Trilinear Gauge Couplings at TESLA Photon Collider

Trilinear Gauge Couplings at TESLA

Photon Collider

Ivanka Božović - Jelisavčić&

Klaus Mönig

DESY/Zeuthen

12 -15 April, St. Malo, France ECFA/DESY Linear Collider Workshop

• TGCs in W-pair production - sensitive observables

• Measurement of anomalous and at a photon

collider at TESLA

• Summary


TGCs in W-pair production - sensitive observables

In order to understand the mechanism EW symmetry is broken,

self interaction of the gauge bosons can be used to reveal a

structure of the underlying gauge group. In particular, W-pair

production at high energies is a sensitive process to the possible

scenarios of EWSB.

W-pair production in collisions is dominated by single diagram.

Due to spin-1 t-channel exchange, the cross-section for W-pair production becomes flat in the ‘picobarn region’, above threshold up to 1 TeV center-of-mass energy.

W

W

W


High production cross section of W pairs in collisions allows cuts to be

successively applied, so the signal can be extracted with a ratio to

background of 27:1


A potential of different observables for analysis of the

anomalous TGCs has been discussed. We study the effect

of anomalous dipole coupling parameter on differential

cross-section for W-pair production in collisions and

on W polarization fractions reconstructed from W

production and decay angle distributions.


• ‘W production angle’ - the polar angle of the outgoing W with respect to the e- beam direction

• ‘W decay angle’ - the angle of fermion with respect to the W flight direction, reconstructed in the W rest frame

• Azimuthal angle of the fermion in the plane orthogonal to W direction, where the axes are defined in such a way that the electron beam is contained in X-W plane.

Measurement of triple gauge couplings in W-pair production is in

principle based on several sensitive angular observables:

These information can be combined (i.e. by using the optimal observables

method)

in order to determine anomalous coupling parameters.


Tools:

– SIMDET V3 - A Parametric Monte Carlo for a TESLA Detector

– Analytic formula for total (differential) cross-section (E.Yehudai, Phys. Rev. D11(44)1991) based on helicity amplitudes for different initial and final states (G. Belanger et al., ENSLAPP-A-473/94, hep-ph/9405359)

– WHIZARD Monte Carlo generator based on tree-level matrix elements for multi particle processes. Sample of 105 mixed W decays is generated at = 500 GeV s


The shape and the size of the total cross-section for W-pair production is dominated by transverse W mode


• The cross-section to produce one L and one T W from a Jz=0 initial two-photon state is 0 in the Standard Model.

• Production of two

longitudinal Ws (LL)

for both initial two-

photon states is

suppressed in the

Standard Model.

TT/LL/LT: 99.9/0.1/0.0, =0

TT/LL/LT: 96.3/1.4/2.3, =0

TT

LL

LT

TT

LT

LL


TT/LL/LT: 99.9/0.1/0, =0 TT

LL

LT

/SM

=1.01

=0.99

Although Ws from initial

two-photon state Jz=0

do not exhibit a

polarization structure,

differential

cross-section is sensitive

up to 2% to 10-2 variation

of parameter . This

effect is more pronounced

in the forward region.


However, Jz=2 state

exhibits a

structure over the

W polar angle

range. This

implies that

longitudinal Ws

could be partially

recovered in the

central region.

Fra

cti

on


TT

LL

LT

6.0cos W

TT/LL/LT: 70/16/14, =0

Differential cross-section as a function of W production and

decay angle, can be used to extract the polarization

fractions from simulated (experimental) data, even without identification of

charges of the decay products.

2

4

3

)(cosd)(cosd)(cosd

d

21

2

21

2TT cos1cos14

1

)(cosd

d[

)]cos1(sin2

1

)(cosd

d)cos1(sin

2

1

)(cosd

dsinsin

)(cosd

d1

22

2TL2

21

2LT2

21

2LL

(M. Bilenky et al., Nuc. Phys. B409(1993)22)


6.0cos W Jz=2


J z= 2 J z= 0

TT/ LL/ LT

97.90.4

96.3

2.20.7

1.4

0.10.8

2.3

99.50.3

99.9

-0.60.8

0.1

1.10.9

/ 2 of the 3-param eter fi t

1.07 1.04

TT/ LL/ LT Wcos < 0.6

782

70

253

16

-24

14

101.70.9

99.9

02

0.1

-33

/ 2 of the 3-param eter fi t

1.14 1.03

Similarly, one-dimensional decay angle distribution can be fitted in order to determine fraction of polarization states T=TT+TL and L=LL+LT


• WHIZARD results follow the trend predicted using numerical function, however they deviated for several sigmas.

