ATLAS, Lambda_B Studies
Over the past year the University of Michigan ATLAS Group has made considerable strides in preparing for the measurement of the polarization of inclusively produced L b hyperons in the Atlas experiment. We expect to acquire approximately 70,000 L b ‘s which decay into four final state charged particles through the reactions L b -> J/ y (-> 2m) + L o (-> p p). Angular information between the four final state charged particles is sufficient to determine both the parity violating parameter a ( L b) and the polarization of the L b itself, at a statistical level of a few percent. The Working Group pursuing these studies is led by Homer Neal and includes Eduard de la Cruz Burelo(Michigan), Natasha Panikashvili(Technion/Michigan), Shlomit Tarem(Technion) and Maria Smizanski (Lancaster, ATLAS B-Physics Group Coordinator).
The proposed measurements are motivated by the continuing mystery of large (~20%) L hyperon polarizations observed at energies up to 800 GeV, a region where all existing models would suggest that such polarization should be vanishingly small. It is very intriguing to ask if such effects are due to some novel manifestation on existing physics or, in fact, the result of new physics. Based on general principles applied to quark-quark scattering, some authors have argued that the substitution of a s quark in the L hyperon with the much more massive b quark might lead to polarization effects that are even larger than those observed in the case of the L . The prospect of inclusively hyperons being produced in 14 TEV p-p collisions with a polarization of, say, 50%, is tantalizing; this would pose an enormous challenge to all theories.
Theoretical interest in L b polarization measurements also derives from the fact that HQET models are able to predict the a parameter based on parton distribution functions and PQCD. A direct measurement of a can thus pose a significant test for these models. Furthermore, since we will have the capability of measuring a for both the L b particle and its antiparticle, we will be able to make a CP test in a régime not heretofore explored.
One of the initial challenges of the group has been to demonstrate that the polarization magnitude of polarized L b ‘s can indeed be measured in the Atlas detector. This required us to master the large particle generator EVTGen and to interface this package with the Atlas Pythia generators. This has been accomplished and we have generated over 400,000 polarized L b.'s inside the Atlas detector with different polarizations, and reconstructed them to demonstrate that we can recover the known polarization. This undertaking has been very successful, and now our code is being inserted into official ATLAS releases.
Another challenge has been to develop techniques to ensure that the maximum likelihood analyses of the angular distribution of the four final state charged particles can lead to precise extraction of the polarization parameter without biases arising related to the fitting techniques.
As we have prepared for the measurement of L b polarization in the Atlas experiment we have also recognized that a modest number of such events should be available for analysis in DZERO. Indeed, we have been able to extract roughly 100 clean L b events from DZERO data and are proceeding to conduct an analysis of the average polarization and a parameter. More details of that analysis can be found in the DZERO section of this report. This analysis will provide an excellent opportunity for tuning our techniques in preparation for the Atlas measurements, as well as making new fundamental measurements, albeit statistically limited.
Topics of study in the coming year include a careful assessment of the anticipated background events derived from the channels b bbar--> J/ y + X and p p -> J/ y +X, a understanding of fitting biases for low event samples, and an analysis to make sure on that any dependence of the detector acceptance on polarization is negligible. We note that techniques we have developed for analyzing the four final state charged particles to extract spin information will also have relevance to Higgs -> 4l studies, both for the purpose of confirming the JCP of the Higgs, as well as a possible discovery tool. In the coming year the application of these techniques will be carried out on Higgs samples, whose generation is presently in progress.
Results of our studies have been presented at numerous meetings of the ATLAS b-physics group, at the ATLAS Rome Physics Workshop, at the USATLAS Tuscon North American Physics Workshop, Greek Physical Society (Thessanoliki, Greece), at CKM 2005 and at Beauty 2005. We will also present an update at BEACH2006 and SPIN2006.
In the figures below, we illustrate a) the lambda B. decay pattern, b) the mass distribution of the L b. hyperons simulated and reconstructed in ATLAS, c) our ability to generate polarized L b in the Atlas detector using EvtGen.
| Figure b: Reconstructed mass distribution of the L b |
Figure b): Reconstructed L b mass distribution
Figure: a; Illustration of L b decay topography
Figure c): Distribution of cos( q ) for different values of L b polarization for events produced with EvtGen, where q is the angle momentum (( L between the scattering plane normal and the in the L b rest frame.