Triple Gauge Couplings in diboson production at LHC

Samira HassaniCEA/DAPNIA/Saclay

France

(On behalf of the ATLAS Collaboration)

Introduction Theoretical framework Form Factor considerations Diboson production: WZ, W, ZZ and Z Analysis strategies

Channel and selections Expected sensitivity Summary

• non-abelian SU(2)LxU(1)Y W W Z ,W W vertices

• Open window to EW symmetry breaking mechanism

• Probe tool: sensitive to low energy remnants of new physics

• Compliment to direct searches for new physics

Motivation for TGC’s

Probe W W V vertex with W Z, W , W W Under general assumptions (Gauge, C, P invariance) 5 parameters specify the anomalous W W V vertex

g1z, Z, Operators of dimension 4 grow like √ŝ

Z, Operators of dimension 6 grow like ŝg1

z, Z, , Z, = zero in S.M.

LEPMoriond 2003 TeVatron expected Run II

95 C.L. limits are O (0.02-0.10)

Charged Triple Gauge Couplings (TGC)

NTGC

Standard Model Physics beyond the S.M.

/ /

/

The SM has no interaction between (Z,)Higher order corrections through virtual loop contribute at the level of 10-4

Virtual effects from new heavy fermions and supersymetric models

In Model independent, the Z V* and ZZV* (V*=Z, ) vertex are described by 12 parameters requiring Lorentz + EM gauge invariance, Bose symmetry

hi Z , (i=1,…,4) : Z V* vertex (8 parameters ) in Z final state

fi Z (i= 4, 5) : Z Z V* vertex (4 parameters ) in ZZ final state

f4, f5, h1, h3 operators of dimension 6 grow like (ŝ)3/2

h2, h4 operators of dimension 8 grow like (ŝ)5/2

Neutral Triple Gauge Couplings (NTGC)

Form Factors• Constant non-standard TGC (NTGC)

would lead to a unitarity violation of the s-matrix

Form Factor req’d for TGC

• Many valid choices (and interpretation) of FF

• Ideally the limit should be given as function of the scale

• At high Λ there is a asymptotic limit, because of machine/energy/luminosity limitations convoluted with analysis sensitivity

For LHC this gives Λ=8 to10 TeV

Diboson production at the LHC• At LHC energies, higher order QCD

corrections (NLO) becomes dominant (a factor 1.4 to 3 on total cross section)

• At high PT(V) (V=Z, W, ), the NLO corrections are largest. Qualitatively, this is precisely what one expects for TGC and NTGC

Lowering TGC and NTGC sensitivity

However• Dominant channel, qg, does not

contain TGC and NTGC

• A jet veto is very efficient in recovering the qualitative shape of the LO distribution

Restore TGC and NTGC sensitivity

WZ

Z

Analysis strategies Experimental sensitivity to TGC and NTGC comes from three

different types of information : cross section energy dependence polarization

• Cross section : Parabolic increase of cross section with TGC and NTGC due to the

linear Lagrangian : σ ~ (TGC)2

• Energy dependence TGC (NTGC) lead to a broad increase in the differential cross

section at large invariant mass M WV, ZV (V=Z, ) and transverse momentum PT(V) (V= W, Z, )

ZZ ZZ

Polarization• Production angular information of the bosons for TGC: A Born ~ cos Θ ± 1/3 “ Radiation Zero ”

• Since different TGCs contribute to different helicity configuration, and NTGC lead to primarily longitudinally polarized Z boson angular information can be used as “ projectors”

W

WZ and W production at LHC

W±Z → l ±l± l± ν ( l = , e) W± → l± ν ( l = , e)

Consider leptonic channels only : ± / e ±

Number of events for 30 fb-1

• Expected number of events : ~ 2000• almost background free

• Expected number of events : ~ 4300• Jets faking photons is large background(signal/backgrounds = 1.62)

ZZ and Z production at LHC

Z → l± l± ( l = , e)

Number of events for 100 fb-1

• Expected number of events : ~ 2050 with 5 % background

• Expected number of events for

PT(Z) > 150 GeV : ~ 580 with 6 % background

• Z Z → l ±l± l ±l± ( l = , e) : ~ 780 events ( almost background free)• Z Z→ l ±l± ν ν ( l = , e)

Extracting the confidence intervals for TGC

-0.0035 < < +0.0035-0.0073 < Z < +0.0073-0.075 < < +0.076

0.0086 < g1Z < 0.011

-0.11 < Z < +0.12For 30 fb-1, systematics included.

• Binned maximum likelihood fit to PT(V) distribution

• Sensitivity lies mainly in high-end PT(Z/ ) spectrum

• Investigated: Optimal observables Multi-variant fits Other 1-D distributions

• 95 % Confidence Intervals are derived by averaging over large “mock” ATLAS experiments

Extracting the confidence intervals for NTGC

•Multi-parameter show a large correlation (50 %) between fZ

5 (fZ4) and f 5 (f

4)

• Sensitivity for 10 fb-1 are of the order Ο(0.001)

ZZ

Extracting the confidence intervals for NTGC

• The limits obtained for h Z, 2,4 and h Z, 2,4 are different because their ŝ dependence

• Theoretical errors dominate the systematics (PDF, scale)

Z

What can we do if we observe anomalous couplings??

• Structure and scale of form-factor must be determined to have meaningful results• This can be done at LHC by measuring bare-couplings in ŝ bin and fit to form factor parameterization

Summary• The study of boson pair productions at LHC provides an opportunity to probe

gauge boson self-interaction in direct way

• At LHC, the luminosity will allow fits using multi-dimensional distribution

• NLO effects increase cross sections and tend to reduce sensitivity to TGC and NTGC, however a jet veto restores cross section to a fair approximation to born level

• In case TGC or NTGC are observed, the scale and the functional behavior of the form factor can be assessed

• The Triple Gauge-boson couplings can be measured at LHC at the level of 10-2 to 10-3 (order of magnitude improvement over LEP TeVatron)

• The Neutral Triple Gauge-boson couplings can be measured very accurately at LHC at the level of 10-4 to 10-7 ( 3 to 5 orders of magnitude improvement over LEP TeVatron)

Become almost sensitive to radiative corrections and contributions from supersymetric models