High p T Charged Hadron Production at RHIC Claus O. E. Jørgensen Ph.D. defense Friday the 17 th of September, :30 in Auditorium A - a search for the quark gluon plasma -

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Thanks to… …and many more!

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Outline QCD and the Quark Gluon Plasma Heavy Ion Collisions High p T Particle Production The BRAHMS Experiment The Results - What is it that we want to understand? - How do we compress/heat the matter? - What can we use to probe the dense matter? - How do we measure the high p T particles? - What have we found out?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 The Structure of Matter quarks leptons “force mediators” (bosons) photons: electromagnetic gluons: strong force  Z and W bosons: weak force Building blocks of The Standard Model: 0.1m m m m m Can we get a better understanding of the forces of nature? size:

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Quantum Chromo Dynamics (QCD) QCD is the theory of the strong interaction The force mediators (the gluons) carry the color charge (red, green, blue)  gluons can interact. Confinement: the quarks (and gluons) are confined in white objects (hadrons) The electromagnetic force: The strong force: r V(r) qualitative difference between the q-q and the Coulomb potentials If you try to separate two quarks, you will just get two new ones… If you compress the QCD matter… … the q-q potential is screened and the hadrons disolve  quark gluon plasma

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 ice water steam Phase Transitions nucleus quark gluon plasma hadron gas The transition to the quark gluon plasma (QGP) is predicted by lattice QCD… temperature  degrees of freedom The first phase transition has been observed. temperature [MeV] excitation energy [MeV] Where can we find a quark gluon plasma? (H 2 O and QCD matter)

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 The History of the Universe Big Bang (15 billion years ago) sec.: Inflation Quark Gluon Plasma (in danish: ursuppe) 1 million years: Atoms are formed 1 billion years: Galaxies are formed 3-15 min: Atomic nuclei are formed sec: Hadronization 15 billion years: Claus defends his thesis

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 The QCD Phase Diagram Nuclear Matter early universe BB T T C ~170 MeV 940 MeV baryon chemical potential temperature SIS AGS SPS RHIC crab nebula neutron stars hadron gas Where can we find QGP? The early Universe Neutron stars Heavy Ion Collisions quark gluon plasma

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Outline QCD and the Quark Gluon Plasma Heavy Ion Collisions High p T Particle Production The BRAHMS Experiment The Results - What is it that we want to understand? - How do we compress/heat the matter? - What can we use to probe the dense matter? - How do we measure the high p T particles? - What have we found out?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Heavy Ion Collisions Collision zone is small (volume  cm 3 ) is hot (  1000 billion  C) is dense (  1 billion kg/cm 3 ) evolves over short time-scales (life-time  sec) Only the “final state” particles are observable  information on the earlier stages can only be extracted via models.

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Evolution and Observables before collision initial collisions quark matter production collective expansion hadronization collective expansion thermal freeze out time  A small but important piece of the puzzle! elliptic flow high p T hadrons thermal photons, J/Ψ relative hadron abundances (stat. model) transverse spectra (blast wave model)

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 (Definition of Variables) We need to characterize the collision… lower impact parameter  more particles produced  lower centrality  b multiplicity (N charged particles) Centrality measured in % (0-10%, 10-20%, 20-40%…) Energy available? …and the particles emitted from it. Center-of-mass energy  s NN measured in GeV (=1.6  J) Ex:  s NN =200GeV ( %c) central Au+Au collisions beam  Transverse momentum: p T = p sinθ Pseudo-rapidity: η = -ln(tan(θ/2)) Spectrum (example)

