summer student lecture, june 17, 2010 – g. david, bnl heavy ion physics: an overview g. david, bnl...

Summer Student Lecture, June 17, 2010 – G. David, BNL

Heavy Ion Physics: an overview

G. David, BNL

With lots of inspiration by (and some outright plagiarism from)

Bill Zajc, former spokesman of PHENIX


Why?


History / 0 -- “In the beginning…”

Nature’s first experiment with ultra-high density and temperature matter freely expanding in vacuum (quite succesful )

RHIC’s attempt to duplicate it


History / 1 -- how fundamental are “elementary” particles?

1950s (slowly) growing number of elementary particles

1960s (rapidly) growing number of “elementary” particles

1970s thousands of elementary particles

Hadron 'level' diagram

0

500

1000

1500

0 10 20 30 40

Degeneracy

Mass (MeV)

Kfo

Density of States vs Energy

0

50

100

150

200

250

0 500 1000 1500 2000

Mass (MeV)

Number of available

states

This just cannot be true! - should I really believe in thousands of different building blocks??? - and even if so, what is this exponential rise of their number (with mass)???


History / 2 -- QCD …

The roaring 60s: - DIS (deep inelastic e-p scattering, probing on much smaller length-scale than the size of the proton) there is a substructure (partons)! - could neatly explain the thousands of “elementary” particles assuming just a few building blocks (but has to assume strange properties) - frantic search for fractional charge none found

The victorious 70s: - quarks are confined by an interaction that is just the opposite of QED: the further apart the quarks, the stronger the attractive force! (Uhm, just like a rubber string…) - QCD (Quantum Chromodynamics) was born

QCD potential at T=0

r -->

V(r

)


History / 3 -- …and Hagedorn

Meanwhile… - increased energies and luminosities produce newer and newer particles the “density” of new states increases exponentially with mass

Density of States vs Energy

0

50

100

150

200

250

0 500 1000 1500 2000

Mass (MeV)

Number of available

states

HT /maem~dmdn

(m)ρ

It looks as if there were a “temperature” TH (numerically: 170MeV) above which one cannot “heat” the system, no matter how much energy is “pumped in” “Hagedorn limiting temperature”


The quest for free (unconfined) quarks

Thus, confinement prevents us from seeing free quarks in the vacuum.

However: what if we compress and heat up many nucleons in a sufficiently large volume for sufficiently long time such that the individual quarks “forget” where they belonged and roam freely?


Caveat: (almost) everything we see is indirect

Even if we produce a system of free quarks and gluons, this lives only for extremely short times, ~10-23s – after that the quarks team up again in doublets and triplets (mesons and baryons, long-lived, observable particles)

Ideally one would insert an external “probe” into the medium, but that’s impossible.The next best thing is to use “internal probes” – created in processes we know are happening in elementary particle collisions (like p+p). Therefore, understanding this baseline is crucial.

Direct photon

One example: photon-jet pair production in p+pThe rate can also be calculated and it factorizes - momentum distribution of partons before scattering (PDF) - parton-parton scattering cross-section - probability of the parton to hadronize into a particular species (and momentum) “fragmentation function”


In some more detail

Direct photon

Prob. that a parton carries fraction x of the total proton momentum

Parton-parton scattering

Probability that a u-quark “fragments” into a + carrying fraction z of the original quark momentum


Warning

So we have a decent understanding (and a theory, QCD) to interpret and calculate how do partons interact when confined in nucleons We will try to understand a system of free partons supposedly produced in collisions of relativistic nuclei by looking at the same processes All the time we should ask ourselves

Direct photonAre these still the same distributions?

Are these still the same Feynman-graphs?

Isn’t this influenced by the medium?

The only bright spot: this is almost certainly unchanged


A few words on jets, leading particles

Jets (borne 1982) in theory are all particles generated from a single parton experimentally somewhat ill defined (“jets are a legal contract”) all particles within a cone and attributed (right or wrong) to the same parent parton

In principle they have a well-defined axis, total energy, etc. in practice these quantities depend on the jet algorithm

Sometimes we (casually) equate the jet with its leading particle (the one carrying the highest momentum)


What?

Summer Student Lecture, June 17, 2010 – G. David, BNL VNI Simulations: Geiger, Longacre, Srivastava, nucl-th/9806102

Space-time and visualization of a nucleus-nucleus collision


PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Initial hard (parton)scattering: jetsHigh momentumhadrons, photons correlations

Phase transition(constant T?)Hadronization,deexcitation

Large pressures,anisotropiesin particle emission

Thermalized system ofquarks, gluonsFreely escaping e.m. probes:photons, dielectrons, quarkonia

Tremendous yieldof particles,10-15,000/eventover 4

Different stages, different signals


But all we see are the end-products – except for photons

(Electromagnetic probes, to be more correct, which, when created, come out almost freely because em << s).And they are created in all stages of the collision! That surely is the magic bullet, the ideal probe, right?

Unfortunately, they come equipped with their own detection problems, like very low production rates to begin with…


Parton energy loss – single particles

We know at what rate are high transverse momentum (pT) hadrons produced in p+p collisions. These come from fragmentation of “hard scattered” partons – a very rare process (because it involves large momentum exchange)If the same hard scattering occurs in a nucleus-nucleus collision and a medium of deconfined partons is present, the scattered parton may re-interact and lose energy before leaving the medium and hadronizing at the far end we observe less high pT hadrons than expected

g

“jet quenching”observed with single particle distributions


Parton energy loss – control experiment

When a deuteron collides with a gold ion, you do not expect a partonic medium be formed.Therefore, if this deficit is really due to energy loss in the medium, there should be no deficit in d+Au collisions

“Nuclear modification factor” – the observed yield divided by the properly normalized yield in p+p collisions


Parton energy loss – is the normalization sane?

