1 Andrea Dainese - Physics Forum – 01 03 05 Measurement of the beauty production cross section in Pb-Pb collisions via single electrons F. Antinori, A

Andrea Dainese - Physics Forum – 010305

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Measurement of the beauty production cross section

in Pb-Pb collisions via single electrons

F. Antinori, A. Dainese, M. Lunardon and R. TurrisiPadova – University and INFN


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Contents

Detection strategy for the semi-electronic channel and simulation details (quick review)

Estimate of the uncertainties on the pT-differential cross section of B-decay electrons

Extraction of the B-level cross section

Conclusions and perspectives


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The semi-electronic and semi-muonic decay channels have a good B.R.:

Open Beauty detection in AA at LHC:the semi-leptonic decay channel

B±/B0 l + + X 10.7 ± 0.3% (l = e or )

Good detection and identification capabilities for muons (MUON ARM) and electrons (TRD, TPC and ITS) with ALICE down to low pT


44

X-section from NLO calculations : flavor N qq

in Pb-Pb @ 5.5 TeV (5% tot) charm 115beauty 4.6

in pp @ 14 TeV charm 0.16beauty 0.007

high uncertainty:

1.8 - 7.3

Assumption on beauty production at LHC:

Open Beauty detection in Pb-Pb at LHC with ALICE:

perspectives for the semi-electronic decay channel

Semi-electronic channel ~ 10 % , ALICE accept. for these electrons ~ 24 % in Pb-Pb ~ 0.22 beauty electrons / event

Statistics for 107 central events (~ 1 month Pb-Pb run): ~ 2 M beauty electrons


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Semi-electronic beauty detection:simulation details

Most relevant background sources included:

1) hadrons misidentified as electrons2) e from conversions3) e from charm4) e from Dalitz and strange particles

Separated generation of beauty, charm and background:beauty:

Pythia6 with MSEL=5, CTEQ4L, forced semi-electronic decay

charm:similar to beauty

background:HIJING central (b < 2 fm) events (dNCH/dy|y=0 = 6000)

Normalizations according to the NLO cross section calculations

Magnetic field: 0.4 T


66

d0 and pT distributions for “electrons” from different sources:

Semi-electronic Beauty:detection strategy

Distributions normalized to the same integral in order to compare their shapes


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• electron identification in TRD+TPC• reject 99.99% of pions and “all” heavier hadrons;

lose ~40% of electrons

• impact parameter cut• reject “photonic” electrons (conv.s, Dalitz …)

• pT cut• reduce electrons from charm

Semi-electronic Beauty:detection strategy


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Semi-electronic Beauty detectionsimulation results

Expected statistics (107 Pb-Pb events)

Signal-to-total ratio and expected statistics in 107 Pb-Pb events

pT > 2 GeV/c , 200 < |d0| < 600 m 90% purity

40,000 e from B


99

En

trie

s in

107

even

ts

• Statistical uncertainty corresponding to the number of electrons expected for a sample of 107 Pb-Pb central events (5% tot)

• The impact parameter cut is 200 |d0| 600 m

• 11 pT bins of different width. For the i-th bin:

Estimation of uncertainties on the pT-differential cross section of beauty electrons

statistical error

otherei

charmei

beautyeii NNNT ,,,

beautyeii NS ,

where

i

i

i

i

S

T

S


1010

Main systematic uncertainties:

• Monte Carlo corrections (detector acceptance, tracking efficiency, selection efficiency) ~ 10% (at the moment assumed to be pT-indep.)

• Normalization to one NN collision (error on <Ncoll>: definition of the centrality range, WS Pb density profile, inelastic cross section from TOTEM) ~ 11%

• Charm contribution to be subtracted

from the total electron spectrum

(see following slides)

Note: we assume to be able to subtract

the residual misid. and photonic e

with negligible systematic error

Estimation of uncertainties on the pT-differential cross section of beauty electrons

systematic error


1111

- Charmed hadrons (Hc=D0,D+,D+s,+

c) cross section assumed to be proportional to the

D0 one. The Hc/D0 ratio is assumed to be 1.70 0.07 (*)

Errors propagated from Hc to e level:

- Monte Carlo corrections for the D0 measurement ~ 10%

- Statistical error on the D0 pT distribution

- NN normalization not considered at this level (same as beauty)

- The 69% uncertainty of D0 from b should become negligible after the beauty direct measurement

The charm contribution to the total electron spectrum is evaluated using the MC by introducing the charmed hadron pT distributions deduced from the D0K-+ measurement.

