# Presentation: 426377

## Richard Seto
University of California, Riverside
Physics 39
Nov 2, 2002

- Richard Seto
- University of California, Riverside
- Physics 39
- Nov 2, 2002

**Quantum Chromodynamics**

**Relativistic Heavy Ion Physics **

**and the Structure of the Vacuum**

## Relativistic Heavy Ion Physics – an outline

- Going back to the big bang
- What was it like? – use some theory
- What is the vacuum? We have a theory! – Can we believe it? Does it explain mass?
- How are theories built?
- Weird...
- Can we recreate the big bang??
- The experiment
- Some results
- The future?

- Richard Seto

## Why Heavy Ion Physics? Recreating the Big Bang

- Very early universe - 1 microsecond after the big bang
- Temperature -1 million times hotter than sun
- Universe was a soup of free quarks and gluons
- Questions
- What was early universe like?
- How did it go from free quarks and gluons to regular “Stuff” (protons,neutrons...) as we know it?
- Relativistic Heavy Ion Physics seeks to recreate this                                       in the  laboratory by colliding heavy nuclei (e.g. Gold)                                   at high energies

- Richard Seto

**Notes:**
Define the field
UCR - historic strength
Group of 10 research people
 supported by DOE

## RHIC: A Doomsday Machine?

- Richard Seto

## -

- 1 microsecond after big bang
- Size of universe ~ 10 km size of riverside
- At the time of strong interaction phase transition
- How do we push back there?
- Theories... what do they say? Can we trust them?

- Richard Seto

## Early universe  Different Vacuum?

- Present theories (well tested ones!) indicate that the vacuum is NOT empty but is filled with a quark condensate “goo”
- Explains why particles (quarks)  stick together -”confinement”
- is the origin of  hadronic particle masses (protons, neutrons, pions etc)
- In the early universe this Vacuum was very different than it is now.
- Particles (hadrons) had different (zero) masses!
- Study of Heavy ions (RHIC) allows us to study this vacuum directly

- Vacuum
- =mass?

- Richard Seto

## Why do we have mass (F=ma) ?

- Clever Idea!!
- All things massless
- a complicated vacuum
- MASS !!!
- WIERD

- Richard Seto

## How does a theory come to be?

- Experimental Test

- Old “Classical” ideas
- e.g. F=ma

- New Ideas (might be crazy)

- Unanswered questions
- Puzzling Experimental Facts

- New Theory (A guess)

- A New “Good”
- Theory !

- Yes

- No

- Richard Seto

## “Once upon a time”

- Pre-1900 “The standard model”
- Newton, Kepler
- Maxwell
- Pretty good
- Explained orbits of planets
- Explained static electricity
- Maxwell –”We have a complete theory. Physics will be over in 10 years...”
- But there were a few minor problems

- Richard Seto

## The bad guys

- Spectra of Hydrogen atom
- Strange – like a merry-go-round which only     went v=1mph, 5mph, 10mph,....
- THE WHITE KNIGHT – QUANTUM MECHANICS

- Richard Seto

## Classically

To “quantize”
E,p,V Change to operators           (EH, pp, VV)
But operate on what??
Answer: (x) 
the “wave function”
 is the “real thing”

- To “quantize”
- E,p,V Change to operators           (E*H,* p*p,* V*V*)
- But operate on what??
- Answer: (x)
- the “wave function”
-  is the “real thing”

- Getting to QM

- Richard Seto

## What does it predict?

- Energy levels!
- BUT!

- Zero probablility!

- Richard Seto

## What is  ?

- Answer ************* is a probability
- We have lost the normal notion of a particle with position and velocity
- Crazy, but it EXPERIMENT tells us it works
- E.g. double slit experiment with electrons

- electrons

- Richard Seto

## What tools to we have to build a theory?

- To Build theories (an effective Lagrangian) we invoke fundamental symmetries.
- Why? -
- because it works
- and it seems right somehow???  “ H. Georgi”
- Examples
- translational invariance momentum conservation
- rotational invariance  angular momentum conservation
- Local gauge invariance (phase change)  EM forces
- In the standard theories we essentially always start with a massless theory - for QCD this comes from a requirement of “chiral symmetry”
- Mass comes about from a breaking of symmetry giving rise to a complicated vacuum.

