**LDRD DR Project 20060049DR**

**\"Heavy Quarks as a Probe of a New State of Matter\"**

**Self --Assessment**

**Collaborators**

# Accelerator Operations and Technology

Mark Prokop

# Physics Division

Melynda Brooks, Sergey Butsyk, Gary Grim, Jon Kapustinsky, Andi Klein,
Gerd Kunde, Patrick McGaughey, Anuj Purwar, Hubert van Hecke

*Theory Division*

Rajan Gupta, Emil Mottola, Ivan Vitev

# With Collaborators from

Brookhaven National Laboratory, Columbia University, Fermi National
Accelerator Laboratory, Lawrence Livermore National Laboratory, New
Mexico State University, University of New Mexico, University of Utah,

Intriguing evidence that a new state of matter, the quark-gluon plasma
(QGP), is formed in collisions of ultra-relativistic heavy ions has been
presented by physicists working at the Relativistic Heavy Ion Collider
(RHIC) at Brookhaven National Laboratory. We are developing a unique
experimental capability for the direct identification of heavy quarks at
RHIC, which will be used to accurately determine the properties of this
new state of matter. We are constructing a silicon micro-vertex detector
(iFVTX) covering the forward collision region, which will detect heavy
quarks in the high multiplicity environment of nuclear collisions using
four layers of precision tracking. The iFVTX will provide
state-of-the-art position resolution, speed and low power consumption.
Developing the proposed detector is a natural extension of our
leadership role in the PHENIX experiment at RHIC.

In parallel, we are developing the tools necessary for the full
interpretation of the new data, including state-of-the-art perturbative
QCD, lattice QCD and non-equilibrium field theory calculations. We are
addressing the current pressing questions in heavy ion physics: Are the
interactions in the plasma so strong that heavy quarks are quickly
equilibrated and exhibit hydrodynamic flow? What is the mechanism of
energy loss for heavy quarks in the plasma? What is the equation of
state for the QGP? How does the thermalization of the QGP proceed?

This project directly addresses one of LANL's grand challenges: "Physics
beyond the standard model." Research at RHIC is one of the highest
priorities of the national nuclear physics program. Relativistic Heavy
Ion Physics is also a strategic priority of the LDRD DR program. The
experience we've acquired performing this work has led directly to a
proposal we have recently submitted to the Department of Energy for the
construction of a much larger vertex detector.

**Total funding** for the theoretical and experimental work is \$1.25M
per year for FY06, FY07 and FY08.

**Experimental Program**

##### Charm and beauty particles can be cleanly identified by measuring their lifetimes. These are \~1 picosecond, which translate into a decay distance of \~1 mm at forward angles. At present, none of the existing RHIC detectors have sufficient spatial resolution to pinpoint these decay vertices. Our strategy is to use a silicon micro-vertex detector to precisely measure the decay distance, together with a PHENIX muon arm to identify the decay muon and record its momentum. Factors such as occupancy, radiation dose, power and cost dictate that the detectors be composed of small strips or pixels, with strip sizes of 50 μm by about 1 mm. These detectors and the associated readout electronics are very challenging to build, requiring custom silicon integrated circuits. Working together with Fermilab, we have adapted the BTeV pixel detectors and FPIX front-end electronics for our use. {#charm-and-beauty-particles-can-be-cleanly-identified-by-measuring-their-lifetimes.-these-are-1-picosecond-which-translate-into-a-decay-distance-of-1-mm-at-forward-angles.-at-present-none-of-the-existing-rhic-detectors-have-sufficient-spatial-resolution-to-pinpoint-these-decay-vertices.-our-strategy-is-to-use-a-silicon-micro-vertex-detector-to-precisely-measure-the-decay-distance-together-with-a-phenix-muon-arm-to-identify-the-decay-muon-and-record-its-momentum.-factors-such-as-occupancy-radiation-dose-power-and-cost-dictate-that-the-detectors-be-composed-of-small-strips-or-pixels-with-strip-sizes-of-50-μm-by-about-1-mm.-these-detectors-and-the-associated-readout-electronics-are-very-challenging-to-build-requiring-custom-silicon-integrated-circuits.-working-together-with-fermilab-we-have-adapted-the-btev-pixel-detectors-and-fpix-front-end-electronics-for-our-use. .unnumbered}

The application of this state-of-the-art technology requires us to
overcome a number of technical challenges including:

1.  Fabrication of custom silicon pixel sensor and front-end wafers.

2.  Bump bonding of pixel sensors to front end electronics.

3.  Development of complex circuit boards and flex circuits (HDI) that
    precisely support the detectors and transmit the data.

