Charmonia production and suppression at the SPSresults and perspectives

• The J/ψ suppression saga⇒ results and puzzles

• Near future perspectives⇒ with proton and ion beams

Carlos Lourenço - CERN-EPIWHQ, CERN, Nov. 8-10, 2002

1 nb

1 pb

Μµµ (GeV)

p-U → µµ at 20-30 GeV

Christensen et al.,Phys. Rev. D8 (73) 2016

First observationof J/ψ suppression,

in the late 60’s, by Lederman

??

1986 : J/ψ suppression proposed as a signal of the QCD phase transitionfrom confined hadronic matter to a deconfined partonic plasma

“we thus conclude that

• there appears to be no mechanism for J/ψ suppression in a nuclear collisionexcept the formation of a deconfining plasma

• and if such a plasma is produced, there seems to be no way to avoid J/ψ suppression”

[Matsui & Satz]

1987/88 : NA38, approved in 1985 to search for thermal dimuon production,finds that the J/ψ 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

When p-U was the only p-A data, and high mass Drell-Yan had poor statistics

NA38pp

p-U

1988/89 : Could the J/ψ be suppressed by normal nuclear matter ?or by hadronic “comovers” (produced secondaries: pions, rhos, etc) ?or by both processes ?

Normal absorption of charmonia production :

→ Aα parametrization :

→ < ρ L > parametrization :

→ Glauber calculation :

NA3 had measured αψ = 0.95 for 0.0 < xF < 0.4 ; equivalent to σabs ~ 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)

ψ + N/h → D + D + X

ασσ ApA 0=

)(exp0 ><−= LA abspA ρσσσ

[ ]∫ −= AabsA

abspA sTsd σ

σσ

σ )(10 rr

Remark : Comparison between three formulations of charmonia nuclear absorption

[Shahoyan]

Aα : widely used but very rough: the lighter is the first target, the higher is the extracted α

<ρL> : 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 σabs

1990/91 : Fermilab E772 experiment :⇒ measures αψ = 0.92, or σabs ~ 6-7 mb⇒ observes the same nuclear absorption for J/ψ and ψ ’

xF

α

NA3E772

DY

E772

The lightest target of NA3 was Hydrogen, while E772 used Deuterium

1992 : Nuclear absorption with σabs ~ 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 ⇒ σabs = 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

1992 : Puzzles :• Why is σabs much larger than the J/ψ geometrical cross section ?• Why is σabs the same for the J/ψ and for the ψ ’ mesons,

in spite of their different sizes ?

αψ’ − αJ/ψ = 0.00 ± 0.02

xF > 0.

Ratio ψ ’/ ψ does not seem to depend on target or energy or beam (p, p, π, γ)

0.23

1993/94 : heavy quark (short distance) QCD rediscovered :

⇒ σ (h-J/ψ) ~ 0 mb until quite high hadron’s energy ⇒ hadronic matter cannot dissociate J/ψ formed states⇒ suppression of physical J/ψ 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 ?⇒ σ(N-J/ψ) must be measured experimentally :

the inverse kinematics experiment !slow J/ψ ’s in the nucleus rest frame can onlybe measured if the nucleus is the beam

[Kharzeev & Satz, 1995]

1995 : Fermilab CDF experiment :⇒ J/ψ and ψ ’ absolute cross sections (excluding beauty and χc decays)

are much higher than predicted by the colour singlet model⇒ substantial direct J/ψ production comes from ccg states

[Bodwin, Braaten, ...]

CSM

NRQCD

x 50

x 6-7

CSM

CDF

Also E789 has seen that J/ψ and ψ ’ production require K factors of 7 and 25 relative to the colour singlet model calculations

⇒ S-U suppression reproduced by Glauber with a ccbreak 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

1996 : Implications of colour octet ideas for charmonia in media :⇒ J/ψ and ψ ’ 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/ψ and ψ ’ (for xF > 0)

[Kharzeev et al.]

p-A

S-U Pb-Pb

1996 : Status of the charmonia suppression saga

CDF dataαψ’ − αJ/ψ = 0.00 ± 0.02

σabs ~ 6-7 mb understood

colour octet ideas

S-U data compatiblewith p-A extrapolation

No hint for deconfinement or comover absorption up to S-U

1999 : Anomalous suppression pattern : evidence of deconfinement in Pb-Pb collisions

Open questions :⇒ Onset of what exactly ?⇒ As a function of which variable ?⇒ Is there really a second step ?⇒ Are peripheral Pb-Pb collisions normal ?

χc suppression

Mea

sure

d / E

xpec

ted J/ψ suppression ?

Pb-Pb

centrality

1999/2000 : E866 and NA50 news (from high statistics data) :⇒ stronger nuclear absorption for ψ ’ than for J/ψ (for low xF )⇒ lower values for σabs : 4.7 mb instead of ~ 6-7 mb ...

Normal J/ψ nuclear absorption (NA50) :

α = 0.927 ± 0.012 from Aα fit

σabs = 4.7 ± 0.8 mb from Glauber fit

σabs = 4.3 ± 0.7 mb from <ρL> fit

Bσ

(ψ’)

/ B

σ(ψ

)

α = 0.95

2001/02 : Further NA50 news from high statistics p-A data :

α(DY) = 0.995 ± 0.016 ± 0.019

Glauber fit :

σabs (J/ψ) = 4.4 ± 1.0 mb

σabs (ψ’) = 6.4 ± 1.5 mb

Correcting for the feed-down from the ψ ’ decays :

6.3 ± 2.9 mb

σabs (S-U) :

7.1 ± 3.0 mb

Simultaneous Glauber fitto p-A and S-U J/ψ data :⇒ σabs = 4.3 ± 0.6 mb

J/ψ 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 ψ ’ feed-down)• Drell-Yan in the mass range 4.2-7.0 is less sensitive to fitting systematics

Measuring charmonia suppression in a lighter collisionsystem will confirm or rule out specific models

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 ψ’, χc and J/ψ suppression ?

♠ Which fraction of J/ψ comes from χc 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 ?

• Track matching through the muon filter• Improved mass resolution• Improved signal / background ratio (rejection of π and K decays)

• Muon track offset measurement • Separate charm from prompt (thermal ?) dimuons

µ

π, Κ → µ

D → µ

{offset

vertex

The detector concept of the NA60 experiment

Adding pixel detectors

D+ : cτ = 317 µm

D0 : cτ = 124 µm

D

D meson tagging using the impact parameter of the muon tracks

Improved dimuonmass resolution

interaction vertex

Beamscope

Pixel Telescopeprimary vertex

beam

The NA60 silicon pixel telescope

• 16 silicon pixel planes• 720 000 pixels of 50 × 425 µm2

• Accurate track and vertex reconstructionin a high multiplicity environment

( Instead of ) Conclusions

ð After considerable progress quarkonia hadro-production remains full of puzzleseven in pp and p-nucleus “elementary” collisions

ð Heavy quarkonia production is a rare processdetailed understanding requires good statistics and controlled systematics

ð A third generation experiment is now ready for precision p-A and A-A studiesbuilt on the shoulders of a 15-year long learning curve

future running requires very good and competitive physics cases

ð The QWG is invited to suggest interesting measurements to be made in NA60in p-A collisions at 400 GeV