charmonium ii: nuclear collisions
DESCRIPTION
Charmonium II: Nuclear Collisions. Thomas J. LeCompte Argonne National Laboratory. Review of Yesterday. Charmonium is a bound state of a charmed quark and antiquark The attractive force between them has two pieces A Coulomb-like part, which dominates at short distances - PowerPoint PPT PresentationTRANSCRIPT
1
Charmonium II:Nuclear Collisions
Thomas J. LeCompteArgonne National Laboratory
2
Review of Yesterday
Charmonium is a bound state of a charmed quark and antiquark
The attractive force between them has two pieces A Coulomb-like part, which dominates at short distances A spring-like part, which dominates at long distances
The charmonium wavefunctions Are Hydrogen-atom like Can be described with H-atom like quantum numbers, e.g.
3S1 Drive the production properties
Charmonium production Is not a simple story Cannot be understood without considering the color
quantum numbers
3
Three Kinds of Confinement
1. Uncle Jake’s “two-to-five”stint in Leavenworth
2. Magnetic Poles: break a magnet intwo, and you still have a N-S pair.
3. Quark confinement – freequarks seem not to exist, andonly colorless hadrons are seen.
#2 is often used as an analogy for #3 In my opinion, this is a bad analogy
4
Semi-Classical Quark Confinement
Yesterday’s not-too-terrible model of the quark-antiquark force law:
Brr
AF
2
A Coulomb-like part
A spring-like part
This piece comes from the non-Abelian nature of QCD: the fact that you have 3-gluon and 4-gluon couplings.
In QED, there is no coupling, sothis term is absentThis is just like QED:
(sometimes called the“chromoelectric”
force)
4 E
QCDQCDE 4
In the interest of full disclosure:
The same thing happens with the quark-diquark forces. [Diquarks are also in a 3bar representation of SU(3)]
There are MUCH better potential models than what I have shown. These models use the quarkonia spectra to fit their parameters.
5
More Quark Confinement
Start with a quark antiquark pair
Pull them apart, and the color lines stretch. Potential energy increases by ½Br2
You can think of these color lines as “strings” connecting the quarks
Eventually, ½Br2 > ~m() and one can pop an antiquark-quark pair from the vaccuum
Instead of a free quark, now you have two colorless hadrons
This is not the way to observefree quarks – ironically, one of the best things to do is the reverse. You try and get the quarks close to each other – so as is small. This is called “asymptotic freedom” and is the subject of another talk.
6
Deconfinement
Suppose we repeat the same experiment in an environment gradually increasing the surrounding energy density The additional energy needed to pop a quark-antiquark pair
decreases It becomes progressively easier to “break the string” holding
quarks together The quarks act more and more like free particles
The other ends of their “color/glue strings” dance from quark to quark Having the quark connected to an ensemble of particles (instead of just
one) causes it to behave more like a free quark From experience, I can say working for seven bosses is like working for
yourself. It’s the same for quarks.
We have a deconfined state a quark gluon plasma
Last talk – the left hand piece of the force lawThis talk – the right hand piece
7
Phases of Nuclear Matter
From the Nuclear Physics Wall Chart
(“You don’t have to be a nuclear physicist to understand nuclear science”)
In a QGP, the coulombpart of the interactiondominates; the confinement part is overcome by the manynearby color charges.
8
What Is A Quark-Gluon Plasma?
My recollection of what a QGP was supposed to be prior to RHIC data, was a thermally and chemically equilibrated system of non-interacting quarks and gluons which were deconfined over distance scales much larger than a typical hadron. - Lanny Ray, University of Texas
Physicists believe that RHIC collisions will compress and heat the gold nuclei so much that their individual protons and neutrons will overlap, creating an enormously energetic area where, for a brief time, a relatively large number of free quarks and gluons can exist. This is the quark-gluon plasma. – BNL Public Web Page
… very hot, dense matter can dissolve into a mass of loosely associated quarks and gluons known as a quark-gluon plasma, or QGP, where particles would behave differently than they do in normal nuclei – Physical Review Focus
In … ordinary matter … quarks are never free of other quarks or gluons … at the heart of these collisions, the ties that bind quarks and gluons may have melted, creating a soup-like plasma of free-floating individual particles – LBL Public Web Page
Many opinions – but deconfinement is a (the) common thread
9
Reaching the QGP
Two beams of nuclei traveling towards each other.
(in the COM frame)
Upon collision, the pressure andenergy density is enormous.
If large enough, a deconfinedstate is produced.
As the systemexpands and cools, Hadrons form and leave the interactionregion.
Detectors measurethese hadrons andinfer the collisionproperties.
