experimental studies of qcd in p/d/e-a collisions at rhic, the lhc, and e-rhic
DESCRIPTION
Experimental Studies of QCD in p/d/e-A Collisions at RHIC, the LHC, and e-RHIC. Prof. B.A. Cole Columbia University. p-A Physics Goals. Nuclear effects (hard) Shadowing / saturation @ low x A . Jet structure / mono-jets @ low x A . p T broadening / energy loss. - PowerPoint PPT PresentationTRANSCRIPT
Experimental Studies of QCD in p/d/e-A Collisions at RHIC, the LHC, and e-RHIC
Prof. B.A. ColeColumbia University
1. High (moderate) - pT physics in p-p @ RHIC 2. High (moderate) - pT physics in d-A @ RHIC 3. Theoretical analyses of d-A forward suppression 4. Future: p-A @ LHC (with a focus on ATLAS) 5. Further future: e-A, p-A with e-RHIC detector 6. Summary
p-A Physics Goals• Nuclear effects (hard)
– Shadowing / saturation @ low xA.
– Jet structure / mono-jets @ low xA.
– pT broadening / energy loss.– Modifications of baryon production.– Tests of pQCD: factorization / universality.
• Nucleus as a filter (soft)– Diffraction.– Proton break-up, color transparency.– Baryon junction excitation.– Soft phenomenology.
• In this talk: focus on “hard” effects (?)
Why p-A (d-A) Collisions ?• Probe Initial-State Effects at RHIC
– Shadowing of Nuclear PDF’s– Parton saturation– Cronin effect
p broadening of hard processes
– It’s becoming clear that these are all due to or reflect the same underlying physics
• Unique feature of RHIC measurements– Ability to constrain “centrality” – i.e. impact
parameter range of d-A collisions.
peripheral
central
Hard Scattering in p-p Collisions
–Factorization: separation of into•Short-distance physics: •Long-distance physics: ’s
p-p di-jet Event
STARSTAR
a/A
b/B
A
B
ab̂
From Collins, Soper, Sterman Phys. Lett. B438:184-192, 1998
Single High-pt Hadron Production
–NLO calculation agrees well with PHENIX 0 spectrum (!?)• BUT, FF dependence ?• Lore: KKP better for gluons • Includes soft-gluon resummation!
KKP
Kretzer
data vs pQCDdt
d
z
QzD
QxQxdxdxdp
dE
c
abcaBbaAaba
ˆ),,(
),,(),,(
2
/
2/
2/3
3
0
Phys. Rev. Lett. 91, 241803 (2003)a/A
b/B
A
B
ab̂
D(z)
But QCD is not Nearly So Simple …
• Initial and final state radiation leads to QCD evolution– Parton distributions– Fragmentation func’s
• Well-controlled (infrared safe) evolution depends on cancellation of real and virtual radiation.
• Why does this matter?– Radiation broadening of transverse momenta– Phase space restrictions inhibit the real/virtual cancellation.
• High pT hadron production at large xT (low s).• Heavy quark production at low transverse momenta.
• “Re-summation” of large logarithms needed.
Application of pQCD vs s
– How well does NLO pQCD work as we go down in energy from RHIC ?
– Clearly describes data more poorly for decreasing s.
• And for more forward production.
– Also, sensitivity to factorization scale also grows.
Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004
Threshold re-summation• Threshold & soft
gluon resummation (NLL) improves agreement with data at lower s.
• Much smaller effect for RHIC at mid-rapidity.– But still a factor of
~2!
– pT dependence ??
• What about at forward rapidity??
Forward Production at RHIC
• NLO pQCD works at RHIC @ large xF
• But ~40% scale error (=pT vs =pT/2)
• Re-summed NLO (Vogelsang) also agrees with data.
• But, scale error in non-resummed NLO:– Strong sensitivity to
“nuclear” effects???
Soffer and Bourrely, Eur. Phys. J. C36:371-374,2004
PHENIX: 200 GeV p-p Prompt • Background removed
via combination of:– (Jet) isolation cuts 0 decay tag– Statistical subtraction
• Spectrum and yield well-described by NLO pQCD (w/ threshold & recoil resummation).
