1. 7th Feb 2011 1 Prospects for untriggered di-hadron correlations in Pb+Pb 2.76 TeV collisions… Anthony Timmins for the ALICE collaboration

2. 7th Feb 2011 2 Outline… oUntriggered di-hadron correlations… oCorrelation function… oFit decomposition… oResults at RHIC… oWhat can the LHC bring? oProgress report from ALICE… oSummary…

3. 7th Feb 2011 3   Untriggered di-hadron correlations… oUntriggered correlations provide map of bulk correlation structures: –Look at all pairs over all p T –Number of physical mechanisms contribute in heavy-ion collisions oMain contributors at RHIC energies: –Elliptic flow Shows up as cos (2  ) structure…. –“Soft ridge” Shows up as elongated nearside 2D Gaussian.. arXiv:0806.2121 (STAR) Au+Au 200 GeV

4. 7th Feb 2011 4 Correlation function… STAR definition o“Auto-correlation” based on Pearson’s coefficient… –  sib signal +background (real events), –  ref background (mixed events) oDesigned to be independent of multiplicity –R the same for p+p and Pb+Pb if Pb+Pb is superposition of p+p oMeasures # of correlated pairs per particle

5. 7th Feb 2011 5 Correlation function… CMS vs STAR oPrefactor only thing that differs: –CMS: Mean number of tracks - 1 –STAR: Mean number of tracks per  width per  with JHEP 1009:091,2010 (CMS) M arXiv:0806.2121 (STAR)

6. 7th Feb 2011 6 Correlation function… Evolution with centrality oChange in Cu+Cu correlation structure with centrality… –p+p like in peripheral collisions –Correlations become stronger.. –Nearside gaussian structure widens… arXiv:0910.5938v1 (STAR)

7. 7th Feb 2011 7 Fit decomposition… Sharp exponential HBT, e+e- Nearside 2D Gaussian Soft ridge Awayside Not directed flow v 2 Elliptic flow   arXiv:0806.2121 (STAR)

8. 7th Feb 2011 8 Fit decomposition… Nearside 2D gaussian Sharp exponential Awayside v2v2 oFits generally good –Residuals show no structure… arXiv:0806.2121 (STAR)

9. 7th Feb 2011 9 Results at RHIC… 2D Nearside Gaussian = 2  N bin  /  N part  oGaussian gets amplitude grows, gets wider in  –Quantitative characterisation of “soft ridge” Au+Au 200 GeV Au+Au 62 GeV Peak Amplitude Peak η WidthPeak φ Width arXiv:0806.2121 (STAR)

10. 7th Feb 2011 10 Results at RHIC… Is the wide Gaussian due to v 3 ? oIncluding v 3 in fit depletes Gaussian Amplitude and  width: –Doesn’t kill Gaussian though… Cu+Cu 200 GeV STAR Preliminary Peak Amplitude Peak η Width C De Silva (STAR) RBRC workshop

11. 7th Feb 2011 11 Results at RHIC… Soft ridge theories oInitial energy density fluctuations –P. Mishra et.al, Phys. Rev. C 77 (064902) 2008 –B. Alver and G. Roland, arXiv:1003.0194 –P Sorensen, arXiv:1101.1925v1 o CGC flux tubes + radial flow model –Gavin, McLerran and Moschelli: Phys. Rev. C79 (2009) 051902 –Moschelli and Gavin: Nucl.Phys.A836:43-58,2010 oMedium modified mini-jets (pQCD related explanation) –T. Trainor and D. Kettler, arXiv:1008.4759 –T. Trainor, Phys. Rev. C 80 (2009) 044901 oList not exhaustive….

12. 7th Feb 2011 12 Results at RHIC… Soft ridge theories oDescriptions give quantitative predictions for nearside Gaussian amplitude (A 1 ) –CGC + radial flow on left –Initial energy density fluctuations on right… Phys.Rev. C 79 (2009) 051902 arXiv:1101. 1925v1

13. 7th Feb 2011 13 What can the LHC bring? oNew energy, new physics, possibly new correlation structures… oPrevious “soft ridge” descriptions capable of LHC predictions: –Pb+Pb 2.76 TeV data provides essential constraints.. Easy for many theories to fit one dataset Hard for one theory to fit many datasets P Sorensen, PC G Moschelli & S Gavin, PC

14. 7th Feb 2011 14 Progress report from ALICE… oEvents –Online trigger cut loose –Background reject offline –|PVz| < 5 cm –2 million events after all cuts Subset of total possible… oTracks –p T > 0.15 GeV/c –|  | < 0.8 –Various quality cuts… oInitial studies underway: –Di-hadron correlation group meets biweekly –Selection criteria for “first look” at untriggered correlations in Pb+Pb

15. 7th Feb 2011 15 Progress report from ALICE… ALICE performance oPronounced change in correlation structure from peripheral -> central Pb+Pb collisions –STAR definition adopted –Total correlations become stronger –Nearside more elongated in  …

16. 7th Feb 2011 16 ALICE performance Progress report from ALICE… oCentrality evolution: –Nearside Gaussian gets gradually wider… –v 2 rises and falls…

17. 7th Feb 2011 17 Summary… o“First look” at untriggered di-hadron correlations in Pb+Pb 2.76 TeV: –Rapid change in correlation structure from peripheral -> central… –Emergence of the soft ridge at the LHC… oFuture work: –Obtain more events, finer centrality binning… –Fit decomposition Plan to use RHIC functions initially Multi-parameter fits challenging –Systematic uncertainties on parameters need to well understood…. –Things look very promising…