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

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

STARAuto-correlation

3 4 /

2 /

trigt

assoct

p GeV c

p GeV c

J. Putschke, QM 2006

PHOBOS: High pt Triggered Ridgefrom Edward Wenger, RHIC & AGS User’s Meeting, ‘08

PHOBOS: High pt Triggered Ridgefrom Edward Wenger, RHIC & AGS User’s Meeting, ‘08

long range correlations

Ridge in pp collisions from CMS

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

Long-range near-side angular correlations

Peaks aretruncated !

12

G. Roland’s talk

7 TeV

Long-range near-side angular correlations

Peaks aretruncated !

13

G. Roland’s talk

7 TeV

Long-range near-side angular correlations

Peaks aretruncated !

14

G. Roland’s talk

7 TeV

Long-range near-side angular correlations

Peaks aretruncated !

15

new ridge-likestructure at ~ 0

7 TeV

Long-range near-side angular correlations

16

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

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

• 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

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?

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

Φ determined by overlap geometry

0 2 4 60

0.005

0.01

0.015

pT (GeV/c)

R(p

T)

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 hadro-

fluctuations

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?

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 bemediated into phase space well separated.

Heating simultaneously two points at large Δηresults in long range correlations

Lessons from STARPRD74, 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