Bose-Einstein and Fermi-Dirac Correlations at LEP and HERA

Bose-Einstein and Fermi-Dirac Correlations at LEP and HERAIgnacio Aracena University of Bern

Outline Introduction Bose-Einstein and Fermi-Dirac Correlations –One-dimensional analysis –Multidimensional analysis Bose-Einstein Correlations in hadronic WW decays Conclusions

IntroductionBose-Einstein (Fermi-Dirac) Correlations: Enhanced (suppressed) probability for the emission of identical bosons (fermions) with similar momenta. Provides information about space-time characteristics of the particle emission regionThe two-particle correlation function is given by R2(Q)

2 ( p1 , p2 ) : Two-particle density distribution

ρ2(p1 ,p2 ) , with Q ( p1 p2 ) 2 . ρ0(p1 ,p2 )

The reference sample 0 ( p1 , p2 ): Should contain all features which are present in data, except BEC/FDC. Should not contain additional features. Is the main experimental problem. used techniques: Mixed events, unlike-sign, MC, ...Detector inadequacies, effects introduced by the reference sample are obtained from MC without BEC/FDC:MC MC MC R2 N / N ref

R2Data The measured correlation function is given by: C 2 MC R217.8.2004 I. Aracena 2

BEC at LEP – one-dimensional resultsWhy study BEC in one dimension again? During LEP1 run >~ 106 hadronic Z decays Statistically precise measurement and possibility to study BEC invarious particle species.C2mix (Q )

Standard parameterisation C 2 (Q) N [1 exp( ( RQ)2 )](1 Q ...) λ coherence strength factor. R is related to the size of the boson emission region. ALEPH – BEC in pairs: Eur. Phys. J. C36: 2004, 147

C 2 (Q )

Mixed events λ = 0.362±0.001(stat) R = 0.528±0.006(stat) fm χ2/ndf = 513 / 94

Unlike-sign λ = 0.438±0.001(stat) R = 0.777±0.007(stat) fm χ2/ndf = 432 / 72

2 1.8 1.6 1.4 1.2 1 0.8

2.2 2 1.8 1.6 1.4 1.2 1 0.8 1.6 1.4 1.2 1

BEC in

ALEPH

BEC in

0 pairs KS

ALEPH

0.8

BEC in K0K0 pairs (submitted to Phys.Lett.B) Mixed ref. sample R = 0.57 ± 0.04(stat) ± 0.14(sys) fm χ2/ndf = 0.68 MC reference sample

0 0.2 0.4 0.6 0.8 1

0

0.2

0.4

1.2 1.4 1.6 1.8 2 Q (GeV) Q (GeV)

0.6

0.8

1

1.2

main contribution to systematic error

Discrepancy between results obtained from different reference samples

Gaussian does not describe data at low Q217.8.2004 I. Aracena 3

BEC at LEP – one-dimensional resultsFragmentation models currently used:Lund string model: A larger BEC strength and a smaller effective radius of the source for Cluster fragmentation model: No difference between BEC in OPAL Coll. Phys. Lett. B568 (2003) 181 0 BEC in pairs mixed reference sample

0compared to .

0and pairs.C 2 (Q )2

λ = 0.55 ± 0.10(stat) ± 0.10(sys) R = (0.59 ± 0.08(stat) ± 0.05(sys)) fmBEC in pairs: Opal Eur. Phys. J. C16 (2000) 423 unlike-sign reference sample

1.5

1

R = (1.002 ± 0.016(stat) +0.023/ 0.096(sys)) fm

0.50 0

.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 Q (GeV)

No sensitivity to 0 from string Cannot test hadronisation model predictions. BEC exist in 0 pairs in which each 0 is a decay product of a different hadron.

Comparison with previous results difficult due to different reference sample and selection cuts.

17.8.2004

I. Aracena

Fermi-Dirac Correlations at LEPParameterisation of the FD correlation function: C2 (Q) N[1 exp( ( RQ ) 2 )](1 Q ...) The λ parameter contains information about the incoherence of the source and the spin composition of the pair. ALEPH (sub. to Phys.Lett. B): FDC in pp and pp pairs 3526 pp, pp pairs in the region 0 < Q < 10 GeV 74% of pairs from the primary vertex for Q < 5 GeV 65% of pairs for Q < 10 GeV1.2 1 0.8 0.6 0.4 0.2 0 0

DELPHI χ2/ndf = 18/23

pp ppQ (GeV)

DELPHI: FDC in pp pairs

ALEPH

χ2/ndf = 0.40

955 pp pairs 70% purity for pp pairs

1

2

3

4

5

6

7

8

9 10 Q (GeV)

λ ALEPH DELPHI 0.49±0.04(stat)±0.08(sys) 0.67+0.19/ 0.17(stat)±0.18(sys)

R (fm) 0.11±0.01(stat)±0.01(sys) 0.16±0.04(stat)±0.03(sys)

p17.8.2004 I. Aracena 5

One-dimensional BEC and FDC at LEPBEC and FDC have been studied at LEP1 in a large variety of particle species: 0 , , K 0 , K , p ,

Unlike-sign ref. sample

Mixed ref. sample p π K Λ

MC ref. sample

Results indicate a smaller correlation radius for baryons than for mesons: Correlation radius is a function of the particle mass, R(m).

17.8.2004

I. Aracena

BEC in ep neutral current DIS at HERAMotivation: In DIS the production volume may depend on the virtuality of the exchanged photon

Q 2.

Study differences of BEC between the current and the target regions. Studies performed in the Breit frame: momentum of the exchanged photon q = (0,0,0, Qγ = 2xPBreit) Allows separation of outgoing struck quark from the proton remnant ZEUS: Phys.Lett.B583:2004, 231 0.1 <

Q 2 < 8000 GeV2

121 pb-1 assume particles are charged pions unlike-sign reference sample BEC studied in current and target region

λ = 0.475±0.007+0.011/ 0.003 R = 0.666±0.009+0.022/ 0.036 fm

The BEC function measured at HERA shows no dependence on the virtuality of the exchanged photon No significant difference between the BE effects in the current and the target regions. HERA results are compatible with LEP results. 17.8.2004 I. Aracena

Q 2.

Multidimensional BEC analysesMotivation Test predictions of the Lund string model (M. Ringner, B. Andersson, Phys. Lett. B421: 1998, 283):

The longitudinal size of the boson source is bigger than the transve …… 此处隐藏：8938字，全部文档内容请下载后查看。喜欢就下载吧 ……