1. Noise Analysis for PREx - Pb Radius Experiment
Presented by:
Luis Mercado
UMass - Amherst
6/20/2008

2. Overview Part I: About the experiment Part II: My current work
Introduction – What is PREX?
Motivation / Physics Background
Applications in nuclear and astrophysics.
Part II: My current work
Luminosity Monitors
Recent Results
Looking Forward

3. PART I
About the Experiment…

4. 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.

5. Motivation
(T.W. Donnelly, J. Duback, I. Sick).
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 form factor.
The value of Rn is currently known to 5% accuracy.
PREx aims to achieve an accuracy of 1% or less.
(C. J. Horowitz)

6. Physics Background
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)

7. Heavy Nuclei and Neutron Stars
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.

8. PART II
My current work…

9. 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.

10. Luminosity Monitor Setup

11. 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.

12. January ’08 Test Run
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).

13. Test Run Configuration1
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. 8 2 7 3 6 4 5

14. 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
112.4 4
816.3 5
105.7 6
152 7 8
383.6

15. 1
’08 RMS Values I (odd) 7 3 5 1 3 5 7

16. 8 2
’08 RMS Values II (even) 6 4 2 4 6 8

17. ’08 Combo Noise V D1 C
Sum H D2 X
Ave

18. Analysis of ’08 Run (cont.)V D2 D1
Analysis of ’08 Run (cont.) H
Statistics for 4-Lumi combo is ~50 ppm
For each odd-numbered Lumi, noise should be ~100 ppm, but we get higher value.
When averaging Lumis, width does not scale like 1/sqrt(N).
Lumi ID
Reg. RMS [ppm] V
55.7 H
62.8 D1
149.6 D2
451.2 C
51.7 X
241.1
Sum
126.7
Ave
126.3

19. What are the source of noise?
Blinded – Backgrounds.
Electronics noise in BPM and BCM.
Unaccounted Beam Noise.
PMT pedestals.
Correlated noise.
Possible Higher order effects.

20. Summary 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.

21. Acknowledgements Robert Michaels (JLab) Kent Pachke (UVA)
Krishna Kumar (UMASS)
Dustin McNulty (UMASS)
Charles Horowitz (IU-Bloomington)
The rest of the HAPPEx/PREx Collaboration…