1. Charmonia production and suppression at the SPS results and perspectives
1 nb
1 pb
Mmm (GeV)
Christensen et al., Phys. Rev. D8 (73) 2016
The J/y suppression saga  results and puzzles
Near future perspectives  with proton and ion beams ?
First observation of J/y suppression,
in the late 60’s, by Lederman
p-U ® mm at GeV
Carlos Lourenço - CERN-EP
IWHQ, CERN, Nov. 8-10, 2002

2. 1986 : J/y suppression proposed as a signal of the QCD phase transition from confined hadronic matter to a deconfined partonic plasma
“we thus conclude that
there appears to be no mechanism for J/y suppression in a nuclear collision except the formation of a deconfining plasma
and if such a plasma is produced, there seems to be no way to avoid J/y suppression” [Matsui & Satz]

3. in S-U, from peripheral to central collisions
1987/88 : NA38, approved in 1985 to search for thermal dimuon production, finds that the J/y is suppressed w.r.t. the dimuon continuum
from p-U to O-U and S-U
in S-U, from peripheral to central collisions
NA38 pp
p-U
When p-U was the only p-A data, and high mass Drell-Yan had poor statistics

4.  < r L > parametrization :  Glauber calculation :
1988/89 : Could the J/y be suppressed by normal nuclear matter ? or by hadronic “comovers” (produced secondaries: pions, rhos, etc) ? or by both processes ?
NA3 had measured ay = 0.95 for 0.0 < xF < 0.4 ; equivalent to sabs ~ 4 mb
 Not enough to explain the observed suppression
Adding absorption by hadronic comovers could describe the p-U / O-U / S-U data
(but requiring very high pion densities)
Normal absorption of charmonia production :
 Aa parametrization :
 < r L > parametrization :
 Glauber calculation :
y + N/h  D + D + X

5. Remark : Comparison between three formulations of charmonia nuclear absorption
[Shahoyan]
Aa : widely used but very rough: the lighter is the first target, the higher is the extracted a
<rL> : average amount of matter seen by the meson from its production until exiting the nucleus
Glauber : meson is produced in NN interaction and absorbed in nuclear matter with cross section sabs

6. 1990/91 :. Fermilab E772 experiment :.  measures ay = 0
1990/91 : Fermilab E772 experiment :  measures ay = 0.92, or sabs ~ 6-7 mb
 observes the same nuclear absorption for J/y and y ’ DY
NA3
E772 a
E772 xF
The lightest target of NA3 was Hydrogen, while E772 used Deuterium

7. 1992 : Nuclear absorption with sabs ~ 6-7 mb describes the p-A / O-U / S-U data  No room left for suppression by produced hadronic comovers
[Gerschel & Hufner]
Glauber fit  sabs = 6.4  0.8 mb
However, the p-A data points are rather poor :
no points between pd and p-W
pp/pd at different energy from p-W/p-U
poor statistics

8. 1992 : Puzzles :
Why is sabs much larger than the J/y geometrical cross section ?
Why is sabs the same for the J/y and for the y ’ mesons, in spite of their different sizes ?
ay’ - aJ/y = 0.00  0.02
xF > 0.
0.23
Ratio y ’/ y does not seem to depend on target or energy or beam (p, p, p, g)

9. 1993/94 : heavy quark (short distance) QCD rediscovered :
 s (h-J/y) ~ 0 mb until quite high hadron’s energy
 hadronic matter cannot dissociate J/y formed states
 suppression of physical J/y states requires deconfinement
 something else is being absorbed in the p-A, O-U, S-U data
caveat :
short distance QCD only works for heavy quarks
 is the charm quark heavy enough ?
 s(N-J/y) must be measured experimentally : the inverse kinematics experiment ! slow J/y ’s in the nucleus rest frame can only
be measured if the nucleus is the beam
[Kharzeev & Satz, 1995]

10. 1995 : Fermilab CDF experiment :
 J/y and y ’ absolute cross sections (excluding beauty and cc decays)
are much higher than predicted by the colour singlet model
 substantial direct J/y production comes from ccg states
[Bodwin, Braaten, ...]
x 50
x 6-7
CSM
CDF
CSM
NRQCD
Also E789 has seen that J/y and y ’ production require K factors of 7 and 25 relative to the colour singlet model calculations

