Forward Physics(Experiments)
Tatsuya Chujo
Univ. of Tsukuba
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」
Mar 26, 2015
理化学研究所（和光市）

Outline
1. Introduction
2. Forward physics for HIC• QCD Factorization theorem• Nucleon parton distributions at small x• Gluon saturation (CGC)• Nuclear parton distributions

3. Experimental results from RHIC/ LHC
4. Future experiments
5. Summary
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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1. Introduction

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

“My” motivation of this field• It is often said that CGC is the initial state of heavy
ion collisions at RHIC/LHC.
• Do we see a gluon saturation effect at RHIC andLHC?
• How clearly see the effect?
• Are there any links from the initial condition to theearly thermalization process of QGP, “Glasma”?
• What is the best probe(s) of CGC in the future
experiments?

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo4

Initial condition and thermalization%Ini(al%condi(on,%CGC,%%early%thermaliza(on%Color%Glass%
Collision%Condensate%(CGC)

“Glasma”

sQGP%produc<on,%thermal%
dynamics%QGP

strongly%coupled%%
Quark%Gluon%Plasma%% Hadron%gas
(sQGP)

t%=%@1%fm%%%%%%%%%%%%%%%%%%t%=%0%fm%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%t=%0.6%fm/c%%%%%%%%%%%%%%%%%%%%t=%20%fm/c%
Time

RHIC/LHC data suggests an early thermalization of QGP(~ 0.6 fm), and it is still a big missing link between initialcondition to QGP.
➠ Need a direct probe to access to initial condition
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

52. Forward Physics forHeavy Ion Collisions

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

Factorization theorem of QCD
Hadron production by hard scattering processes

Parton (a, b), incoming nucleon (A, B), hadron hpA(Β) : momentum for nucleon A (Β)pa(b) : momentum for parton a (b)x1 = pA / pa
x2 = pB / pbf(x,Q2): parton distribution function (PDF)
Q2: momentum transfer
Ek dσab→k/d3pk :Elementary cross section to produce parton “k” from a and b, in the final
momentum pk
Dh|k(z): fragmentation function from parton k to hadron h.
z = ph/pk: momentum fraction of hadron h carried by parton k
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

7Factorization theorem of QCD

PDFs are determined empiricallyby the best fit to the data, assuming
the factorization theorem andDGLAP (Q2) evolution.
NNPDF collaboration:
DIS, Drell-Yan, inclusive jet, weak
vector boson, and charm→determined “NNPDF2.1” PDF
R. D. Ball et al., (NNPDF Collaboration),
Nucl. Phys. B 849, 296 (2011).

Strong support for factorization
assumption with universal PDFs (w/DGLAP evolution).

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

8Gluon Density
distribution function

high energy

hard process

momentum fraction

At small x and small Q2, the parton density
will become large by non-linear effects due to gluon fusion
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

9Gluon saturation (CGC)
Gluon density per area
(nucleon cross section: π r2)
Gluon fusion cross section

When;➡Gluon density saturate, called;• Gluon Saturation, or• Color Glass Condensate (CGC)
Saturation scale:gluon saturation: Qs < Q→ Exclusion of small x and Q2 region,
better PDF determination by the exp. data fit(NNPDF)

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

10kinemacis (x-Q2)• Large increase in beam energy
at LHC,• Measurements at the LHC probe
the nucleon PDFs over a wide
region of x and Q2 that is muchgreater than that previouslyaccessible.

Bulk properties in HI
and gluon saturation physics→ low Q2 and low x
only PDF determined by DIS,
but there are uncertainties of gluon PDF
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

11Uncertainty of Gluon PDF at low x and Q2Gluons are uncharged they are not directly probed in the DIS process,Constrained only indirectly way→ gluon PDFs have rather large uncertainties in the low-x and low Q2.
Ratio of the gluon distribution function xg(x;Q2) to the nominal value with shaded region

low Q2 (=2GeV2)high Q2 (=100GeV2)
R. D. Ball et al., (NNPDF Collaboration),
Nucl. Phys. B 849, 296 (2011).

