Observation of Z g nng at DØ
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Observation of Zgnng at DØ
Yurii Maravin (KSU)On behalf of the DØ Collaboration
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DØ Collaboration
• 18 countries• 82 institutions• 500 authors
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DØ Detector
protons
antiprotons
Electronics
Tracker Solenoid Magnet
3 LayerMuon System
Preshowers
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Data taking efficiency
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The irresistible rise of the SM
• Wonderful agreement with experimental results– What lies ahead?
From Tom Diehl’s office wall
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Diboson physics• Physics with multiple bosons in
the final state– Such as WW, WZ, Zg, gg, …
• A number of important measurements and searches– Cross section– Search for resonant production
• Such as Higgs, or fermiophobic higgs, or whatever…
– Self-interaction boson couplings are the least well known parameters of the EW sector of the standard model
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top
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Integrated luminosity
3.6 fb-1 of integratedluminosity
Thanks to Tevatron!
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Zg production• SM predicts only two tree-level diagram of Zg via
initial and final state radiation– No final state radiation in nng final state
– ZZg and Zgg couplings are almost zero• QED corrections are at the 10-4 level
?
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New phenomena in Zg• Numerous possible extensions of the standard
model result in non-zero ZZg and Zgg couplings
Compositeness
X
Something else? • Follow effective Lagrangian approach– Parameterize the ZZg/Zgg vertex in
the most general way
X
X
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Most general parameterization• ZVg vertex can be parameterized by 8 complex
couplings and where is 1-4– h1, h2 are CP-odd,
h3, h4 are CP-even– Unitarity is violated at high ŝ, use form-factor ansatz to
enforce good energy behavior
• Here, L is a new physics scale that is responsible for conserving unitarity at high ŝ – Customary, n = 3 for and, and 4 for
€
hiZ
€
hiγ
€
i
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hiV = hi0
V
(1+ ˆ s /Λ2)n
Low energy approximation
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h1,3V
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h2,4V
Baur & Berger, 1993
de k
Mh h
e kM
h h
ZZ
Z Z
ZZ
Z Z
T
T
2
2
2
3 30 40
2
3 10 20
( )
( )
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Effect of anomalous coupling• Any non-zero coupling result in increase of the
cross section and harder pT spectrum of the photon and Z– Produced from Baur MC 4-vector output (LO)
Standard model
Anomalous coupling
Standard modelAnomalous TGC
Photon pT
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Previous results on Zg: LEP
€
−0.056 < h1γ < 0.055
−0.045 < h2γ < 0.025
−0.049 < h3γ < −0.008
−0.002 < h4γ < 0.034
€
−0.13 < h1Z < 0.13
−0.078 < h2Z < 0.071
−0.20 < h3Z < 0.07
−0.05 < h4Z < 0.12
LEP EWWK 2003, Preliminary
• Measured ZZg and Zgg couplings agree with SM at 10-1 – 10-2 level
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Previous Tevatron results Zgllg• DØ set limits Zgllg in Run I– Observed 29 events– The pT
g spectrum agree with standard model prediction
Agreement with SM at 10-1 – 10-2 level
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Previous Run II results on Zgllg• Both CDF and DØ performed extensive studies of the
Zg production in Zgllg– Both cross section and pT
g spectrum agree with standard model prediction
– Limits are
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h3V < 0.085
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h4V < 0.0047
Agreement with SM at 10-1 – 10-3 level
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Can we do better?• Precision is still dominated by statistics– Sensitivity is in the tail of the pT
g distribution
• Major limiting factors:– Three particle final state– Low Zll branching fraction
• Challenging alternative: Zgnng– Much higher acceptance– No FSR processes!– Neutrino branching fraction is three times the Zll
Precision should double for the nng channel alone!
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Zgnng• Very challenging– Has not been seen at Tevatron!
