# FSI and Mw(qqqq)

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FSI and Mw(qqqq) 1

FSI and Mw(qqqq)FSI and Mw(qqqq)

Marie Legendre, Djamel Boumediene,Patrice Perez,Oliver Buchmüller

… an alternative approach …

PFCUT and PCUT update fromfrom PISA

FSI and Mw(qqqq) 2

Motivation

W mass combination for Winter 2001:W mass combination for Winter 2001: The “statistical sensitivities” of the hadronic and semileptonic channel are roughly equal.

The reduced weight of the qqqq channel (27%) in the combination is a consequence of the large estimate of the

systematic errors due to CR(40 MeV) and BE(25 MeV) !!!

Progress since Winter 2001: Progress since Winter 2001: BE: All four LEP experiments disfavour correlation between different W’s (Inter-W) as implemented in the LUBOEI models by 2-4 Sigma

The current value of MThe current value of Mww(BE)=25 MeV (BE)=25 MeV is is overestimated overestimated and will go down (Common believe ?!) and will go down (Common believe ?!)

Ongoing work of the LEP W group but things are converging …..

FSI and Mw(qqqq) 3

Motivation

CR:So far the only observable we found (excepted Mw and w)sensitive to CR effects is the “ Particle Flow” Particles in the “middle region” are clearly sensitiveto CR effects as they are implemented in SKI

Adding all ALEPH data (189 -207) yields:(see talk from Thomas last Monday)

Evidence for CR ……… or statistical fluctuation

… or not understood systematic ..?

wait for LEP combination ..?!

ki3.5 and thisexcludes ki=0.0by roughly 2 Sigma

FSI and Mw(qqqq) 4

How to use Mw in order to measure FSI effects …

Standard cross check:Standard cross check:Mw(qqqq)- Mw(qql)= 18 46 MeV (Winter 2001)

No evidence for FSI effects… but that is the only “plausibility argument” we have so far for the mass ….

Try to use Mw(qqqq) in order to measure FSI effects:Try to use Mw(qqqq) in order to measure FSI effects:

Mw(qqqq) is at least as sensitive to CR effects as the Particle Flow

… why not using Mw(qqqq) in order to measure FSI in data …

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Requirements on the Method

a) “tunable sensitivity” to FSI effects => in order to quantify the effect a dynamical range of 100% FSI effect to 0% FSI effect would be ideal

b) smallest possible modification to the standard 4q mass analysis => in order to reach the “required statistical significance” a combination of all LEP Experiments might be needed. Hence the modification should be as easy as possible to allow other experiments with less man power to catch up … (same argumentation as for the Particle Flow)

FSI and Mw(qqqq) 6

How to achieve the …..

… “tunable sensitivity” to FSI effects exclude soft particles in the inter jet regions in a well defined way …

Two approaches tested so far:Two approaches tested so far:a) cut in Particle Flow distribution (PFCUT) no cut [0.55,0.65] [0.9,0.1]FSI effect : 100% x% 0% (ideal)

b) cut in Particle momentum (PCUT) no cut 0.5 GeV x.x GeVFSI effect : 100% x% 0% (ideal)

Cutout

Varying the cuts should lead to the desired“dynamical range” of 100% to 0% (ideal) FSI effect

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How to achieve the …..

… smallest possible modification to the standard 4q mass analysis

Use the standard 4q mass analysis as base (but 3D RW!):=> force event to four jets using the standard jetfinder DURHAM PE. • Apply soft particle selection as defined by PCUT or PFCUT

• Recalculate jet energies and directions by using the remaining particles. The assignment of the particles to the jets is still based on the original cluster process (Note: the event is not reclustered !) . • Recalculate jet parametrisation for the Kinematic fit in order to correct for the additional energy loss due to cutting

... that’s in principle all what we need ! Remark: All Experiments have already the Remark: All Experiments have already the same definition for the Particle Flow distributionsame definition for the Particle Flow distribution and also the cut in particle momentum should be straightforward to do. and also the cut in particle momentum should be straightforward to do. => very easy implementation=> very easy implementation (should fulfill our LEP combination requirement easily) (should fulfill our LEP combination requirement easily)

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FSI Robust Method ... a nice byproduct

The PFCUT or PCUT methods provides also this possibility. Due to the easy tunable sensitivity to FSI effects a combination of [MW(CR) ;MW(stat.) ] which optimizes the ALEPH (LEP) W mass combination can be chosen.