• ~ 15% scale error above 5 GeV/c.
• More work needed to go below 5 GeV/c.
A-A Hard Scattering Rates–Parton flux density “thickness”
•
–For point-like interactions:
• dNhard / dA product of nuclear T’s
• Integrate over transverse area
• Then
• Nbinary (also known as Ncoll) is fiction
– no successive nucleon-nucleon scattering !
– Just a convenience (pure number not fm-2)
),()(
rzdzrT nucleon
A
rT
b
|)(||)(|)( rbTrTrdbT BAAB
binary
NNhardNN
inelAB
NNhard
AB
NNhard
ABhard N
dp
dnbT
dp
dnbT
dp
d
dp
dn2222
)()(
Coherence in p/d-A @ RHIC• View in nucleus rest frame
– For mid-rapidity jet with MT
• Relative to nucleus, y=5.4
• E pL = MT cosh(y) 100 MT
• Lorentz boost: = cosh(y) 100
– Also, Jet formation time: ~1/ mT
– Giving (jet) formation length (LF )
• LF = 20 GeV fm / mT
• From this simple analysis we can conclude:– All for the “action” for mid-rapidity particle production (and
forward) occurs along the straight path of the incoming nucleon.
– Even high-pT and heavy quark production processes may be affected by coherence in the multiple scattering process.
• New at RHIC:– Ability to select on “centrality” (poor man’s impact parameter)
text
d-Au “Centrality” • # soft scatters of n/p:
–
• Parameterize multiplicity at large vs n, p.
– Cut data according to fraction of total dA.
– For each, determine TdAu
– e.g for PHENIX (in %)• 0-20, 20-40, 40-60, 60-88
• Define:
NNpnAupn bT )( //
22
22
/
/1
dp
dn
dp
dn
T
TR
dp
d
dp
dn
TR
periphhard
centhard
centdA
periphdA
cp
NNhard
dAhard
dAdA
STAR d-Au @ High-pT
– Beware: • Top plot is RdA
• Bottom plot is Rcp
– Strong enhancement in charged hadron production at =0.
– Enhancement larger for baryons than for mesons.• Ks similar to
similar to
PHENIX: d-Au Neutral Mesons
• Now evaluate consistency with pQCD:– TAB scaling (factorization)
0 production vs centrality production vs centrality
PHENIX d-Au 0 vs Centrality
–Small Cronin effect (not expected to be large)–It is now known that preliminary data suffer from small trigger bias (central will go peripheral ).
PHENIX d-Au Production
–PHENIX sees small Cronin effect• Approx. consistent within errors with STAR Ks result • Enhancement seen in charged (baryons) all the more striking!
PHOBOS: d-Au h RdA
• Clearly the “enhancement” of charged hadron production in d-Au depends on rapidity ().
• dependence suggests suppression for >1
nucl-ex/0406017, PRC in press nucl-ex/0406017, PRC in press
PHENIX d-Au Forward/Backward h
– PHENIX observes similar trend in hadron spectra• Suppression relative to “expected” TAB scaling
• Suppression greater for more central collisions• Suppression NOT confined to large only!
BRAHMS: d-Au RdA or Rcp vs
–BRAHMS also sees suppression of (h-) yields at larger (beware “isospin” effect for =2.2, =3.2)
–Suppression increases for more central collisions.
BRAHMS – A closer look– Rcp shows suppression
increases with TAu
• Clearer than RdA (pp data?)
– Suppression smooth in – But see h+/h- difference !
• Reflects Z=+1 of d ??
– Rcp with (h++h-)/2 still shows suppression.
Rcp
=3
Forward Suppression (CGC ??)• Kharzeev, Kovchegov, Tuchin (Phys.Lett.B599:23-31,2004)
• Evolution from enhancement (Cronin effect) at mid-rapidity to suppression at forward rapidity.
• h- RdA modified by charge bias in p-p coll’s.
• Rcp less sensitive.