• Possible explanation for it might be due to the fact that function, differently from WHIZARD, generates cross-sections for on-shell Ws. If we generated on-shell W pairs with WHIZARD as well, the numerical prediction of the function will be confirmed: TT/LL/LT, 70±3/16±1/15±1.

• As well, by restricting W mass narrower around it’s nominal value (i.e. |MW|

<2 (1) GeV) , the WHIZARD prediction for W mixed decays converges towards on-shell approximation. This implies that contribution from tree-level non-resonant processes to production of off-shell W pairs could possibly explain the effect.


Recovering longitudinally polarized Ws (LL, LT) from initial two-photon state Jz=2, their fraction can be determined with a statistical error of order ~10-3 for an integrated luminosity of ~ 120 fb-1. This would make accessible the ~2% effect of deviation of W’s longitudinal fractions from their SM values, for anomalous =0.01 coupling.

=1.01

=1.01

=0.99

=0.99

Fac

/FS

MF

ac/F

SM


= 0.90 = 1.10 = 0.99 = 1.01

Dominance of the transverse W modes

even when angular cuts are imposed, makes

Jz=0 channel not promising to exploit the sensitivity of W

polarization fractions to anomalous

dipole coupling parameter .


Measurement of and

From the 2D decay

angle distributions of

the accepted data the

dipole and quadrupole

coupling parameters

can be fitted. The

statistical error and

systematic effect of the

error of luminosity

have been estimated.

WcosA

ccep

tan

ce

Wcos

Decay angle W1

Decay angle W2

3D-Acc.


With an integrated luminosity of 110 fb-1 , that would correspond to one

year of running TESLA photon collider at center-of-mass energy of

400 GeV, C and P conserving couplings (, ) could be measured with a

statistical error of order of 10-4.

E=400 GeV; L=110 fb-1

L ·10-4 ·10-4

3.1(TDR)

4.3(TDR)1% 6.0

4.7

4.3

3.3

0.1% 5.3 4.2

accurate 3.1 3.5

E [GeV] 160 400 640

·10-4 9.6 6.0 5.0

·10-4 7.5 4.3 3.4


• Systematic error on dipole coupling parameter introduced by the error of luminosity of ~1% is of the same order as the statistical error. Measurement of can not be significantly improved by more precise luminosity measurement.

• By increasing a center-of-mass energy of photon collisions

from 400 GeV to 800GeV, total error of these parameters

can be reduced for approximately 20% (less then in e+ e-

case).


Summary

• In collisions, both differential cross-section for W-

pair production (Jz=0 state) and W polarization

fractions (Jz=2) are sensitive observables to anomalous

dipole coupling. With an integrated luminosity of order

of 100 (120) fb-1, these observables can be measured

with a statistical error of 10-3. This would make

accessible the ~2% effect of their deviation from the

Standard Model values for anomalous =0.01

coupling.


• With an integrated luminosity of 110 fb-1 , that would correspond

to one year of running TESLA photon collider at center-of-mass

energy of 400 GeV, C and P conserving couplings (, ) could be

measured with a statistical error of order of 10-4.

• By increasing a center-of-mass energy of photon collisions from

400 GeV to 800GeV, total error of these parameters can be

reduced for approximately 20%.

• Taking into account the fact that the full information on W

polarization structure (i.e.azimuthal angle ‘’) in not included yet

into analysis, as well as more realistic simulation of data (variable

photon energy spectrum), there is no yet definite answer on

photon collider potential to study anomalous TGCs.

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Trilinear Gauge Couplings at TESLA Photon Collider