- Richard Seto

## Spontaneous symmetry breaking

- Which wine glass is yours?
- Or think of a nail sitting on its head

- Richard Seto

## Where Does Mass come from?

- Massive quarks in lite QCD?  (u,d)
- Chiral (R-L) Symmetry
-  Massless quarks!
- So Where does mass come from?

##   The Vacuum: Source of Mass

-   The Vacuum: Source of Mass

- Start at high Temperature with massless quarks

##   The Vacuum: Source of Mass

-   The Vacuum: Source of Mass

- Start at high Temperature with massless quarks

- Assume a background field =
-  - goo of quarks and gluons
- Similar to the higgs field for E-W theory
- Couples to quarks(massless for now) and gluons
- Potential term for  has special Temperature Dependence

- The Vacuum

##   The Vacuum: Source of Mass

-   The Vacuum: Source of Mass

- As Temperature Cools past Tc

- The Vacuum

##   The Vacuum: Source of Mass

-   The Vacuum: Source of Mass

- As Temperature Cools past Tc
- Spontaneous symmetry breaking      (I.e. chiral)
- quark condensate at low Temperature
- generates hadron masses

- The Vacuum

##  Weird!

- The idea that empty space should be full of complicated material is wilder than many crackpot theories, and more imaginative than most science fiction...
- F. Wilczeck in Physics Today (April 1998)

- Richard Seto

## A simulation of the vacuum

- Richard Seto

## Living in the cold QCD vacuum

- It is generally believed in the fish population that there is an inherent resistance to motion and that they swim in a “vacuum”.

- The vacuum – perceived to
- be empty by the general
- fish population

- Richard Seto

## A clever (and crazy idea)

- One clever young fish  is enlightened. “The vacuum is complicated and full of water!” he says –” really there is no resistance to motion!”
- “Phooey” say his friends, “we all know the vacuum is empty.”

- Richard Seto

## How does he prove it?

- Answer – he builds a machine to boil the water into steam – to “melt” the vacuum
- In steam his “friends” move freely.
- OK OK – they die because
- They can’t breathe in air
- They are poached because of the heat
- So maybe he boils only what’s in a small bottle as an experiment...
- you get the point...

- Richard Seto

## Phases of water

- Richard Seto

## Phases of Nuclear Matter

- TWO different phase transitions!
- The deconfinement transition - particles are roam freely over large volume
- The chiral transition - masses change
- All indications are that these two are at or are very nearly at the same TC

**Phase Transition:**
**    ****T = 170 **** 15% MeV**
**    ********  ****~ 0.6 GeV/fm****3**

- Richard Seto

## What is the hot vacuum like?

- How hot is it? (Temperature)
- How sticky is it?
- How much energy can it store? (Latent heat)
- What is its equation of state?
- What is (are) the phase transition(s) to a cold vacuum like?
- 1st, 2nd order, cross over?
- How does it generate mass?
- How/why does it confine?
- What interesting properties does it exhibit?
- ...

- Richard Seto

## How can a Hot Vacuum aka QGP be made?

**Image from Hubble**
** ****Collision between two galaxies (Oct 21, 1997)**

**Colliding galaxies**

- Richard Seto

## Relativistic Heavy Ion Collider

- New Accelerator at the Brookhaven National Laboratory - 4km circumference
- 100100 GeV2 Au-Au collisions
- spin polarized proton collisions at 500 GeV

- Richard Seto

## Slide 31

- Richard Seto

## How do we hope to see this phase transition?

- Relativistic Heavy Ion Collisions
- It would nice if we created a bottle of compressed quark and gluon gas gas – but it isn’t
- Better analogy – early universe, exploding star
- Time evolution
- Lorenz contracted pancakes
- Pre-equilibrium < ~1fm/c ??
- QGP and hydrodynamic expansion  ~ few fm/c ??
- Hadronization and freezout    ~ 5-20 fm/c??