Model Comparisons (I)• Vitev(nucl-th/0302002)
– pQCD w/ shadowing
– Include self-consistent p broadening, dE/dx
• Both elastic & radiative
correct enhancement at mid-rapidity
• But EKS anti-shadowing overestimates RdA
– Predict RdA >1 at y = 3
• dE/dx small effect.
– But significant dE/dx effects at y = -3.
Vitev and Qiu: Higher Twist
• “Higher Twist”: – multiple exchanges
between projectile & target.
• Vitev & Qiu: coherent multiple scattering
• Effective rescaling of x of parton from deuteron.
Model Comparisons (II)• Describe hard scattering
in nuclear rest frame.– “Cronin effect” from multiple
semi-hard scattering
• With unitarity corrections:
• Fit to p-p + Fermilab p-A– Reproduces y=0 0 Rcp
– But not y=3 ***– Even if opacity increased x3
• BRAHMS data changed– But p dependence wrong …
A. Accardi nucl-th/0402101
(Semi) Hard Scattering in d-A @ RHIC
• We don’t have to look very hard to see the effects of coherence.
• Effects near mid- disappear by pT ~ 6 (?)
• @ = 3.2 kinematic limit: pT 8 GeV/c.
• Limited phase space for truly high-pT physics
Brahms
d-A J/ Production (from M. Leitch)
Klein,Vogt, PRL 91:142301,2003 Kopeliovich, NP A696:669,2001
RdA Low x2 ~ 0.003(shadowing region)
(in gold)
•Not universal versus X2 : not shadowing !??
– BUT does scale with xF ! - why?– Initial-state gluon energy loss depends
on x1~xF - weak at RHIC energy?
•But Kopeliovich: – Effect can be due to “energy loss”
compared to lower sE866: PRL 84, 3256 (2000)NA3: ZP C20, 101 (1983)
xF = xd - xAu
•Data favors (weak) shadowing + (weak) absorption ( > 0.92)
•With current statistics hard to separate different nuclear effects
•Will need more d-Au data!
Summary of d-A @ RHIC• Observe clear suppression of forward
hadron production at pT >~ 4 GeV/c.– Continuation of trend over large y range.– Does not fit within pQCD calculations
• Issue: EMC (0.2 < x < 0.9) suppression of gluons typically included in calculations, valid???
• weak Cronin effect at mid-y for mesons.– But, also clearly depends on rapidity.
• Some crucial aspect of physics is missing in “pQCD” calculations.– Kopeliovich: factorization breaking?
• “Sudakov suppression” – but at low/negative xF ?
Centrality in d(p)-A• The ability to select on centrality in d(p)-A
collisions is NEW and very important.• Potentially the first opportunity to measure
the impact parameter dependence of:– Initial-state broadening, Shadowing, …
• Observations of centrality dependence have already been important.
• But, there are some limitations:– Rely on Glauber model to indirectly relate
“centrality” observables to impact parameter.– Kopeliovich: Flaw in Glauber models due to
neglect of diffraction – which I think is a real issue.
– May be important for understanding RCP.
Di-hadron Azimuthal () Correlations
• jT represents hadron pT relative to jet
• kT represents the di-jet momentum imbalance
• “y” implies projection onto transverse plane.
yTj
yTj
Jet
yTk
PHENIX d-Au/p-p, - h, Correlations
–“Trigger” pion pT > 5 GeV/c
–Four different associated hadron pT bins
–Clearly see role of constant jT, contribution from kT
0.4-1 GeV/c1-2 GeV/c
2-3 GeV/c 3-5 GeV/c
PHENIX preliminary
p-pd-Au
PHENIX: Di-jet KT• No jet reconstruction
in PHENIX (yet)• But can measure KT
via two-hadron correlations.
• Additional broadening from fragmentation.– But can be measured
in single jet.
• Then:
• Study vs ph1
• KT Same in p-p, d-Au?– Sensitivity ??– More work needed.
