Colour reconnection is the final state interaction between quarks from
different sources. It is not yet fully understood and is a source of systematic
error for W-boson mass and width measurements in hadronic \WW decays at LEP2.
The methods of measuring this effect and the results of the 4 LEP experiments
at $183\gev\leq\rts\leq 202\gev$ will be presented.Comment: 6 pages, 1 table, 7 figures, Symposium, ISMD2000, Typos fixed (proof
  reading by editor

Nandakumar, Raja

English

arXiv

Colour reconnection is the final state interaction between quarks from different sources. It is not yet fully understood and is a source of systematic error for W-boson mass and width measurements in hadronic \WW decays at LEP2. The methods of measuring this effect and the results of the 4 LEP experiments at $183\gev\leq\rts\leq 202\gev$ will be presented.Colour reconnection is the final state interaction between quarks from different sources. It is not yet fully understood and is a source of systematic error for W-boson mass and width measurements in hadronic \WW decays at LEP2. The methods of measuring this effect and the results of the 4 LEP experiments at $183\gev\leq\rts\leq 202\gev$ will be presented

CERN Document Server

COLOUR RECONNECTION AT LEP2RAJA NANDAKUMARTata Institute of Fundamental Research, IndiaE-mail: Raja.Nandakumar@cern.chColour reconnection is the nal state interaction between quarks from dierentsources. It is not yet fully understood and is a source of systematic error for W-boson mass and width measurements in hadronic W+W− decays at LEP2. Themethods of measuring this eect and the results of the 4 LEP experiments at183 GeV ≤ √s ≤ 202 GeV will be presented.1 IntroductionThe W{boson is a carrier of charged weak interactions and it decays intolepton{neutrino (lν) or quark{antiquark (qq) pairs. The LEP2 W{factory1 atCERN, Geneva has produced thousands of W{pair events which have beencollected by the 4 LEP experiments (Aleph, Delphi, L3 and Opal).The typical distance between W decay vertices at LEP2 (0.1 fm) is muchsmaller than the typical hadronisation distance (1 fm). When both the W’sdecay in the qq mode, this leads to the possibility of gluon exchange betweenquarks from dierent W’s, called Colour Reconnection (CR).CR2 can thus be dened as the strong interaction between independentcolour singlets, before hadron formation. This causes a change in the stringconguration (gure 1) leading to changes in the reconstructed 4{momentumof the W’s. Thus, a study of CR tells us about the space{time evolution ofhadronic systems and about the systematic uncertainty of the MW and ΓWmeasurements at LEP2.2 Methods of measuring CRChanges in the string conguration change the momentum distribution ofthe particles and hence, also the expected event multiplicity. Thus, thesequantities carry information about CR. The typical analysis procedure tostudy CR is to compare the data with Monte Carlo events simulated usingvarious models3 (SKI, SKII, GH, etc). We note that the qq sector of qql νlevents is a source of W’s without CR.In qqqq events, there is an additional problem of pairing the jets correctly,due to the existence of 3 dierent ways of pairing up 4 jets into two W{bosons.This problem does not arise in the qql νl events.0101026: submitted to World Scientic on May 2, 2002 13 Multiplicity based measurementsCR typically leads to a reduction in the string length. This implies that theaverage multiplicity is decreased. This is a small eect, as shown for chargedparticles in OPAL data (gure 2).One can dene the average charged multiplicitieshN4qch i : from qqqq events, hN2qch i : from qq system of qql νl eventshNchi = hN4qch i − 2hN2qch i (6= 0 for CR)Averaging over all LEP data 4, the author obtains for hNchi, a value ofabout 0.22  0.20  0.35. This result is consistent with 0 and the error isdominated by systematics (mainly fragmentation modelling).Another approach4 is to study the fragmentation functions xp = 2p/psand ξ = −logxp, where p is the momentum of the particle under considerationandps is the center of mass energy.The sensitivity of the fragmentation function to the presence of CRis shown in gure 3. Sensitivity is maximum in the region 4.5  ξ 6.1 ) 0.001  xp  0.005. Similar studies can also be performed withheavy hadrons (eg. protons, kaons) which will be more sensitive to CR eects.However, the net result is that the systematic error of the method is greaterthan the expected eect and no signicant non-zero result is seen in the data.4 Particle flow based measurementsThe particle flow method5 studies the event’s string topology by looking atthe particle momentum distribution across the jets. The event (satisfying thetopology in gure 1) is forced into 4 jets to reconstruct the 4 parent quarks.Two adjacent jets { the most energetic jet (jet 1) and the jet associated to it(jet 2) { are used to form a plane and the particles in the event are projectedonto this plane. The angle of the projection (φ) from jet 1 is called the particleflow of the event (gure 4). The distribution for φ is shown in the gure 5(a).The angle φ is rescaled to the jet{jet angle to obtain φresc = φφjj . Thedistributions are symmetrised between the 3 adjacent pairs of jets, as shownin gure 5(b). The energy flow distributions correspond to the same