Progress on tt spin correlations at CPPM

Goal :

Update the tt spin correlation study in semileptonic channels using production and decay density matrices

First :

Using the same approach, reproduce our Prague colleagues dileptonic results

Since last meeting :

• Independent tool developed

• Dileptonic channel analysis performed

F. Hubaut, E. Monnier, P. Pralavorio

CPPM10/2003 Top WG meeting

• Standard PYTHIA does not include spin correlations

• Interfaced Toprex3.28-Pythia6.156 with Atlfast2.22 (thanks to K. Assamagan)

• Then moved to Toprex4.1-Pythia 6.214

Tools

• Use TOPREX* (developed by CMS) which includes full density matrices

• Pythia for hadronization, fragmentation and decays

• Tauola package for the τ decay

• Independent tool

• Generalist: can do single top, Higgs resonance, rare top decays (FCNC, t H+b), …

• Evolutive (future integration in Athena framework…)

URL = http://cmsdoc.cern.ch/~spitskyc/toprex/toprex.html and note hep-ph/0201292

*

• 2 processes with different theoretical spin correlation implementations are well suited in Toprex :

• CTEQ5L structure function set

• ~20fb-1 of events (~1M events) in the dileptonic channel with process 20, with process 50, and with standard Pythia6.2

• Events reconstructed with Atlfast2.22

Event generation and reconstruction

• process 20 : Standard model tt production (Breit-Wigner approach with off-shell W bosons)

• process 50 : tt production in the on-shell approximation, with the possibility to switch on an intermediate Higgs resonance (switched off in the following)

can help to understand the theoretical systematics

Theoretical framework and notations largely discussed by Prague colleagues (e.g. see K. Smolek talk given at Prague Atlas week or Atlas note ATL-PHYS-2003-012) :

• Θ+ (Θ-) = angle between chosen spin axis and lepton direction in the t(t) rest frame. Spin axis is t(t) direction in the parton c.m.s. (helicity basis)

• 9 <CosΘ+ · CosΘ-> = unbiased estimator for the asymmetry coefficient A, which measures the spin correlations

Asymmetry coefficient

9 * CosΘ+ · CosΘ-

En

t ri e

s Standard Pythia

Mean = -0.0010.003

= A

In the following, statistical errors < 0.01

Standard Pythia vs Toprex

Toprex reproduces the LHC expected asymmetry

A=0.00

A=0.01 A=0.46

A=-0.45

A=0.00 A=-0.32

Toprex 20 vs Toprex 50

Very good agreement between both processes

A=-0.45

A=0.46 A=0.46

A=-0.45

A=-0.32 A=-0.32

• Signal : events with both W e or μ

• Background : events with at least one W τ e or μ

• Event topology reconstruction in the dileptonic channel is out of our scope (done by Prague colleagues)

Event analysis

Look at the angles between partons

• without any cut (S/B ~ 1/0.382)

• with standard cuts on partons • with standard cuts on reconstructed objects (S/B ~ 1/0.116) – only acceptance effects, no reconstruction efficiency (lepton id., B-tagging, …)

and compute the related asymmetry (for S+B, S, B)

Standard Pythia vs Toprex

Necessary to study the systematics induced by the cuts

A=0.00

A=0.09 A=-0.14

A=0.04 A=-0.17

A=-0.32

Signal + background

Toprex 20 vs Toprex 50

Very good agreement at all levels

A=-0.32

A=-0.14 A=-0.14

A=-0.32

A=-0.18 A=-0.18

Signal

Idem with ISR/FSR

Changes are small with respect to previous plot

A=-0.32

A=-0.13 A=-0.16

A=-0.33

A=-0.18 A=-0.19

Signal

• All these results (at the parton level) are in fair agreement with Prague results :

• Comparisons between Prague, Toprex20 and Toprex50 give a first guess on some systematics :

Systematics (1)

No cut

Prague Toprex20 Toprex50

S+B -0.332 0.002

-0.321 0.003

-0.331 0.003

Signal -0.341 0.002

-0.323 0.004

-0.330 0.004

No spin

-0.005 0.004

-0.001 0.003

Even before cuts, differences ~0.02

(statistical errors only)

• Besides this systematics due to event generation, other systematics

– related to the cuts – related to the event topology reconstruction – …

still need to be studied (out of our scope for dileptonic channel)

• Systematics are much larger than statistical errors

Systematics (2)

Control of systematics is one of the key point of this kind of

study

• A new tool (Toprex+Pythia6.2+Atlfast) is under control for tt spin correlation studies

• Results on dileptonic channel have been reproduced with 2 generation processes : good agreement with Prague, but necessity to study the systematics

Conclusions and outlook

Same study will be done on semileptonic channels including :

• event topology reconstruction

• study of systematics