There exist both fixedtarget and collidingbeam experiments.
See S. Bass’ talk
10
Some QGP Questions
Is this actually a phase? Is there a phase transition? What is the order paramater? Is this a 1st order or 2nd order phase transition?
Is there a latent heat? Or just a “state”?
An analogy might be a block of glass, a piece of fiberglass insulation and a pile of sand
Macroscopically, very different properties, but not because of properties at the microscopic level
Is QGP an event-wide phenomenon? Is it all or nothing within an event? Or is there a
mixed phase? With a specific set of initial conditions, do you
always form a QGP, or are there two categories of an event: events with and without a QGP
Is there a discontinuity? What are the exact conditions required to
form a QGP? How are you sure you’ve made one?
These questionswill occupy theexperimental programs forsome time to come.
(i.e. I don’t have answers)
The hints thathave been obtainedfrom the SPS andRHIC so far point to a rich and complex phenomenon.
11
Charmonium Suppression
Start with a J/ This works with other charmonium
states as well The J/ is easiest to observe – lamppost
physics
Put it in a sea of color charges
The color lines attach themselves to other quarks This forms a pair of charmed mesons
These charmed mesons “wander off” from each other
When the system cools, the charmed particles are too far apart to recombine Essentially, the J/ has melted
1.
3.
2.
c.f. Matsui &Satz (1986)
Often called Debye screening, in analogy with E&M
12
More on Charmonium Suppression
Another way of looking at it is in terms of energy density: When the energy density is large enough, the color string
between the quark and antiquark pair can break This is exactly the same condition that causes quarks in
hadrons to deconfine in a plasma A simple (simplistic?) view is “All hadrons melt. The J/ is a
hadron.”
The more color charges that can get between the charmed quark and antiquark, the more the attraction is disrupted It’s easier to “melt” large hadrons than small ones:
The (2S) is bigger than the J/ (radial quantum number k = 2 vs. k = 1)
The J/ is bigger than the upsilon family, and so on This provides a very interesting prediction that can be tested
experimentally
13
The NA50 experiment
A closed-geometrymuon spectrometer,like many earlycharmonium experiments.
(see yesterday’s talk)
14
The Plot That Got Everyone Excited
This is a complicated plot: there is a lot of information in a small space!
I will spend the nextfew slides going through it.
From the CERN NA50 Experiment
15
How Excited?
A NEW FORM OF NUCLEAR MATTER has been detected at the CERN lab in Geneva – Physics News #470
CERN claims quark-gluon first10 February 2000 physicsweb
First data on the quark-gluon plasma reported at CERNEurophysics News
'Little Bang' creates cosmic soup – BBC News
Britney Spears set to release second album - Newsweek
16
The y-axis
A nucleus is a hadron – a big hadron It has p.d.f’s, just like nucleons (and mesons too!) G(x) for a nucleus may or may not be simply A x G(x) for a nucleon
Complications: shadowing, anti-shadowing, Cronin effect… [see Dave Soper’s talk]
Normalizing to Drell-Yan is a good idea because It tries to compensate for the fact that a nucleon in a nucleus is different
from a free nucleon The dimuon final state is similar to the J/ signal – many systematics cancel
Normalizing to Drell-Yan is a bad idea because Drell-Yan is an quark-antiquark induced process J/ production is a gluon-gluon induced process
Despite the problems this normalization procedure probably does more good than harm.
)(
)/()/(
DYAB
JBFJAB
The plot uses the ratio of
the measured numbers ofJ/’s normalized to Drell-Yan dimuons
17
The x-axis
The x-axis is conceptually simple: the average distance the J/ travels through the medium The amount of J/ dissociation should be proportional to the
distance it travels The sudden drop in yield is then interpreted as an increase
in the J/ dissociation probability per unit length Unfortunately, this is not a directly measured
quantity It has to be inferred Not everyone is happy with the chain of reasoning leading
to this inference Averages of distributions can be misleading
F(<x>) may not equal <F(x)>.
Eventually NA50 dropped it Later measurements are plotted against more directly
measured quantities
18
The Problem of Impact Parameter
b is large
In a glancing blow or“peripheral” collision, not much of excitement happens.
b is small
In a head on or “central” collision, there is a lot of energy transferred – forming a QGP?
Impact parameter cannot be measured directly. So experiments have to use a measured quantity as a proxy for impact parameter: multiplicity, total transverse energy, etc…
19
Estimating Centrality
Increasing Multiplicity in Detector
Many forwardneutrons:MID-central collisons
Few forward neutrons – EITHER a peripheral or a highly central collision
20
Another Plot that Caused Excitement
This is the ratio of the (2S) production relative to the J/. Note that you alwayshave more (2S) suppression than J/ suppression – exactly what you expect in a QGP
Also from the CERN NA50 Experiment
Apology: there was a lot more going around CERN at that time than just the J/ suppression story.
