Eventi Adronici a LEP

Transcription

1 Eventi Adronici a LEP Alessandro De Angelis Università di Udine and INFN Trieste 1. Many things have been done Phenomenology of hadronic events QCD and the color structure of hadronic events α s q/g jets More in the grey area (between perturbative & soft) 2. A few still to be done (a kind of a shoplist), and more in detail Interconnection effects (If there will be time) Photon radiation off partons 3. From LEP to the next experiments? How-to IFAE Lecce, April 2003

2 2/53 Hadronic events at LEP ( ) GeV E cm (GeV) kevts>.85e cm Plus: High quality detectors data analysis tools and methods theory M(Z)

3 3/53 Anatomy of an annihilation event A solution of the theory is not yet available, and its applications require further assumptions, ( ) in particular, ( ) that quantities that can be reliably computed in perturbation theory ( ) do correctly predict corresponding measurable quantities, up to effects which are suppressed by inverse powers of the characteristic energy scale of the process. This assumption (often called parton-hadron duality), implies that a description of a phenomenon in terms of constituents correctly describes the behavior of the hadrons that form the final state. (Nason 2002) p Hadronization effects suppressed as 1/Q (n)

4 4/53 LEP has been the benchtest for pqcd Between 1989 and today, > 250 papers on hadronic physics at LEP Very important contributions to Establishing pqcd Developing the intermediate regime (LPHD) Tuning the fragmentation MC models And more (power corrections) Exploring the fully nonperturbative region

5 5/53 And the story is not over: many more papers (~50) still to be finalized α s in 4-jets events Angular distr. in 4-jet events Running m b W hadronic decays CR in WW BEC in WW Fragm. functions and α s Multiplicity in 3-jet events Gluon fragmentation QCD overview paper Soft photon excess in qq

6 6/53 The LEP contribution: establishing pqcd as part of the SM Hadronization effects suppressed as 1/Q (n)

7 7/53 Establishing pqcd: color factors C F C F C A T F

8 8/53 The one parameter e + q e - Z/γ* q g α s σ de α qqg d s 1 σ 0 gluon 2π Egluon Born x-section for Z qq Bremsstrahlung

9 9/53 Experimental procedure to measure the one parameter Conventionally computed at m(z) Data Theory Calculate perturbative predictions Calculate the hadronization corrections using Monte Carlo (JETSET,HERWIG,ARIADNE) Common level Correct to the hadron level accounting for acceptance, resolution, ISR etc. ( Detector corrections ) Measure the distributions from data Observables : T, M h2, C par, B tot, B w, lny 3,...

10 10/53 1/2 1 Thrust.. non-perturbative =soft gluons 3 jet region T << 1 perturbative = energetic gluons 2jet region T >> 0.5

11 11/53 Combination by observable

12 12/53 Combination by energy Using only LEP LEP I: α s (M Z ) = ±0.0002(stat)±0.0008(ex) ±0.0010(had)±0.0048(th) LEP II: α s (M Z ) = ±0.0005(stat)±0.0010(ex) ±0.0007(had)±0.0044(th) All-energies doesn t beat LEP II <= high correlation of theory uncertainty

13 13/53 The one parameter and its uncertainty The value is stable α s ~ 0.120

14 14/53 Running established χ 2 /dof = 11.6/

15 15/53 And a few important results in the gray area : multiplicity

16 16/53 momentum spectrum

17 17/53 also for q/g jets separately

18 18/53 and many very nonperturbative features modeled/understood (?) Amazingly accurate thermodynamicsinspired formulae to predict the multiplicity of identified hadrons (Chliapnikov, Becattini, Pei) And then LPHD for the momentum spectrum And one should not neglect the validation & tuning of MC (Lund, Herwig) We have a theory... so why can t we test it right away to see if it s right or wrong? Because what we have to do is calculate the consequences of the theory to test it. This time, the difficulty is this first step. (Feynman)

19 Two open topics - I Interconnection effects Soft photon radiation

20 20/53 W mass In the absence of Higgs signal,m W determination Within the SM, link between M W, M t and m H The present LEP accuracy on M W is ±42 MeV => < m H < 211 GeV (95% CL)

