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Measurement of Z/γ+jet+X and γ+b/c+X Cross
Sections with the D0 Detector
Murilo Rangel
To cite this version:
Measurement of Z/
γ
∗+jet+X and
γ
+b/c+X Cross
Sections with the D0 Detector
Murilo RANGEL∗
Laboratoire de l’Accélérateur Linéaire E-mail:rangel@fnal.gov
on behalf of D0 collaboration
We present measurements of differential cross sections for the inclusive Z/γ∗+ jet production and
the inclusive photon plus heavy flavor production in a data sample of 1fb−1collected with the DØ detector in proton-antiproton collisions at√s=1.96 TeV. In the first measurement, we compare
kinematic distributions of the Z/γ∗ and the jets as well as various angles of the Z+jet system
with different Monte Carlo event generators and next-to-leading order perturbative QCD (NLO pQCD) predictions with non-perturbative corrections applied. In the second measurement, we compare the results with NLO pQCD predictions, covering photon transverse momenta 30-150 GeV, photon rapidities|yγ| < 1.0, jet rapidities |yjet| < 0.8, and jet transverse momenta pjet
T > 15
GeV.
European Physical Society Europhysics Conference on High Energy Physics July 16-22, 2009
Krakow, Poland
Measurement of Z/γ+jet+X andγ+b/c+X Cross Sections with the D0 Detector Murilo RANGEL
1. Introduction
Understanding the background productions in searches for new physics is very challenging at hadron colliders. Therefore, measurements of differential cross sections represent important
milestones in the discovery road. In this note, we present measurements of Z/γ∗+ jets production
and photon plus heavy flavor jets at the Fermilab Tevatron with the DØ detector [1].
The production of vector bosons is an important signal at hadron colliders, providing unique information about the production mechanism of heavy bosons with additional hard partons. The electron and muon decay modes are distinct experimental signatures, and can be identified with low background rates.
Photons produced in association with heavy quarks (c or b) at hadron colliders provide valu-able information about the heavy quark and gluon content of the initial state hadrons, . Although the background rates are not negligible, the statistics are high enough to constrain accurately the parton distribution functions (PDFs).
2. Z/
γ
∗+jetsThe DØ collaboration has studied the production of Z/γ∗ + jets in both the electron
chan-nel and the muon chanchan-nel [2, 3, 4]. The jets were reconstructed using a seeded mid-point cone
algorithm [5] with cone size of 0.5, and they are required to have|y| < 2.8 and pT > 20 GeV.
The muons were selected to have opposite charge, pT > 15 GeV, |η| < 1.7 and the di-muon
invariant mass ranging between 65 GeV and 115 GeV. Isolation requirements were used to reduce the background rates to negligible levels. The electrons were selected to have opposite charge,
pT > 25 GeV, |η| < 1.1 or 1.5 < |η| < 2.5, and the di-electron invariant mass ranging between
65 GeV and 115 GeV. In the muon decay channel, we measured differential cross section in the
leading (transverse momenta) jet pT and the Z/γ∗pT [2]. In Figure 1, the NLO pQCD predictions
and from three event generators (ALPGEN [6], using PYTHIA [7] for the showering; SHERPA [8]; PYTHIA, with all jets coming from the parton shower) are compared to data.
The∆ϕ between the Z/γ∗ and the leading jet were also measured in the muon channel
us-ing two different pT thresholds (25 GeV and 45 GeV), since the production of additional jets is
essentially uncorrelated with the Z/γ∗ production [4]. We compared the measurement with NLO
pQCD and LO pQCD predictions, and the event generators: SHERPA; HERWIG using JIMMY
[9] for multiple parton interactions; PYTHIA with tune QW and with the Perugia tune using the pT
ordered shower; ALPGEN, using the mentioned PYTHIA tunes and HERWIG for the showering (Figure 2).
In the electron decay channel, we measured the jet pT spectra normalized to the Z/γ∗(→
e+e−) + X cross section in different jet multiplicities [3]. The measurements were compared to
different theory predictions: NLO pQCD; LO pQCD; PYTHIA using tune QW; PYTHIA using Tune S0; HERWIG using JIMMY; ALPGEN using PYTHIA tune QW; and SHERPA (Figure 3).
The pQCD NLO prediction describes the DØ measurements within uncertainties while the event generators show varying agreement. With more data, these results can be extended and tighter constraints can be placed.
