1. Production of vector bosons in association with jets in CMS and ATLAS
Benjamin Trocmé (LPSC Grenoble)
on behalf of the CMS and ATLAS collaborations

2. Introduction Motivations for these studies:
Control of the background for numerous searches (Higgs boson from vector boson fusion, single top, SUSY…).
Test of perturbative QCD.
Sample with large statistics for performance studies.
Different channels considered:
Z+j, W+j, Z+b (ATLAS+CMS), W+b (ATLAS), W+c (CMS)
Studies based on whole 2010 statistics : 33-36pb-1 (uncertainty on luminosity between 3.4% and 3.8%)
Final states with e/m considered (plots shown indifferently for one or the other channels: similar conclusions).

3. Object reconstruction methods
Well tuned and known from W/Z cross section measurements (tag and probe methods).
Pileup concerns:
simulated by superimposing minimum bias event generated by Pythia.
CMS treatment: subtraction of an event energy density not related to the hard interaction, estimated as r = median (ptjet/area(jet))
ATLAS treatment:
reject jets with less than 75% of charged tracks associated to primary vertex (JVF: jet vertex fraction).
Jet energy scale uncertainty increased to take into account the additional energy
Leptons
ATLAS
CMS
Pt lower cut
>20 GeV
>20(1st) / 10(2nd)GeV
h coverage
e:|h| <2.47 ( excl.)
m: |h| < 2.4
e:|h| <2.5
( excl.)
m: |h| < 2.1
Trigger/reco efficiencies
e: 94%
m: ~85% (T)
90% (R)
e: 80%(1st)/95% (2nd)
m: 85%
Jets
ATLAS
CMS
Anti kt
R = 0.4
R = 0.5
Pt lower cut
Pt>30 GeV (W+ J : 20)
Pt>30 GeV
h coverage
|h| < 2.8
|h| < 2.4
Jet energy scale uncertainty
4-8% (Pt & h depend.)
3-6% (Pt & h depend.)
Lepton-jet sep.
Rlj > 0.5
Rlj > 0.3

4. Z + jets : event selection + control plots
2 opposite charged leptons with invariant mass in range (66-116) GeV.

5. Z + jets : signal extraction
Subtraction of main sources of background :
electroweak (VV, W+J, tt, Z(→tt)+J) + QCD multijets (jet faking electron + m from heavy flavour jets): shapes derived from simulation.
ATLAS (e channel): QCD shape derived from an orthogonal sample of dielectrons (loose identification criteria).
Relative proportion of signal/background derived from a fit of Mdileptons.
Low background -
Very good agreement

6. Z+jets : fiducial cross section computation
Fiducial cross section not corrected for acceptance to avoid model dependence (acceptance within lepton/jet fiducial and kinematic cuts beforementioned).
Unfolding coefficient to correct for detector effects: efficiencies (trigger/ reconstruction/selection), resolution.
a: ptjet / Njets
Source
ATLAS
CMS
Jet energy scale
10-20%
8-16%
Pile up 4%
Unfolding
5-7%
Not avail.
Lepton selection
5-6%
2-10%
QCD background 2% -
Jet energy resolution
<1%
Total
13-24%
10-25%
Similar systematic uncertainties in ATLAS
and CMS: ~3xstatistical uncertainty (njet=1)

7. Z+jets : ATLAS result
Fiducial cross section compared with different generators, corrected for:
Parton to hadron effects;
QED effects (“Dressed” leptons).
Pythia (LO pQCD) do not reproduce the data (even rescaled for Njet=1).
Correct description of data by:
Alpgen/Sherpa: LO matrix element generators for multipartonic final states.
BlackHat: NLO pQCD calculations up to Njet=3 (LO for Njet=4).

8. Z+jets : CMS results Measurement of s(Z+≥n jets)/ s(Z+≥n-1 jets):
Correct description of data by Madgraph (matrix element generator).
Possibility to test hypothesis of Berends-Giele scaling
LO term
NLO term

9. W+jets : event selection + control plot
Only one reconstructed lepton + cut on transverse mass:
Overall good agreement.

10. W+jets : signal extraction
Background treatment very similar to Z+jets analysis:
Shapes derived from simulation except the QCD multijets derived from data in ATLAS (e channel).
Global fit of Etmiss (ATLAS) / Mt+nb-jet(CMS) to derive level of background.
Unfolding method used again to derive fiducial cross sections:
Systematic error still dominated by jet energy scale (additional contributions by jet energy resolution/lepton energy scale/ Etmiss ).
Systematic error largely dominant (vs statistical one) for n=1;2.

