Ridge Formation and Long Range
Correlations in pp Collisions at CMS
C.B. Yang
Institute of Particle Physics
Central China Normal University
Wuhan 430079, China
Based on PRC83, 024911 (2011)
by R.C. Hwa and C.B. Yang
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
Ridge from RHIC
• STAR: auto-correlation, w/wo trigger
• PHOBOS: triggered analysis
STAR
Auto-correlation
3  pt
trig
assoc
pt
J. Putschke, QM 2006
 4 G eV / c
 2 G eV / c
PHOBOS: High pt Triggered Ridge
from Edward Wenger, RHIC & AGS User’s Meeting, ‘08
PHOBOS: High pt Triggered Ridge
from Edward Wenger, RHIC & AGS User’s Meeting, ‘08
long range
correlations
Ridge in pp collisions from CMS
Long-range near-side angular correlations
Signal = same event pairs
 pT-inclusive two-particle angular correlations
in minimum bias collisions
 S (  ,   )

N
R (   ,   )  ( N  1)
 1
B N (   ,   )

N
S N (  ,  ) 

Ratio Signal/Background
d N
signal
N ( N  1) d  d 
B N (  ,  ) 

2
1
2
1
N
d N
2
bkg
d  d 
Background = mixed-event pairs
   1   2

   1   2

JHEP 09 (2010) 091
G. Roland’s talk
Long-range near-side angular correlations
7 TeV
Peaks are
truncated !
12
G. Roland’s talk
Long-range near-side angular correlations
7 TeV
Peaks are
truncated !
13
G. Roland’s talk
Long-range near-side angular correlations
7 TeV
Peaks are
truncated !
14
Long-range near-side angular correlations
7 TeV
Peaks are
truncated !
new ridge-like
structure
at f ~ 0
15
G. Roland’s talk
Long-range near-side angular correlations
 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
16
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
Theoretical explanations for ridge
in AuAu collisions
hard ridge explanations -- jet interactions with matter
• 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
soft ridge -- similar but no jet -- collective behavior
• 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]
Ridge in pp collisions
by 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?
Our consideration on ridge
for 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
Φ
determined by overlap geometry
R(pT)
0.015
0.01
0.005
0
0
2
4
pT (GeV/c)
6
From the azimuthal dependence of R, v2 can be calculated
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
Long-range correlations
• CMS data
• Initial fluctuations
• Transverse momentum
conservation
• Correlation induced by hadrofluctuations
Initial fluctuations
Cross sectional
slices are the same
• Correlated
Particles
come from the same
tube
• originate at the earliest stages of the collision
• information on particle production mechanism
What’s the pT dependence of correlations?
Why long range correlations can be seen only in
high multiplicity events?
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
T’
T’
ΔT=T’-T depends on multiplicity, because
the initial fluctuations at some point need to be
mediated into phase space well separated.
Heating simultaneously two points at large Δη
results in long range correlations
Lessons from STAR
PRD74, 032006 (06)
• Hard component increases with multiplicity
• ΔT larger for higher multiplicity
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
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!!
Thank you !
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