Eventi Adronici a LEPAlessandro De Angelis

Università di Udine and INFN Trieste

1. Many things have been done…Phenomenology of hadronic eventsQCD and the color structure of hadronic events αs

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

Lecc

e, A

pril

2003

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Hadronicevents at LEP(1989-2000†)70-208 GeV

Plus: High quality detectorsdataanalysis tools and methods theory

Ecm(GeV) kEvts>.85Ecm

M(Z) 17000133 3.6161 1.4172 1.2183 5189 15192-202 18204-208 16

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p

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)

Hadronization effects suppressed as 1/Q(n)

Anatomy of an annihilation event

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LEP has been the benchtest for pQCD

Between 1989 and today, > 250 papers on hadronic physics at LEP

Very important contributions toEstablishing pQCDDeveloping the intermediate regime (LPHD)Tuning the fragmentation MC models

And more (power corrections)Exploring the fully nonperturbative region

5/53 And the story is not over: many more papers (~50) still to be finalized

αs in 4-jets eventsAngular distr. in 4-jet eventsRunning mb

Fragm. functions and αs

Multiplicity in 3-jet events Gluon fragmentation

Soft photon excess in qq

W hadronic decaysCR in WW BEC in WW

QCD overview paper

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The LEP contribution: establishing pQCD as part of the SM

Hadronization effects suppressed as 1/Q(n)

7/53 Establishing pQCD:color factors

TF

CF CF

CA

8/53

The one parameter

e+

e-

Z/γ*

q

q

gsα∝

Born x-section for Z qq

gluon

s

gluon

qqg

EdEd 1

20 πασσ

∝

“Bremsstrahlung”

9/53 Experimental procedure to measure the one parameter

Calculate perturbative predictionsCalculate 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, Mh2, Cpar, Btot, Bw, lny3 , ...

Dat

aT

heor

y

Conventionally computed at m(Z)

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Thrust..

3 jet region T << 1

2jet regionT >> 0.5perturbative

= energetic gluons

non-perturbative=soft gluons

1/2 1

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Combination by observable

12/53 Combination by energy

Using only LEPLEP I: αs(MZ) =0.1197

±0.0002(stat)±0.0008(ex)±0.0010(had)±0.0048(th)

LEP II: αs(MZ) =0.1196±0.0005(stat)±0.0010(ex)±0.0007(had)±0.0044(th)

All-energies doesn’t beat LEP II <= high correlation of theory uncertainty

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The one parameter and its uncertainty

The value is stable

αs ~ 0.120

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χ2/dof = 11.6/13

Runningestablished

1989

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

16/53 …momentum spectrum…

17/53

…also for q/g jets separately…

18/53 …and many very nonperturbativefeatures modeled/understood (?)

Amazingly accurate thermodynamics-inspired 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)

Two open topics - I

Interconnection effects

Soft photon radiation

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W mass

The present LEP accuracyon MW is±42 MeV=> 114.4 < mH < 211 GeV (95% CL)

In the absence of Higgssignal,MW determination

Within the SM, link betweenMW, Mt and mH

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W production at LEP 2

Well modeled by SM; pair production dominates the cross section

e+

e-

νe

w+

w-

ff

ff

e+

e- ff

ff

Z/γw+

w-

Year 1996 1997 1998 1999 2000 total/exp.√s(GeV) 161-172 183 189 192-202 203-209 161-209L (pb-1) 20 55 170 230 220 ~700

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W decay properties

WW branching~ 4/9 in qqqq, ~ 4/9 in qq lν

The W is a goodQCD object in itshadronic decay

19.4 0.3chW

n = ±

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W pair selection

Based onIn qqqq (hadronic, 4q)4 jets ~back-to-backIn qq lν (semileptonic, 2q)2 jets , 1 lepton and pmiss

Performance:4q, eff. ~80%, pur. ~80%2q, eff. ~70%, pur. ~90%

Topological criteriastill OK @ 208 GeV

The 4 LEP collaborations analyzed ~ all data

WW->4q WW-> 2qlν

WW ~20000 ~16000(4 exp)

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W mass @ LEP2 (4q)

The most precisedetermination might comefrom (4q)

Larger statisticsMore constrained

Systematics-dominated

Due to FSI sustematics, ourdetermination (δMW ~42 MeV) comes almost exclusively from(2q)

Hope δMW ~30 MeV if we cancontrol FSI

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WHAT ARE FSI ?

26/53 (Anatomy) simplified view of the system e+e- → W+W-→ qqqq

Hadronic models: separation between1. ElectroWeak phase2. « Hard QCD » phase 3. « Soft QCD » phase 4. Decays

chn ~ 39e+

e-

W+

W-

q1

q2

q3

q4

27/53 Limits of the simplified view:QM-inconsistent ?

Hadronization products can mix in spacetime:• No independent W decay• Final State hadrons might interconnect

Separation between Ws~ /ΓW ~ 0.1 fm/c

Hadronization radiusτhad ~ 0.5-1.0 fm/c

h

e+

e-

W+

W-

q1

q2

q3

q4

? ?

