Robin Smith, Swarthmore College

Advisor: Professor Homer A. Neal, University of Michigan

University of Michigan Summer 2003 REU at CERN

August 5, 2003

A Study of Lambda-b Polarization in the LHC-ATLAS Experiment

For nearly thirty years, it has been experimentally known that lambda

hyperons produced inclusively in proton-proton collisions can be highly

polarized. Still not understood, this phenomenon remains one of the

significant mysteries in particle physics. It is hoped that a study of

the production and decay of lambda-b hyperons in the ATLAS experiment at

LHC can help resolve this dilemma. My project has aimed to examine the

major features of lambda inclusive polarization, assess applicability of

existing models to lambda-b studies in ATLAS, and review techniques for

extracting lambda-b polarizations from data collected in the ATLAS

experiment.

An examination of two simple hyperon production models provides a

reminder that hyperon polarization is not yet understood. For example,

two distinct approaches include the Lund model, in which lambda

polarization occurs as a result of s and sbar production requiring their

spins to counteract the angular momentum produced in the snapping of

color strings, and a quark-quark scattering model, in which polarization

of incoming u- and d-quarks is responsible for the lambda's spin.

Agreement between experimental lambda polarization measurements with 400

GeV/c incident protons and lambda polarization predicted from

proton-proton elastic scattering supports the latter, where the multiple

quark-quark scattering mechanism explains different polarization

behavior in distinct transverse scattering momentum regimes. While

explanation of the dramatic lambda polarization saturation for

transverse scattering momenta exceeding 1 GeV/c eludes the quark-quark

scattering model, a rather natural explanation of this effect emerges

from the Lund model.

We have developed a Monte Carlo program to determine if a saturation

effect might result from various spin flip protocols, subject to the

condition that the u and d quarks end up in a singlet state, with the s

quark ultimately determining the lambda spin. No such mechanism has yet

been found, though this work continues. In the Lund model, saturation

results from the s and sbar spins alining with 100% polarization to

offset one unit of angular momentum created during pair production.

Interestingly, new measurements reported just this summer suggest that

the spins of quark/anti-quark pairs produced from the vacuum are

anti-aligned. If true, this would invalidate the claims of the Lund

model and, once again, leave unanswered the question of the dramatic

saturation.

While it is commonly believed that spin effects disappear at higher

energies, the thirty percent lambda polarization at 400 GeV could

extrapolate to even larger lambda-b polarization at LHC energies,

depending on the model assumed. Since the b-quark is more massive than

the strange quark by a factor of ten, comparison of lambda and lambda-b

results will likely provide insight into unexplained hyperon

polarization phenomena. Current efforts focus on simulating lambda-b

production, developing methods for extracting lambda-b polarization from

experimentally measurable parameters, and evaluating relevant model

predictions.