- Richard Seto

## A Gold-Gold collision at RHIC

- Richard Seto

## What does it look like in our detectors?

- Richard Seto

**    ****11 Countries          **
**    ****51 Institutions  **
**~450 Collaborators**

- Richard Seto

## Culture of large collaborations

- Large, complicated detector facilities are needed to address the exciting physics available at RHIC.
- The resources of large collaborations allow for unique learning experiences and access to state of the art technologies.
- Project management
- state of the art computing
- databases
- object oriented computing environment
- State of the art electronics
- custom designed ASIC’s.
- real time applications
- Cross-disciplinary
- Strong ties with mechanical, electrical, and chemical engineering. Same experimental technics as high energy physics.
- Physics ties with high energy and condensed matter physics.

- Richard Seto

## 2 Arm central spectrometers                + 2 muon endcaps
-0.35<  <0.35 (e,, hadrons)
1.2<|  |<2.5 (muons-2nd year) 
Tracking : DC, PC, TEC
Particle ID : RICH, TEC , EMCal, TOF
Global Detectors (centrality)
Zero Degree Calorimeters (ZDC)
Beam-Beam Counter (BBC)

- 2 Arm central spectrometers                + 2 muon endcaps
- -0.35<  <0.35 (e,, hadrons)
- 1.2<|  |<2.5 (muons-2nd year)
- Tracking : DC, PC, TEC
- Particle ID : RICH, TEC , EMCal, TOF
- Global Detectors (centrality)
- Zero Degree Calorimeters (ZDC)
- Beam-Beam Counter (BBC)

- PHENIX: electrons,muons, photons, hadrons

- Central Magnet

- Muon ID
- Muon Magnet

**Central**** ****Arms**

**Beam**

- Richard Seto

- Richard Seto

## What have we learned so far?

- Is it like the vacuum? **yes**
- How hot does it have to be to melt the vacuum? **170 MeV**
- How hot is it? **hot!**
- How sticky is it? **very sticky**
- Are masses zero in a hot vacuum? **?????? **

- Richard Seto

## Is it like the Vacuum? Baryon Density at RHIC

- Early Universe (a vacuum):
- Anti-proton/proton = 0.999999999

- We’ve created “pure” matterapproaching this value

*PHENIX **        preliminary*

- E866
- Au+Au

- NA44
- Pb+Pb

- Richard Seto

## How hot does it need to be? Lattice Calculations

- T

- Tc

- Transition – Sharp Crossover at RHIC
- That’s OK – 1st order for all practical purposes

- Lattice Calculations:
- Tc = 170  15% MeV
-  ******critical ** ~ 0.6 GeV/fm3

## Hot hot is it?

- Plenty HOT!

- Bjorken formula for thermalized energy density

- time to thermalize the system (0 ~ **1.0** – 0.3  fm/c)

- For the most central events:

- PHENIX preliminary

- Bjorken~ 5.0 GeV/fm3

- critical~ 0.6 GeV/fm3

- Lattice phase transition:

- ~6.5 fm

- EMCAL

- What is the energy density achieved?
- How does it compare to the expect phase transition value from lattice QCD?

- Richard Seto

## We need to worry about Geometry Measuring Centrality (impact parameter)

- Zero Degree Calorimeters (ZDC)
- Sensitive to spectator neutrons
- common to all four RHIC experiments
- Beam Beam Counters (BBC)
- Using a combination of the ZDC’s and BBC’s we can define Centrality Classes

- .

- Zero Degree Calorimeter

- n

- n

- n

- p

- p

- p

- Beam-Beam Counter (BBC)

**Impact Parameter**

- ZDC

- BBC

- Richard Seto

- How sticky should it be?