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Outline QCD and the Quark Gluon Plasma Heavy Ion Collisions High p T Particle Production The BRAHMS Experiment The Results - What is it that we want to understand? - How do we compress/heat the matter? - What can we use to probe the dense matter? - How do we measure the high p T particles? - What have we found out?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 nucleon-nucleon collisions: - high p T particles are created in jets - well described High p T Particle Production Like using X-rays to measure the (dense) bones. (fracture on the 4 th metacarpal) q q hadrons leading particle leading particle hadrons jet q q hadrons leading particle nucleus-nucleus collisions: - high p T particles can be used to probe the medium (tomography) We only measure the yields (p T spectra), so how can we compare the nucleon-nucleon collisions to the nucleus-nucleus collisions?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Nuclear Modification Factor How do we compare the high p T particle production in nucleus-nucleus collisions and nucleon-nucleon collisions? The number of binary collisions Naïve expectation, i.e. no nuclear effects (no funny stuff going on)  binary scaling at high p T (R AB = 1) High p T particles are created in hard collision: large energy transfer  short collisions time  incoherent collisions Yields in nucleus-nucleus collisons Yields in nucleon-nucleon collisions Mean number of binary collisions

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Explained by multiple scattering in the initial state. Modeled by transverse momentum broadening. Cronin Enhancement Nuclear effects: Shadowing Cronin enhancement Enhancement of high p T particles first observed by Cronin in the 70’s Nuclear Modifications Jet quenching

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Shadowing Low x partons are depleted (x is the momentum fraction carried by the parton) The structure of a nucleon changes when it’s put inside a nucleus. Nuclear Modifications Nuclear effects: Shadowing Cronin enhancement Jet quenching 1 R Explained by: multiple coherent scattering saturation (boosted nuclei) low energy higher x high energy smaller x  gluon density can saturate. Effect below the saturation scale Q S  A 1/3 e y It’s the Color Glass Condensate!

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Energy loss of jets by medium induced radiative energy loss (gluon bremstrahlung) hadronic rescattering (is not enough to quench jets) Jet quenching Note that the medium is expanding! We need to model the dynamics to extract information on the density. Nuclear Modifications Nuclear effects: Shadowing Cronin enhancement Jet quenching

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 How can we disentangle the effects? p+p collisions: we need this as reference d+Au collisions: Cronin enh. due to mult. scattering? shadowing in the Au nucleus? jet quenching must be a small effect! Au+Au (central): Cronin enh. due to mult. scattering? shadowing in the Au nuclei? jet quenching?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Outline QCD and the Quark Gluon Plasma Heavy Ion Collisions High p T Particle Production The BRAHMS Experiment The Results - What is it that we want to understand? - How do we compress/heat the matter? - What can we use to probe the dense matter? - How do we measure the high p T particles? - What have we found out?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Relativistic Heavy Ion Collider RHIC

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 RHIC pictures Two concentric rings 6 interaction regions 3.8 km long 1740 superconducting magnets booster injector tandem RHIC blue and yellow rings BRAHMS

Ph.D defence, Claus Jørgensen, Friday 17 th of September, ° 30° 15° 2.3° The BRAHMS Experiment Midrapidity Spectrometer Front Forward Spectrometer Back Forward Spectrometer Global Detectors Zero Degree Calorimeter | 18 m It’s the Broad RAnge Hadron Magnetic Spectrometers!

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 A BRAHMS Event TPM2 TPM1 D5 D1 T1 D2 T2 beam collision point (vertex) MRS at 90 degrees FFS at 6 degrees Reconstructed tracks

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Event Counting We want to make spectra,  we need to count the number of collisions (events) N events and the number of tracks N tracks …still we need the centrality from the multiplicity (MA)… multiplicity [a.u.] …and the collision point (vertex) from the BB, ZDC and INEL counters. MA INEL ZDC BB We measure (almost) all the collisions and counting is easy!

