1. Boost Angles Measurement Technique Crossing Angle Measurement
Characterization of the PEP-II Colliding-Beam Phase Space
by the Boost Method
M. Weaver, SLAC, CA 94025, USA
W. Kozanecki, CEA/Saclay, F91191 Gif-sur-Yvette, France
B. Viaud, Université de Montréal, Montréal, Québec, Canada
Abstract
We present a novel approach to characterize the colliding-beam phase space at the interaction point of the energy-asymmetric PEP-II B-Factory. The method exploits the fact that the transverse-boost distribution of e+ e– → μ+ μ– events reconstructed in the BABAR detector reflects that of the colliding electrons and positrons. The mean boost direction, when combined with the measured orientation of the luminous ellipsoid, determines the e+-e– crossing angles. The average angular spread of the transverse boost vector provides an accurate measure of the angular divergence of the incoming high-energy beam, confirming the presence of a sizeable dynamic-β effect. The longitudinal and transverse dependence of the boost angular spread also allow to extract from the continuously monitored distributions detailed information about the emittances and IP β-functions of both beams during high-luminosity operation.
Introduction
PEP-II collides 9.0 GeV electrons upon 3.1 GeV positrons.
The high resolution BABAR particle tracking detector records e+ e– → μ+ μ– event tracks.
The boost vector angles {xB’,yB’} can be reconstructed from the μ± tracks.
The e± trajectories, and therefore IP parameters, are reflected in the boost vector angles.
Observables are luminosity-weighted
- Introduces z-dependent correlations
Boost Angles Measurement Technique
μ± tracks acquire only a small curvature in the solenoidal detector field
μ± momentum is poorly measured
μ± trajectory is measured precisely
Measure the boost angles using the vector n normal to the μ+ μ– decay plane. ^ n n l y n z m- f x U m+
Crossing Angle Measurement
Mean x’B is an E-weighted sum of beam x’ angles
Mean x’L is a size-weighted sum of beam x’ angles
Their difference is a measure of the x’ crossing angle
Dynamic β Observation
Measurements are performed in collision
Dynamic effects (beam-beam) are clearly visible
X-size of luminous region is proportional to βx*’s
Size of boost x’ (RMS) is inversely related to βx*’s
Boost z-Dependence
Fit for zw,β, ε-, ε+
waist
Amplitude2
Asymptotic
Limit2
Peak indicates waist location
Time of x-tune change towards ½-integer
x-size decrease
Optimum found during crossing angle experiment
z = -1
z = +1
x-divergence increase
IP Parameter Results
Individual vertical emittances for the electron beam (HER) and positron beam (LER) are also fit to the data. The combined fit to the σy’(z) and dy’/dy (z) measurements has considerably better statistical precision for the determination of the positron beam emittance. Initial investigations also point towards the presence of x-y coupling as the source of the observed systematic shift. An understanding of this systematic would allow a precise determination of the vertical emittances.
A vertical β-function waist location (common for both beams) is fit to the measurements. The longitudinal collision location can be seen to sometimes deviate.
● βy waist location
○ mean collision point
● σy’(z) fits only
○ dy’/dy(z) + σy’(z) fits
● σy’(z) fits only
○ dy’/dy(z) + σy’(z) fits
● σy’(z) fits only
○ dy’/dy(z) + σy’(z) fits
A common βy-function IP value is fit to either the σy’(z) measurement or both the σy’(z) and dy’/dy (z) measurements. A systematic shift is seen which might be explained by a more complete consideration of x-y coupling.