SLIDE 1
Collective Behaviour in a small system
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~30000 particles ~1000 particles ~100 particles
Reveal collective/flow like behaviour via the two particle correlation method
Angular Correlations as a function of multiplicity in pp and p-Pb - - PowerPoint PPT Presentation
Angular Correlations as a function of multiplicity in pp and p-Pb - - PowerPoint PPT Presentation
Angular Correlations as a function of multiplicity in pp and p-Pb collisions in the ATLAS experiment Deepak Kar On behalf of ATLAS collaboration MPI@LHC Workshop, Chiapas, Mexico, November 2016 Collective Behaviour in a small system ~30000
SLIDE 2
SLIDE 3
13 TeV pp Ridge
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In high multiplicity events there is an enhancement in the particle production at Ỏφ ≈ 0 over wide range of Ỏỷ
- Phys. Rev. Lett. 116, 172301 (2016)
First ATLAS Run 2 paper
Leading Jet Ridge Recoil Jet
SLIDE 4
Extracting the Ridge
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- Phys. Rev. Lett. 116, 172301 (2016)
Suppress dijet system
SLIDE 5
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40 < Nch < 50
- Phys. Rev. Lett. 116, 172301 (2016)
Enhancement of the high Nch region due to a dedicated high-multiplicity track trigger
SLIDE 6
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60 < Nch < 70
- Phys. Rev. Lett. 116, 172301 (2016)
SLIDE 7
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Nch > 90
- Phys. Rev. Lett. 116, 172301 (2016)
SLIDE 8
8
Nch > 120
- Phys. Rev. Lett. 116, 172301 (2016)
Narrowing of the distribution due to cosine component
SLIDE 9
In more Systems
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arXiv:1609.06213
SLIDE 10
Azimuthal Correlations
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Characterized by Fourier coefficients to describe relative amplitudes of sinusoidal components Measured by 2 pc:
Long range part subtracted by using peripheral events
arXiv:1609.06213
SLIDE 11
Factorization?
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While v2,2 values vary, v2 agrees quite well. Similarly for p-Pb case.
arXiv:1609.06213
SLIDE 12
Multiparticle Cumulants
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Useful tool to study the global nature
- f correlations
Cumulants:
SLIDE 13
Results
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Nreftrk: Ref particles: within a narrow pT range Method 1: for a fixed Nreftrk Method 2: fixed multiplicity for fluctuating Nreftrk For lower ref pT range, method 2 gives smaller values, multiplicity fluctuations lead to negative contributions to cumulants
ATLAS-CONF-2016-106
SLIDE 14
Comparison with MC
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As compared to the data, Pythia 8 overestimates the values of cumulants measured for events with high charged particle multiplicities
ATLAS-CONF-2016-106
SLIDE 15
Multiplicity Dependence
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arXiv:1606.08170
SLIDE 16
Observables
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- ppositely
charged pairs same charged pairs Ratio
Disentangles statistical and dynamical fluctuations
arXiv:1606.08170
SLIDE 17
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Short Range Correlations Δη = η1-η2 ≅ 0 Within same source Long Range Correlations large |Δη| FB asymmetry of number of sources
SRC (decays, jet frag, BEC)
LRC
Correlation function decomposed into Legendre Polynomials
arXiv:1606.08170
SLIDE 18
Legendre Spectra
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More +- pairs than ++— pairs in each source No charge dependence, reflects global symmetry
arXiv:1606.08170
SLIDE 19
Multiplicity vs Shape
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Strong system dependance for SRC Weak or no dependance for LRC
arXiv:1606.08170
SLIDE 20
Summary
- Strong collectivity observed in high-multiplicity pp collisions, pp
ridge described by cos2Ỏϕ, has (surprisingly) weak dependence on event activity and √s
- LRC controlled by Nch, not by collision systems or charge
combination, SRC depends strongly on collision system and charge combination
- Nch dependence of LRC and SRC follows power-law with an index
close to 0.5 - information on the number of sources for particle production?
- v2 from 2PC in pp is independent of √s while vn are consistent with
no Nch dependance in pp, but increase in p+Pb
- Multiplicity fluctuations are important and tend to shift cumulants
to more negative values, as such, can mimic the collective-like effects
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