Quarkonium suppression at the SPS

or

Are the SPS J/



and



’ suppression patterns “smoothy” or “steppy” ?

Carlos Louren ço International Workshop on Heavy Quarkonium, DESY, Oct. 2007

The QCD phase transition QCD calculations indicate that, at a critical temperature around 170 MeV, strongly interacting matter undergoes a phase transition to a new state where the quarks and gluons are no longer confined in hadrons hadrons quarks and gluons

The phase diagram of QCD T c

quark-gluon plasma hadron gas nucleon gas

 0

nuclei net baryon density

The first QCD Phase Diagram N. Cabibbo and G. Parisi, Phys. Lett. B59 (1975) 67

The

really

first QCD Phase Diagram So Dark the Confinement of Man

Not easy to see...

unless you know what you are looking for

For details, see: “The Da Vinci c o l o u r Code”

How do we study

bulk

QCD matter?

Pb 208 Pb 208 We heat and compress a large number of hadrons, in the lab, by colliding heavy nuclei at very high energies p p p +

One Pb-Pb collision seen by NA49 at the CERN SPS

A

very large

volume of compressed QCD matter

“Seeing” the QCD matter formed in heavy-ion collisions We study the QCD matter produced in HI collisions by seeing how it affects well understood probes as a function of the temperature of the system (centrality of the collisions)

Matter under study Probe

Calibrated “probe source”

QGP ?

Calibrated heat source

Calibrated “probe meter”

Challenge: find the good probes of QCD matter vacuum The good QCD matter probes should be: Well understood in “pp collisions” hadronic matter Only slightly affected by the hadronic matter, in a very well understood way, which can be “accounted for” QGP Strongly affected by the deconfined QCD medium...

Heavy quarkonia

(J/  ,  ’ ,  c ,  ,  ’ , etc) are good QCD matter probes !

Challenge: creating and calibrating the probes The “probes” must be

produced

together with the system they probe (!) and

very early

, to be there

before

 quarkonia the matter to be probed: We need “trivial” (reference) probes,

not affected

 by the dense QCD matter: photons, Drell-Yan dimuons We need “trivial” collision systems, to understand how the probes are affected in the

absence

 of “new physics”: pp, p-nucleus, light ions

“Tomography” of the produced QCD matter Tomography in medical imaging: Uses a calibrated probe and a well understood interaction to derive the

3-D density profile of the medium

from the absorption profile of the probe.

“Tomography” in heavy-ion collisions: Jet suppression gives the density profile of the matter Quarkonia suppression gives the confinement state ( hadronic or partonic ) of the matter

In the beginning was the Verb, and the Verb was… The suppression of the J/  production yield in nuclear collisions should be

a clear signal of the QCD phase transition

from confined hadronic matter to a deconfined plasma of quarks and gluons “... colour screening prevents c-cbar binding ...”

The “melting” of the heavy-quarkonia states In the deconfined phase the QCD potential is screened and the heavy quarkonia states are “dissolved” into open charm or beauty mesons.

Different heavy quarkonium states have different binding energies and, hence, are dissolved at successive thresholds in energy density or temperature of the medium; their suppression pattern woks as a “thermometer” of the produced QCD matter.

T

A “smoking gun” signature of QGP formation:

steps

state Mass [GeV} B.E. [GeV] T d /T c J/  3.096

0.64

 c 3.415

0.2

0.74

 ' 3.686

0.05

0.15

The feed down from higher states leads to a “step-wise” J/  suppression pattern 

’



c

“Well known” J/  60% direct J/  cocktail: 30% from  c decays 10% from  ’ decays HERA-B J/  cocktail: 72% direct J/  21% from  c decays 7% from  ’ decays

J/  suppression: from theory predictions to SPS data p-Be p-Pb central Pb-Pb J/  normal nuclear absorption curve The yield of J/  mesons (per DY dimuon) is “slightly smaller” in p-Pb collisions than in p-Be collisions; and is

strongly

suppressed in central Pb-Pb collisions Drell-Yan dimuons are not affected by the dense medium they cross Interpretation: strongly bound c-cbar pairs by the deconfined medium (our probe) are “anomalously dissolved” created in central Pb-Pb collisions at SPS energies

The J/  and  ’ “normal nuclear absorption” in p-A collisions NA50 NA50 p-A 400 GeV J/   2 /ndf = 0.7

 ’  2 /ndf = 1.4

The Glauber model describes the J/  and  ’ “normal nuclear absorption”, in p-A collisions, in terms of the average path length, L, which they traverse in the target nucleus, from the production point to the nuclear “surface”

NA60: a third generation J/  experiment at the SPS NA60 collected less J/  events in In-In than NA50 in Pb-Pb but they are directly compared to the

normal nuclear absorption

curve, calculated with “Glauber”, without using the “statistically challenged” Drell-Yan yield normal nuclear absorption ~ 29 000 J/  dimuons The calculation of N part for each E ZDC bin uses the Glauber model, which reproduces distributions collected with minimum bias triggers (no dimuons)

J/  suppression at the SPS: In-In vs. Pb-Pb patterns There is a good agreement between the Pb-Pb and In-In suppression patterns when plotted as a function of the N part variable, determined from the (same) ZDC detector. The statistical accuracy of the In-In points is, however, much better...

The pink box represents the ±6% global systematic uncertainty in the

relative

normalization between the In-In and the Pb-Pb data points.

Question: Is there a “step” in the SPS J/  suppression pattern measured at the SPS ?