(pb / GeV) jet T /dp σ d -3 10 -2 10 -1 10 1 Data NLO pQCD + corr. Z T p ⊕ Z = M F µ = R µ CTEQ6.6M PDF ALPGEN Z T p ⊕ Z = M F µ = R µ CTEQ6.1M PDF -1 DØ Run II, L=1.0 fb ) + jet + X µ µ → *( γ Z/ | < 1.7 µ < 115 GeV, |y µ µ 65 < M | < 2.8 jet > 20 GeV, |y jet T =0.5, p cone R (a) (GeV) jet T p 2 10 Ratio 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 Data / ALPGEN NLO pQCD / ALPGEN Scale and PDF unc.
SHERPA / ALPGEN PYTHIA / ALPGEN (b) 20 50 100 200 0 20 40 60 80 100 120 140 160 180 200 (pb / GeV) Z T /dp σ d -2 10 -1 10 1 Data NLO pQCD + corr. Z T p ⊕ Z = M F µ = R µ CTEQ6.6M PDF ALPGEN Z T p ⊕ Z = M F µ = R µ CTEQ6.1M PDF -1 DØ Run II, L=1.0 fb ) + jet + X µ µ → *( γ Z/ | < 1.7 µ < 115 GeV, |y µ µ 65 < M | < 2.8 jet > 20 GeV, |y jet T =0.5, p cone R (a) (GeV) Z T p 0 20 40 60 80 100 120 140 160 180 200 Ratio 0.5 1 1.5 2 2.5 Data / ALPGEN NLO pQCD / ALPGEN Scale and PDF unc.
SHERPA / ALPGEN PYTHIA / ALPGEN
(b)
Figure 1: The measured cross section (a) and the ratio of data and predictions to ALPGEN (b) are shown in
bins of leading jet pT (left) and in bins of Z/γ∗pT (right).
(1/rad) φ∆ /d Z+jet σ d × Z σ 1/ -4 10 -3 10 -2 10 -1
10 DataNLO pQCD + corr.
SHERPA -1 , L=1.0 fb ∅ D < 115 GeV µ µ 65 < M > 25 GeV Z T | < 1.7, p µ |y | < 2.8 jet > 20 GeV, |y jet T =0.5, p cone R (a) Ratio to SHERPA 1 Data HERWIG+JIMMY PYTHIA Tune P PYTHIA Tune QW
SHERPA scale unc. 2 0.7 3 0.5 (c) (Z, jet) (rad) φ ∆ 0 0.5 1 1.5 2 2.5 3 Ratio to SHERPA 1 Data NLO pQCD Scale & PDF unc.
LO pQCD Scale & PDF unc.
SHERPA scale unc. 2 0.7 3 0.5 (b) (Z, jet) (rad) φ ∆ 0 0.5 1 1.5 2 2.5 3 Ratio to SHERPA 1 Data ALP+HER ALP+PY Tune P ALP+PY Tune QW
SHERPA scale unc. 2 0.7 3 0.5 (d) (1/rad) φ∆ /d Z+jet σ d × Z σ 1/ -5 10 -4 10 -3 10 -2 10 -1 10 Data NLO pQCD + corr. SHERPA -1 , L=1.0 fb ∅ D < 115 GeV µ µ 65 < M > 45 GeV Z T | < 1.7, p µ |y | < 2.8 jet > 20 GeV, |y jet T =0.5, p cone R (a) Ratio to SHERPA 1 Data HERWIG+JIMMY PYTHIA Tune P PYTHIA Tune QW
SHERPA scale unc. 2 0.7 3 0.5 (c) (Z, jet) (rad) φ ∆ 0 0.5 1 1.5 2 2.5 3 Ratio to SHERPA 1 Data NLO pQCD Scale & PDF unc.
LO pQCD Scale & PDF unc.
SHERPA scale unc. 2 0.7 3 0.5 (b) (Z, jet) (rad) φ ∆ 0 0.5 1 1.5 2 2.5 3 Ratio to SHERPA 1 Data ALP+HER ALP+PY Tune P ALP+PY Tune QW
SHERPA scale unc. 2
0.7 3
0.5
(d)
Figure 2: The measured normalized cross section in bins of∆ϕ(Z, jet) for Z pT > 25 GeV (left) and for
pZ
T> 45 GeV (right).
