How can we prove/disprove the relevance of Color Glass Condensate/saturation physics at the LHC? Javier L Albacete

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1 How can we prove/disprove the relevance of Color Glass Condensate/saturation physics at the LHC? Javier L Albacete Hard Probes, Cagliari, Italy, 7 May- June

2 What the CGC is about : coherence effects in high energy QCD (small-x) High gluon densities in the projectile/target Saturation: gluon self-interactions tame the growth of gluon densities towards small-x φ(x, k t ) ln(x /x) K φ(x, k t) φ(x, k t ) radiation recombination k t Q s (x) Breakdown of independent particle production A(k Q s ) g ga O()

3 What the CGC is about : coherence effects High gluon densities in the projectile/target Saturation: gluon self-interactions tame the growth of gluon densities towards small-x φ(x, k t ) ln(x /x) K φ(x, k t) φ(x, k t ) radiation recombination k t Q s (x) Breakdown of independent particle production initial scale Q which are then evolved by NLO DGLAP tion are the diffractive PDFs measured in DIS with proto dominatedhic by gluons, phenomenology resulting in a stronger shadowing fo for quarks. In Fig. the results from this approach for differences at small-x become even larger at smaller virtu Nuclear shadowing, String fusion, percolation ) =.69 GeV (x,q Pb Rg Resummation of multiple scatterings Rkt-broadening Pb Energy dependent cutoff in event generators GeV. EPS (x > 6 HIJING. ) for nuclear distributions from DIS and DY d a p+a 5 benchmarking programme removes these uncertain RHIC have already proven to be an appropriate testing gro straints for gluons; npdfs are also extensively used in phe -5 HKN nds - energy dependence of Pt-cutoff in EG HYDJET HERWIG PYTHIA Qs Pb - NUCLEUS See, however, Ref. [8] for contradicting PROTONresults. x - - s ) = GeV (x,q Pb Rg Fig. : Current knowledge of nuclear PDFs, shown as the ratio of g (x, Q ), obtained by the NLO global fits EPS9 [5], HKN7 [ Q =.69 GeV and Q = GeV. Also shown for Q = GeV which gluon shadowing is computed from the DIS diffraction cross se It is worth noticing that in contrast to RHIC, where unexplored regions of phase space. This complicates the transverse momentum particles observed in []. The exper before, data on inclusive production at high-p T has been i sqrt(s) (GeV)

4 What the CGC is about : coherence effects High gluon densities in the projectile/target Saturation: gluon self-interactions tame the growth of gluon densities towards small-x φ(x, k t ) ln(x /x) K φ(x, k t) φ(x, k t ) radiation recombination k t Q s (x) Breakdown of independent particle production ) =.69 GeV (x,q Pb Rg. EPS HIC phenomenology Nuclear shadowing, String fusion, percolation HKN nds - - x - - Resummation of multiple scatterings kt-broadening Energy dependent cutoff in event generators GeV energy dependence of Pt-cutoff in EG HYDJET HIJING. HERWIG PYTHIA Qs Pb NUCLEUS PROTON s sqrt(s) (GeV)

5 OUTLINE Coherence effects are essential for the description of data in HIC collisions (RHIC, LHC) Is the CGC effective theory (at its present degree of accuracy) the best suited framework to quantify those coherence phenomena in LHC HI collisions? Pros and Cons: - Derived from QCD within a controlled approximation -> Theory driven predictive power - Systematic unified description of different observables/collision systems - Limited degree of applicability: High-(x,Q) effects not accounted for small-x d.o.f (dynamical) valence d.o.f (static) fields! +! + sources P + k + LHC RHIC SPS Can you give us predictions for some observable that would allow to unambiguously distinguish the CGC approach from others? 5

6 (brief and incomplete) CGC Theory Status: Entering the NLO era Evolution Equations: φ(x, k) ln(/x) = K φ(x, k) φ (x, k) - Running coupling kernel in BK evolution for the -point function - Full NLO kernel for BK-JIMWLK [Balitsky Chirilli] - Analytic [D. Triantafyllopoulos s talk] and numerical [T. Lappi s talk] solutions of full B-JIMWLK hierarchy for n-point functions -... LO: αs ln(/x) NLO Running coupling W [ρ] Y =... Kovchegov Weigert Gardi Balitsky Production processes dn AB X d p... [φ(x, k); W Y [ρ]] - Running coupling and full NLO corrections to kt-factorization [Kovchegov, Horowitz, Balitsky, Chirilli] - Inelastic terms in the hybrid formalism [Rezaeian s talk] - Hadron-hadron, hadron-photon* correlations [Heikki s talk, Jalilian Marian s talk] - Factorization of multiparticle production processes at NLO [Lappi s talk] - DIS NLO photon impact factors [Chirilli] -... Used in phenomenological works? Yes No A bit :) 6