• Final state is a single photon and a missing transverse energy (MET) consistent with Znn production– Backgrounds: • QCD processes and W production (e g)• Beam-halo, bremsstrahlung cosmic muons
• A crucial ingredient to this analysis is identification of photons
Z
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Photon identification• Unconverted photons do not have much
redundancy: just a shower in the calorimeter– Handles to suppress backgrounds:• Isolation in tracker and hadron
calorimeter• Shower profile should be
consistent with that of a photon• No track pointing to the photon candidate
– No additional hits in the vicinity of a photon candidate is consistent with not reconstructed track
-
++
0
0
-
normal jet fluctuated jet: most energy is carried by 0
+
0
0
0
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Calibrating photons• We must find Higgs to produce a photon
calibration signal! • Use data for calibration: Zee– Use Monte Carlo to
describe the difference between photon and electron shower well
– Tune Monte Carlo so that itdescribes electrons well• Cross checked in data with FSR Zg events
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Zllg as a standard candle• FSR Zg is the cleanest source of photons– One can use FSR production
to make photon sample veryclean and to infer photonenergy scale!
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Suppression of electrons• In addition to the standard matching algorithm in f and h
space, require tracker hits density along the EM trajectory to be consistent with noise– Hit density and resolution is determined in data
• Improves electron track matching efficiency and decreases the eg misidentification rate by a factor of four!
IP Shower
Calorimeter
Tracker
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Non-pointing background
DØ
DØ
• One of the major backgrounds to the nng final state is cosmic muon that radiated a photon in the calorimeter
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DØ Calorimetry• DØ calorimeter is highly segmented– Use it to pinpoint the
shower direction in 3D!
FH
CH
EM
OH
MH
IH
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Pointing in z-coordinate• Cosmics and beam-halo photons would not
point to the primary vertex of the event– Exploit this to identify and reject non-collision
backgrounds
Turn-offs due to reconstruction acceptance
Single g sample
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• Dz distributions from data– Extremely important for the
MET measurement– Electrons and photons have
narrow showers and thus small Dz resolution• Use Zee sample
– Misidentified jets have widershower profile and thus larger pointing resolution • Use “bad” EM sample and g+jet
– Non-collision is pretty flat• Use cosmics-enriched data
Electrons
EM Jets
Cosmics-enriched sample
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Identification of non-pointing g• Determine DCA distributions
from dataPRL 101, 011601 (2008)
~ 12 cm
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lukewarm cells
Bremsstrahlung dominates
Applying the pointing algorithm• Select data sample with photon pT > 90 GeV– Sample is dominated by muon Bremsstrahlung– Applying pointing requirements reduces cosmics and
beam-halo considerably!
Kinematics requirements only
Applying CPS information
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Selecting Zgnng • Select events with single EM triggers– Fully efficient at 40 GeV
• Require event to have missing ET > 70 GeV• Require a clean event– No jets with pT > 15 GeV, isolated tracks, cosmic rays,
muons…
• Photon candidate has pT > 90 GeV, |h| < 1.1, isolated, and have shower profile consistent with that of a photon
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nng candidate event
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Backgrounds• Wen: electron is misidentified as a
photon– Estimated from W data sample
• Non-collision: cosmic or halo muon Bremsstrahlung– Estimated from DCA template method
• W or Z + jet: jet is misidentified as a photon– Estimated from the DCA template method
• W+g ln + g: lepton is lost– Small, estimated from Monte Carlo simulation
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W → eν 9.67 ± 0.30 (stat.) ± 0.48 (syst.)non − collision 5.33± 0.39 (stat.) ±1.91 (syst.)W /Z + jet 1.37 ± 0.26 (stat.) ± 0.91 (syst.)Wγ 0.90 ± 0.07 (stat.) ± 0.12 (syst.)
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Simulation• There is a number of Monte Carlo generators on
the market: use Baur generator– The generator of choice for CDF, DØ, CMS,…
• Use both NLO and LO generators to simulate the process kinematics and acceptance and calculate theoretical cross section– NLO generator is used to calculate NLO k-factor– Detector simulation is done by using Parameterized
Monte Carlo Simulation (PMCS)• Very fast and reliable!
