Fluctuations and Correlations in Relativistic Heavy Ion Collisions
Ridge Formation and Long Range Correlations in pp Collisions at CMS
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Transcript of Ridge Formation and Long Range Correlations in pp Collisions at CMS
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Ridge Formation and Long Range Correlations in pp Collisions at CMS
C.B. Yang
Institute of Particle PhysicsCentral China Normal University
Wuhan 430079, China
Based on PRC83, 024911 (2011) by R.C. Hwa and C.B. Yang
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Outline
• Ridge from experiments Ridge in AuAu collisons from RHIC Ridge and long range correlations in pp collisions at
CMS
• Existing theoretical model explanations• Our model for ridge and long-range correlations• Discussion
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Ridge from RHIC
• STAR: auto-correlation, w/wo trigger• PHOBOS: triggered analysis
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STARAuto-correlation
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3 4 /
2 /
trigt
assoct
p GeV c
p GeV c
J. Putschke, QM 2006
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PHOBOS: High pt Triggered Ridgefrom Edward Wenger, RHIC & AGS User’s Meeting, ‘08
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PHOBOS: High pt Triggered Ridgefrom Edward Wenger, RHIC & AGS User’s Meeting, ‘08
long range correlations
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Ridge in pp collisions from CMS
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Long-range near-side angular correlations
JHEP 09 (2010) 091
pT-inclusive two-particle angular correlations in minimum bias collisions
Background = mixed-event pairs
Signal = same event pairs
1 2
1 2
Ratio Signal/Background
SN (,) 1N (N 1)
d 2N signal
dd
BN (,) 1N 2
d 2N bkg
dd
R(,) ( N 1)SN (,)BN (,)
1
N
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Long-range near-side angular correlations
Peaks aretruncated !
12
G. Roland’s talk
7 TeV
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Long-range near-side angular correlations
Peaks aretruncated !
13
G. Roland’s talk
7 TeV
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Long-range near-side angular correlations
Peaks aretruncated !
14
G. Roland’s talk
7 TeV
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Long-range near-side angular correlations
Peaks aretruncated !
15
new ridge-likestructure at ~ 0
7 TeV
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Long-range near-side angular correlations
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G. Roland’s talk
No such structure is seen in Monte Carlo simulations : PYTHIA8, PHYTHIA6, Herwig++, MadGraph
The effect is small, but clearly seen for large and multiplicities > 90 It is most pronounced at intermediate pT (1–3 GeV/c) It is the first observation of such an effect in pp (or p-pbar) collisions Further studies ongoing for a better understanding of the effect The heavy ion run will provide additional input
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Experimental summary
• A peak at relative angle• Wide distribution in pseudo-rapidity• Most obvious for particles with 1<pT<3 GeV/c
• Spectrum of secondaries harder than a bulk one• A composition very different from jets
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• N. Armesto, C.A. Salgado, U.A. Wiedemann, Phys. Rev. Lett. 93, 242301 (2004)• P. Romatschke, Phys. Rev. C 75, 014901 (2007)• A. Majumder, B. Muller, S. A. Bass, Phys. Rev. Lett. 99, 042301 (2007)• C. B. Chiu, R. C. Hwa, Phys. Rev. C 72, 034903 (2005)• C. Y. Wong, arXiv:0712.3282 [hep-ph]• R. C. Hwa, C. B. Yang, arXiv:0801.2183 [nucl-th]• T. A. Trainor, arXiv:0708.0792 [hep-ph]• A. Dumitru, Y. Nara, B. Schenke, M. Strickland, arXiv:0710.1223 [hep-ph]• E. V. Shuryak, Phys. Rev. C 76, 047901 (2007)• C. Pruneau, S. Gavin, S. Voloshin, Nucl.Phys.A802:107-121,2008
• S. Gavin and M. Abdel-Aziz, Phys. Rev. Lett. 97, 162302 (2006)• S. A. Voloshin, Phys. Lett. B 632, 490 (2006)• S. Gavin and G. Moschelli, arXiv:0806.4366 [nucl-th]• A. Dumitru, F. Gelis, L. McLerran and R. Venugopalan, arXiv:0804.3858 [hep-ph]• S. Gavin, L. McLerran, G. Moschelli, arXiv:0806.4718 [nucl-th]• F. Gelis, T. Lappi, R. Venugopalan, arXiv:0807.1306 [hep-ph]
hard ridge explanations -- jet interactions with matter
soft ridge -- similar but no jet -- collective behavior
Theoretical explanations for ridgein AuAu collisions
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Ridge in pp collisionsby Edward Shuryak
• Independent string breaking -> small , arbitrary
• If the string moves as a whole->a ridge can be seen in all events. But the data show ridge is there only for events with high multiplicity
• Explosion for Nch>100 in CMS? Can hydrodynamics be applied to pp collisions?
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Our consideration on ridgefor AuAu collisions
• Without using hydrodynamics explicitly• Semihard scattering near the surface is the
driving force of the azimuthal anisotropy• The lost energy from (mini)jets heats the
medium system• This heating effect depends on the position of
the semihard scattering point• The enhanced soft medium->ridge
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Φ determined by overlap geometry
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0 2 4 60
0.005
0.01
0.015
pT (GeV/c)
R(p
T)
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From the azimuthal dependence of R, v2 can be calculated
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Ridge in pp collisions shows
• Soft partons of high density created (?)• Those soft partons affect passage of jets• Origin of ridge in pp collisions at CMS may be
the same as in AuAu collisions at RHIC
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Long-range correlations
• CMS data
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• Initial fluctuations• Transverse momentum
conservation• Correlation induced by hadro-
fluctuations
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Initial fluctuations
•originate at the earliest stages of the collision
•information on particle production mechanism
• Correlated Particles come from the same tube
Cross sectional slices are the same
What’s the pT dependence of correlations?Why long range correlations can be seen only in high multiplicity events?
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Transverse momentum conservation
• With suitably parameters one can fit the data• Momentum is conserved in all elementary
processes in MC codes. Why PYTHIA cannot explain experimental data?
• The conservation effect is stronger for events with low multiplicity. Contrary to experimental discovery
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T’ T’
ΔT=T’-T depends on multiplicity, because the initial fluctuations at some point need to bemediated into phase space well separated.
Heating simultaneously two points at large Δηresults in long range correlations
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Lessons from STARPRD74, 032006 (06)
• Hard component increases with multiplicity• ΔT larger for higher multiplicity
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If dense medium is produced in pp collisions at CMS
• anisotropy in the spectrum->v2
• harder spectrum in ridge• high p/πratio, about 0.5 at 3-5 GeV/c
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Summary
• Experimental data on ridge are revisited• Model explanations are introduced briefly• Long range correlations in pp collisions at CMS
may be induced by ridge• Possible phenomena are predicted• More work needed!!
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Thank you !
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Discussion
• Hydrodynamic influence to ridge formation at CMS
• Dependence of ridge yield and correlations on direction of triggered particle
• Particle spectra in ridge• pT dependence of particle ratio in ridge