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Heavy flavor measurements with high-pT electrons
in the ALICE EMCalMark Heinz (for the ALICE Collaboration)
Yale University
26th Winter Workshop on Nuclear DynamicsOcho Rios, Jamaica, Jan 2-9 2010
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Winter Workshop 2010, Jamaica 2M.Heinz (Yale)
Physics Motivation (p+p)• Beauty physics in p+p (examples):
– Transverse momentum spectrum, X-section, (normalization for “Quarkonia” studies)
– Jet-Shapes – Fragmentation function
X-section vs NLO
CDF (PRD78,2008)
Y(r
/R)
Jet-Shape
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Winter Workshop 2010, Jamaica 3M.Heinz (Yale)
Physics Motivation (Heavy Ion)• Investigate energy loss of quarks vs. gluons• Investigate differences between the predictions of pQCD
vs ADS/CFT
Horowitz, Gyulassy (arXiv:0804.4330)
RHIC (200 GeV)
S.Wicks et al (Nucl.Phys.A784 (2007)
RAA(charm)/RAA(beauty)
pQCD
ADS/CFT
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Winter Workshop 2010, Jamaica 4M.Heinz (Yale)
Electron Identification in Alice
Barrel (TPC/TRD/TOF) Superior electron/hadron
discrimination up to p~10 GeV/c
EMCal Very good electron/hadron
discrimination for p > 10 GeV/c. PID parameters tuned.
4 Super-modules installed in ALICE
5th SM at calibration stage at Yale University
Energy calibration of detector with p0 currently underway
Part of routine data-taking Sufficient coverage for day 1
physics Run 2010 configuration(Pb+Pb low luminosity)
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Winter Workshop 2010, Jamaica 5M.Heinz (Yale)
Electron Rates (PYTHIA)• PYTHIA Rates are scaled to 1 month of Pb+Pb running (0.5mb-1s-1, 106
sec, A2)• Significant beauty-electron yield out to pT~50 GeV/c• Dominant background are W-electrons above 20 GeV/c• Hadron/Electron ratio is 500-800. Sets scale for PID performance.
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Winter Workshop 2010, Jamaica 6M.Heinz (Yale)
EMCAL PID: E/p• For electron identification we use the ratio of Ecluster/ptrack. • Track-Cluster matching efficiency ~65%• The electron peak can be cleanly separated from pions. Material
interaction in front of the EMCAL does deteriorate the performance.
Simulation Simulation
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Winter Workshop 2010, Jamaica 7M.Heinz (Yale)
Hadron Rejection Power (HRP)• Results from CERN test-beam for
energies 40-80 GeV. These compare well to ideal simulation (ie. no additional detector material in front of EMCAL)
• Results from simulation for a set of optimized cuts for electron identification and full ALICE detector simulation. For ~65% electron efficiency we obtain a HRP between 400-1000.
From published NIM paperarXiv: 0912.2005
Simulation
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Winter Workshop 2010, Jamaica 8M.Heinz (Yale)
Electron Spectra• Reconstructed conversion electron vertices are found by the V0-finder
and tagged using an invariant mass cut (Minv<100 MeV). These are then subtracted from the inclusive electrons.
• After EMCAL PID non-photonic electron candidates dominate misidentified hadrons by at least factor 10.
• Dominant background to b-electrons above 20 GeV/c from W-electrons. This can be further reduced by secondary vertex strategies.
PYTHIA
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Winter Workshop 2010, Jamaica 9
Corrected Spectra
• Efficiency corrected rates of non-photonic electron candidates
• Systematical errors indicate the variation of EMCAL PID criteria
M.Heinz (Yale)
PYTHIA Simulation
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Winter Workshop 2010, Jamaica 10M.Heinz (Yale)
B-Tagging: Displaced Vertex Method
• The method previously used by CDF to identify secondary vertices from semi-leptonic decays. Further development and implementation in ALICE.
• Breakdown of method:– Find a high-pt electron trigger particle in
EMCAL– Pair with all charged hadrons within a
cone of radius dR.– Reconstruct “approximate” (average
between B-D decay) secondary vertex using an identified electron and all associated hadrons
– apply more cuts to reduce backgrounds from other leptonic decays.
– Plot Lxy distribution and obtain B-contribution from positive/negative imbalance
D0
B-
e-
Kp/e
Pe+K
r
PrimVtx
SecVtx
Lxy= r .pe+k / |pe+k|
Lxy
CDF Phys.Rev.D66 (2002))
M.Heinz, arXiv: 0712.2422
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Winter Workshop 2010, Jamaica 11M.Heinz (Yale)
Method Cuts• Tracks
– Track Impact Parameter < 0.5 cm (electron)– Silicon Hits: ITS>=4– PT > 1.0 GeV/c (associated hadrons)
• Secondary Vertex– pairDCA < 0.02 cm– R= √Dh2+Df2 (electron-hadron) < 1.0 rad– Vertex length < 1.0 cm– SignDCA Lxy > 0.1– Invariant Mass (e+k) > 1.5
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Winter Workshop 2010, Jamaica 12M.Heinz (Yale)
Sign-Dca Lxy
• The method relies on the fact that the B-meson creates a displaced vertex with ct~500mm.
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Winter Workshop 2010, Jamaica 13M.Heinz (Yale)
Electron tag efficiency• Tagging algorithm can be tuned by requiring a
given number of heavy flavor vertices which pass our cuts
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Winter Workshop 2010, Jamaica 14M.Heinz (Yale)
B-Jet Tagging performance in p+p• FASTJET Jet-Finding Algorithm: kT, R= 0.4 (anti-kT comparable)• Standard definitions:
– Efficiency = tagged B-jets/MC-B-jets– Fake Rejection = Non B-Jet efficiency
• Algorithm currently tuned for maximum fake rejection. Studies ongoing to map out full cut-space
• Results comparable to CMS Soft Lepton Tag (Note 2005/058)
PYTHIA jets (pThard 12-150 GeV/c) PYTHIA jets (pT
hard 12-150 GeV/c)
Signal Efficiency Fake rejection
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Winter Workshop 2010, Jamaica 15
Expected tagged Jet yield
M.Heinz (Yale)
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Winter Workshop 2010, Jamaica 16M.Heinz (Yale)
Conclusions & OutlookConclusions• EMCAL PID (using E/p method) works well and we expect a hadron
rejection power of 400-1000 (65% efficiency) for 10 GeV/c< pT <80 GeV/c. Hadron/Electron ratios from MC in this range are ~500.
• Reconstructed electrons after subtraction of the photonic component and EMCAL PID dominate the background of mis-identified hadrons by at least a factor 10.
• Preliminary studies of B-jet tagging algorithms in simulated p+p events have been performed. First results for efficiency and fake rejection using the displaced vertex method have been calculated and are comparable to similar methods used in other experiments.
• Tagged Jet-yields should be sufficient for measurements of B-jets to pT~50-60 GeV/c.
Outlook • More optimization of B-tagging algorithms• First look at real data …
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Backup
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Winter Workshop 2010, Jamaica 18
CMS Electron Tag performance
c uds g
-
M.Heinz (Yale)