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Elena Plotnikova, for the ATLAS Collaboration

Two–particle Bose–Einstein CorrelationsTwo–particle Bose–Einstein Correlations in in pppp–collisions at –collisions at √s√s = 0.9 and 7 TeV = 0.9 and 7 TeV measured with the ATLAS detectormeasured with the ATLAS detector

LHCC Poster Session - CERN, 4 March 2015

[1] ATLAS Collaboration, JINST 3, S08003 (2008)[2] ATLAS Collaboration, New J. Phys. 13, 053033 (2011)[arXiv:1012.5104][3] ATLAS Collaboration, Phys. Lett. B 688, 21 (2010) [arXiv:1003.3124][4] T. Sjostrand, S. Mrenna, P.Z. Skands, J. High Energy Phys. 05, 026 (2006) [hep-ph/0603175][5] CMS Collaboration, J. High Energy Phys. 05, 029 (2011) [arXiv:1101.3518][6] C. Albajar et al. (UA1 Collaboration), Phys. Lett. B 226,410 (1989)[7] M. G. Ryskin and V. A. Schegelsky, Nucl. Phys. Proc.Suppl. 219-220, 10 (2011)[8] T. Alexopoulos et al., Phys. Rev. D 48, 1931 (1993)[9] M.M. Aggarwal et al. (STAR Collaboration), Phys. Rev. C 83, 064905 (2011) [arXiv:1004.0925]

References

The multiplicity dependence of the BEC parameters characterizing the correlation strength λ and the correlation source size R is investigated at charged–particle multiplicities of up to 240. As n

ch

increases λ decreases and R increases. The saturation effect in the R(nch

) dependence is

observed using the high–multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair k

T is also investigated. As k

T increases λ and R are decrease. The same behaviour for the n

ch & k

T

dependencies is observed, and R becomes larger for increasing charge multiplicity, consistently with the results of the R(n

ch) dependence.

Minimum Bias Trigger Scintillators (MBTS) [1]: ∙ 32 independent wedge–shaped plastic scintillators (16 per side of the ID) ∙ read out by PTMs ∙ 2.09 < |η| < 3.84 ∙ for triggering on min bias events, > 99% efficiency

The MBTS are used for triggering events in 0.9 TeV and 7 TeV analyses.

MBTS trigger

Study based on the pp-collision datasets at the centre-of-mass energies √s = 0.9 and 7 TeV [2,3] and MC samples: MC09 PYTHIA6 (main)[4], Perugia0 PYTHIA, PhoJet and EPOS. The integrated luminosities are 7 ∼ μb−1,

∼ 190 μb−1, 12.4 ∼ nb−1 for 0.9 TeV, 7 TeV Minimum Bias and 7 TeV High Multiplicity, res-pectively.

Main track and event selection criteria: ∙ p

T ≥ 100 MeV, |η| < 2.5

∙ At 0.9 TeV, 7 TeV MB: ‣ Min Bias (MB) trigger relies on the MBTS ‣ one primary vertex with ≥ 2 tracks

∙ At 7 TeV HM: ‣ High Multiplicity (HM) trigger ( ≥ 124 tracks with p

T > 400 MeV on a single vertex at the trigger level)

‣ one primary vertex with ≥ 108 tracks

The data are corrected for detector effects, such as the resolution and inefficiencies, etc.

Data and MC samples

SummarySummary

The two–particle correlation function:

The double R2(Q) correction function:

The "natural choice" for ρ(+−) — the unlike–charged particles pairs of the event (UCP). The C

2

MC(Q) is used in the R2(Q) Ratio to

correct ρ(+−) to minimise effect of resonances and doesn't include BEC effects.

