palau - 29 sept. 2008 iv convegno nazionale sulla fisica di alice 1 study of the b j/ decays in pp...
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Palau - 29 Sept. 2008 IV Convegno Nazionale sulla Fisica di ALICE
1
Study of the B J/ decays in pp collisions
(B J/ + X e+e- + X)
Carmelo Di Giglio
Università di Bari & INFN - Italy
OUTLINE: Physics motivations. Analysis strategy. Status of the analysis:
measuring the fraction of J/ from B (i.e secondary J/ ). background analysis.
Outlook
IV Convegno Nazionale sulla Fisica di ALICE
2Palau - 29 Sept. 2008
Physics motivations
The study of the channel B J/ +X is an important issue since:
A large fraction ( ≈ 40% ) of total J/ come from beauty hadron decays.
It provides another measurement of the bb cross-section in the central barrel besides the single electron channel (B e +X)
e+e-
IV Convegno Nazionale sulla Fisica di ALICE
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B J/ w.r.t B e + + X Being the J/ much heavier than the electron, it
resembles more an “exclusive” channel. It can allow a determination of the dbb/dpt cross-
section down to pt≈0. Invariant mass analysis is possible. Branching ratio:
B e + + X 10.8%
B J/ + X 1.16 ± 0.10 %
J/ e+e- 5.94 ± 0.06 %
B J/ e+e- 0.069 ± 0.007 %
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Analysis of total J/ relies on: Combinatorics between + and - tracks. PID selection of e+e- pairs (in this analysis based
on the parametrised response of TRD + TPC). Invariant mass analysis.
Strategy
Goal: separate secondary J/ from prompt J/ it relies on the spatial resolution of the ITS.
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Measuring the fraction of J/ from HB
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MC samples used for the analysis
7.3*104 PDC’06 p-p events with secondary J/ equivalent to 9.9·109 min. bias pp events
3*105 PDC’06 p-p events with prompt J/ equivalent to 14.9·109 min. bias pp events.
dedicated PDC’06 prod in the Bari
farm.
2.8*106 min. bias pp events. from the Cern Analisys Facility (CAF) for the bkg. study
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PID scheme adopted
Each particle is assigned a weight, depending on the specie:
e+ and e- 0.9(TPC)* 0.9(TRD)
+ and - effTPC(p) * eff
TRD(p)
All the others 0.
TPC-TRD parametrization of the probability to misidentify a as an electron:
TPC same as for the B in the semielectronic channel
TRDfrom ALICE Physics Week (March 2008)
10-1
1
10-2
10-3
eff
Pio
n c
on
tam
ina
tion
IV Convegno Nazionale sulla Fisica di ALICE
8Palau - 29 Sept. 2008
Kinematical distributions: prompt J/ Vs. secondary J/
rapidity and transverse momentum
(all distribution normalized to 1 m.b. event)
y
%1.40 /tot
b from /
J
Jbut pT dependent
Primary J/
Secondary J/
Impact parameters d0(e)
e+ e-
Primary J/
Secondary J/
Product of impact parameters d0(e+)*d0(e-)Secondary dominant at high pt
IV Convegno Nazionale sulla Fisica di ALICE
9Palau - 29 Sept. 2008
Kinematical distributions: prompt J/ Vs. secondary J/
Pseudo-proper decay time: )/(
)/( /
Jp
MJLx
T
Jxy
)/(
)/()/(
Jp
JpLJL
T
Txy
prim
vtxsec
vtx rrL
Primary J/
Secondary J/
Invariant Mass
Primary J/
Secondary J/
The shape is due to: detector resolution bremsstrahlung
process on the path through the detector material
long tail at low invariant mass values.
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Background components The dominant background component in the J/ invariant
mass window (after PID), is e+e-
e+e-
eh
h+h-
totN ~ 106 pp min biasEvents on CAF
The available statistics 3*106 pp @ 14 TeV MB events (stored on CAF) was not enough to study combinatorial background properly.
Bkg equal to h+h- (normalized to the total bkg) used so far to develop analysis tools.
What is expected from a real MB sample of N ~109 events:
634 BS
S
In the selected mass window
J/ (prompt+secondary)
Bkg (h+h- norm)
Tot
Normalized to 1 m.b event
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In order to extract the fraction fB of J/ from b-hadron decays, one should fit simultaneously: the invariant mass spectrum one distribution which can discriminate
prompt from detached J/(e.g. x or product of impact parameters d0
e+·d0e-)
the analysis should be performed in pt bins.