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 x y TPM1 D5 TPM2 (side view of MRS) TOFW Not detected! Track Counting We make sure that the tracks come from the collision. We want to make spectra,  we need to count the number of collisions (events) N events and the number of tracks N tracks Acceptance corrections are done by Monte Carlo simulations (vertex bins of 5 cm) + corrections for resolution and bin size effects… Inefficiencies of the detectors Decay of particles Limited acceptance. We miss some tracks due to:

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Outline QCD and the Quark Gluon Plasma Heavy Ion Collisions High p T Particle Production The BRAHMS Experiment The Results - What is it that we want to understand? - How do we compress/heat the matter? - What can we use to probe the dense matter? - How do we measure the high p T particles? - What have we found out?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Results – The Spectra Mmmh - very nice Claus, but it’s not easy to conclude anything from this! Why don’t you show the nuclear modification factors R AB ? Au+Au at  s NN =200GeV d+Au and p+p at  s NN =200GeV

Ph.D defence, Claus Jørgensen, Friday 17 th of September, % Nuclear Modification at η= %10-20%0-10%Approximate binary scaling in 40-60% central Strong supp. in central collisions What causes the suppression shadowing or jet quenching?  s NN =200GeV Cronin enhancement!!!  shadowing in the gold nucleus is not important Suppression in the central Au+Au must be due to jet quenching!  s NN =200GeV We made it to the cover of PRL!

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Energy systematics (p T =4GeV/c) central heavy ion collisions  s NN = 17 GeV: Large Cronin enhancement (R AB >1)  s NN = 62.4 GeV (NEW!): Slight suppression in central collisions.  s NN = 200 GeV: Large suppression in central collisions. What does it tell us about the medium? This is also what other experts say: “…jet tomography analysis at RHIC presents strong evidence for the creation of the deconfined state of QCD.” [Vitev,qm04] HIJING, Vitev-Gyulassy and Hirano-Nara: medium induced radiative energy loss Cassing et. al.: hadronic rescattering & pre-hadronic interactions. “…there should be some additional and early partonic interaction in the dense and possible colored medium.” [Greiner, qm04] It must be due to radiative energy loss in a deconfined medium! Could the strong suppression be explained by jet quenching in a hadronic medium?

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 q q q q 40-60%20-40%10-20% Nuclear Modification at η=2.2 Reaction dynamics are important! The medium extended in the longtudinal direction. [Hirano&Nara, PRC68, (2003)] Suppression is similar to η=0 Does it simply scale with the density? dN/dη η How important is saturation? Not easy to say in a symmetric collision system… maybe d+Au can help?  s NN =200GeV 0-10%

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Nuclear Modification in d+Au  s NN =200GeV η=0 Cronin enhancement disappears at higher pseudo-rapidities. η=1 Higher η means lower x of the Au nuclei. Is the suppression at higher η an effect of low-x gluon saturation? Does the enhancement/suppression cancel out in the Au+Au collisions?

Summary High p T particles provide a unique window to the early stages of heavy ion collisions at RHIC. It’s not due to shadowing - we have made the d+Au check! We’ve discovered a new effect: High p T particles are suppressed in central Au+Au collisions at RHIC  the jets are quenched in the dense medium! Energy loss studies show that the medium must be deconfined – we’ve made “free” quarks and gluons! So what do you think? Have we discovered the Quark Gluon Plasma? The medium is extended in the longitudinal direction, but I think we need more data to understand the saturation effect. Yes, I think so…

Come and have a drink and a snack in the T-villa…

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Backup - Models HIJING: pQCD (hard) + strings (soft), shadowing, very schematic jet quenching (1992) Vitev-Gyulassy: pQCD (hard), no soft Cronin k T broadening, shadowing and (GLV) jet quenching Cassing et al: pQCD (hard) + strings (soft) k T broadening, shadowing and energy loss (pre-hadronic and hadronic) Hirano-Nara: pQCD (hard) + hydro (soft) Cronin k T broadening, shadowing and (GLV) jet quenching

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004 Backup – the CGC ln(1/x)

Ph.D defence, Claus Jørgensen, Friday 17 th of September, 2004  s NN =62.4GeV At  s NN =62.4GeV: Cronin enhancement in semi-peripheral collisions. Canceled out by energy loss in central. At  s NN =200GeV: Strong suppression in central collisions %20-40%10-20% At SPS (  s NN =200GeV): R AB is “consistenly above1 due to strong Cronin effect via initial multiple scattering, leaving not much room for parton energy loss…” [Wang, nucl-th/ ] 0-10%  s NN =62.4GeV