iv convegno nazionale fisica alice, palau, 29.09.08 andrea dainese 1 andrea dainese (infn legnaro)...

IV Convegno Nazionale Fisica ALICE, Palau, 29.09.08 Andrea Dainese 1

Andrea Dainese Andrea Dainese (INFN Legnaro)(INFN Legnaro)

Charm as a probe Charm as a probe of small-x gluons at LHCof small-x gluons at LHC


Layout

Small-x gluon dynamics, the onset of saturation and nuclear shadowing

Study of small-x effects with charm in ALICE

Appendix: status of open charm reconstruction code

open items

more details: presentation at PWG3 meeting, Sept 15th


Parton densities at small-xDIS ep collisions probe distributions of partons in the proton:

HERA: strong rise of F2(x,Q2) at small-x:

Q2

Q 2 = “resolving power”

F2 ,F

3 ,F

L= proton structure functions, (y = inelasticity)

Bjorken x = momentum fraction carried by parton

∂lnF2 /∂lnQ2 gluons

couples directly

only to (sea) quarks


(x,Q2) evolution of PDFsQ2 - DGLAP (kT-ordered emission): F2(Q2)~sln(Q2/Q0

2)n, Q02 ~1 GeV2

x - BFKL (pL-ordered emission): F2(x)~sln(1/x)n

Linear equations (single parton splitting) fail at low x (even more for multi-parton systems = nuclei) Onset of Saturation:

(i) Standard (linear) approach to PDF

evolution invalid nonlinear gluon-

gluon fusion balances gluon splitting

(ii) Factorisation assumptions (pert. QCD)

invalid parton scattering not

incoherentColor Glass Condensate:

Collision of 2 classical saturated

fields (full parton coherence)


Saturation scale Qs2

Onset of non-linear QCD when gluons are numerous enough (low-x) & extended enough (low-Q2) to overlap:

Pb/Au nucleus (larger parton tranverse density) amplifies saturation effects by factor about 6 (2001/3)

Qs2~2 GeV2 (RHIC, dAu @ 200 GeV) x < 10-3-10-2

Qs2~5 GeV2 (LHC, pPb @ 8.8 TeV) x < 10-4-10-3

~0.3)

Saturation for: low x, large s, large y, large A

[GA(x,Q2)=A g(x,Q2)]

poorly knownPDFs in p and A


Low-x (<10-2) gluon PDF (proton)

Most of our current knowledge of low-x gluons comes indirectly from F2 “scaling violations”:

Large uncertainties below x~10-2 at moderate Q2 (<5 GeV2)R.D. Ball et al. arXiv:0808.1231

Q2 = 2 GeV2


Low-x (<10-2) gluon PDF (nucleus)Current data on low-x gluons from: nuclear F2, nuclear Drell-Yan (eA)

DGLAP analysis: linear evolution + nuclear shadowing Shadowing low-x gluon “fusion” or “recombination”: gx1 + gx2 gx1+x2

Shadowing factor for PDFs: xGA(x,Q2) = A xg(x,Q2) RGA(x,Q2)

Most of data in non-pert. range (Q2<1-2 GeV2) limited applicability of pQCD analysis large uncertainties (> factor 2!)

D.d’Enterria,JPG30 (05)S767Armesto, JPG32(06)R367


D.d’Enterria,JPG30 (05)S767

Qs2

RHIC=2

Experimental study of saturation:(x,Q2) acceptances & probes

Probes at RHIC (Q2<2):• bulk dNch/dy• forward hadrons (pT<3-4 GeV)


LHC: study “saturation” with perturbative probes

LHC (Pb): unexplored region (A=208, x<10-3, Q2 < 5 GeV2) large uncertainties in p and Pb PDFs


Qs2

RHIC=2

Experimental study of saturation:(x,Q2) acceptances & probes

Probes at RHIC (Q2<2):• bulk dNch/dy• forward hadrons (pT<3-4 GeV)

Qs2

LHC=5

LHC y=0

LHC y=4

Probes at LHC (Q2<5):• bulk dNch/dy• forward J/ • charm

new!


Heavy quark production at LHC:a probe of small-x gluons

Probe small-x region with HQs at low pT and/or forward y

Charm production at low pT: Q2 (4mc2 ~ 5 GeV2) ~ Qs

2

Down to x~10-4 with charm at y=0, x~10-6 at y4

Large pert. yields: ~0.1 cc/ev in pp, ~100 cc/ev in Pb-Pb

Cartoon

char

mbea

utyPYTHIAcharm


Low-x at LHC: forward J/ in ALICE (2.5 <||< 4)

J/ measurement in -spectrometer xg(x) at x2~10-5 :

d/dy J/: NLO CEM, varying PDFs

QQbar: Sensitive to different PDFs and DGLAP versus CGC predictions

(Note: mJ/~Q

s at the LHC)

pp @ 14 TeV

D.Stocco


Heavy quark production at LHC:a probe of small-x gluons (pp)

Large pQCD uncertainties for charm pT0

onset of non-linear effects / saturation ?

