Helicity puzzle

After recent discussion with Jacques van de Wiele we have realized that general double Lorentz boost is not trivial because it does not result in the Lorentz tranformation only, but also involves Wigner rotation (i.e. see the attached document at the bottom). Therefore all procedures of helicity calculation are affected.

The distribution 1 + cos2θ is expected in γ* reference system. In fact, the helicity calculations are done in the Δ at rest (therefore only one boost of leptons to γ*). In our procedure we have included more boosts (Δ not at rest) distorting the proper angular distribution. Please, read also nice and short explanation of helicity from BaBar experiment, found by Beatrice.

This brings the following questions:
  • how the angular distributions are treated in PLUTO (in which reference frame it is 1 + cos2θ)
  • what is the impact of the (possibly) wrong procedure on the result, mainly on the correction matrix but also on the uncorrected data

The measured objects

We measure in LAB: proton, e+, e-. We create Δ (= proton + electron + positron or beam + target - proton) in the LAB. We create γ* (= electron + positron) in the LAB.

Helicity "old" (incorrect) procedure

We boost in the following way:
  • Δ stays in the LAB
  • γ* boosted to the Δ rest frame
  • e+ / e- boosted to the Δ first, then to the γ*
  • helicity: scalar product (cosine of angle) of e+ / e- and γ* is calculated
Sometimes is addition all objects (Δ, γ*, e+, e-) are boosted to CM first, what biases the anisotropy even more!

Helicity "new" (correct) procedure

We boost in the following way:
  • Δ stays in the LAB
  • e+ / e- boosted to the γ* (at the moment γ* in the LAB)
  • γ* boosted to the Δ rest frame
  • helicity: scalar product (cosine of angle) of e+ / e- and γ* is calculated
In this procedure there is only one Lorentz boost (of leptons, and later, of γ*). This is exactly in line with BaBar definition: in the reaction Δ -> γ* -> e+ + e-, the helicity angle of particle e+ / e- is the angle measured in the rest frame of the decaying parent particle, γ*, between the direction of the decay daughter e+ / e- and the direction of the grandparent particle Δ.

Investigation and nomenclature

In the pictures below "one boost" (usually green) means the correct boosting procedure while "sequence boost" or "two boosts" (usually red) means the old, incorrect procedure.

First, I draw the Δ helicity in the 4pi (PLUTO ver.5.30c simulation). It should indicate what kind of boosting / production is implemented inside the PLUTO generator. Please note, that I do not select the proton coming from the Δ, I assume ambiguity in proton selection (the mean of both). Red distribution seems to indicate, because the defined 1 + cos2θ distribution has been reconstructed, that in PLUTO the decaying objects are boosted to the parent rest frame and within the parent reference frame a given distribution is applied. The result is such that if we apply correct boosting procedure we get significantly less anisotropic helicity distribution (compare anisotropy factor 0.97 to 0.48).

  • simulation in 4pi:
    simulation in 4pi

  • simulation (uncorrected) - Delta helicity:
    simulation (uncorrected) - Delta helicity

  • acc&eff correction (1-dim) matrix for Delta helicity:
    acc&eff correction (1-dim) matrix for Delta helicity

  • Delta helicity (uncorrected) - experiment:
    Delta helicity (uncorrected) - experiment

  • Delta helicity (corrected) with a sequence of boosts - experiment:
    Delta helicity (corrected) with a sequence of boosts - experiment

  • Delta helicity (corrected) with one boost - experiment:
    Delta helicity (corrected) with one boost - experiment

The same as above but to limit possible systematic error in correction procedure, matrices with more bins (30 bins) have been produced and the experimental data (in both procedures) corrected event by event, see pictures below:

  • Delta helicity correction acc+eff with more (30) bins:
    Delta helicity correction acc+eff with more (30) bins

  • Delta helicity (corrected event by event with 30-bin matrix) with a sequence of boosts - experiment:
    Delta helicity (corrected event by event with 30-bin matrix) with a sequence of boosts - experiment

  • Delta helicity (corrected event by event with 30-bin matrix) with one boost - experiment:
    Delta helicity (corrected event by event with 30-bin matrix) with one boost - experiment

Helicity angle separately for electron and postrion, the case of Δpee and Δmiss:

  • Δ helicity components: sequence boost:
    Δ helicity components: sequence boost

  • Δ helicity components: one boost:
    Δ helicity components: one boost

  • opening angle (Δpee and Δmiss): lepton ("one boost") and lepton ("two boosts") versus cos θ (helicity in the one boost case):
    opening angle (Δ<sub>pee</sub> and Δ<sub>miss</sub>): lepton (

