# Presentation: 712904

## Successes and Failures with Hard Probes

- M. J. Tannenbaum
- Brookhaven National Laboratory
- Upton, NY 11973 USA

- 3rd International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions--Hard Probes 2008
- Illa da Toxa, Galicia, Spain                   June 8-14, 2008

## Successes: Many discoveries. Failures: Arrogance by some in thinking that we understand everything when we are far from understanding, e.g. the many effects observed, the underlying physics of QCD in a medium, and the properties of the medium.  

## Away-side correlations to a particle which is a fragment of a jet do NOT measure the fragmentation function!!!

- MJT Moriond 2008. Not selected for parallel talk at HP2006.

## PHENIX-compared measured xE distribution to numerical integral using LEP fragmentation functions-not sensitive!

-  independent of pTt

- xE ~ pTa/pTt ~z/<ztrig>

- <ztrig>=0.85 measured

- Dq (z)~e-6z

**ISR-CCOR 1979**

**LEP**

- PHENIX PRD** 74** (2006) 072002

**b****q****=8.2**

**b****g****=11.4**

## Contradicts Feynman, Field and Fox Nucl Phys B128 (1977) 1--65

- “There is a simple relationship
- between experiments done with single-particle triggers and those performed with jet triggers. The only difference in the opposite side correlation is due to the fact that the ‘quark’, from which a single-particle trigger came, always has a higher p than the trigger (by factor 1/ztrig). The away-side correlations for a single-particle trigger at p should be roughly the same as the away side correlations for a jet trigger at p (jet)= p (single particle)/ <ztrig>”.

## 0 production in p-p collisions at RHIC

- 0 production in p-p collisions at RHIC

- No surprise (to me) that NLO pQCD agrees with data

- NLO-pQCD precison agreement Strattmann Vogelsang hep-ph/0702083

- hard scattering dominates after ~3 orders of  magnitude power-law

- PHENIX,  PRD**76**(2007)051006(R)

## ISR 0 vs RHIC p-p  RHIC pp vs AuAu

- ISR 0 vs RHIC p-p  RHIC pp vs AuAu

- 0 invariant cross section in p-p at s=200 GeV is a pure power law for pT > 3 GeV/c, n=8.100.05. New 62.4 GeV measurement agrees with CCOR shifted up in pT by 3.3%

## Suppression of 0 is arguably the major discovery at RHIC. Energy loss in medium?

- Suppression of 0 is arguably the major discovery at RHIC. Energy loss in medium?

**Au Au ********s****NN****=200 GeV-QM 2005**

- Non-identified h and 0 are different for pT< 6 GeV/c particle ID is important. 0 suppressed by a factor of 5 compared to point-like scaling for 3< pT< 20 GeV/c!

- Original 0 discovery, PHENIX PRL **88 **(2002)022301; latest preprint 0801.4020 [nucl-ex]

## RAA vs. Npart integrated over pT
 NA50 at SPS 
 0<y<1
 PHENIX at RHIC
 |y|<0.35

- Bar: uncorrelated error
- Bracket : correlated error
**Global error = 12% is not shown**

- RAA vs. Npart integrated over pT
-  NA50 at SPS
-  0<y<1
-  PHENIX at RHIC
-  |y|<0.35

- BUT-J/ Suppression (RAA) is the **same** at  mid-rapidity (PHENIX e+e-) as at lower sNN!!!

- PHENIX PRL **98**, 232301 (2007)

- This was CERN-Heavy Ion’s main claim to fame in the infamous press conference of 2000 claiming observations ``consistent with the predicted signatures of a QGP.’’ Will have to wait for LHC to find out whether J/ merely act like ordinary hadrons at low pT or whether they are actually probes of deconfinement as predicted by Matsui and Satz PLB**178**(1986)416. Also see discussion by MJT in Rep.Prog.Phys. **69 **(2006)2005

**Au+Au**

## The 2nd major discovery at RHIC-ID’d particles 

- 2) xT scaling in AuAu 200 cf 130 GeV showed 0 xT scale  Energy loss scales. h do not xT scale  the baryon anomaly

- 1)h and 0 behave differently at intermediate pT 2--6 GeV/c

- PHENIX PR C **69**, 034910 (2004)

- Discovery-PHENIX PRL **88 **(2002) 022301

## Baryon Anomaly Ratios different from `Bulk’ 

- ps: If this is ‘recombination’  QGP: Fries,Muller, Nonaka PRL **90** 202303 (2003)

- PHENIX PRL 91(2003) 172301

- M. Kaneta, N. Xu  nucl-th/0405068

- Bulk particle ratios at midrapidity and thermal model for Au+Au central at sNN=200 GeV

- p/ ratio much larger than from jet fragmentation or bulk: The Baryon Anomaly-still not understood

## Status of RAA in AuAu at sNN=200 GeV QM05

- Implies a strong medium effect (energy loss) since  not affected.
- Suppression is flat at high pT. Are data flatter than theory?

