Andrea Dainese

1
Open Heavy Flavour reconstruction in ALICED0 ?
Kp and B ? e X
ALICE-Italia, Cagliari, 4-5 Maggio 2005

• Andrea Dainese
• per il gruppo ALICE Padova
• F.Antinori, A.D., J.Faivre, M.Lunardon, R.Turrisi

2
Contents

• Physics motivations
• Heavy-quark production at the LHC
• c and b energy loss
• ALICE cross sections baseline
• Exclusive reconstruction of D0 ? Kp decays in pp
  and Pb-Pb collisions in ALICE
• Inclusive beauty via single electrons
• Future developments

3
HVQs production in pp at 14 TeV

• Important test of pQCD in a new energy domain
• remember the 15-years saga of b production at
  the Tevatron
• and c production not yet fully reconciled
• NLO predictions for LHC uncertainties of factor
  gt 2--3

CDF, hep-ex/0307080 FONLL Cacciari, Nason
Cacciari, Frixione, Mangano, Nason and Ridolfi,
hep-ph/0312132
beauty
charm
HERA-LHC Workshop Proceedings using Mangano,
Nason, Ridolfi, NPB 373 (1992)
M.Mangano
4
HQs as a probe of small-x gluon dynamics

• Probe unexplored small-x region with HQs at low
  pT and/or forward y
• down to x10-4 with charm already at y0

• Window on the rich phenomenology of high-density
  PDFs
• gluon saturation / recombination effects
• non-linear terms in PDFs evolution
• Possible effect enhancement of charm production
  at low pT w.r.t. to standard DGLAP-based
  predictions

Eskola, Kolhinen, Vogt, PLB 582 (2004) Gotsmann,
Levin, Maor, Naftali, hep-ph/0504040
5
Final-state effects in nucleus-nucleus Parton
Energy Loss

• Partons travel 4 fm in the high colour-density
  medium
• Energy loss by gluon radiation

(BDMPS)
Casimir coupling factor 4/3 for quarks 3 for
gluons
Medium transport coefficient ? gluon density and
momenta
Probe the medium
R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne'
and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997)
291.
6
Lower E loss for heavy quarks?

• In vacuum, gluon radiation suppressed at q lt
  mQ/EQ
  
• ? dead cone effect
• Dead cone implies lower energy loss
  (Dokshitzer-Kharzeev, 2001)
• Detailed calculation confirms this qualitative
  feature, although effect is small and
  uncertainties significant (Armesto-Salgado-Wiedema
  nn, 2003)
• Exploit abundant massive probes at LHC
  study the effect by measuring the nuclear
  modification factor for D and B

Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett.
B519 (2001) 199 hep-ph/0106202. N.Armesto,
C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69
(2004) 114003 hep-ph/0312106.
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Latest from RHIC Charm Suppressed!
RAA
light-flvred hadrons
electrons from c (b) decay
pT (GeV)
Effect of charm mass is small similar
suppression as for light-flvred hadrons
Calculations Armesto, Dainese, Salgado,
Wiedemann, PRD71 (2005) 054027.
8
Color-charge and mass dep. of E loss with
heavy-to-light ratios at LHC
Heavy-to-light ratios Compare g ? h , c ? D
and b ? B
For 10 lt pT lt 20 GeV, charm behaves like a m0
quark, light-flv hadrons come mainly from
gluons RD/h enhancement probes color-charge dep.
of E loss RB/h enhancement probes mass dep. of E
loss
Armesto, Dainese, Salgado, Wiedemann, PRD71
(2005) 054027.
9
D0? K-p reconstruction

• Exclusive channel direct measurement of
  the pt distribution ideal tool to
  study RAA
• Weak decay with mean proper length ct 124 mm
• Large combinatorial background (dNch/dy6000 in
  central Pb-Pb!)
• STRATEGY invariant-mass analysis of
  fully-reconstructed topologies originating from
  (displaced) secondary vertices
• Measurement of Impact Parameters (ITSTPC)
• Measurement of Momenta (ITSTPC)
• Particle identification (TOF) to tag the two
  decay products

