Exploring QCD at the Large Hadron Collider

Transcription

1 Exploring QCD at the Large Hadron Collider Lanny Ray University of Texas at Austin High Energy Heavy Ion Nuclear Physics Group UT Austin April 15, 014 UT Physics Pizza Seminar April 15, 014 1

2 Outline Principal physics goals of the UT High Energy Heavy-Ion Group Chiral symmetry restoration in QCD Using resonance hadrons to study CSR at the LHC Opportunities for new graduate students Longer term plans UT Physics Pizza Seminar April 15, 014

3 The Standard Model Quantum Chromodynamics UT Physics Pizza Seminar April 15, 014 3

4 with (3-1) gauge fields A Gell - Mann matrices 1 L QCD i - m F F q λ A 16 q F A A A A Yang - Mills theory for fermion (quark) fields for color (r,g, b) and flavor (u,d,c, s,t,b) Quantum Chromodynamics (QCD) (gluons). λ ensure that correct quark - gluon color coupling free, massive fermions : i free, massless gauge - m A A A A quark - gluon vect or interaction : q λ A 3- gluon interactions : 4 - gluon interactions : bosons : A A A A A A A A quark g r g r gluon quark UT Physics Pizza Seminar April 15, 014 4

5 Consider the transformation : QCD: chiral symmetry e i1(1 ) i 1(1 ) 5 where is a constant,"1"is a unit matrix in the fermion (color) space, The mass term is not invariant, where : e i1 5 e i1 however, the remainder of the QCD Lagrangian is invariant. 5 The operator (1 ) projects right - handed or left - handed helicity states: 1 e 5 5, L, R L, R (1 ) 5 (1 ) right-handed quark left-handed quark Invariance of (massless) L QCD under this transformation is Chiral Symmetry ( handedness). Chiral symmetry is relevant in QCD for light quarks because m q << hadronic masses. For example, in QED CS is irrelevant because m e >> typical atomic mass scales. 5

6 Expand, QCD: chiral symmetry (1 ) (1 ) (1 ) 1 (1 ) terms containing (1 ) (1 ) ( or in other words components 5 5 ) and therefore all vector terms involving the quark fields separateinto left and right - handed terms : i i i q λ A L L L q 0 (q - g interaction does not change helicity) But themass term does not separateinto left and right - handed terms. L R L R q λ A 0, L L L R R R R R R R L λ A R L 5 Strong, non - perturbative coupling with gluons a dynamical mass term, resulting in : dynamically generates effective L R coupling, Spontaneous (dynamical) Chiral Symmetry Breaking in the quark fields The dynamical mass (quark condensate) L R R L 50 MeV 3 UT Physics Pizza Seminar April 15, 014 6

7 If the quark fields are CS, then combinations of quark fields chiral operation should remain constant,e.g. mass eigenstates should not change. Consider :spin 0, isospin These are the pions (pseudoscalar 0 Transformation, e angle corresponding to an aribtrary mixing of u, d components gives i i i, or 0 which means that arbitrary mixtures of,, should have the same mass, or that m m i 1 m multiplet constructed with u,d, I 1) and sigma, where is an arbitrary isospin rotation which is approximately true. when transformed under the light quarks. - meson (scalar 0 5 u i, where, includes light quark flavors u,d; u d d are the ( ) Pauli isospin matrices. i QCD: pion & sigma-meson fields i j ijk k :, I 0), anti - quarks; e i 1 UT Physics Pizza Seminar April 15, 014 7

8 Now consider a pseudoscalar, isospin rotation e 5 i ie where 5 i 5 5, 5 5 i i i i i For exact chiral symmetry this The paltry light quark masses Only thecombination, A Lagrangian i i 5 i i 5 i m combination of fields, where the quark condensate appearsas an interaction : L 1 j QCD: pion & sigma-meson fields j i 1 ij 5 e i result requires m which is not realized in nature: 140 MeV ~ 500MeV (Higgs) are too small to explain these huge violations of chiral symmetry. is invariant under the chiral transformation. for the pion & sigma meson fields (bosons) can be constructed using this CS invariant i 4 j i j j i e i 5 (Linear model) Quark condensate Stabilizing higher-order interactions 8

