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