Ridge Formation and Long Range Correlations in pp Collisions at CMS C.B. Yang Institute of Particle Physics Central China Normal University Wuhan , China Based on PRC83, (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

J. Putschke, QM 2006

PHOBOS: High p t Triggered Ridge from Edward Wenger, RHIC & AGS User’s Meeting, ‘08

PHOBOS: High p t 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 JHEP 09 (2010) 091  p T -inclusive two-particle angular correlations in minimum bias collisions Background = mixed-event pairs Signal = same event pairs Ratio Signal/Background

Long-range near-side angular correlations Peaks are truncated ! 12 G. Roland’s talk 7 TeV

Long-range near-side angular correlations Peaks are truncated ! 13 G. Roland’s talk 7 TeV

Long-range near-side angular correlations Peaks are truncated ! 14 G. Roland’s talk 7 TeV

Long-range near-side angular correlations Peaks are truncated ! 15 new ridge-like structure 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 p T (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<p T <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, (2004) P. Romatschke, Phys. Rev. C 75, (2007) A. Majumder, B. Muller, S. A. Bass, Phys. Rev. Lett. 99, (2007) C. B. Chiu, R. C. Hwa, Phys. Rev. C 72, (2005) C. Y. Wong, arXiv: [hep-ph] R. C. Hwa, C. B. Yang, arXiv: [nucl-th] T. A. Trainor, arXiv: [hep-ph] A. Dumitru, Y. Nara, B. Schenke, M. Strickland, arXiv: [hep-ph] E. V. Shuryak, Phys. Rev. C 76, (2007) C. Pruneau, S. Gavin, S. Voloshin, Nucl.Phys.A802: ,2008 S. Gavin and M. Abdel-Aziz, Phys. Rev. Lett. 97, (2006) S. A. Voloshin, Phys. Lett. B 632, 490 (2006) S. Gavin and G. Moschelli, arXiv: [nucl-th] A. Dumitru, F. Gelis, L. McLerran and R. Venugopalan, arXiv: [hep-ph] S. Gavin, L. McLerran, G. Moschelli, arXiv: [nucl-th] F. Gelis, T. Lappi, R. Venugopalan, arXiv: [hep-ph] hard ridge explanations -- jet interactions with matter soft ridge -- similar but no jet -- collective behavior Theoretical explanations for ridge in AuAu collisions

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 N ch >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

From the azimuthal dependence of R, v 2 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’ 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, (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->v 2 harder spectrum in ridge high p/ π ratio, 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