11. W+jets : ATLAS results Correct description of data by:
Alpgen/Sherpa : LO ME generators for multipartonic final states normalized by NNLO inclusive sW.
Blackhat-Sherpa: NLO pQCD calculations up to Njets=3 (LO for Njets=4).
MCFM: NLO pQCD calculations up to Njets=2 (LO for Njets=3).

12. Electrons only. Same level of precision with m
W+jets : CMS results
Measurement of s(Z+≥n jets)/ s(Z+≥n-1 jets):
Correct description of data by Madgraph
Hypothesis of Berends-Giele scaling also tested.
Electrons only. Same level of precision with m

13. W+jets : asymmetry in CMS
Pythia not able to describe AW asymmetry for n ≥ 1 jet
W charge asymmetry well described by MadGraph.
Systematic errors includes: uncertainty on jet energy scale, difference of efficiencies for positive and negative leptons and charge misidentification (lower than 1% for e and 1‰ for m).

14. Ratio sZ+j/sW+j
Stringent test of standard model with a reduced systematic error:
cancellations of different sources of systematic errors : jet energy scale, jet energy resolution, lepton efficiency (partially), generator.
ATLAS (1 jet only): 4-6% total systematic error (vs ~13-15% in V + ≥ 1jets).
Event selection/background subtraction/unfolding very similar to the single boson analysis:
Experimental results well described by the different generators.

15. Z+b jets: context and event selection
Interests:
Important background to H→bb search.
Main probe of the description of the b content of proton.
Standard reconstruction of lepton/jets; b jet taggers based on the identification of a secondary vertex :
Efficiency~35-40% - Mistag rate<~1% (ATLAS SV0 + CMS SSV HE - “high efficiency”)
Efficiency~20% - Mistag rate <~0.1% (CMS SSV HP - “high purity”)
Fixed flavour scheme
(massive ME)
Variable flavour
scheme (“b pdf” –
collinear approximat.)

16. Z+b jets: background subtraction and results
Background treatment very similar to Z+jets analysis:
QCD multijets contribution neglected (estimate by ATLAS with data driven method : ~1event). Top background estimated from simulation.Shapes of different flavours of Z+J from simulation.
Global fit of secondary vertex mass to extract composition.
In agreement with :
MCFM (NLO) : 3.88±0.58(theory) pb
Alpgen (LO–Fixed flavour scheme) : 2.23 ± 0.01(stat)pb
Sherpa (LO–Variable flavour scheme) :3.29 ± 0.04(stat)pb
In agreement with :
MCFM (NLO) : 4.3±0.5 (theory)
MadGraph (LO – Fixed flavour scheme):
5.1 ± 0.2(stat) ± 0.2(syst) ± 0.6(theory) (also in agreement with LO + variable flavour scheme).

17. W+b/c jet : context and event selection
Complementary measurements:
W+b jet (ATLAS) : sizeable background for WH channel and top production. Largely Cabibo suppressed process.
W+c jet (CMS): valuable probe of the strange quark content of the proton. Main background for W+b process.
Selection similar as before with an additional requirement on ≥1 jets with displaced vertex (SV0/SSV HE taggers).
W+b
Likelihood fit to extract W+c yield
Likelihood fit to extract W+b yield

18. W+b/c jet : results Two ratios derived from the W+c jet yield (CMS):
Usual method (background subtraction + unfolding) to extract the production cross section of W+b jet (ATLAS).
Good agreement with MCFM predictions
Measurement of cross section slightly above NLO expectation (1.5s effect).
Similar disagreement also observed in CDF (with even larger amplitude).
Origin not yet understood

19. Conclusion
Characteristics of the (W,Z)+jets production have been heavily tested at LHC in several areas.
Pythia unable to reproduce correctly data.
Data well described within the present precision by generators with high multiplicity LO matrix element (matched to the parton shower) and NLO pQCD:
W+jets production cross sections up to 4-5 jets
Z+jets production cross sections up to 3-4 jets
Asymmetry in W+jets production.
Heavy quarks (c/b) sector also well described, except in W+b jets.
Crucial control of background in many searches (Higgs, Susy).
Large statistics of data in 2011 should allow to refine the measurement and constraint the parton density functions:
Reduced systematic uncertainty associated to the jet energy scale.
Increased pileup may complicate the analysis.