Measurement of associated Z plus charm production in pp collisions at s= 8 TeV

Measurement of associated Z plus charm production in pp collisions at  s= 8 TeV

CMS Collaboration

Juan Alcaraz Maestre, Alberto Escalante del Valle, Juan Pablo Fernández Ramos, Isabel Josa Mutuberría(CIEMAT).

CPAN VIII – Physics at LHC

Nov. 28, 2016

Introduction



Measurement of σ(pp⇾Z+c) and its relative production w.r.t. Z+b both inclusively and differentially



Tests QCD predictions and allow (charm) PDF studies.



Intrinsic Charm-quark component inside the proton enhances σ(Z+c) @ ↑pt(Z)



Z+Heavy Flavour often as background in searches



Study the performance of c-tagging algorithms (new in CMS since 2015)

c-tagging = identification of jets coming from charm quarks

2

Z+charm Analysis strategy:

Identify charm in three independent final states:

• Semileptonic: lepton in jet

• D^*+/- and D^+/- exclusive decays in jet

Z → l⁺l⁻ with p_T(l) > 20 GeV and |η(l)| < 2.1

Dilepton invariant mass: [71,111] GeV

Take advantage of the relatively long life of charm hadrons p_T^jet >25 GeV & |^jet|<2.5

Charm selection from std. CMS tools (b-tagging) hard to calibrate in charm jets… Different strategy than Z+b that purely rely on tagger.

Exclusive Final states

Samples

MC: DY+jets generated w. MADGRAPH5 + PYTHIA6 using PDF set CTEQ6L Cross section normalized to σ(pp⇾Z+X) at NNLO calculated with FEWZ and PDF MSTW2008NNLO

Main backgrounds:

-

Missing transverse energy < 40 GeV (to reduce ttbar background) DATA: 2012 8 TeV ( 19.7 fb-1 )

Contributions from ttbar, diboson, Z+light processes (from simulations except ttbar from data)

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Semileptonic selection

The muon candidate is one of the constituents of the jet and of the tracks of a secondary vertex (SSV or IVF).

With moderate p_T, p_T^μ<25 GeV, with pT(μ)/p_T^jet<0.6 & |η^μ|<2.5 μ not-isolated from hadron activity, I_comb/p_T^μ>0.2

Z+c: ~25% Z+b: ~65% Z+light: ~5% Others:~5%

4145 Z ⇾ e⁺e^- Semileptonic candidates:

5258 Z ⇾ μ⁺μ^-

Jet with a 3 tracks vertex (SSV, IVF) Search for D^± → K^∓π^±π^± resonant peak

D

channel – Selection

Signal region :

|m(D^±) − 1.87| < 0.05 GeV Sideband region :

0.05 < |m(D^±)−1.87| < 0.10 GeV

After sideband subtraction:

Non resonant background in the signal region subtracted from the neighboring sidebands

Z+c: ~60% Z+b: ~36% Z+light: <1% Others:<3%

375 ± 44 D^± (Z⇾e⁺e^- ) 490 ± 48 D^± (Z⇾μ⁺μ^- )

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D

channel - Selection

Search for D^*± → D0 π_s^± [D0 → K^-π⁺(+c.c.)] decay chain

Loop over all tracks in the jet (instead of using SSV or IVF col.) D0 → K^-π⁺(+c.c.) ; Kaon: track with sign opposite to third track the π_s (D^*∓ → D0 π_s^∓) .

◮ |ΔR(D^∗, jet)| < 0.5, |ΔR(D0, π_s )| < 0.1.

◮ |m(D0) − 1.865| < 0.1 GeV,

|Δm − 145| < 5 MeV

◮ Signal region :

1.97 < D^∗ < 2.05 GeV sidebands : adyacent bands

After sideband subtraction:

Non resonant background in the signal region subtracted from the neighboring sidebands

309 ± 22 D^*±(Z⇾μ⁺μ^-) 234 ± 22 D^*±(Z⇾e⁺e^-) Z+c:~65% Z+b:~30% Z+light:<1% Others:<4%

Discriminant Variable: Z+b/Z+bc separation

• Vertex mass (for semileptonic mode)

• Jet Probability (for D hadron modes): likelihood estimate

• of prob. of jet tracks to come from primary vertex

Template modeling:

- Z+c : data driven (W+charm)

- Z+b : from MC but corrected with data (ttbar) - Z+light and Dibosons: from MC

- ttbar: Data driven

The larger the IP of a track the more incompatible w.r.t. PV

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JP discriminant from BTV-15-001

Template Modeling for Z+c

• Evaluated in a clean sample of c-jets from W+c production.

• W e plus jets with similar selection to Z+c

Same identification of heavy flavor: μ in jet or D-hadron

• OS–SS subtraction to remove symmetric backgrounds

After OS-SS subtraction the purity in W+c of the resulting sample is > 90%

(semil. channel) and >98% (D-hadron chan.)