Estimation of uncertainties on the pT - differential cross section of beauty electrons

evaluation of charm background

(*) deduced by comparing the PYTHIA value with the ALEPH measured value [D.Abbaneo et al., Eur. Phys. J. C16 (2000) 597]


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Effect of the D0 statistical error on the electron pT distribution (1)



n

T

T

T

D

T p

pA

dp

dN

p

2

01

10

1) the initial D0 pT distribution (the measured one) is fitted to the expression:

Monte Carlo method


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2) the points are smeared according to the estimated statistical standard deviation and the fit is recalculated

3) the ratio of the new fit to the reference one is used as a weight for the charm generated electron

the electron generated from a D with pT = 2 GeV/c is counted as 0.94 electrons


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4) the refit procedure is repeated many times and the standard deviation of the content in each electron pT-bin is evaluated

Relative error as a function of pT


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Analytic method

1) for each D0 pT bin, the corresponding electron pT distribution is extracted and

the relative errors of all the electron pT bins are set equal to that of the D0


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2) the different electron spectra are summed with a quadratic error propagation

The analytic result compared to the numerical one


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Total relative error on the charm electron pT distribution to be subtracted


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Final B-decay electron pT distribution

statpt-dep. syst11% norm. err.(not shown)

E loss calculations:

N. Amesto, A. Dainese, C.A. Salgado, U.A. Wiedemann, hep-ph/0501225

(11% norm. err. not shown)


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Extraction of a minimum-pT-differential cross section for B mesons

Using UA1 MC method (*), also adopted by ALICE

(*) C. Albajar et al., UA1 Coll., Phys Lett B213 (1988) 405 C. Albajar et al., UA1 Coll., Phys Lett B256 (1991) 121

The B meson cross section per unit of rapidity at midrapidity with pTB > pT

min is

obtained from a scaling of the electron-level cross section measured within a given electron phase space e

MC

eB

TBT

B

meas

ebeautyeT

BT

B ppdy

d

ppdy

d

)(

)()()(

min

,min

},,{ 0dpTe

Bemeas

ebeautye F)(,

The semi-electronic B.R. is included here

The phase space used is where pT are the previously used bins,

= [-0.9, 0.9] and d0 = [200,600] m

(thanks to R.Guernane for useful discussions)


2020

BeFSystematic error for

- semi-electronic decay B.R.: ~ 3 %

- dependence on the shape of the B meson pT distribution used as input in the MC: can be minimized using a proper choice of pT

min for a given phase space e see following slides

- Monte Carlo correction for the efficiency of the selection cuts: this is, in principle, depending on the B meson pT distribution, and should be then evaluated at this stage of the analysis. For the present feasibility study we account for it with a 10% systematic.


Using UA1 MC method, also adopted by ALICE


2121

BeFEvaluation of and determination of the optimal pTmin

1) we used the B e + X decays from PYTHIA.

is the ratio of the red area to the blue one.

BeF

here pTe = [3,4] GeV/c


Using UA1 MC method, also adopted by ALICE


2222


2) in the HVQNMR program we changed the theory parameters:

a) quark mass and scales

b) nuclear modification of the PDFs

c) b B fragmentation (Peterson)

d) add the quenching (q = 100 GeV2/fm (*))

(*) N. Amesto, A. Dainese, C. A. Salgado, U. A. Wiedemann, hep-ph/0501225



2323

Can find an optimal pTmin

for pTe > 2 GeV/c


F ~ 1 %



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E loss calculations:

N. Amesto, A. Dainese,

C.A. Salgado, U.A. Wiedemann,

hep-ph/0501225

Using electrons in2 < pT < 16 GeV/c

obtain B-meson2 < pT

min < 23 GeV/c

statpt-dep. syst11% norm. err.(not shown)


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Summary• The semi-electronic channel offer a good tool for the study

of the beauty production at LHC• In the present study we evaluated:

– purity of the electron sample (~90% for pT>2 GeV/c)

– uncertainties on the pT-differential beauty electron spectrum, including: statistical error, systematic from MC corrections, AANN normalization, charm background (using the D0K-+ measurement results)

– extraction of a min-pT-differential cross section for the B mesons

• Coming up– more detailed study of B D e effect on the estimated result

– study of the sensitivity of the pTmin-differential B

distribution to different energy loss scenarios


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THE END


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BACKUP SLIDES


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- 13% semi-electronic decay and much more charm than beauty expected significant background

- softer pT spectrum and narrower d0 distribution (c (D0) ~ 100 m, c(D+) ~ 300 m)

Semi-electronic beauty detectionbackground analysis: direct charm

Distributions normalized to the same integral in order to compare their shapes


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B D e effect on the electron spectrum

Fraction of the b c e withrespect to the direct b e

Estimated effects:increase the statistics of beauty originated electrons +introduce a small uncertainty in the deconvolution =

improvement on the measurement sensitivity


3030


B D e effect on the electron spectrum

d0 distributions pT distributions


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Semi-electronic Beauty detectionpT quark distribution

Analysis of the electron pT distribution useful for beauty production cross

section measurement. But, what about the quark pT distribution?


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From test beam results:90% electron efficiency

1% misidentified pions (constant in 1-6 GeV/c pT range)

pion contamination ~ 1%

Semi-electronic beauty detectionbackground analysis: misidentified pions

Electron identification with Transition Radiation Detector (TRD)


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PID used in this simulation:

- can assume complete rejection of K,p and heavier particles from TRD and TOF

- 80% electron reduction factor for identification efficiencies of TRD (0.9) and TPC (0.9)

- pion contamination less than 0.01% at low momentum

Semi-electronic beauty detectionbackground analysis: misidentified pions

relative magnitudes correct

Effect of the PID on the pion backgound

Number of pions much greater than number of electrons good rejection using

combined PID technique

pT > 1 GeV/c

Documents

1 Andrea Dainese - Physics Forum – 01 03 05 Measurement of the beauty production cross section in Pb-Pb collisions via single electrons F. Antinori, A