PPR Meeting - May 16, 2003 Andrea Dainese 1 Measurement of D 0 production in pp Andrea Dainese...

PPR Meeting - May 16, 2003 Andrea Dainese 1

Measurement of D0 production in pp

Andrea Dainese

University of Padova


Outline

One step back: study of feed-down from beauty (b B D0), important also for systematic error

Estimation of statistical error on dN(D0)/dpT

Estimation of systematic errors:contribution from beautyuncertainty on D0 cMC corrections & uncertainty on B.R. to K

Combination of errors and error on total cross section

totcc

T

cc σdp

d &

PHYSICS PERFORMANCE: SENSITIVITY TO


Feed-down from Beauty (1)

At LHC energies

Fraction of D0 from chain b B0/B+ D0 not negligible

“Standard” NLO pQCD + Pythia fragmentation + PDG:

Important to use the correct ratio “D from b” / “D from c”:Selection may change the ratio

Results have to be eventually corrected for feed-down

estimate systematic error from b cross section

%5~/ ccbb

%5.5)(/

)/()/(/

)(/

)(/0

000

0

0

DcdydN

XDBBBRBBbdydN

DcdydN

DbdydN0.640.0017

0.0196



How D0 signal was generated:generate standard pp minimum bias with Pythiakeep only those events that have a D0

weight D0s according to their pT in order to reproduce distribution given by NLO pQCD

In that sample: factor 4 too high!

All candidates have been re-weighted:c D: to reproduce pT distr. by NLO

b D: to have correct ratio (5.5%) AND reproduce THEIR pT distr. by NLO (harder than for c D !)

%22)(

)(0

0

DcN

DbN


Feed-down from Beauty (3)After event reconstruction (tracking + vertexing): D0 from b / D0 from c = 5.8 % Different pT slopes

After selection (impact parameter + pointing angle): D0 from b / D0 from c = 16 %


Feed-down from Beauty (4)Selection on pointing angle expected to suppress D0 from b(they point to the decay vertex of the B and not to the primary vertex)

Resolution on pointing angle is worse for D from c pointing angle enhances D from b

D

D

B

V1 V1

K

K

2<pT<3 GeV/c



Also selection based on impact parameter enhances beauty

20% !

Systematic error from b =uncertainty on b cross section fraction of D0 from b

D0 from b / D0 from c = 16 %



new cut |d0|< 500 mcan suppress beauty

impact parameter distributionafter selection

D0 from b / D0 from c = 10 %


Results

Essentially unchanged w.r.t. to last shown (March)

K id


Binning & “Smoothing” in dN/dpT

Optimize binning in the range 0-14 GeV/c: bin size increasing with pT

good precision on measurement of pT slope

small statistical error up to high pT

Fit pT distributions to remove fluctuations due to reduced statistics of simulation at high pT (especially for Bkg)

Use fitted function to obtain “smooth” dN/dpT for S and B


Statistical Error (1)

Rel. Statisitical error on S expected to be 1/Significance:

S = (S+B)observed – Bestimated

error on (S+B)observed = sqrt(S+B)

error on Bestimated negligible (uses info from wide range mass distr.)

error on S = error on (S+B)observed = sqrt(S+B)

relative error on S = sqrt(S+B)/S = 1/Significance

This has been checked by generating and fitting a large number of invariant mass distributions, using as input parameters the results of the simulation, namely:

statistics for S and B VS pT

width of D mass distribution VS pT

slope of exponential bkg VS pT


Statistical Error (2)

0.5<pT<1 GeV/cSignificance = 11

Fit with expo + gauswith 4 free parameters:• expo slope• gaus integral (S)• gaus mean• gaus sigma(total integral fixed)

Many (10,000) iterations: study resolution on S (statistical error) study robustness of fit

2<pT<2.5 GeV/csignificance = 2112<pT<14 GeV/csignificance = 11

Pul

ls a

naly

sis

no systematic shift of integral (S)

err. given by fit = stat. err. on S


Systematic Errors

When the data are corrected for efficiency, acceptance, etc… systematic errors are introduced

Main sources of systematic errors have been considered

CORRECTION ERRORS INTRODUCED

subtraction of contribution from b uncertainty on b cross section

selected reconstructed

(MC correction)

error on D0 c

correction for PID efficiencyerrors from MC description of ALICE

(TOF eff. & contaminations)

extrapolation from D0 reconstructed to D0 produced in |y|<1

errors from MC description of ALICE (tracking eff. & resolutions)

from D0 K to total D0 error on B.R. D0 K

~10%

2.4%


Systematic from b subtraction

Fraction of D0 from b will be estimated from MC and subtracted to obtain D0 from c

Input to MC:

Beauty cross section in pp will be measured by all 4 expts

Now: pessimistic case use theoretical uncertainty (80%)

Error on N(c D0) = 0.8 N(b D0)

Relative error = 0.8 N(b D0)/N(c D0) 0.810% = 8%

mb )4.05.0( bb presentth. uncertainty


Systematic from c = (123.4 0.8)m

From selected to reconstructed D0:

For each D, calculated “equivalent” true value of ct = LM/p

MCSele

RecoSeleReco

depends on c

in MC input

beauty

assuming it is primary

charm

lower cut on d0

upper cut on d0

error ~ 0.5 % negligible


Errors combined

dydp

DdN

T

)( 0

Note:Error from MC may be lowerthan 10% and dep. on pT

Error from b may be much lower after direct measureError on BR doesn’t alterpT shape

dN(D0)/dy for|y| < 1 and pT > 0.5 GeV/c (91% of pT > 0) statistical error = 3 % systematic error = 13 % from b = 8 % MC correction = 10% B.R. = 2.4 %


Comparison with CDF-Run II

CDF Run II: preliminary measurement of charm cross section using the Secondary Vertex Trigger

sigma(D0, pT >= 5.5 GeV/c) = (13.3 ± 0.2 ± 1.5) µb

1.5% 11%

“We could measure the charm cross section in pp @ LHC

from pT > 0.5 with the ~ same precision/accuracy”


Conclusions

Feed-down from b studied – important for systematic error

Statistical errors on pT distribution estimatedMain systematic errors consideredPhysics performance on charm X section measurement in pp:

statistical error: 3 %systematic error: 13 % (only indicative value)

Systematic error from MC corrections will dominate

Next steps:try D0 impact parameter to measure (b D0) / (c D0) “a la CDF”sensitivity for comparison with NLO pQCD and/or Pythiaerror analysis for Pb-Pb

play a bit with comparisons (RAA (pT) ….)

PPR Meeting - May 16, 2003 Andrea Dainese 1 Measurement of D 0 production in pp Andrea Dainese...

Documents

Transcript of PPR Meeting - May 16, 2003 Andrea Dainese 1 Measurement of D 0 production in pp Andrea Dainese...