Universita` degli Studi di Torino Facolta` di Scienze M.F.N. Laurea Specialistica in Fisica delle Interazioni Fondamentali K0 Study of L identification with BaBar detector Mario Pelliccioni Relatore: Controrelatore: Prof. Diego Gamba Dott. Marco Monteno Corelatore: Dott.ssa Marcella Bona Anno Accademico: 2004-2005 ii MARIO PELLICCIONI Contents Introduction . ii 1 Theoretical Motivations 5 1.1 Standard Model and CKM matrix . 5 1.2 Measurement of CP Violation . 9 1.3 Pure-penguin B-meson decays . 11 2 The BaBar experiment 15 2.1 Introduction . 15 2.2 The PEP-II B factory . 16 2.2.1 Luminosity . 19 2.2.2 Machine background . 19 2.2.3 Detector Overview . 21 2.3 Tracking System . 21 2.3.1 Silicon Vertex Tracker . 22 2.3.2 Drift Chamber . 24 2.3.3 Detector of Internally Reflected Cerenk˘ ov Light (DIRC) . 25 0 3 KL detection and identification 31 3.0.4 Introduction . 31 3.1 The Electromagnetic Calorimeter and Instrumented Flux Return . 31 3.1.1 The Electromagnetic Calorimeter EMC . 31 3.1.2 The Instrumented Flux Return IFR . 36 3.1.3 The IFR Upgrade Project . 39 iv 3.2 EMC cluster shape variables . 41 4 Data-MC EMC cluster shape variables comparison 45 4.0.1 Introduction . 45 4.1 K sample . 47 4.2 Data-MC comparison . 48 4.3 Conclusions . 48 0 5 A low momentum KL sample 51 5.1 D* sample . 51 5.2 Event selection . 52 5.3 ∆m fit . 58 0 6 A high momentum KL sample 61 6.1 The e+e φγ sample . 61 − ! 6.1.1 Event selection . 62 0 6.1.2 KL cluster selection . 63 6.1.3 Missing mass fit . 64 0 7 KL selector implementation and performances 69 7.1 Neural Network selector . 69 7.1.1 An introduction to the neural networks . 69 7.1.2 NN configuration . 72 7.1.3 Neural Network training and validation . 73 7.2 Likelihood selector . 75 0 A A KL sample feasibility study 85 A.1 Event selection . 86 A.1.1 Event selection for D0 K ρ . 87 ! ∗ MARIO PELLICCIONI 1 A.1.2 Event selection for D0 K0! . 89 ! L Bibliography . 95 2 MARIO PELLICCIONI Introduction The primary physics goal of the BABAR experiment is to study CP violation in neutral B decays. The Standard Model (SM) theory with three quark generations leads naturally to a CP asymmetry, which is represented by an irreducible complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix [1, 2]. Charmless B decays through hadronic states supply an important way of determining the angles of the Unitarity Triangle (UT ); in many of them, neutral kaon production is expected. For instance, CP asymmetry in B0 J= K0 has given one of the most precise measurements of sin 2β [3]. Decays B 0 K+K K0 ! ! − are dominated by b sss gluonic penguin amplitudes, suppressed by elements of the CKM matrix, but they may also be affected! by new physics amplitudes, thus bringing to evidences of new physics beyond SM. 0 This thesis mainly concerns KL identification in BABAR. Neutral kaons are detected, through hadronic interaction, in the Electromagnetic Calorimeter (EMC) and in the Instrumented Flux Return (IFR). The former is however calibrated for electromagnetic showers, which differ from hadronic ones mostly beacuse of their shape; thus it is mandatory to define cluster shape variables in order to distinguish these two kinds 0 of showers. These variable distributions will be then used to implement a selector for K L, using both a Likelihood function and a Neural Network. IFR information may be used to better discriminate kaons from other neutral particles, and to have a more precise reconstruction of particle trajectories. However, the time-dependent and decreasing RPC efficiency avoids a massive use of this subdetector. In the first chapter, a general description of B physics is given. It is mostly oriented to show the importance of studies involving B decays in a K 0. The second chapter is a layout of PEP-II B factory and BABAR detector, their features and performances, 0 except for those subdetectors involved in KL identification. In the third chapter, a focus on EMC and IFR is given, including a description of the new IFR upgrade to Limited Streamer Tubes, in which I personally 0 took part. The main tecniques for KL identification are discussed too. Chapter four shows a comparison between data and Monte Carlo for EMC cluster shape variables using a charged kaon sample. 