7 (brief and incomplete) CGC Phenomenology Status Empiric information needed to constrain: - Non-perturbative parameters: initial conditions for BK-JIMWLK evolution, impact parameter - K-factors to account for higher order corrections (effectively also for missing high-(x,q) contributions, energy-conservation corrections etc) :) proton :( nucleus Abundant high quality data at small-x Good simultaneous description of e+p and p+p data Global rcbk fits to constrain gluon distribution modelling! Few data at small-x LHC Pb+Pb data and RHIC dau forward data troublesome (more later) 7

8 The baseline: proton collisions. Global fits to e+p data at small-x Talk by P. Quiroga. Extract NP fit parameters.5 Q =.85 GeV Data Theory Q =.5 GeV Q =. GeV Fit including heavy quarks Q =. GeV Q =8.5 GeV Q =. GeV φ(x, k t ) k γ t γ > Q =5. GeV Q =5 GeV Q =8. GeV Q =5 GeV Set Q s,proton (GeV ) γ MV. h.68.9 h x x 8

9 The baseline: proton collisions. Global fits to e+p data at small-x Talk by P. Quiroga. Extract NP fit parameters.5 Q =.85 GeV Data Theory Q =.5 GeV Q =. GeV Fit including heavy quarks Q =. GeV Q =8.5 GeV Q =. GeV φ(x, k t ) k γ t γ > Q =5. GeV Q =5 GeV Q =8. GeV Q =5 GeV Set Q s,proton (GeV ) γ MV. h.68.9 h x x. Run consistency and stability checks Q [GeV ] combined HERA data DGLAP evolution DGLAP fitted region rcbk evolution rcbk fitted region unfitted region rcbk (xcut, Q ) x DGLAP d rel - DGLAP Q =.5 GeV Q =6.5 GeV Q =. GeV Q =5. GeV DGLAP d rel - rcbk x x Q =.5 GeV Q =6.5 GeV Q =. GeV Q =5. GeV Q =5. GeV rcbk rcbk fits more stable than DGLAP fits at small-x 9

10 The baseline: proton collisions. Global fits to e+p data at small-x Talk by P. Quiroga. Extract NP fit parameters.5 Q =.85 GeV Data Theory Q =.5 GeV Q =. GeV Fit including heavy quarks Q =. GeV Q =8.5 GeV Q =. GeV φ(x, k t ) k γ t γ > Q =5. GeV Q =5 GeV Q =8. GeV Q =5 GeV Set Q s,proton (GeV ) γ MV. h.68.9 h x x dn ch /d!d p T (/GeV ). Apply gained knowledge in the study of other systems (theory driven extrapolation) dn g dηd K α s (Q r ) φ(x, k t ) φ(x, k t pt) FF(Q f ) p t CDF! <,.96 TeV CMS! <., 7 TeV LO kt-factorization: proton-proton MV model "= e+p data "> dn ch /d!d p T (/GeV ) proton-proton MV model "= e+p data "> p T (GeV) p T (GeV)

11 Nuclear ugd s and nuclear modification factors Setting up the evolution φ Pb (x, k t, B) =φ p (x, k t ; {Q s,p Q s,pb(b)); γ} B φ Pb (x, k t, B) =rcbk[φ Pb (x, k t, B)] RHIC forward data does not constraint the NP parameters for the initial condition Other approaches based on high-x energy loss effects also account for data Rapidity dependent K-factors Fujii-Itakura-Nara dn/d!d p T [GeV ].... e-5 e-6 dau BRAHMS h!=. (x) BRAHMS h!=. (x) 5 p T [GeV/c] STAR "!= param h rcmv

12 Nuclear ugd s and nuclear modification factors Setting up the evolution φ Pb (x, k t, B) =φ p (x, k t ; {Q s,p Q s,pb(b)); γ} B φ Pb (x, k t, B) =rcbk[φ Pb (x, k t, B)] A) Most natural option: Q s,pb(b) = T A (B) Q s,p γ Pb = γ p (> ) PROBLEM: yields RpPB > at high transverse momentum B) Possible solution Q s,pb(b) = T A (B) /γ Q s,p and/or.8.6 RpPb(y=) rcbk-mc EPS9 npdf IPSat A) rcbk.6.6 B).8.8 γ Pb = (MV)+ # A / npdf EPS9 results by P Quiroga RpPb(y=) rcbk-mc EPS9 npdf rcbk IPSat pt (GeV) Preliminary results. JLA-Dumitru-Fujii-Nara pt (GeV).8.6.

