Transcript What is PREx?

Noise Analysis for
PREx - Pb Radius Experiment
Presented by:
Luis Mercado
UMass - Amherst
6/20/2008
Overview

Part I: About the experiment
–
–
–

Introduction – What is PREX?
Motivation / Physics Background
Applications in nuclear and astrophysics.
Part II: My current work
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–
–
Luminosity Monitors
Recent Results
Looking Forward
PART I
About the Experiment…
APV
R L

~ 10 6
R L
Introduction – What is PREx?


PREx is a Parity Violation experiment to be
conducted in Hall A in 2010.
Will measure PV electroweak asymmetry in elastic
scattering of polarized electrons from 208Lead.
 d 
 d 

 

GF Q 2
 d  R  d  L
A 

2 2
d

d






 

 d  R  d  L
Motivation


Because the Z boson couples mainly to neutrons,
ALR can provide a clean measurement of Rn by
studying the ratio of proton and neutron form factors.
Proton form factor is well known, so we can extract
the neutron density distribution from the neutron
1
F (Q ) 
d r j (qr )  (r )
form factor.
4 
The value of Rn is currently known to 5% accuracy.
PREx aims to achieve an accuracy of 1% or less.
2

0
(T.W. Donnelly, J. Duback, I. Sick).
3
N


FN (Q 2 ) 
2
 1  4 sin W 

FP (Q 2 ) 

dA
 3% 
A
0
dRn
 1%
Rn
(C. J. Horowitz)
N
Physics Background
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Heavy nuclei such as 208Pb
are believe to have a larger
neutron-weak radius than its
proton-charge radius.
This “neutron skin” is a result
of a large neutron excess and
a large Coulomb barrier.
Finding Rn will be a
fundamental test of nuclear
theory and provide clues
about neutron-rich matter.
(C. J. Horowitz)
Heavy Nuclei and Neutron Stars
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The 208Pb nucleus is 18 orders smaller and 55
orders lighter than a neutron star.
Rn -> Sv -> E(np/nn) -> Pressure
Lead’s neutron skin is similar to the crust of a
neutron star, made up of neutron-rich matter at
similar densities.
To understand these systems, the equation of state
of dense matter is essential.
Transition density between crust and outer core
depends on the neutron skin of 208Pb.
PART II
My current work…
Luminosity Monitor Setup



Used as diagnostic for experimental setup
(beam-line, instrumentation and target).
Located 7m from the target.
Consists of eight symmetrically placed
detectors.
Luminosity Monitor Setup
Reason for Analysis



Lumis are sensitive to very small scattering
angles (~0.5°), where any measured
asymmetries should go to zero.
Width of integrated signal gives an idea of
the intrinsic noise of the experimental setup.
Want noise to be < 100ppm. Noise is defined
as width of asymmetry distribution for 15Hz
pulse pair.
January ’08 Test Run
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Had week long test period for diagnosing several
components of the experiment.
Used several beam energies/currents on Carbon
and Lead targets.
Tested target stability up to 100uA successfully.
New Lumi design was made to include a filter box.
Some of the goals were to study backgrounds,
diagnose noise and understand linearity of Lumi
setup (PMTs and ADCs).
Test Run Configuration
1

2
8


7
3

6
4
5
1, 3, 5 & 7 see full
signal.
2 & 6 have 10% filters.
4 & 8 were ‘blinded’
with aluminum sheets.
Bottom three were
shielded by Lead
bricks.
Analysis of ’08 Run #10296


Data acquired on
01/25/08 with beam
current of 60uA on a
Thin Lead target.
RMS values are
regressed with
respect to position
and current.
Lumi ID Reg. RMS [ppm]
1
119.1
2
270.2
3
4
5
112.4
816.3
105.7
6
7
8
152
112.4
383.6
1
7
3
’08 RMS Values I (odd)
5
1
3
5
7
’08 RMS Values II (even)
2
4
6
8
8
2
6
4
’08 Combo Noise
V
D1
C
Sum
H
D2
X
Ave
D2
V
H
Analysis of ’08 Run (cont.)
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

Lumi ID Reg. RMS [ppm]
Statistics for 4-Lumi
combo is ~50 ppm
V
55.7
For each oddH
62.8
numbered Lumi, noise
D1
149.6
should be ~100 ppm,
D2
451.2
but we get higher
C
51.7
value.
X
241.1
When averaging
Sum
126.7
Lumis, width does not
Ave
126.3
scale like 1/sqrt(N).
D1
What are the source of noise?
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Blinded – Backgrounds.
Electronics noise in BPM and BCM.
Unaccounted Beam Noise.
PMT pedestals.
Correlated noise.
Possible Higher order effects.
Summary
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Lumi data is very useful for studying noise.
Seems like we will be able to achieve
required noise levels.
Can try to reduce it further by over-sampling
or by using a thicker target.
Problem: thicker target introduces
nonlinearities in PMTs and ADCs.
Need further tests and analysis.
Acknowledgements
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Robert Michaels (JLab)
Kent Pachke (UVA)
Krishna Kumar (UMASS)
Dustin McNulty (UMASS)
Charles Horowitz (IU-Bloomington)
The rest of the HAPPEx/PREx
Collaboration…