The exponential function of a static source in the plane–wave approach:

Analysis

the 4–momenta of two identical bosons in the event

C2(p1 , p2)=ρ( p1 , p2)

ρ0( p1 , p2)

the two–particle density functions

the "Reference sample" without BEC

Q=√−(E1−E2)2+( p⃗1− p⃗2)

2

R2(Q)=C2(Q)

C2MC

(Q)=

ρ(++,−−)/ρ(+−)

ρMC

(++,−−)/ρMC

(+−)

the track pair 4–momentum differencethe like–sign and unlike–sign

charged–particle pairs distributions

aaaaaa⏟R2(Q)=C0(1+λ e−R Q)(1+ϵQ)

a normalization factor

the incoherence factor (0÷1)

the effective radius of the source size

accounts for the long–range momentum correlations

R2(Q) correlation functions

A clear signal of BEC is observed in the region of small 4–momentum difference Q.

↑ Figure 1. The R2

UCP(Q) correlation functions

at 0.9 TeV, 7 TeV MB and 7 TeV HM

The bump at 0.5 ≤ Q ≤ 0.9 GeV is due to an overestimation in the MC simulation for ρ meson (more decays).

λ and R of the R2

UCP(Q) exponential fits:

0.9 TeV : λ = 0.74 ± 0.11, R = 1.83 ± 0.25 [fm] for nch

≥ 2

7 TeV MB: λ = 0.71 ± 0.07, R = 2.06 ± 0.22 [fm] for nch

≥ 2

7 TeV HM: λ = 0.52 ± 0.06, R = 2.36 ± 0.30 [fm] for nch

≥ 150

BEC

BEC

BEC

The Bose–Einstein correlations (BEC) lead to an enhancement of the production of identical bosons that are close together in phase space. The measurement of the two–particle BEC allows to study characteristics of the boson source and hadronization features.

IntroductionIntroduction

↑ Figure 3. kT dependence of λ and R

Comparison R(kT) with:

∙ C2

MixEv(Q) E735 [8] and

∙ C2

MixEv(Q) STAR [9] experiments

Main results:

∙ As kT increases λ and R decrease

∙ Within the uncertainties λ and R are energy–independent

kT dependence

↑Figure 4. nch

& kT dependencies of λ and R

kT=| ⃗pT ,1+ ⃗pT ,2|

2

nch

& kT dependencies of 7 TeV MB

nch

dependence of the BEC parameters

The average transverse momentum kT of the

pair of particles:

↑ Figure 2. nch

dependence of λ and R

The result of the p0

3√nch

fit for nch

≤ 55:

‣ 0.9 TeV: p0 = 0.64 ± 0.07 [fm]

‣ 7 TeV MB: p0 = 0.60 ± 0.05 [fm]

The result of the Constant fit for nch

≥ 55:

‣ 7 TeV MB + HM: p0 = 2.28 ± 0.31 [fm]

Main results:

∙ As nch

increases λ decreases and

R increases ∙ Within the uncertainties λ and R are energy–independent ∙ R exhibits a saturation at high–multiplicities and for n

ch > 55 is inde-

pendent of the multiplicity

↙ The saturation of R for high–multiplicity particles is observed for the first time.

A bit of theory

The saturation of R at high nch

predicted by the Pomeron–based mo-del [7], due to highly overlapping of colliding protons, i.e. at n

ch ≈ 70, re-

markably close to the one obtained in our studies (see the Constant fit result for n

ch ≥ 55). However, the

model predicts R decreases with multiplicity for n

ch ≥ 70 in contradiction

with our analysis.

∙ The R2

MixEv(Q) of CMS [5] and

∙ The C2

MixEv(Q) of UA1 [6] experiments

— good agreement up to nch

≅ 50

The results of the p0

3√nch

fit are close to

the analogous p0

3√nch

CMS fit results [5].

The total errors are presented. The statistical errors for p0 ~ 1 – 5%.

Main results:

∙ The behaviour is similar to the kT depende-

nce of λ and R ∙ R values increase with n

ch, in agreement

with the feature observed in Figure 2

↘ Comparison R(nch

) with:

The total errors are shown. The statistical errors for λ ~ 3 – 8%, R ~ 2 – 6%. The leading source of systematic error (> 50%) is due to differences in the MC generators used, for λ ~ 5 – 14.6%, R ~ 8 – 13%.

ρ→π+π

−

Main progress

Reference:

arXiv:1502.07947