1 EvtGen and  b Spin Studies in ATLAS First North American ATLAS Physics Workshop December 19-21, 2004 Michela Biglietti, Jacob Bourjaily, Homer A. Neal, Natalia Panikashvili, Maria Smizanska, Shlomit Tarem

2 Outline EvtGen overview  b Polarization and decay model in EvtGen Validation of Generators Acceptance studies Data processing First look to reconstruction Plans

3 EvtGen Overview Decay model package Originated in BaBar; authors Anders Ryd, David Lange et al Tuned against new data from BaBar, Belle, CLEO Large collaboration of experiments has accepted EvtGen as their principal B-decay package – LHCb, ATLAS, CMS, CDF, D0, BaBar, Belle Features works with helicity amplitudes uses spinor algebra Polarization Handles with sequential decay decays are independent each decay is described by a model the model is a class providing the decay amplitude users can easily write their models calculation of probabilities and spin density matrices are done by the framework

4 EvtGen in ATLAS External software interfaced with Generators package receives the un-decayed top particle from PythiaB Event written in HepMC format and saved in POOL, ready for simulation generates the decay chain according to the user input card PYTHIA Conversion to HepMC B-mesons HepMC B-mesons PDG Decay files HepMC Event Record HepMC decay products User input EvtGen

5 Probability Function   As a first step the probability function has been implemented in EvtGen and in a dedicated Monte Carlo generator (as a cross-check) Angular distribution  b  J/  (  )  (  p) depends on 5 angles + 6 parameters of 4 complex helicity amplitudes and polarization P b. Helicity amplitudes and P b have to be simultaneously determined (Maximum likelihood method). No software tools for implementing polarization models available in Pythia. Pythia uses a probabilistic approach to produce baryons and spin information is not used

6 Polarization in EvtGen Get unpolarized baryons from Pythia EvtGen 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 PythiaB HepMC EvtGen 1. Set polarization 2. Decay with helicity amplitudes b-Baryons B hadrons can be only polarized perpendicular to the production plane Calculate the polarization vector Calculate the spin density matrix Associate the density matrix to the particle Adopt EvtGen class “HELAMP” to introduce a theoretical model for Λ b decay

7 Results 5 angular distributions are generated using both the probability function and EvtGen   cos(   ) Slope~  0 cos(   ) Probes J/    HelAmp Prob cos(  ) Slope~  b ∙P From PQCD model (*): a + = i a - = i b + = i b - = i  b =-.457, P=-40% Blue squares are events generated according to probability function (in EvtGen) Red circles are events generated with helicity amplitudes in EvtGen *  b =|a + | 2 -|a - | 2 +|b + | 2 -|b - | 2

8 Input/Output Parameters Comparison cos(  1 ) and cos(  2 ) do not depend on Polarization

9 Acceptance Corrections Kinematics do not depend on the input model Proton p T Pion p T It’s possible to correct distorted angular distributions with weight factors original distributions after cuts after corrections MeV   cos(   )cos(   ) cos(  ) Generator Level: p T (p,  )>0.5 GeV p T (  )>2.5, 4 GeV |  |<2.7 To calculate the corrections, phase space events (without any model or cuts) are used

10 DC2/Rome Workshop Processing Production using Michigan processor clusters Pacman installation of patches for Generators and Muon Digitization 100k of events generated for EvtGen tests Helicity amplitudes and probabilty function with different polarization values acceptance and phase space studies. 300k of events generated for full simulation and reconstruction Different input models 30K of them simulated and digitizated with DC2 software (preproduction) shipped to CERN for reconstruction/analysis test Waiting for Rome software validation to start a massive simulation

11 First look to Reconstruction and Analysis J/ψ  μ + μ - reconstruction Dedicated low p T muon identification algorithm Λ  π - p reconstruction π and proton reconstruction in Inner Tracker Vertexing algorithms Analysis within AOD/Physics Analysis Tool Preliminary!

12 Mass Distributions J/   bb Mass (MeV)

13 Decay Length Resolution bb J/      p  bb mm

14 The need for identifying low p T muons Muons from J/ψ are rather soft (a few GeV) The lower the threshold for a second muon - the better our efficiency MOORE/MuId are inefficient for low p T muons Di-muon efficiency is too low 30% J/ψ  μ2.5 μ4 are identified, with purity 52% The algorithm The purpose is to identify low p T muons and increase the efficiency of J/ψ identification by improving the reconstruction of muons with low p T in events where one muon had sufficient momentum to trigger the event. The principle: Start with ID tracks and match muon digits to them 1.If we select for maximum efficiency, accepting each candidate that either has precision chamber digits or has η and  trigger chambers digits, the resulting efficiency is 80% with the purity ~ 32%. 2.If we require each candidate to have both precision chamber hits and trigger chamber hits, 60% of J/ψ particles are identified with purity 69%. Low p T Muon Identification Pt(MeV) Mass(MeV)

15 Plans Continue generation of events with different models and start a large scale simulation and digitization as soon as possible Understand which will be the good release for physics simulation Pacman installation of ATLAS release 9.0.X Reconstruction and analysis for polarization extraction Within the Physics Analysis Tools Inner detector particle candidates Muon particles candidates Interface to vertexing algorithms Calculation of the 5 angular distributions Production of histograms for data quality Produce ntuples for maximum likelihood studies Maximum likelihood studies Acceptance corrections in 5-D Background channels studies Understand the dangerous channels with fast simulations Full simulation of the dangerous channels Target : ATLAS Physics Workshop in Rome

16 Backup

17 Background studies Λ0 channels: Can’t find this channels after LVL1 trigger The trigger for this is a μ from b  J/ψ, with the Σ in coincidence. That means we don't produce it separately but search in b  J/ψ Λ0 production from Σ baryons: Σ*-/ Σ*bar  Λ0+π- Σ0/ Σbar0  Λ0+gamma Σ*0/ Σ*bar0  Λ0+π0 Σ*+/ Σ*bar-  Λ0+π+ Λ0 production from Ξ baryons: Ξ- / Ξbar+  Λ0+π - Λ0+νebar+e- Λ0+νμbar+μ- Generate using EvtGen (PHSP) Ξb-  Λ0+J/ψ+π – Λ0+J/ψ+νebar+e Λ0+J/ψ+νμbar+μ-

18 Background studies Λ0 channels: Λ0 production from Ξ baryons: Ξ0 / Ξbar0  Λ0+π0 Λ0+gamma Generate using EvtGen (PHSP) Ξb0  Λ0+J/ψ+π0 Λ0+J/ψ+gamma Λ0 and J/ψ production from bottom baryons: Λb0 / Λb bar0  Λ0+eta_c Λ0+J/ψ Λ0+chi_1c (Only in Pythia) chi_1c  J/ψ + gamma Generate using Pythia (PHSP) Λb0 / Λb bar0  Λ0+J/ψ+gamma