- 4 Fundamental interactions
- Gravity  ~_**10**__**-36**_
- Weak interaction         ~ _**10**__**-3**_
- decays

- Electromagnetic Interaction ~ _**1**_
- Your body
- Cooking
- cars

- Strong interaction~ _**10**_
- The sun
- Atom bombs
- Nuclear energy

- Richard Seto

## The probe - Jets

- proton

- proton

- quark

- quark

- jet

- jet

- To detectors

- To detectors

- Jets-back to back to conserve momentum

- Richard Seto

- How sticky is it?, aka Jet  quenching

- Deep Inelastic Scattering of the QGP?
- “hard” probes
- Formed in initial collision
- penetrating
- sensitive to state
- dE/dx by strong interaction
- jet quenching
- Jets by leading particles
- Look for* *a suppression of high pT hadron production.

- Richard Seto

## Peripheral: 



   Consistent with Nbin scaling

- Peripheral:
- 
-    Consistent with Nbin scaling

- Consistent with Nbin scaling? NO

- Central:
- 
-    Consistent with Nbin scaling

- Consistent with Nbin scaling

**PHENIX Preliminary**

- NO!

**Scaled pp**

- Richard Seto

## Nuclear Modification Factor

-   Effect of nuclear medium on yields

- Run 2 Data
- Shows a many
- Sigma Effect!

- 0-10%

- (dE/dx)initial~7 GeV/fm
- 15x Cold matter (Hermes)

- RHIC
- central -Suppression
- peripheral – Nbin scaling

**PHENIX Preliminary**

- Richard Seto

## How sticky?

- Energy loss 10 GeV per 10-15m
- How much is this
- Take a 22 bullet
- Shoot out of a gun 1000x strength of ordinary gun
- Stops in tissue paper!
- The strong interaction on the loose!

- Richard Seto

##  Can you look at Di-jets?

- Richard Seto

## Two-particle azimuthal correlations

- Identify jets on a *statistical* basis
- Trigger particle with pT>pT(trigger)
-  associate particles with  pT>pT(associated)
- C2 is probability to find another particle at (,)
- pT(associated)>2 GeV/c
- pT(trigger): 4-6 GeV/c, 3-4 GeV/c, 6-8 GeV/c
- ||<0.7  ||<1.4

- Jet

- Away side Jet

- Richard Seto

## Away side-Jet Suppression

- Away side-Jet Suppression

- Away side-jet
- strong suppression with centrality
- jet quenching?

- trigger-jet

- Centrality  dependence similar to quenching of neutral pion spectrum!

- Richard Seto

## Aside: Saturation and the CGC and the Vacuum- The Approximation

- The Approx
- Non-perturbative (high density)
-  Small coupling
- Requires S(QS) to be “small”
- QS larger – more central, higher energy, mid-rapidity
- Expected to fail for QS small
- Low energy
- Peripheral
- High y

- At RHIC, QS~1-2 GeV
- S(QS)~0.3 –0.4

- Richard Seto

## Prediction at 22 GeV – touching the vacuum?

- Prediction at 22 GeV – touching the vacuum?

- Do we expect it to work? Isn’t QS too small?

- Works ??
- mid-rapidity corrected for 19.6 vs 22
- Note- 2 component model works too

- Kharzeev, Levin, Nardi
- Hep-ph/0111315

- Idea
- S “freezes” at ~ 0.4
- iteraction with “vacuum”!
- quarks pop out of vacuum
- Sparking or better
-  semi-conductor

- Richard Seto

## Can we see the mass disappear?

- “Light” Vector mesons are ideal probes (,,)
- Like putting a scale to measure                                                                                                 mass inside the fireball
- Short lifetime ~ few fm/c
- Decay inside hot fireball
- Electrons (and muons) are ideal messengers
- Don’t interact strongly (e.g. neutrinos from the sun)

- e.g. In Medium 
- shows low mass tail -
- With its good mass resolution PHENIX should be able to see this
- R. Rapp (Nucl. Phys A661(1999) 238c

- Richard Seto

## The Vacuum

- Richard Seto