10,474 research outputs found
An Upper Bound on the Decay tau -> mu gamma from Belle
We have performed a search for the lepton-flavor-violating decay tau -> mu
gamma using a data sample of 86.3fb^{-1} accumulated by the Belle detector at
KEK. No evidence for a signal is seen, and we set an upper limit for the
branching fraction of B(tau -> mu gamma) < 3.1 x 10^{-7} at the 90% confidence
level.Comment: 6 pages, 4 figuresm, submitted to Phys. Rev. Let
Chiral magnetic effect search in p+Au, d+Au and Au+Au collisions at RHIC
Metastable domains of fluctuating topological charges can change the
chirality of quarks and induce local parity violation in quantum
chromodynamics. This can lead to observable charge separation along the
direction of the strong magnetic field produced by spectator protons in
relativistic heavy-ion collisions, a phenomenon called the chiral magnetic
effect (CME). A major background source for CME measurements using the
charge-dependent azimuthal correlator () is the intrinsic
particle correlations (such as resonance decays) coupled with the azimuthal
elliptical anisotropy (). In heavy-ion collisions, the magnetic field
direction and event plane angle are correlated, thus the CME and the
-induced background are entangled. In this report, we present two
studies from STAR to shed further lights on the background issue. (1) The
should be all background in small system p+Au and d+Au
collisions, because the event plane angles are dominated by geometry
fluctuations uncorrelated to the magnetic field direction. However, significant
is observed, comparable to the peripheral Au+Au data, suggesting
a background dominance in the latter, and likely also in the mid-central Au+Au
collisions where the multiplicity and scaled correlator is similar. (2)
A new approach is devised to study as a function of the particle
pair invariant mass () to identify the resonance backgrounds and hence
to extract the possible CME signal. Signal is consistent with zero within
uncertainties at high . Signal at low , extracted from a
two-component model assuming smooth mass dependence, is consistent with zero
within uncertainties.Comment: ISMD 2017 conference proceeding contributio
The role of the triangle singularity in production in the and processes
We have investigated the cross section for the and reactions paying attention to a
mechanism that develops a triangle singularity. The triangle diagram is
realized by the decay of a to and the decay into , and the finally merges into . The mechanism is
expected to produce a peak around MeV in the invariant
mass. We found that a clear peak appears around MeV in the
invariant mass which is about MeV lower than the
expectation, and that is due to the resonance peak of a resonance which
plays a crucial role in the production. The mechanism studied
produces the peak of the around or below 1400 MeV, as is seen
in the HADES experiment.Comment: 12 pages, 9 figure
Isolating the chiral magnetic effect from backgrounds by pair invariant mass
Topological gluon configurations in quantum chromodynamics induce quark
chirality imbalance in local domains, which can result in the chiral magnetic
effect (CME)--an electric charge separation along a strong magnetic field.
Experimental searches for the CME in relativistic heavy ion collisions via the
charge-dependent azimuthal correlator () suffer from large
backgrounds arising from particle correlations (e.g. due to resonance decays)
coupled with the elliptic anisotropy. We propose differential measurements of
the as a function of the pair invariant mass (), by
restricting to high thus relatively background free, and by
studying the dependence to separate the possible CME signal from
backgrounds. We demonstrate by model studies the feasibility and effectiveness
of such measurements for the CME search.Comment: 16 preprint pages 5 figures. v2: added a test with a broad
"instanton/sphaleron" peak, and added clarifying texts; v3: added event-shape
engineering (and two new figures) and expanded discussions on the low
invariant mass region; v4: repeated cautionary discussions in introduction
and conclusion sections, published versio
Triangle singularity in the decays
We study the reaction and
find that the mechanism to produce this decay develops a triangle singularity
around ~MeV. The differential width
shows a rapid growth around the
invariant mass being 1515~MeV as a consequence of the triangle singularity of
this mechanism, which is directly tied to the nature of the and
as dynamically generated resonances from the interaction of
pseudoscalar mesons. The branching ratios obtained for the decays are of the order of , accessible in
present facilities, and we argue that their observation should provide relevant
information concerning the nature of the low-lying scalar mesons.Comment: 12 pages, 8 figures, published in EPJ
Sqrt{shat}_{min} resurrected
We discuss the use of the variable sqrt{shat}_{min}, which has been proposed
in order to measure the hard scale of a multi parton final state event using
inclusive quantities only, on a SUSY data sample for a 14 TeV LHC. In its
original version, where this variable was proposed on calorimeter level, the
direct correlation to the hard scattering scale does not survive when effects
from soft physics are taken into account. We here show that when using
reconstructed objects instead of calorimeter energy and momenta as input, we
manage to actually recover this correlation for the parameter point considered
here. We furthermore discuss the effect of including W + jets and t tbar+jets
background in our analysis and the use of sqrt{shat}_{min} for the suppression
of SM induced background in new physics searches.Comment: 23 pages, 9 figures; v2: 1 figure, several subsections and references
as well as new author affiliation added. Corresponds to published versio
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