Two attempts to include non-linear

terms in evolution equationsDGLAP+GLRMQ

BK

indicate charm as a sensitive probe in pp

Eskola et al., NPB660 (2003) 211

Kutak, Kwiecinski, Martin, Stasto

BK: Charm suppression due to non-linear effects

DGLAP: Charm enhancementdue to non-linear effects (GLR-MQ term)

|y| < 1


Saturation scale Qs2(x) ~ xg(x)A/RA

2 ~ xg(x)A1/3

At LHC for x~10-4, Qs~2 GeV > mc

For mT,c~Qs, charm prod. CGC-dominated:• scales with Npart in pA (not Ncoll)• harder pT spectra, since typical kT~Qs~2 GeV, while in standard factorization kT~QCD~0.2 GeV

CGC: very similar RpA for p, D, B, ~indep. of pt and rapidity

Heavy quark production at LHC:a probe of small-x gluons (pPb)

Kharzeev, Tuchin, NPA 735 (2004) 248 + LHC Predictions Workshop


Heavy quark production at LHC:a probe of small-x gluons (pPb)

Charm production at low pT sensitive to shadowing parametrisations

Sensitive to multi-parton

interactions in pPb:

cccc / cc ~ 10%

probe “many-body” PDFs

ppN

pPbR

coll

DpPb

Cattaruzza, Del Fabbro, Treleani, PRD 70 (2004) 034022

K.Eskola et al. JHEP 0807 (08)102


Low-pT charm in ALICE (|η|<1)

Open charm reconstruction down to pT=0 in the ALICE central barrel

D0K-+

1 LHC year (2009)

ppN

pPbR

coll

DpPb

R.Grosso


Status of software toolsfor open charm analysis


Invariant mass analysis and significance maximization (in bins of pt, y, RP…)

Charm Analysis Scheme

Charm “production” (ESD/AOD AOD for Charm)

Signal selection

Correction for efficiencies, acceptance, BR

KDselectedN 0

1||

0

yt

Dproduced

dydp

dN

Cross section normalisation1||

0

yt

D

dydp

d


AliAnalysisVertexingHF classsingle-track cuts on pt and d0, (PID...)

[common cuts for all analyses]

build all (+,-) pairs and compute secondary vtx

create AliAODRecoDecayHF2Prong apply reco cuts

store D0

loop on all tracks (+ & -): build triplets, create AliAODRecoDecayHF3Prong,apply reco cuts (common for the 3 particles?)

loop on all tracks (+ & -) …create AliAODRecoDecayHF…

store D+, Ds+, c store D0

for D+,Ds+,c

+

for D0K

for D0K, J/ee

store J/

tight D0 mass cutloop on all tracks for D* candidates

store D*+

BA,LNL,TO


Output in AliAODEvent

All the I/O managed by AliAODHandler & AliAODInputHandler

Macro to read the candidates: ReadAODVertexingHF.C

AliAODEvent

AliAODTrackAliAODTrackAliAODTrack

...

AliAODVertexAliAODVertexAliAODVertex

...

tracksvertices (from ESD)

aodTree in AliAOD.root(standard AOD, unchanged )

AliAODRecoDecayAliAODRecoDecay

...

AliAODVertex...

verticesHF D0toKpi, ...

friend aodTree in AliAOD.VertexingHF.root

not yet ...


Open items (hadronic charm)

AliAnalysisVertexingHF: possibility to start from AOD (AODs will be are smaller than ESDs faster analysis; will be replicated in more Tiers faster access)

ITS alignment & impact parameter resolutionalignment result from cosmics suggest that we can reach the target precision ( S.Moretto)estimate residual misalignment & impact parameter resolution from data

User analysis toolsevent mixing for background subtraction: try using official event mixing framework (AliAnalysisTaskME)

PIDoptimize cuts on TOF PID & play with Bayesian method (priors)

Correction Frameworkpossibility start from AOD, adapt to charm (selection cuts...)