Answer:

J/  suppression at the SPS: model

tuning

on the Pb data CF: suppression by “comovers” Pb-Pb @ 158 GeV DFS: percolation phase transition GR: dissociation and regeneration in QGP and hadron gas, inc. in-medium properties of open charm and charmonium states Digal, Fortunato and Satz EPJ C32 (2004) 547 Capella and Ferreiro hep-ph/0505032 Grandchamp and Rapp NP A715 (2003) 545 PRL 92 (2004) 212301

Data vs. theoretical

predictions

: the results

S. Digal et al. EPJ C32 (2004) 547 R. Rapp EPJ C43 (2005) 91 centrality dependent

t

0

In-In 158 A GeV

R. Rapp EPJ C43 (2005) 91 fixed termalization time

t

0 A. Capella, E. Ferreiro EPJ C42 (2005) 419

None of these

predictions

describes the measured suppression pattern...

Homework exercise: calculate the  2 /ndf for each of these curves (ndf = 8) 49 9 14 ) = erreiro = 0 ) = t Solutions: (variable ed t 0 (fix 21 al. = et & F Rapp Rapp Capella Digal Note: the In-In data set was taken at the same energy as the Pb-Pb data...

to minimise the “freedom” of the theoretical calculations  Note: by moving up or down all the data points, within the 6% uncertainty on their global normalisation, we can get better (and worse) agreements...

but even in the best cases, the  2 /ndf remains very high...

Data vs. theoretical

post-dictions

...

Nuclear plus hadron gas absorption L. Maiani

et al

., NP A748 (2005) 209 F. Becattini

et al

., PL B632 (2006) 233 Nuclear absorption only...

plus largest possible absorption in a hadron gas (T = 180 MeV) This figure

In-In 158 A GeV Charmonium dynamics in heavy ion collisions O. Linnyk

et al.

, SQM 2007, June 28, 2007 and Nucl. Phys. A 786 (2007) 183.

HSD

 2 /ndf = 7.4

 2 /ndf = 16 ndf = 8 The probability that the measurements

should really be

on any of these two curves and “statistically fluctuated” to where they were in fact observed is...

zero

In-In data vs. a step function in Npart 1

A1 A2 Step position N part

Step at N part = 86 ± 8 A1 = 0.98 ± 0.02

A2 = 0.84 ± 0.01



2 / ndf = 0.75

(ndf = 8 3 = 5) Taking into account the E ZDC resolution, the measured pattern is perfectly compatible with a step function in N part

A step function in Npart or in another “physics variable” ?

N part is convenient to compare the measured In-In and Pb-Pb data, since it is derived from the same E ZDC variable (measured by the same detector) using the same Glauber formalism (except for different nuclear density functions).

Also, the derivation of N part from E ZDC is trivial and essentially model independent.

Maybe the “real variable” driving charmonia suppression is

measured not

smearing”, due to the conversion from the “real variable” to N N part . Then, the smearing is the convolution of the detector resolution with the “physics part But the N part “detector resolution” is 20 and the “total resolution” from fitting the measured pattern is 19 (!) indicating that the “physics smearing” is negligible with respect to the detector resolution...

In summary, the In-In pattern

indicates

that: 1) there is a step and 2) the “physics variable” is N part

What about the Pb-Pb pattern? Another step?

1

A1 A2 Step positions A3 N part

Steps: N part = 90 ± 5 and 247 ± 19 A1 = 0.96 ± 0.02

A2 = 0.84 ± 0.01

A3 = 0.63 ± 0.03



2 / ndf = 0.72

(ndf = 16 5 = 11) If we try fitting the In-In and Pb-Pb data with one single step we get  2 /ndf = 5 !..

In summary, the Pb-Pb pattern 1) rules out the single-step function and 2) indicates the existence of a second step...

 ’ suppression in heavy-ion collisions at the SPS The  ’ suppression pattern also shows a

significantly stronger drop

than expected from the “normal extrapolation” of the p-A data  ’  ’  abs ~ 20 mb The “change of slope” at L ~ 4 fm is very significant and looks

very

abrupt...

The third step of the day ! Starts to look like a “stairway to heaven”...

Back to the ideal world The predicted patterns,

before

any data points were available, were quite different from each other  We thought it was going to be very easy to discriminate between the two theories...

normal nuclear absorption suppression by QGP 

c

Energy density We made measurements, to rule out one of these two scenarios (or both)

Observations made at CERN Can any of the models describe the experimental data points?

Data versus the “no new physics” model normal nuclear absorption “outlier” point; to be rejected  All

kept

data points agree with the expected

normal nuclear absorption

pattern!

Data versus the “new physics” model calibration error anomalous suppression  All

kept

data points agree with the expected

QGP suppression

pattern!

A more detailed theoretical framework gluon anti-shadowing direct J/  suppression  ’,  c suppression B decays recombination of uncorrelated ccbar pairs Energy density New and improved data points are needed

Improved model versus new experimental data Once again, the model

prediction

agrees with the measurements...

Take-home messages...

1) There is a

BIG

difference between “ the measurements are

compatible with the model expectations

...

” and “ the measurements

show beyond reasonable doubt

that the model is good ” 2) “Nature never tells you when you are right, only when you are wrong” You only learn something when the theory

fails

to describe the data...

[Bacon, Popper, Bo Andersson]  Be happy if your model is shown to be wrong… 3) Today, none of the “fancy theories” describes the In-In suppression pattern  All the “fancy theorists” should be happy  4) The very simple step function gives a

perfect

description of the In-In pattern; a second step is needed to describe the Pb-Pb pattern 

We found what we were told to look for, as a “smoking gun QGP signal”

the experimentalists should be happy 