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Observation of W decay in 500GeV p+p collisions at RHIC
Kensuke Okada for the PHENIX collaborationLake Louise Winter Institute
February 20, 2010
2/20/2010
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W at RHIC
• An unique probe to access the flavor dependence of polarized sea quarks.
• Parity violating single spin asymmetry (AL).
2/20/2010
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RHIC-PHENIX 2009 run
• First 500GeV p+p run. (March to April, 2009)• Polarization is <P>=0.39±0.04 • Integrated luminosity (with vertex cut) is
Ldt=9.3 8.6/pb
Confirmation of electron decay of W (central)Commissioning of new detectors for muon
decay of W (forward)
2/20/2010
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PHENIX Central arm
2/20/2010
For We channel
EMCal + DC/PC1 trackingAcceptance : 0.35 in rapidity 0.5*2arms in
EMCal Trigger Fully efficient at 12GeV
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Expected W decay signal
2/20/2010
pT [GeV/c]
We signalJacobean peak
Cartoon
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Expected W decay signal
2/20/2010
pT [GeV/c]
We signalCharm/bottom decay Zee
Cartoon
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Expected W decay signal
2/20/2010
pT [GeV/c]
We signalCharm/bottom decay Zeeh miss ID
Cartoon
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Expected W decay signal
2/20/2010
pT [GeV/c]
We signalCharm/bottom decay Zeeh miss IDPhotons (conversion & miss association)
Cartoon
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Measured spectra
2/20/2010
Data driven BG estimation: EMCal cluster x (conversion & mis association probability) The rest is explained by the h shape (NLO pQCD+EMCal response)
The bump is W+Z signal.
(shape & yield)
Data (9.3/pb)
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Charge separated spectra
2/20/2010
Charge sign : from DC angle ( 2.3 separation for 40GeV/c track)
The same factor was used for signal shape. W-e- signal has less acceptance than W+e+ signal.
+ -
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Parity violating single spin asymmetry
• RHIC has 120 bunch cycle (each bunch 106ns). It reduces systematic uncertainties of detector time dependence.
• The sample are sorted by the spin state and calculate the asymmetry.
2/20/2010
Raw asymmetry
N+ : helicity +N- : helicity –normalized by L
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Isolation cut
2/20/2010
E < 2GeV
We can apply any cut to improve S/N ratio,if it’s spin independent
+ -
90+% of signal is kept (red histograms)
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Raw asymmetries (positive particle)
2/20/2010
BGSignal
pT range [GeV/c]
raw asymmetry
Background 12-20 0.0350.047
Signal 30-50 -0.290.11
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Physics asymmetry
2/20/2010
Raw asymmetry Physics asymmetry (AL)
x 1/<P> beam polarizationx Dilution factor (BG from Z, hadron)
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Summary
• PHENIX observed W e decay at mid rapidity region.
• First attempt to measure single spin asymmetry has detected a parity violating asymmetry leading to a preliminary value of AL
• ALW+e+ is measured.
-0.830.31 (scale uncertainty) It is consistent with the predictions.
2/20/2010
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Outlook
• 2009 run was a short test. • We plan to accumulate more 500GeV p+p data
in the next few years. • RHIC expects higher polarization. • The PHENIX detector is undergoing
considerable upgrades to enable a program of measurements of W± in the forward direction
2/20/2010