# Presentation: 271247

## A personal and surely biased view on

- only a few of the many

- Carlos Lourenço, Hard Probes 2006, June 9–15 2006

- Open questions on quarkonium and EM probes

## Recent progress

- Since Hard Probes 2004 there has been significant progress in the topics quarkonia and EM probes, with new results from NA50, NA60, HERA-B and PHENIX, as well as new developments in the theory sector
- However, the number of open questions has not decreased
- Optimistic view:
- “The scientific value of a discovery is also measured by the number of interesting new questions it raises”
- Pessimistic view:
- Some of today’s open questions are pretty old...

## Some open questions

-   The present SPS results are very interesting but **important issues remain unclear** :
-   Is the  meson modified by the medium ?   measure the  !     Is there a centrality threshold in the dilepton enhancement ?
-   Is the observed  enhancement a specific feature of heavy ion collisions ?     Is the  sensitive to flow ?  What is the “inverse pT slope”, T, of the  meson ?
-   What is the origin of the intermediate mass dimuon excess ?      Thermal dimuons ?
-   Is the open charm yield enhanced in nucleus-nucleus collisions ?     How does it compare to the suppression pattern of  bound charm states ?
-   What is the variable that rules the onset of ’, c and J/ suppression ?
-   What is the physical origin of the ’ suppression ?     If it is Debye screening, what is its melting temperature ( value of Tc) ?
-   What fraction of J/ come from c decays ?     Does it change from p-Be to p-Pb ?

- Slide from talk at Quark Matter 2001

- NA60

- NA60

- PHENIX
- STAR

- HERA-B

- ?

- ?

- ?

- ?

- ?

## What are we missing ?

- Done!

- Slide from talk at Quark Matter 2001

- Done!

- Done!

**NA49**
-   A detailed centrality dependence of the Pb-Pb data
-   A careful comparison of the existing 40, 80 and 158 GeV data
-   A solid baseline reference from hadron production in h-A collisions
**NA57**
-   A robust p-A (and light ion) reference line
-   A scan of the energy density region between p-Pb and Pb-Pb
**NA50**
-   Accurate low ET Pb-Pb data to compare with the normal absorption curve
-   High precision p-A data for the J/ and ’ suppression patterns
**CERES**
-   Improved resolution, statistics and signal to background
-   A study of the excess as a function of the energy density

## 

-  If so, do we see a shift of the pole mass or a broadening of its width?
-  Are these observations related to chiral symmetry restoration?
-  Needed: improved statistics, mass resolution and signal to background ratio;     and to study the signal versus collision centrality and pT

- Low mass dimuon production

- CERES

- NA60

- see NA60 talkS. Damjanovic

****

## 

-  Or is the open charm yield enhanced in nucleus-nucleus collisions?
-  Needed: to measure muon offsets with ~ 50 m precision     to separate prompt dimuons from D meson decays

- Intermediate mass dimuons

- dN/dΔ

**central****collisions**

- NA50

- NA60

- see NA60 talkA. David

****

## ?

-  Needed: compare the J/ suppression pattern measured in In-In collisions     with the Pb-Pb pattern, as a function of several physics observables, and     with theory predictions, previously *tuned* on the Pb-Pb data...

- J/ suppression

- see NA60 talkR. Arnaldi

- NA38 / NA50 / NA60

- ’

- J/

- NA38 / NA50

- From ET bins

**?**

## J/ absorption

-  What is the impact of the c feed-down on the observed J/ suppression pattern? Needed: a study of the nuclear dependence of c production in p-A collisions

- J/ absorption

- see NA60 & HERA-B talksat Hard Probes **2008**

## 

-   Higher energies improve the signals ... but also increase the backgrounds !
-      no replacement for systematic and careful studies
-   Do not underestimate the importance of the baseline reference    take accurate p-A and light ion data
-   To improve our understanding, we need falsifiable models !    when a model is disproved by data, we have learned something
-   Heavy ion physics is a complicated field    to study the critical nature of QCD... we must keep a critical mind