Final goal: measure fB(pt)
How to measure the fraction of secondary J/
Even
ts/1
0 M
eV
t
bb
dp
d
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How to measure the fraction of secondary J/
Approach used: simultaneous mass and lifetime fit using the log-likelihood function [1] :
N
ieemxL
1
),(Flnln
N is the number of events in |mee-MJ|<=200 MeV
)()()1( )()(),(F eeBkgBkgSigeeSigSigSigee mxfmxfmx MFMF
[1] Acosta et al.,CDF Collaboration, PRD 71 032001 (2005)
IV Convegno Nazionale sulla Fisica di ALICE
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How to measure the fraction of secondary J/
)()1( )()( xfxfx PBBBSig FFF fB fraction of secondary J/
)()( )( xxxRx MCB F
)( )( xRxP F R(x) resolution function
MC(x,pt) MonteCarlo templates (as obtained by PYTHIA) of the x distribution
)()()1( )()(),(F eeBkgBkgSigeeSigSigSigee mxfmxfmx MFMF
More details in the extra slides.13 parameters at all.
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Improvements on the method to extract the fraction of secondary J/:
New parametrization of the signal mass distribution Msig(mee).
New parametrization of the x resolution R(x).
Analysis performed in pt bins (9 bins).
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Parametrization of )(xR
)(xR resolution function determined from prompt J/ distribution of X / RMS(X) parametrized as sum of
2 Gaussian + 1 exponential centered at 0 (5 params) to account for the long tails observed at high values
Pt > 0
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To reduce the n. of parameters the fit in individual pt bin is performed by:
fixing, from fit on the distribution integrated over pt , two parameters: sigma of one gaussian and of the exponential
leaving free the remaining three, i.e:
the sigma of the main gaussian. the relative weights among two components of the fitting function.
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Pt < 1GeV
1 < Pt < 1.5 1.5 < Pt < 2
2 < Pt < 2.5
3.5 < Pt < 4.53 < Pt < 3.5
2.5 < Pt < 3
4.5 < Pt < 5.5 Pt > 5.5
Result of the fit
Pt>0
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Parametrization of )( eemsigM New parametrization:Based on the Crystall Ball function as introduced for the first time in [2]
[2] J.E.Gaiser, Charmonium Spectroscopy from Radiative Decays of the J/Psi and Psi-Prime,SLAC-R-255, (1982)
),,,;( mnmF nmm
BA
mm
2
2
2
)(exp
mm
mm
N
for
for
4 params
2exp
2
n
nA
n
B
N = normalization factor
= 0.5 = 1. = 1.5 = 2. = 2.5
= 0.1 = 0.2 = 0.7 = 0.9
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Total J/
The fit has been successfully performed on all the three samples:
Secondary J/Primary J/
whole pt
range… Extracted
invariant massM = 3.091GeV
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… and in nine pt bins
Pt < 1GeV
1 < Pt < 1.5GeV 1.5 < Pt < 2GeV
2 < Pt < 2.5GeV
3.5 < Pt < 4.5GeV3 < Pt < 3.5GeV
2.5 < Pt < 3GeV
4.5 < Pt < 5.5GeV Pt > 5.5GeV
Total J/ Pt < 1GeV
1 < Pt < 1.5 1.5 < Pt < 2
2 < Pt < 2.5
3.5 < Pt < 4.53 < Pt < 3.5
2.5 < Pt < 3
4.5 < Pt < 5.5 Pt > 5.5
Secondary J/Pt < 1GeV
1 < Pt < 1.5 1.5 < Pt < 2
2 < Pt < 2.5
3.5 < Pt < 4.53 < Pt < 3.5
2.5 < Pt < 3
4.5 < Pt < 5.5 Pt > 5.5
Prompt J/
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a C++ program has been implemented to perform the fit, based on the Root module Minuit2
The fit is a multidimensional one:
13 parameters.
Analytical calculation of convolution integrals.
The code written to execute minimization in pt bins automatically.
How to measure the fraction of secondary J/
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Optimization is ongoing:
A first check performed to test convergence of
the minimization procedure:
consider in input prompt J/ only, sec. J/ only,
bkg only and check weather the algorithm
converges to reasonable values (i.e fB 0, fSIG 1,
ecc…)
Still some problems of convergence occurred in
few bins.
How to measure the fraction of secondary J/
IV Convegno Nazionale sulla Fisica di ALICE
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To study bkg:
Like Sign (LS) pair technique:
Uncorrelated bkg in the unlike-sign pair sample estimated by the number
of like-sign pairs ++ and -- within each event. Event Mixing technique:
Uncorrelated bkg in the unlike-sign pair sample estimated by an unlike-
sign pair spectrum for which the two particles are taken from different but
similar (as far as vtx-position and multiplicity is concerned) events.
In order to reduce the number of parameters one can:
determine a priori the shape and (eventually) the amount of the background invariant mass spectrum under J/ peak.
impose (fix) them in the likelihood fit.
The likelihood fit provides the amount of bkg as well
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Pros & Cons of Like Sign w.r.t of Event Mixing
Can introduce bias in this analysis, which aims at separating prompt from detached (hundreds of microns) since it mixes events with diff primary vtx-positions.