distri-butions, with each particle in the distribution being weighted with its energy,normalised to the visible energy in the event. The background is subtractedfrom the data distribution before comparison with Monte Carlo.The regions A and B in gure 5(b) arise when the two jets come from thesame W and the regions C and D arise when the two jets are wrongly paired.Hence, one expects CR to cause a depletion in A and B, and an enhancement0101026: submitted to World Scientic on May 2, 2002 2in C and D. This can be clearly seen by taking the ratio A+BC+D as shown ingure 6. This ratio also has the advantage of cancellation of some systematics.The distributions shown in gure 6 are integrated over their most sensitiveregion, to obtain R =∫ 0.70.3A+BC+D dφresc. This R is a good estimator of thepresence of CR in the data. As seen from the L3 results in table 1 for 189GeV, the systematics for this method are much smaller than the statisticalerror and the data are inconsistent with the result of no CR at the 2σ level.For this method to work, one would obviously need jets that are clearlyidentied and with a high probability of correct pairing. This leads to a loweciency (’ 15%) in selection of the WW events.Similar analyses have been performed by Aleph and Opal. One can denea χ2 to estimate the best t of data to Monte Carlo for dierent values of CRprobability (gure 7). This is done using the SKI model, where the parameterk is proportional to the probability of CR. The data from Aleph, L3 and Opal,prefers in general, the existence of CR with probabilities ranging from 15% to60% with an error from each collaboration, of about 30%. A combination ofthe results will result in an improvement of numbers.5 SummaryCR can be measured in hadronic WW decays at LEP. The multiplicity andfragmentation function based methods do not yield signicant results, andnone are expected due to high systematic errors inherent to the method.However, rst indications of CR have been seen using the particle flow method.The W{boson mass and width measurements are major aims for LEP2.Current estimates of systematic uncertainties from CR for the mass is 50 MeV,fully correlated across experiments, while it varies from 40 to 70 MeV for thewidth. It is hoped that further studies on CR, especially through the particleflow method will help in improving these numbers.AcknowledgementThe author would like to thank TIFR, CERN and KFKI for their supportduring ISMD 2000. Many thanks too to Som Ganguli, Martin, Paul, Luca,John, Dominique, Gagan and the LEP collaborations for their kind help dur-ing preparations.References1. Physics at LEP-II, CERN 96{01, Vol.10101026: submitted to World Scientic on May 2, 2002 32. S. Todorova-Nova, DELPHI 96{158 PHYS 6513. T. Sjo¨strand and V. Khoze, Z. Phys. C 62 (1994) 281T. Sjo¨strand and V. Khoze, E. Phys. J. C 6 (1999) 271G. Gustafson and J. Ha¨kkinen, Z. Phys. C 64 (1994) 6594. ALEPH Collaboration, Contr. to ICHEP 2000, ALEPH 2000 { 058DELPHI Collaboration, CERN-EP/2000-023L3 Collaboration, Contr. to ICHEP 2000OPAL Collaboration, CERN EP / 98 { 196 ; Physics note, PN 4125. ALEPH Collaboration, Contr. to ICHEP 2000L3 Collaboration, Contr. to ICHEP 2000OPAL Collaboration, Physics note, PN 412R Data Monte Carlo : No CRParticle flow 0.771 0.049 0.029 0.868 0.007Energy flow 0.593 0.058 0.020 0.696 0.009Table 1. 189 GeV results from L3 for particle flowqqqq1423stringstringqqqq1423stringstringNo Reconnection Immediate ReconnectionFigure 1. Illustration of Colour Reconnection (CR) between two W’s decaying into q1q2and q3q4, respectively.0501001500 10 20 30 40 50 60 70 80nch for qq_qq_No. of eventsOPAL dataPYTHIASK ISK IISK II/BackgroundOPALPreliminaryFigure 2. Charged particle multiplicity0101026: submitted to World Scientic on May 2, 2002 4ALEPH preliminary combination 183-202GeVξ1/Nev (dn/dξ)0.80.911.11.21.31 2 3 4 5 6Figure 3. Sensitivity of the fragmentation function to CR (Aleph)Jet 1Jet 4Jet 2Jet 3particleφpiFigure 4. The particle flow method025050075010000 100 200 300φ (deg)nclus/Ntot / 8 deg particle flow with clusters 189 GeV (preliminary)L3 DataWWZZqqγ(a) The raw distribution10-110 1 2 3 4←→W1 ←→W2A C B Drescaled angle (φresc)1/N evt dn/dφ particle flow with clusters 189 GeV (preliminary)L3 Data- WW → qqqq(b) The symmetrised distributionFigure 5. The particle flow distributions from L30101026: submitted to World Scientic on May 2, 2002 50.20.40.60.811.20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1OPAL PreliminaryχR=χ1/χ0RflowOPAL 189 GeV dataAriadne KoralwHerwig(a)χR=χ1/χ0RflowSK I, kI=100SK IIAr2Ar3OPAL 189 GeV data(b)0.20.40.60.811.20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.7511.251.50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1φr1/Nevt dσ/dφ r(A+B)/(C+D)ALEPH preliminarydata 189, 196, 200 GeV combinedKoralW, ki = 0.KoralW, ki = 0.6KoralW, ki = 2.3KoralW, ki = 1000.Figure 6. Distributions for A+BC+D15202530χ20.60.70.80.90 0.25 0.5 0.75 1L3 preliminarySKI modelProbability of reconnectionRN01230 0.5 1 1.5 2 2.5 3ki∆χ2 /dof189, 196, 200 GeV combinedALEPH preliminaryFigure 7. Chisquared vs SKI CR probability for L3 and Aleph data0101026: submitted to World Scientic on May 2, 2002 6

Colour Reconnection at LEP2

Abstract

Similar works

Full text

Available Versions

CERN Document Server