FSI Robust Method :=FSI Robust Method := Minimize the FSI effect by excluding soft particle in the inter jet region.

The price to pay is a loss in statistical sensitivity.

... a nice byproduct but not the main intention

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Performance of “FSI robust Method”

=> only PFCUT results are ready (PCUT is still running!) (cut in ten bins of the Particle Flow distribution)

SKI predictions are calculated using 600K at 189 GeV(additional 100K are still running)

=> clear FSI sensitivity change as function of PFCUT and

acceptable loss in statistic sensitivity

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Performance of “FSI robust Method”

Table for SKI 100% and ki=0.65 withstatistical loss and CR reduction!!!!!!

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Reminder: Last talk from Djamel

PFCUT at 189 GeV:PFCUT at 189 GeV:

=> 189 ALEPH data seems to be compatible with no FSI effect but very pure statistical power!

Add all high energy data!!Add all high energy data!!

FSI and Mw(qqqq) 12

PFCUT for all data

PFCUT for all ALEPH data:PFCUT for all ALEPH data:

=> The data seem to be not compatible with the flat hypothesis. Is this now evidence for a FSI effect?

Three question have to be answered:a) what is the statistical significance of the effect?b) would we measure a flat response by using MC pesudo experiments (bug check)?c) are there maybe other systematic sources (rather than FSI) which could lead to such a behaviour?

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Statistical Significance

1 -1. 18 -21 2 +25. 25. -39 3 -11. 28. -69 4 -52. 42. -91

5 -99. 57. -121 6 -81. 50. -160 7 -70. 67. -193 8 -145. 72. -208

PFCUT Data shift uncorrelated error

SKI 100% shift

Shifts in MeV

=> e.g. for PFCUT 8: 2 Sigma effect and more if you combine all cuts but fitting the slope!!!!! (not yet done)

=> calculated using Pseudo experiments at all energies

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Flat Response in MC

Using MC Pseudo Experimentsleads to a flat response.

Therefore, the observed shift can only be explained with

either a statistical fluctuationor a systematic data Monte Carlo

difference

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Possible Systematic Sources

One obvious source could be FSI. However, it could alsobe that the Particle Flow distribution has some systematicerror which we have not yet taken into account. This might

also explain the results obtained from Thomas using the Standard Particle Flow method

=> Check performance of cut in momentum (Pcut)this might give some hints … (is it flat or not)

FSI and Mw(qqqq) 16

Summary

Using the PFCUT and the PCUT method provides an alternative way Using the PFCUT and the PCUT method provides an alternative way of an (nearly) model independent measurement of FSI effects.of an (nearly) model independent measurement of FSI effects.

This methods only require very simple modifications of the standard This methods only require very simple modifications of the standard 4q measurement and hence very easy to apply for everybody4q measurement and hence very easy to apply for everybody.

Applying the PFCUT method to the full ALEPH data setApplying the PFCUT method to the full ALEPH data setleads to an x-y% deviation from the non FSI Hypothesis.leads to an x-y% deviation from the non FSI Hypothesis.

Further work will be needed to understand the origin of thisFurther work will be needed to understand the origin of thiseffect (statistical fluctuation, unknown systematic effect (statistical fluctuation, unknown systematic

source or FSI). source or FSI).

As a “byproduct” the PFCUT and PCUT methods can alsobe used as “FSI Robust Method”.