9 QCD: pion & sigma-meson fields Pion -sigma potentialis V which has the familiar form : V q-condensate higher-order Physicalsolutions: m m 0 (neglectin g Higgs) Chiral symmetry spontaneously broken in the states (meson fields) even though the Lagrangian is invariant under chiral transformation! No direct evidence of chiral symmetry in hadronic states. This is what our UT group is looking for at the LHC. UT Physics Pizza Seminar April 15, 014 9

10 QCD at finite Temperature, phase transition, chiral symmetry Numerical solutions of non-perturbative QCD are possible on a 4D space-time Lattice but require massive parallel computing centers, yet still are only approximations to QCD, e.g. the pion mass is always (so far) too large. Nevertheless these calculations provide insight into the mechanism of confinement, hadron spectra and possible phase transitions from bound hadrons to a free quark-gluon plasma (QGP). Fermi Natl. Lab Lattice QCD farm 10

11 Dynamic quark and gluon fields on the Lattice 11

12 Lattice QCD predicts a deconfined QGP Lattice QCD shows a rapid increase in the entropy associated with the deconfinement of quarks and gluons. Critical temperature (phase transition) Tc = 173 MeV RHIC LHC SPS Quark Hadronic Nuclear Gluon Matter Plasma deconfined (confined)! Temperature Slide from Christina Markert, UT Austin 1

13 Lattice QCD predicts vanishing chiral condensate dynamical mass decreases at higher energy, q, T Chiral symmetry restored Lattice QCD predictions deconfinement increases Slide from Christina Markert, UT Austin 13

14 QCD chiral symmetry: pion & sigma fields and m At T > T crit lattice QCD predicts that the quark condensate g 0, ( g 0) resulting in restoration of chiral symmetry in the meson states. The linear sigma model Lagrangian simplifies to: L ( )( ) 1 and the pion -sigma potential V 1 4 ; the minimum potentialenergy occurs for 0, 0, m Massof the - meson drops to 0. A major goal of the UT high energy heavy-ion group is to search for direct evidence of this restoration of Chiral Symmetry. UT Physics Pizza Seminar April 15,

15 Search for chiral symmetry restoration using hadronic resonances produced in heavy ion collisions Relativistic Heavy-Ion Collider at BNL s NN 7 00 GeV (CM energy per NNpair) p p,d Au, Au Au e e 15

16 Search for chiral symmetry restoration via hadronic resonances at RHIC & LHC Prof. Christina Markert s research (100) K e e K Resonance hadrons are short lived (few to tens of fm/c) and predominantly decay by the strong interaction. Strong decays (K + K ) in the medium are washed out by scattering. E&M decays (e + e ) in the medium can be observed and serve as messengers of possible CSR. 16

17 Signals of chiral symmetry restoration: particle mass reduction in medium Example for (100) resonance width broadening and mass shift due to medium modification (100) Eur.Phys.J.A17:83-87,003. Slide from Christina Markert, UT Austin 17

18 Resonance modifications at RHIC with STAR (about 500 members) E-M Calorimeter Projection Chamber Time of Flight 18

19 Resonances: Time-of-Flight essential for electron identification Au-Au 00 GeV, 0-80%, 50M events from Run 10 TOF de/dx in TPC Entries e e p : GeV/c T STAR Preliminary signal=31461 Remove detector resolution; fit with B.W. S/stat.Err= Invariant Mass [GeV/c ] Particle Data Group values 19

20 Resonances at the Large Hadron Collider at CERN: ALICE proton+proton to 14 TeV, Pb+Pb to ~7 TeV per N-N pair UT Physics Pizza Seminar April 15, 014 0

21 ALICE A Large Ion Collider Experiment Prof. Christina Markert at UT is an active member and supports a post-doc to work on ALICE; currently no graduate student. 1

22 Proposed observation of -meson in medium Compare leptonic decays of resonances associated with trigger and recoil jets in Pb+Pb collisions at ALICE K K away e + side 1 e - near side Trigger Jet surface bias e + e - -meson decays outside medium -meson decays within medium; daughter electrons are minimally affected by the medium enabling the parent meson and its mass & width to be reconstructed. e Analysis of in Pb+Pb collisions underway; more data needed, still lots of work to do. e Modified slide from Christina Markert, UT Austin

23 Projects for New Graduate Students Data analysis of jet + resonance yields and correlations, paper writing, shifts at CERN about 1 month per year. Data analysis of charm and bottom quark meson production and interaction; heavy flavor resonances D* ALICE hardware upgrade work: silicon pixel sensor testing in our lab on 10 th level of RLM. UT Physics Pizza Seminar April 15, 014 3