Even though I’m going to concentrate on one part of the story, there’s a lot more that I am not going to talk about.
This is S-U data; it was Pb-Pb that showed the large J/ suppression
21
The Line on The Plot
Even ignoring the last point, it’s clear that the J/ yield relative to Drell-Yan is falling as the nuclei get heavier.
This is called “A-dependence”, and it is often parameterized as ~ A
What the plot shows is for light nuclei, there is already some J/ suppression
The point of the NA50 data is not that therewas J/ suppression at all. The point was thatthere was too much in Pb-Pb collisions. The argument was quantitative, not qualitative.
This point often got lost by non-experts trying to understand and evaluate the NA50 results.
22
J/ A-dependence
0.8
0.85
0.9
0.95
1
1.05
1.1
10 15 20 25 30 35 40 45
Center of Mass Energy (GeV)
E-537
NA-3 NA-38 E-789
E-772
Drell-Yan
OpenCharm
The best fit is something like ~ 0.92
The (2S) has a similar A-dependence
(see next slide)
23
(2S) A-dependence
The [(2S)]/[J/] yield ratio is roughly independent of target A Conclusion: A-dependence is
similar Error bars still allow some
difference in the A-dependence
NUSEA has measured this as ~ 0.02-0.03
(2S) measurements are a tough business Yield is only ~2% of the J/ In a closed-geometry/high-rate
experiment, resolution smears the J/ into the (2S) and you have a shoulder, not a peak
In an open-geometry experiment, you’re fighting the 2% factor.
A hard choice to make
NA-50
C. Lourenco
24
Understanding the J/ A-dependence
Drell-Yan has ~ 1 Therefore the nuclear quark and antiquark p.d.f’s can’t
have changed by too much Sum rules limit how much the gluon can change
Open charm production has ~ 1 That tells us we are making about as many charmed quarks
as before – they just aren’t ending up paired as J/’s.
Inference: we are making as many J/’s as = 1 would predict, but they are disappearing in the medium Remaining Hypotheses:
QGP Absorbtion
Even before we come to the last data point, there are mysterious goings-on with charmonium in nuclei.
25
J/ in Nuclear Media
Could it be suppression from Debye screening? Note that the word “plasma” never appeared in my discussion Are there enough color charges in cold nuclear matter to dissociate
a J/? This is a quantitative question, and the answer appears to be “no”.
More to the point – this model predicts a rather substantial and unobserved difference in A-dependence between J/ and (2S):
If J/ = 0.92, (2S) ~ 0.79 [based on relative size of hadrons]
What else can this be? If this loss in yield is due to an interaction between the J/ and
nucelons, the cross section inferred is about 6 mb: a little larger than the J/ itself!
You will read that the J/ is “blacker than black”: that’s not quite true; many of the comparisons of cross-section and size drop a factor of . Nevertheless, the J/ is quite black.
One would expect the tightly bound J/ to be relatively non-reactive An analogy from chemistry: a tightly bound helium atom is also non-
reactive
26
Color Octet to the Rescue
From the last talk… Color Octet Model suggests that the charm-antiquark pair
forms in a color octet state Later this state emits a soft gluon and forms the J/
The suppression can occur at any time: it does not have to wait until the J/ is formed
A color-octet state will be very reactive To take the helium analogy, He is very non-reactive But the He+ ion is VERY reactive: it will oxidize oxygen! This can explain the large cross-section
Before the J/ is formed, you have an interacting octet precursor with a strong A dependence
After the J/ is formed, the relatively inert J/ sails through
27
CoMovers
There’s [at least] one other source of suppression: co-movers: hadrons moving near the J/ with small relative velocity.
You might ask “how can something moving slowly impart enough energy to disrupt a J/?”
It takes less energy than you think – all you have to do is (for example) flip the spin of one quark. Then you have turned a 3S1 J/ into a 1S0 c – invisible to the
experiments.
J/
hadron
J/
Hadron’s color field disrupts the J/.
Hadron approaches the J/.
1.
2.
3.
J/ remnants move apart.
