Quarkonia polarization in ALICE

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Quarkonia polarization in ALICE

• Physics motivations
• Analysis technique for the measurement of the
  quarkonia (J/?, ?) polarization in the dimuon
  channel
• Feasibility study under different running
  conditions

R. Arnaldi L. Bianchi - E. Scomparin
Terzo Convegno Nazionale sulla Fisica di Alice,
12-14 Novembre 2007
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Basic definitions

• Quarkonia polarization is reconstructed from
  the angular distribution of the decay products
  (J/? ? ??- ) in the quarkonia rest frame
• The polarization axis z can be chosen as the
  J/? direction in the target-projectile center
  of mass frame (Helicity frame)
• The angular distribution is parameterized as

• gt 0 Transverse polarization
• 0 No polarization
• lt 0 Longitudinal polarization

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Physics motivations
p-p collisions
Polarization measurements are a test for
different quarkonia production mechanisms, since
different models predict different polarizations

• CSM predicts transverse polarization
• CEM predicts no polarization
• NRQCD predicts transverse polarization at
  large pT

A-A collisions
An increase of J/? polarization in heavy-ion
collisions is expected in case of QGP
B.L. Ioffe and D.E. Kharzeev Phys. Rev. C68
061902 (2003) Quarkonium Polarization in
Heavy-ion collisions as a possible signature of
the QGP
The physics picture emerging from several
experiments (E866, CDF, HERA-B, PHENIX
and NA60) is not very clear
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Experimental results J/? polarization
E866 (pA_at_800GeV)
HERA-B (p-A _at_ 900GeV)
PRL 99, 132001 (2007)
HERA-B
Large transverse polarization at high pT
predicted by NRQCD NOT seen
NA60 (In-In _at_ 158GeV)
Phenix (d-Au and Au-Au _at_ vs 200GeV)
No significant polarization effects
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Experimental results ? polarization
E866 experiment (pA_at_800GeV)
? (1S) small transverse polarization at high
pT ? (2S) and ? (3S) strong transverse
polarization ? (1S) NRQCD predicts ? 0.28
0.31 Measured value ? 0.07 ? 0.04
Results for ?(1s) show no evidence for transverse
polarization (no discrepancy with NRQCD since
polarization predicted for pT(?)gtgtm?)
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How to extract ? (J/?) kinematical distributions
Quarkonia distribution for a certain kinematical
variable obtained

• determining NJ/? (?) (y, cos?, pT)
• correcting for acceptance effects
• integrating on the other kinematical variables

Acceptances obtained on a 3D grid

• generation and reconstruction of J/? (?) events
  (with flat y, pT and cos? distributions) over
  the kinematical region with a fine binning
• 0 lt pT lt 20 GeV/c, -4 lt y lt
  -2.5, -1 lt cos? lt 1

Acut 0.05
Definition of a fiducial region
Example pT vs y region where AgtAcut for
-0.8ltcos?lt0.8, covering the largest pT range
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? acceptance matrices - Helicity
Similar acceptances for the positive values of
cos?HE y, pT coverage depends on the cos? range
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Generated and acc. corrected distributions
realistic y, pT distributions fixed degree of
polarization
Generation of test distributions for the ? with