JHEP 02 013(W+c)

Modeling for Z+c

Templates after subtraction of remaining (little) background

Agreement in general distributions (p_T^jet , N_SV ) SV-mass W+c MC and Z+c MC agree

SV-mass W+c MC and W+c data do not agree

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Template modeling for Z+b

Evaluated in a clean sample of b-jets from tt production.

Using a sample where the two W bosons from the top(antitop) quark decay leptonically into leptons of different flavour.

Identification of heavy flavor: muon inside the jet

-

Not enough statistics to validate the Z+b templates in the D-hadron channels

Total # of observed Z+c/Z+b extracted from a χ² minimization fit of the Z+c/Z+b templates to the experimental distributions of vertex mass and JP discriminants.

Signal extraction

n_i = Number of events in data N_i^Z+c, N_i^Z+b = Number of Z+c, Z+b Z+light, ttbar, Dibosons subtracted Parameters to fit: μ_z+c & μ_z+b

Z ⇾ μμ Z ⇾ ee

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Fitted signal

strength:

μ_z+c & μ_z+b In the 0.9- 1.1 range

D

D

Z ⇾ ee

Z ⇾ ee

Z ⇾ μμ

Z ⇾ μμ

D

D

13

Cross section determination

14

Cross section determination

 (Z+c)

Only the

semileptonic channel is used

Cross section ratio Z+c/Z+b

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 (Z+c)/ (Z+c)

Differential cross sections as a function of p

Bins [GeV] : 25-40, 40-60 and 60-200

Differential cross sections as a function of p

Bins[GeV] : 0-30, 30-60 and 60-200

Conclusions

(Z+c) = 8.6 ± 0.5 ± 0.7 pb

Evaluated (Z+c) y ratio (Z+c)/(Z+b) inclusive and differential for two opposite sign leptons from the Z with p_T^lepton > 20 GeV and

|^lepton| < 2.1 and p_T^jet >25 GeV & |^jet|<2.5

(Z+c)/(Z+b) = 2.0 ± 0.2 ± 0.2

CMS-PAS-SMP-15-009 (https://cds.cern.ch/record/2202823)

In agreement with predictions from MadGraph5 amc@nlo and Madgraph renormalized to a FEWZ calculation.

Back up

Systematic uncertainties

PU : uncertainties in the evaluation of the pileup profile (assuming a different inelastic cross section ) PDFs (PDFNNPDF & PDFCT10) : reweighted DY MC according to the new PDF's : NNPDF23_nnlo & PDFCT10 Jet Energy scale and Resolution (JES & JER) :

Change scale and resolution correction factors by their uncertainties

Branching ratios of D⇾Xμ hadrons and fragmentation c⇾D (BRFRAG): We reweight the simulation to match the PDG on D-hadron channels. The syst comes from uncertainty on those reweighting factors For semileptonic channel the systematic

comes from the difference between the BR from inclusive or exclusive individual contributions

Missing-et : Misestimations on the Missing transverse energy: modify the missing E_t by 10% of the unclustered missing E_t.

Cgluon-splitting, misestimation of the contribution to our background from gluon- splitting from charm : increase a factor 50 % the weight on events with 2c with

ΔR (jet,c) < 0.5

Bgluon-splitting, increase 50% the weight on events with 2b w. ΔR(jet,b) < 0.5 c-(b-) tagging efficiency. Repit analysis with efficiency increased by its error Shape (semileptonic mode) : change Z+b template correction factor by its error Lepton efficiencies :change efficiencies by their errors

Preselection

Preselection of Z ee and Z  plus jets

• Jets:

– Jets anti-kT, R=0.5

– JEC corrected, both in data and MC – pT(jet) > 25 GeV, |(jet)| < 2.5

• Z ee:

– Dielectron trigger thres-hold of 17 and 8 GeV

– Electron Id Medium WP – pT(e)> 20 GeV, |(e)| < 2.1 – Isolated: Icomb/pT <0.15 – Energy scale corrected

– MC corrected for differences in Electron Id and Iso

efficiencies

• Z μμ :

– DiMuon trigger thres-hold of 17 and 8 GeV

– Tight selection

– pT(e)> 20 GeV, |(e)| < 2.1 – Isolated: Icomb/pT <0.2 – Momentum scale and

resolution corrected (Rochester correction)

– MC corrected for differences in Muon Trigger, Id and Iso

Previous measurements at hadron colliders:

1)D0@Tevatron:σ(ppbar - Z+c+X)/σ(ppbar-Z+b+X) and σ(ppbar- Z+c+X)/σ(ppbar-Z+jets+X), pT(jet) > 20 GeV

2) CDF@Tevatron:σ(ppbar-Z+D*+X)/σ(ppbar-Z+X), pT(D*) > 3 GeV [σ(ppbar-W+D*+X)/σ(ppbar-W+X) as well]

3)LHCb@LHC: Evidence and σ( pp-Z+D0+X) andσ( pp-Z+D+X) in the forward region