Transcript Slide 1

Systematic Errors Studies in the RHIC/AGS
Proton-Carbon CNI Polarimeters
Andrei Poblaguev
Brookhaven National Laboratory
The RHIC/AGS Polarimetry Group:
I. Alekseev, E. Aschenauer, G. Atoian, A. Bazilevsky, A. Dion, H. Huang,
Y. Makdisi, A.Poblaguev, W. Schmidke, D. Smirnov, D. Svirida, K. Yip, A. Zelenski
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Layout of the RHIC facility
BRAHMS(p)
Absolute Polarimeter (H jet) RHIC pC Polarimeters
Siberian Snakes
Spin flipper
PHENIX (p)
STAR (p)
Spin Rotators
(longitudinal polarization)
Spin Rotators
Solenoid Partial Siberian Snake (longitudinal polarization)
LINAC
Pol. H Source
200 MeV Polarimeter
BOOSTER
AGS
Helical Partial
Siberian Snake
AGS pC Polarimeter
Strong AGS Snake
• H jet (pp) polarimeter provides absolute polarization measurements at RHIC
• RHIC pC polarimeters provide polarization monitoring including polarization
profile measurements
• AGS pC polarimeter provides polarization monitoring (mainly used for technical
control and special beam studies)
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Proton-Carbon Polarimeter kinematics
Plan view
Event selection in RHIC/BNL
pC polarimeters:
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Polarization Measurement
Spin dependent amplitude:
Rate in the detector:
1. Spin Flip (one detector):
A theoretical model for AN(t)
(a fit to the BNL E950 data)
2. Left-right asymmetry
(two detectors)
Square-root formula:
Combining “spin flip” and “left/right asymmetry”
methods allows us to strongly suppress systematic errors
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AGS CNI Polarimeter 2011
3 different detector types:
1,8
- Hamamatsu, slow preamplifiers
Larger length (50 cm)
2,3,6,7 - BNL, fast preamplifiers
Regular length (30 cm)
4,5
- Hamamatsu, fast preamplifiers
Silicon Strip Detectors:
Strip orientation
Dead Layer
90 degree detectors
(2,3,6,7)
45 degree detectors
(1,4,5,8)
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Schema of Mesurements
WFD
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α-source measurements
(241Am , 5.486 MeV)
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“Banana fit”
t-t0 = tA(xDL,αA)
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An example of data selection
If t0 is known, a model
independent calibration
can be done
Wrong determination
of mean time
It must be a vertical line
if detector is properly
calibrated
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The AGS pC polarimeter is succesfully used for
the relative measurements
Beam Intensity, I
Polarization profile measuremens
(jump quads study)
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Study of Polarization dependence
on beam intensity
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Is absolute polarization measurement possible
with a proton-Carbon polarimeter ?
A systematic errors study is necessary to answer this question.
• Are results dependent on detector configuration ?
• Do we know the Analyzing Power AN(t) ?
• Could we properly calibrate detectors ?
• Do we understand energy losses in the target ?
• Can we control rate dependence of polarization measurements ?
•…
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Polarization dependence on detector type
Polarization vs Beam Intensity (Late CBM),
Vertical Target3, all 2011 runs
Polarization measured by
all 3 types of detectors is
consistent within 1-2%
accuracy !
Can we explain slope
difference for 90 and 45
degree detectors by rate
effect ?
All 2011 data was
included in the fit.
Results of the fit
should be used for
comparison only
Polarization,
P(1.2) , is given for
intensity 1.2×1011
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Polarization dependence on detector type
Hamamatsu (45 degree) vs. BNl (90 degree) detectors
No visible variations of the polarization ratio during 4-month Run 2011!
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Analyzing Power AN(t)
AN measurement for assumed 65% polarization
• Poor consistency between theory and measurements
• Wrong energy calibration and energy losses in the
target may contribute to the discrepancy
• Results depend on the target (rate ?, energy losses ?)
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Potentially, analyzing power
may be measured by the pC
polarimeter (up to a
normalization constant)
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Enrgy Calibration
Dead-Layer corrections
L0 is stopping range derived
from MSTAR dE/dx (used in
“standard” calibration)
Stopping range parametrization:
“standard parametrization”, p=1/d
constant energy loss, p=Eloss
polinomial
Carbon Energy from measured amplitude:
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Enrgy Calibration
Inverse task:
If E(αA) is known then we can determine L(E) and dE/dx
If t0 is know then we can measure Carbon energy as a function of the amplitude αA
A model independent
calibration of the
amplitude
and thus we can measure dE/dx (in deadlayer length units)
WARNING: In such a way we measure effective dE/dx which may be different
from ionization losses dE/dx.
If t0 is unknown we can make a fit, that is to try all possible t0 and select one which
provides best data consistency. It might provide us with value of t0 and calibration
of the measured amplitude ECarbon = E(αA) .
WARNING: the fit may work incorrectly if parameterization of stopping range
L(p, αA) can not approach well true effective dE/dx.
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New calibration method vs standard one
• The function L(E) = p0L0(E) + p1L02(E) fits data much
better then “standard” calibration function p0L0(E)
• Significant difference in the value of t0
• Significant difference (up to 15% ) in the energy
scale
Better fit of data does not guarantee better
calibration !
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Comments about t0 determination in the fit
Including t0 to the fit:
(τ is time of flight for 1 MeV carbon )
If
then
However, if
(good calibration)
may be approximated by variations of the
then result of the calibration is unpredictable
may be masked by faked
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correction
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Rate effect
An estimate of the rate effect
Simplified example
More realistic example
Only one carbon signal may be taken by the DAQ
Detection efficiency:
is rate of good events
is total DAQ rate
where r is average rate per bunch.
- is a strip pair number
- is average rate per strip (millions events per spill)
- is rate in strip i (events per bunch), n = 0.0528
- is relative rate in the strip I
Rate contribution
Machine contribution
assume factor k is the same for all strips
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Rate effect
Vertical Target3, all 2011 runs: Strip Pairs
The measured value of the rate effect factor
agrees well with a pileup based estimate
Polarization dependence on beam intensity
(averaged over all 2011 runs) :
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Enrgy losses in the target
Target dependence of the Polarization measurements
Polarization vs intensity, Horiz. target #1, JQ-on
Polarization vs intensity, Vertical target#3, JQ-on
AGS pol., during H-jet meas. at injection
Intensity -1.5
• Slope difference is consistent with our estimates
• We can explain 4±1 % of polarization difference
by rate effect. Where the rest 4.6±1.7% come
from?
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Enrgy losses in the target
Energy losses in the target
125 μm target
φ
Target
Beam
(d ~ 30 nm)
Measured/True Polarization
Angle
dE/dx
Calculation
AN(t)
Energy range 400-900 keV
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Target Thickness (μg/cm2)
4
8
16
0
0.991
0.982
0.965
45
0.987
0.975
0.951
80
0.950
0.903
0.825
85
0.903
0.802
0.610
Effect of energy losses in
the target
• may be significant
• may be unpredictable
0 - 360
0.970
0.948
0.911
Results are independent on target width !
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Summary
• Different types of detectors were tested in the Run 2011
• Results of polarization measurements were consistent within 1-2% accuracy
• No significant variation of the results of measurements were observed during the
whole 4 month run.
• The polarimeter has a capability to measure analyzing power up to the arbitrary
normalization factor, but accurate study of the systematic errors is needed for that.
• Standard energy calibration method was found to be unreliable, new method of
calibration are suggested but more development is still needed.
• Experimental evaluation of the rate effect is consistent with estimation of pileup
contribution.
• More accurate control of energy losses in the target is needed.
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