A CP violatjon measurement of B s mesons at ATLAS and the LHC Adam Barton ATLAS Collaboration
The LHC The large hadron collider is the world's largest and highest energy synchrotron collider in the world. It is built and run by CERN (the European/Everyone Organizatjon for Nuclear Research) It can collide protons at energies of 14 TeV, (currently running at 13TeV It is located in a 27 kilometer tunnel under Geneva near the Jura mountains. 2
The ATLAS (A Toroidal LHC ApparatuS) detector ATLAS is a 45 by 50 metres in size Muon Spectrometer: (1) Monitored Drift Tube (2) Thin Gap Chamber Magnet system: (3) End-Cap Toroid Magnet (4) Barrel Toroid Magnet Inner Detector: (5) Transition Radiation Tracker (6) Semi-Conductor Tracker (7) Pixel Detector Calorimeters: (8) Electromagnetic Calorimeter (9) Hadronic Calorimeter At the start of run 2 (2015) an insertable B-layer was installed to give better vertex and lifetime resolution 3
Data Collectjon 4
B-physics and Light-States ● ATLAS B-physics and Light-States programme: – Comprehensive measurements across a variety of decay modes: Precise property measurements including CPV (Bs->J/ψ φ) ● Cross-sectjon measurements including Quarkonium ● Rare decay processes; e.g FCNC B (s,d) →μμ ● Spectroscopy, exotjc states (e.g pentaquarks) ● Charged lepton fmavour violatjon (τ -> 3μ) ● ● Typically rely on low-pT di-muon signatures. 5
Introductjon to the CP violatjon ● Charge Parity (CP) symmetries mean that partjcle interactjons should produce matuer and antjmatuer in equal quantjtjes ● In 1967 Soviet Nuclear Physicist Andrei Sakharov proposed CP violatjon: ● Since the observed universe seems devoid of stable antjmatuer there must be baryon number violatjng transitjons in partjcle physics. ● CP has to be violated otherwise there would be equal amounts of antj matuer ● CP violatjons must occur during interactjons and not in thermal equilibrium 6
3 Types of CP violatjon 7
Exclusive decay chain ● While φ s can be accessed a number of ways the easiest way at ATLAS is through the exclusive decay B s → J/ψ ϕ where – J/ψ → μ + μ - selected nicely from the muon system – ϕ → K + K - ATLAS has no partjcle ID so this is diffjcult to isolate 8
CP Violatjon in neutral B s system Mixing of flavour eigenstates are governed by: The mass eigenstates ● Δm S = m H – m L ≈ 2|M 12 | ● φ SSM =arg(-M 12 /Γ 12 ) ≈ -0.04 - CP violating phase ● Γ is the average lifetime of the two states (Γ L +Γ H )/2 ● ΔΓ =Γ L -Γ H ≈ 2 |Γ 12 | cos(2 φ SSM ) – Can be considered the difference of the two lifetime ● states 9
Measuring a partjcle lifetjme 10
Angular Systems for B s → J/ψ ϕ ● You can access the key physical variables for this decay using one of 2 angular defjnitjons Transversity Basis Helicity Basis 11
What the signal looks like 12 Provisional MC Generation – no cuts applied so no acceptance effects
ATLAS Publicatjons Time dependent untagged ϕ s and ΔΓ s from B s →J/ψϕ JHEP 1212 (2012) 072 – ● 02-AUG-12 Time dependent fmavour-tagged ϕs and ΔΓs from B s →J/ψϕ at 7 TeV Phys. Rev. ● D. 90, 052007 (2014) 05-JUL-14 Time dependent fmavour-tagged ϕ s and ΔΓ s from B s →J/ψϕ in Run 1 JHEP 08 ● (2016) 147 13-JAN-16 Measurement of the CP violatjon phase ϕ s in B s →J/ψϕ decays in ATLAS at 13 ● TeV 23 Mar 2019 (Conf-Note going to publicatjon) Next paper will include all Run-2 data. ● 13
Deciding cuts 14
Deciding Cuts ATLAS-CONF-2019-009 ● This analysis follows are previous measurement using 19.2 fc -1 of √s=7 TeV and 8 TeV (“run 1”) ● The new analysis uses datasets from 2015 to 2017 with √s=13 TeV totalling 80.5 fc -1 . ● Full decay reconstructjon using inner detector and muon detectors, no K/pi separatjon: – J/ ψ selectjon – di-muon vertex χ2/ NDF <10, J/ ψ invariant mass windows width 0.27 ... 0.48 GeV (barrel → endcap) – ϕ selectjon – p T (K ± ) > 1 GeV, Invariant mass window 22 MeV – B candidates – 4-track vertex χ2/ NDF <3, (5.15 – 5.65) GeV, no proper decay tjme cut. 15
Flavour Tagging ● The analysis gains precision with tagging informatjon. We use opposite-side tagging (OST). ● We use 4 tagging methods: “Tight” muons, electrons, Low-p T muons, Jet ● Charge of p T -weighted tracks in a cone around the opposite primary object, used to build per-candidate B s tag probability. ● Calibrated from B + → J/ψ K + sample 16
Tagging: weighted sum of charge in a cone In events where multiple methods are available the highest dilution is selected. 17
