599 research outputs found
Measurement as a shortcut to long-range entangled quantum matter
The preparation of long-range entangled states using unitary circuits is
limited by Lieb-Robinson bounds, but circuits with projective measurements and
feedback (``adaptive circuits'') can evade such restrictions. We introduce
three classes of local adaptive circuits that enable low-depth preparation of
long-range entangled quantum matter characterized by gapped topological orders
and conformal field theories (CFTs). The three classes are inspired by distinct
physical insights, including tensor-network constructions, multiscale
entanglement renormalization ansatz (MERA), and parton constructions. A large
class of topological orders, including chiral topological order, can be
prepared in constant depth or time, and one-dimensional CFT states and
non-abelian topological orders with both solvable and non-solvable groups can
be prepared in depth scaling logarithmically with system size. We also build on
a recently discovered correspondence between symmetry-protected topological
phases and long-range entanglement to derive efficient protocols for preparing
symmetry-enriched topological order and arbitrary CSS (Calderbank-Shor-Steane)
codes. Our work illustrates the practical and conceptual versatility of
measurement for state preparation.Comment: 22 pages, 9 figures, restructured, relation to Ref. [7, 8] clarifie
Mixed axion/neutralino cold dark matter in supersymmetric models
We consider supersymmetric (SUSY) models wherein the strong CP problem is
solved by the Peccei-Quinn (PQ) mechanism with a concommitant axion/axino
supermultiplet. We examine R-parity conserving models where the neutralino is
the lightest SUSY particle, so that a mixture of neutralinos and axions serve
as cold dark matter. The mixed axion/neutralino CDM scenario can match the
measured dark matter abundance for SUSY models which typically give too low a
value of the usual thermal neutralino abundance, such as models with wino-like
or higgsino-like dark matter. The usual thermal neutralino abundance can be
greatly enhanced by the decay of thermally-produced axinos to neutralinos,
followed by neutralino re-annihilation at temperatures much lower than
freeze-out. In this case, the relic density is usually neutralino dominated,
and goes as \sim (f_a/N)/m_{axino}^{3/2}. If axino decay occurs before
neutralino freeze-out, then instead the neutralino abundance can be augmented
by relic axions to match the measured abundance. Entropy production from
late-time axino decays can diminish the axion abundance, but ultimately not the
neutralino abundance. In mixed axion/neutralino CDM models, it may be possible
to detect both a WIMP and an axion as dark matter relics. We also discuss
possible modifications of our results due to production and decay of saxions.
In the appendices, we present expressions for the Hubble expansion rate and the
axion and neutralino relic densities in radiation, matter and decaying-particle
dominated universes.Comment: 31 pages including 21 figure
Strong disorder fixed points in the two-dimensional random-bond Ising model
The random-bond Ising model on the square lattice has several disordered
critical points, depending on the probability distribution of the bonds. There
are a finite-temperature multicritical point, called Nishimori point, and a
zero-temperature fixed point, for both a binary distribution where the coupling
constants take the values +/- J and a Gaussian disorder distribution. Inclusion
of dilution in the +/- J distribution (J=0 for some bonds) gives rise to
another zero-temperature fixed point which can be identified with percolation
in the non-frustrated case (J >= 0). We study these fixed points using
numerical (transfer matrix) methods. We determine the location, critical
exponents, and central charge of the different fixed points and study the
spin-spin correlation functions. Our main findings are the following: (1) We
confirm that the Nishimori point is universal with respect to the type of
disorder, i.e. we obtain the same central charge and critical exponents for the
+/- J and Gaussian distributions of disorder. (2) The Nishimori point, the
zero-temperature fixed point for the +/- J and Gaussian distributions of
disorder, and the percolation point in the diluted case all belong to mutually
distinct universality classes. (3) The paramagnetic phase is re-entrant below
the Nishimori point, i.e. the zero-temperature fixed points are not located
exactly below the Nishimori point, neither for the +/- J distribution, nor for
the Gaussian distribution.Comment: final version to appear in JSTAT; minor change
Light Sneutrino Dark Matter at the LHC
In supersymmetric (SUSY) models with Dirac neutrino masses, a weak-scale
trilinear A-term that is not proportional to the small neutrino Yukawa
couplings can induce a sizable mixing between left and right-handed sneutrinos.