  • opening angle (Δpee and Δmiss): lepton ("one boost") and lepton ("two boosts"):
    opening angle (Δ<sub>pee</sub> and Δ<sub>miss</sub>): lepton (

* TwoLorentzBoosts: Two Lorentz boosts and Wigner rotation

-- WitoldPrzygoda - 15 Dec 2009
Topic attachments
I Attachment Action Size Date Who Comment
TwoLorentzBoostsEXT TwoLorentzBoosts manage 133.9 K 15 Dec 2009 - 14:39 WitoldPrzygoda Two Lorentz boosts and Wigner rotation
helicity_all_boost30_exp-sim.gifgif helicity_all_boost30_exp-sim.gif manage 11.6 K 16 Dec 2009 - 13:09 WitoldPrzygoda Delta helicity (corrected event by event with 30-bin matrix) with a sequence of boosts - experiment
helicity_all_boost_exp-sim.gifgif helicity_all_boost_exp-sim.gif manage 12.3 K 15 Dec 2009 - 14:05 WitoldPrzygoda Delta helicity (corrected) with a sequence of boosts - experiment
helicity_all_ok_exp.gifgif helicity_all_ok_exp.gif manage 10.4 K 15 Dec 2009 - 14:04 WitoldPrzygoda Delta helicity (uncorrected) - experiment
helicity_all_ok_sim.gifgif helicity_all_ok_sim.gif manage 9.2 K 15 Dec 2009 - 14:02 WitoldPrzygoda simulation (uncorrected) - Delta helicity
helicity_all_ok_sim_4pi.gifgif helicity_all_ok_sim_4pi.gif manage 10.6 K 15 Dec 2009 - 14:01 WitoldPrzygoda simulation in 4pi
helicity_components_all_boost.gifgif helicity_components_all_boost.gif manage 16.4 K 15 Dec 2009 - 17:38 WitoldPrzygoda Δ helicity components: sequence boost
helicity_components_ok_boost.gifgif helicity_components_ok_boost.gif manage 10.4 K 15 Dec 2009 - 17:38 WitoldPrzygoda Δ helicity components: one boost
helicity_correction.gifgif helicity_correction.gif manage 8.7 K 15 Dec 2009 - 14:03 WitoldPrzygoda acc&eff correction (1-dim) matrix for Delta helicity
helicity_correction30.gifgif helicity_correction30.gif manage 13.9 K 16 Dec 2009 - 13:08 WitoldPrzygoda Delta helicity correction acc+eff with more (30) bins
helicity_ok_boost30_exp-sim.gifgif helicity_ok_boost30_exp-sim.gif manage 10.8 K 16 Dec 2009 - 13:10 WitoldPrzygoda Delta helicity (corrected event by event with 30-bin matrix) with one boost - experiment
helicity_ok_boost_exp-sim.gifgif helicity_ok_boost_exp-sim.gif manage 11.8 K 15 Dec 2009 - 14:06 WitoldPrzygoda Delta helicity (corrected) with one boost - experiment
oa_2dim.gifgif oa_2dim.gif manage 16.3 K 15 Dec 2009 - 19:29 WitoldPrzygoda opening angle (Δpee and Δmiss): lepton ("one boost") and lepton ("two boosts") versus cos θ (helicity in the one boost case)
oa_electrons.gifgif oa_electrons.gif manage 8.0 K 15 Dec 2009 - 17:33 WitoldPrzygoda opening angle (Δpee): electron ("one boost") and electron ("two boosts")
oa_electrons_miss.gifgif oa_electrons_miss.gif manage 8.8 K 15 Dec 2009 - 17:34 WitoldPrzygoda opening angle (Δmiss): electron ("one boost") and electron ("two boosts")
oa_leptons.gifgif oa_leptons.gif manage 9.0 K 15 Dec 2009 - 19:30 WitoldPrzygoda opening angle (Δpee and Δmiss): lepton ("one boost") and lepton ("two boosts")
oa_positrons.gifgif oa_positrons.gif manage 8.0 K 15 Dec 2009 - 17:35 WitoldPrzygoda opening angle (Δpee): positron ("one boost") and positron ("two boosts")
oa_positrons_miss.gifgif oa_positrons_miss.gif manage 8.4 K 15 Dec 2009 - 17:36 WitoldPrzygoda opening angle (Δmiss): positron ("one boost") and positron ("two boosts")
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Topic revision: r6 - 16 Dec 2009, WitoldPrzygoda
 
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