- Slide from talk at Quark Matter 2001

- 

- 

- 

## Examples of critical questions

- Do we have, after 20 years of high-energy heavy-ion collisions, _convincing evidence _of *anomalous* J/ suppression?  Do we have a logical case which convinces, *beyond reasonable doubt*, people outside of our community?
- If so, is the anomalous suppression smooth or “step”-wise?  With one or two “steps”?
- Is the ’ suppressed from p-A to S-U/Pb-Pb because of newly produced *hadronic* or *deconfined* matter?  Is the c *not* suppressed in S-U while the ’ is?
- Do we *really* understand the charmonia production mechanisms, their formation times, and their interactions with *normal hadronic matter* (in pp and p-A collisions)?
- Should the Glauber formalism, with a *single* abs variable, describe “J/” absorption?
- What are the s, pT and xF dependences of the *normal nuclear absorption* of the J/, ’ and c states?  What is the normal nuclear absorption of the c at 158 GeV?
- How can we ensure that significant progress will be made on these issues in the coming 10 years?  Will the LHC experiments solve all remaining open questions?

- Easy to get into trouble by asking such questions !

## Since this talk is “not a summary talk”...

- I will not try to summarise the many new results presented at this conference
- I will just mention a few points which deserve further emphasis:

-  The “elementary” pp and p-nucleus (or d-Au) collisions
-  The pT dimension of the J/ absorption and suppression
-  The ’ suppression

## p-A data on low mass dimuons production

- NA60 collected data in 2002,with Be, In and Pb targetsMuch more statistics in 2004...

- Already in p-nucleus collisions,  production is enhancedwith respect to the 

- see H. Wöhri talkat Hard Probes **2004**

- M

## The p-A input to the low mass dimuon analysis

- _Not yet well known:_
-  B(**→ **)
-   form factor
-   interference
-  etc
-  How do we go from pp and p-A to A-A?  What is the equivalent of “Glauber” for light meson production?
-  Is there a “normal nuclear enhancement” of low mass dimuons?  It will be with respect to that baseline that we will be able to separate  the “anomalous” excess from the “cold nuclear matter” effects

- see NA60 talkat Hard Probes **2008**

## The p-A input to the intermediate mass dimuon analysis

- _Not yet known:_
-  Why is the charm cross-section derived from  the NA50 p-A *dimuon* data *two times higher* than  the world average from hadronic decays?   Use CLEO or PDG form factors instead of Pythia’s (Y. Zhang)
-  What is the real DY contribution at these low dimuon masses  and how does it increase from pp to AA?  Jianwei: send us your code!
-  Is there a “normal nuclear enhancement” of intermediate mass dimuons?  It will be with respect to that baseline that we will be able to separate the  “anomalous” excess (thermal dimuons?) from the “cold nuclear matter” effects

## The p-A input to the understanding of the quarkonia data

- Let’s open an historical parenthesis

## The Matsui and Satz paper... 20 years ago

- In 1986 J/ suppression is proposed as a signal of the QCD phase transition from confined hadronic matter to a deconfined partonic plasma

## The J/ is suppressed in heavy-ion collisions...

- NA38 finds J/ suppression with respect to the “dimuon continuum” in the mass range 1.7–2.7 GeV (very poor statistics of high mass dimuons)
- 1) from p-U to O-U and S-U
- 2) in O-U and S-U, from peripheral to central

- Note: p-U and O-U data collected in 1986;first results shown in Moriond ’87 and inQuark Matter 87... very fast data analysis!No attempt was made to understand thephysics contributions to the “continuum”

