Petrakou Eleni


Petrakou Eleni
Anomalous Trilinear Gauge Couplings
in the Z(Z(->ll) γ channel at the CMS experiment
Eleni Petrakou (NCSR Demokritos)
XXVIII Workshop on Recent Advances in Particle Physics and Cosmology
Thessaloniki, 25-28 March 2010
Trilinear Gauge Couplings (1/4)
SelfSelf-interactions of gauge bosons enter the SM because of
the nonnon-Abelian character of the electroweak sector.
+/-: WWZ, WWW, WWγ
Trilinear couplings involving the W+/WWγ
Their existence was an important prediction of the SM.
However, interactions involving only electrically neutral
vector bosons are prohibited at Born level.
(At one-loop level, fermionic loops contribute much less than the current
detector sensitivity at LHC energies.)
Simultaneous Zγ
Zγ production would require such couplings,
-- therefore, its detection would be a signal of new physics.
Trilinear Gauge Couplings (2/4)
In the specific case of the Zγ
Zγ channel with leptonic Z
decay, the
the only detectable SM contributions are:
γ: initial-state radiation
γ: final-state radiation
And they act as backgrounds to the present study.
Trilinear Gauge Couplings (3/4)
It is possible to study TGCs in a modelmodel-independent way:
Starting from all possible terms in the lagrangian and
keeping only very basic symmetries (EM gauge invariance,
Lorentz invariance of on-shell photon, assuming negligible lepton masses),
we arrive at an effective Lagrangian.
The result for the Zγ
ZγV vertex includes 4 general anomalous
coupling parameters, hiZ:
CP violating
Z (q1)
V (P)
CP Conserving
γ (q2)
Trilinear Gauge Couplings (4/4)
U.Baur et al. used helicity amplitudes summation to calculate
the contribution of the Zγ
ZγV diagrams to the Zγ
Zγ crosscross-section.
The main signatures of anomalous TGCs would be the
enhancement of the total production cross-section, and
the photon Pt distribution towards higher values; there
are also expected differences in angular distribution etc.
In addition, they provided software for the calculation at
leading and nextnext-toto-leading logarithmic order, and QCD corrs.
In this study we use an implementation of the "Baur generator"
in the framework of the CMS experiment.
Baur & Berger, PhRevD 47 (93)
Baur et al, PhRevD 57 (98)
Zgamma Baur generator
The signal samples (both SM and beyond) were generated with the "Baur Zgamma"
generator, hadronized with Pythia, and passed through full CMS simulation.
showers, FSR
of event
Of the four parameters in the ZγV vertex, the CPCP-conserving h3,4Z were set to the values
h3Z: {±5
{ 5‧10-2, 0}, h4Z: {±1
{ 1‧10-3, 0}
0}, giving eight nonnon-SM combinations.
The SM sample was produced by setting all hiZ = 0.
Baur Production:
• centre of mass E = 10TeV
both Z->ee and µµ decays
• cut-off scale Λ = 2TeV
lepton Pt > 3GeV, |η| < 2.7
• pdf: CTEQ Set 5L
photon Pt > 50GeV, |η| < 3
• only ZZγ vertex enabled
~9,500 events per sample
• leading-order calculations
The CMS experiment
Event selection (1/7
(1/7) : Samples
"Zγ Baur" SM production
σ @10TeV
0.12 pb
Background ("official"):
Muon QCD
EMEM-enriched QCD
σ @10TeV
3.7 nb
0.5 mb
0.1 mb
Baur Production:
• centre of mass E = 10TeV
• cut-off scale Λ = 2TeV
• pdf: CTEQ Set 5L
• only ZZγ vertex enabled
• leading-order calculations
both Z->ee and µµ decays
lepton Pt > 3GeV, |η| < 2.7
photon Pt > 50GeV, |η| < 3
~9,500 events per sample
C.Karafasoulis, A.Kyriakis, A.Markou, E.P.
NCSR Demokritos, Athens
P.Adzic, M. Djordjevic
Vinca Institute, Belgrade
electron candidates with Pt>14 GeV/c
1(2) muon candidates with Pt>9(3) GeV/c
** √s=10TeV ** L = 200pb-1 **
1. SM analysis:
Electrons selection
Muons selection
+ brem. correction
Photon selection
2. Anomalous TGCs analysis
Event selection (1/6) : Electrons
Electron selection
• Pt > 20 GeV/c, |η| < 2.5 excl. gaps
• |Δηin|< 0.0040 / 0.0066 (barrel/endcaps)
• |Δφin|< 0.025 / 0.020 (barrel/endcaps)
• H / E < 0.01
• σiηiη< 0.0099 / 0.028 (barrel/endcaps)
• |Mee – MZ| < 10 GeV/c2
• Isolation: (Tracker+Ecal+Hcal)/Pt < 0.15
• Exactly one pair of e+,e-
Efficiency 95%
Rejection 93%
Event selection (2/6) : Muons I
Muon selection
• Pt > 10GeV/c, |η| < 2.5 excl. gaps
• silicon detector track fit, χ2/n.d.f. < 2
• silicon hits > 10, d0 < 0.2 mm
• |Mμμ – MZ| < 10 GeV/c2,
after correcting Mμμ with brem (if any)
• Isolation: (Tracker+Hcal)/Pt < 0.2
• Exactly one pair of μ+,μ-
Efficiency 99%
Rejection 80%
Event selection (3/6) : Muons II
Bremsstrahlung selection
• Pt > 10 GeV
• Distance from closest muon, dR<0.5
• H / E < 0.2
• No pixel hits
(97% brem., 3% "contamination")
Dimuon invariant mass for signal
before (red) and after (blue) brem correction..
Dimuon invariant mass for Z+Jets background
before (red) and after (blue) brem correction..
Event selection (4/6) : Photons
Photon identification
• Not identified as brem. (for Z->μμ)
• Pt > 60 GeV, |η| < 2.5 excl. gaps
• H / E < 0.2
• No pixel hits
• Isolation: (Tracker+Hcal)/Pt < 0.1
• Exactly one photon passing all criteria
Efficiency 96%
Rejection 96%
Event selection (5/6) : Event yields for electrons
Event selection (6/6) : Event yields for muons
Anomalous TGCs (1/4)
The "discovery variable of choice" is photon's Pt:
Anomalous TGCs (2/4)
The vertex amplitude is a linear function of the couplings, so the number of
predicted events can be expressed as a function of the couplings' values {h3 h4}.
More specifically, it is an elliptical paraboloidal function:
Ni(h3, h4) = NiSM + Ai·h3 + Bi·h4 + Ci·h32 + Di·h42 + Ei·h3·h4
Given only some specific combinations of h3, h4, this permits the prediction of
the number of events for a larger range of coupling values.
To predict the coefficients we fit over the number of events for the eight pairs of
h3, h4 in each Pt bin:
Muller et al, CMS NOTE 2000/017
Anomalous TGCs (3/4
Using this function for predicting the number of events –not only for the fixed
pairs h3, h4– we can employ the extended log-likelihood method,
for calculating the likelihood that the data from anomalous TGC's are consistent
with SM and find the sensitivity of CMS to anomalous values.
Anomalous TGCs (4/4)
CMS sensitivity on h3 and h4 values @ 95% CL:
L = 0.2 fb-1
L = 1.0 fb-1
Limits from previous
arXiv:0810.3766v1 [hep-ex] (08)
D0 (1.1fb-1)
this study
Current work – To do
Repeat the analysis at 7 TeV
Data-driven background estimation ("fake-rate method")
Next-to-leading order: reweighting
Next-to-leading order: included in new production
Systematics' analysis.