. B0candidatesareselectedusingthedifferencebetweentheenergyofthe candidate andthebeamenergy √s/2inthecenterofmassframe,andthebeamenergysubstituted mass, calculated from √s/2 and the reconstructed momentum of the B0. After the selection, two kinds of backgrounds remain: combinatorial background from random combination of tracks in 2 BABAR: sin(2b +g ) CeciliaVoena the events, estimated from data, and background coming from misreconstructed B decays, that maypeakintheenergy substituted massneartheBmassandisdetermined fromsimulation. WedeterminetheB decayvertexfromitscharged tracks. TheB decayvertexisobtained rec tag byfittingtracksthatdonotbelongtoB usingconstraints fromtheknownbeamenergyandfrom rec theposition oftheluminousregion. TheD t resolution isapproximately 1.1ps. Weusemultivariatealgorithmsthatidentifysignatures intheBdecayproductsthatdetermine the flavor of the B to be either a B0 or a B0 as primary leptons from semi-leptonic B decays, tag kaonsandsoftpionsfromD decays. Wecombinethemusinganeuralnetworkandweassigneach ∗ event with mistag probability less than 45% toone of six hierarchical, mutually exclusive tagging categories. Theeffectiveefficiencyofthetaggingalgorithm, definedasQ=S e (1 2w)2,where i i i − e andw aretheefficiencyandthemistagprobability forthetagging categoryi,is(30.5 0.4)%. i i ± An unbinned maximum-likelihood fit is performed to the D t distribution of Eq. 1.1, appro- priately modified to take into account the finite detector D t resolution, the mistag probability, and everysignificant sourceofbackground. Theresolution function andthemistagparameters aswell asmanybackgroundparametersaredeterminedfromthefittothedatasimultaneouslywiththeCP parameters. Fromtheunbinnedmaximumlikelihoodfitweobtain: aDp = 0.013 0.022(stat.) − ± ± 0.007(syst.),cDp = 0.043 0.042(stat.) 0.011(syst.),aD∗p = 0.043 0.023(stat.) 0.010 lep − ± ± − ± ± (syst.), cD∗p = 0.047 0.042(stat.) 0.015(syst.), aDr = 0.024 0.031(stat.) 0.010(syst.) lep ± ± − ± ± and cDr = 0.098 0.055(stat.) 0.019(syst.). Figure 1 shows the D t distribution for events lep − ± ± taggedwithleptonsforthesamplewiththehighestpurity, B0 D p . Thebiggest contribution ∗± ∓ → tothesystematicuncertainty comesfromeffectsreleated tothemeasurementofD t. 3. Partiallyreconstructed B0 D( ) p decays ∗ ± ∓ → Theanalysis is based on 89290 B0 D p decays selected from 232 million¡ (4S) BB ∗± ∓ → → decays [5]. The B are partially reconstructed using the hard pion from the B decay and the soft pionfromtheD applyingkinematicconstraintsconsistentwiththesignaldecaymode. Signaland ∗ background areobtained fromfitstothecomputed Dmassonseveraldatasamples. Wedetermine the B decay vertexby fittingthehard pion track andthe B decay vertex from afitofallother rec tag tracksintheeventexcludingalltrackswithin1radoftheDmomentuminthecenterofmassframe. The flavor of the B is determined using leptons or kaons. The purity of the sample is 54% and tag 31%foreventstaggedwithleptonsandkaonsrespectively. TheCPparametersareobtainedwitha unbinned maximum likelihood fit and the result is: aD∗p = 0.034 0.014(stat.) 0.009(syst.), − ± ± cD∗p = 0.025 0.020(stat.) 0.013(syst.). The dominant systematic uncertainty comes from lep − ± ± possibleCPviolating contributions inthebackgrounds. 4. Summary andinterpretation ofthe result Wehavemeasured thetime-dependent distribution ofB0 D p decays using bothafully ∗± ∓ → reconstructed andapartially reconstructed sampleandofB0 D r decays usingafullyrecon- ± ∓ → structed sample, based on 232 million BB¯ pairs. We combine the above measurements to obtain constraints onsin(2b +g )usingbothafrequentistic andaBayesianapproach[6]. Giventhesmall sizeoftherDp ,rD p andrDr parameters( 0.02)wearenotabletoextractthemfromthefittothe ∗ ∼ 3 BABAR: sin(2b +g ) CeciliaVoena ps )ps ) 220000 ps )ps ) 220000 vents / ( 1 vents / ( 1 111111468468000000 vents / ( 1 vents / ( 1 111111468468000000 EE 112200 EE 112200 110000 110000 8800 8800 6600 6600 4400 4400 2200 2200 00 00 --1100 --55 00 55 1100 --1100 --55 00 55 1100 DD tt ((ppss)) DD tt ((ppss)) 1 ps )1 ps ) 2255 1 ps )1 ps ) 2255 nts / ( nts / ( 2200 nts / ( nts / ( 2200 veve veve EE EE 1155 1155 1100 1100 55 55 00 00 --1100 --55 00 55 1100 --1100 --55 00 55 1100 DD tt ((ppss)) DD tt ((ppss)) Figure1: D t distributionondataforthedecaymodeB0 D p intheleptontaggingcategoryfor,from ∗± ∓ → upper left going clockwise, B =B0 and B =D p +, B =B0 and B =D +p , B =B0 and tag rec ∗− tag rec ∗ − tag B =D p +,B =B0 andB =D +p . Theresultofthefitissuperimposed.Ineachplot,thedashed rec ∗− tag rec ∗ − curverepresentsthebackgroundcontribution. data. Instead we estimate these parameters using SU(3) symmetry relations [5] and we assign a 30% (frequentistic approach) or100% (Bayesian approach) uncertainty to SU(3)breaking effects and other theoretical uncertainties. We find: sin(2b +g ) >0.64(0.42) at 68%(90%) confidence | | leveland 2b +g =(90 43)o inthefrequentistic andintheBayesianapproach, respectively. | | ± References [1] N.Cabibbo,Phys.Rev.Lett.10,531(1963);M.KobayashiandT.Maskawa,Prog.Theor.Phys.49, 652(1973). [2] I.Dunietz,Phys.Lett.B427,179(1998);I.Dunietz,R.G.Sachs,Phys.Rev.D37,3186(1988). [3] BABARCollaboration,B.Aubertetal.,Nucl.Instrum.Meth.A479,1(2002). [4] BABARCollaboration,B.Aubertetal.,hep-ex/0507075. [5] BABARCollaboration,B.Aubertetal.,Phys.Rev.D71,112003(2005). [6] CKMFitterGroup(J.Charlesetal.),Eur.Phys.JC41,1-131(2005);M.Bonaetal.,JHEP0507,028 (2005). 4