NSD_P6
5.4 Forward-backward correlations
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Figure 5.12: Forward-backward multiplicity correlation plots for standalone PYTHIA simulated pp events at √
s = 10 TeV. The forward and backward η intervals are both 0.2 units wide. The results for the η windows centered at ±0.1, ±0.3, and ±0.5 are shown in the upper, middle and bottom pan-els, respectively as correlation plots (left) and its fitted projections (right).
Pseudorapidity gaps ∆η are defined from the center of the forward interval to the center of backward interval. No pt cut is applied.
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Figure 5.13: Forward-backward multiplicity correlation plots for the Monte Carlo Truth (MCT) of reconstructed tracks (ESD) of ALICE simulated pp events at √
s = 10 TeV. The forward and backward η intervals are both 0.2 units wide. The results for the η windows centered at ±0.1, ±0.3, and ±0.5 are shown in the upper, middle and bottom panels, respectively as correlation plots (left) and its fitted projections (right). Pseudorapidity gaps ∆η are defined from the center of the forward interval to the center of backward interval. No pt cut is applied.
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Figure 5.14: Forward-backward multiplicity correlation plots from recon-structed tracks (ESD) of ALICE simulated pp events at √
s = 10 TeV. The forward and backwardηintervals are both 0.2 units wide. The results for the η windows centered at ±0.1, ±0.3, and±0.5 are shown in the upper, middle and bottom panels, respectively as correlation plots (left) and its fitted pro-jections (right). Pseudorapidity gaps ∆η are defined from the center of the forward interval to the center of backward interval. No pt cut is applied.
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Fb
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g/b Bratio b
0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1
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ESD MCT
Figure 5.15: Forward-backward correlation strength in simulated pp events at √
s = 10 TeV as a function of the pseudorapidity gap. The values and their ratios are shown in the upper and lower parts of the figure, respectively.
The three dispersion method flavors agree within 2%.
The comparison between the two methods of the correlation strength determination is shown in Fig. 5.16. The dispersion method gives values that are 10-15% lower than the direct method. The pt > 0.2 GeV/c cut reduces the correlation strength by 3%.
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1 Direct Method
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t= 0 p
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Figure 5.16: Forward-backward correlation strength in simulated pp events at √
s = 10 TeV as a function of the pseudorapidity gap. The values and their ratios are shown in the upper and lower parts of the figure, respectively.
The direct and the dispersion methods agree within 15%.
The correlation strength determined directly can be compared to the pp¯collision results obtained by E735 at Fermilab [Alex95] where the same method was used (Fig. 5.17). The dependence of our b on the size of the pseudorapidity gap is weaker than in the case of the E735 data. The pure PYTHIA correlations, on the other hand, rapidly decrease between ∆η = 0 and ∆η = 1.0. This might be interpreted as a signature of short range cor-relations present in PYTHIA. Concerning the magnitude of the corcor-relations, our values are comparable to the ones observed by E735 at a collision
en-!
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b
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E735 data 0.300 TeV 0.546 TeV 1.00 TeV 1.80 TeV ALICE TPC 10 TeV
= 0.2 ESD, pt
= 0.2 MCT, pt
= 0.2 P6.3, pt
Figure 5.17: Dependence of the correlation strength on the size of the pseu-dorapidity gap. We compare E735 p¯p results [Alex95] with the output of an analysis of simulated data at √
s = 10 TeV (ESDs) and the corresponding Monte Carlo Truth (MCT) for the TPC central acceptance |η|<0.9 using a pt cut of 0.2 GeV/c. The PYTHIA 6.319 dependence (P6.3) is also shown.
ergy of √
s = 0.3 TeV. An extrapolation to 10 TeV would exceed our result significantly.
The pseudorapidity range over which the multiplicity fluctuations are cor-related can be quantified by fitting an exponential to theb(∆η) dependence:
b(∆η)∼e−∆ηλ . (5.3)
The pseudorapidity correlation length λ can be used to estimate the range of correlations in the coordinate space assuming that the rapidity and the space-time rapidity are roughly equal to each other. In classical systems, the correlation length is expected to diverge at the critical point. To get some feeling about the reliability of the so determined correlation length we compare, in Fig. 5.18, the fit of Eq.5.3to the correlation strength determined via the direct and the dispersion methods.
The correlation length obtained is on the order of 10-20 and, in nature, might point to existence of extended objects like color flux tubes or percolat-ing strpercolat-ings [Brog09]. Unfortunately, the discrepancy between the correlation length obtained from Monte Carlo and from reconstructed events is signifi-cant. The correlation strength is nearly constant over the analyzed pseudo-rapidity gap range and thus the exponential fit is not stable. Obviously, the correlation length can only be measured reliably if its value is not much larger than the experiment’s acceptance. We postpone the quantitative analysis of the statistical and systematic errors to the next section.
The fact that the correlation strength we obtained is lower than the ex-perimental one is also visible in Fig. 5.19where we plotb versus the collision energy. The multiplicity correlations shown in this figure were obtained for wide pseudorapidity intervals without a gap. Our value is 15% below the a0+a1ln√
sextrapolation from the existing ISR, UA5, and E735 experimen-tal data. The disagreement does not originate from the difference between the collision systems, p¯p versus pp. In fact, Pythia even predicts that the correlation strength inppshould be some 3% higher that inp¯pat the 10 TeV collision energy. The difference increases to about 30% when going down to ISR energies. This could be because pp collisions have more quark-diquark strings which contain more energy than quark-antiquark ones and thus lead to increased forward-backward correlations.
On the other hand, the UA5 and E735 results can be rather well repro-duced by Pythiap¯psimulations once the pseudorapidity coverage is extended to −3.0 < η < 3.0, similar to the actual acceptance of the two experi-ments. This means that the difference between the simulated ALICE points in Fig. 5.19 and the extrapolation line from lower energies is primarily a trivial consequence of the ALICE central barrel acceptance being narrower than the acceptances of UA5 and E735.
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= 16.1
#
Figure 5.18: Fits of Eq. 5.3 to the dependence of the correlation strength b on ∆η for reconstructed tracks (ESD) and the corresponding MC Truth (MCT), for the direct (labeled b) and dispersion (labeledbg) methods. Two values of the pt cut were studied. Results are summarized in Table5.2.
(GeV) s
10
210
310
4b
0 0.2 0.4 0.6 0.8
1
ISR, |!|<3.6|<4 UA5, |!|<3.25 E735,|!
|>1.0 NOFUS,|!
|>1.0 FUS,|!
|<0.9 ALICE TPC,|!
> 0.2 (ESD) pt
> 0.2 (MCT) pt
s
1ln
0+a a
Figure 5.19: Energy dependence of the correlation strength b. Shown are experimental data from ISR pp collisions [Uhli78], UA5 p¯p [Anso88], E735 pp¯[Alex95], and the results of a string fusion model (labeled FUS when the string fusion is included, NOFUS when it is not included [Amel94]) together with the information from the analysis of the reconstructed tracks (ESD) and the corresponding MC Truth (MCT) of simulated pp collisions in the ALICE TPC.