Heavy Ion Physics Hot and dense QCD after the first LHC running period Carlos A. Salgado Universidade de Santiago de Compostela The European School of High-Energy Physics Garderen - the Netherlands - June 2014 @CASSalgado @HotLHC
Some of the questions accessible with heavy-ion collisions nucleus A What is the structure of hadrons/nuclei at high energy? color coherence effects in the small-x partonic wave function fix the initial conditions in well-controlled theoretical framework Initial State Is the created medium thermalized? How? presence of a hydrodynamical behavior what is the mechanism of thermalization in a non-abelian gauge theory? What are the properties of the produced medium? Final State identify signals to characterize the medium with well-controlled observables what are the building blocks and how they organize? is it strongly-coupled? quasiparticle description? phases? ESHEP - Garderen - June 2014 Heavy Ion Collisions 2
Menu ESHEP - Garderen - June 2014 Heavy Ion Collisions 3
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ESHEP - Garderen - June 2014 Heavy Ion Collisions 5
Splitting probability: the building block The splitting probability of an off-shell parton computed in pQCD Soft and collinear divergent Large probability to emit soft and collinear gluons Divergencies need to be resumed (renormalization techniques) The picture is a shower of partons produced by subsequent splittings ESHEP - Garderen - June 2014 Heavy Ion Collisions 6
Heuristic: Collision “counts” partons (Incoherent) cross section proportional to the number of partons in hadron Quantum fluctuations put on-shell by the probe Lifetime of the fluctuation of the order of the size of the probe The probe cannot resolve smaller fluctuations (stay virtual) Harder probes resolve smaller components (basic idea of pQCD factorization) ESHEP - Garderen - June 2014 Heavy Ion Collisions 7
Heuristic: Collision “counts” partons II Coherent cross section: the probe can interact with more than one parton TAMES the cross section Saturation of partonic densities (gluon fusion) - aka Color Glass Condensate Color correlations among different partons in the proton/nucleus ESHEP - Garderen - June 2014 Heavy Ion Collisions 8
Quantum fluctuations: Linear/non-linear dynamics Different kinematical regions: dominated by different dynamics Large-Q : Linear Small-x : Non-linear (eventually) Where is the boundary? (Information from experiment needed) ESHEP - Garderen - June 2014 Heavy Ion Collisions 9
ESHEP - Garderen - June 2014 Heavy Ion Collisions 10
Collinear factorization Collinear factorization A hard cross section is the convolution of universal PDFs and partonic cross sections Factorization of long-distance and short distance terms in the cross section Short-distance (perturbative) in the partonic cross section Long-distance (non-perturbative) in the PDFs and Fragmentation Functions (FF) ESHEP - Garderen - June 2014 Heavy Ion Collisions 11
Global fits for nucleus Plots from Hannu Paukkunen Ratios of the PDF of a proton inside a nucleus over that in a free proton Isospin effects may be important (e.g. W production in pPb@LHC) i ( x, Q 2 ) = f p/A ( x, Q 2 ) R A i f p i ( x, Q 2 ) ESHEP - Garderen - June 2014 Heavy Ion Collisions 12
DGLAP approach - Some recent results I Agreement of EPS09 with neutrino DIS data Neutrino beam Antineutrino beam 1.2 1.2 NuTeV NuTeV CTEQ6.6 CTEQ6.6 1.1 1.1 EPS09 EPS09 R Average R Average 1 1 0.9 0.9 1.2 1.2 CHORUS CHORUS CTEQ6.6 CTEQ6.6 1.1 1.1 EPS09 EPS09 R Average R Average 1 1 0.9 0.9 1.2 1.2 CDHSW CDHSW CTEQ6.6 CTEQ6.6 1.1 1.1 EPS09 EPS09 R Average R Average 1 1 0.9 0.9 0.8 0.8 10 -2 10 -1 10 -2 10 -1 1 1 x x [Paukkunen, Salgado, 2013] Collinear factorization works - universal set of nPDFs Neutrino data important for proton global fits ESHEP - Garderen - June 2014 Heavy Ion Collisions 13
