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First-time Determination:Angular Dependence of Beam Helicity Asymmetry for
γp → ppp̄
Brandon S. TumeoFaculty Advisor: Lei Guo, Ph. D.Dept. of Physics and Astronomy
Florida International University, Miami FL
Introduction
Goal in nuclear physics: understanding of nuclear structure
QCD: theory of the strong interaction
Simplest QCD particle: protons
Introduction
1932: Positron observed (Carl Anderson, cloud chamber)
1933: Antimatter theorized, antiprotons predicted (Paul Dirac, Nobel Lecture)
1955: Antiprotons observed (Emilio Segre and Owen Chamberlain, UC Berkeley’s Bevatron)
Posited: Baryonium (baryon-antibaryon bound state)
Search: pp̄ excited states
Introduction
1999: reanalysis of WA56 experiment data from CERN Ω spectrometer; narrow pp̄ state at 2.02 GeV
2001: higher statistics work (one order of magnitude higher) done with CLAS g6c
2001: CLAS g6c data, showing no pp̄ resonance.1999: Reanalysis of WA56 data in pion production, showing a pp̄ resonance at 2.02 GeV/c².
Introduction
The CLAS registers theparticles that pass through it.
For each particle, it measuresq, E, p, and other quantities.
These measurements allow usto determine how many timesthe reaction of interest hasoccurred in the experiment.
Introduction
Our reaction: γp → ppp̄
Interested in pp̄ excited states
Polarization observables: measurements and calculations we can make from polarization of photon beam
Sensitive to interference between intermediate states:
− i.e. γp → pX
X can be bound states of: (p p̄), (K+ K-) , (π+ π-), and more
This research project focuses on polarization observable “beam helicity asymmetry”
Introduction
Photon helicity defined as 1,-1,0 corresponding to photon spin
Beam Helicity Asymmetry defined as: A =
Plane-angle φ defined as angle between momentum-planes:
Goal:To determine the beam
helicity asymmetry, A, as a function of φ
Missing Mass
MX = √(pγ + pp0 – pp1 – pp2)2
MX [Gev/c2]
Asymmetry: Method 1
To measure asymmetry:
Fit MM(p̄) with polynomial + gaussian for helicities 1,-1
Integrate gaussian parts to count
Polarization is averaged across each φ-bin
Asymmetry measurements taken for bins of beam energy, cos(θp̄), and φ
Antiproton Missing-Mass
[GeV/c2]
Asymmetry: Method 1
Check background asymmetry via integration of polynomial
Result: background asymmetry approx. zero
So
Asymmetry: Method 2
Data was kinematically fit; confidence-level distribution employed
Signal: α > 5%, Background: α < 5%
Diluted # events vs “true” # events:
“True asymmetry” vs. “diluted asymmetry”
Background counting employed to calculate diluting background B and dilution factor X
Confidence Level Distribution
α
Asymmetry: Method 2
We want to calculate “diluting background”, B
− Take polynomial from total fit
− Re-fit to missing-mass below confidence level (1-parameter fit)
− Integral = I; #events = J; (integral and #events within μ ±3σ)
− B = I – J, σB = √(I+J)Antiproton Missing-Mass Antiproton Missing-Mass, α < 5%
Results: Asymmetry and Dilution Factor
Asymmetry @ backward region (left) and forward region (right) Asymmetry @ lower energy (left) and higher energy (right)
Results: Coefficients
Statistical Errors
Final stage of analysis
− Currently being worked on
Rough calculation:
− Total Sys. Error, Method 1: 18.88%− Total Sys. Error, Method 2: 16.46%
Summary
Asymmetry determinations help probe for intermediate states
Sin(2φ) dominance throughout beam energy
Possible future work:
− Partial Wave Analysis: attempt at determination of state amplitudes
− Asymmetry determinations for other γp reactions