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M. Pepe

Publications and source records attributed to M. Pepe.

At least 19 recordsLinked to original sources
arXiv2025

Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II

The rare kaon decay $K_L\to\pi^0\nu\bar{\nu}$ is extremely sensitive to new physics, because the contribution to this decay in the Standard Model (SM) is highly suppressed and known very accurately; the branching ratio is $3\times 10^{-11}$ in the SM with a theoretical uncertainty of just 2%. The measurement of this branching ratio could provide essential new information about the flavor structure of the quark sector from the $s\to d$ transition. The decay is being searched for in the KOTO experiment at J-PARC, which has obtained the current best upper limit on the branching ratio of $2.2\times 10^{-9}$; a sensitivity to branching ratios below $10^{-10}$ is achievable by the end of the decade. A next-generation experiment at J-PARC, KOTO II, was proposed in 2024 with 82 members worldwide, including significant contributions from European members. The goal of KOTO II is to measure the $K_L\to\pi^0\nu\bar{\nu}$ branching ratio with sensitivity below $10^{-12}$ in the 2030s. Discovery of the decay with $5\sigma$ significance is achievable at the SM value of the branching ratio. An indication of new physics with a significance of 90% is possible if the observed branching ratio differs by 40% from the SM value. Another important goal of KOTO II is to measure the branching ratio of the unobserved $K_L\to \pi^0e^+e^-$ decay, which can give an input to flavor structures of new physics. Other rare $K_L$ decays and hidden-sector particles are also in the scope of the study. After 2026, KOTO will be the only dedicated rare kaon decay experiment in the world, and KOTO II is the only future rare kaon decay project currently proposed. We would like to lead a global initiative for the experimental study of rare kaon decays, with significant contributions and support from the European community.

hep-ex↗
arXiv2024

Baryonic screening masses in QCD at high temperature

We compute the baryonic screening masses with nucleon quantum numbers and its negative parity partner in thermal QCD with $N_f=3$ massless quarks for a wide range of temperatures, from $T \sim 1$ GeV up to $\sim 160$ GeV. The computation is performed by Monte Carlo simulations of lattice QCD with $O(a)$-improved Wilson fermions by exploiting a recently proposed strategy to study non-perturbatively QCD at very high temperature. Very large spatial extensions are considered in order to have negligible finite volume effects. For each temperature we have simulated $3$ or $4$ values of the lattice spacing, so as to perform the continuum limit extrapolation with confidence at a few permille accuracy. The degeneracy of the positive and negative parity-state screening masses, expected from Ward identities associated to non-singlet axial transformations, provides further evidence for the restoration of chiral symmetry in the high temperature regime of QCD. In the entire range of temperatures explored, the baryonic masses deviate from the free theory result, $3 \pi T$, by $4$-$8\%$. The contribution due to the interactions is clearly visible up to the highest temperature considered, and cannot be explained by the expected leading behavior in the QCD coupling constant $g$ over the entire range of temperatures explored.

hep-lat↗
arXiv2023

First observation and study of the $K^{\pm} \rightarrow \pi^{0} \pi^{0} \mu^{\pm} \nu$ decay

The NA48/2 experiment at CERN reports the first observation of the $K^{\pm} \rightarrow \pi^{0} \pi^{0} \mu^{\pm} \nu$ decay based on a sample of 2437 candidates with 15% background contamination collected in 2003--2004. The decay branching ratio in the kinematic region of the squared dilepton mass above $0.03$~GeV$^2/c^4$ is measured to be $(0.65 \pm 0.03) \times 10^{-6}$. The extrapolation to the full kinematic space, using a specific model, is found to be $(3.45 \pm 0.16) \times 10^{-6}$, in agreement with chiral perturbation theory predictions.

hep-ex↗
arXiv2022

HIKE, High Intensity Kaon Experiments at the CERN SPS

A timely and long-term programme of kaon decay measurements at a new level of precision is presented, leveraging the capabilities of the CERN Super Proton Synchrotron (SPS). The proposed programme is firmly anchored on the experience built up studying kaon decays at the SPS over the past four decades, and includes rare processes, CP violation, dark sectors, symmetry tests and other tests of the Standard Model. The experimental programme is based on a staged approach involving experiments with charged and neutral kaon beams, as well as operation in beam-dump mode. The various phases will rely on a common infrastructure and set of detectors.

hep-ex↗
arXiv2020

Light Detection System and Time Resolution of the NA62 RICH

A large RICH detector is used in NA62 to suppress the muon contamination in the charged pion sample by a factor of 100 in the momentum range between 15 and 35 GeV/c. Cherenkov light is collected by 1952 photomultipliers placed at the upstream end. In this paper the characterization of the photomultipliers and the dedicated Frontend and Data Acquisition electronics are described, the time resolution and the light detection efficiency measurement are presented.