1jetp
2jetp 1hp
2hp
1jetp
2jetp
2hp
1hp
22jetjetdiTT pk
y
Studying Jet Properties @ RHIC
– Use hadron pairs to study jet properties
– pout dist. has both non-pert. (Gaussian) + hard (power) contributions.
pp
Radiative tails
PHENIX, From J. Jia, DNP’04 Talk
PHENIX Preliminary
JetPout
Pout
Jet Properties in d-Au• Compare pout dist’s in
p-p and d-Au.• Evidence for effects of
re-scattering, modified radiation, … ?– Not so far!– But this is just the
beginning!
• Such measurements w/ one jet @ > 2 would be very interesting!!– But not possible yet
Radiative Effects on (di)Jets
– Conclude: large radiative component to di-jet kT
• Also see Vitev, Qiu : Phys.Lett.B570:161-170,2003.
– Without accounting for radiation initial parton intrinsic kT ~ 2 GeV/c (RMS).
– After accounting for radiation ~ 1 GeV/c
Analysis of STAR di-hadron distribution by Boer & Vogelsang,
Phys. Rev. D69 094025, 2004
Hard Scattering – IS/FS Radiation• Radiative contributions from initial & final state• Initial state radiation due to parton shower prior
to the hard scattering
• The development of the initial-state shower must be different in nucleus (?).– “Quantum evolution” an
important part of CGC– Treatment of soft radiation in
co-linear vs kT factorization?
• “Model-independent approach”• Measure di-jet acoplanarity • Better: -jet and - (hard) processes
Direct Photon Production• kT broadening and
evolution of parton distributions will modify production.
• If there are mono-jets, are there mono-photons??
-jet angular correlations more sensitive because less broadening from jet.
• Di- production even more interesting – kinematics completely determined.
• Need good photon/0 separation.
J. Jalilian-Marian, hep-ph/0501222
p-A Collisions @ LHC
• Summary of LHC “Yellow Report” on p-A
Physics Motivation / Goals
•From DOE LHC Heavy Ion Review (2002)
p-A @ LHC
• p-A @ LHC can reach low x at high Q2
• Rates for high-pT processes are enormous
• Concerns– No p-p measurements at same s (?).– Centrality selection will require care.– Little particle (baryon) identification away from mid-rapidity
Parameters from LHC Yellow Report
Rates for pT > 100 GeV/c
2QQ
Low-x Effects @ LHC
•Measurable shadowing even at 100 GeV.
• Modest effects at mid-rapidity (but going away slowly)
Q=100 GeVQ=10 GeVQ=2 GeV
Frankfurt, Strikman: Shadowing Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005)
Di-jet / -jet / - Acoplanarity (2)
• d-A measurements @ RHIC limited by– Luminosity and Acceptance
• Both of these limitations are removed in (e.g.) ATLAS @ LHC
• Isolate initial-state radiation effects (modified in p-A) by comparing:– Di-jets, (isolated) -jets, (hard) di-photon
• Prediction from saturation:– “disappearance” of di-jet signal at pT ~ Qs
– But, presumably measurable (calculable?) effects at higher pT?? (precision vs “discovery”)
Example: from CDF
p-A in ATLAS (CMS)
• p-p detectors @ LHC ideal for studying high-pT physics in p-A collisions.
Hadronic CalorimeterElectromagnetic Calorimeter
Inner Detectors Silicon Pixels Silicon Strips Transition Radiation Tracker
SuperconductingSolenoid
Muon chambers
Superconducting Coils for Toroidal Field for Muon System
For CMS:EMCal covers ||<5
Had. Cal: ||<5
TOTEM: ||<7
ATLAS Calorimeter System (1)
Hadronic TileCalorimeters
EM AccordionCalorimeters
Silicon Tracker in Inner Detector
Forw
ard
LAr
Calo
rimete
rsH
adro
nic
LA
r End C
ap
Calo
rim
ete
rs
p-A Collisions: Soft “Background”
• Some numerology:– @ LHC energies, p-Pb collisions ~ 7– Due to coherence (wounded-nucleon scaling) ~ 7 4 times soft multiplicity (on average)
– In p-p @ high-, ~ 25 collisions/bunch crossing– Typical p-Pb collision has 1/6 the soft
background of high- p-p collision.
• Conclusion: for high-pT measurements ATLAS p-Pb performance better than p-p.
