8.882 LHC PhysicsExperimental Methods and Measurements

Heavy Ion Physics Overview[Lecture 4, February 17, 2009]

with a 'thank you' to Bolek and Gunther for material and explanations

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Organizational Issues• Course and recitation

• new students: Michael and Erik• please make sure to catch up on recitation• any questions• Matthew is expert in setting up windows

machines if needed•

• Recitation• Friday at 12:00 noon in 24-414

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Lecture Outline Heavy Ion Physics Overview•general introduction•the strong force and QCD•state diagram•real life heavy ion physics

•variables and their implementation•measurements

•experimental status

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Particle Physics Searching for the smallest constituents – elementary particle•un-dividable unit(s)• the atomos in the true sense of the word

•water droplet → water molecule → hydrogen atom → proton → quarks

Search for the fundamental forces or interactions

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Current Elementary Particles Matter particles• fermions (half integer spin)(

Force carriers•bosons (integer spin)

Fermions organized•generations, families•higher generations unstable: decay to lowest

•1st generation makes up almost all we see

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Particle Physics and the Universe

Heavy ion physics after elementary particle formation but before nucleon formation (~1 GeV)

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The Strong Force Heavy Ion Physics is all about the strong force

Examples of strong force•binding of nucleons into the atom core: protons repel each other (electromagnetic charge), neutrons need to be added

•strong force in core let's proton decay (weak decay)•binding force of the proton itself (three quarks inside)

•binding force of the pion (two quarks inside)• in fact binds all hadrons

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Quantum Chromo Dynamics (QCD) What is QCD? Theory of the strong force!• fermions = quarks: fractional electric charge: u +2/3, d -1/3• force carrier is the gluon (8)• charge (QED) → color charge (QCD): red-green-blue• asymptotic freedom

• quarks free to move when they are close• coupling large: no perturbative solution

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Nobel Price 2004 The Nobel Prize in Physics 2004•Gross, Politzer, Wilczek: “for the discovery of asymptotic freedom in the theory of the strong interaction”

H. David Politzer Frank Wilczek

Interesting to read• http://nobelprize.org/nobel_prizes/physics/laureates/2004/gross-autobio.html• http://nobelprize.org/nobel_prizes/physics/laureates/2004/politzer-autobio.html• http://nobelprize.org/nobel_prizes/physics/laureates/2004/wilczek-autobio.html

David J. Gross

the younger Wilczek

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Discovery of the Quarks Repeat of Rutherford experiment

Finding in a nutshell•high energy electrons scatter of point-like quasi-free particles inside the proton

•proton has sub-structure (quarks)

1990 Nobel Prize to

Jerry Friedman (MIT), Henry

Kendall (MIT),

Richard Taylor (SLAC)

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Nobel Price 1990 The Nobel Prize in Physics 1990•Friedman, Kendall, Taylor: “for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics”

Henry W. Kendall Richard E. Taylor

Interesting to read• http://nobelprize.org/nobel_prizes/physics/laureates/1990/friedman-autobio.html• http://nobelprize.org/nobel_prizes/physics/laureates/1990/kendall-autobio.html• http://nobelprize.org/nobel_prizes/physics/laureates/1990/taylor-autobio.html

Jerome I. Friedman

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Strong Force Paradox•weakly bound proton constituents can be seen in high-energy scattering, but cannot be liberated even in most violent collisions

Confinement•direct search for quarks were performed without success

•why can they not be found by themselves?•answer: confinement - objects are always colorless•mesons: quark-antiquark= no color, ex. pion:•baryons: green-blue-red = white, ex. proton [uud], neutron[udd]

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The Color String – Flux Tube• The Color String

• overall colorless• stores energy when quark-antiquark are pulled apart• breaks up when enough energy stored = fragmentation

→ quarks look like “jets”• baryons are formed when three quarks are close in

phase space

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Observation of the Gluon Gluon confined as the quarks• indirect observation only•gluon looks like a quark = “jet”• three jet events are signature• first shown by TASSO experiment at PETRA (1978-86)

•detailed flow of fragmentation very interesting

• independent versus string fragmentation

•PETRA built to discover top

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Heavy Ion Physics Goals• find regime to set the quarks and gluons free•we know, they are asymptotically free (QCD)•matter has to be extremely dense that protons break up

•recreates phase of the universe close to big bang•quark-gluon-plasma (quark gluon gas, weakly coupled)

Implementation•accelerates many neutrons and protons to very large energies and collide them