10
Impact parameter resolution

• Mainly provided by the 2 layers of silicon
  pixels in the ITS

? 9.8 M
Systematic study of track impact parameter
resolution A.D. and R.Turrisi,
ALICE-INT-2003-028.
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D0? K-p Selection of D0 candidates
increase S/B by factor 103!
12
D0? K-p Results
S/B initial (M?3s) S/evt final (M?1s) S/B final (M?1s) Significance S/?SB (M?1s)
Pb-Pb 5 ? 10-6 1.3 ? 10-3 11 37 (for 107 evts, 1 month)
pp 2 ? 10-3 1.9 ? 10-5 11 44 (for 109 evts, 7 months)
Note with dNch/dy 3000, S/B larger by ? 4 and
significance larger by ? 2
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D0? K-p d2s(D0)/dptdy and ds(D0)/dy
ds(D0)/dy for y lt 1 and pt gt 1 GeV/c (65
of s(pt gt 0)) statistical error 7
systematic error 19
from b 9 MC
correction 10 B.R.
2.4 from AA to NN 13
14
Sensitivity to NLO pQCD params
pp
?s 14 TeV
15
Charm Quenching
Dainese, EPJC33 (2004) 495.
16
Beauty via single electrons in central Pb-Pb
collisions

• Main sources of background
• ? misidentified as e
• charm-decay electrons
• Dalitz decays
• photon conversions
• strangeness decays

• Detection strategy
• electron ID in TPCTRD
• impact parameter cut (B c? 500 ?m)
• pt cut (large b mass ? hard spectrum)

17
Electron Identification in TPC and TRD
Background from charged pions suppressed by
factor 10-4
Example e ID in TPC
18
Purity and statistics
Signal-to-total ratio and expected statistics in
107 central Pb-Pb events
80 purity 8 ? 104 e from B
pt gt 2 GeV/c , 200 ?lt d0 lt 600 ?m
19
Statistical error on electrons from B

• Statistical errors for 107 0-5 central Pb-Pb
  events
• 200 ?lt d0 lt 600 ?m

electrons
20
Systematic errors

• Error on MC corrections for tracking, selection,
  ID efficiencies
• Error on subtraction of charm electrons (curr.tly
  from D0 measure)
• Error on normalization to cross section per NN
  collision

Error on e from charm (propagated from D0 meas.)
21
Cross section of electrons from B
inner bars stat. errors outer bars stat. ?
pt-dep. syst. errors not shown 11 normalization
error
22
Extraction of a ptmin-differential cross section
for B mesons
Using electrons in 2 lt pt lt 20 GeV/c obtain B
meson 2 lt ptmin lt 30 GeV/c
inner bars stat. errors outer bars stat. ?
pt-dep. syst. errors not shown 11 normalization
error
Note quenching curves for illustration Pb-Pb
vs. pp comparison needed to study E loss
E Loss Calculation Armesto, Dainese, Salgado,
Wiedemann, PRD71 (2005) 054027.
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Future developments D0 ? Kp

• Optimization of selection strategy
• Maximize signal selection efficiency minimize
  background contamination
• Not trivial,especiallly at low pt, where
  background is large
• Use guidance from (semi-)automatized methods for
  pattern recognition
• We started from Linear Discriminant Analysis
  (LDA)
• our new postdoc, J.Faivre, did LDA for X and W at
  STAR (next slide)
• Strategy use LDA as a guide for optimization of
  classical cuts, under better human control

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Linear Discriminant Analysis (LDA)
Classical cuts either high efficiency or low
contamination
BKG
BKG
SGN
SGN
The idea of LDA cut on best linear combintion of
the variables
Example W in STAR
classical LDA
25
Future developments beauty

• In progress internal note and PPR contribution
  on Beauty via single electrons in Pb-Pb
• Next step dedicated study for pp collisions
• Physics performance goals
• sensitivity on measurement of b cross section (vs
  ATLAS/CMS, vs QCD calculations)
• combined with Pb-Pb, sensitivity on high-pt
  suppression of B, and comparison with
  light-flavoured hadrons and D mesons
• Study pp-specific issues
• define displaced electron track in pp scenario
  with broad beams
• use global event topology for better b selection?

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Summary

• Study for D0 reconstruction in Pb-Pb and pp (p-Pb
  in Trieste by R.Grosso) carried out
• strong simulation requirements fast simulation
  partially employed
• many (most) critical items addressed PID,
  primary vertex in pp, feed-down from beauty
• results are very promising
• analysis code skeleton prepared (in
  ALICE_ROOT/ANALYSIS/)
• starting second iteration, using LDA to optimize
  selection strategy
• Performance for beauty production measurement in
  Pb-Pb assessed, in terms of pt coverage and
  expected errors
• fast simulation partially employed
• 2 lt B ptmin lt 30 GeV/c, similar coverage with
  single m (Guernane et al.)
• starting study for pp
• Main studies for Charm and Beauty PPR section
• http//alice.web.cern.ch/Alice/ppr/chap
  6_6dir/current/alice_chap6_6.ps