4.  Design of high-speed readout electronics to receive, buffer and
    format the data to be compatible with the PHENIX data acquisition
    system.

Our original plan (simplified) from the DR proposal was:

##### FY06: {#fy06 .unnumbered}

##### Complete design and produce front-end chip.

##### Design and procure silicon pixel detectors. Design support structure.

##### Have industry bond the detectors and front-end chips. Design readout bus structure.

**FY07:**

1.  Assemble detector/readout, bus and fiber optic readout. Test
    detector/readout assemblies.

2.  Assemble the iFVTX from four layers of mini-strip detectors.

3.  Install the iFVTX in front of a PHENIX muon arm at RHIC. Connect
    readout to PHENIX data acquisition system.

**FY08:**

4.  Record charm and beauty quarks data during the Au+Au run with a
    fully operational iFVTX.

5.  Establish the elliptic flow of heavy versus light quarks.

6.  Determine the suppression of the heavy quarks in central heavy ion
    collisions.

#####  {#section .unnumbered}

**At present**, about half way through the 3 year project, we have
achieved the following:

1.  Fabricated and tested all of the custom silicon pixel detectors.
    Completed conceptual design of support structure.

2.  Fabricated and tested all of the custom front end chips.

3.  Bump-bonded \~15 (of the \~80 total) pixel detectors to the front
    ends. Designed and fabricated working samples of the readout bus
    (HDI).

4.  Designed and prototyped the readout electronics and fiber optics.
    Successfully tested a few bump-bonded assemblies on HDIs. Designed
    and prototyped the printed circuit board that supports the pixel
    detectors.

and

-   Completed simulations of the iFVTX performance in heavy ion
    collisions, demonstrating good separation of heavy quarks from
    backgrounds.

**Remaining Issues:**

We have not yet operated several pixel detector modules simultaneously.
Potential issues such as how to minimize the electronic crosstalk or
optimize the analog operating conditions remain to be addressed.

**In the future**, we expect to meet our goal of completing the
construction of the iFVTX, but not until well into FY08. In any case,
the PHENIX experiment will not be ready for us to install the iFVTX
until at least FY09, due to funding delays for both RHIC running and for
required PHENIX upgrades. Therefore, data collection and analysis will
not be possible during the three year term of this project.

**Follow-on Project** - "PROPOSAL for a Forward Silicon Vertex Tracker
(FVTX) for the PHENIX Experiment," submitted to the DOE, 2006. Melynda
Brooks (P-25) is the spokesperson.

**Theoretical Program**

##### FY06: {#fy06-1 .unnumbered}

-   Determine mean field evolution of collective gluon fields.

##### Study particle production and pre-thermalization stage of the plasma.

##### Investigate charm particle production and correlations in light colliding systems.

-   Investigate improved lattice actions and different observables to
    reduce the systematic uncertainties in lattice QCD calculations.

-   Develop a US collaboration and utilize the power of the Blue Gene L
    supercomputer at Lawrence Livermore Lab for the lattice QCD
    simulations.

**FY07:**

-   Apply new quasi-particle resummation methods to compute an improved
    EOS.

-   Identify quasi-particle lifetime and scattering during the approach
    to equilibrium.

-   Calculate the energy loss of charm and beauty quarks in the plasma.

```{=html}
<!-- -->
```
-   Perform finite temperature lattice QCD simulations of QCD in the
    range 0.8Tc-3Tc.

-   Determine thermodynamic observables and QCD EOS as a function of
    temperature.

**FY08:**

-   Compute transport coefficients and mean free path of plasma
    excitations.

