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

Cristina Oropeza-Barrera 1 , Dr. Salvador Carrillo-Moreno 1 , M. Sc. César Castromonte-Flores 2 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 + π

0

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

Polarizatio n

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:

How do we calculate it?

The equation that allows us to determine the polarization value is: where, dN/dcosθ : angular distribution calculated at the center of mass of Λ c

N o

: total number of particles produced in the decay Λ o + π

α Λc

: weak asymmetry parameter of Λ c P : polarization θ : angle between the normal to the production plane and the Λ o momentum measured from Λ c ’s center of mass

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.

iii. Study of Λc  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.

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.

PARTICLES MC Rec MC Gen

Bin 1 Bin 2 14690 1171650

Efficiency

0.0125 ± 0.0001

8375 1215546 0.0069 ± 7.50274E-05 Bin 3 8485 1217178 0.0069 ± 7.54141E-05 Bin 4 14510 1170902 0.0124 ± 0.0001

iv. Fitting the signal The function used for the fit is: 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:

y = b(1 + mx) m =

α

P

i. Reconstruction of Λc 

Procedure and Results

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: lsig > 4 0 < pt < 4 2.1 < lc_mass < 2.45

Λ c → Λ o + π

+ π 0

Λ c →

Σ o (→

Λ o

+γ) +

π Λ c → Λ o + π

α PDG P Λc+ P Λc-

- 0.0308

±

0.1339

0.2839

±

0.1521

P

Λc+

- 0.0386

±

0.1679

±

0.1106

α

FOCUS

P

Λc-

0.3566

±

0.1926

±

0.1485

Conclusion s

• 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.