16 research outputs found
Hadronic matrix elements of proton decay on the lattice
We report on our on-going project to calculate proton decay matrix elements
using domain-wall fermions on the lattice. By summarizing the history of the
proton decay calculation on the lattice, we reveal the systematic errors of
those calculations. Then we discuss our approach to tackle those uncertainties
and show our preliminary results on the matrix elements.Comment: 4 pages, 2 figures, talk presented at CIPANP2003: Conference on the
Intersections of Particle and Nuclear Physics, New York City, May 19-24, 200
Transition temperature in QCD with physical light and strange quark masses
We present results from a calculation of the transition temperature in QCD
with two light and one heavier (strange) quark mass on lattices with temporal
extent N_t =4 and 6. Calculations with improved staggered fermions have been
performed with a strange quark mass fixed close to its physical value and for
various light to strange quark mass ratios that correspond to light
pseudo-scalar masses in the range (150-500) MeV. From a combined extrapolation
to the chiral (m_l -> 0) and continuum (aT -> 0) limits we obtain for the
transition temperature at the physical point T_c = 192(7)(4) MeV.
We also present first results from an ongoing calculation of the QCD equation
of state with almost realistic light and strange quark masses.Comment: 4 pages, 4 figures, to appear in the proceedings of the 19th
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions,
Shanghai, Nov. 200
Heavy quark thermodynamics in full QCD
We analyze the large-distance behaviour of static quark-anti-quark pair
correlations in QCD. The singlet free energy is calculated and the entropy
contribution to it is identified allowing us to calculate the excess internal
energy. The free energy has a sharp drop in the critical region, leading to
sharp peaks in both excess entropy and internal energy.Comment: talk given at Quark Matter 200
Chirally improving Wilson fermions - I. O(a) improvement
We show that it is possible to improve the chiral behaviour and the approach
to the continuum limit of correlation functions in lattice QCD with Wilson
fermions by taking arithmetic averages of correlators computed in theories
regularized with Wilson terms of opposite sign. Improved hadronic masses and
matrix elements can be obtained by similarly averaging the corresponding
physical quantities separately computed within the two regularizations. To deal
with the problems related to the spectrum of the Wilson--Dirac operator, which
are particularly worrisome when Wilson and mass terms are such as to give
contributions of opposite sign to the real part of the eigenvalues, we propose
to use twisted-mass lattice QCD for the actual computation of the quantities
taking part to the averages. The choice for the twisting angle is
particularly interesting, as O() improved estimates of physical quantities
can be obtained even without averaging data from lattice formulations with
opposite Wilson terms. In all cases little or no extra computing power is
necessary, compared to simulations with standard Wilson fermions or
twisted-mass lattice QCD.Comment: 71 pages, Latex, Keywords: Lattice, Improvement, Chirality. Version
v2: mistake corrected in transformation properties under \omega -> -\omega,
sect. 5.3.1 (see also sect. 6.1). Minor corrections in App. D and argument
clarified in App. F. Version v3: minor modifications in sect. 2 (pag. 8-10:
on the odd r-parity of M_crit(r)), in sect. 3.1.3 and 5.4.1 (few sentences
about cutoff effects at small quark mass) and in sect. 3.2 (details of
discussion below eq. 3.17); updated/added some reference
Reducing Residual-Mass Effects for Domain-Wall Fermions
It has been suggested to project out a number of low-lying eigenvalues of the
four-dimensional Wilson--Dirac operator that generates the transfer matrix of
domain-wall fermions in order to improve simulations with domain-wall fermions.
We investigate how this projection method reduces the residual chiral
symmetry-breaking effects for a finite extent of the extra dimension. We use
the standard Wilson as well as the renormalization--group--improved gauge
action. In both cases we find a substantially reduced residual mass when the
projection method is employed. In addition, the large fluctuations in this
quantity disappear.Comment: 18 pages, 10 figures, references updated, comments adde
On Unification and Nucleon Decay in Supersymmetric Grand Unified Theories Based on SU(5)
We investigate the unification constraints in the minimal sypersymmetric
grand unified theories based on SU(5) gauge symmetry. The most general
constraints on the spectrum of minimal supersymmetric SU(5) and flipped SU(5)
are shown. The upper bound on the mass of the colored Higgs mediating proton
decay is discussed in detail in the context of the minimal supersymmetric
SU(5). In the case of the minimal SUSY SU(5) we show that if we stick to the
strongest bound on the colored triplet mass coming from dimension five proton
decay contributions there is no hope to test this model at future nucleon decay
experiments through the dimension six operators. We find a lower bound on the
partial proton decay lifetime for all relevant channels in the context of
flipped SUSY SU(5). We conclude that flipped SUSY SU(5) might be in trouble if
proton decay is found at the next generation of experiments with a lifetime
below 10^{37} years.Comment: 17 pages, 6 figures, some corrections and references adde
Baryon structure from Lattice QCD
We present recent lattice results on the baryon spectrum, nucleon
electromagnetic and axial form factors, nucleon to transition form
factors as well as the electromagnetic form factors. The masses of the
low lying baryons and the nucleon form factors are calculated using two
degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV.
We compare to the results of other collaborations. The nucleon to
transition and form factors are calculated in a hybrid scheme, which
uses staggered sea quarks and domain wall valence quarks. The dominant dipole
nucleon to transition form factor is also evaluated using dynamical
domain wall fermions. The transverse density distributions of the in
the infinite momentum frame are extracted using the form factors determined
from lattice QCD.Comment: Invited talk at the Workshop on the Physics of excited nucleons
NSTAR2009, Beijing, April 19-22, 2009, 9 page
Progress in finite temperature lattice QCD
I review recent progress in finite temperature lattice calculations,
including the determination of the transition temperature, equation of state,
screening of static quarks and meson spectral functions.Comment: 8 pages, LaTeX, uses iopart.cls, invited talk presented at
Strangeness in Quark Matter 2007 (SQM 2007), Levoca, Slovakia, June 24-29,
200
Quarkonium in Hot Medium
I review recent progress in studying quarkonium properties in hot medium as
well as possible consequences for quarkonium production in heavy ion
collisions.Comment: Invited talk at SQM 2009, Buzios, Brazil, Sep. 27 -Oct. 2 2009,
LaTeX, 8 pages,3 figures; typos corrected, references adde