Transcript Slajd 1

STAR
Односпиновая асимметрия в образовании π0мезонов в области фрагментации
поляризованного протонного пучка на установке
СТАР в Брукхейвене
Л.Ногач, ИФВЭ, Протвино
the STAR Collaboration
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Motivation
• Contrary to simple pQCD predictions, first
measurements of  production in p↑p collisions found
large transverse single-spin asymmetries. Similar large
effects have recently been found in e+e- and semiinclusive deep-inelastic scattering.
• Significant developments in theory in the past few years
suggest common origins for these effects, but large
transverse spin asymmetries in p↑+p →  + X production
are not yet fully understood.
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Possible mechanisms
• Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]:
Flavor dependent correlation between the proton spin (Sp), proton momentum
(Pp) and transverse momentum (kT) of the unpolarized partons inside. The
unpolarized parton distribution function fq(x,kT) is modified to:
• Collins effect [Nucl. Phys. B396, 161 (1993)]:
Correlation between the quark spin (sq), quark momentum (pq) and transverse
momentum (kT) of the pion. The fragmentation function of transversely polarized
quark q takes the form:
According to the latest theoretical developments, both mechanisms
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contribute to  AN
RHIC Polarized Collider
RHIC pC Polarimeters
BRAHMS & PP2PP
Absolute Polarimeter (H jet)
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
Pol. H Source
LINAC
BOOSTER
Helical Partial Siberian Snake
200 MeV Polarimeter
AGS
AGS pC Polarimeter
Strong AGS Snake
1 MHz collision rate
Polarization ~ 0.5-0.6
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STAR
detector
layout
Forward
Meson
Spectrometer
•
•
•
•
•
TPC: -1.0 <  < 1.0
FTPC: 2.8 <  < 3.8
BBC : 2.2 <  < 5.0
EEMC: 1 <  < 2
BEMC: -1 <  < 1
FPD/FPD++:
 ~ -3.7/3.3
FPD/FMS:
 ~ -4.1/2.5-4.0
1/2 of the FMS
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Transverse spin runs at STAR
with forward calorimetry: 2001→2008
run/year
Run2
Run3
2001-2002
Run5
Run6
Run8
2008
detector
EEMC
and FPD
prototypes
6 matrices
of FPD
full FPD
(8 matrices)
East FPD
West
FPD++
East FPD
FMS
~15
~30
~45
~55
~45
0.15
0.25
0.1
6.8
~7.8
3.8
±3.3/±4.0
±3.7/±4.0
-3.7/3.3
-4.1/
2.5-4.0
Pbeam, %
sampled
 Ldt , pb
η
1
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Single Spin Asymmetry
•
Definition:
AN 
d  d
d   d 


Two methods of measurements:
• Single arm calorimeter:
AN 
dσ↑(↓) – differential cross section of
 when incoming proton has spin
Pbeam
 N   RN  

  
 
 N  RN 
R
L
L
R – relative luminosity (by BBC)
Pbeam – beam polarization
up(down)
Left
π0, xF<0
1
π0, xF>0
• Two arm (left-right) calorimeter:
p
p
Right
positive AN: more 0 going
left to polarized beam
 N  N  N  N
L
R
R
L
AN 

PBeam  N   N   N   N 
L
R
R
L

1




No relative luminosity needed
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Detectors – from FPD to FMS
Run8 and beyond: FMS
• FMS provides full azimuthal coverage for
Runs 3-6: FPD, FPD++
range 2.5    4.0
• broad acceptance
in xF-pT plane for
0

• Inclusive
cross
sections in p+p and
inclusive g,
,… production
 ,w,K
0
d(p)+Au
N
• A for inclusive  production