Reconstruction
Acceptance corr. and comparison to the generated
spectra in the fiducial region
-0.6 lt cos?HE lt0.6 0 lt pT lt 20 GeV/c -3.6 lt y lt -3
Good agreement between the generated and
acceptance corrected kinematical variables
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? polarization in p-p _at_ 14 TeV - Helicity
According to the ALICE-INT-2006-029, assuming a
Luminosity 3 1030 cm2s-1 and 107 s, we expect
27000
Polarization can be studied in the whole pT range
for -0.6 lt cos?HE lt0.6 and -3.6 ltylt -3 ? 13000
reconstructed events after kinematical cuts
Generated and Acc. corrected cos?HE distributions
are in good agreement for all the polarization
values
Statistical error on ? 0.06 0.11
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? polarization in p-p _at_ 14 TeV vs pT
Polarization is expected to depend on pT
Starting from 27000
Define 4 pT bins, as done in CDF
0ltpTlt3 GeV/c 3ltpTlt5 GeV/c 5ltpTlt8 GeV/c 8ltpTlt20
GeV/c
For each pT range a fiducial range is defined
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? polarization in p-p _at_ 14 TeV vs pT
0ltpTlt3 GeV/c
8ltpTlt20 GeV/c
3ltpTlt5 GeV/c
5ltpTlt8 GeV/c
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? polarization in p-p _at_ 14 TeV vs pT
a 1
a -1
a 0
in each pT bin
Statistical errors on the ? 0.03 -0.20
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J/? polarization
The same approach can be used to study J/?
polarization However, to better take into
account the background under the J/?, another
approach was used, similar to the technique used
by CDF
ALICE p-p _at_ 14 TeV ? S/B 12.0
ALICE Pb-Pb _at_ 5.5 TeV ? S/B 0.2 - 3
The amount of background is negligible
The amount of background is important
the subtraction of the bkg is less crucial, since
the bkg should not bias the J/? polarization
measurement
the bkg contribution has to be subtracted, since
it may bias the J/? polarization measurement
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Comparison J/? ?Gen and ?Calc p-p _at_ 14 TeV

• The bias on the evaluation of the J/?
  polarization due to the background is not
  very large (as expected)
• Even in this case, the subtraction of the
  background improves the measurement,
  compensating for the small discrepancy between
  ?Gen and ?Calc
• With this statistics (200K) the error on ?J/? is
  lt 0.02

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Comparison J/? ?Gen and ?Calc - Pb-Pb _at_ 5.5
TeV
S/B 0.2 central Pb-Pb
S/B 3.13 peripheral Pb-Pb

• The background clearly washes out the original
  J/? polarization
• In both cases, the subtraction of the background
  allows to correct for the bias on the J/?
  polarization measurement
• Small systematic effect still visible

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Feasibility of the J/? polarization study in
running scenarios
p-p _at_ 14 TeV Luminosity 3 1030 cm-2 s-1 time
107 s J/? 2.8 106
The number of J/? is enough to perform a detailed
study as a function of pT.
Assuming 200000 reconstructed J/? in p-p _at_ 14 TeV
(all the statistics we have)

• 1ltpTlt4 GeV/c ? -0.02 0.02
• 4ltpTlt7 GeV/c ? -0.03 0.04
• pTgt7 GeV/c ? -0.03 0.05

when injecting ?0 we get
Pb-Pb _at_ 5.5 TeV Luminosity 5 1026 cm-2
s-1 time 106 s J/? 133000 (central
events) J/? 21700 (peripheral events) Total
J/? 6.8 105
The number of J/? is enough to perform a study as
a function of centrality. Absolute statistical
error 0.05 for all centralities (for
peripheral, smaller statistics compensated by the
smaller background)
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Conclusions
Polarization study is clearly feasible in ALICE
for the J/? and
Various algorithms to extract and J/?
polarization have been implemented and applied to
the foreseen data taking conditions
J/?

• Polarization can be determined both as a
  function of pT (p-p collisions) and centrality
  (Pb-Pb)
• p-p _at_ 14 TeV
• ? negligible errors on ?J/?
• Pb-Pb _at_ 5.5 TeV
• ?large background may lead to systematic
  error on ? ?statistical errors expected
  0.05

• p p _at_ 14TeV
• polarization can be studied as a function of pT.
• Assuming 4 pT the statistical error is 0.02
  0.2

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Backup
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Error on the polarization degree
The error on the ? parameter (for a fixed number
of events) depends on the degree of
polarization it is smaller for LONGITUDINAL
polarization, larger for TRANSVERSE polarization
same number of events
It depends on the lever arm of the fit