3. Photon+heavy flavor
Using the DØ detector, we studied events with at least one photon candidate and at least one
heavy-flavor jet candidate [10]. The photons were selected to have pT> 30 GeV with |y| < 1.0 and
the leading jet pT > 15 GeV and|y| < 0.8. To suppress background events coming from
cosmic-ray muons and W leptonic decays, the total missing transverse energy was required to be less than
70% of the photon pT. The remaining background from dijet events, containingπ0andη mesons
that can mimic photon signatures, is rejected using an artificial neural network (ANN) with the requirement that the ANN output be > 0.7. Light jets are suppressed using another dedicated ANN (b-ANN), trained to discriminate light flavor from heavy flavor jets. The leading jet is required to have a b-ANN output value > 0.85.
The fraction of c and b jets in the final data sample is determined using a fitting technique,
where the discriminant is Pb= −ln∏iPi , where Pi is the probability of a track in the jet cone to
Measurement of Z/γ+jet+X andγ+b/c+X Cross Sections with the D0 Detector Murilo RANGEL [1 / GeV] jet) st (1T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 10-6 -5 10 -4 10 -3 10 -2 10 || -1 D0 Run II, L=1.04 fb (a) [1 / GeV] jet) st (1T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 10-6 -5 10 -4 10 -3 10 -2
10 Data at particle levelMCFM NLO
| | ee) + 1 jet + X → ( * γ Z/ | | < 115 GeV ee 65 < M | | e / y e T Incl. in p | | | < 2.5 jet = 0.5, | y cone jet R jet) [GeV] st (1 T p 20 30 40 50 100 200 300 Ratio to MCFM NLO jet) [GeV] st (1 T p 20 30 40 50 100 200 300 Ratio to MCFM NLO Data MCFM NLO Scale unc. (b) MCFM LO Scale unc. 0.5 1.0 1.5 2.0 Ratio to MCFM NLO (c) Ratio to MCFM NLO Data HERWIG+JIMMY PYTHIA S0 Scale unc. PYTHIA QW Scale unc. 0.5 1.0 1.5 2.0 jet) [GeV] st (1 T p 20 30 40 50 100 200 300 Ratio to MCFM NLO (d) jet) [GeV] st (1 T p 20 30 40 50 100 200 300 Ratio to MCFM NLO Data ALPGEN+PYTHIA Scale unc. SHERPA Scale unc. 0.5 1.0 1.5 2.0 [1 / GeV] jet) nd (2T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 -6 10 -5 10 -4 10 -3 10 | | -1 D0 Run II, L=1.04 fb (a) [1 / GeV] jet) nd (2T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 -6 10 -5 10 -4 10 -3 10
Data at particle level MCFM NLO | | ee) + 2 jets + X → ( * γ Z/ | | < 115 GeV ee 65 < M | | e / y e T Incl. in p | | | < 2.5 jet = 0.5, | y cone jet R jet) [GeV] nd (2 T p 20 30 40 50 60 100 200 Ratio to MCFM NLO jet) [GeV] nd (2 T p 20 30 40 50 60 100 200 Ratio to MCFM NLO Data MCFM NLO Scale unc. (b) MCFM LO Scale unc. 0.5 1.0 1.5 2.0 2.5 Ratio to MCFM NLO (c) Ratio to MCFM NLO Data HERWIG+JIMMY PYTHIA S0 Scale unc. PYTHIA QW Scale unc. 0.5 1.0 1.5 2.0 2.5 jet) [GeV] nd (2 T p 20 30 40 50 60 100 200 Ratio to MCFM NLO (d) jet) [GeV] nd (2 T p 20 30 40 50 60 100 200 Ratio to MCFM NLO Data ALPGEN+PYTHIA Scale unc. SHERPA Scale unc. 0.5 1.0 1.5 2.0 2.5 [1 / GeV] jet) rd (3T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 -6 10 -5 10 -4 10 | | -1 D0 Run II, L=1.04 fb (a) [1 / GeV] jet) rd (3T d p | *γ Z/ σ d ×|*γ Z/ σ | 1 -6 10 -5 10 -4 10
Data at particle level MCFM LO | | ee) + 3 jets + X → ( * γ Z/ | | < 115 GeV ee 65 < M | | e / y e T Incl. in p | | | < 2.5 jet = 0.5, | y cone jet R jet) [GeV] rd (3 T p 20 30 40 50 60 Ratio to MCFM LO jet) [GeV] rd (3 T p 20 30 40 50 60 Ratio to MCFM LO Data MCFM LO Scale unc. (b) 0.5 1.0 1.5 2.0 3.0 Ratio to MCFM LO (c) Ratio to MCFM LO Data HERWIG+JIMMY PYTHIA S0 Scale unc. PYTHIA QW Scale unc. 0.5 1.0 1.5 2.0 3.0 jet) [GeV] rd (3 T p 20 30 40 50 60 Ratio to MCFM LO (d) jet) [GeV] rd (3 T p 20 30 40 50 60 Ratio to MCFM LO Data ALPGEN+PYTHIA Scale unc. SHERPA Scale unc. 0.5 1.0 1.5 2.0 3.0