Event Mixing:
Appropriate to study comb. bkg in case of low statisticsand in the continuum.
Like Sign
Easier from a computational point of view.
Feasible to reproduce the shape of the bkg. invariant mass spectrum under J/ peak.
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3*105 pp @ 14 TeV events with 1 prompt J/ per event forced to dielectrons (in the “barrel”, | y | < 2)
2.5*104 pp @ 14 TeV minimum bias events
Data samples used:
1.6*106 pp @ 14 TeV events with 1 CC / event (PWG3 production 2006 generated on the Italian Grid, Torino-CNAF-Padova).
Is not a minimum bias sample. No CC couples explicitly present in the sample
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Log scale M (Gev)
Like Sign
Unlike Sign
J/D0
No PID
N.B: Normalization performed so far on the integral of the curves tot #
of US and LS pairs respectively. D0 mass also shown: LS should reproduce well bkg spectrum under
corresponding peak (not shown here).
prompt J/
Ratio US/LS
M (Gev)
IV Convegno Nazionale sulla Fisica di ALICE
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Normalization performed integrating
curves in the region:
0.5 Gev < M < 1.3 GeV
M( GeV)
Like Sign
Unlike Sign
M( GeV)Log scale
After PID
M( GeV)
Under J/ peak
IV Convegno Nazionale sulla Fisica di ALICE
30Palau - 29 Sept. 2008
Like Sign
Unlike Sign
No PID
Only e-e and e-h couples have been selected:
h-h contribution after PID is negligible.
e+e-
eh
h+h-
tot
Results for sample with 1 cc/ev.
Ratio US/LS
normalized to 1 MB event.
IV Convegno Nazionale sulla Fisica di ALICE
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After PID
M (GeV)
Invariant mass
Like Sign
Unlike Sign
2 of the ratio US/LS w.r.t r = 1 curve:
20 = 2/ ndf = 1.099
P(2 > 20 ) ~ 40-50%
Good result from Kolmogorov test:
probability P Kolm~ 81%
Two plots have the same variable binning.
Plots normalized to 1 Min Bias event
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Conclusions & Outlook Method to extract the fraction of secondary J/improved:
New parametrization of the signal mass distribution and x resolution R(x).
Code algorithm improved: multidimensional fit (13 parameters) performed in about one hour for a statistics equivalent to 10*109 min bias event
Try other variables to separate J/ from B, e.g. d0xd0
Robustness of the minimization to be investigated: Try to reduce n. of parameters (as for the bkg invariant mass
shape study). Performance on GRID data(2*105 pp events with J/ from
Beauty hadrons) Performance in Pb-Pb.
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Experience at colliders: CDF @ Tevatron , √sNN=1.96TeV
CDF measures J/ in the dimuon channel using SVX II (five layers) to separate primaries from secondaries (i.e. from B decays). the channel e+e- has a worsen mass resolution
than the , due to bremsstrahlung.
Performance of CDF: CDF resolution on primary vertex is 30.
CDF (pt)/pt2 = 0.0017 GeV/c-1 .
IV Convegno Nazionale sulla Fisica di ALICE
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)()()1( )()(),(F eeBkgBkgSigeeSigSigSigee mxfmxfmx MFMF
2
1)(
minmax
minmaxeeee
eeslopeeeee
eeBkg
mmmM
mmmM
The mass background is modeled using a linear distribution function normalized to unity over the mass range
[meemin, mee
max]
How to measure the fraction of secondary J/
The only parameter is Mslope
IV Convegno Nazionale sulla Fisica di ALICE
36Palau - 29 Sept. 2008
)()()1( )()(),(F eeBkgBkgSigeeSigSigSigee mxfmxfmx MFMF
)()(exp2
)()(exp2
)()(exp
)()(exp)()1()(
xxRxxf
xxRxxf
xxRxxf
xxRxxf
xRfffx
symsym
sym
symsym
sym
symBkg
F
How to measure the fraction of secondary J/
Parameters: f+ ,f- ,fsym ,+, -, sym, params of R(x)
IV Convegno Nazionale sulla Fisica di ALICE
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Old parametrization of )( eemsigM Parametrization previously used:Ad hoc parametrization taken as the sum of 2 Landau with same mpv, one of them square-rooted and reflected about mpv (4 params)
LL RRL
Failed when trying to perform the fit also in pt bins.
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Simple cuts on reconstructed tracks, excluding tracks with any kink (GetKinkIndex(0) > 0).
tracks not reconstructed at the level of:
• ITS• TPC• TRD
Like Sign candidates ++ and -- constructed within each event:
using the same classes in AliROOT code (in PWG3)used to build unlike-sign pairs candidates, i.e.
• AliBtoJPSItoEleAnalysis (search for candidates)• AliBtoJPSItoEle (stores & describes candidates)
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