prospects for untriggered di-hadron correlations in pb+pb 2.76 tev collisions…
DESCRIPTION
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…. - PowerPoint PPT PresentationTRANSCRIPT
7th Feb 2011 1
Prospects for untriggered di-hadron correlations in Pb+Pb 2.76 TeV
collisions… Anthony Timmins for the ALICE collaboration
7th Feb 2011 2
Outline… Untriggered di-hadron correlations…
Correlation function…
Fit decomposition…
Results at RHIC…
What can the LHC bring?
Progress report from ALICE…
Summary…
7th Feb 2011 3
Untriggered di-hadron correlations… 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)
Au+Au 200 GeV
7th Feb 2011 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
7th Feb 2011 5
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
JHEP 1009:091,2010 (CMS)
M
arXiv:0806.2121 (STAR)
7th Feb 2011 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)
7th Feb 2011 7
Fit decomposition…
Sharp exponentialHBT, e+e-
Nearside 2D GaussianSoft ridge
Awayside Not directed flow
v2
Elliptic flow
arXiv:0806.2121 (STAR)
7th Feb 2011 8
Fit decomposition…
Nearside 2D gaussian
Sharp exponential
Awaysidev2
Fits generally good– Residuals show no
structure…
arXiv:0806.2121 (STAR)
7th Feb 2011 9
Results at RHIC…2D Nearside Gaussian
= 2Nbin/Npart
Gaussian gets amplitude grows, gets wider in – Quantitative characterisation of “soft ridge”
Au+Au 200 GeVAu+Au 62 GeV
Peak Amplitude Peak η Width Peak φ Width
arXiv:0806.2121 (STAR)
7th Feb 2011 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…
Cu+Cu 200 GeVSTAR Preliminary
Peak Amplitude Peak η Width
C De Silva (STAR)RBRC workshop
7th Feb 2011 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….
7th Feb 2011 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…
Phys.Rev. C 79 (2009) 051902
arXiv:1101.1925v1
7th Feb 2011 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
7th Feb 2011 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
7th Feb 2011 15
Progress report from ALICE…ALICE performance
Pronounced change in correlation structure from peripheral -> central Pb+Pb collisions– STAR definition adopted
– Total correlations become stronger
– Nearside more elongated in …
7th Feb 2011 16
ALICE performance
Progress report from ALICE…
Centrality evolution:– Nearside Gaussian gets gradually wider…
– v2 rises and falls…
7th Feb 2011 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…