- Status of RAA in AuAu at sNN=200 GeV QM05

## AuAu sNN=200 GeV RAA vs centrality   Energy loss in medium?

- AuAu sNN=200 GeV RAA vs centrality   Energy loss in medium?

**Au Au ********s****NN****=200 GeV arXiv:0801.4020**

- RAA is ~ constant for pT> 5 GeV/c  for all centralities

- RAA (pT>5, 10 GeV/c) does not saturate with increasing centrality, continues to decrease to 5%, Npart=351

## Comparison to Model(s) Including Systematic errors

- Many models explain RAA. All have different assumptions about nuclear overlap geometry, medium expansion, parton propagation, etc, and use a parameter to characterize the medium. For example, we give a fit to the PQM model, Dainese, _Loizides_,Paic, EPJC**38**, 461 (2005)

- The derived transport coefficient     , the mean-4-momentum transfer2/mean free path, is strongly model dependent and under intense theoretical debate, e.g. see Baier,Schiff JHEP09(2006)059.

- Fit by PHENIX including systematic errors arXiv:0801.1665

## Comparison to Model(s) Including Systematic errors

- Many models explain RAA. All have different assumptions about nuclear overlap geometry, medium expansion, parton propagation, etc, and use a parameter to characterize the medium. For example, we give a fit to the PQM model, Dainese, _Loizides_,Paic, EPJC**38**, 461 (2005)

- The derived transport coefficient     , the mean-4-momentum transfer2/mean free path, is strongly model dependent and under intense theoretical debate, e.g. see Baier,Schiff JHEP09(2006)059.

- Fit by PHENIX including systematic errors arXiv:0801.1665

## QM2006-Direct e in Au+Au indicate a theoretical crisis

-  heavy quarks suppressed the same as light quarks, and they flow, but less.             This disfavors the hypothesis of energy loss by gluon bremsstrahlung in medium  but brings string theorists into the game, see references in PRL 98 (2007) 172301.

- p-p beautiful agreement of e with *c b*  production PHENIX PRL**97**(2006)252002

**Au+Au **PHENIX   PRL **98 **(2007)172301

**Au+Au**

**p-p**

##  RAA 0 in AuAu sNN=200 GeV vs. Reaction Plane to probe details of the theory-learn something new!

-  RAA 0 in AuAu sNN=200 GeV vs. Reaction Plane to probe details of the theory-learn something new!

**PHENIX nucl-ex/0611007**
**Phys Rev C 76, 034904 (2007)**

- Little/no energy loss for L< 2 fm

- L =  distance from edge to center of participants calculated in Glauber model

- RAA is absolute,  v2 is relative so no hint of this in v2 measurements. This result also suggests that v2 for pT>2GeV/c is due to anisotropic energy loss not flow.

- RAA() vs. centrality varies density of and distance through medium

**3 < p****T**** < 5 GeV/c**

## Theorists are rising to the challenge

- Agreement of RAA vs pT is quite good at both centralities. But  dependence (pT=10-15 GeV/c) is still much too shallow compared to data at pT=5-8 GeV/c

- e.g. S. A. Bass, et al., arXiv:0805.3271

- Also, other work by many of the co-authors, e.g. PRC 76 (2007) 064907, PRL 100 (2008) 072301

## So are experimentalists

- Y.Nakamiya-QM08

- M. Naglis-PHENIX

## QM2005-direct  in AuAu via internal conversion

- QM2005-direct  in AuAu via internal conversion

- Kroll Wada PR**98**(1955) 1355

- PHENIX NPA**774**(2006)403

- Eliminating the 0 background by going to 0.2<mee<0.3 GeV enables direct  signal to be measured for 1<pT <3 GeV/c in Au+Au. It is exponential, does that mean it is thermal. We must see whether p-p direct  turns over as pT 0 as in Drell-Yan or exponential like for 0

## QM2008 direct  in p-p via internal conversion

- QM2008 direct  in p-p via internal conversion

- Lowest pT direct  ever measured in p-p (and Au+Au). Curves are pQCD extrapolated (W.Vogelsang)