13 Moving forward (pt, yh>>) Yet another issue: Where to switch from kt-factorization to hybrid formalism? x () m t s exp(± y h ) Midrapidity: kt-factorization: dn g dηd p t φ p (x ) φ Pb (x ) Forward rapidity: hybrid formalism dn dηd p t pdf p (x ) φ Pb (x ) npdf EPS9 results by P Quiroga Preliminary results. JLA-Dumitru-Fujii-Nara RpPb(y=) rcbk-mc EPS9 npdf rcbk-mc-hybrid pt (GeV).8.6.

14 Moving forward: Testing the evolution (pt, yh>>).8.6 RpPb(y=) rcbk-mc EPS9 npdf rcbk IPSat RpPb(y=) rcbk-mc EPS9 npdf rcbk-mc-hybrid RpPb(y=) Preliminary results. JLA-Dumitru-Fujii-Nara. pt (GeV) npdf EPS9 results by P Quiroga rcbk-mc-hyb EPS9 npdf pt (GeV) RpPb(y=6) rcbk-mc-hyb EPS9 npdf pt (GeV) 6 8 pt (GeV)

15 Moving even forward: LHCf (?).8 At large rapidities (very small-x) the sensitivity to i.c is reduced (scaling regime) Fixed dn/d!/d p t [GeV - ] Running DHJ,sqrt(s)=7TeV, p+p->h+x,!=8.5,cteq6msbar+ KKP(NLO) p t [GeV] Fujii-Itakura-Nara ", K=. (K + K bar)/, K=. (n + nbar)/, K=. Hadron production (π K and n) at η =8.5 is being studied in this framework BFKL BFKL k RpPb should approach its universal limit at.5 very forward rapidities. Scaling properties. generated by evolution should be clearly. visible in this region BK. ϕ(x, k t,b). ϕ(k t /Q s (x, b)) BK - - k BK - 5 BK BK 5 5 k LBL Feb 5 p. 5

16 Forward di-hadron angular correlations Marquet 7, Dominguez et al CGC description: A quark (gluon) emits a gluon. The pair scatters off the target x p = k e y + k e y s ( ) At small-x, the transverse momentum transfer is controlled by the saturation scale Angular decorrelation happens if Q Pb s (x A ) (k, k ) trigger φ Coincidence probability x A = k e y + k e y s CP( φ) = dn pair N trig d φ Ergo, decorrelation should be stronger with Increasing rapidity of the pair Increasing collision centrality Decreasing hadron momentum 6