The lighter sneutrino mass eigenstate can hence become the lightest SUSY
particle (LSP) and a viable dark matter candidate. In particular, it can be an
excellent candidate for light dark matter with mass below ~10 GeV. Such a light
mixed sneutrino LSP has a dramatic effect on SUSY signatures at the LHC, as
charginos decay dominantly into the light sneutrino plus a charged lepton, and
neutralinos decay invisibly to a neutrino plus a sneutrino. We perform a
detailed study of the LHC potential to resolve the light sneutrino dark matter
scenario by means of three representative benchmark points with different
gluino and squark mass hierarchies. We study in particular the determination of
the LSP (sneutrino) mass from cascade decays involving charginos, using the mT2
variable. Moreover, we address measurements of additional invisible sparticles,
in our case the lightest neutralino, and the question of discrimination against
the MSSM.Comment: 25 pages, 16 figure
Fine-tuning implications for complementary dark matter and LHC SUSY searches
The requirement that SUSY should solve the hierarchy problem without undue
fine-tuning imposes severe constraints on the new supersymmetric states. With
the MSSM spectrum and soft SUSY breaking originating from universal scalar and
gaugino masses at the Grand Unification scale, we show that the low-fine-tuned
regions fall into two classes that will require complementary collider and dark
matter searches to explore in the near future. The first class has relatively
light gluinos or squarks which should be found by the LHC in its first run. We
identify the multijet plus E_T^miss signal as the optimal channel and determine
the discovery potential in the first run. The second class has heavier gluinos
and squarks but the LSP has a significant Higgsino component and should be seen
by the next generation of direct dark matter detection experiments. The
combined information from the 7 TeV LHC run and the next generation of direct
detection experiments can test almost all of the CMSSM parameter space
consistent with dark matter and EW constraints, corresponding to a fine-tuning
not worse than 1:100. To cover the complete low-fine-tuned region by SUSY
searches at the LHC will require running at the full 14 TeV CM energy; in
addition it may be tested indirectly by Higgs searches covering the mass range
below 120 GeV.Comment: References added. Version accepted for publication in JHE
Geographic Clustering of Leishmaniasis in Northeastern Brazil1
Different forms of this disease are spreading rapidly in distinct geographic clusters in this region
Coupled Boltzmann calculation of mixed axion/neutralino cold dark matter production in the early universe
We calculate the relic abundance of mixed axion/neutralino cold dark matter
which arises in R-parity conserving supersymmetric (SUSY) models wherein the
strong CP problem is solved by the Peccei-Quinn (PQ) mechanism with a
concommitant axion/saxion/axino supermultiplet. By numerically solving the
coupled Boltzmann equations, we include the combined effects of 1. thermal
axino production with cascade decays to a neutralino LSP, 2. thermal saxion
production and production via coherent oscillations along with cascade decays
and entropy injection, 3. thermal neutralino production and re-annihilation
after both axino and saxion decays, 4. gravitino production and decay and 5.
axion production both thermally and via oscillations. For SUSY models with too
high a standard neutralino thermal abundance, we find the combined effect of
SUSY PQ particles is not enough to lower the neutralino abundance down to its
measured value, while at the same time respecting bounds on late-decaying
neutral particles from BBN. However, models with a standard neutralino
underabundance can now be allowed with either neutralino or axion domination of
dark matter, and furthermore, these models can allow the PQ breaking scale f_a
to be pushed up into the 10^{14}-10^{15} GeV range, which is where it is
typically expected to be in string theory models.Comment: 26 pages with 12 .eps figure
WZ plus missing-E_T signal from gaugino pair production at LHC7
LHC searches for supersymmetry currently focus on strongly produced
sparticles, which are copiously produced if gluinos and squarks have masses of
a few hundred GeV. However, in supersymmetric models with heavy scalars, as
favored by the decoupling solution to the SUSY flavor and CP problems, and
m_{\tg}> 500 GeV as indicated by recent LHC results, chargino--neutralino
(\tw_1^\pm\tz_2) production is the dominant cross section for m_{\tw_1} \sim
m_{\tz_2} < m_{\tg}/3 at LHC with \sqrt{s}=7 TeV (LHC7). Furthermore, if
m_{\tz_1}+m_Z \lesssim m_{\tz_2}\lesssim m_{\tz_1}+m_h, then \tz_2 dominantly
decays via \tz_2\to\tz_1 Z, while \tw_1 decays via \tw_1\to \tz_1 W. We
investigate the LHC7 reach in the WZ + MET channel (for both leptonic and
hadronic decays of the W boson) in models with and without the assumption of
gaugino mass universality. In the case of the mSUGRA/CMSSM model with heavy
squark masses, the LHC7 discovery reach in the WZ+MET channel becomes
competetive with the reach in the canonical MET + jets channel for integrated
luminosities \sim 30 fb^-1. We also present the LHC7 reach for a simplified
model with arbitrary m_{\tz_1} and m_{\tw_1} \sim m_{\tz_2}. Here, we find a
reach of up to m_{\tw_1}\sim 200 (250) GeV for 10 (30) fb^-1.Comment: 15 pages including 9 .eps figure
- …