j. alcaraz (ciemat) 7 march 2011
DESCRIPTION
Measuring the W+charm Cross Section in CMS J. Alcaraz , I. Josa, J. Santaolalla (CIEMAT, Madrid) V+HF Working Meeting 31 May 2011. J. Alcaraz (CIEMAT) 7 March 2011. Why is W+c interesting. - PowerPoint PPT PresentationTRANSCRIPT
Measuring the W+charm Measuring the W+charm Cross Section in CMSCross Section in CMS
J. Alcaraz J. Alcaraz, I. Josa, , I. Josa, J. SantaolallaJ. Santaolalla(CIEMAT, Madrid)(CIEMAT, Madrid)
V+HF Working MeetingV+HF Working Meeting
31 May 201131 May 2011
Wc analysis, EWK Working Meeting, 27 May 2011 2
Why is W+c interestingWhy is W+c interesting In “W+c”, the W production proceeds predominantly
via “gluon + s-quark”: g + s -> cc + s -> c + W-”. This means that this channel gives direct access to the s-quark PDFs:
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Non-strange contributions to W+cNon-strange contributions to W+c Valence quark contribution for W-: g + d -> cc + d -> W- + c . This is
strongly Cabibbo suppressed (|Vcd
|2 / |Vcs
|2 ~ 0.05), but it is partially
compensated by the fact that a “d” is a valence quark -> its contribution is: ~ 15 %.
Valence quark contribution for W+: g + d -> cc + d -> W+ + c , but an “anti-d” is not a valence quark: it is much more suppressed in the W+ case (i.e. there may be small differences depending on the charge of the W). Contribution: ~ 5%.
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Non-strange contributions to W+cNon-strange contributions to W+c Gluon splitting of the type: u + d -> W+ + g -> W+ + cc . In this case,
there are two c-quarks in the final state, but they are confused with our signal. These contributions are small, but not fully negligible. At the end of the day, (W+ + c)/(W- + c) ~ 1.0-1.1 according to our MCs (POWHEG-MADGRAPH). More gluon splitting pushes the ratio slightly up, more g+d → W- + c pushes the ratio slightly down.
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Simple: use the standard VBTF W selection and apply b-tagging criteria to the observed jets in the event.
This will work for W production because it is almost impossible to produce W+b in the final state. For instance, g + u -> bb + u -> W+ + b is very strongly suppressed (~|V
ub|2 ~ 10-5), as well as g + c ->
bb + c -> W+ + b (strong charm PDF suppression and |Vcb
|2 ~ 2*10-
3).
Main backgrounds in practice will be ttbar and single top (giving W + b quarks in the final state).
There is still gluon splitting of the type: u + d -> W+ + g -> W+ + bb . But this contribution is at the 1-2% level and not visible in the final distributions compared with top backgrounds.
Strategy to measure W+c+XStrategy to measure W+c+X
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Analysis of W+c in the muon channelAnalysis of W+c in the muon channel Current set of VBTF cuts to select W->mu nu, 38X processing (Nov 4th):
Single-mu triggered (HLT_Mu15_v1 at the end of 2010 data-taking),
One muon with PT > 25 GeV, ||<2.1,
VBTF tracker+muon quality cuts (|dxy
|<2 mm, minimal number of hits, at
least two segments, 2 cut), Z-> veto (two global muons with ptmax>20 GeV, ptmin>10 GeV) ISO variable <0.1
MT > 50 GeV
I.e. no fit to the MT distribution to extract the cross section (unnecessary complication)
pT(hadron jet) > 20 GeV, ||<2.1, no more than 3 jets above 40
GeV We use particle-flow jets, L2+L3 corrected according to official
calibrations
Decay length uncertainty < 0.15 (cm)
We finally plot the b discriminator of the most significant jet
NEW
NEW
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Reference MC for this studyReference MC for this study We use the POWHEG MC WITH PILEUP for W production. This should
provide a reliable prediction for W + 1 hard jet + soft/collinear jets. POWHEG has some advantages: Straightforward access to 'single-charm' productionl: “W+c” or
“W+nonc” information is directly accessible in the generator information with status=3
More direct comparison with theory calculations (pp -> W+c (+1 jet)). PDFs are already NLO (a sensible NLO comparison can not be done
with MCs like Alpgen, MadGraph or Sherpa). And one disadvantage:
W + ≥ 2 hard jets are not so reliably predicted by POWHEG. But we cross-check with W+jets MadGraph MC samples too
We finally plot the b discriminator of the most significant jet (no implicit cut on jet E