21 21/53 W production at LEP 2 Well modeled by SM; pair production dominates the cross section e+ e- ν e w + w - f f f f e+ f Z/γ w+ w - e- f f f Year total/exp. s(gev) L (pb -1 ) ~700

22 22/53 W decay properties WW branching ~ 4/9 in qqqq, ~ 4/9 in qq lν The W is a good QCD object in its hadronic decay ch n = 19.4 ± 0.3 W

23 23/53 W pair selection Based on In qqqq (hadronic, 4q) 4 jets ~back-to-back In qq lν (semileptonic, 2q) 2 jets, 1 lepton and p miss Performance: 4q, eff. ~80%, pur. ~80% 2q, eff. ~70%, pur. ~90% Topological criteria still 208 GeV The 4 LEP collaborations analyzed ~ all data WW->4q WW-> 2qlν WW ~20000 ~16000 (4 exp)

24 24/53 W LEP2 (4q) The most precise determination might come from (4q) Larger statistics More constrained Systematics-dominated Due to FSI sustematics, our determination (δm W ~42 MeV) comes almost exclusively from (2q) Hope δm W ~30 MeV if we can control FSI

25 25/53 WHAT ARE FSI?

26 26/53 (Anatomy) simplified view of the system e + e - W + W - qqqq q 1 e + W + q 2 ch n ~39 e - W - q 3 Hadronic models: separation between 1. ElectroWeak phase 2. «Hard QCD» phase 3. «Soft QCD» phase 4. Decays q 4

27 27/53 Limits of the simplified view: QM-inconsistent? q 1 Separation between Ws ~ /Γ W ~ 0.1 fm/c h e + W + q 2?? Hadronization radius τ had ~ fm/c e - W - q 3 q 4 Hadronization products can mix in spacetime: No independent W decay Final State hadrons might interconnect

28 28/53 Interconnection mechanisms Color (Re)connection Between primary quarks Due to soft gluon exchange during the hadronization (nonperturbative) q 1 Statistical Correlations (in particular Bose-Einstein, BEC) Identical pions from the Ws cannot be labeled: concentration in small phase space regions favoured e + W + q 2?? Exogamous correlations (Ellis-Geiger 1997) e - W - q 3 q 4

29 29/53 Reconnection exists somewhere Branching fraction B J/ψ X (Fritzsch 79, Gustafson and Zerwas 85) BEC: shape of the ρ 0 (770) in a multihadron environment (Chliapnikov et al. 92) DELPHI But it has been computed at the perturbative level (Khoze, Gustafson): negligible

30 30/53 CR Models At the hadronization scale (~1fm) models Connection probability decreasing with distance, interference In the string models (Lund, Khoze et al.) extra dimensions to the string P rec V overlap de/dr skin core Lund Type I : skin << core Lund Type II : skin >> core r

31 31/53 (by the way CR could distinguish the ZH -> 4b channel from the ZZ -> 4b) Z Z Z H otherwise an irreducible background

32 32/53 FSI and «W mass» in 4q Truth Measured Value gen level Measured value in MC Corrections for detector effects not well known CR: M W ~ MeV BEC: M W ~ 0-30 MeV Does the concept of W mass in (4q) have a meaning? Particles «belonging to neither W»

33 33/53 A surprising prediction from Bo Andersson Exchange of information is driven by color exchange; two strings fragment independently Hakkinen & Ringner 98, Andersson 98: There could be little or no correlations after all when q 1 E >> Mq e + W + q 2?? e - W - q 3 q 4

34 34/53 Direct measurement... w + w - q q q w + w - q ν Experimental status: q q l M W = M W (4q) - M W (2q lν) = (22 ± 43) MeV/c 2 (stat + syst without FSI) Was 152 ± 74 MeV/c 2 in Moriond δ( M W ) ±25 MeV/c 2 exp d from 2fb => No hope

35 35/53 Observables for CR A set of model-independent observables For whatever observable, a clean experimental technique: the comparison with (2q) -2 The effect of CR should lower the multiplicity in (4q) Look to the interjet regions in particular... E k = 0 E k > 0

