Beauty05 biglietti

Lb Polarization Measurement at ATLAS: 

Lb Polarization Measurement at ATLAS Michela Biglietti1 on behalf of the ATLAS Lb working group: Eduard De La Cruz Burelo2, Claudio Ferretti2, Homer A Neal2, Natalia Panikashvili2,4 Mária Smizanská3, Shlomit Tarem4 1. INFN Naples, Italy 2. University of Michigan, USA 3. University of Lancaster, UK 4. Technion Institute of Technology, Israel Beauty 2005 - 10th International Conference on B-Physics at Hadron Machines - June 20 - 24 2005, Assisi, Italy.

Lb Production at LHC: 

Lb Production at LHC Due to the parity conservation in the strong interactions, at LHC Lb can be produced with polarization orthogonal to the production plane ... but why polarization ? Large number of unanswered questions about the role of spin in production of hyperons at high energies which we wish to explore L hyperon (uds) Why is the polarization is so large ? Why is the shape so unusual ? Lb hyperon (udb) Are the mechanisms of L and Lb polarization different ? Are the s and b production mechanisms similar ? Does the polarization depend on quark mass? K. Heller, Procedings of the 9th International Symposium on High Energy Physics, Bonn (1990)

Lb Decay: 

Lb Decay The considered decay, Lb J/y(mm) L(pp), is characterizes by 4 helicity amplitudes and the asymmetry parameter ab caused by the parity non conservation of the weak interactions 1/21/2 +1 helicity amplitudes and asymmetry parameter ab not yet measured they can, in principle, be predicted using PQCD, and thus be a tool to test theoretical models and to search for new physics Other interesting studies for this decay channel Lifetime of Lb hyperon CP violation in baryons

Lb in ATLAS: 

Lb in ATLAS For Low luminosity operations L= 1033 cm-2 s-1 ∫ L= 10fb -1  O(1012) Lb per year In the first 3 years O(105) Lb J/y (m+m-) L (pp) (ATLAS TDR estimation) The world’s biggest sample so far is ~ O(102) Lb J/y (m m) L (p p) p(7 TeV) p(7 TeV)

Measurement of Lb J/y L : 

Measurement of Lb J/y L The Lb polarization (Pb) and decay parameters can be determined from the angular distributions p.d.f. of the cascade decay LbJ/y(mm)L(pp) p.d.f. depends on 5 angles + 6 parameters of 4 complex helicity amplitudes and polarization Pb (  7 unknowns).

What angular distributions do we expect?: 

What angular distributions do we expect? P = 0% P = -40% P = 40% Cos(q) Probability distributions:

A Model for Lb Decay: 

A Model for Lb Decay The general amplitude for the decay b  J/ is given by: em* - polarization vector of the vector meson (J/) A1, A2, B1, B2 - complex decay amplitudes are calculable within the framework of PQCD models using factorization theorems (Phys. Rev. D65:074030, 2002, hep-ph/0112145) Helicity amplitudes related to A1, A2, B1, B2: Units x10-10 ab = - 0.457 a+ = -0.0176 - 0.4290i a- = 0.0867 + 0.2454i b+ = -0.0810 - 0.2837i b- = 0.0296 + 0.8124i

Generation of Polarized Lb: 

Generation of Polarized Lb In order to study the feasibility of measuring Lb polarization in ATLAS we have developed a way to make polarized Lb in EvtGen EvtGen: originally written in BaBar and CLEO, now also in MC/LCG project package providing a framework for implementation of processes relevant to B physics uses spinor algebra and helicity amplitudes it’s possible to set the polarization of the particle before the particle is decayed .. and obtain the correct angular distributions First ever adaptation of EvtGen as a generator of polarized baryons

EvtGen Cross Check: 

