David Morrison

Jet Physics at RHIC

Focusing high-energy tools on nuclear collisions

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The BNL lecture last month …

PET Imagingby

Dave Schlyer

electron positron

photonE = 500,000 eV

photon

2 mm

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The BNL lecture this month …

quark quark

photonE = 500,000 eV

photon

2 mm

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The BNL lecture this month …

quark quark

quarkE = 500,000 eV

quark

2 mm

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The BNL lecture this month …

quark quark

quarkE > 5,000,000,000 eV

quark

2 mm

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The BNL lecture this month …

quark quark

quarkE > 5,000,000,000 eV

quark

0.000000000002 mm

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Exploring different territories

PETbiological tissues

RHICdense mix of quarks, gluons

particles

particles

brain nuclearcollision

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Back to 1964

people were trying to understand origins of so many discovered particles

Gell-Mann and Zweig propose quarks as underlying structure

quark concept focuses on kinship relations among particles

quarkshadrons

mesons baryons

pions,kaons,

...

protons,neutrons,

...

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While others were at Woodstock Feynman, in 1969,

proposes “partons” as way to explain experimental results from SLAC at Stanford

parton concept focuses on dynamics, the way things behave when then interact

With advent of QCD in 1973 partons are identified with quarks and gluons

hadron

pL

pT

hadron

Free quarks?

Strong nuclear force has some very unusual properties doesn’t get weaker with

distance! So what happens when

you try to send two quarks flying apart?

anti-quark

quark

EM force decreases with distance

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Fragmentation

q q

quarks, anti-quarks appear, break

original connection into more and more and more particles

q qqq

qqqqq q

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A directed “spray” of particles as connection between

quarks breaks up, most of the motion stays close to direction of the original quarks

the fragmented bits appear as normal subatomic particles pions, kaons, protons, ...

kaon

pion

pion

pion

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Origin of the word “jet”

“sprays” of particles had been seen in experiments before

original term “core”, came from cosmic ray experiments

first use of “jet” seems to be by Bjorken in 1970

high-energyproton

14N

corecore

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Jet properties

cone-like spray of particles surrounding direction of each outgoing parton

quark-quark scattering leads to back-to-back structure

high-energy parton-parton interaction can be calculated with precision

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Many sources of low pT particles many ways to create

particles in electron-positron or proton-proton collisions that don’t involve jets e.g., create an unstable

particle that then decays typical transverse

momentum (pT) few hundred MeV/c

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At low energy, jets hard to discern

?

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At higher energy, jets stand out

!

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First evidence for jets was subtle By 1975 at SLAC (DESY

too) energy of electron-positron collisions high enough for jets to appear ... statistically

As collision energy was raised, average “sphericity” decreased

Gradual appearance of back-to-back jets in Mark I experiment

collision energy

sphe

ricity

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Jets in electron-positron collisions

qqee

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Jets & proton-antiproton collisions e+e- one thing; hadron

collisions another incoming partons vary

International conference on high-energy physics, Paris, 1982

Results from CERN experiment UA2 really convinced everyone that jets in hadron-hadron collisions had been seen

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Very selective timeline

quarks1964

partons1969

“jets”1970

jets in e+e–

1975QCD1973

Fermilab(NAL)CERN ISR

CERN SPS

UA2jets inp +p1982

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Jets and the period 1969-1982 It took time for suitable facility to be available

high enough energy for jets to stand out It took time to design and build the right sort

of experiment Fundamental theory was developed part-way

through the period

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RHIC Physics Program

RHIC proposed 1983 One of the main emphases is study of

properties of matter under extreme conditions huge energy densities enormous temperatures (over 1 trillion C)

To achieve these conditions we collide heavy nuclei at very high energies

Extremely useful to have probes with known properties

1984 BNL note about RHIC physics

Jets in nuclear collisions

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Jets at RHIC

Not starting at “square one” properties of jets in electron-positron, proton-

proton, proton-antiproton collisions well-measured relying on over 30 years of jet physics results