physics.ins-det↗
arXiv2018

Precise Mirror Alignment and Basic Performance of the RICH Detector of the NA62 Experiment at CERN

The Ring Imaging Cherenkov detector is crucial for the identification of charged particles in the NA62 experiment at the CERN SPS. The detector commissioning was completed in 2016 by the precise alignment of mirrors using reconstructed tracks. The alignment procedure and measurement of the basic performance are described. Ring radius resolution, ring centre resolution, single hit resolution and mean number of hits per ring are evaluated for positron tracks. The contribution of the residual mirror misalignment to the performance is calculated.

physics.ins-det↗
arXiv2013

Measurement of the branching ratio of the decay $Ξ^{0}\rightarrow Σ^{+} μ^{-} \barν_μ$

From the 2002 data taking with a neutral kaon beam extracted from the CERN-SPS, the NA48/1 experiment observed 97 $Ξ^{0}\rightarrow Σ^{+} μ^{-} \barν_μ$ candidates with a background contamination of $30.8 \pm 4.2$ events. From this sample, the BR($Ξ^{0}\rightarrow Σ^{+} μ^{-} \barν_μ$) is measured to be $(2.17 \pm 0.32_{\mathrm{stat}}\pm 0.17_{\mathrm{syst}})\times10^{-6}$.

hep-ex↗
arXiv2012

Topological Lattice Actions for the 2d XY Model

We consider the 2d XY Model with topological lattice actions, which are invariant against small deformations of the field configuration. These actions constrain the angle between neighbouring spins by an upper bound, or they explicitly suppress vortices (and anti-vortices). Although topological actions do not have a classical limit, they still lead to the universal behaviour of the Berezinskii-Kosterlitz-Thouless (BKT) phase transition - at least up to moderate vortex suppression. Thus our study underscores the robustness of universality, which persists even when basic principles of classical physics are violated. In the massive phase, the analytically known Step Scaling Function (SSF) is reproduced in numerical simulations. In the massless phase, the BKT value of the critical exponent eta_c is confirmed. Hence, even though for some topological actions vortices cost zero energy, they still drive the standard BKT transition. In addition we identify a vortex-free transition point, which deviates from the BKT behaviour.

hep-lat↗
arXiv2012

Drastic Reduction of Cutoff Effects in 2-d Lattice O(N) Models

We investigate the cutoff effects in 2-d lattice O(N) models for a variety of lattice actions, and we identify a class of very simple actions for which the lattice artifacts are extremely small. One action agrees with the standard action, except that it constrains neighboring spins to a maximal relative angle delta. We fix delta by demanding that a particular value of the step scaling function agrees with its continuum result already on a rather coarse lattice. Remarkably, the cutoff effects of the entire step scaling function are then reduced to the per mille level. This also applies to the theta-vacuum effects of the step scaling function in the 2-d O(3) model. The cutoff effects of other physical observables including the renormalized coupling and the mass in the isotensor channel are also reduced drastically. Another choice, the mixed action, which combines the standard quadratic with an appropriately tuned large quartic term, also has extremely small cutoff effects. The size of cutoff effects is also investigated analytically in 1-d and at N = infinity in 2-d.

hep-lat↗
arXiv2011

Linear broadening of the confining string in Yang-Mills theory at low temperature

The logarithmic broadening predicted by the systematic low-energy effective field theory for the confining string has recently been verified in numerical simulations of (2+1)-d SU(2) lattice Yang-Mills theory at zero temperature. The same effective theory predicts linear broadening of the string at low non-zero temperature. In this paper, we verify this prediction by comparison with very precise Monte Carlo data. The comparison involves no additional adjustable parameters, because the low-energy constants of the effective theory have already been fixed at zero temperature. It yields very good agreement between the underlying Yang-Mills theory and the effective string theory.

hep-lat↗
arXiv2011

The Width of the Color Flux Tube at 2-Loop Order

The color flux tube connecting a static quark-anti-quark pair in Yang-Mills theory supports massless transverse fluctuations, which are the Goldstone bosons of spontaneously broken translation invariance. Just as in chiral perturbation theory, the dynamics of these Goldstone bosons is described by a systematic low-energy effective field theory. We use the effective theory to calculate the width of the fluctuating string at the 2-loop level, using both cylindrical and toroidal boundary conditions. At zero temperature, the string width diverges logarithmically with the quark-anti-quark distance r. On the other hand, at low but non-zero temperature T = 1/β, for r >> β, the string width diverges linearly.