- This is a major discovery, p-p result turns over as pT0, follows the same function B(1+pT2/b)-n used in Drell Yan [Ito, et al, PRD23, 604 (1981)] .                                      Fit to Au+Au is [A e-pT/T + TAA Bpp(1+pT2/bpp)-npp]. Significance of exponential (thermal?) is > 3 

- arXiv:0804.4168v1

## Direct  with respect to the reaction plane

- Turbide, Gale & Fries, PRL 96 (2006) 032303 predict that if jet(parton) suppression is due to g+q-->g+q (+g) in the medium then the reaction g+q--> +q should create a source of direct photons proportional to the distance traversed through the medium-fewer on the mid-plane more vertical, the opposite of 0 and other hadronic jet fragments

**PHENIX preliminary**

**Not seen to within errors.   Same sign of v****2**** for ******** and ********0**** more likely from present data.**

## Correlations

- e.g. p + p  jet + jet                     c.f. Au + Au  jet + jet

## STAR-PRL91(2003)072304  4< pTt< 6 GeV/c  2<pTa<pTt  xEpTa/pTt~0.5

- STAR-PRL**91**(2003)072304  4< pTt< 6 GeV/c  2<pTa<pTt  xEpTa/pTt~0.5

- STAR-PRL**95**(2005)152301         4< pTt< 6 GeV/c  0.15<pTa<4 GeV/c xE  pTa/pTt~0.04

- _Au+Au_: STAR (PRL91) claimed that the away jet vanished but realized in PRL95 that it really didn’t vanish--it just lost energy and widened

## Away side correlations in Au+Au much wider than in p-p

- D

- Away side distribution much wider in A+A than p-p in correlation fn. C() Subtraction of v2 (flow?) effect J() causes a dip at 180o which gives 2 peaks at D~1 radian independent of system and centrality for Npart >100. This is also seen for (auto) correlations of low pT particles. _Is this the medium reaction to the passage of a color-charged parton? Why no dependence on centrality?_ Stay tuned, much more study needed.

- With the exception of the low pT measurement, all data which show the ``cone’’ are non-identified h-h corelations in the range 1< pT < 5 GeV/c where the particle ratios are strongly varying and are anomalous. Is there a connection?

## One of the few definitive results

- Trigger mesons and baryons in the region of the baryon anomaly both show the same trigger (near) side and away side jet structure. This ‘kills’ the elegant recombination model of the baryon anomaly

- PHENIX PRC **71** 051902  2.4<pTt<4 GeV/c 1.7< pTa<2.5 GeV/c

## Away-side yield  vs pTa/pTt (xE PHENIX) (zT STAR)            is steeper in Au+Au than p-p indicating energy loss

- Away-side yield  vs pTa/pTt (xE PHENIX) (zT STAR)            is steeper in Au+Au than p-p indicating energy loss

- h(4<pTt<5 GeV/c)--h

- The away side pTa/pTt  xE distribution triggered by a leading particle with pTt was thought to be equal to the fragmentation function but we found that it is NOT sensitive to the shape of the fragmentation function but only to the shape of the inclusive pTt spectrum  with power n (=8.1). Formula derived in PRD 74 (2006) 072002 works for pp and AA

- PHENIX AuAu PRC **77**, 011901(R)(2008)

## STAR dAu, AuAu

## Newest STAR-Horner-QM2006-JPG34 (2007) S995

- Two-component distribution (punch-through) is now clear for 6< pTt< 10 GeV/c

## If this is punch-through due to tangential emission, why does it depend on pT?

- Tangential emission

- We must carefully map out how this effect depends on pTt and pTa and particle type and angle to the reaction plane...

**Notes:**
Start off with 2 assoc, then move to 6 assoc

## Prediction of Jet shape in vacuum and medium

- Would be much easier to understand if they also plotted z in addition to =ln(1/z):  e.g =3.0z=0.050

- Borghini & Wiedemann, hep-ph/0506218

## The Holy Grail: -h correlations in Au+Au

- The Holy Grail: -h correlations in Au+Au

- X-N. Wang and Z. Huang PRC 55, 3047 (1997)

- MJT-should scale the z axis to see the energy loss (not take the ratio)

## Can make the =ln(1/z) plot from -h correlations Tannenbaum-Ting-Borghini-Wiedemann-Wang plot 

- Can make the =ln(1/z) plot from -h correlations Tannenbaum-Ting-Borghini-Wiedemann-Wang plot

**Modulo k****T****, the x****E**** distribution opposite the direct-******** IS the fragmentation function of a jet of  p****T****=-p****T******

