1 Л.Ногач, ИФВЭ, Протвино the star collaboration Односпиновая...
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Л.Ногач, ИФВЭ, Протвиноthe STAR Collaboration
Односпиновая асимметрия в образовании π0-мезонов в области фрагментации
поляризованного протонного пучка на установке
СТАР в Брукхейвене
STASTARR
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Motivation
• Contrary to simple pQCD predictions,Contrary to simple pQCD predictions, f first measurements of production in production in pp↑p↑p collisions found collisions found large transverse single-spin asymmetries. Similar large large transverse single-spin asymmetries. Similar large effects have recently been found in effects have recently been found in ee++ee-- and semi- and semi-inclusive deep-inelastic scattering.inclusive deep-inelastic scattering.
• Significant developments in theory in the past few years Significant developments in theory in the past few years suggest common origins for these effects, but suggest common origins for these effects, but large large transverse spin asymmetries intransverse spin asymmetries in pp↑+p↑+p → → + X+ X productionproduction are not yet fully understood.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 contribute to AN
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RHIC Polarized Collider
BRAHMS & PP2PP
STAR
PHENIX
AGS
LINACBOOSTER
Pol. H- Source
Spin Rotators(longitudinal polarization)
Siberian Snakes
200 MeV Polarimeter
RHIC pC PolarimetersAbsolute Polarimeter (H jet)
AGS pC PolarimeterStrong AGS Snake
Helical Partial Siberian Snake
PHOBOS
Spin Rotators(longitudinal polarization)
Siberian Snakes
1 MHz collision ratePolarization ~ 0.5-0.6
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STAR detector layout
• TPC: -1.0 < < 1.0• FTPC: 2.8 < < 3.8• BBC : 2.2 < < 5.0• EEMC: 1 < < 2• BEMC: -1 < < 1
Forward Meson Spectrometer
FPD/FPD++: ~ -3.7/3.3
FPD/FMS: ~ -4.1/2.5-4.0
1/2 of the FMS
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run/year Run2
2001-2002
Run3 Run5 Run6 Run8
2008
detector EEMC
and FPD
prototypes
6 matrices
of FPD
full FPD
(8 matrices)
East FPD
West
FPD++
East FPD
FMS
Pbeam, % ~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
1, pbLdtsampled
η
Transverse spin runs at STAR with forward calorimetry: 2001→2008
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Single Spin Asymmetry
• Definition:
dσ↑(↓) – differential cross section of when incoming proton has spin
up(down)
Two methods of measurements: • Single arm calorimeter:
R – relative luminosity (by BBC)
Pbeam – beam polarization
• Two arm (left-right) calorimeter:
No relative luminosity needed
dd
ddAN
L
LR
RNN
RNN
PA
beamN
1
LRRL
LRRL
BeamN
NNNN
NNNN
PA
1
π0, xF<0
π0, xF>0
Left
Right
p p
positive AN: more 0 going
left to polarized beam
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Runs 3-6: FPD, FPD++
• Inclusive 0 cross sections
• AN for inclusive 0 production
Detectors – from FPD to FMS
Run8 and beyond: FMS • FMS provides full azimuthal coverage
for range 2.5 4.0
• broad acceptance in xF-p
T plane for
inclusive ,,,K,… production in p+p and d(p)+Au
• broad acceptance for and from forward jet pairs
20x more acceptancethen previous detectors
three different positionsof modular detectors
single monolithic detector
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Published measurements with FPD
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 model calculations
PRL 92, 171801 (2004)
√s=200 GeV, <η> = 3.8
Asymmetry revealed at lower energiespersists at √s=200 GeV
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Event selection done with:
• 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
minbias condition
hightower ADC threshold
(400/200 cts. for small/large cells)
(for small cells only)
fiducial volume cut (0.5 cell)
Run8 data analysis -calibration
7.0)(||
/2.007.0
2
2121
2
EEEEz
cGeVm
N
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Calorimeter stable at the level of a few percents
Minimal time dependence in π0 peak position is observed
LED system: critical calibration tool
LED monitoring system
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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
Octant subdivision of FMS for inclusive spin sorting
• AN comparable to prior measurements
• Azimuthal variation appears to be as expected
STAR preliminary
stat.errors only75% of Run8 data
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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 measurementsPRL 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
Collins mechanism: asymmetry in the forward jet fragmentation
Sivers mechanism: asymmetry in the forward jet or γ production
SPkT,q
p
p
SP
p
p
Sq kT,π
To discriminate between the two effects we need to go beyond π0 detection to direct photons or jet-like events
Sensitive to proton spin – parton transverse motion correlations
Sensitive to transversity
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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
gensim EEE ,, • comparison of generated quantities to GEANT simulations
• we consider
Eliminating energy dependence in mass peak gives the correct average
neutral pion energy
UNCORRECTED CORRECTED
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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