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Interconnection mechanisms

Color (Re)connectionBetween primary quarksDue to soft gluon exchange during thehadronization (non-perturbative)

Statistical Correlations(in particular Bose-Einstein, BEC)

Identical pions from the Wscannot be labeled: concentration in small phase space regions favoured

“Exogamous” correlations (Ellis-Geiger 1997)

e+

e-

W+

W-

q1

q2

q3

q4

? ?

29/53 Reconnection existssomewhere

Branching fractionB → J/ψ X(Fritzsch 79, Gustafson and Zerwas 85)

BEC: shape of the ρ0(770) in a multihadron environment(Chliapnikov et al. 92)

DELPHI

But it has been computed at the perturbative level (Khoze, Gustafson): negligible

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CR Models

At the hadronization scale (~1fm) ⇒ models

Connection probability decreasing withdistance, interference…

In the string models (Lund, Khozeet al.) extra dimensions to thestring

Prec ∝ Voverlap

Lund “Type I”: skin << coreLund “Type II”: skin >> core

dE/dr

r

“core”

“skin”

31/53 (by the way CR could distinguish the ZH -> 4b channel from the ZZ -> 4b)

Z

Z

ZH

otherwise an irreducible background

32/53 FSI and «W mass» in 4q

Corrections for detectoreffects not well known

Does the concept of W mass in (4q) have a meaning?Particles«belonging to neither W»

MC @gen level

Measured valuein MC

“Truth” MeasuredValue

⇒ CR: ∆MW ~ 0-300 MeVBEC: ∆MW ~ 0-30 MeV

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 E >> Mq

e+

e-

W+

W-

q1

q2

q3

q4

? ?

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Direct measurement...

Experimental status:

∆MW = MW(4q) - MW(2q lν) = (22 ± 43) MeV/c2

(stat + syst without FSI)

qq

qq

w+

w-

νl

qq

w+

w-

Was 152 ± 74 MeV/c2 in Moriond 2000...

δ(∆MW) ±25 MeV/c2 exp’d from 2fb-1 @Run2 => No hope

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Observables for CRA set of model-independent observables

For whatever observable, a clean experimental technique: thecomparison with (2q)

The effect of CR should lower themultiplicity in (4q)

Look to the interjet regions in particular...

Ek = 0 Ek > 0

-2

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Observables for CR: The measurement

For realistic models, the decrease in multiplicity is belowthe experimental sensitivity (~0.5 units) In any case, not observed

Also for heavy hadronsAlso at small x

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“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)

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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 MW

Observables at the limit of sensitivity...

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10-1

1

0 1 2 3 4rescaled angle (φresc)

1/N

evt d

n/dφ particle flow with clusters

189 GeV (preliminary)

L3 Data- WW → qqqq

Oriented flux - I

Make optimal use of the information available: study energy and particle flow between jetsAssociate jets to original quarks (85% purity)Plot energy/particle as a function of angle φ wrt most energetic jet

Φ

pi

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0.75

1

1.25

1.5

1.75

0 0.2 0.4 0.6 0.8 1rescaled angle (φresc)

1/E

dE

/dφ(

A+

B)/

(C+

D)

189 GeV energy flow (particle level) particles < 1 GeV

No CRSK IGH

Compare inter- to intrajet distributions

integration region for RE

Sensitivity needed for SKI: 0.04 Sensitivity reached so far : 0.12 (stat.)

Oriented flux - II

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Mass, again

Still model-dependent

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

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

44/53 Direct searches for interconnection: summary

No signal from model-independent studiesNo signal of CR from charge multiplicity

|<∆n>| / <n>WW < 1% No signal from low-p, identified particles

Marginal results from the «oriented flux» analysesAlso at the ultimate precision, many models can’t be tested. If the effect has to be found,

New theoretical inputsLook somewhere else

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Looking for indirect signatures - I

MZ in ZZ

With 4 perfect detectors, LEP2 accuracy ~60 MeV

A good idea defeated by statistics !

Wait for NLC

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Looking for indirect signatures - II

OPAL: 439 events in thefull LEP1 statisticsJetset, Herwig w/o CR OKARIADNE OK w/o CR, 5σ out w/ CR

e+

e-

Zq

q

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Looking for indirect signatures - III

Bose-Einstein: different strength in quark and gluon jets? (Andersson, De Angelis, Giordani and Vitale 1996)

48/53 Interconnection:Thoughts and perspectives

No unambiguous signature of interconnection seen, the limit is notstatistics. 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 mightnot exist (and for which we have no indication, apart our belief on QM)

FSI is the most important point left for MWThe hadronic channel, expected to be the best, is practically unused

Interconnection studies are going beyond the interest related to QCD and W mass determination

Interconnection depends on the environment?What if there is no interconnection ?

(The experimental answer from ZZ events at NLC?)

Two open topics - II

Interconnection effects

Soft photon radiation

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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 FSRInvestigations 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?

51/53 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 ~ 1/10First indications!

p ~ 20 MeV/c

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From LEP to new experiments

PLEASE ARCHIVE

And do it in the simplest possible way !4-vectors are not a shame…

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Conclusione

C’è ancora gente che chiede una tesi suglieventi adronici a LEP…

…giustamente.

ARCHIVIAMO PER IL FUTURO !(e in modo da poter fare l’analisi sul nostro portatile)