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J. Alcaraz (CIEMAT) 7 March 2011

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.

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J. Alcaraz (CIEMAT) 7 March 2011

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  1. Measuring the W+charm Cross Section in CMS J. Alcaraz, I. Josa, J. Santaolalla(CIEMAT, Madrid)V+HF Working Meeting31 May 2011 J. Alcaraz (CIEMAT)7 March 2011

  2. Why 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:

  3. Non-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%.

  4. Non-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, s(W+ + c)/s(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.

  5. Strategy to measure W+c+X • 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 (~|Vub|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.

  6. Analysis 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, |h|<2.1, • VBTF tracker+muon quality cuts (|dxy|<2 mm, minimal number of hits, at least two segments, c2 cut), • Z->mm 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, |h|<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

  7. Reference 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 ET for the moment (effective cut is ~ 20 GeV) MC PLOTS ARE NORMALIZED TO LUMI * XSECTION ((N)NLO), UNLESS 'FITTED'

  8. SSVHE as our default W+ W- 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

  9. Fit procedure W+ W- • 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 L contributions to positive vertices in light-quark jets) • Data-driven top templates

  10. Results • 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

  11. Some additional distributions W-: Invariant mass at vertex (GeV) W+: Invariant mass at vertex (GeV) • Good agreement with MC, but no sensitivity to improve the analysis (except to reject a few top events in the tail)

  12. Some additional distributions W-: decay length uncertainty (cm) W+: decay length uncertainty (cm) • 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)

  13. Some additional distributions W-: number of tracks at vertex W+: number of tracks at vertex • 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)

  14. Systematics for charm charge ratio • Details described in CMS-AN-11-156 (being finalized now) • Result: Rc± = 0.99 ± 0.21 (stat.) ± 0.18 (syst.)

  15. Systematics for charm ratio • Details described in CMS-AN-11-156 (being finalized now) • Result: Rc = 0.496 ± 0.097 (stat.) ± 0.134 (syst.)

  16. Cross checks • Redo the analysis with TCHE tagging in the 3 < DISCR < 20 region W- W+ • Consistent with the SSVHE result within systematics

  17. Cross checks • Redo the analysis with a MadGraph MC instead of POWHEG W- W+

  18. TO DO LIST • Cross check results and systematic uncertainties • Compare with different set of cuts • Do 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

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