Correction for b feed-downprepare subtraction using MC with QCD cross section and errors (collab. muon people and theorists)


SummaryGluon saturation & non-linear evolution must set-in at (some) low-x in the hadrons → Basic info on high-energy limit of QCD

LHC = unique lab to study high parton density dynamics in p,Pb down to x~10-6 using perturbative probes (charm)

some hints already in pp

crucial to have p-Pb run

LHeC (Large Hadron electron Collider – under discussion within EFCA) would be “the” machine for this physics

ALICE has good capality with charm(onia) at low pT and forward rapidity

Preparation of D0 / charm analysis in progress (need to focus on background estimate and on corrections)


EXTRA SLIDES


Charm at s = 14 TeV: x,Q2 range (1)

Simple estimate: ccgg

2

121

2 ln2

1 ˆ

x

xysxxsM cccc

c

c2/1 sx

2/2 sx)exp( )exp( 21 cc

cccc

cc ys

Mxy

s

Mx

0 ,0 GeV, 2.1 TeV, 14 tccc pyms

421

2222

102~

GeV 8.5)2(

xx

mMQ ccc


How to detect the enhancement due to nonlinear effects?

The idea is that the effect (enh. only at very low pt) cannot be mimicked by NLO pQCD

In practice: consider ratio “Data/Theory” for all reasonable choices of

theory parameters

Dain

ese

, Bon

di la

, Esk

ola

, Kolh

inen, V

ogt ,

JPG

30

(200

4) 1

787

Data:mc = 1.2 GeV, Q2 = 4mT

2

and enhancement

Data:mc = 1.3 GeV, Q2 = mT

2

and enhancement


Experimental study of saturation:acceptances & probes

A=200



Qs2

RHIC=2

Saturation in heavy-ion collisions:(x,Q2) acceptances & probes

Probes at RHIC (Q2<Qs2=2):

• bulk dNch/dy• forward hadrons (pT<3-4 GeV)


Saturation hints at RHIC (1):hadron multiplicity dNch/d

AuAu (200 GeV) 0-5% most central collisions:

Reduced multiplicity predicted by saturation models: gluon recombination reduces incoming parton flux

dNch

/d

Predicted multiplicites:

dNch

/d~ 650 charged particles at y=0


Saturation hints at RHIC (2): geometrical scaling of dNch/d

CGC: final hadron multiplicity initial number of released gluons Qs

2:

Centrality & √s dependence well described:

+ “local parton-hadron duality” (1 gluon = 1 final hadron)Collision of 2 classical

(saturated) fields

Armesto, Salgado, Wiedemann, PRL94 (2005) 022002


Saturation hints at RHIC (3): forward hadron suppression

~0 (x~10-2): hard hadroprod. described by NLO pQCD (+little shadowing)

=3.2 (x~10-3): suppressed hadron production (pT<4 GeV/c) better described by CGC than pQCD: reduced partonic flux in Au at low-x CGC

pQCD+shadowing

< 0.5

= 3.2


Saturation at LHC:charged multiplicity

LHC Predictions Workshop, compiled by N.Armesto

Saturation-drivenpredictions: dNch/d = 1500

day-1 measurement for all experiments

+ “local parton-hadron duality” (1 gluon = 1 final hadron)

CGC:


GLR-MQ vs BK: gluons

Linear (dashed) vs non-linear (solid) in BK with R = 4 GeV-1

Linear (blue) vs non-linear (red) inDGLAP

R = 4 GeV-1 = 0.8 fmR = 5 GeV-1 = 1 fm

Eskola, Honkanen, Kolhinen, Qiu, Salgado, NPB660 (2003) 211 Kutak, Kwiecinski, Martin, Stasto

Caveat: LO!


Effect of ITS misalignment on d0 resolution

Impact parameter resol: track = ascatter/pt bmeas cmisalign

Effect studied with full simulation of exptected initial (full+) and residual (after realignment) misalignments

nullresidualfullfull+


Corrections & Errors:Beauty feed-down (~10%)

Methods:

1. Monte Carlo with state-of-the-art pQCD input

2. use single measurement from ALICE?

Systematic error: uncertainty on b-bbar cross section from theory (estimated to be ~8-10%)


Corrections & Errors:Acceptance, Reco & Selection EfficiencyEmbedding of MC signal in MC (or real?) events, and calculate all corrections in one go (selected in-acceptance). Average correction in {pt, y} grid

use ALICE common correction framework

needed tuned MC (good descr. of tracking effs and resols) d0 resol. to be evaluated from data (cosmics and pp) vs pt,

Alignment and calibration: simulation: same as for data reconstruction reconstruction: same as for data reconstruction + estimated residual

misalignment and miscalibration

Systematic error:compare weights in different runs, and with the two field orientations (+z and -z)

check stability of extracted yield VS variation of cuts

repeat everything with different sets for alignment corrections?

iv convegno nazionale fisica alice, palau, 29.09.08 andrea dainese 1 andrea dainese (infn legnaro)...

Documents

x gluons

x effects

q2q2 q

inelasticity bjorken

saturation scale q s

evolution of pdfs q

layout smallx gluon

parton coherence slide