Recent Results on Hard Probes of the Quark-Gluon Plasma with the ATLAS Experiment at the LHC Tom´ aˇ s Kosek for the ATLAS collaboration 53 rd International Winter Meeting on Nuclear Physics 26-30 January 2015 IPNP, Charles University in Prague Tom´ aˇ s Kosek 29 January 2015 1 / 21
Motivation The main goal of the presented measurements is to study properties of the strongly coupled medium created in Heavy-ion (HI) collisions Hard probes are ideal tools for these studies ◮ created in the early stages of the collision ◮ relatively low background from the underlying event Tom´ aˇ s Kosek 29 January 2015 2 / 21
ATLAS experiment ATLAS is multi-purpose detector well capable of measuring heavy-ion collisions Excellent tracking performance within | η | < 2 . 5. Combination of silicon pixel and strip detectors and transition radiation tracker. Powerful calorimeter system with fine segmentation with η coverage up to | η | < 4 . 9 Tom´ aˇ s Kosek 29 January 2015 3 / 21
More about calorimetry Cells in Layer 3� ∆ϕ×� ∆η = 0.0245 ×� 0.05 T r g i g e r T o w e r ∆ η 2X 0 = 0 . 1 η = 0 m m 0 16X 0 7 4 T r i g g e r T o w e r ∆ ϕ = 0 . 0 9 8 2 4.3X 0 m m 0 1.7X 0 0 5 1 ∆ ϕ = 0 . 0 2 4 5 x 4 3 6 . 8 m � m x 4 � = 1 4 7 3 . m m Square cells in � Layer 2 ∆ ϕ = 0 . 0 2 4 5 ϕ 3 ∆ η 7 . 5 m = 0 m / . 0 2 5 8 = 4 . 6 9 ∆ η m m � = 0 . 0 0 3 1 Strip cells in Layer 1 η Calorimetry system is composed of electromagnetic, hadronic and liquid-argon (LAr) forward calorimeters High granularity LAr electromagnetic calorimeter covers range of | η | < 3 . 2 and is composed of barrel and end-cap modules EM calorimeter is backed by hadronic calorimeter Allows for precise measurement of photons, electrons and jets Forward calorimeters are located in the range 3 . 1 < | η | < 4 . 9, used for centrality bin selection Tom´ aˇ s Kosek 29 January 2015 4 / 21
Centrality in Pb+Pb collisions Centrality expresses measure of overlap of two colliding nuclei Is closely related to the average number of participant nucleons and number of binary inelastic collisions Centrality determined by the sum of E T deposited in the FCAL calorimeter (3 . 1 < | η | < 4 . 9) Events divided into successive percentiles of the � E FCal T Tom´ aˇ s Kosek 29 January 2015 5 / 21
EW probes Since EW bosons don’t interact strongly, they aren’t influenced by the medium We can look at the EW boson+jet events - is p T balanced? Or we can test modification of the PDF’s caused by the nuclear effects Tom´ aˇ s Kosek 29 January 2015 6 / 21
