charmonia production and suppression at the sps results ... · charmonia production and suppression...
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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