Signal Likelihood The solution with a negative CP +1 CP +1 ΔΓ S is excluded using another CP -1 LHCb measurement which determines the ΔΓ S to be Interfer positive ence terms S- wave terms 18
Mass spectrum including Direct Background descriptjon background To make a precision measurement it is necessary to ● either exclude or accurately describe the background The difgerent backgrounds present are: ● Direct pp → J/ψ background ● Misreconstructed complete decays such as B d → J/ψK* ● and Λ b →J/ψ Λ*(Kp) Miscellaneous combinatorics from bb→ J/ψX ● Mass spectrum excluding direct background by 19 lifetime cut
Unbinned Maximum Likelihood Fit B d →J/ ψ K*(K П ) and Λ b →J/ψΛ*(Kp) decay reflections , derived from MC, PDG and the LHCb Λ b →J/ΛKp Measured variables: measurement; fixed shape and relative contribution in the fit B s mass m i Combinatorial background description, derived from data B s proper decay time t i sidebands; angular distribution described by spherical and its uncertainty σ ti harmonics and fixed in the fit 3 angles Ω i ( θ T ,ψ T ,φ T ) Weights accounting for proper decay time trigger B s momentum p T efficiency (muons track d 0 reconstruction efficiency bias); B s tag probability p B|Qi 20 estimated from MC tagging method M i
Background with Monte Carlo 21
Direct jpsi background Background representatjon in the fjt Time component of background: ● Prompt background: delta function at 0, convoluted by Gauss per- – candidate resolution σ ti Two exponentials representing longer-lived backgrounds Total background – Small negative exponential component for events with poor vertex – resolution Background angular shapes ● Arise from detector and kinematic sculpting – Described by empirical functions with parameters determined in the fit – Background mass model – linear function ● ∗ and Λb→J/ψ Λ*(Kp) contamination treated separately B 0 → J/ψK 0 ● fractions are determined from MC – mass, angular shapes - from MC – 22 used in PDF but no free parameters of fit –
Angular Background The angular component of the background is shaped by detector and ● acceptance efgects producing a non-trivial 3D shape that is also p T dependent The mass side bands are taken and a Legendre polynomial functjon is used to ● fjt the shape. The resultjng parameters are fjxed and used in the main fjt. The dedicated backgrounds are simulated with monte carlo, their shaping ● applied and also fjt by spherical harmonics 23
Angular Background 24
Kinematjc Acceptance ● It is necessary to exclude (cut) low energy tracks to exclude large quantjtjes of background. ● The muon trigger applies at least a 4GeV cut on the muons (triggers vary according to the luminosity) ● Kaon cuts are applied afuer reconstructjon to reduce the background. ● This biases the angular distributjons distortjng the “true” distributjon. ● This is atuained by simulatjng a naïve level of physics so the angular distributjons are fmat, and then feeding these events through the detector simulator and applying the standard cuts. 25
What Acceptances look like (mu4mu4) Helicity Transversity Chi (mix) Costheta Phi (mix) Costheta1 Costheta2 (phi) Cospsi (phi) (muon) (muon) All pT Pt > 21000 Pt < 21000 26
Fit Projectjons 27
Systematjc Uncertaintjes Uncertainty in the calibration of the B s -tag probability; MC statistical uncertainty included in fit stat. error Alternative detector acceptance fit-functions and binning determined from MC Radial expansion uncertainties determined from their effect on tracks d 0 in the data Background angles model (fixed in UML fit) extracted from data with varying sidebands size and binning Uncertainties of relative fraction; fit-model and P-wave contribution Uncertainties of relative fraction; fit-model and contributions from Λb→J/ψΛ* decays 28 Toy-MC studies; pulls of the default fit model, default fit on toy-data generated with modified PDFs
→ ϕ Study Result of the CPV Bs J/ψ Fit correlation matrix: 29
Combination with 7 TeV and 8 TeV results ● We present a combined result (BLUE) of this result with our previous “run-1” result. 30
LHCb - 2019 LHCb have recently released an updated result. ● LHCb has partjcle ID hardware allowing them to signifjcantly reduce background, but ● cannot record as much luminosity reducing statjstjcs Resultjng in a worse statjstjcal error but betuer systematjc error. ● Φ s = −0.083 ± 0.041 ± 0.006 rad ΔΓ s = 0.077 ± 0.008 ± 0.003 ps−1 ATLAS 31
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