Λ b Polarization Measurement at ATLAS

Transcription

1 Λ Polaization Measuement at ATLAS Beauty th Intenational Confeence on B-Physics at Hadon Machines - June Assisi Italy. Michela Biglietti 1 on ehalf of the ATLAS Λ woking goup: Eduad De La Cuz Buelo Claudio Feetti Home A Neal Natalia Panikashvili 4 Máia Smizanská 3 Shlomit Taem 4 1. INFN Naples Italy. Univesity of Michigan USA 3. Univesity of Lancaste UK 4. Technion Institute of Technology Isael

2 Λ Poduction at LHC Due to the paity consevation in the stong inteactions at LHC Λ can e poduced with polaization othogonal to the poduction plane... ut why polaization? Lage nume of unansweed questions aout the ole of spin in poduction of hypeons at high enegies which we wish to exploe Λ hypeon (uds) Why is the polaization is so lage? Why is the shape so unusual? Λ hypeon (ud) Ae the mechanisms of Λ and Λ polaization diffeent? Ae the s and poduction mechanisms simila? Does the polaization depend on quak mass? K. Helle Pocedings of the 9th Intenational Symposium on High Enegy Physics Bonn (1990)

3 Λ Decay The consideed decay Λ J/ψ(µµ) Λ(pπ) is chaacteizes y 4 helicity amplitudes and the asymmety paamete α caused y the paity non consevation of the weak inteactions 1/ 1/ 1 a = Α(1/0) a = Α( 1/0) = Α( 1/ 1) = Α(1/1) α = helicity amplitudes and asymmety paamete α not yet measued they can in pinciple e pedicted using PQCD and thus e a tool to test theoetical models and to seach fo new physics Othe inteesting studies fo this decay channel Lifetime of Λ hypeon CP violation in ayons a a a a 3

4 Λ in ATLAS Fo Low luminosity opeations L= cm - s -1 p(7 TeV) L= 10f -1 O(10 1 ) Λ pe yea p(7 TeV) In the fist 3 yeas O(10 5 ) Λ J/ψ (µ µ ) Λ (πp) (ATLAS TDR estimation) µ µ π The wold s iggest sample so fa is ~ O(10 ) Λ J/ψ (µ µ) Λ (π p) Λ J/ψ Λ Λ p 4

5 Measuement of Λ J/ψ Λ The Λ polaization (P ) and decay paametes can e detemined fom the angula distiutions p.d.f. of the cascade decay Λ J/ψ(µµ)Λ(πp) p.d.f. depends on 5 angles 6 paametes of 4 complex helicity amplitudes and polaization P ( 7 unknowns). 5

6 What angula distiutions do we expect? Poaility distiutions: Cos() Cos( 1 ) Cos( ) P = 0% P = -40% P = 40% φ 1 φ 6

7 A Model fo Λ Decay The geneal amplitude fo the decay Λ Λ J/ψ is given y: M = Λ(p Λ )ε * µ (p J ψ ) A γ p µ µ µ Λ µ Λ 1 γ5 A γ5 B1γ B Λ (pλ ) m Λ m Λ p ε µ * - polaization vecto of the vecto meson (J/ψ) A 1 A B 1 B - complex decay amplitudes ae calculale within the famewok of PQCD models using factoization theoems (Phys. Rev. D65: hep-ph/011145) Helicity amplitudes elated to A 1 A B 1 B : a = i a - = i = i - = i Units x10-10 α =

8 Geneation of Polaized Λ In ode to study the feasiility of measuing Λ polaization in ATLAS we have developed a way to make polaized Λ in EvtGen EvtGen: oiginally witten in BaBa and CLEO now also in MC/LCG poject package poviding a famewok fo implementation of pocesses elevant to B physics uses spino algea and helicity amplitudes it s possile to set the polaization of the paticle efoe the paticle is decayed.. and otain the coect angula distiutions Pythia -Bayons EvtGen uses spino algea and helicity amplitudes 1. Set polaization. Two ody decay y specifying the helicity amplitudes fo the final state paticles HepMC Fist eve adaptation of EvtGen as a geneato of polaized ayons 8

9 EvtGen Coss Check 5 angula distiutions ae geneated using oth the poaility function and using the full amplitude featue in EvtGen cos() Slope~α P cos( 1 ) Slope~α 0 cos( ) Po HelAmp φ 1 φ Vey good ageement etween the angula distiutions geneated with EvtGen and the poaility density function! P =-40% Blue squaes ae events geneated accoding to poaility function Red cicles ae events geneated with helicity amplitudes in EvtGen 9

10 Data Simulation Reconstuction and Analysis Events geneated with Pythia/EvtGen seveal input models Poduction cuts: p T (µ 1 ) >.5GeV p T (µ ) > 4.0GeV p T (π/p) > 0.5 GeV η <.7 Full simulation in ATLAS detecto with Geant4 Recostuction Dedicated algoithm fo low p T muons developed Bette efficiency fo J/ψ identification Inne Detecto tacking and vetexing algoithms fo Λ and Λ identification Angula distiutions detemination Analysis MC toy fo "fast" pedictions Maximum Likehood method to measue P and Λ decay paametes 10