28
Three phases of suppression
Before Before the J/ formation Color-octet precursor interacts strongly, even with cold
nuclear matter Gives rise to the observed A-dependence: ~ A0.92
During While the J/ is in the nuclear medium This is the Debye screening signature of Matusi and Satz
After As the hadrons escape the scene of the crime Co-movers can disrupt or destroy J/’s after they have
exited the nuclear medium
29
Conclusions from NA50 and Aftermath
Charmonium in nuclear media has a very rich phenomenology – richer than anticipated at the beginning of this enterprise. There is a lot of interesting physics going on! It’s hard to be “simple” and “interesting” at the same time.
The down side of this is that charmonium suppression is no longer the smoking gun than it was once thought to be. It’s still a powerful piece of evidence for understanding what is going on in nuclear matter.
We have – for good or ill – moved past the phase where qualitative understanding is good enough. We need quantitative modeling of all three phases - before, during and after – so they can be compared them with data.
Warning: Personal Opinion Here
30
More Recent NA50 Data
More data provides a clearer picture of what NA50 saw earlier
The overall picture is similar, but there are more data points That makes the
transition look less abrupt.
See next slide…
L has been dropped in favor of a more “experimental” variable.
31
Do We Expect an Abrupt Transition?
There are multiple sources of J/’s 8½% come from (2S) decays
Determined from yield in channel + branching fractions
35-40% of them come from decays See Talk #1
The Debye screening model says that large mesons should be destroyed by the QGP more readily than small ones The (2S) is bigger than the ’s, which are bigger than
the J/ Barring overlap, there should be three transitions – not
one
32
Next Steps: RHIC
Our understanding of charmonium hadroproduction improved enormously once the Tevatron collider data entered the picture
Similarly, heavy-ion physics now has a colliding beam accelerator, RHIC (at BNL) available to probe charmonium in nuclear matter
RHIC is a two-ring collider, able to
accelerate nuclei as heavy as gold (A = 197) to 100 GeV/nucleon
Can (and does) accelerate different ions in different rings
has 4 experiments (STAR, PHENIX, BRAHMS and PHOBOS)
33
The PHENIX Experiment
Large Acceptance Colliding Beam Detector
Emphasizes Leptons Forward and Backward Muon
Spectrometer Arms Central 2-Arm high resolution
electromagnetic calorimeter (for electron identification)
Probably the detector best suited for charmonium studies.
Painful for me to say: I used to be the physics coordinator for STAR
The Other RHIC Experiment
34
The Real PHENIX
Magnets and Muon Shielding
Calo
rim
ete
rM
uon
Dete
ctor
35
Other RHIC Experiments
STAR A 4 solenoidal detector
(like CDF, D0, CMS…) Emphasizes hadron
identification A calorimeter that is less
capable than PHENIX’s But with larger coverage
Tracker is a Time Projection Chamber (TPC)
These have very slow readout
Makes triggering on the J/ difficult
BRAHMS & PHOBOS Two small acceptance
detectors
STAR probably has the ability to“confirm” a PHENIX result.
BRAHMS and PHOBOS probably don’t have the acceptance/yield
36
Life at RHIC is Hard
A picture is worth a thousand words. I’ll let the pictures of PHENIX and STAR Au-Au events speak for themselves.
37
J/ in d+Au Collisions at PHENIX
ee invariant mass
invariant mass
Top: all dataBottom: wrong sign subtracted
Yield is ~ 1250 events
38
Physics with J/’s
With many hundreds of J/’s, PHENIX can measure distributions Plots to the left show
kinematics of J/ production
PHENIX is already moved from showing a signal to making measurements
Thus far, everything shown is for pp or dAu collisions What about AuAu?
Tra
nsve
rse
mom
entu
m d
istr
ibut
ion
Rap
idit
y di
stri
buti
on
39
Go For The Gold! (Au-Au Collisions)
PHENIX has a hint of a signal already of J/’s in this year’s Au+Au collisions.
STAR wants very badly to show a similar plot: I expect them to be able to do
this eventually
axes frompresent Au-Au run…
Once the signals have been established, both experiments will be investigating the yield of J/’s, particularly as a function of collision centrality.
Theoretical predictions can and will be compared with these measurements as a way of characterizing the properties of the RHIC collisions: the yield is related to the color charge density.Stay
tuned!
40
Summary
The good news: the phenomenology of charmonium in nuclear collisions is richer than anyone supposed There is enough interesting physics going on to support a
few dozen careers
The bad news: the phenomenology of charmonium in nuclear collisions is richer than anyone supposed Things are not as simple as first supposed
The goal of the field has shifted from “discovering the quark-gluon plasma” to “characterizing the nuclear medium under extreme conditions” This is a plus – we’ve moved past presupposing how things
will behave and towards measuring and understanding what really happens
Charmonium is a critical probe in this wider effort RHIC data in Au+Au collisions is right around the corner