##  from single inclusive 0 at Z0--L3. Thank you Sam Ting

- We should be able to do this from -0 -h away side correlations in p-p (almost there, convert xE plot to ) and Au+Au to get a  Tannenbaum,Ting,Wang,Wiedemann (in alphabetical order) plot

## Zhang, Owens Wang, Wang Model

**2******

**1******

- Zhang, Owens, Wang and Wang, PRL 98 (2007) 212301 found in their model, 0=1.6-2.1 GeV/fm

- Fit by PHENIX including systematic errors arXiv:0801.1665

- Again a precision of 20-25% (1)

## But-physics and statistics Issue

- Zhang, Owens, Wang and Wang, PRL 98 (2007) 212301 found in their model, 0=1.6-2.1 GeV/fm. Good, but...

-  can’t get error on best fit from 2/d.o.f curves, need 2. N standard deviation errors on fit parameters are given by 2= 2min + N2, so depending on d.o.f can’t really tell from 2/d.o.f whether  IAA gives better constraint than RAA
-  However  2min/d.o.f=2.8 for IAA fit seems too large to be acceptable. (?)

## CGC?‘Monojets’ in d+Au?

## I could go on for hours, but

## Brahms RdAu, RCP in d+Au vs rapidity

- Brahms RdAu, RCP in d+Au vs rapidity

- BIG effect in RCP; some tendency in RdAu. BRAHMS PRL**93** 242303

## d+Au is it CGC?`Monojets’?  correlations

**widths and conditional yields the same for triggers in all 3 spectrometers for pp and dAu**

- PHENIX PRL** 96 **(2006)** **222301

##  are suppressed at small <xF> and <pT,>      

-  are suppressed at small <xF> and <pT,>**      **
**S****pp****-S****dAu****= (8.0 ± 1.9) %**
- consistent with     CGC picture
** **are consistent in d+Au and p+p at larger <xF> and <pT,>** **
- as expected by HIJING** **

**STAR-Correlations in d+Au PRL 97, 152302 (2006)**

- Is this inconsistent with PHENIX? New d+Au soon.

## h-h correlations - Reaction plane dependence

- We now see a nice jet shape without a dip at 0o. Need new fit function since obviously not gaussian. Nowhere except at RHIC did anybody ever claim jet shapes were gaussian.

- Allows improved background subtraction. 1) Acceptance correction does not depend on event plane, so use non-event plane selected mixed events for this. 2) The flow background, which is a non-jet correlation to the reaction plane, should then be given by mixed events as a function of event plane corrected for acceptance. No need to know v2, right? Try it, please!

- PHENIX QM2008

## A STAR special---A -independent near-side correlation in addition to the jet is observed in central Au+Au--``The Ridge”

- pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig

- d+Au, 200 GeV

- What is the ridge?
- 1) Medium heating and parton recombination
-       *Chiu & Hwa PRC 72  (2005) 034903 *
- 2) Radial flow + high-pT trigger particle
-       *Shuryak, **Phys.Rev.C76** (2007) **047901*
-       *Voloshin, nucl-th/0312065  NPA 749,  (2005) 287 *
- 3) Parton radiation and its coupling
-     to the longitudinal flow
*     **Armesto, Salgado, Wiedemann, PRL 93 (2004) 242301 *

- 4) Momentum broadening in anisotropic QGP
-       *Romatschke, PRC 75 (2007) 014901 *
- 5) Longitudinal broadening of quenched
-     jets in turbulent color fields
-       *Majumder, Mueller, Bass, **PRL** **99** (2007) **042301*
- 6) Momentum kick imparted on partons in medium
-        *Wong, PRC**  **P**R**C** **76** (2007) **054908** *
*    *

- Jana Bielciekova XLIII Rencontres de Moriond

- STAR preliminary

## The biggest result at QM2006??!

- QM2006-
- pp dir  reference is run 5 msmt
- If RAA= RAA the whole concept of energy loss changes: perhaps no effect for pT>20 GeV

- QM2005
- -I wanted to make a T-shirt
- pp dir  reference is pQCD

## For Au+Au min bias direct  RAA is simple

- For Au+Au min bias direct  RAA is simple

- Au+Au minimum bias

- Do the structure function ratios actually drop by ~20% from x=0.1 to x=0.2?

- Eskola,Kolhinen,Ruuskanen Nucl. Phys. B**535**(1998)351

## Central Collisions---no theory counterpart-yet

- Au+Au Central Collisions

- Nobody has seriously measured nor calculated structure function ratios as a function of centrality!!!