EvtGen Cross Check 5 angular distributions are generated using both the probability function and using the full amplitude feature in EvtGen f1 f2 cos(q1) Slope~a0 cos(q2) HelAmp Prob cos(q) Slope~ab∙P Blue squares are events generated according to probability function Red circles are events generated with helicity amplitudes in EvtGen Very good agreement between the angular distributions generated with EvtGen and the probability density function! Pb=-40%

Data Simulation, Reconstruction and Analysis: 

Data Simulation, Reconstruction and Analysis Events generated with Pythia/EvtGen several input models Production cuts: pT(m1) > 2.5GeV, pT(m2) > 4.0GeV, pT (p/p) > 0.5 GeV, |h|< 2.7 Full simulation in ATLAS detector with Geant4 Recostruction Dedicated algorithm for low pT muons developed Better efficiency for J/y identification Inner Detector tracking and vertexing algorithms for L and Lb identification Angular distributions determination Analysis MC toy for "fast" predictions Maximum Likehood method to measure Pb and Lb decay parameters

Event Reconstruction: 

Event Reconstruction Lb mass e = 80% = 40.5 MeV e = 40% s = 2.8 MeV e = 28% s = 21.5 MeV e = rec efficiency s from Gaussian fits

Angular distributions in ATLAS detector: 

Angular distributions in ATLAS detector Cos(q) Cos(q1) Cos(q2) f1 f2 Reconstructed angle vs Generated angle P=-100% ATLAS acceptance modifies the angular distributions and increases the difficulty of the measurement

Angular Resolutions: 

Angular Resolutions Cos(qrec)-Cos(qgen)

Maximum Likelihood (ML) Method : 

Maximum Likelihood (ML) Method where: θ’: Measured angles θ : Generated angles R(θ,θ’) is the resolution ε(θ,θ’) is the acceptance correction ML Mehod : Acceptance correction normalization : i=0...19

Maximum Likelihood Results: 

The ML results (Toy MC sample + angular resolution from reconstruction + detector acceptance) only statistical uncertainties are shown Pb = - 20% Pb = - 40% Parameter Pulls Parameter Pulls Maximum Likelihood Results In the problem we have 9 parameters: P and four complex amplitudes The normalization + global phase constraint  7 independent parameters

Slide16: 

Decoupling P and b bPcos(q) – dominant term The 5 angle method allows to decouple P and b. Estimating the statistical uncertainties b can be measured with good precision even if P~0. Small uncertainty (P), even for P~0. statistical uncertainty of ab statistical uncertainty of P P P correletion of ab and P

Summary and Outlook: 

Summary and Outlook We have done an extensive study of the feasibility of measuring Lb parameters in ATLAS: Generator model in EvtGen for producing polarized Lb in ATLAS Lb reconstruction from their decay into L’s and J/y Polarization and amplitudes extraction from the decay angles Effects of angular resolution on the determination of the parameters Correlations between ab and polarization Future plans make background studies continue to develop likelihood fitting procedures incorporate trigger in analysis make Lifetime feasibility studies develop plans for CP studies

Backup...: 

Backup...

Acceptance Effects: 

Acceptance Effects ATLAS acceptance modifies the angular distributions It’s possible to correct distorted angular distributions with weight factors generated distributions generated after cuts after corrections f1 f2 cos(q1) cos(q2) cos(q) Cuts @ Generator Level: pT(p,p)>0.5 GeV pT(m)>2.5, 4 GeV |h|<2.7 To calculate the corrections, phase space events (without any model or cuts) are used proton pt (MeV) cos(q) cut

Backgrounds : 

Backgrounds Dominated by J/y from b-hadrons + L from heavier hadrons and fragmentation fake m and combinatorial background found negligible in similar topologies (B0 J/y Ks) real J/y from cascade decay estimated at percent level (ex. Ξb-,0 Ξ-,0 J/y L p-,0 J/y) B0 J/y Ks where a proton mass is given to a p rejected with mass constraints for non direct Lb from Sb; try to reconstruct Sb to reduce the diluition of polarization