- ET > 85 GeV

## The p-A input to the understanding of the quarkonia data

- End of the historical parenthesis

## 20 years later...

- _Some good progress on the understanding of p-nucleus collisions:_
- - NA50 collected much better data, with up to 6 targets, at 400 and 450 GeV
- - Excellent statistics for the J/ and quite reasonable for the ’
- - The continuum is understood as Drell-Yan dimuons and D decays
- _Some of the remaining problems:_
- - The NA50 p-A data were collected at 400 and 450 GeV;  not at the energy of the heavy-ion data (158 GeV)
-   NA60 collected a *very small* p-A data sample at 158 GeV
- - The c nuclear dependence is not yet known;  this is important because of its feed-down to J/  New data exists; analyses in progress:
-   HERA-B at 920 GeV  NA60 at 400 GeV

- see NA60 & HERA-B talksat Hard Probes **2008**

## The J/ is absorbed in p-nucleus collisions...

- NA50 saw that the J/ is absorbed in p-A collisions, with respect to the high-mass Drell-Yan dimuons or in terms of production cross-sections per target nucleon

- Glauber calculation exp(-L abs)

- The lines are the result of Glauber calculations, assuming that the drop is due to final state absorption of the charmonium state in the cold nuclear matter it crosses

- The J/ is absorbed in p-nucleus collisions...

- see G. Borges talkat Hard Probes **2004**

## More from fixed-target p-A collisions

- 10–15 times more data at 450 GeV

## Feed-downs from the ’ and the c

**7%** of the J/ mesons observed by HERA-B are due to ******’ decays**

- Feed-downs from the ’ and the c

- see HERA-B talkP. Faccioli

**(7.0 ****±**** 0.2 ****±**** 0.4****BRs****) %**

## Feed-downs from the ’ and the c (cont.)

**21 ****± 5 ****%** of the J/ mesons observed by HERA-B are due to ******c**** decays**
- Lower than the 30–40% values coming from earlier data (inc. earlier HERA-B data)
- Based on 1300 c’s reconstructed in the dimuon channel; <10% of the full statistics!

- Of the observed J/ mesons: 7% are from ’ decays, ~20% from c decays**** _**more than 70%**__ are directly produced_

- Feed-downs from the ’ and the c (cont.)

- see HERA-B talkP. Faccioli

## Absorption of J/ production versus p-A collision energy

- The 158 GeV p-A data of NA60 will clarify if the trend continues to lower energies

- Absorption of J/ production versus p-A collision energy

## Absorption of J/ and ’ production versus xF and pT

- Why then the J/ is *not* strongly absorbed at RHIC?

- and it increases with pT(a la Cronin effect?)

- Absorption of J/ and ’ production versus xF and pT

## xF

- E866	38.8 GeV	Be/Fe/W
- E789	38.8 GeV	Be/C/Cu/W
- E772	38.8 GeV	H2/C/Ca/Fe/W
- NA50	29.1 GeV	Be/Al/Cu/Ag/W
- NA3	22.9 GeV	H2/Pt

******

- - R. Vogt, PRC 61 (2000) 035203, NP A700 (2002) 539
-  K.G. Boreskov & A.B. Kaidalov, JETPL **77 **(2003) 599

- B&K

- Models (with variants):

**1.0**

**0.9**

**0.8**

**HERA-B*****preliminary***

- If you have enough models... one should describe the data...

- Vogt: final state absorption

**!!!**

- see HERA-B talkP. Faccioli

- Negative xF HERA-B data rule out all models of cold matter J/ absorption
- We don’t need A-A data to find anomalies...

## Normal nuclear absorption of J/ production at RHIC

- abs = 3 mb

- PHENIX

- No final stateabsorptionabs = 0 mb

- Such a *y-dependent effect* is not expected to be seen in the SPS p-A data

- Normal nuclear absorption of J/ production at RHIC

## What’s known about the J/dN/dy in SPS p-A collisions?

- What’s known about the J/dN/dy in SPS p-A collisions?

- The J/ y distributions are *not* centered at 0, even for the p-Be collision system !Why is there a significant change from pp to p-Be but not from p-Be to p-W ?