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Cross section measurement• Using 3.64 fb-1 of data we observe 51 Zgnng
candidate events with an estimated17.3 ± 0.6 (stat.) ± 2.3 (syst.) background events
– Theory predicts 39 ± 4 fb (NLO)• Perform 108 pseudo-experiments with background-
only hypothesis to find out that the probability for for background to fluctuate up is 3.1 x 10-7 which corresponds to 5.1s– First observation of Zgnng at the Tevatron!
s Br(Znn) = 32 ± 9 (stat+syst) ± 2 (lumi) fb.
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Measuring ZZg/Zgg couplings
• Data are consistent with standard model production– Proceed with setting
limits on anomalouscouplings
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Setting limits on ZZg and Zgg
• The observable (photon pT) is sensitive to the strength of the coupling– We present results for CP-even couplings:
sensitivity to h1 is similar to h3, and similarly h2 is similar to h4
• Generate a 2D grid of simulation with different values of couplings (h30 and h40)– Set CP-odd couplings to zero
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• Assume Poisson statistics for signal and Gaussian statistics for systematic uncertainties and background, calculate the likelihood of data to be described by aTGC simulation and background– Repeat for every point of the generated grid
h 30Z
h40 Z
L ≈
3
-log(
L)
-log(
L)
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Limits on anomalous couplings
L = 1.2 TeV L = 1.5 TeVZgllg 1 fb-1 |h30
g|<0.085 |h40g|<0.0054 |h30
g|<0.079 |h40g|<0.0036
Zgllg 1 fb-1 |h30Z|<0.083 |h40
Z|<0.0054 |h30Z|<0.075 |h40
Z|<0.0037Zgnng 3.6 fb-
1|h30
g|<0.042 |h40g|<0.0029 |h30
g|<0.037 |h40g|<0.0020
Zgnng 3.6 fb-
1|h30
Z|<0.041 |h40Z|<0.0029 |h30
Z|<0.036 |h40Z|<0.0020
Zg combination |h30g|<0.038 |h40
g|<0.0025 |h30g|<0.033 |h40
g|<0.0017Zg combination |h30
Z|<0.037 |h40Z|<0.0025 |h30
Z|<0.033 |h40Z|<0.0017
• Set 1D limits by setting all the other aTGCs to zero
• Best limits from Tevatron!
Submitted to PRL
This result!
Old DØ result!
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Limits on anomalous couplings• The most probable values of the ZZg and Zgg
couplings is at the standard model predictions
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Comparison with LEP• These results: |h30
V| < 0.033, |h40V| < 0.0017
– Similar results for CP-odd couplings• LEP results
– LEP does not scale couplings with the form-factor, which makes direct comparison more complex• Additional ei/2 factor from Baur MC
€
−0.056 < h1γ < 0.055
−0.045 < h2γ < 0.025
−0.049 < h3γ < −0.008
−0.002 < h4γ < 0.034
€
−0.13 < h1Z < 0.13
−0.078 < h2Z < 0.071
−0.20 < h3Z < 0.07
−0.05 < h4Z < 0.12
€
hiV = hi0
V
(1+ ˆ s /Λ2)n
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Summary of these results• We observed Zgnng for the first time at the
Tevatron and measured the cross section to be in excellent agreement with the standard model
• We set the tightest limits on anomalous ZZg and Zgg couplings at the Tevatron
|h30g|<0.033 |h40
g|<0.0017|h30
Z|<0.033 |h40Z|<0.0017
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What comes next?
• It is exiting time to do HEP at the Tevatron• We have all the necessary ingredients to
perform rather sophisticated data analyses– Ever-increasing integrated luminosity– Well-understood detectors– Well-developed analysis tools
• We have a good shot at making more discoveries at the Tevatron!
Prediction is very difficult, especially if it is about the futureMark Twain, Niels Bohr, Yogi Berra