17 fw T N/d(# " ) dp... Forward di-hadron angular correlations in RHIC dau data d$ fwd /N midd fwd T N/d(# " ) dp.6.5-! GeV/c.6 p+p b =.88.5 d+au 6-88 b = d+au - b =.8.. central-forward. -! GeV/c.6 b (pp) =.86 b (da 6-88) = b (da -) # " (rad) = ! GeV/c b (pp) =.8 b (da 6-88) =. b (da -) = FIG.. : (color online)..pedestal-subtracted π -π per-trigger correlation. functions. for, as indicated,. p+p, d+au peripheral (6 88% centrality) and d+au central ( % centrality) collisions at s NN = GeV; the associated π s... of p T =.5.75 GeV/c are measured at forward rapidity (. < η <.8) and 5 6 the triggered 5π 6 s are measured 5 6at midrapidity ( η <.5) for# the " (rad) indicated p T ranges. The subtracted pedestal values, forward-forward b, are also indicated. FIG. : (color online). Pedestal-subtracted π -π per-trigger correlation..-.6 functions! GeV/c for,..6- as! indicated, GeV/c. p+p, -5! d+au GeV/c peripheral (6 88% centrality) and d+au central ( % central- p+p ity) collisions at b pp pp =.79 b =.59 da 6-88 da d+au b =.7.8 b =.6 d+au - s NN = GeV; the associated π da - da - b s =.6 b =. of p.6 T =.5.75 pp GeV/c.6 are measured.6 at forward rapidity b =.95 (. < η <.8) and the triggered π da 6-88 b s are measured at =. midrapidity. da - ( η b <.5). for the indicated. =.76 p T ranges. The subtracted pedestal values, b, are also indicated. /N midd d$% % d$% % T N/d(# " ) dp T N/d(# " ) dp d /N clus d /N clus ! GeV/c..6-! GeV/c. -5! GeV/c p+p pp pp b =.79 b =.59 da 6-88 da d+au b =.7.8 -b =.6 d+au - da - da - b # = ".6 (rad) b =..6 d+au - pp da 6-88 b b b b = =.95.6 =. =.76. b (da -) =.85 FIG. : (color online). Pedestal-subtracted cluster-π pertrigger correlation. da -. functions measured at forward rapidity (.. < η <.8) for,.as indicated, p+p,. d+au peripheral (6 88% centrality) and d+au central (-% centrality) collisions at s NN = GeV; the correlation functions are for associated - π s of p T = GeV/c and - trigger clusters # " (rad) over the indicated p T ranges. Systematic uncertainties of up to % on the near side ( φ <.5) are not shown. FIG. : (color online). Pedestal-subtracted cluster-π The pertrigger correlation functions measured at forward rapidity subtracted pedestal values, b, are also indicated. (. < η <.8) for, as indicated, p+p, d+au peripheral....6 b (da -) =.8 tweenthe midrapidity relative yield, andjforward-rapidity da [], of correlated π pairs, back-to-back per π trigger hadron detected pairsat inmidrapidity, d+au collisions in p+p, compared peripheral to p+p d+au, collisions central scaled d+au with collisions N for varying trigger π coll, p T. and Figure shows the same correlations for trigger clusters where the cluster-π J pairs are both detected at forward tuned to match da = I the data. da R t da da = /σ da rapidity. The constant pedestal, Nb coll, was σ pair, () pp subtracted /σ pp from the Figure correlation shows function. the azimuthal The correlations angle correlations were corrected between for the where midrapidity forward RdA t π = and detection (/ N peripheral forward-rapidity coll ) efficiency (σda t /σ da)/(σ and π for pairs, pp t /σ the pp) per combinatoric background beneath the inπp+p, forperipheral trigger particles d+au, t, is πthe trigger usual detected nuclear at modification midrapidity, factor peaks in the photonpair invariant d+au t, and σ and central and σ, σ masscollisions pair are the for cross varying sections trigger (or normalized spectra. This background is π determined shows the same correlations for trigger clusters p T. yields) for the full event selection, trigger particle event Figure selection, by measurement and dihadron of the where the cluster-π pairs central pair azimuthal event selection. correlations for photon-pair mass selections are adjacent both detected to the πat I forward da is the ratio of conditional hadron yields, CY, for mass d+auwin- dow and p+p and rapidity. The from collisions: constant studies of pedestal, simulated b, jet was events subtracted from pythia from the events correlation processed function. throughthe correlations fwd were corrected fwd d+au 6-88 p d+au - p for the- forward π T T PISA. For the midrapidity/forward-rapidity detection d( φ)[dn/d( φ) efficiency and for b ] the combinatoric background beneath thegev/c π peaks in the GeV/c CY = correlations (Fig. ), due to the large N t pseudorapidity ɛ a η a pgap a, () T ofphoton- pair invariant mass spectra This GeV/c background.75-. is GeV/c η. between determined by measurement.-.5 corrected with the the acceptance hadrons, only an away-side of thegev/c dihadron azimuthal.-.5 correlation peak ( φ=π) correlations GeV/c functionfor dn/d( φ), the forward-forward the number ofcorrelations trigger particles a near-side is seen. for photon-pair mass selections - adjacent frag - x to the π N mass window and from studies of simulated jet events from pythia a and yields the t, the peak detection ( φ=) efficiency is also present for the associated (see Fig. Au particle ). The ɛ and widths level of the of the uncorrelated correlated pedestal pairs in arethe extracted correlation byfunc- processed away-side b through GaussianPISA.. The integral issignal taken shape over the plus Gaussian a constant fit of fits events to antions background Forthethe away-side (b midrapidity/forward-rapidity correlations (Fig. Observed ). peak. The fit ThetoJ the forward-forward correlationtematic has an uncertainty additional from Gaussian the ZYAM signal pedestal for the η. subtrac- near- ), due to the decorrelation da uncertainties include a sys- large pseudorapidity IS stronger gap of with between side peak. tion. Increasing the The determining hadrons, pedestal rapidity only this an uncertainty determined of away-side the pair itfrom was peak assumed a ( φ=π) fit in that the is midrapidity/forward-rapidity seen. Increasing For the forward-forward collision correlations centrality correlations and a near-side is consistent with ( φ=) Decreasing the pedestal is also present hadron level found (see momentum based Fig. ). on the Theassump- yields peak and tionwidths that the ofsignal the correlated yield is pairs at theare minimum extracted of the by fits correlation an away-side functiongaussian - zero yield signal at minimum shape plus(zyam) a constant []. to background In the forward-forward (b ). The fitcorrelations to the forward-forward the ZYAM pedestal correlations is used has in the an additional yield extraction. Gaussian Additional signal for systematic the nearside uncertainties peak. The of pedestal up to % is determined (not shown from in a Fig. fit in ) the are midrapidity/forward-rapidity ascribed to the near-side peak correlations due to corrections and is consistent for resonance with the decays pedestal that level contaminate found based the on jet the signal, assumption and due that to the the acceptance signal yield loss is around at the the minimum trigger of particle the correlation of φ η function.5 - zero.5 yield rad, resulting at minimum from (ZYAM) the minimum []. In the forward-forward correlations the ZYAM pedestal J da PHENIX data σ pair separation cut of one tower between cluster peaks in the lat tro I da wi Fo for ra an J d lis th ing for ra su nu in pr cle ( p in tw th av 7