W bosons (1) arXiv:1408.4674 10 10 〉 〉 coll coll 9 9 ∫ 10 10 -1 N ATLAS N ≈ ATLAS 9 9 Ldt 0.14-0.15 nb 〈 〈 events events 8 8 + → + ν - → - ν 1 W l 1 W l N N 7 7 dN η dN η d d 6 6 ∫ 5 5 -1 ≈ Ldt 0.14-0.15 nb 4 4 Pb+Pb s = 2.76 TeV Pb+Pb s = 2.76 TeV NN NN 3 3 Data 2011 Data 2011 Data 2011 Data 2011 Data 2011 2 2 POWHEG CT10 Pb+Pb POWHEG CT10 Pb+Pb POWHEG CT10 Pb+Pb POWHEG CT10 Pb+Pb POWHEG CT10 Pb+Pb 1 1 CT10+EPS09 Pb+Pb CT10+EPS09 Pb+Pb CT10+EPS09 Pb+Pb CT10+EPS09 Pb+Pb CT10+EPS09 Pb+Pb 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 η η | | | | l l Differential production yield per binary collision for W + and W − integrated over centralities and compared to theoretical predicitons Tom´ aˇ s Kosek 29 January 2015 7 / 21
W bosons (2) 40 0.4 fiducial events l Lepton Charge Asymmetry A ATLAS Pb+Pb s = 2.76 TeV ATLAS ∫ NN N N 35 0.3 ≈ -1 〉 Ldt 0.14-0.15 nb 9 coll 10 N 30 0.2 〈 25 0.1 20 0 15 -0.1 ± ± W + Data W + POWHEG CT10 10 -0.2 W - W - Data 2011 Data 2011 W W 5 ∫ -0.3 POWHEG CT10 Pb+Pb POWHEG CT10 Pb+Pb -1 ≈ Ldt 0.14-0.15 nb Pb+Pb s = 2.76 TeV CT10+EPS09 Pb+Pb CT10+EPS09 Pb+Pb NN 0 -0.4 0 50 100 150 200 250 300 350 400 0 0.5 1 1.5 2 2.5 〈 〉 η N | | part l W production yield per binary collision doesn’t show any dependence on � N part � and is consistent with POWHEG prediction Lepton charge asymmetry agrees with theoretical predictions Tom´ aˇ s Kosek 29 January 2015 8 / 21
Photons (1) ATLAS-CONF-2014-026 [pb/GeV] [pb/GeV] ATLAS Preliminary ATLAS Preliminary 7 7 10 10 Pb+Pb s =2.76 TeV Pb+Pb s =2.76 TeV NN NN η η | |<1.37 1.52<| |<2.37 JETPHOX 1.3 JETPHOX 1.3 6 6 10 10 E (R=0.3) < 6 GeV E (R=0.3) < 6 GeV AA AA T,iso T,iso × 3 × 3 Data 0-10% 10 Data 0-10% 10 T T × 2 × 2 Data 10-20% 10 Data 10-20% 10 )/ )/ × 1 × 1 T Data 20-40% 10 T Data 20-40% 10 p p 0 0 × × /d Data 40-80% 10 /d Data 40-80% 10 γ γ )(dN (dN 4 4 10 10 evt (1/N 2 2 10 10 1 1 -2 -2 10 10 2 × 2 2 30 40 50 10 2 10 30 40 50 60 70 10 photon [GeV] photon [GeV] p p T T Fully corrected yields of prompt photons in four centrality intervals as a function of p T Compared to JETPHOX calculations Tom´ aˇ s Kosek 29 January 2015 9 / 21
Photons (2) 2 2 2 2 ) pp JETPHOX PDF+scale err. ATLAS Preliminary JETPHOX Pb+Pb/ pp Ratio to JETPHOX ( Pb+Pb s =2.76 TeV NN JETPHOX EPS09/ pp & err. L = 0.14 nb -1 int 1.5 1.5 1.5 1.5 1 1 1 1 0.5 0.5 0.5 0.5 η η η η 40-80%, | |<1.37 20-40%, | |<1.37 10-20%, | |<1.37 0-10%, | |<1.37 0 2 0 2 0 2 2 0 ) pp Ratio to JETPHOX ( 1.5 1.5 1.5 1.5 1 1 1 1 0.5 0.5 0.5 0.5 η η η η 40-80%, 1.52<| |<2.37 20-40%, 1.52<| |<2.37 10-20%, 1.52<| |<2.37 0-10%, 1.52<| |<2.37 0 0 0 0 2 2 2 2 10 10 10 10 photon p [GeV] photon p [GeV] photon p [GeV] photon p [GeV] T T T T The ratio of the data to the JETPHOX pp prediction Data agree well with JETPHOX predictions in all centrality and η regions Tom´ aˇ s Kosek 29 January 2015 10 / 21
