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ATLAS measurement of dipolar flow (v 1 ) in Pb-Pb collisions

ATLAS measurement of dipolar flow (v 1 ) in Pb-Pb collisions. Jiangyong Jia for the ATLAS Collaboration. Based on results in 1203.3087 (v 1 -v 6 summary) https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-01/. WWND 2012 April 7 th - 14 rd. Motivation. Alver, Roland etc. ε 2.

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ATLAS measurement of dipolar flow (v 1 ) in Pb-Pb collisions

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  1. ATLAS measurement of dipolar flow (v1) in Pb-Pb collisions Jiangyong Jia for the ATLAS Collaboration Based on results in 1203.3087 (v1-v6 summary) https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-01/ WWND 2012 April 7th- 14rd

  2. Motivation Alver, Roland etc ε2 ε3 ~400 nucleons >20000 particles ε4 • Initial geometry has multi-pole shape due to fluctuations. • Fourier expansion of azimuthal distribution in momentum space • Also measure with two-particle correlation (2PC) Probe shape of initial geometry and transport properties

  3. Eccentricity from Glauber model • Sizable eccentricities for all order • vn~εn in linearized hydro, but • Complicated by dynamic mixing during expansion, especially for n>3. • Higher order vn damped more by viscosity. • ε1 is smaller, but v1 is not affected by dynamic mixing and less affected by viscosity.

  4. Two-particle correlation (2PC) method |Δη|>2 • Long range structure (“ridge”, “double-hump”) well described by v1,1-v6,6. • Factorization works for n=2-6 • Soumya Mohapatra’s talk • Not for n=1.

  5. v1 physics Even component: ~boost invariant in η Odd component: vanish at η=0 Fig from P. Stankus • v1(η) dependence has a rapidity-odd and a rapidity-even component • rapidity-odd v1 reflect sideward bounce off, small at mid-rapidity • rapidity-even v1 is associated with the dipole asymmetry in initial geometry • v1 also affected by global momentum conservation • Balance of pT of one particle by all other particles: N. Borghini nucl-th/0004026 • Inversely proportional to multiplicity M, linear in pT.

  6. Dipole in Cosmic Microwave Background The CMB is dominated by a dipole, representing the Doppler shift of observer (600km/s)

  7. Rapidity-even v1 and expected trend in v1,1 Luzum et.al • Expected v1,1 contribution from rapidity-even v1 • a,b both at high pT  positive and increase with pTa,b (convex shape) • a,b both at vey low pT positive • a at low pT, b at high pT negative, more negative at higher pTb (concave shape) Do we see these trends in the data? Values inferred from STAR 2PC data by estimating the second term.

  8. Δη dependence of v1,1 • Peripheral collisions(GMC dominated): v1,1 is always negative at large Δη. More negative at higher pT. Magnitude decrease at large Δη • Influence of jets and dijets • Central collisions(flow dominated): v1,1 is negative at low pT, become positive at large pT. Magnitude flat in Δη. • Consistent with a rapidity-even v1. • Integrate over 2<|Δη|<5 and look at the pT dependence

  9. pT dependence of v1,1 data Can we account for both with a two-component fit? Peripheral collisions (GMC dominated): v1,1 negative, linear in pTa,pTb. Central collisions (flow dominated): v1,1 becomes positive at 1.5-6 GeV range, but on top of a negative momentum conservation component Cross each other at low pT where flow driven v1,1 ~ zero.

  10. Two-component fit • Simultaneous fit of v1,1 data of each centrality with a function • Simple χ2 minimization • v1Fit(pT) defined at 15 pT, and interpolate in between. Total 16 parameters • Systematic checks: • Interpolation form: Linear or cubic spline. • Number of interpolation points (vary within 9-21 points) • Vary pT range of fitting (0-5 to 0-10 GeV) • Account for correlations between data points and fitting parameters. Similar fit in arXiv:1203.0931

  11. Fit for 0-5% centrality Agrees with data within 1σ at pT<6 GeV. Slightly more deviation ~ 2σ in some higher pT bin.

  12. Understanding v1,1=<cosΔϕ> in 2PC (0-5%) Most of v1,1 is due to momentum conservation ~1.5 : 1 ~3:1 Most of v1,1 is due to dipolar flow Correlation function well described by v2-v6 and v1,1

  13. Fit for 40-50% centrality Despite that the v1,1 is always negative, significant positive v1 can still exist.

  14. Fit result vs pT and centrality Glauber • v1Fit(pT) peaks around 4-5 GeV, peak-value increases with centrality by about 20%. • Less viscosity damping, reflecting the increase in ε1?

  15. Compare with v2 and v3 1203.3265 L3 • v1 comparable to v3 but peak at higher pT. • High pT v1 seems drop slower than v2,v3. • Limitation of two-component assumption? • Both L dependent eloss become important? • v1>v2 in jet absorption model calculation in central collisions.

  16. About momentum conservation component ? M. Lisa 0807.3569 • The system that conserves momentum may only be a subset of the event • c dN/dη but decrease toward peripheral by 20-30% • For <pT2>=1 GeV2, M=5000 in 0-5% events, about 3 units in rapidity • Increases for peripheral collisions, about ~4 unit for 40-50% centrality

  17. Comparison with AMPT model: arxiv:1203.3410 Arxiv1203.3410 v1,1 calculated for pairs with |Δη|>1.5 • AMPT=HIJING +F.S scattering. • Interaction strength controlled by αs and μ. • HIJING only need momentum conservation, while AMPT need both • The complex pT dependence of v1,1 can naturally be generated from final state interaction

  18. Centrality and energy dependence pT dependence is qualitatively similar to what is seen in data and hydro predictions Weak dependence on centrality Increases from RHIC to LHC

  19. Dependence on the strength of interaction More sensitive to changing αs than changing screening mass μ Values from a larger screening mass and smaller coupling constant is closer to the data from ATLAS: αs=0.33, σ=1.5mb

  20. Summary • The cos(Δϕ) component of the 2PC data suggests contributions from rapidity-even dipolar flow and global momentum conservation. • A two-component fit is used to extract the individual contribution from these two components • Extracted v1 cross zero at pT~1 GeV, reaches a value of 0.1 (comparable to v3), and decreases at higher pT. The pT at which it reaches maximum is 1 GeV higher than other vn. • Extracted v1 shows a mild increase with centrality (~20%) • The system conserving momentum only involves a subset of the event • AMPT transport model calculation confirmed qualitative trend at low pT. • Dipolar flow is indeed associated with final state interaction • Flow magnitude is sensitive to the strength of the interaction

  21. Backup

  22. Extracting the η dependence: 1203.3410 • Extend the procedure to study rapidity dependence by using a simultaneous fit of the 4-D v1,1 data. • Only v1,1 data satisfying a certain η gap is used (|Δη|>2) • The number of independent c values can be restricted by symmetry • Impose the constraint v1(η) = v1(−η)

  23. η dependence of v1 and c from AMPT Weak η dependence at RHIC energies but has a dip at mid-rapidity at LHC energy strong longitudinal flow? c is not constant: contribution from momentum conservation is not constant across whole η and |∆η| range and shows a strong dependence on |∆η|

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