PPR Meeting - May 16, 2003 Andrea Dainese 1
Measurement of D0 production in pp
Andrea Dainese
University of Padova
PPR Meeting - May 16, 2003 Andrea Dainese 2
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
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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
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Feed-down from Beauty (2)
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
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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 %
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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
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Feed-down from Beauty (5)
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 %
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Feed-down from Beauty (6)
new cut |d0|< 500 mcan suppress beauty
impact parameter distributionafter selection
D0 from b / D0 from c = 10 %
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Results
Essentially unchanged w.r.t. to last shown (March)
K id
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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
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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
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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
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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%
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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
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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
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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 %
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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”
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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) ….)
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