Spin PWG

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Relative Luminosity Systematic


Documentation for a systematic uncertainty introduced in the Run 5 charged pion A_{LL} based on uncertainties in our measurement of the relative luminosity.

Combinatoric Systematic

For the combinatoric background systematic we first estimate the background contribution (or contamination factor) to the signal reigon.  That is we integrate our simulated background to disce

Low Mass Systematic

For the low mass background systematic we first estimate the background contribution (or contamination factor) to the signal reigon.  That is we integrate our simulated background to discern t


We need to worry about a number of systematic effects that may change our measurement of ALL.  These effects can be broadly separated into two groups: backgrounds and non background


The measurement of ALL for inclusive neutral pion production is seen below along with statistical error bars and a systematic error band.  This asymmetry was calculated using a clas

Combinatoric Background

The last piece of the invariant mass distribution is the combinatoric background.  This is the result of combining two non-daughter photons into a pion candidate.  Since each photon in an

Low Mass Background

The low mass background is the result of single photons being artifically split by the detector (specifically the SMD.)  The SMD fails in it's clustering algorithm and one photon is reconstruc

Eta Peak

I treat the eta peak in a similar way as the pion peak.  I throw single etas, flat in Pt from 2 - 25, and reconstruct the two-photon invariant mass distribution for the results.  The thro

Yield Extraction

 After all the pion candidates have been found and all the cuts applied, we need to extract the number of pions in each bin (in each spin state for ALL.)  To do this we simply

Pt Dependent Mass

The two-photon invariant mass is given (in the lab frame) by

M = Sqrt(2E1E2(1 - Cos(theta)))


I am using the final polarization numbers from run 6, released by A. Bazilevsky to the spin group on December 4, 2007.  The files can be found below.

Run List

Below you will find the runlist I used for all of the studies leading up to a preliminary result.  For a more detailed look at how I arrived at this runlist please see my run

Updated Charged Pion A_{LL} predictions

Received some new NLO pQCD predictions from Werner incorporating DSS fragmentation functions.  Here's a comparison of these predictions to the old mocked up ones used in the SPIN 2006 preliminary result:

No really dramatic changes here.  The increased asymmetries might mean that g-g scattering is even more important than predicted originally.  Of course, this goes in the wrong direction if we're looking for better agreement with our Pythia.

g-g suppression in Pythia

I'm concerned about an apparent deficit of g-g scattering events in our CDF Tune A Pythia samples.  I first noticed this deficit when I was looking at MC asymmetries for pi-minus production, where the difference between asymmetries from g-g and q-g events is extreme.  For reference, here is Vogelsang's prediction for the subprocess mixture for inclusive pi0 production:

Compare that to the following plot from Pythia for pi-plus production after combining partonic samples starting at 3_4:

If I add 2_3 GeV the gg contribution at low pT gets a boost, but I'm still seeing ~zero pions from g-g scattering at 8 GeV, where Vogelsang predicts 20% g-g. 

Renee suggested skipping the observables and just plotting the event partonic pT.  The turnover from gg to qg occurs at 3 GeV ... seems too low to me.  On the other hand, the mixture is ~20% g-g at 16 GeV, which seems about right.

One other tidbit for any Pythia tuners out there.  When I was running standalone Pythia to get the various partonic xsections for weighting purposes I saw a number of advisory warnings like

Advisory warning: maximum violated by 1.120D+00 in event 625
XSEC(28,1) increased to 5.982D-01

in this case, I was running the 5_7 sample.  The initial max value for the q-g xsec was 5.3392D-01, and the final measured xsec was 1.515D-01.  Not sure if it's significant.

Update 11-19-2007

I looked into the relationship between charged pion p_{T} and event partonic p_{T} in more detail.  Here are plot of charged pion multiplicity and mean pion p_{T} versus event p_{T}:

so g-g events actually produce more charged pions per event, but these pions are all at low p_{T}.  The second conclusion is reinforced by the PDF I've linked at the bottom of the page, which shows pion p_{T} spectra split by subprocess for a range of partonic p_{T} bins.  The g-g events have a much steeper slope.  Those plots are not normalized by the # of events per subprocess, so the conclusion about the number of pions per event is not immediately evident.

Update 11-27-2007

I tried manually rescaling the g-g scattering contribution to the PYTHIA asymmetries; it looks like increasing the g-g by a factor of 5 does a decent job of reproducing the NLO theoretical predictions:

no rescaling

multiply g-g by 3.0

multiply g-g by 5.0


Agreement Between "Polarized" Pythia and NLO pQCD
Updated Charged Pion A_{LL} predictions

Agreement Between "Polarized" Pythia and NLO pQCD

We're using an afterburner framework that turns Pythia into something like a LO polarized event generator to study any biases introduced by our triggers on A_LL measurements.  This page compares the asymmetries generated by Pythia to theoretical predictions from GRSV.


  • MC and reco vz positions inside 60 cm
  • |eta| of reco primary track < 1.0
  • dca of associated global < 1.0
  • fit points > 25
  • select pions using geant ID 8 (pi-plus) or 9 (pi-minus)

No trigger requirements are imposed.


I combined MC samples 3_4 through 55_65 using the following cross sections and event counts

xsec = {
'3_4' : 1.287, 384593
'4_5' : 3.117*10**-1, 586568
'5_7' : 1.360*10**-1, 380791
'7_9' : 2.305*10**-2, 404272
'9_11' : 5.494*10**-3, 413651
'11_15' : 2.228*10**-3, 418547
'15_25' : 3.895*10**-4, 407427
'25_35' : 1.016*10**-5, 99998
'above_35' : 5.299*10**-7, 119995
'45_55' : 2.830*10**-8, 119995
'55_65' : 1.433*10**-9 119998


First off, here are the asymmetries integrated over all subprocesses.  The left column is pi-plus, the right column is pi-minus.  The agreement for postive charges seems basically acceptable to me, but pi-minus is off by quite a lot:

If I restrict to g-g, q-g, and q-q subprocesses individually, the difference between the two is obviously in the q-g subprocess contribution (as expected):

It seems to me that the lack of agreement between our Monte Carlo and the theoretical predictions could be due in part to a lack of gg / over-abundance of qg in the subprocess mix.  Here are plots of the subprocess mix from Pythia:

compare that to the prediction from Vogelsang et al. for inclusive pi0 production, where gg is the primary contributor until ~3 GeV:

In particular, only the 3_4 GeV partonic sample has more g-g than q-g at any pion p_T.  I'm going to try including the 2_3 GeV sample in my studies and see if that bolsters the gg contribution.

Old Studies

Outdated or obsolete studies are archived here

Charged Pions

Charged pion analysis

Di jets

Speaker : Tai Sakuma

Talk time : 09:00, Duration : 00:30

DNP 2007 Inclusive Hadron Talk

Here are the powerpoint and pdf versions of my slides for DNP.

2006 Neutral Pion Update (9/27/07)

Click on the link to download my slides for the PWG meeting.