- All five distributions are well described by Gaussians of mean y0  0.2 and  = 0.85
- Forcing y0 = 0, the 2/ndf increases from 1–3 to 20–50, depending on the data set (target)

- (NA50, to appear in Eur. J. Phys. C)

## Nuclear effects on the PDFs and the J/absorption

- Is the p-A absorption extracted from p-A data directly applicable to A-A?
- “Not taken care of, but absorption does a splendid job from pp to peripheral Pb-Pb”
- abs at RHIC: after removing the nuclear effects on the PDFs assuming EKS98: ~ 1-3 mb
- abs at SPS: effective parameter, convoluting absorption and nuclear PDFs: 4.2 mb
- If the nuclear effects on the PDFs change from 450 GeV to 158 GeV, the value extracted from the 450 GeV p-A data is not exactly the value to be used in the In-In and Pb-Pb studies
- We need to extract abs from the p-A data at 158 GeV ! (work in progress)
- For now, we can make a rough evaluation of the importance of this issue

- see talk ofR. Granier de Cassagnac

- Nuclear effects on the PDFs and the J/absorption

## Nuclear effects on the PDFs and the J/absorption (cont.)

- For p-Pb collisions, the nuclear modification factor is:
- M(158) = 1.06; M(450) = 1.12

- Nuclear effects on the PDFs and the J/absorption (cont.)

- abs,conv = 4.2 mb    abs,real = 5.9 mb

- Assuming the same abs at 158 as at 450 GeV:

- = M(158) exp(-L abs,real)= 0.71 (instead of 0.75)

- The final state absorption increases to compensate for the *anti*-shadowing
- This is the abs value directly comparableto the PHENIX 1-3 mb values

## Nuclear effects on the PDFs: from p-A to A-A

-  compensating for the stronger abs value ?
- Questions:
- 1) Who believes the EKS98 model at the percent level for the *gluon* anti-shadowing?
- 2) How should we parameterize EKS98 in A-A versus centrality?
-     Remember Eugene Wigner:
-     “Give me two parameters and I can fit an elephant,
-     give me three and I make its tail wiggle”
-     Can the centrality dependence of the nuclear effects on the PDFs be fixed by    independent data?
- 3) How is all this affected by the feed-down sources, which have a higher abs value?

## What about the J/ transverse momentum dimension?

- What about the J/ transverse momentum dimension?

**〈****p****T****2****〉 ****(GeV/c)****2**

- J/

- J/

**p-A 400 GeV**

**S-U 200 GeV**

**Pb-Pb 158 GeV**

**L (fm)**

******s (GeV)**

- p

- pp

**〈****p****T****2****〉****pp**** ****(GeV/c)****2**

**Pb-Pb 158 GeV**
**p-Pb 400 GeV**

- (NA50, submitted to EJPC)

- The average pT of the J/ increases with the thickness of colliding nuclear matter and with the s of the collisions

## Absorption of J/ production versus pT in p-A collisions

- The increase of  with pT seems to be *identical* at 400, 800 and 920 GeV
- (at mid-rapidity)
-  Maybe the increase of  from NA50 to E866 to HERA-B to PHENIX is     due to the increase of the average pT of the J/ when s increases...

- Absorption of J/ production versus pT in p-A collisions

- NA50

## The transverse momentum view of J/ suppression

- The transverse momentum view of J/ suppression

- The J/ RCP *strongly* depends on pT (at the SPS)
- Significantly different from what happens with light hadrons...
-  Only the low pT J/ mesons get suppressed !

- see talk ofC. Blume

- (NA50, in preparation)

## No extra J/ suppression from p-nucleus to O-U and S-U

- But the J/ suppression pattern changes significantly between S-U and Pb-Pb...

- No extra J/ suppression from p-nucleus to O-U and S-U

- see G. Borges talkat Hard Probes **2004**

## The ’ is suppressed from p-nucleus to S-U...

- The “change of slope” looks very abrupt...The abs of the ’ increases from 450 to 158 GeV?  Maybe, but *not* from 8 to 20 mb...

- The ’ is suppressed from p-nucleus to S-U...

