Test of color-reconnection models in Z 3 jets G.Rudolph - - PowerPoint PPT Presentation

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Test of color-reconnection models in Z 3 jets G.Rudolph - - PowerPoint PPT Presentation

May 07, 2023 339 likes •489 views

Test of color-reconnection models in Z 3 jets G.Rudolph Inst.f.Experimentalphysik, Uni Innsbruck Fachtagung Kern- u. Teilchenphysik, Weyer, 26 Sept 2004 Motivation: precise measurement of M W at LEP-2 (to better than 5 10 4 ) Color

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SLIDE 1

Test of color-reconnection models in Z → 3 jets

G.Rudolph Inst.f.Experimentalphysik, Uni Innsbruck Fachtagung Kern- u. Teilchenphysik, Weyer, 26 Sept 2004

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SLIDE 2

Motivation: precise measurement of MW at LEP-2 (to better than 5 · 10−4)

Color reconnection (CR) can modify the hadronic final state in the process e+e− → W +W − → (q¯ q)(q¯ q) and thus the reconstructed W mass. CR contributes the largest systematic uncertainty (90 MeV at present). Direct information from

  • particle momentum spectra
  • interjet particle flow (LEP comb.)
  • MW (q¯

qq¯ q) − MW (lνq¯ q) (LEP comb.)

  • differential behaviour of MW with Pcut and Cone (new development)

shows no convincing evidence for CR. Data consistent with no CR. Data not sensitive enough to test all models. Only extreme models (SK1 100%) can be excluded.

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SLIDE 3

How to calculate CR effects ?

Perturbative QCD prediction for WW is of order ((αs/NC)2 · ΓW /MW ), from which δMW = few MeV (negligible). Possible non-pert. CR ⇒ phenom. models

General case : multiple gluon emission

Leading Log parton shower determines a color sequence (in the large NC limit) for drawing the string : R − ¯ RG − ¯ GB − ¯ BG − ¯ GR − ¯ R.... Some models assume that non-perturbative rearrangement is possible among identical colors, with probability 1/N 2

C,

and takes place if the total string length (e.g. λ) decreases. Leads to effects also within color singlet systems.

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SLIDE 4

CR Monte Carlo models :

All but the last run with Pythia

model criterion of free effect in reconnection parameter value Z → q¯ q SK1 space-time overlap kI 0.6 not

  • f flux tubes

implemented SK2 crossing of

  • NO

vortex lines ARIADNE reduce total Preco 1/9 Y AR1, AR2 string length λ GAL reduce area with prob. R0 0.1 Y (Rathsman) P = R0(1 − exp(−b∆A)) HERWIG reduce cluster size Preco 1/9 Y in space-time

Predicted MW bias < 100 MeV for reasonable parameter values

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SLIDE 5

Test of CR in Z → hadrons :

possible ? CR effect localised in gluon jet because it often fragments in isolation = ⇒ select 3 jet events Sensitive variables :

  • Interjet particles (L3, PLB 581 (2004) 19)

problem: no-CR Jetset and Ariadne predictions differ

  • Particle momentum spectra in jets :

not clear what is tested.

  • Rapidity gaps, Jet charge ⇒ clear signal

(OPAL, Eur.Phys.J.C35 (2004) 293, and Eur.Phys.J.C11 (1999) 217)

String drawing :

normal reconnected

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SLIDE 6

ALEPH data analysis :

From 3.4 million multihadronic Z events (LEP-1, 1992-1995) select 3-jet events using Durham cluster algorithm, with resolution ycut = 0.02, ⇒ R3 = 0.228

Energy-ordered analysis :

Order the jets as x1 > x2 > x3, with xj = 2Ej/Ecm More cuts : require Φjk > 40◦, x3 > 0.1, | cos Θj| < 0.9 → 539000 events

< Ejet > gluon purity GeV Pg (from MC) jet 3 17.7 0.69 gluon enriched jet 1 40.8 0.06 quark enriched

Rapidity : refers to respective jet axis

Charged particles: pion mass assumed; Neutral particles: pseudorapidity used.

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SLIDE 7

Multiplicity distributions in fixed rapidity interval (0 - 1.5) : Rate of jet 3 with a gap (7% in data) is sensitive to CR, but not used as observable.