• Beware: this argument neglects rapidity dependence of soft p-Pb/p-p.– Observe: best performance in low XA direction.
Simulated (& Recon) Hijing p-Pb Event
Simulated (& Recon) Hijing p-Pb Event #2
•Jet at forward (actually backward) rapidity
Detecting Forward jets (from Takai)
Event Characteristics• Use Hijing to simulate (central) p-Pb
events– Apply ideal ATLAS acceptance cuts to
particles.– Study what ATLAS “sees” in typical events
• e.g. charged multiplicity
Fra
ctio
n o
f eve
nts
Charged part. multiplicity
ATLAS does not measure a large fraction of charged particles
1) coverage
2) Magnetic bend (minimum pT ~ 0.5)
Pseudo-rapidity ()
dN
chg/d
ALL
ATLAS
Event Characteristics (2)• Instead, look at ET (electromagnetic)
Electromagnetic ET (GeV)
Fra
ctio
n o
f Eve
nts
Need serious analysis of effects of noise on ET measurement
pseudo-rapidity ()
dE
T/d
(Ge
V)
ALL
ATLAS
Centrality Measurement• The ability to measure centrality has
been an important feature of RHIC d-Au.
From Mark Baker (BNL), Talk @ RHIC pA/eA workshop
Centrality Measurement (2)• Problem w/ centrality measurement:
– Measurements at mid-rapidity are biased• By hard processes• By the very low-x physics we want to study
– At RHIC, measurements @ || > 3 are “safe”• Hard processes suppressed by phase space.
– How far out in is “safe” at LHC (6, 7, 9?)
• Zero-degree calorimeter(s) are useful– But evaporation neutron yield saturates.– Can distinguish peripheral from central but
…
• p-A Centrality determination @ LHC needs careful study by all experiments.
p-A in ATLAS: Studies Needed• Basically everything ! But specifically:
– Real simulations of mult. and ET measurement
– Centrality determination.
– Forward jet measurement @ moderate pT
– Measurement of < 20 GeV jets at mid-rapidity. isolation efficiency and rejection vs pT
– Analysis of -jet kinematic (x1, x2) reconstruction
– Sensitivity to changes in di-jet/ -jet/ … acoplanarity.
– Double b-tag efficiency, rate (moderate pT).
– Jet overlap, double parton scattering events.– …
p-A @ LHC: Plans• p-A is considered an “upgrade” @ LHC
– Straight-forward but needs second timing system.
– Cabling will be in place but the $ are not yet committed (but small change: few 100k$)
• Meeting on p-A @ LHC May 25-28.– We will know much more then.
• “Guaranteed” that there will be p-A @ the LHC but when?– Presumably all three experiments will run.
• LHC p-A complementary to RHIC p-A and e-RHIC.
e-A in Target Rest Frame• q q-bar pair + …
(evolution) interacts with target
• In many ways similar to p-A collision.
• But:– Transverse size
controlled by Q2
– Kinematics much better determined
• For moderate Q2 get multiple scattering– Shadowing– “Centrality” !!?
• Connection between structure function & unintegrated PDFs
• But only at leading twist!• Can we directly measure
violation using p-A/e-A ?
dxdk
NdQxxG
Qd
d
T2
22
2),(
log
p-A: What is Unique to RHIC• At LHC we will not be able to measure
into the fragmentation region.• At RHIC, we could in principle cover a
large part of the fragmentation region • Simultaneously measure
– Proton break-up– Hard/semi-hard processes with good
efficiency for capturing di-jet, -jet, -– Nuclear break-up
• In a detector that would look much like an e-A detector w/ similar requirements (?)
Semi-inclusive DIS• HERMES has very
interesting results on modifications of quark fragmentation in nuclei.
• In target rest-frame:– Nucleus as filter of
different dipole+… configurations
Nuclear Modification Factor RAA
evt 2AB AB T
AB 2AB pp T
1 N d N /dydpR
T d /dydp
strongsuppression
Phys. Rev. Lett. 91, 072301 (2003)
integrated 0 yield above pT = 4 GeV/c