Bulk observables vs. hard probes bulk high- p T probes Only few - - PowerPoint PPT Presentation

bulk high-pT probes

Bulk observables vs. hard probes

Only few particles with high transverse momenta (or containing heavy quarks), but their production mechanism is a priori better understood (perturbative QCD) can probe their environment ≡ the “bulk”.

STAR Collaboration, Phys. Rev. Lett. 91 (2003) 172302

Lclassical = −1 4F A

µνF µν A +

• flavors

¯ qa(iD − m)abqb F A

µν = ∂µAA ν − ∂νAA µ − gf ABCAB µ AC ν

(Dµ)ab = ∂µδab + ig(tCAA

µ )ab

QCD Lagrangian

[tA, tB] = if ABCtC a, b = 1 … Nc = 3; A, B, C = 1 … Nc

• 1

= 8 Effective coupling:

2

αs ≡ g2 4π

Running of αs

plot from Bethke, Prog. Part. Nucl. Phys. 58 (2007) 351

Q2 ∂αs ∂Q2 = β(αs) ≡ −bα2

s

• 1 + b′αs + b′′α2

s + O(α3 s)

• b = 11CA − 2Nf

12π

• ≡ β0

4π

Jets in heavy-ion collisions

[…] …

Jets in heavy-ion collisions

[…] …

(unfortunately, effect overestimated by a factor ≈100)

Time evolution of a heavy-ion collision

space time

Time evolution of a heavy-ion collision

hard scatterings

space time

γ QQ – hard parton

e x p a n s i

• n

Time evolution of a heavy-ion collision

hard scatterings

space time

γ μ± e± Quark-Gluon Plasma few fm/ - c QQ –

e x p a n s i

• n

Time evolution of a heavy-ion collision

hard scatterings

space time

γ μ± e± Quark-Gluon Plasma few fm/ - c jet hadronization p K Λ π J/ψ,Υ? QQ –

e x p a n s i

• n

Time evolution of a heavy-ion collision

stolen from Steffen Bass & John Harris

hard scatterings

space time

γ μ± e± Quark-Gluon Plasma few fm/ - c jet hadronization p K Λ π J/ψ,Υ? freeze-out QQ –

“Jet quenching”: basic picture

A fast parton propagating through a dense medium will “lose” part of its energy. The resulting jet of hadrons (if any!) is distorted: “quenching”. in vacuum in medium

Hadrons at large pT at RHIC

Nuclear modification factor

(=1 if AA collision is a superposition of independent NN collisions )

*

* up to isospin corrections…

√s

NN

In central Au+Au collisions at = 200 GeV , one misses 80% of the high-transverse-momentum hadrons! uncertainty Rh

AB ≡

1 N AB

coll dNh

AB

dpT dy dNh

pp

dpT dy

N AB

coll

PHENIX Coll., Phys. Rev. Lett. 101 (2008) 232301

Hadrons at large pT at RHIC

Nuclear modification factor

(=1 if AA collision is a superposition of independent NN collisions )

*

* up to isospin corrections…

√s

NN

In central Au+Au collisions at = 200 GeV , one misses 80% of the high-transverse-momentum hadrons! uncertainty uncertainty on pp-normalization Rh

AB ≡

1 N AB

coll dNh

AB

dpT dy dNh

pp

dpT dy

N AB

coll

PHENIX Coll., Phys. Rev. Lett. 101 (2008) 232301

Photons should not dissipate energy like colored particles : RAA≈1

*

* yet photon production is modified: Bremsstrahlung, photons from parton fragmentation…

Hadrons at large pT at RHIC

Photons should not dissipate energy like colored particles : RAA≈1

*

* yet photon production is modified: Bremsstrahlung, photons from parton fragmentation…

computations of are not off by a factor 5! Ncoll

Hadrons at large pT at RHIC

Photons should not dissipate energy like colored particles : RAA≈1

*

* yet photon production is modified: Bremsstrahlung, photons from parton fragmentation…

deviation from 1 not unexpected (isospin…) computations of are not off by a factor 5! Ncoll

Hadrons at large pT at RHIC

Photons should not dissipate energy like colored particles : RAA≈1

*

* yet photon production is modified: Bremsstrahlung, photons from parton fragmentation…

deviation from 1 not unexpected (isospin…) embarrassingly close to the pion value? computations of are not off by a factor 5! Ncoll

Hadrons at large pT at RHIC

≡ 1 Ncoll

d2NAA dPT dy d2Npp dPT dy

RAA less quenching more quenching

PHENIX Coll., Phys. Rev. Lett. 101 (2008) 232301

Hadrons at large pT at RHIC

PHENIX Coll., Phys. Rev. Lett. 101 (2008) 232301

The scaled yield of high-pT hadrons decreases with growing centrality: increasing quenching

Hadrons at large pT at RHIC

Hadrons at large pT at RHIC

taken from A.Sickles’ talk at QM’09

Hadrons at large pT at RHIC

“Azimuthal correlation” between a “trigger” particle (4 < pT,trig < 6 GeV /c) and “associated” particles (2 < pT < pT,trig).

In central collisions, the “back jet” (= peak at 180° from the trigger particle) disappears.

Hadrons at large pT at RHIC

“Azimuthal correlation” between a “trigger” particle (4 < pT,trig < 6 GeV /c) and “associated” particles (2 < pT < pT,trig).

Hadrons at large pT at RHIC

STAR Coll., Phys. Rev. Lett. 91 (2003) 072304

Trigger particle: 4 < pT,trig < 6 GeV /c; associated particles: 2 < pT < pT,trig.

Hadrons at large pT at RHIC

STAR Coll., Phys. Rev. Lett. 97 (2006) 162301

The “back jet” reappears when one changes the momentum cuts...

Hadrons at large pT at RHIC

STAR Coll., Phys. Rev. Lett. 97 (2006) 162301

Trigger particle: 8 < pT,trig < 15 GeV /c. The “back jet” reappears when one changes the momentum cuts...

pictures taken from T.Ullrich’ s student lecture at QM’09

Jets?

UA2, ca.1982 DELPHI, 1991

pictures taken from T.Ullrich’ s student lecture at QM’09

Jets?

CDF D0, ca.2003

Observing jets in heavy-ion collisions

Needle in a haystack… About 8000 hadrons in a central Au+Au collision at = 200 GeV: √s

NN

≃ 22 GeV /c η φ

pT per grid cell (GeV /c)

Jets in Au-Au collisions at RHIC

Audaces fortuna juvat… very preliminary “results”

talks by J.Putschke & S.Salur @ Hard Probes 2008

Jets in Au-Au collisions at RHIC

Audaces fortuna juvat… very preliminary “results”

talks by J.Putschke & S.Salur @ Hard Probes 2008

≃ 22 GeV /c η φ

pT per grid cell (GeV /c)

≃ 47 GeV /c η φ

pT per grid cell (GeV /c)

(with cone or kT reconstruction algorithms)