24 Previous STAR hardware projects: Time of Flight (TOF) Muon Telescope Detector (MTD) Heavy Flavor Tracker (HFT) 4

25 TOF: Tray Assembly 5

26 TOF: Cosmic Test Stands Cosmic Stand Cosmic Stand 1 THUBs 6

27 MTD Tray Assembly and Testing 7

28 HFT: STAR Heavy Flavor Tracker ( ) (Main Collaborator LBNL) UT participates in Pixel readout hardware and firmware development UT is responsible for Pixel slow controls, online monitoring software UT participates in the SSD readout firmware development Installed in STAR for Engineering Run May 013 8

29 ALICE: Barrel Tracking Upgrade Install new multi-layer silicon tracking detectors 9

30 ALICE: Barrel Tracking Upgrade Each stave will be bench tested. Sensor testing of all Staves may be done in our 10 th floor labs. Great opportunity for students to learn about state-of-the-art silicon detectors, on-board electronics, and cosmic-ray testing. 30

31 Longer Term (00s) Electron+Ion Collider 31

32 Longer Term (00s) Electron+Ion Collider Relativistic Heavy Ion Collider at the Brookhaven Natl. Lab, Long Island, NY 3

33 Physics Goals of EIC: Deep Nuclear Structure gluons/0 quarks Accessible at higher collision energies But eventually the gluon density increases until gluon fusion dominates and limits (saturates) the gluon density; perhaps forming a color condensate. UT Physics Pizza Seminar April 15,

34 Gluon saturation Parton effective area ( Q ) / Q Nuclear area in transverse plane R R If #partons N ~ A A, then partons begin to overlap and g g g dominates. S A, Q is parton p t R A Parton area Gluon density saturates at Q S ~ ( Q S ) N A R A ~ A 1/3, saturation scale Low-x partons projected onto transverse plane A+A overlap may increase Q S, leading to observable effects probed at higher s NN 34

35 Color Glass Condensate* (an effective theory of QCD at high energy) ln 1 x EIC *(see arxiv: for a recent review and 35

36 High Energy Heavy Ion Nuclear Physics Group Faculty: G. W. Hoffmann, Christina Markert Research Scientists: Lanny Ray, Jo Schambach Post-Doc: Anders Knopse, Martin Codrington Graduate Students: Prabhat Bhattarai, Alex Jentsch UT Physics Pizza Seminar April 15,

37 Extras 37

38 Resonances: Searching for evidence of chiral symmetry restoration (Ph.D. Thesis data of M. Wada) e Resonances produced and decay within medium serve as a probe Leptonic decay particles less affected by re-scattering If CSR occurs in HIC then mass, widths, BR s may be affected For (c = 46 fm) medium effects diluted for ~10 fm/c system lifetime e width increase e e within medium can be reconstructed Rapp, Wambach, Van Hees, arxiv: Hadronic medium predictions; no QGP included DOE Comparative Review UT Austin Gaithersburg, MD, 9 May

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40 Resonances: Comparison with Rapp et al. e e Fit mass peak with sum of: free space decays + width broadened decays (inside medium, Rapp) Total STAR Preliminary Rapp et al. predicted fraction of in-medium decays: No statistically significant change in mass or width Predicted width increase is not observed Data may permit some contribution of broadened decays Complete e e Complete e e paper analysis and paper (thesis topic for new student) DOE Comparative Review UT Austin Gaithersburg, MD, 9 May

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42 STAR ToF Assembly and Testing RLM , Prof. G. W. Hoffmann and J. Schambach STAR Time of Flight detector upgrade: p/k separation p=1.6gev/c, p/(k+p) p=3gev/c Improves statistics for L(150) by a factor of 11 And extends measurement out to higher p T regions With large UT Austin Nuclear Physics group involvement 4

43 UT STAR detector upgrades Heavy Flavor Tracker Muon Telescope Detector Uses same RPC technology as TOF UT: construction, electronics, testing UT doing read-out electronics 43

44 Color Glass Condensate* (an effective theory of QCD at high energy) ln 1 x Bzdak and Skokov, PRL 111, (013); CGC prediction for p+pb at LHC CGC *(see arxiv: for a recent review and 44

45 Gluon saturation 45