DGLAP approach - Some recent results II Dijet data in proton-nucleus collisions at LHC - CMS Preliminary CMS data “by eye” [Eskola, Paukkunen, Salgado, 2013] [Plots from Paukkunen - LHeC workshop - Jan 2014] Doga Gulhan, IS2013, Spain Provide new constraints to gluon distributions ESHEP - Garderen - June 2014 Heavy Ion Collisions 14
DGLAP approach - Some recent results II Dijet data in proton-nucleus collisions at LHC - CMS Preliminary CMS data “by eye” Preliminary CMS data “by eye” [Eskola, Paukkunen, Salgado, 2013] [Plots from Paukkunen - LHeC workshop - Jan 2014] Doga Gulhan, IS2013, Spain Provide new constraints to gluon distributions ESHEP - Garderen - June 2014 Heavy Ion Collisions 14
ESHEP - Garderen - June 2014 Heavy Ion Collisions 15
ESHEP - Garderen - June 2014 Heavy Ion Collisions 15
Saturation in the dipole picture A convenient way of discussing the problem is the dipole picture A dipole measures the color correlations in transverse plane Propagator of the quark - Wilson line � ⇥ ⇤ dx − A + ( x ⊥ , x − ) W ( x ) = P exp i | α � ; β � ⇥ � S α � β � αβ | α ; β ⇥ = W α � α ( x ⇥ ) W † So that the S-matrix is β � β ( ¯ x ⇥ ) | α ; β ⇥ and the total interaction probability (cross section w/ needed factors) ⇤ ⇥⌅ 2 P q ¯ q W ( x ⊥ ) W † ( ¯ � tot = 2 − Tr x ⊥ ) N C Students day - QM14 IS: observables and concepts 16
Medium averages All the medium properties are encoded in the averages of Wilson lines Several prescriptions used. Here, just focus on a simple one ⇤ ⌅ 1 − 1 2 W ( x ⊥ ) W † ( ¯ x ⊥ ) 2 � ⇥ N Tr x ⊥ ) ≈ exp 8 Q sat ( x ⊥ − ¯ The dipole “counts” the number of gluons, the unintegrated gluon distribution d 2 r � ⇥ − 1 ⇤ 8 Q 2 sat r 2 2 π r 2 e i r . k N ( r ) N ( r ) = 1 − exp = φ ( k ) = ⇒ [up to logs: McLerran, Venugopalan 1994] Two important consequences Qsat cuts-off the low momentum Geometric scaling φ = φ ( k 2 /Q 2 sat ) Students day - QM14 IS: observables and concepts 17
QCD evolution A way of including QCD evolution in the dipole picture (in x ) Boost the dipole: the splitting probability can be computed Use the large-N limit [Balitsky-Kovchegov eqs] Students day - QM14 IS: observables and concepts 18
Fits using non-linear evolution eqs. [AAMQ S - 2010] [Albacete, Dumitru, Marquet 2013] 2 2 2 2 Q =0.85 GeV Data Q =2.0 GeV 1.5 2 2 Theory ch rcBK-MC, min bias 1.8 p-Pb s = 5.02 TeV R pPb ( η =4) NN 1 rcBK-MC, Npart >10 σ 1.6 ALICE, NSD, charged particles, | η | < 0.3 cms cme= 5 TeV r rcBK-MC, LO+inelastic term α=0.1 0.5 1.4 1.5 1.5 EPS09 nPDF 1.2 − 5 − 4 − 3 − 2 − 5 − 4 − 3 − 2 2 2 2 2 Q =4.5 GeV Q =8.5 GeV 1 1.5 1 1 0.8 1 Saturation (CGC), rcBK-MC 0.6 σ Saturation (CGC), rcBK r Saturation (CGC), IP-Sat 0.4 0.5 0.5 0.5 0 2 4 6 8 10 12 14 16 18 20 1.8 0 Shadowing, EPS09s ( π ) − 5 − 3 − 5 − 3 2 2 LO pQCD + cold nuclear matter 2 2 Q =12.0 GeV 1.6 Q =10.0 GeV 1.5 0 0 1.4 0 2 4 6 8 10 12 pPb 1 p t (GeV/c) 1.2 σ R r 1 2 2 0.5 ch rcBK-MC, min bias R pPb ( η =6) 0.8 rcBK-MC, Npart >10 − 5 − 3 − 5 − 3 2 2 0.6 2 2 Q =15.0 GeV cme= 5 TeV rcBK-MC, LO+inelastic term α=0.1 Q =28.0 GeV 1.5 1.5 1.5 0.4 EPS09 nPDF 0 2 4 6 8 10 12 14 16 18 20 s =0.28 1 1.8 g HIJING 2.1 σ DHC, s =0.28 g r 1.6 DHC, no shad. 0.5 1 1 DHC, no shad., indep. frag. 1.4 − 5 − 3 1.2 − 5 − 3 − 4 − 2 − 4 − 2 2 2 2 2 1.6 Q =35 GeV Q =45 GeV 1.5 1 1.4 0.5 0.5 1.2 0.8 1 1 σ 0.8 0.6 r 0.6 0.5 0.4 0.4 0 0 0 2 4 6 8 10 12 0.2 0 2 4 6 8 10 12 14 16 18 20 p t (GeV/c) − 5 − 3 − 3 − 4 − 2 − 4 − 2 p (GeV/c) 10 10 10 10 10 10 10 x x T Checks of validity of the formalism with proton-nucleus data ESHEP - Garderen - June 2014 Heavy Ion Collisions 19
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