Multiparticle azimuthal correlation with subevent method Mingliang Zhou
Multi-particle azimuthal correlation with subevent method Mingliang Zhou for the ATLAS collaboration Quark Matter 2017, Feb. 5 -11, Chicago ar. Xiv: 1606. 08170 ATLAS-CONF-2017 -002
Long-range pseudo-rapidity correlation Nature of sources seeding the long-range collective behavior? Event 1 Event 2 PRC 87, 024906 (2013) PRC 93, 044905 (2016) • Not Dominated by linear fluctuation! enough FB sources in models. • Independent of collision system! ar. Xiv: 1606. 08170 1
Long-range azimuthal correlation Nature of sources seeding the long-range collective behavior? • Azimuthal correlation: well understood in A+A. 2
Long-range azimuthal correlation PRL 116 (2016) 172301 • Near-side SRC: jet, HBT… • Away-side ridge: mainly from dijet. • Near-side ridge • Ridge structure also observed in small systems! Competing theories: • Initial stage interaction (CGC, …)? PRD 87 (2013) 094034 • Initial stage fluctuation + final stage interaction? PRC 88 (2013) 014903 • Fluctuation in proton can be constrained in pp and p+Pb collisions. • However, ridge hard to extract in data: larger non-flow in small system. From the experimental perspective, central question about the ridge is whether it involves all particles (collectivity? ). 2
Standard cumulant • Cumulant method: natural to probe collectivity: PRL 115, 012301 (2015) 3
Limitations of standard cumulant ar. Xiv: 1701. 03830 Energy dependence? Large residual non-flow in PYTHIA. 6% flow ATLAS-CONF-2016 -106 Sign depends on event class definition? 4
Subevent cumulant 5
Subevent cumulant 5 ar. Xiv: 1701. 03830
Test of residual non-flow Non-flow changes greatly Eby. E Flow changes little Eby. E 6
4% flow non-flow fluctuation Standard cumulant v. s. Subevent cumulant ? ? ? 3 subevent cumulant is a more reliable method in pp! 7
New method tested in 5. 02 Te. V p+Pb 4% flow ? 8
13 Te. V pp: comparison of three methods 4% flow 9
10 4% flow
Comparison among three collision systems 11
12
Comparison with 2 -particle correlation method 13
Number of sources in initial stage 14
Summary 15 Weak energy dependence. Less non-flow in 3 subevent. Hope the new results can better constrain models in small systems. Less non-flow fluctuation in 3 subevent.
## Back Up
Data sets tector e D r e n n I ATLAS HMT 5. 02 Te. V pp
Formulas without particle weight
non-flow fluctuation Multiplicity fluctuation ? ? ?
Residual non-flow check in 2 subevent
Summary 16 Nature of sources seeding the long-range collective behavior? Longitudinal correlation Azimuthal correlation PYTHIA CGC? Hydro? PRC 94, 044918 (2016) Models underestimated data. • Fluctuation of saturation scale.
Physics signal and background • 2 -particle pseudo-rapidity correlation: • Short-range correlation (SRC) reflects correlation in the same source: jet fragmentation, resonance decay… • Since SRC has a strong charge dependence, a data-driven way was developed to remove the SRC. • An alternative way is through multiparticle correlation (cumulant) in pseudo-rapidity. PRC. 93. 024903 • Long-range correlation (LRC) From the experimental perspective, the key is to extract the physics signal from the background.
• Particles from the same source (SRC) have strong charge dependence. Opposit e charges Same charges
Estimation of short-range correlation • To estimate SRC, LRC pedestal is estimated first. SRC Same charges •
Why it is important to remove SRC? Before SRC removal After SRC removal Oppo pairs All Same pairs • Higher order coefficients observed; • Coefficients have charge dependent; • Results hard to interpret: due to SRC! • Simpler picture after SRC removal! • LRC dominated by linear fluctuation; • LRC is charge independent.
Why it is important to remove SRC? Short-range correlation • SRC increases towards peripheral; • SRC is stronger in small systems; Long-range correlation • Dominated by linear fluctuation! • Independent of collision system! • Follow-up model studies • Viscous hydro: PLB 2015. 11. 063 • Length of sources fluctuation: PRC 93, 064910 (2016) • Saturation scale fluctuation: PRC 94, 044918 (2016)
- Slides: 32