•best done by using heavy ions (heavy = large A)• ions to accelerate, electrons completely removed

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Key Word: State Transition• From Thermodynamic

• phase or state transition:• sudden change of an observable with

respect to a state parameter

• first order: discontinuous• second order: discontinuous first

derivative• cross over: smooth transition

• Random example• Argon ice

• gas-fluid-liquid

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State Transition: Quark-Gluon Plasma What would one expect to happen?•phase transition of some observable:

• hadron regime → free gluon-quark regime•observable should show sudden change of behavior

What are observables in heavy ion collisions?•normalized number of particles produced•ratio of kaons to pions•normalized number of heavy onia•smart ideas might make you a hero in HI physics!

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Quark Gluon Plasma or what? Expected to find Quark-Gluon Plasma•gas in which quarks and gluons are free•naïve starting point: put quarks/gluons close together but give them lots of energy (10-20 times than in proton)

•expect asymptotic freedom to do the rest•subtle balance between energy and force required•problem: calculations are close to impossible

Experiments find• no quark-gluon plasma• instead: quark-gluon conglomerate behaves like a liquid

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State Diagram

RHIC

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A Typical Heavy Ion Collision Sequence of events

Consider● luminosity low: one collision per event ●need to extrapolate back from freeze out●detailed collision parameters are crucial

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Practical Problems Collision objects• no elementary particles• no protons either• every collision needs to be characterized

Participants• nucleon can be observer or participant• normalize to number of participants

nucleus 1 nucleus 2 spectators

participants

spectators

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Number of Participants

Number of participants also referred to as• impact parameter b, b = 0 (full participation)•centrality: fully central means b = 0

b Number of collisions•each participant can interact more than once

•≥ number of participants

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Determine Number of Participants Fixed target experiments•measure energy of spectators•nParticipants = (1 – E

cal/E

beam) A

Non trivial problem for collider (boot strap)•use Monte Carlo to determine impact parameter•measure tracks in forward region, N (different process)

Determine b impact parameter when fMC = fData•assume N related with b by monotonous function•systematics has to be evaluated

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Impact Parameter from N Particles

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Variables of Interest Number of particles seen from collisions•very straight forward quantity to measure•natural comparison is pp collision

• neutron or proton should have very similar behavior concerning the strong force

• independent collisions could be directly compared•normalize to number of participants (participant can have several collisions

•phase transition should appear by adding all experiments together

Our first measurement•use CDF data at 2 TeV proton-antiproton collisions

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Status after RHIC Published

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Jet Quenching

Independent collisions should makethis ratio exactly 1

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Jet Quenching, Confirmed Analysis outline•define main direction by leading trigger particle

•count number of particles in azimuthal angle

•expect to find opposite side jet as expected from pp

Analysis result• find particles around leading particle (jet)•opposite side activity significantly reduced compared with corresponding pp data

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Conclusions Heavy Ion physics•create matter state close to the beginning of the universe: this is about strong interaction (QCD)

•no sharp state transition observed• number of particle produced on a smooth curve

•quark-gluon plasma not so gas like but rather like liquid• jet quenching• elliptic flow (not discussed here)

•a surprise but not inconceivable: theory could not make precise predictions

•exciting experiences expected from the LHC

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Next Lecture Charge multiplicity measurements• introduction to observables and experimental status• the CDF data and how they are organized

• trigger conditions and information• contents of the ntuple

•prototype analysis•main components for full analysis

• pile up events• secondary interactions

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Particle Physics

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Explain in 60 Seconds• Quarks are fundamental building blocks of matter.

They are most commonly found inside protons and neutrons, the particles that make up the core of each atom in the universe. Based on current experimental evidence, quarks seem to be truly fundamental particles; they cannot be further subdivided.

Protons and neutrons mainly contain two types of quarks. These are called up and down quarks. For reasons still unknown, nature also designed two copies each of the up and down quarks, identical except for having larger masses. The heavier copies of the up quark are called charm and top quarks; the copies of the down quark are named strange and bottom quarks. Converting energy into mass, accelerators produce these heavier, short-lived quarks through particle collisions.

Quark masses span an enormous range. The heaviest quark is the top quark, which is about 100,000 times more massive than the two lightest types, up and down. The explanation for this hierarchy is a deep mystery, but the top quark’s huge mass could turn out to be a virtue. Probing the detailed properties of the top may shed light on the origins of mass itself in the universe.

Jay Hubisz, Fermilab

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The Universe