(data, Jetset, GAL) (data, AR0, AR1)

ID Entries 3022331 538775

N(c+n), y in 0 - 1.5, jet 3

ID Entries 3022331 538775

N(c+n), y in 0 - 1.5, jet 3

ID Entries 3022311 538775

N(c+n), y in 0 - 1.5, jet 1

ID Entries 3022311 538775

N(c+n), y in 0 - 1.5, jet 1

10000 20000 30000 40000 50000 60000 70000 80000 2 4 6 10000 20000 30000 40000 50000 60000 70000 80000 2 4 6 10000 20000 30000 40000 50000 60000 70000 80000 90000 2 4 6 10000 20000 30000 40000 50000 60000 70000 80000 90000 2 4 6

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SLIDE 8

Jet charge Qj =

i qi

distributions, normalized to same area, for all jets and for those with a rapidity gap (i.e. no particles in 0 ≤ y ≤ yu, with yu = 1.5)

Several factors determine Qj :

  • charge compensation in fragment.
  • cluster algorithm
  • jet environment
  • detection effects

Fraction f(Qj = 0) of neutral gluon jets is sensitive to CR !

data, Jetset, GAL

ID Entries Mean RMS 3020032 538775 0.4268E-01 1.595

Q(jet 3), all

ID Entries Mean RMS 3122332 39037 0.1885E-01 1.055

Q(jet 3), y gap

ID Entries Mean RMS 3020012 538775 0.5470E-01 1.463

Q(jet 1), all

ID Entries Mean RMS 3122312 42307 0.1066E-01 0.9716

Q(jet 1), y gap

200 400 600 800 1000 1200 1400 1600 1800 x 10 2

  • 5
  • 2.5

2.5 5 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

  • 5
  • 2.5

2.5 5 200 400 600 800 1000 1200 1400 1600 1800 x 10 2

  • 5
  • 2.5

2.5 5 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

  • 5
  • 2.5

2.5 5

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SLIDE 9

Similar for data vs. Ariadne

data, AR0, AR1

ID Entries 3020032 538775

Q(jet 3), all

ID Entries 3122332 39037

Q(jet 3), y gap

ID Entries 3020012 538775

Q(jet 1), all

ID Entries 3122312 42307

Q(jet 1), y gap

200 400 600 800 1000 1200 1400 1600 1800 x 10 2

  • 5
  • 2.5

2.5 5 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

  • 5
  • 2.5

2.5 5 200 400 600 800 1000 1200 1400 1600 1800 x 10 2

  • 5
  • 2.5

2.5 5 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

  • 5
  • 2.5

2.5 5

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SLIDE 10

Result for relative model-data difference δ = f(Qj = 0)MC − f(Qj = 0)data f(Qj = 0)data as a function of yu : Data vs Jetset and Ariadne :

  • jet 1: well described
  • jet 3: CR models clearly disfavoured.
  • jet 3: interesting effect (??) :

More neutral jets with a y-gap seen than expected without CR. Also reported in Delphi 2002-053. Data vs Herwig :

  • cannot describe jet 1 ⇒ doubtful MC
  • no CR sensitivity in jet 3

model-data difference

JETSET ARIADNE JETSET+GAL ARIADNE AR1

yu of gap (c+n), jet 3 δ

HERWIG HERWIG CR

yu of gap (c+n), jet 3 δ

JETSET ARIADNE JETSET+GAL ARIADNE AR1

yu of gap (c+n), jet 1 δ

HERWIG HERWIG CR

yu of gap (c+n), jet 1 δ

  • 0.1
  • 0.05

0.05 0.1 0.15 0.2 1 2

  • 0.1
  • 0.05

0.05 0.1 0.15 0.2 1 2

  • 0.1
  • 0.05

0.05 0.1 0.15 0.2 1 2

  • 0.1
  • 0.05

0.05 0.1 0.15 0.2 1 2

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SLIDE 11

Systematic checks:

Variation of the jet definition :

  • jet resolution parameter ycut varied from 0.005 to 0.05
  • a different jet finder : Durham =

⇒ Jade

  • re-assigning particles to jets on basis of smallest angle
  • define y-gap with charged only

⇒ result does not change qualitatively Systematics from detector simulation :

  • varied track selection cuts and | cos Θj| cut ⇒ δ remains within 1 σ
  • event charge Qev =

i qi (= 0 ideally) used as a test quantity

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SLIDE 12

B-tag analysis :

select Z → b¯ bg events by requiring lifetime signals in 2 jets and no signal in 1 jet ⇒ the gluon jet High purity : Pg = 97 % < Ejet >= 19.8 GeV Less statistics : 24600 events Effects numerically larger. Same result, but statistically inferior to the energy-

  • rdered analysis

JETSET ARIADNE JETSET+GAL ARIADNE AR1

yu of gap (c+n), gluon jet δ = (f(Q=0)MC - f(Q=0)data) / f(Q=0)data

  • 0.3
  • 0.2
  • 0.1

0.1 0.2 0.3 0.4 0.5 1 1.5 2 2.5

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SLIDE 13

Discussion

The CR models AR1 and GAL fail to describe gluon jet data at the Z (confirming L3 and OPAL results) Does this have consequences for WW ?

  • Yes, according to the above models, the physics is the same
  • Not necessarily, according to Sj¨
  • strand

Note also: ALEPH does not see Bose-Einstein correlations between pions from different W’s. My conclusion: large CR effects are unlikely