palau - 29 sept. 2008 iv convegno nazionale sulla fisica di alice 1 study of the b j/ decays in pp...

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-



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 %

Dire a voce che questo è diventato il canale standard per lo studio del decadimento di adroni con B ai collider: es. CDF in cui JPSI ricostruita nel canale dimuonico



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.



Measuring the fraction of J/ from HB



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

Descrivere megli i campioni



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



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

Dire che frazione dipende dal pt e che ad alti pt domina la componente di JPSI secondarie da adroni con Beauty



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.

Qui spendere qualche prola in più sul significato fisico della grandezza x


10Palau - 29 Sept. 2008

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


11Palau - 29 Sept. 2008

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


12Palau - 29 Sept. 2008


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)


13Palau - 29 Sept. 2008


)()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


More details in the extra slides.13 parameters at all.


14Palau - 29 Sept. 2008

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).


15Palau - 29 Sept. 2008

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


16Palau - 29 Sept. 2008

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.


17Palau - 29 Sept. 2008

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


18Palau - 29 Sept. 2008

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


19Palau - 29 Sept. 2008

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


20Palau - 29 Sept. 2008

… 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/


21Palau - 29 Sept. 2008

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.



22Palau - 29 Sept. 2008

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.



23Palau - 29 Sept. 2008

Background shape analysis


24Palau - 29 Sept. 2008

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


25Palau - 29 Sept. 2008

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.


26Palau - 29 Sept. 2008

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


28Palau - 29 Sept. 2008

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)


29Palau - 29 Sept. 2008

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


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.


31Palau - 29 Sept. 2008

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


32Palau - 29 Sept. 2008

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.


33Palau - 29 Sept. 2008

Extra slides


34Palau - 29 Sept. 2008

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 .


35Palau - 29 Sept. 2008


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]


The only parameter is Mslope


36Palau - 29 Sept. 2008


)()(exp2

)()(exp2

)()(exp

)()(exp)()1()(

xxRxxf

xxRxxf

xxRxxf

xxRxxf

xRfffx

symsym

sym

symsym

sym

symBkg

F


Parameters: f+ ,f- ,fsym ,+, -, sym, params of R(x)


37Palau - 29 Sept. 2008

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.


38Palau - 29 Sept. 2008

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)


40Palau - 29 Sept. 2008

Contributions to the background

tot

e+ e-e hh h

MB

Contribution to the total bkg from events with 1 cc pair.

The band accounts for the inefficiency factor ~ 15 %due to TPC parametrization in the used sample.

palau - 29 sept. 2008 iv convegno nazionale sulla fisica di alice 1 study of the b j/ decays in pp...

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