-   Determine heavy quark mass dependence of departures from local
    thermodynamic equilibrium.

-   Make realistic predictions for the suppression of high p~T~ charm
    and beauty mesons in the plasma.

```{=html}
<!-- -->
```
-   Compare predictions of Debye-screening and dissociation effects in
    the QGP with the measured yields of charm and beauty bound states.

-   Finish the lattice QCD calculation of Tc and publish the results.

-   Carry out the required finite T and T=0 simulations of lattice QCD
    to estimate the EOS

**At present**, in the theoretical program we have achieved the
following:

1.  We have thoroughly investigated the higher order corrections to the
    Hard Thermal Loop (HTL) expansion in hot gauge theories. The
    two-loop contributions to the electron self-energy in high
    temperature QED have been shown to contain infrared divergences and
    dependence upon the gauge-fixing parameter. This necessitates
    inclusion of the first order HTL self-energies and vertices into
    corrected propagators and vertices, for systematic extension beyond
    the leading order. We have shown that these corrections give the
    full next-to-leading order (NLO) contributions to the quasi-particle
    dispersion relation, necessary to compute transport coefficients
    such as the electrical conductivity of the plasma. We have used this
    resummation method to calculate the gauge invariant imaginary part
    of the electron self-energy in high temperature QED, for the first
    time, namely, Imag Σ= γ = (2.70) α T.

2.  The analogous scattering processes and resummation method for the
    self-energies and decay rates have been identified for the thermal
    propagators and proper vertices of dressed quarks and gluons, making
    possible the systematic calculation of transport coefficients from
    first principles in QCD.

3.  We have carried out perturbative QCD calculations of the single and
    double inclusive D- and B-meson cross sections at the Tevatron and
    at RHIC, established the importance of including the charm and
    beauty parton distributions from global pQCD analysis in leading
    order calculations for improved convergence of the perturbative
    expansion., and proposed observables related to heavy meson
    triggered jets to determine the dominant heavy flavor production
    mechanism.

4.  We have generalized the dynamical calculations of nuclear shadowing,
    which arises from the coherent final-state scattering of the struck
    parton in the nucleus, to the case of heavy quarks, and demonstrated
    the shadowing is not the dominant effect for the nuclear
    modification of particle production at forward rapidities.

5.  We have calculated initial-state energy loss in cold nuclear matter
    for the first time, demonstrating that it is large and dominates
    over final-state energy loss contrary to earlier conjectures. We
    have identified cold nuclear matter energy loss as a dominant source
    of forward rapidity particle suppression at RHIC, and generalized
    the results to charm and beauty quarks.

6.  We have calculated the drag and diffusion coefficients for heavy
    quarks from radiative and collisional processes in the quark-gluon
    plasma, and carried out preliminary studies of the thermalization,
    suppression and elliptic flow of charm and beauty quarks. Charm
    quarks undergo partial thermalization; beauty quarks are largely
    unaffected.

7.  We have developed a comprehensive plan to simulate lattice QCD at
    finite temperature that addressed three major sources of systematic
    errors in existing calculations. The continuum limit of lattice
    results will be based on simulations with N~t~ =4, 6 and 8 lattices.
    We have investigated three different lattice discretization schemes
    to have further control over the lattice spacing errors, simulate
    with realistic values of light quark masses, and reduce statistical
    errors below systematic errors.

8.  These simulations required major computational resource of over 10
    teraflop year sustained. We therefore developed a collaboration with
    LLNL and two large national collaborations (MILC and RBC) to carry
    out these simulations on the BlueGene/L supercomputer at LLNL. This
    required NNSA approval for which we prepared a detailed science and
    computing case. This was successfully funded.

9.  We have ported and optimized codes for p4 and AsqTad staggered
    fermions and domain wall fermions on the BlueGene/L.

10. We have started simulations at finite temperature and have finished
    about 35% of targeted statistical sample, and started optimizing
    code and simulations for the zero temperature calculations.

**Major Issues:**

We budgeted for one-half of a post-doctoral research assistant, expert
in equilibrium and non-equilibrium fie1d theory, energy loss
calculations, and/or lattice QCD. Although two individual canditates
were identified, they both declined offers. This has been compensated in
part by having several active external collaborations and visitors. We
will continue to seek and recruit a qualified research associate for the
theoretical effort of the project.