## Extra ’ suppression from p-nucleus to S-U and Pb-Pb

- But it is very unfortunate that the “drop” happens between p-A and S-U/Pb-Pb, when we change collision systems and energies, from 400/450 to 200/158 GeV.
- Poor statistics prevents the NA60 In-In data from defining the ’ suppression pattern.
- If the extra (strong) ’ suppression isdue to the dissolution of the bound ccstate by the QGP, Lattice QCD saysthat this would indicate that Tc sitsin the most *peripheral* S-U or Pb-Pbcollisions at SPS energies...
- The ’ suppression measurements
- deserve more attention...
- And have the advantage of not being affected by feed-down sources

- ’

- Extra ’ suppression from p-nucleus to S-U and Pb-Pb

## ’ vs. J/ suppression from p-nucleus to S-U and Pb-Pb

- Are the ’ and J/ “departures” from the expected normal nuclear absorptionoccurring at the same energy densities? Should we ignore the S-U data points?
- Asymmetric collision system, different energy, different isospin corrections, etc

## J/ suppression: S-U vs. Pb-Pb vs. In-In

-  S-U points seem to be 10% too high
-  In-In and Pb-Pb patterns do not seem to be overlapping
-  Important to get a Pb-Pb suppression pattern with much better accuracy

## J/ suppression: Pb-Pb vs. Pb-Pb

- J/ suppression: Pb-Pb vs. Pb-Pb

-  Patterns vs. ET and EZDC are perfectly compatible
-  The error bars must be taken seriously

- see L. Kluberg talkat Hard Probes **2004**

## J/ suppression: In-In vs. In-In

- It was not easy... but after a lot of checks and improvements, the plateau is gone...
- A challenge to the “in medium formation” models:it works already at the SPS... in In-In but not in Pb-Pb

- J/ suppression: In-In vs. In-In

## Are we nearly there yet ? 

- _1996__–__2005_ : **NA50** believes that the J/ suppression pattern is consistent with                     QGP formation **in Pb-Pb collisions**                     (but *not* in S-U collisions...)

- _1986__–__1995_ : **NA38** believes that the J/ suppression pattern is consistent with                      QGP formation **in S-U collisions**

- _Prediction for the next 10 years:_

- _2006__–__2015_ : **NA60** believes that the J/ suppression pattern is consistent with                     QGP formation **in In-In collisions**

-                      (but *not* in S-U or Pb-Pb collisions...)

- Are we nearly there yet ?

## Back to the ideal world

- normal nuclearabsorption

- anomaloussuppression

- J/  survival  probability

- _Predicted_ patterns, before any data points were available. It must be very easy to discriminate between the two!

## A new quarkonium state found at CERN: the elmouton

- In 1997, Helmut predicted the existence of a new particle

- The elmouton was now observed at CERN

- The measurement was not expensive at all; in fact, it was quite sheep

## Too much data?

- Several elmoutons have been seen, with different masses, colors, spin...
- Can any of the models describe all experimental data?

**C**olorful**G**rass of**C**ERN

- X

- skin

## Theory versus data: the “no new physics” model

- All data sets overlap, showing the “expected” *normal nuclear absorption* pattern!

- normal nuclearabsorption

## Theory versus data: the “new physics” model

- anomaloussuppression

- All data sets overlap, showing the “expected” *anomalous suppression* pattern!

## A more detailed theoretical framework

- A next generation experiment must be established...

- B decays,shadowing

- direct J/suppression

- ’,csuppression

- gluonanti-shadowing

- recombination

- J/  survival  probability

## Improved experimental data

- Once again, the model prediction (?) describes the new measurements...
- There is a BIG difference between “the measurements are *compatible with*...”and “the measurements *show beyond reasonable doubt* that...”

## The End

- The measurements are done in exceedingly difficult conditions (not always valued) and often must (should) be redone in significantly improved conditions. Remember: extraordinary claims require extraordinary evidence; or at least a second _*good*_ look.
- There are no miracles: solid progress requires people, effort... and beam time.

- Hot and dense QCD Matter... matters