36 36/53 Observables for CR: The measurement For realistic models, the decrease in multiplicity is below the experimental sensitivity (~0.5 units) In any case, not observed Also for heavy hadrons Also at small x

37 37/53 Exogamous BE: how to investigate? Models are even more uncertain in this sector No evidence from experimental data (but the estimation of the sensitivity is model dependent)

38 38/53 Partial summary FSI in e + e - WW hadrons are an expected quantum phenomenon, which could affect the W mass in this channel The effect could be important both at the level of color reconnection (soft gluon exchange) and at the level of BEC Predictions for the effect? The effect could be large and O(50 MeV) on M W Observables at the limit of sensitivity...

39 39/53 Oriented flux - I Make optimal use of the information available: study energy and particle flow between jets Associate jets to original quarks (85% purity) 1/N evt dn/dφ 1 particle flow with clusters 189 GeV (preliminary) Plot energy/particle as a function of angle φ wrt most energetic jet Φ p i 10-1 L3 Data - WW qqqq rescaled angle (φ resc )

40 40/53 Oriented flux - II Compare inter- to intrajet distributions 1/E de/dφ(a+b)/(c+d) GeV energy flow (particle level) particles < 1 GeV integration region for R E 0.75 No CR SK I GH rescaled angle (φ resc ) Sensitivity needed for SKI: 0.04 Sensitivity reached so far : 0.12 (stat.)

41 41/53 Mass, again Still modeldependent

42 42/53 As a matter of fact, Andersson could still be right for CR

43 43/53 As a matter of fact, Andersson could still be right for BE as well Warning: very hot topic! The different experiments give different indications

44 44/53 Direct searches for interconnection: summary No signal from model-independent studies No signal of CR from charge multiplicity < n> / <n> WW < 1% No signal from low-p, identified particles Marginal results from the «oriented flux» analyses Also at the ultimate precision, many models can t be tested. If the effect has to be found, New theoretical inputs Look somewhere else

45 45/53 Looking for indirect signatures - I M Z in ZZ With 4 perfect detectors, LEP2 accuracy ~60 MeV A good idea defeated by statistics! Wait for NLC

46 46/53 Looking for indirect signatures - II e + e - Z q q OPAL: 439 events in the full LEP1 statistics Jetset, Herwig w/o CR OK ARIADNE OK w/o CR, 5σ out w/ CR

47 47/53 Looking for indirect signatures - III Bose-Einstein: different strength in quark and gluon jets? (Andersson, De Angelis, Giordani and Vitale 1996)

48 48/53 Interconnection: Thoughts and perspectives No unambiguous signature of interconnection seen, the limit is not statistics. Very little hope to see something along this path Maybe there is no interconnection after all? The largest uncertainty on the W mass is due to a phenomenon which might not exist (and for which we have no indication, apart our belief on QM) FSI is the most important point left for M W The hadronic channel, expected to be the best, is practically unused Interconnection studies are going beyondtheinterestrelatedto QCD and W mass determination Interconnection depends on the environment? What if there is no interconnection? (The experimental answer from ZZ events at NLC?)

49 Two open topics - II Interconnection effects Soft photon radiation

50 50/53 The problem The decay of an unstable particle of width E into charged particles can be thought as the sudden creation of rapidly moving charges in a time h/ E. Such a variation of the em field is accompanied by the emission of a soft FSR Investigations between 1976 and 1995 in hh collisions. (E. Amaldi & al., Darriulat & al., Sonderegger & al., Chliapnikov & al.) Generally, an excess of a factor 3 to 6 Something fundamental?

51 51/53 Maybe something to look at in e+e- Where things are clean But: needs accurate use of VD Photons below 10 MeV to 70 MeV OR s/b ~ 1/10 First indications! p ~ 20 MeV/c

52 52/53 From LEP to new experiments PLEASE ARCHIVE And do it in the simplest possible way! 4-vectors are not a shame

53 53/53 Conclusione C è ancora gente che chiede una tesi sugli eventi adronici a LEP giustamente. ARCHIVIAMO PER IL FUTURO! (e in modo da poter fare l analisi sul nostro portatile)