18 Forward di-hadron angular correlations in RHIC dau data CP(!").6... Central dau collisions STAR Preliminary p t < GeV/c <# t > =. p t > p a > GeV/c <# a >=. CGC calculations Stasto et al Albacete-Marquet "non-cgc" calculations Kang et al Uncertainties in current CGC phenomenological works: Need for a better description of n-point functions: [D. Triantafyllopoulos s and T. Lappi s talk] Better determination of the pedestal: K-factors in single inclusive production? Role of double parton scattering? [Heikki Mäntysaari s talk] correlated! " # "!!" " " # $%& uncorrelated Alternative descriptions including resummation of multiple scatterings, nuclear shadowing and cold nuclear matter energy loss seem possible... π π! # π $'& Strikman, Vogelsang, 9.6 π 8

19 di-hadron angular correlations at the LHC Analogous decorrelation phenomena should be seen at the LHC The increase in collision energy implies that they should be visible at * Lower rapidities of the produced pair * Higher transverse momentum All previously mentioned details are been taken care of. Stay tuned!!! [Heikki Mäntysaari s talk] 9

20 hadron-photon* correlations in ppb collisions at the LHC hadron-dilepton pair hadron-photon Stasto et al.86 Jalilian-Marian s talk Y γ = Y π = M = GeV M = 8 GeV These processes are theoretically cleaner: Only knowledge of -point needed!!

21 Miscellanea CGC gives a very good descriptions of bulk features of multiparticle production dn ch /d! / N part rcbk MC viscosity? LHC ALICE MV i.c. MV "=.9 i.c N part CGC: Non-linear and non-local Knowledge of the hard part of nuclear UGD would further constrain the description of the initial state!

22 dn ch /d! / N part Miscellanea CGC gives a very good descriptions of bulk features of multiparticle production rcbk MC viscosity? LHC ALICE MV i.c. MV "=.9 i.c N part CGC: Non-linear and non-local Knowledge of the hard part of nuclear UGD would further constrain the description of the initial state! Can we see the saturation scale?? Semihard scale in p+p ridge Scaling laws?.5 UGD s at x=!!.5 Q s =. GeV Q s =. GeV Q s =. GeV.5.5 McLerran Praszalowic.5 k t Dumitru et al τ = p t /Q s (x)

23 Outlook Important steps have been taken in promoting GCG to an useful quantitative tool - Continuos progress on the theoretical side - Phenomenological effort to systematically describe data from different systems (e+p, e+a, p+p, d+au, Aa+Au and Pb+Pb) in an unified framework Most solid CGC predictions for the upcoming p+pb run: - Suppression of nuclear modification factors at moderate pt already at mid-rapidity - Stronger suppression at more forward rapidities (evolution) - Suppression of di-hadron and photon-hadron angular correlations Current predictions carry some uncertainty due to lack of data to constrain NP aspects of nuclear UGD. This problem can be largely fixed through the measurement of simple observables (i.e. single inclusive spectra) in p+pb collisions The CGC will not be neither proven nor disproven at the LHC, it will be improved GRAZIE!

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