LHCC Poster Session - CERN, 4 March 2015 Two–particle …...‣ High Multiplicity (HM) trigger (...
1
Elena Plotnikova, for the ATLAS Collaboration Two–particle Bose–Einstein Correlations Two–particle Bose–Einstein Correlations in in pp pp –collisions at –collisions at √s √s = 0.9 and 7 TeV = 0.9 and 7 TeV measured with the ATLAS detector measured 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(n ch ) 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 Summary Summary The two–particle correlation function: The double R 2 (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 R 2 (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 C 2 ( p 1 ,p 2 ) = ρ( p 1 ,p 2 ) ρ 0 ( p 1 ,p 2 ) the two–particle density functions the "Reference sample" without BEC Q= √ −( E 1 −E 2 ) 2 +( p 1 − p 2 ) 2 R 2 ( Q )= C 2 ( Q ) C 2 MC ( Q ) = ρ(++ , −−) / ρ(+−) ρ MC (++ , −−) / ρ MC (+−) the track pair 4–momentum difference the like–sign and unlike–sign charged–particle pairs distributions aaaaaa ⏟ R 2 ( Q ) =C 0 ( 1 + λ e −R Q ) ( 1 +ϵ Q) a normalization factor the incoherence factor (0÷1) the effective radius of the source size accounts for the long–range momentum correlations R 2 (Q) correlation functions A clear signal of BEC is observed in the region of small 4–momentum difference Q . ↑ Figure 1. The R 2 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 R 2 UCP (Q) exponential fits: 0.9 TeV : λ = 0.74 ± 0.11, R = 1.83 ± 0.25 [fm] for n ch ≥ 2 7 TeV MB: λ = 0.71 ± 0.07, R = 2.06 ± 0.22 [fm] for n ch ≥ 2 7 TeV HM: λ = 0.52 ± 0.06, R = 2.36 ± 0.30 [fm] for n ch ≥ 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. Introduction Introduction ↑ Figure 3. k T dependence of λ and R Comparison R(k T ) with: ∙ C 2 MixEv (Q) E735 [8] and ∙ C 2 MixEv (Q) STAR [9] experiments Main results: ∙ As k T increases λ and R decrease ∙ Within the uncertainties λ and R are energy–independent k T dependence ↑Figure 4. n ch & k T dependencies of λ and R k T = | p T, 1 + p T, 2 | 2 n ch & k T dependencies of 7 TeV MB n ch dependence of the BEC parameters The average transverse momentum k T of the pair of particles: ↑ Figure 2. n ch dependence of λ and R The result of the p 0 3 √n ch fit for n ch ≤ 55: ‣ 0.9 TeV: p 0 = 0.64 ± 0.07 [fm] ‣ 7 TeV MB: p 0 = 0.60 ± 0.05 [fm] The result of the Constant fit for n ch ≥ 55: ‣ 7 TeV MB + HM: p 0 = 2.28 ± 0.31 [fm] Main results: ∙ As n ch 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 n ch 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 R 2 MixEv (Q) of CMS [5] and ∙ The C 2 MixEv (Q) of UA1 [6] experiments — good agreement up to n ch ≅ 50 The results of the p 0 3 √n ch fit are close to the analogous p 0 3 √n ch CMS fit results [5]. The total errors are presented. The statistical errors for p 0 ~ 1 – 5%. Main results: ∙ The behaviour is similar to the k T depende- nce of λ and R ∙ R values increase with n ch , in agreement with the feature observed in Figure 2 ↘ Comparison R(n ch ) 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
Transcript of LHCC Poster Session - CERN, 4 March 2015 Two–particle …...‣ High Multiplicity (HM) trigger (...
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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
