Polarization study of Λ c Baryon using data from the Fermilab E831 Experiment

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Polarization study of Λ c Baryon using data from the Fermilab E831 Experiment

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Polarization study of Λ c Baryon using data from the Fermilab E831 Experiment

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Λc Σo+π

This decay does influence our signal, so it was necessary to study it in MonteCarlo (MCFocus) and to adjust it using a Spline function. This was done for particles and antiparticles.

y = b(1 + mx)

m = α P

Polarization study of Λc Baryon using data from the Fermilab E831 Experiment

Cristina Oropeza-Barrera1, Dr. Salvador Carrillo-Moreno1, M. Sc. César Castromonte-Flores2

1 Universidad Iberoamericana A.C., 01219, México D.F., México

2 Centro Brasileiro de Pesquisas Físicas, 22290-180, Río de Janeiro, Brasil

FOCUS Beamline and Spectrometer

ii. Study of contamination from other Λc decay modes.

Λc Λo + π + π0

The contribution of this decay to our signal is not representative, hence we don’t take into account when fitting.

iii. Study ofΛc Λπ in MonteCarlo

We generated and reconstructed MC for our decay. To the signal obtained, a fit was done using a double Gaussian for the signal and a second degree polynomial for the background. With the parameters obtained by these fitting procedure, we fixed the width of each Gaussian and their weight factor in the experimental data fitting. The sample for both particles and antiparticles was divided into 4 cosine bins and the same procedure was used.

Polarization

By dividing the number of reconstructed events by the number of generated events we were able to obtain the efficiencies for each cosine bin and its errors.

Polarization is a phenomena associated with the spin of particles and reflects the asymmetry in its distribution. Theoretically there should not be a preferred direction for this intrinsic property, however, the production of heavy baryons (hyperons) has been widely studied and significant values of polarization have been observed. In this study we are focused on the decay:

iv. Fitting the signal

How do we calculate it?

The function used for the fit is:

The equation that allows us to determine the polarization value is:

where,

We project the number of events in each cosine bin into a histogram so to get an angular distribution. The next step is to fit this distribution, normalized and efficiency corrected, using a linear function:

dN/dcosθ : angular distribution calculated at the center of mass of Λc

No : total number of particles produced in the decay Λo + π

αΛc: weak asymmetry parameter of Λc

P : polarization

θ : angle between the normal to the production plane and the Λo momentum measured from Λc’s center of mass

Procedure and Results

i. Reconstruction of Λc Λπ decay with initial cuts (ntuple) and more restrictive ones.

lsig > 3

πcon > 6

nlife < 5

lc_mom > 40

1.09 < lo_mass < 1.14

2.09 < lc_mass < 2.49

cls > 0.01

clp > 0.01

v. Final Results

After studying the systematic errors, and repeating the methodology for antiparticles, the final values for polarization are:

Λc → Λo + π+ π0

Λc → Λo + π

lsig > 4

0 < pt < 4

2.1 < lc_mass < 2.45

Λc→Σo (→ Λo+γ)+ π

Conclusions

- We found that, in photoproduction, Λc is produced with a small polarization which, due to the fact that we have very low statistics and the magnitude of the errors are large, is compatible with zero within 2σ.
- The study of polarization could help understanding the hadronization process, that is, the mechanism by which quarks join together to produce hadrons. Also it could shed some light in how spin is distributed in non-elementary particles.
- Apparently, particles produced by hadroproduction tend to get polarized, while in photoproduction this phenomena is negligible.