Figure 3: The measured normalized cross section for the leading jet in Z/γ∗+ jet + X events (upper left)
compared to the predictions of NLO and the ratios of data and theory predictions to NLO compared to various event generator models. The measured normalized cross section for the second leading jet in Z/γ∗+ 2 jets +
X events (upper right) compared to the predictions of NLO and the ratios of data and theory predictions
to NLO compared to various event generator models. The measured normalized cross section for the third leading jet in Z/γ∗+ 3 jets + X events (bottom) compared to the predictions of LO and the ratios of data and
theory predictions to LO compared to various event generator models.
The measured differential cross sections and their ratios to theoretical predictions are presented
in five bins of pγT and two regions of yγyjet (yγyjet> 0 and yγyjet< 0), and can be seen in Figure 4
for photon + b jets and photon + c jets. Theoretical predictions are from NLO pQCD calculations using the CTEQ 6.6M PDFs. For photon + c jets, comparisons with CTEQ 6.6M PDFs based on the models with an intrinsic charm component (IC) were also done.
The NLO pQCD prediction agrees with the measured cross sections for photon+b production
over the entire pγT range, and with photon+c production for pγT < 70 GeV. For pγT > 70 GeV, the
measured photon + c cross section is higher than the NLO pQCD prediction by about 1.6 - 2.2 standard deviations (including only the experimental uncertainties) with the difference increasing
with growing pγT.
4. Conclusion
Important measurements have been performed with the DØ detector, testing NLO pQCD, and the modeling of these complex final states by event generators. The understanding of the discrepancies observed between data and predictions is vital to the sensitivity to new physics at the Tevatron and LHC.
(GeV) γ T p 0 20 40 60 80 100 120 140 ) (pb/GeV) jet dy γ dyT γ (dp ⁄ σ 3 d -4 10 -3 10 -2 10 -1 10 1 10 | < 1.0 γ |y | < 0.8 jet |y > 15 GeV T jet p > 0 jet y γ y < 0 jet y γ y NLO QCD CTEQ 6.6M γ T = p R,F,f µ -1 = 1.0 fb int DØ, L +c+X γ +b+X γ (x3.0) (x1.0) (x0.3) (x0.1) Data / Theory 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 -1 = 1.0 fb int DØ, L |yjet| < 0.8 > 0 jet y γ y | < 1.0 γ |y > 15 GeV jet T p + b + X γ Data / Theory 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 data / theory CTEQ6.6M PDF uncertainty IC BHPS / CTEQ6.6M IC sea-like / CTEQ6.6M Scale uncertainty 40 60 80 100 120 140 0.5 1 1.5 2 2.5 3 3.5 > 0 jet y γ y + c + X γ 40 60 80 100 120 140 0.5 1 1.5 2 2.5 3 3.5 < 0 jet y γ y + b + X γ (GeV) γ T p 40 60 80 100 120 140 < 0 jet y γ y + c + X γ (GeV) γ T p 40 60 80 100 120 140
Figure 4: The γ+ b and γ+ c differential cross sections as a function of pγT for both rapidity regions.
The data points include the overall uncertainties from the measurement, and the theoretical predictions are displayed as dotted lines. The uncertainties from the theoretical predictions include those from the CTEQ 6.6M PDFs (yellow band) and from the choice of scale (full line). The ratio of two intrinsic charm models to the standard theoretical predictions are also included (dashed lines).
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