a study of the kπ system mikhail kozyulin, (cern, binp) 1
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A study of the Kπ system
Mikhail Kozyulin, (CERN, BINP)
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Introduction
In the latest Bs J/ψK* paper the Kpi spectum was shown, but analysed only qualitatively
S-wave mass lineshape is still not well understood
https://twiki.cern.ch/twiki/bin/view/LHCbPhysics/Swave
We would like to have better control of the waves that can enter K* region
Some isobar test Some K-matrix test
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Introduction
In addition, a higher Kpi peak is clearly visible, but neither BR(branching rations) nor polarization fractions were reported
Mass fit favoured K*2(1430) rather than K*0(1430). But difficult to prove or extract exact yield without angular analysis that separates spin
My summer student project:
Perform an angular study of the Kpi spectrum in BdJ/psiKpi in order to see the actual mass shape of the different waves This will allow us to:
Search for an Spin-2 signal K*2See the mass lineshape of the S-wave….
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Strategy
Angular analysis in bins of Kpi mass.– MC test– data
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MCImplemented RooFit model (in helicity basis) with S and P waves and tested in MC @ generator level.Good agreement with true values for the amplitudes (including As compatible with 0).
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Angular acceptance
To make our life easy for this round, we fit using per event weight
In LHCb-ANA-2011-071 (published paper) we have the acceptance function evaluated in several binsusing MC10 BdJpsiK* MC(never used in old analysis, but maybe ok for this initial study)
1200-1650
We will use:
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We interpolate the coefficient values between different mass bins using error function parameterizations• Bin position in is taken as the mean of distribution in that bin• We add a loose asymptotic property for Acc(ψ): at high masses
• Acc(Psi) ~ 1 + c*cos(ψ)2 (inspired by the trend at lower masses as well as by the J/ψ acceptance)
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Data1. Fit in angular and B mass spectrum. Model=Bs+Bd+bkg, each Bs and Bd =Swave+Pwave. Acceptance with per event weights.
2. Cut in Bmass – M(Bd)+-20MeV
M(K*) +- 40 MeV
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Fit with S-,P-wave
Blue: fit functionRed: Bd Pink: backgroundBlack: Bs
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Blue: fit functionRed: Bd Pink: backgroundBlack: Bs
Fit with S-,P-wave
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Construct the fit model using Urania’s tools
Use Urania.Helicity to get symbolic pdf
given the spin list for intermediate XKπ states : 0, 1, 2
Use Urania.RooInterfaces to create a RooFit C++ class out of the symbolic pdf
op2 = D.RooClassGenerator(func, [x,y,z]+TransAmpModuli.values()+TransAmpPhases.values(),"RooB")op2.makePdf(integrable = kTRUE)op2.doIntegral(1,(x,-1,1))…..op2.doIntegral(7,(x,-1,1),(y,-1,1),(z,-Pi,Pi))op2.overwrite()
We can use this procedure for higher and higher waves if necessary
Adding now the D wave …..
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S - wave P - wave
fL = 0
fL = 1
fL = 0.5
D - wave
fL,2 = 0
fL,2 = 1
fL,2 = 0.5
Altogether
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Fit with S-,P-,D-waves
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P - wave
fL = 0
fL = 1
fL = 0.5
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P - wave
fL = 0
fL = 1
fL = 0.5
S - wave
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P - wave
fL = 0
fL = 1
fL = 0.5
S - wave
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D - wave
fL,2 = 0
fL,2 = 1
fL,2 = 0.5
S - wave
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D - wave
fL,2 = 0
fL,2 = 1
fL,2 = 0.5
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0.55
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Conclusion
• We have performed an angular analysis of Bd->J/ψKπ in bins of Kπ mass.
• Decided to neglect background.• Angular acceptance maybe not very accurate (MC10,
interpolated parameters,…) but hope is good enough to identify the dominant wave in each bin.
• Clear evidence of spin 2, favouring K*2(1430) instead of K*0(1430).
• S-wave seems to be the main amplitude in (1000-1300 MeV), but small variation in phases causes big changes in wave shapes, probably interferences play a dominant role in that region.