Energy high enough that jets not too rare Experiments designed with jets in mind The plan in a nutshell

show that RHIC experiments can “see” jets look for changes in expected jet properties

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STAR

PHENIX

PHOBOS BRAHMS

you are here

PHENIX

STAR

Each collaborationabout 400 physicists

and engineersMuch of the research

driven by students

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Finding jets using correlations Method 1

find a high-momentum charged particle and look nearby for others

Method 2 main particle in jet is very

often a pion a 0 usually decays into

photons find a high-energy photon

and look nearby for others

pion

photons

pion, kaon, ...

triggerparticle

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Correlations outside particle physics One way to find “el

encierro” (the running of the bulls) in Pamplona, Spain:

1. start by finding one high-momentum bull

2. look near that bull for others moving in the same direction

3. if the bull density is high, you’re likely standing in the middle of the bull run

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PHENIX

See a pattern consistent with jets!

part

icle

trac

k de

nsity

angle of track away from photon

+-

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Candidate jets in RHIC p+p collisions

PHENIX

STAR

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Collisions of larger objects

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Hiding in plain sight

gold ion

gold ionAu+Au at full RHIC energy

in STARdetector

jets can be difficult to

find,even when you know

they’re there

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What is a nucleus?

If you ask a very high energy proton, it’s a huge collection of partons

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Collision geometry

gold nucleus

gold nucleus

gold is a large nucleus, lots of

partons (quarks, anti-quarks and gluons)

“pancake” thin due to special

relativity

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A “peripheral” (glancing) collision

a bit like a proton-proton

collision

quark quark

jet

jet

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A “central” (head-on) collision

quark quark

jet?

jet?

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Single pions and energy losspr

obab

ility

of c

reat

ing

0

PHENIX Preliminary

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A “lighthouse” of parton fragments

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Using this tool to study QCD “fog” seeing one beam, know

the other should be there collide two fog banks,

wait for spontaneous appearance of working lighthouse

look for changes intensity wavelength angular spread

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High-energy physics in vacuum

parton parton

parton

parton

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High-energy physics in medium

parton parton

parton

parton

hot, dense systemof quarks, gluons

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What about the other jet?

jet “

stre

ngth

”

glancing head-ontype of collision

STAR results: PRL 90, 082302 (2003)

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Interpreting the observation

quark quark

jet!

strongly suggests that “stuff” created during collision is very unusual, very unlike normal

nuclear matter

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A (very) loose comparison

accelerator RF cavity: 10 MV/m

parton in hot, dense “QCD matter”: 5 GeV/fm

factor of 500 quadrillion different

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Parallels with 1970’s high-energy RHIC is creating nuclear collisions at particle

physics energies C-AD runs a machine with unprecedented

capabilities The experiments have been designed with

the benefit of previous efforts acceptance, resolution, calorimetry, particle

identification Very active exchanges between experiment

and theory; active development of theory

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Fin

RHIC experiments detect jets convincingly Jets used as sophisticated probe of very

complex environment of nuclear collision study of jets also important for spin physics!

Only the more straightforward jet analyses have been published so far

Jet measurements contributing to very lively interplay between theory and experiment

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We’d like to “X-ray” nuclear collisions However, analogy is

slightly flawed Medical X-ray source is

(usually) outside the system being studied

Properties of X-ray beam can be prepared precisely

Can study X-ray photos with and without a sample in place

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STAR

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What is a nucleus?

+

Depends how you ask the question. If you ask what it looks like to a not too high energy electron, it’s a heavy blob of positively charged matter

-

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What is a nucleus?

If you ask a medium energy proton, it’s largely a collection of protons and neutrons

+

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What about the other jet?

jet “

stre

ngth

”

glancing head-on

type of collision

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Yes, almost completely.

jet “

stre

ngth

”

glancing head-on

type of collision

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RHIC proton-proton collisions We haven’t discovered jets; we’ve shown that

the experiments can detect the expected jets Provide a solid baseline for studying nucleus-

nucleus collisions Fine-tune techniques and the understanding

of detectors to prepare for finding jets in nuclear collisions