Plans for Lb Polarization Measurements in ATLAS

1
Plans for Lb Polarization Measurements in ATLAS

• North American ATLAS Physics Meeting
• December 20, 2004
• Homer A. Neal
• University of Michigan

On behalf of the ATLAS Lb Polarization Working
Group Michela Biglietti, Jacob Bourjaily, Homer
A. Neal, Natalia Panikashvili, Maria Smizanska,
Shlomit Tarem
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Outline

• Project Goals
• Theoretical Motivation
• Lb Production
• Lb Decay
• Simulation Studies and Validation of
  Generators (next talk by M. Biglietti)
• Planned background studies
• Rome milestones

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Ultimate Project Goals

• to measure the polarization of inclusively
  produced Lb hyperons in ATLAS in order to test
  models of Q/QBAR production, HQET, PQCD,
  Factorization, CP violation, and to search for
  new physics beyond the Standard Model

4
Intermediate Project Goals

• assess ability to detect, reconstruct, and
  acceptance-correct decay angular distributions
  from the reaction Lb-gt L J/y in ATLAS (with L-gt
  pp and J/y -gt mm )
• develop protocols for extracting the Lb
  polarization from these events
• estimate sample size, develop triggering and
  reconstruction strategies
• study backgrounds and develop algorithm for
  dealing with them
• outline physics analysis strategies

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Link between Heavy Quark Production and Resulting
Hyperons

• ASSUMED HYPERON PRODUCTION MECHANISM?
• heavy quark is produced by some interaction
  (maybe q-qbar with large relative momentum).
• this production process finishes quickly,
  compared to the time scale of non-pertubative
  strong interactions
• fragmentation occurs over a longer time scale,
  eventually forming a physical hadron containing
  the heavy quark
• velocity, spin...of heavy quark remains unchanged
  once produced
• WHAT CAN BE LEARNED FROM STUDYING HYPERON
  POLARIZATION?
• ...measuring the Lb polarization may tell us
  something of significance about how the b-quarks
  are produced, how they decay, what mechanisms
  (e.g., Higgs doublets) influence their a, whether
  PQCD and factorization work, what is responsible
  for the huge polarizations previously observed at
  several hundred GeV, etc...

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Example of Large L Hyperon Polarization at 400
GeV/c. Could the Lb Polarization be Even Larger?
What causes this? How does this effect depend on
quark mass?
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Attempt to predict Lambda Inclusive Polarization
from p-p Elastic Polarization Data (H. Neal,
Coral Gables, 2001)
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Lb Decay
The most general form of the amplitude for the
decay of Lb ? J/y L is
The coefficients A1 A2 B1 B2 may be calculated
from models that utilize the Heavy Quark
Effective Theory (HQET) with factorization. The
model assumptions permit the decay amplitude to
be expressed as convolutions of the Lb baryon, L
baryon and J/y meson distribution amplitudes
along with other factors that are calculable via
PCQD. The distribution amplitudes are universal
and may be extracted from other
reactions. Knowledge of M(l) permits the
calculation of the asymmetry parameter a ,
branching rates, and other observables.
Comparison of the predictions with experimental
data will provide a test of PQCD formalism.

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Lb Decay Probability
Lb Decay Probability
Angular distribution Lb? J/y(mm) L(pp) depends
on 5 angles 4 complex helicity amplitudes and
the polarization Pb . Normalization and overall
phase permit reduction of unknown parameters by
2. Helicity amplitudes (and a ) and Pb have to
be simultaneously determined.

• No software tools for implementing polarization
  models available in Pythia.
• Pythia uses a probabilistic approach to produce
  baryons and spin information is not used (see
  next talk by M. Biglietti on EvtGen adaptation)

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Helicity Amplitudes and the Lb Decay Probability
Distribution
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A Simple Test of EvtGen Results
P-0.4 ltcos?gt0.062
P0.9 ltcos?gt-0.130
cos(q)
cos(q)
Vs 2.25
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Background Issues

• WHAT DO WE MEAN BY BACKGROUND ?
• Is the assumed J/y really a J/y ?
• Is the assumed Lo really a Lo ?
• Is the Lb produced directly, or is it the result
  of some higher level decay?

CDF
DZERO

• Studies are underway on each of the above topics
• Encouraged by clean signals in CDF and DZERO

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Work in Progress

• the production of large samples of polarized Lb
  with a range of polarization and helicity
  amplitude values is underway.

• the investigation of a number of L and J/y
  reconstruction strategies and various protocols
  for extracting polarization from angular
  distributions is ongoing

• an extensive analysis of relevant background
  channels is underway

• low pt muons trigger studies to enhance the rate
  of Lb captured are ongoing

• discussions with theorists are underway regarding
  physics analysis strategies

• Target for full report ATLAS Physics Workshop in
  Rome

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Summary

• There exists considerable potential for
  interesting physics to be derived from the study
  of Lb polarization
• We believe we now know how to use EvtGen and how
  to construct bridges between it and any proposed
  decay model
• Tests have been made with EvtGen using polarized
  Lb that seem to confirm its proper functioning
• Encouraging avenues exist for trigger
  improvements and enhancement of J/y
  reconstruction strategies
• Previous studies suggest very clean Lb signals
  and rates that should permit early polarization
  measurements to be made at the few percent level
• There is much work to do, but the path seems clear