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Nuclear PDF (nPDF) for gluons• Ratio of the gluon distributionfunction of a Pb nucleus to thatof the proton

C. A. Salgado et al., J. Phys. G 39, 115010 (2012)• A large spread for small x→ lack of constraints due to thelimited set of relevantmeasurements (nuclear targets)• Gluon density of a nucleus ofmass number A can be written
as (approximately);

Effects of gluon saturation in nucleus:
set in at a factor 6 x higher Q2 with a heavy nucleartarget• Impact parameter dep. in p+A collisions• central: A1/3 ~ 6 (for Pb)• peripheral: ~1
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Probing CGC at RHIC/ LHC
RHIC

LHC

Larger kinematic reach in saturation region at LHC, compared to that at RHIC.
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3. Experimental resultsfrom RHIC/ LHC

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

RHIC d+Au (2003)

PRL 91, Number 7 (2003)
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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d+Au: “Control” Experiment
Au+Au= hot and dense medium
Initial + Final
State Effectsd+Au= cold medium
Initial State
Effects Only

• The “Color Glass Condensate” model predicts the suppression inboth Au+Au and d+Au (due to the initial state effect).
• d+Au experiment can tell us whether the observed hadronsuppression at high pT central Au+A is the final state effect or
initial state effect.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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RAA vs. RdA for charged hadrons and π0d+Au

Initial State
Effects Only

Au+Au
PHENIX (d+Au)
PRL 91, 072303 (2003)

Initial + Final
State Effects•No Suppression in d+Au, small enhancement observed (Cronin effect).

•d-Au results rule out CGC as the explanation for high pT Suppression ofhadrons in AuAu central.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Azimuthal Distributions in d+Au (STAR)* Pedestal andflow subtracted• Near-side: p+p, d+Au, Au+Au similar• Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au

Suppression of the back-to-back correlationin central Au+Au is a final-state effect
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

19• Wide kinematic coverage (|η| < 3.5)
• Particle Identification by TOF

Access to forward region(η < 3.2)

PRL 93, 242303 (2004)
BRAHMS

BRAHMS Rd+Au (MB) at forward rapidity

PRL 93, 242303 (2004)
BRAHMS•

Stronger suppression at low pT hadrons at forward rapidity.

•

At forward rapidities, data show a suppression at all pT.

•

Consistent with dN/dη ratio in d+Au and p+p (red dashed lines)•Consistent with CGC pictures.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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BRAHMS RCP at forward rapidity
PRL 93, 242303 (2004) BRAHMS• RCP: Central to Perial ratio (Ncoll scaled)• Substantial evolution form η = 0 to 3.2• Suppression larger at forward and central
0-20%.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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STAR Rd+Au: π0 at forward rapidity (<η>=4.0)
J. Adams et al. (STAR), Phys. Rev. Lett. 97, 152302 (2006)• Hadron productions aresuppressed at forward
region.
• Suppression evolves as afunction of η
• Qualitatively consistent withthe expectations for gluon
saturation.

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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STAR: π0-π0 correlations at forward rapidity
E. Braidot et al. (STAR collaboration),arXiv:1005.2378

p+pd+Au• p+p: a strong peak at Δφ = π, associated with the recoil jet opposite tothe trigger particle• d+Au (central): suppressed and/or broadened.
➡ qualitative agreement with expectations from gluon saturation.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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PHENIX, JdA of away side peak (π0-π0) in d+Au
PRL 107, 172301 (2011), PHENIX• π0-π0Correlations w/ pT and rapidity cut on near & away side jets.
→ tight constraint on x2 and Q2 of hard process.
• JdA: correlated pair yield suppression factor on away side.
• x2 like property:• Strong suppression at low x, consistent with CGC expectation.
• Little or no suppression for peripheral.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Now, go back to RdA, RAA (RHIC) again…
π0 (d+Au)

PRL 98, 172302 (2007) PHENIX

Direct γ (Au+Au)•Direct γ (Au+Au): no deviation from unity, no indication of initial (or final) state
nuclear effect.

•π0 (d+Au): suppression at high pT (> 10 GeV/c) in central (0-20%)• initial state effect should be similar to that of direct γ in dAu, AuAu.
• Suggesting the final sate effect in “π0 data” shown here.
• Illustrate the difficulty to extract initial effect from the hadronic
observables,• Difficult to constraint the parton kinematics by hadrons
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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In case of LHC…
CMS

ATLAS

ALICE

• Charged hadron RpPb
• Enhancement or unity at high pT?
• (direct photon: unity at LHC in PbPb)チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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J/ψ Production in pA at LHC
J/ψ → μ+ + μ•

Hadron suppression on forward(proton-going) side at low pT.

•J/ψ yield: not described by nPDFs
nor by a CGC calculation•Uncertainties on:–Production mechanism (xsensitivity etc.)–

Other nuclear modifications (e.g.
energy loss, thermalization inpA?)

ALICE, 10.1007/JHEP02(2014)073, arXiv:1308.6726

Difficult to obtain conclusive data by hadrons only.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Towards understanding
from initial state to QGP
evolution%Ini(al%condi(on,%CGC,%%early%thermaliza(on%Color%Glass%
Collision%
Condensate%(CGC)

“Glasma”

sQGP%produc<on,%thermal%
dynamics%QGP

strongly%coupled%%
Quark%Gluon%Plasma%% Hadron(sQGP)

t%=%@1%fm%%%%%%%%%%%%%%%%%%t%=%0%fm%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%t=%0.6%fm/c%%%%%%%%%%%%%%%%%%%%t=%20%f

Medium properties in AA (“Ridge”)
RHIC (STAR, Au+Au 200 GeV)

LHC (ALICE, pPb, 5.02 TeV)

• long range Δη correlations (ridge) at RHIC and LHC.
• Originated from CGC ?