• broad acceptance for g and   from
forward jet pairs
three different positions
of modular detectors
20x
single
more
monolithic
acceptance
then previous
detectordetectors
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Published measurements with FPD
PRL 92, 171801 (2004)
PRL 101, 222001 (2008)
• Large transverse single-spin
asymmetries at large xF
• xF dependence matches Sivers
effect expectations qualitatively
• pT dependence at fixed xF is not
consistent with 1/pT expectation
of theoretical
√s=200 GeV,model
<η> = calculations
3.8
Asymmetry revealed at lower energies
persists at √s=200 GeV
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Run8 data analysis calibration
• the calibration methodologies
employed for the FPD have
been successfully adapted to
the FMS
• offline cell-by-cell calibration
using π0 peak position in the
sorted by high tower mass
distributions
Event selection done with:
minbias condition
hightower ADC threshold
(400/200 cts. for small/large cells)
Ng  2
0.07  mgg  0.2 GeV/c 2
z gg  | Eg 1  Eg 2 | ( Eg 1  Eg 2 )  0.7
(for small cells only)
fiducial volume cut (0.5 cell)
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LED monitoring system
Minimal time dependence in π0 peak position is observed
LED system: critical calibration tool
Calorimeter stable at
the level of a few percents
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Data to simulations comparison
• Full PYTHIA/GEANT simulations have adequate statistics to reach moderate xF
• Mass resolution in data is reasonable, given Run6 FPD performance
• Simulations have somewhat better resolution than data
DATA
SIMULATION
Present understanding sufficient; further investigations to be done
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First look at π0 AN from Run8 data
STAR preliminary
75% of Run8 data
Octant subdivision of FMS for
inclusive  spin sorting
stat.errors only
• AN comparable to prior measurements
• Azimuthal variation appears to be as
expected
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Summary
• High precision AN measurements with the FPD allows for
a quantitative comparison with theoretical models.
• FMS was commissioned and fully operated in Run8. It
has 20x the acceptance of FPD (1264 channels
compared to 98 for the pair of FPD modules).
• Reconstruction and calibration procedures successfully
ported from FPD to FMS.
• Calibration is completed and data show good agreement
with the simulated sample of events.
• Inclusive 0 AN(xF) from the FMS is in agreement with the
FPD measurements.
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Outlook
 Complete analysis of π0 asymmetry
 Going beyond π0 detection:
- AN for a state that contains 0 pairs
- AN for states with heavier mesons
(η, ω, J/ψ…)
- direct photons, “jet-like” events – can help
to discriminate between Sivers and
Collins effects
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BACKUP
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π0 cross section measurements
PRL 97, 152302 (2006)
Cross-section is consistent
with NLO pQCD calculations;
it was used in a recent global
analysis of e+e-, SIDIS and pp data
to determine fragmentation
functions [PR D76, 074033 (2007)]
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Separating Sivers and Collins effects
Sivers mechanism: asymmetry
in the forward jet or γ production
SP
Collins mechanism: asymmetry
in the forward jet fragmentation
SP
kT,q
p
p
p
Sensitive to proton spin –
parton transverse motion
correlations
p
Sensitive to
transversity
Sq
To discriminate between the two effects we need to go
beyond π0 detection to direct photons or jet-like events
kT,π
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Energy dependent corrections
• π0 peak position depends on the
energy
• linear correction extracted from π0
peak position and being applied to
photon energies works for both π0s
and ηs, and significantly decreases
shift from zero in δEγγ = Esimu Ereco.
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Association analysis
• comparison of generated quantities to GEANT simulations
• we consider
Egg  Egg ,sim  Egg , gen
UNCORRECTED
CORRECTED
Eliminating energy dependence in  mass peak gives the correct average
neutral pion energy
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Simulation and search algorithm for π0π0 in
FMS and its engineering prototype
• it is possible to find events where
the π0 pair originated at a
significant distance from the vertex
• one source of such events are
decays KS→π0π0 (31% branching
fraction)
• plot shows the mass distribution
for displaced vertices above 100
cm from the BBC vertex; a
pronounced KS mass bump is
visible
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