0 The fifth chapter concerns the selection of a low momentum KL sample, while in the sixth one the descrip- 0 tion of a high momentum KL sample is reported. The differences between the two samples have brought to 4 distinct selector implementations. In the last chapter, the two selectors produced are described, along with their validation. MARIO PELLICCIONI 1 Theoretical Motivations 1.1 Standard Model and CKM matrix In the Standard Model (SM), elementary fermion fields are grouped by their chirality. By demanding local gauge invariance, weak interactions are introduced as coupling of left-handed spinor doublets to the W boson vector triplet. By the same principle, particle masses are generated with the Higgs mechanism [11] by couplings of the left-handed doublets and the remaining right-handed singlets to a scalar doublet with spontaneously broken symmetry. In the quark sector, where all particles have non-vanishing masses, the u d Higgs mechanism produces two independent mass matrices Mαβ and Mαβ. They generate the masses of the “up”-type and the “down”-type members of the quark doublets, respectively, in the three-family space αβ. The fact that up-type and down-type quarks are grouped in left-handed doublets implies that only one matrix Mαβ can be diagonalized at a time. It follows that only one type of quarks can be turned into mass eigenstates. For the other type, mass eigenstates and chiral eigenstates are separated by a unitarity transformation. By definition, left-handed doublets u c t = A d0 ! s0 ! b0 ! contain the mass eigenstates u,c,t, while the Cabibbo-Kobayashi-Maskawa (CKM) matrix Vαβ can be con- sidered as a rotation transformation from the quark mass eigenstates d,s and b to d0, s0 and b0 states; its most general representation is: d 0 Vud Vus Vub d s = V V V s : (1.1) 0 0 1 0 cd cs cb 1 0 1 b 0 Vtd Vts Vtb b @ A @ A @ A If the quark generations are mixed by the matrix Vαβ, CP T invariance requires that the associated antiquark generations are mixed by the complex conjugated matrix element Vαβ∗ . Thus, complex phases in the CKM matrix are the origin of CP violation in the SM. For n generations of quarks, V is a n n unitary matrix that depends on n2 real numbers (n2 complex entries × with n2 unitarity constraints). In the CKM matrix, not all of these parameters have a physical meaning since, given n quark generations, 2n 1 phases can be absorbed by the freedom to select the phases of the quark fields. A phase factor can be applied− to every quark operator so that the current µ could be written as: J 6 Theoretical Motivations iθd Vud Vus Vub de µ iθu iθc iθt 1 µ iθs = (ue− ; ce− ; te− ) γ (1 γ5) Vcd Vcs Vcb se J 2 − 0 1 0 iθb 1 Vtd Vts Vtb be @ A @ A Each u, c or t phase allows for multiplying a row of the CKM matrix by a phase, while each d, s or b phase allows for multiplying a column by a phase: the u, c and t phases can be chosen in order to make real one element of each of the three rows (for example Vus, Vcs and Vts). Therefore all three elements of a column (the second in the example) can be made real. In a similar way, the d, s and b phases can be chosen in order to make real one element of each of the three columns (for example Vud and Vcb). At the end of this redefinition procedure, five of the CKM matrix phases have been re-absorbed with six quarks: in general, with n quark families, 2n 1 phases can be removed. So it is: n2 (2n 1) = (n 1)2. From the latter, − − − − given 3 quark families, 4 real and independent parameters are necessary. A useful representation is obtained using the four Wolfenstein parameters (λ, A; ρ, η) with λ = sin θ C ' 0:22 playing the role of an expansion parameter and η representing the CP -violating phase [12]: λ2 3 1 2 λ Aλ (ρ iη) − 2 − V = λ 1 λ Aλ2 + (λ4): (1.2) 0 − − 2 1 O Aλ3(1 ρ iη) Aλ2 1 @ − − − A This approximate form is widely used, especially for B physics. The six unitarity conditions VαβVαγ∗ = 0 α X for the elements of the CKM matrix can be represented as triangles in the complex plane. The unitarity relationship between the first and the third columns of the CKM matrix, VudVub∗ + VcdVcb∗ + VtdVtb∗ = 0: (1.3) can be graphically visualized in the form of a unitarity triangle (the so called “unitarity triangle”), in which, after rescaling so that the base becomes of unit length (Fig. 1-1), the coordinate of the apex A on the complex plane becomes Re(V V ) Im(V V ) λ2 λ2 A = ud ub∗ + i ud ub∗ ρ(1 ) + iη(1 ) ρ + iη V V V V ≈ − 2 − 2 ≡ j cd cb∗ j j cd cb∗ j where the rotated ρ and η are introduced.