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Unbiased Reconstruction of Λ c baryons in LHCb

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CERN Summer Student Program 2012. Unbiased Reconstruction of Λ c baryons in LHCb. Speaker : Yi Bao Supervisor: Rolf Oldeman Aug 9 th , 2012 @CERN. Introduction. Motivation:

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Presentation Transcript
  • Motivation:
    • All decays of the heavy baryons Λc + and Λb are presently affected by a large uncertainty of 26% in branching ratio measurement of the decay .
    • Whether the decay chain of the Λc(2595) * (an excited state of Λc + ) can provide a tag that is precise enough and has a low enough background to count the total number of Λc + baryons in the data sample.
    • Estimate the precision that can be obtained in the LHCb and identify possible backgrounds.

Level One:

Λc(2595) * →Σc0,++ π+,-

Level Two:

Nominal mass 2286

Level Three:

Total decay: Λb→ Λc +π+,- π-,+μ- ν

resolution momentum
Resolution (momentum)
  • What is Q-value: e.g. Q(Λc *) =m(Λc *)-m(Σc)-m(π)
  • Nominal values: w(Λc *) =2.6 MeV, w(Σc)=2.21MeV, Q(Λc *) = 3.3MeV, Q(Σc)= 32.5 MeV

Plot2. RMS as a function of width(Λc *) and width(Σc) (nominal Q-values)

Plot1. RMS as a function of Q(Λc *) and Q(Σc) (zero widths)

  • Both widths affect resolution
  • Resolution~17.5MeV for nominal widths
  • Both Q values affect resolution
  • Resolution~13.5MeV for nominal Q
b c 2595 on pythia mc
Λb-> Λc(2595) *μν on Pythia MC
  • Construction of signal (Λb-> Λc(2595) *μν ) Monte Carlo events
  • Plot1. Momentum distribution of Pi1 vs. Pi2
  • Plot2. Estimated mass distribution of Λc
pythia bbbar mc events
Pythiabbbar MC Events
  • Plot.2 Distribution of transverse momentum of muons (sig vs. bk)
  • Plot1. Distribution of reconstructed mass of estimated Λc (sig vs. bk)

use them to improve S/B

  • Plot.3 Distribution of reconstructed mass of muons and Λc + (sig vs. bk)
actual reconstruction sig vs bk
Actual Reconstruction (sig. vs. bk)
  • Plot1. Distribution of reconstructed mass of Λc (sig vs. bk)
  • Actual (true) reconstruction of signal (Λb-> Λc(2595) *μν ) and bbbar background Monte Carlo events
  • Detector simulation is introduced (LHCbApp) here
    • Some events are excluded for detector reason
proton tagging
Proton Tagging
  • A remarkable character of Λbis that there are protons come along with them from the same primary vertex. If the protons are tagged, it is hopeful to find signal events.
  • “right sign”:
    • Λb+antiproton/ Λbbar+proton
  • “wrong sign”:
    • Λb+proton/ Λbbar+antiproton
  • Provide another promising way to improve S/B


r-φ view of the decay (from CDF Collaboration)

proton tagging1
Proton Tagging
  • Plot1&2. Distribution of Δη and Q-value between proton and Λb(right vs. wrong)
  • Plot3&4. Distribution of Δφ and ΔR between proton and Λb(right vs. wrong)
  • π± from Λc(2595) * →Σc0,++ π+,- →Λc+ π+π-allow reconstruction with σ(mΛc)~20MeV
  • There is already a non-negligible bbbar background for MC truth, which influences S/B (Plot. Distribution of reconstructed mass of estimated Λc (sig vs. bk))
  • Other possible applications which could be applied
  • Great thanks for the support from CERN Summer Program and LHCb team.
  • Especially for the comprehensive and helpful instructions from my supervisor Rolf Oldeman.
  • Possible applications:
    • High statistics Λc + studies (not requiring the usual decay wins at least a factor 1/Br~20)
    • Measuring p/pbar reconstruction efficiency (by requiring the K and the π but not the proton - this seems very important for CP violation studies with heavy baryons)