- Theorists, HELP!

- Very few attempts so far for structure function measurements or theory as a function of impact parameter:                                  E665, ZPC 65, 225 (1995)                 Li and Wang, PLB 527, 85 (2002)   Klein and Vogt PRL 91, 142301 (2003) Emel’yanov, et al. PRC 61, 044904 (2000) and references therein.

- Experimentalists: RHIC p+A, eRHIC

## This is a crucial issue for LHC because structure function predictions vary by a factor of >3  

- Predictions from CERN-Yellow-report-hep-ph/0308248

- This is a crucial issue for LHC because structure function predictions vary by a factor of >3

-  LHC could be the CGC factory. Rg(x,Q2) not known for any A or any x or Q2. Forward d+Au measurements at RHIC will be useful.

- Due to lack of knowledge of Rg(x,Q2), direct  could be suppressed in Pb+Pb at LHC by a factor of 3 due to the structure function!!

## Fragmentation  ARE suppressed, confusing some theorists

- Fragmentation  ARE suppressed, confusing some theorists

- e.g I. Vitev, pp 167-169 in Armesto, et al, arXiv:0711.0974 [hep-ph]

- No experimentalist would get this result because we want to use the Compton diagram in p+p as a baseline to measure  the initial state effect in Pb+Pb for the case of an outgoing constituent from the 2-2 hard scattering, the photon, which does not interact with the medium.

- we want

- from 2-2 Compton in p-p

## Many discoveries at RHIC             Lead to many new open questions Like at the ISR in the early 1970’s

## What I still don’t understand-I

- After 8 runs at RHIC, many discoveries have been made in Au+Au collisions but there is much that is still not known or understood:
-  Is the nuclear modification factor RAA for 0 really constant at a factor of 5 suppression over the range 3< pT< 20 GeV/c which would occur for a constant-fractional energy loss analogous to bremsstrahlung, or does the suppression tend to vanish at larger pT? Is dE/dx constant or a constant fraction or something else?
-  Does RAA for direct- really approach that of 0 at large pT~20 GeV/c as indicated by preliminary data? If true this would argue that the suppression due to a medium effect vanishes at large pT> 20 GeV/c and the effect observed is due to the structure function. If this is confirmed, it would be VERY BAD for LHC.
-  The detailed mechanism of jet suppression due to interaction with the medium is not understood. It is not known whether partons lose energy continuously or discretely, whether they stop in the medium so that the only observed jet fragments are those emitted from the surface or whether partons merely lose energy exiting the medium such that those  originating from the interior of the medium with initially higher pT are submerged (due to the steeply falling pT spectrum) under partons emitted at the surface which have not lost energy. In either case, there is a surface bias.

##  The reason why heavy quarks appear to lose the same energy as light quarks is not understood. • It is not known whether a parton originating at the center of the medium can exit the medium without losing any energy. [I think the answer is yes for coherent LPM energy loss. We can discuss]• It is not known where the energy from the absorbed jets or parton energy loss goes or how it is distributed.• The surface bias discussed above complicates the use of two-particle correlations of hard-scattered partons to probe the medium since detecting a particle from an away-side parton changes the surface bias of the trigger parton. This means that detection of both a trigger and away side particle is required in order to constrain the hard-scattering kinematics and the position of the origin of the hard-scattered parton-pair within the nuclear matter. Then, the main correlation information with relatively stable kinematics and origin is obtained by studying correlations with an additional 1 or two particles, i.e. a total of 3 or 4 particle correlations, which is much more complicated and requires much more data than the same studies in p+p collisions.  

- What I still don’t understand-II

## What I still don’t understand-III

- • The ridge is not understood. What causes it? What are its properties?  How does it depend on pTt, angle to reaction plane etc? Why isn’t there an away-side ridge?
- • Finally, J/ suppression, which for more than 20 years has represented the gold-plated signature of deconfinement, is not understood.

## Extras

## ISR 0 vs RHIC 0 p-p 

- ISR 0 vs RHIC 0 p-p

- 0 invariant cross section in p-p at s=200 GeV is a pure power law for pT > 3 GeV/c, n=8.100.05. Power at 62.4 ISR (xT>0.27 is n=11.030.16)

- Moriond 1979

- 5

## Au+Au at √sNN = 200 GeV: Flow

- elliptic flow of e± from heavy-flavor decays
- non-zero v2 (RHIC Run-4)

- Au+Au at √sNN = 200 GeV: Flow

**Run-7**