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

30Initial conditions of Heavy Ion Collisions
IP-Glasma: Color charges and gluon fields fluctuation

AALLIICCEE,,  SSeepp,,

8t

Understanding of initial condition:→key to understand the QGP properties (e.g. η/s),early thermalization.
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Minimum η/s at RHIC?

• IP-Glasma Model (color charge fluctuation)• Higher harmonics ➔ η/s constraints
Minimum η/s at RHIC？

•Temperature dep?
Björn Schenke (BNL) RHIC AGS Users’ Meeting 2013, BNL

C. Gale, S. Jeon, B.Schenke,P.Tribedy, R.Venugopalan, PRL110, 012302 (2013)

LHCf Experiment
PHYSICAL REVIEW C 89, 065209 (2014) (LHCf)

by T. Sako

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo33

LHCf π0 (RpPb @ very forward and low pT)

PRC 89, 065209 (2014) (LHCf)• Strong suppression at very low pT, at very forward region.
• CGC effect?
• (I think) difficult to interpret, due to “too” soft hadron.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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4. Future Experiments

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

Isolated photons
Isolated direct photons canprovide strong constraints on
the gluon PDFs
• LO dominant process: quarkgluon Compton.
• Quark-anti-quark annihilation
contributing mostly at large x.
• NLO: At LHC, the majority ofprompt photons are produced in
the fragmentation process• Fragmentation photon can belargely suppressed by theisolation cut.

→quark-gluon Compton
process dominant, more directaccess to the gluon PDFs.

R. Ichou and D. d’Enterria, Phys. Rev. D 82, 014015 (2010)

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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nPDF/DGLAP vs. CGC (direct photon)•Two scenarios forforward γ production
in p+A at LHC:–

Normal nuclear effects
linear evolution,
shadowing–

Saturation/CGC running
coupling BK evolution• Strong suppression in direct γ RpA.
• Signals expected at forward η, low-intermediate pT.
チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Forward Photon detector (MPC-EX) in PHENIX•• 3.1 < η < 3.8• Start data taking in 2015 (p-Au).

Si tracking detector + pre shower: direct gamma + high mom. π0.

Forward Calorimeter (FoCal) in ALICE•Electromagnetic calorimeterfor γ and π0 measurements,
with Hadron Calorimeter.
• At z ≈ 7m (outside magnet)
3.3 < η < 5.3

FoCal-H
FoCal-E• Proposed schedule:• mini-FoCal: 2018- (after LS2)
• full FoCal: 2023- (after LS3)
Main challenge: separate γ/π0 at high energy• Need small Molière radius, high-granularity read-out• Si-W calorimeter, granularity ≈ 1mm2

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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Uniqueness of FoCal at LHC• FoCal can access:– 3.3 < η < 5.1– few to 100 GeV (lowto high Q2)– Direct photon & π0

チュートリアル研究会「重イオン衝突の物理：基礎から最先端まで」前方の物理, T. Chujo

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質問群 (1)• カラーグラス凝縮とは何か？
• LHC における CGC の役割は? その他の初期条件は排除できるのか• pA衝突におけるカラーグラス凝縮、 早期熱平衡化に関する話題• CGCについて教えてほしいです• 衝突前(CGC)と衝突初期過程 流体化のダイナミクスの現状 ミクロな記述からマクロな記述へ乗り移る際の粗視化の考え方, グルーオンのボーズ−アインシュタイン凝縮と有効化学ポテンシャルによる記述 との違い

• pA衝突, cold nuclear matter effectについて

質問群 (2)• CGCによって、現状、初期の物理をどの程度記述することに成功/失敗しているのか• 初期条件から熱化に至る動的過程をどう実験的に理解できるか？理論の不定性は？それが物性に与える影響はどうなのか？巷にある流体の初期条件と正しくリンクされているのか？

• CGCや原子核効果はどこまで理解されているのか？

Summary• Overview of experimental data at RHIC and LHC, regarding the CGC.
• The data from RHIC and LHC at both mid-rapidity and forward rapidity
are “qualitatively” consistent with CGC picture.

• But they are not conclusive, just because of the strong final state effectby using the hadronic observables.

• Future directions:• From “qualitative agreement” to the conclusive evidence ofCGC.

• Direct (isolated) photon at forward rapidity is one of the good“controlled” probes for CGC effect• Important step for understating of initial condition, QGP properties,
and early thermalization.