Jets Partons from the hard scattering have to traverse through the deconfined medium Do we observe suppression of jet yields or modification of fragmentation functions? Is production of the associated jets influenced by the medium? We can compare to pp data at the same energy or look at differences between central and peripheral collisions Tom´ aˇ s Kosek 29 January 2015 11 / 21
Jet spectra arXiv:1411.2357 [ nb/GeV ] 10 11 [ nb/GeV ] 11 10 10 10 ATLAS ATLAS | y | < 2.1 9 10 × 6 10 0 - 10 % ( 10 ) 10 8 10 × 20 - 30 % ( 10 4 ) anti- k R =0.4, s = 2.76 TeV 10 7 t × 9 30 - 40 % ( 10 2 ) 10 10 6 -1 jet y 2013 pp data, 4.0 pb 60 - 80 % ( × 10 0 ) d N 10 5 T 8 y 10 2 p d 10 4 σ d d 3 10 2 T evt 7 d 10 1 2 p 10 N d 10 6 10 〉 1 s = 2.76 TeV NN AA 1 10 -1 anti- k R = 0.4 jets T t 5 -2 10 10 -1 2011 Pb+Pb data, 0.14 nb 〈 -3 10 2013 pp data, 4.0 pb -1 4 -4 10 10 10 -5 3 10 10 11 [ nb/GeV ] 10 0 - 10 % 10 2 10 9 | | < 0.3 ( × 10 6 ) 10 y 8 ≤ × 4 10 0.3 | y | < 0.8 ( 10 ) 10 10 7 ≤ × 2 0.8 | y | < 1.2 ( 10 ) 10 6 ≤ × 0 jet y 1.2 | y | < 2.1 ( 10 ) 1 d N 10 5 T 8 2 p × d 10 4 | y | < 2.1 ( 10 ) d -1 10 3 10 evt 6 | | < 0.3 ( × 10 ) 1 2 y 10 -2 N 10 10 ≤ × 4 0.3 | y | < 0.8 ( 10 ) 〉 1 -3 AA 10 1 10 -1 ≤ × 2 0.8 | y | < 1.2 ( 10 ) T 10 -2 -4 〈 10 -3 ≤ × 0 10 1.2 | y | < 2.1 ( 10 ) -4 10 -5 10 10 -5 40 60 100 200 400 40 60 100 200 400 p [GeV] p [GeV] T T Differential cross sections for the different rapidity ranges Differential per-event jet yield in Pb+Pb collisions divided by 1 / � T AA � with pp jet cross sections Normalized Pb+Pb yields in central collisions are below the pp yields Tom´ aˇ s Kosek 29 January 2015 12 / 21
Jet R AA AA R ATLAS anti- k R = 0.4 jets 2011 Pb+Pb data, 0.14 nb -1 AA t s = 2.76 TeV -1 2013 pp data, 4.0 pb R NN 0 - 10 % 30 - 40 % 60 - 80 % 1 1 0.5 | y | < 2.1 0.5 0 AA R 0 - 10 % 0 - 10 % 30 - 40 % 30 - 40 % 60 - 80 % 60 - 80 % ATLAS 80 < p < 100 GeV T 1 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 | y | 0.5 AA -1 anti- k R = 0.4 jets 2011 Pb+Pb data, 0.14 nb R t 0.3 < | y | < 0.8 -1 s = 2.76 TeV 2013 pp data, 4.0 pb 0 NN AA 1 R 1 0.5 0.5 80 < < 100 GeV | | < 2.1 p y 1.2 < | y | < 2.1 T 0 0 0 50 100 150 200 250 300 350 400 40 40 40 60 60 60 100 100 100 200 200 200 400 400 400 〈 〉 N p [GeV] part T Variable that expresses the size of the suppression/enhancement is the so called R AA defined as d 2 N jet 1 � � N evt d p T d y central R AA = d 2 σ pp jet � T AA � d p T d y Tom´ aˇ s Kosek 29 January 2015 13 / 21
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