**In the future**, we expect to complete the calculation of the
electrical conductivity of the high temperature QED plasma, extend the
analysis of the quasi-particle quark and gluon damping rates to QCD, and
determine the shear and bulk viscosities of the high temperature QGP. We
will compare our analytic results to lattice QCD simulations and energy
loss parameters. The effective action for the resummation method will
also allow the determination of the eq. of state (EOS) and evolution
neqs. For soft gluonic mean fields coupled to hard scattering processes.

We plan to complete the calculation of thermalization, suppression and
flow of heavy quarks via Langevin simulations, and compare the results
with the analytic resummation method. The two theoretical approaches to
heavy flavor modification, tested in the mid-rapidity region of heavy
ion reactions will be extended to forward rapidity, the part of phase
space covered by iFVTX. Full many-body perturbative QCD calculations
will be carried out, including dynamical shadowing, cold nuclear matter
energy loss and QGP-induced suppression of heavy flavor to map the QGP
properties, such as temperature, density and energy density, on
experimentally measurable observables. We will this meet our goal of
developing a method for precise determination of the properties of the
QGP using heavy quarks as probes.

In the lattice QCD effort, we plan (i) to complete the finite
temperature runs and start full scale runs for the zero temperature
data, (ii) carry out the analysis of the finite temperature data and
estimate the transition temperature Tc, (iii) finish the zero
temperature runs and combine them with finite temperature data to map
the equation of state, and (iv) refine the strategy for the calculation
of heavy quark (onia) spectral functions and implement these
simulations.

**Publications:**

J. Kapustinsky, J. G. Boissevain, M. Brooks, S. Butsyk, G. Cardoso, D.
Chrisitan, B. Cole, G. Grim, R. Gupta, B. Knapp, G. J. Kunde, S. Kwan,
D. Lee, M. Leitch, M. Liu, E. Mannel, P. McGaughey, E. Mottola, A.
Purwar, W. Sondheim, M. Turqueti, H. Van Hecke, I. Vitev, X. Wang, D.
Winter, and W. Zajc. "A pixel-based tracker to probe the quark-gluon
plasma at PHENIX." To appear in Frontier Detectors for Frontier Physics.
(Pisa, Italy,May 21-27, 2006).

P. McGaughey. "Heavy quark detection with a forward silicon micro-vertex
detector in the PHENIX experiment." 2006. In Particles and Nuclei
International Conference. Vol. 842, First Edition, p.1079. American
Institute of Physics.

S.Butsyk. "Forward pixel-based detector for PHENIX." 2006. IEEE Nuclear
Science Symposium, in press.

I. Vitev, T. Goldman, M.B. Johnson, J.W. Qiu, "Open charm tomography of
cold nuclear matter" Phys. Rev. D **74**: 054010**,** 2006.

I. Vitev, "PQCD approach to parton propagation in matter", Nucl. Phys. A
**783**, 31-38, 2007.

I. Vitev, "Recent PQCD calculations of heavy quark production", J. Phys.
G **32**: S317-S324, 2006.

A. Adil, I. Vitev, "Collisional dissociation of heavy mesons in dense
QCD matter", Phys. Lett. B in press, 2007; hep-ph/0611109.

I. Vitev, A. Adil, H. van Hees, "Novel heavy flavor suppression
mechanisms in the QGP", J. Phys. G in press, 2007; hep-ph/0701188.

I. Vitev, "Non-Abelian energy loss in cold nuclear matter", Phys. Rev. C
submitted, 2007; hep-ph/0703002.

T. Bhattacharya, M. Cheng, N. Christ, C. DeTar, S. Gottleib, J. Grandy,
R. Gupta, C. Jung, F. Karsch, L. Levkova, R. Mawhinney, P. Petreczky, C.
Schmidt, R. Soltz, R. Sugar, D. Toussaint, P. Vranas, "Determining the
QCD equation of state on the BlueGene/L supercomputer, Poster at Quark
Matter 2006.