FSI and Mw(qqqq) 1 FSI and Mw(qqqq) Marie Legendre, Djamel Boumediene, Patrice Perez, Oliver Buchmüller … an alternative approach … PFCUT and PCUT update from from PISA

FSI and Mw(qqqq) 2 Motivation 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: 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 M w (BE)=25 MeV is overestimated 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 sensitive to CR effects as they are implemented in SKI Adding all ALEPH data ( ) yields: (see talk from Thomas last Monday) Evidence for CR …… … or statistical fluctuation … or not understood systematic..?  wait for LEP combination..?! ki  3.5 and this excludes ki=0.0 by roughly 2 Sigma

FSI and Mw(qqqq) 4 How to use Mw in order to measure FSI effects … 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:  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 …

FSI and Mw(qqqq) 5 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: 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 GeV FSI effect : 100% x% 0% (ideal) Cutout Varying the cuts should lead to the desired “dynamical range” of 100% to 0% (ideal) FSI effect

FSI and Mw(qqqq) 7 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 same 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 (should fulfill our LEP combination requirement easily) => very easy implementation (should fulfill our LEP combination requirement easily)

FSI and Mw(qqqq) 8 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 [  M W (CR) ;  M W (stat.) ] which optimizes the ALEPH (LEP) W mass combination can be chosen. 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

FSI and Mw(qqqq) 9 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

FSI and Mw(qqqq) 10 Performance of “FSI robust Method” Table for SKI 100% and ki=0.65 with statistical loss and CR reduction!!!!!!

FSI and Mw(qqqq) 11 Reminder: Last talk from Djamel 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!!

FSI and Mw(qqqq) 12 PFCUT for all 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?

FSI and Mw(qqqq) 13 Statistical Significance 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

FSI and Mw(qqqq) 14 Flat Response in MC Using MC Pseudo Experiments leads to a flat response. Therefore, the observed shift can only be explained with either a statistical fluctuation or a systematic data Monte Carlo difference

FSI and Mw(qqqq) 15 Possible Systematic Sources One obvious source could be FSI. However, it could also be that the Particle Flow distribution has some systematic error 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 of an (nearly) model independent measurement of FSI effects. This methods only require very simple modifications of the standard 4q measurement and hence very easy to apply for everybody 4q measurement and hence very easy to apply for everybody. Applying the PFCUT method to the full ALEPH data set leads to an x-y% deviation from the non FSI Hypothesis. Further work will be needed to understand the origin of this effect (statistical fluctuation, unknown systematic source or FSI). As a “byproduct” the PFCUT and PCUT methods can also be used as “FSI Robust Method”.