Remember: we said that the photons do not interact with the medium.So they should not be “suppressed”, the nuclear modification factor should be around 1


Measuring the temperature


Parton energy loss – path-length dependence

In the average collision the nuclei overlap only partially (“almond-shape”).

But this also means that the average pathlength for a parton in the medium is different in different directions:

…and that’s exactly what we observe!


Back-to-back correlations

Au+Au


Parton energy loss – back-to-back jets

Of course if one parton scatters out of the beam direction, there should be another parton scattering in the opposite direction, with the same momentum such that you should see jets always in pairs (many caveats here…)So you should see jets (high pT leading particles) always in pairs

except if one of them loses so much energy that it doesn’t look like a jet any longer!So: look for a jet somewhere, and look for a partner at the opposite side:

Pedestal&flow subtracted

While in p+p and d+Au we see the opposite jet, in central Au+Au it “disappears”


Surface bias

Pedestal&flow subtracted

In the singles spectra we have seen that jets are suppressed, but they don’t disappear completely They can be produced near the surface such that one parton escapes almost without energy loss, but then the other loses most of its energy.In rare cases they can be produced “tangentially” and we observe both jets.


Fine, but then where does the energy go?

“Peripheral” (just grazing) collisions “Central” collisions (full overlap)

The energy is “dissipated” into low (close to thermal) particle production both in (azimuth) and in (beam) direction


Even stranger observables

leads to(Esumi, QM’09)

ASSO TRIG

We have seen on the previous slide that in central events (where the opposite side high pT particles “disappear”) lower pT particles are enhanced at an angle (like a Mach-cone), producing what is called the “shoulder”. For the average event it is symmetric around .Can we actually force a path-length dependence upon them? If so, what do we see?

An asymmetry in the shoulder!


Soft and hard: no strong separation

Isn’t this interesting? All mesons are suppressed, about the same amount, independent of their mass – but baryons are less suppressed!

One possibility: recombination

Due to the steeply falling spectrum to produce a high pT particle is “most expensive” by fragmentation, easier from 2 medium-pT quarks and easiest from 3 low pT quarks


Charmonia – a thermometer

Color Screening

cc

RHIC

Charmonia are bound states of c-cbar pairs with increasing radii. If there is a medium with free color charges, then – depending on its density – it will “screen” the strong interaction above a certain distance.

Once the “screening radius” is smaller than the radius of the charmonium, it “melts”, cannot survive any longer in the plasma!

Watch which one survives tell the temperature…


2cos22cos21 210 vvNd

dN

Fourier expansion of the distribution of produced particle angle wrt reaction plane ():

Bulk behavior

So far we discussed the high pT particles (the realm of perturbative QCD), which os about 0.1% of all The rest 99.9%!) is in the “thermal” region, in fact, thermalized. What’s happening to them>Here is a typical collision:


The anisotropy parameter v2 Scales separately for baryons and mesons once the mass-effect is taken out (middle panel)Scales for all particles if in addition we divide by the number of constutient quarks (right panel)The system exhibits flow on the partonic level!


So, we created…

a medium, which is hot, dense, appears to have partonic degrees of freedom, and in addition behaves like a nearly perfect fluid

pions

PHENIX preliminary data

protons

PHENIX preliminary data

pT (GeV/c)Calculations suggest small Calculations suggest small ηη/s value/s value


But where does it all start?

RHIC low energy scan!


Critical point


First results from low energy scan

STAR, PRC 81 (2010) 024911


Summary

This was an introductory talk, meant to raise interest and to show how multi-faceted the RHIC heavy ion physics is.

It didn’t do justice to many great observations, just to name a few - attempts at full jet reconstruction and measuring fragmentation functions in nnuclei - fluctuations of multiplicity, particle ratios, etc revealing the nature of phase transition - 3-particle correlations - photon-jet correlations, setting the energy scale of jets and allowing true “QGP tomography” - the infinitely rich (and puzzling) world of heavy quarks - hints of parity violation that may help to explain why the universe exists as we know it (overwhelmingly matter, not matter-antimatter) - direct photons, the “historians” of the collisionsEtc. etc. However, one thing is clear:

This is a very rich field, not only promising, but already delivering extremely important physics. Join – you won’t regret it!


Backup


4-momentum can be determined from at least two of these quantities:

energy 3-momentum velocity

calorimetry tracking time-of-flight + pathlength or Cherenkov-effect

Fully stop the particleConvert its energy to - light, charge…Collect and read out

Follow path of chargedparticles in magneticfield – get momentumfrom curvature

p = 0.3B

+

-1 2

B

s

t0

t1

Time of flight

v = s/(t1-t0)

Cherenkov

n

cos() = 1/n

Examples: , K, , p, n, …

Charge (if any!) and 4-momentum needed for PID

Particle identification / 1 – long lifetime (>10-8s)


Why do we emphasize long lifetime? Because the detectors are fairly large, and the particle produced at the vertex has to survive until it reaches the detector! With 10-8s c > 3m

Example: hadron identification with momentum and time-of-flight measurement

y axis: inverse of the momentum x axis: time-of-flight

There are many other methods to identify long-lived particles

Particle identification / 2 – still long lifetime (>10-8s)


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summer student lecture, june 17, 2010 – g. david, bnl heavy ion physics: an overview g. david, bnl...

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