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Prospects for untriggered di-hadron correlations in Pb+Pb 2.76 TeV collisions…

Prospects for untriggered di-hadron correlations in Pb+Pb 2.76 TeV collisions…. Anthony Timmins for the ALICE collaboration. Outline…. Untriggered di-hadron correlations… Correlation function… Fit decomposition… Results at RHIC… What can the LHC bring? Progress report from ALICE… Summary….

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Prospects for untriggered di-hadron correlations in Pb+Pb 2.76 TeV collisions…

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

  2. Outline… • Untriggered di-hadron correlations… • Correlation function… • Fit decomposition… • Results at RHIC… • What can the LHC bring? • Progress report from ALICE… • Summary…

  3. Untriggered di-hadron correlations… Au+Au 200 GeV • Untriggered correlations provide map of bulk correlation structures: • Look at all pairs over all pT • Number of physical mechanisms contribute in heavy-ion collisions • Main 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)

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

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

  6. Correlation function…Evolution with centrality • Change 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. Fit decomposition… Nearside 2D Gaussian Soft ridge Sharp exponential HBT, e+e- Awayside Not directed flow v2 Elliptic flow   arXiv:0806.2121 (STAR)

  8. Fit decomposition… arXiv:0806.2121 (STAR) • Fits generally good • Residuals show no structure… Sharp exponential Nearside 2D gaussian Awayside v2

  9. Results at RHIC…2D Nearside Gaussian arXiv:0806.2121 (STAR) • Gaussian gets amplitude grows, gets wider in  • Quantitative characterisation of “soft ridge” Peak η Width Peak φ Width Peak Amplitude Au+Au 200 GeV Au+Au 62 GeV  = 2Nbin/Npart

  10. Results at RHIC…Is the wide Gaussian due to v3? • Including v3 in fit depletes Gaussian Amplitude and  width: • Doesn’t kill Gaussian though… Peak Amplitude Peak η Width C De Silva (STAR) RBRC workshop Cu+Cu 200 GeV STAR Preliminary

  11. Results at RHIC…Soft ridge theories • Initial 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 • CGC flux tubes + radial flow model • Gavin, McLerran and Moschelli: Phys. Rev. C79 (2009) 051902 • Moschelli and Gavin: Nucl.Phys.A836:43-58,2010 • Medium modified mini-jets (pQCD related explanation) • T. Trainor and D. Kettler, arXiv:1008.4759 • T. Trainor, Phys. Rev. C 80 (2009) 044901 • List not exhaustive….

  12. Results at RHIC…Soft ridge theories • Descriptions give quantitative predictions for nearside Gaussian amplitude (A1) • CGC + radial flow on left • Initial energy density fluctuations on right… arXiv:1101.1925v1 Phys.Rev. C 79 (2009) 051902

  13. What can the LHC bring? • New energy, new physics, possibly new correlation structures… • Previous “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. Progress report from ALICE… • Events • Online trigger cut loose • Background reject offline • |PVz| < 5 cm • 2 million events after all cuts • Subset of total possible… • Tracks • pT > 0.15 GeV/c • || < 0.8 • Various quality cuts… • Initial studies underway: • Di-hadron correlation group meets biweekly • Selection criteria for “first look” at untriggered correlations in Pb+Pb

  15. ALICE performance Progress report from ALICE… • Pronounced change in correlationstructure from peripheral -> central Pb+Pb collisions • STAR definition adopted • Total correlations become stronger • Nearside more elongated in …

  16. Progress report from ALICE… • Centrality evolution: • Nearside Gaussian gets gradually wider… • v2 rises and falls… ALICE performance

  17. Summary… • “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… • Future 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…

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