11 Events Jpsi Inv Mass Event Reconstuction J/ψ mass Enties mass Enties 8498 Mean 3097 ε = 80% σ= 40.5 MeV RMS 8.9 χ / ndf 37.6 / 8 Constant 1.15e004 ± 57 Mean 3094 ± Sigma ± Events ΛΛ mass χ / ndf / 66 N 1.98e004 ± 55 µ ± 0.0 σ 1.3 ± N ε = 40% 3.5e004 ± ± a σ =.8 MeV -.516e005 ± ± 0.1 c ± d ± Mass (MeV) Λ Enties mass 5805 Λ RMS 4.4 mass N e004 ± µ ε 1 = 8% 5638 ± σ ± e004 ± σ ± 0.73 Events σ = 1.5 MeV Mass (MeV) ε = ec efficiency σ fom Gaussian fits

12 Angula distiutions in ATLAS detecto ATLAS acceptance modifies the angula distiutions and inceases the difficulty of the measuement Events cos_theta Cos() Cos() Reconstucted events Geneated h_ctheta events Enties 8391 Mean RMS Events cos_theta Cos( 1 ) Cos( 1 ) h_ctheta1 Enties 8391 Mean RMS Reconstucted events Geneated Events Events cos_theta cos( ) Reconstucted h_ctheta events Cos( Enties 8391 Geneated events Mean RMS ) cos() cos( 1 ) cos( ) Events phi φ φ 1 φ 1 Reconstucted events Geneated events h_phi1 Enties 8391 Mean RMS Events phi φ φ φ Reconstucted events Geneated events h_phi Enties 8391 Mean RMS P=-100% Reconstuct ed angle vs Geneated angle 1

13 Angula Resolutions Events Cos( cos( ec ))-Cos( - cos( gen ) ) gen Enties 5805 Mean 1.959e-005 RMS χ / ndf / 59 N 1.017e004 ± 783 µ e-005 ± 1.573e-005 σ ± N 8568 ± σ ± N 3.737e004 ± 678 σ ± Angles Gaussian fit σ (mad) cos cos 1 1 φ1 3 cos ec gen cos( ) - cos( ) φ 13 13

14 Maximum Likelihood Results = Output - Input 0 In the polem we have 9 paametes: P and fou complex amplitudes iα iα iβ iβ a = a e a = a e = e = e The nomalization gloal phase constaint 7 independent paametes α ; = a 0 a ; 1 = a a ;φ = α β ;φ = α β ; χ = β β ; P The ML esults (Toy MC sample angula esolution fom econstuction detecto acceptance) only statistical uncetainties ae shown Paamete Pulls Toy MC - Likelihood fit esults (P=-0%) : α Λ α - : 0 = 0 a 1 a φ - 3 : 1 = a - a φ χ : φ - 5 : φ : β 7 : Polaization P α Λ = K events = Output - Input Toy MC - Likelihood fit esults (P=-40%) 0 Paamete Pulls 1 : α α Λ 0 1 φ - φ χ : 0 = a a - 3 : 1 = a - a 4 : φ - 5 : φ 6 : β 7 : Polaization P P = - 0% Paametes P = - 40% α Λ = K events

15 Stat. uncetainty Decoupling P and α α Pcos() dominant tem The 5 angle method allows to decouple P and α. Estimating the statistical uncetainties 1) α can e measued with good pecision even if P~0. ) Small uncetainty (P) even fo P~ statistical uncetainty of α Statistical uncetainty of α in function of the polaization P 0.03 α = P) Coelation (α Stat. uncetainty Coelation of α and polaization P in function of P α = Statistical uncetainty σ P in function of the polaization P α coeletion of α and P statistical uncetainty of P = P P P

16 Summay and Outlook We have done an extensive study of the feasiility of measuing Λ paametes in ATLAS: Geneato model in EvtGen fo poducing polaized Λ in ATLAS Λ econstuction fom thei decay into Λ s and J/ψ Polaization and amplitudes extaction fom the decay angles Effects of angula esolution on the detemination of the paametes Coelations etween α and polaization Futue plans make ackgound studies continue to develop likelihood fitting pocedues incopoate tigge in analysis make Lifetime feasiility studies develop plans fo CP studies 16

17 Back-up Slides 17

18 Acceptance Effects Geneato Level: p T (pπ)>0.5 GeV p T (µ)>.5 4 GeV η <.7 ATLAS acceptance modifies the angula distiutions It s possile to coect distoted angula distiutions with weight factos cos() cos( 1 ) cos( ) geneated distiutions φ 1 φ geneated afte cuts afte coections poton pt (MeV) cos() cut To calculate the coections phase space events (without any model o cuts) ae used18

19 19 Maximum Likelihood (ML) Method ML Mehod : ' ') ( ) ( ') ( ) ( ) ' ( α α α d d T A w d T A w A w os = = = = Λ P a a A α α φ φ 1 1 ') ( ') ( ') ( ε : Measued angles : Geneated angles whee: R T = R( ) is the esolution ε( ) is the acceptance coection ε F i )d ( ) ( Acceptance coection nomalization : i=0...19

20 Backgounds Dominated y J/ψ fom -hadons Λ fom heavie hadons and fagmentation fake µ and cominatoial ackgound found negligile in simila topologies (B 0 J/ψ K s ) eal J/ψ fom cascade decay estimated at pecent level (ex. Ξ -0 Ξ -0 J/ψ Λ π -0 J/ψ) B 0 J/ψ K s whee a poton mass is given to a p ejected with mass constaints fo non diect Λ fom Σ ; ty to econstuct Σ to educe the diluition of polaization 0