T for the moment (effective cut is ~ 20
GeV)MC PLOTS ARE NORMALIZED TO LUMI * XSECTION ((N)NLO),
UNLESS 'FITTED'
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SSVHE as our defaultSSVHE as our default
Simple secondary vertices (SSV, discriminator = log(1+decayLengthSignificance)) should be less sensitive to pileup (thinking on 2011 data). Good agreement with POWHEG out-of-the-box. Use negative vertices to control the light-quark contribution
The W+c signal (red dashed histogram) is clearly visible
As well at the ttbar and single-top backgrounds. QCD is negligible
WW++WW--
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Fit procedureFit procedure
Use templates for signal, top, light-quark and “other” contributions. Fit the charm yield for W+ and W- separately. Plots above are “after fit”
Negative vertices help to constrain the light-quark contribution below the charm signal peak (but note that positive and negative contributions are not symmetric: there are also K0 and contributions to positive vertices in light-quark jets)
Data-driven top templates
WW++WW--
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ResultsResults
The measurements are in the expected range (~1 for the charge ratio, ~40% for the charm fraction over the total)
Only statistical uncertainties shown. Systematics is discussed in the next slides
For pT c− jet 20 GeV :
N W + charm=302.18±43 stat.N W - charm=306.49±40 stat. W + charm W -charm
=0.99±0.21 stat.
For pt jet 20 GeV :
N W jets =63799.4±263 stat. c=0.019±0.0003 stat.
Wcharm W jets
=0.496±0.097 stat.
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Some additional distributionsSome additional distributions
Good agreement with MC, but no sensitivity to improve the analysis (except to reject a few top events in the tail)
WW++: Invariant mass at vertex (GeV) : Invariant mass at vertex (GeV) WW--: Invariant mass at vertex (GeV) : Invariant mass at vertex (GeV)
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Some additional distributionsSome additional distributions
Excellent agreement with MC!! We are thinking on cutting in this distribution (a significant fraction of light-
quark decays corresponds to large decay length uncertainties)
WW++: decay length uncertainty (cm) : decay length uncertainty (cm) WW--: decay length uncertainty (cm) : decay length uncertainty (cm)
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Some additional distributionsSome additional distributions
Good agreement with expectations We will use this distribution to assign tracking systematics:
Determine a probability to lose a track that leads to a bad chi2 in the data-MC comparison (we use chi2=12/5 -> 3.5% probability)
WW++: number of tracks at vertex : number of tracks at vertex WW--: number of tracks at vertex : number of tracks at vertex
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Systematics for charm charge ratioSystematics for charm charge ratio
Details described in CMS-AN-11-156 (being finalized now)
Result: Rc± = 0.99 ± 0.21 (stat.) ± 0.18 (syst.)
For pT c− jet 20 GeV :
Rc±=
W+ charm W - charm
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Systematics for charm ratioSystematics for charm ratio
Details described in CMS-AN-11-156 (being finalized now)
Result: Rc = 0.496 ± 0.097 (stat.) ± 0.134 (syst.)
For pT leading jet 20 GeV :
Rc= W charm W jets
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Cross checksCross checks Redo the analysis with TCHE tagging in the 3 < DISCR < 20 region
WW++ WW--
For pT c− jet 20 GeV :
W + charm W - charm
=1.12±0.15 stat.
For pT jet 20 GeV : Wcharm W jets
=0.5±0.08 stat.
Consistent with the SSVHE result within systematics
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Cross checksCross checks Redo the analysis with a MadGraph MC instead of POWHEG
WW++ WW--
For pT c− jet 20 GeV :
W + charm W - charm
=1.15±0.26 stat.
For pT jet 20 GeV : Wcharm W jets
=0.51±0.10 stat.
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Cross check results and systematic uncertaintiesCompare with different set of cutsDo comparisons with predictions from different PDFs
And of course start analyzing 2011 data (things must be more under control now and reprocessing is almost finished)
For the future: do the analysis as a function of different (ptjet, eta) bins
TO DO LISTTO DO LIST