11. 1996 : Implications of colour octet ideas for charmonia in media :
 J/y and y ’ production proceeds via a ccg pre-resonance state
 Gerschel-Hufner fit gives ccg absorption in nuclear matter : 6-7 mb reasonable
 pre-resonance leaves the nucleus before forming final charmonia states
implying the same nuclear absorption for J/y and y ’ (for xF > 0)
p-A
[Kharzeev et al.]
S-U
Pb-Pb
 S-U suppression reproduced by Glauber with a cc break up cross section determined by the p-A data
 No room left for comover absorption from p-A to S-U
 New : anomalous suppression in Pb-Pb collisions, increasing with centrality

12. S-U data compatible with p-A extrapolation
1996 : Status of the charmonia suppression saga
ay’ - aJ/y = 0.00  0.02
CDF data
colour octet ideas
sabs ~ 6-7 mb understood
S-U data compatible with p-A extrapolation
No hint for deconfinement or
comover absorption up to S-U

13. 1999 : Anomalous suppression pattern : evidence of deconfinement in Pb-Pb collisions
cc suppression
Measured / Expected
J/y suppression ?
Pb-Pb
centrality
Open questions :
 Onset of what exactly ?
 As a function of which variable ?
 Is there really a second step ?
 Are peripheral Pb-Pb collisions normal ?

14. 1999/2000 : E866 and NA50 news (from high statistics data) :  stronger nuclear absorption for y ’ than for J/y (for low xF )
 lower values for sabs : 4.7 mb instead of ~ 6-7 mb ...
a = 0.95
Normal J/y nuclear absorption (NA50) : a =  from Aa fit sabs =  0.8 mb from Glauber fit sabs =  0.7 mb from <rL> fit
Bs(y’) / Bs(y)

15. sabs (J/y) = 4.4  1.0 mb sabs (y’) = 6.4  1.5 mb
2001/02 : Further NA50 news from high statistics p-A data :
a(DY) =   0.019
Glauber fit :
sabs (J/y) = 4.4  1.0 mb
sabs (y’) = 6.4  1.5 mb

16. Correcting for the feed-down from the y ’ decays :
Simultaneous Glauber fit to p-A and S-U J/y data :
 sabs = 4.3  0.6 mb
6.3  2.9 mb
sabs (S-U) : 7.1  3.0 mb

17. J/y suppression in peripheral Pb-Pb collisions agrees with normal absorption curve
Peripheral collisions collected in 1996 were contaminated by Pb-air collisions
Year 2000 data collected with target in vacuum
Absorption curve determined from p-A and S-U data (not correcting for y ’ feed-down)
Drell-Yan in the mass range is less sensitive to fitting systematics

18. Some open questions :
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 physical origin of charmonia suppression ? What is the variable that rules the onset of ’, c and J/ suppression ?
Which fraction of J/ comes from c decays ? Does it change from p-Be to p-Pb ?
Is there comover absorption in S-U collisions ? What is the break-up cross section of formed charmonia states in hadronic matter ?
Measuring charmonia suppression in a lighter collision system will confirm or rule out specific models

19. { The detector concept of the NA60 experiment m p, K  m D  m vertex
offset
vertex
Track matching through the muon filter
Improved mass resolution
Improved signal / background ratio (rejection of p and K decays)
Muon track offset measurement
Separate charm from prompt (thermal ?) dimuons

20. D Improved dimuon mass resolution Adding pixel detectors
interaction vertex
D+ : ct = 317 mm
D0 : ct = 124 mm
D meson tagging using the impact parameter of the muon tracks D

21. The NA60 silicon pixel telescope
16 silicon pixel planes
pixels of 50  425 mm2
Accurate track and vertex reconstruction in a high multiplicity environment
Beamscope
beam
primary vertex
Pixel Telescope

22. ( Instead of ) Conclusions
After considerable progress quarkonia hadro-production remains full of puzzles even in pp and p-nucleus “elementary” collisions
Heavy quarkonia production is a rare process detailed understanding requires good statistics and controlled systematics
A third generation experiment is now ready for precision p-A and A-A studies built on the shoulders of a 15-year long learning curve future running requires very good and competitive physics cases
The QWG is invited to suggest interesting measurements to be made in NA60 in p-A collisions at 400 GeV