J. Alcaraz (CIEMAT) 7 March 2011

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:


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%.


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.


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


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


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'


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--


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--


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.


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)



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)



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


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.)


Rc±=

W+ charm W - charm


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


Cross checksCross checks Redo the analysis with TCHE tagging in the 3 < DISCR < 20 region

WW++ WW--


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


Cross checksCross checks Redo the analysis with a MadGraph MC instead of POWHEG

WW++ WW--


W + charm W - charm

=1.15±0.26 stat.

For pT jet 20 GeV : Wcharm W jets

=0.51±0.10 stat.


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

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J. Alcaraz (CIEMAT) 7 March 2011