EvtGen and Lb Spin Studies in ATLASFirst North American ATLAS Physics WorkshopDecember 19-21, 2004Michela Biglietti, Jacob Bourjaily, Homer A. Neal, Natalia Panikashvili, Maria Smizanska, Shlomit Tarem

Outline EvtGen overview Lb Polarization and decay model in EvtGen Validation of Generators Acceptance studies Data processing First look to reconstructionPlans

EvtGen OverviewDecay model packageOriginated in BaBar; authors Anders Ryd, David Lange et alTuned against new data from BaBar, Belle, CLEOLarge collaboration of experiments has accepted EvtGen as their principal B-decay package LHCb, ATLAS, CMS, CDF, D0, BaBar, BelleFeatures works with helicity amplitudesuses spinor algebraPolarizationHandles with sequential decay decays are independent each decay is described by a model the model is a class providing the decay amplitudeusers can easily write their modelscalculation of probabilities and spin density matrices are done by the framework

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 cardPYTHIAConversion to HepMCB-mesonsHepMC B-mesonsDecay filesHepMC Event RecordHepMC decay productsUser inputEvtGen

Probability FunctionAs a first step the probability function has been implemented in EvtGen and in a dedicated Monte Carlo generator (as a cross-check)Angular distribution LbJ/y(mm)L(pp) depends on 5 angles + 6 parameters of 4 complex helicity amplitudes and polarization Pb . Helicity amplitudes and Pb 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

Polarization in EvtGenGet unpolarized baryons from PythiaEvtGen uses spinor algebra and helicity amplitudesits possible to set the polarization of the particle before the particle is decayed .. and obtain the correct angular distributionsPythiaBHepMCEvtGen1. Set polarization2. Decay with helicity amplitudesb-BaryonsB 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 particle1.2.Adopt EvtGen class HELAMP to introduce a theoretical model for b decay

ResultsFrom PQCD model (*):a+ = -0.0176 - 0.4290ia- = 0.0867 + 0.2454ib+ = -0.0810 - 0.2837ib- = 0.0296 + 0.8124iab=-.457, P=-40%Blue squares are events generated according to probability function (in EvtGen)Red circles are events generated with helicity amplitudes in EvtGen5 angular distributions are generated using both the probability function and EvtGen

f1f2cos(q1)Slope~a0cos(q2)Probes J/ymmHelAmpProbcos(q)Slope~abP*ab=|a+|2-|a-|2+|b+|2-|b-|2

Input/Output Parameters Comparisoncos(q1) and cos(q2) do not depend on Polarization

Acceptance CorrectionsKinematics do not depend on the input modelProton pTPion pTIts possible to correct distorted angular distributions with weight factorsMeVMeVf1f2cos(q1)cos(q2)cos(q)Cuts @ Generator Level: pT(p,p)>0.5 GeV pT(m)>2.5, 4 GeV |h|

DC2/Rome Workshop Processing Production using Michigan processor clustersPacman installation of 8.0.5+ patches for Generators and Muon Digitization100k of events generated for EvtGen testsHelicity amplitudes and probabilty function with different polarization valuesacceptance and phase space studies.300k of events generated for full simulation and reconstructionDifferent input models30K 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

First look to Reconstruction and Analysis J/ + - reconstructionDedicated low pT muon identification algorithm - p reconstruction and proton reconstruction in Inner TrackerVertexing algorithms Analysis within AOD/Physics Analysis Tool Preliminary!

Mass DistributionsJ/yLLbMass (MeV)Mass(MeV)Mass(MeV)

Decay Length ResolutionLLbmmmm

The need for identifying low pT muons Muons from J/ are rather soft (a few GeV)The lower the threshold for a second muon - the better our efficiencyMOORE/MuId are inefficient for low pT muonsDi-muon efficiency is too low 30% J/ 2.5 4 are identified, with purity 52%

The algorithmThe purpose is to identify low pT muons and increase the efficiency of J/ identification by improving the reconstruction of muons with low pT in events where one muon had sufficient momentum to trigger the event.The principle:Start with ID tracks and match muon digits to them

If we select for maximum efficiency, accepting each candidate that either has precision chamber digits or has and trigger chambers digits, the resulting efficiency is 80% with the purity ~ 32%.

If we require each candidate to have both precision chamber hits and trigger chamber hits, 60% of J/ particles are identified with purity 69%.Low pT Muon IdentificationPt(MeV)Mass(MeV)

Plans Continue generation of events with different models and start a large scale simulation and digitization as soon as possibleUnderstand which will be the good release for physics simulationPacman installation of ATLAS release 9.0.X Reconstruction and analysis for polarization extractionWithin the Physics Analysis ToolsInner detector particle candidates Muon particles candidates Interface to vertexing algorithmsCalculation of the 5 angular distributionsProduction of histograms for data qualityProduce ntuples for maximum likelihood studies Maximum likelihood studiesAcceptance corrections in 5-D Background channels studies Understand the dangerous channels with fast simulationsFull simulation of the dangerous channelsTarget : ATLAS Physics Workshop in Rome

Backup

Background studies0 channels:

Cant find this channels after LVL1 triggerThe trigger for this is a from b J/, with the in coincidence. That means we don't produce it separately but search in b J/0 production from baryons:*-/ *bar0+-0/ bar00+gamma*0/ *bar00+0*+/ *bar-0++ 0 production from baryons:- / bar+0+ - 0+ebar+e-0+bar+-

Generate using EvtGen (PHSP)b-0+J/+ 0+J/+ebar+e0+J/+bar+-

Background studies0 channels:

0 production from baryons:0 / bar00+0 0+gamma

Generate using EvtGen (PHSP)b00+J/+00+J/+gamma 0 and J/ production from bottom baryons:b0 / b bar0 0+eta_c 0+J/ 0+chi_1c (Only in Pythia)chi_1cJ/ + gamma

Generate using Pythia (PHSP)b0 / b bar00+J/+gamma