hep-lat↗
arXiv2010

Topological Lattice Actions

We consider lattice field theories with topological actions, which are invariant against small deformations of the fields. Some of these actions have infinite barriers separating different topological sectors. Topological actions do not have the correct classical continuum limit and they cannot be treated using perturbation theory, but they still yield the correct quantum continuum limit. To show this, we present analytic studies of the 1-d O(2) and O(3) model, as well as Monte Carlo simulations of the 2-d O(3) model using topological lattice actions. Some topological actions obey and others violate a lattice Schwarz inequality between the action and the topological charge Q. Irrespective of this, in the 2-d O(3) model the topological susceptibility χ_t = ł< Q^2 >/V is logarithmically divergent in the continuum limit. Still, at non-zero distance the correlator of the topological charge density has a finite continuum limit which is consistent with analytic predictions. Our study shows explicitly that some classically important features of an action are irrelevant for reaching the correct quantum continuum limit.

hep-lat↗
arXiv2010

String effects in Yang-Mills theory

We study some features of the confining string connecting a quark-anti-quark pair in Yang-Mills theory. Monte Carlo investigations of the flux tube between two static quarks in the fundamental representation show that its thickness increases with the separation of the sources. The collected numerical data in (2+1)-d SU(2) Yang-Mills theory are in very good quantitative agreement with the next-to-leading order formula derived from the systematic low-energy effective theory of the confining string. At zero temperature, we observe the predicted logarithmic broadening. At finite and low temperature, the flux tube thickness is expected to broaden linearly. We also verify that prediction, finding an excellent agreement with the analytic expression. No adjustable parameter has been used to fit the numerical data. Then we investigate the confining strings connecting color sources in larger representations of the gauge group. Concerning stable strings -- the k-strings -- we study the Luscher term of the fundamental string and of the 2-string in (2+1)-d SU(4) Yang-Mills theory. We find that, at large separation between the two static sources, the coefficient of the Luscher term of the 2-string appraoches the value -pi/24. However, at intermediate distances, there are relevant deviations. This result may suggest that, for SU(N) at large N, a different intermediate string regime could set in. For unstable strings, in (2+1)-d SU(2) Yang-Mills theory we investigate the decay between two different string states and the multiple decays of the confining string connecting color sources in large representations. The multiple decays result from the progressive partial screening of the color sources.

hep-lat↗
arXiv2010

The Width of the Confining String in Yang-Mills Theory

We investigate the transverse fluctuations of the confining string connecting two static quarks in (2+1)-d SU(2) Yang-Mills theory using Monte Carlo calculations. The exponentially suppressed signal is extracted from the large noise by a very efficient multi-level algorithm. The resulting width of the string increases logarithmically with the distance between the static quark charges. Corrections at intermediate distances due to universal higher order terms in the effective string action are calculated analytically. They accurately fit the numerical data.

hep-lat↗
arXiv2010

Empirical parameterization of the K+- -> pi+- pi0 pi0 decay Dalitz plot

As first observed by the NA48/2 experiment at the CERN SPS, the $\p0p0$ invariant mass (M00) distribution from $\kcnn$ decay shows a cusp-like anomaly at M00=2m+, where m+ is the charged pion mass. An analysis to extract the pi pi scattering lengths in the isospin I=0 and I=2 states, a0 and a2, respectively, has been recently reported. In the present work the Dalitz plot of this decay is fitted to a new empirical parameterization suitable for practical purposes, such as Monte Carlo simulations of K+- -> pi+- pi0 pi0 decays.

hep-ex↗
arXiv2009

The decay of unstable strings in SU(2) Yang-Mills theory

We investigate the stability of strings connecting charges Q in the representation {2Q+1} of SU(2) Yang-Mills theory in (2+1) dimensions. While the fundamental {2}-string between two charges Q=1/2 is unbreakable and stable, the string connecting static charges transforming under any other representation Q>1/2 is unstable and decays. A charge Q=1 can be completely screened by gluons and so the adjoint {3}-string ultimately breaks. A charge Q=3/2 can be only partially screened to a fundamental charge Q=1/2. Thus, stretching a {4}-string beyond a critical length, it decays into the stable {2}-string by gluon pair creation. The complete breaking of a {5}-string happens in two steps, it first decays into a {3}-string and then breaks completely. A phenomenological constituent gluon model provides a good quantitative description of the energy of the screened charges at the ends of an unstable string.

hep-lat↗
arXiv2009

From Decay to Complete Breaking: Pulling the Strings in SU(2) Yang-Mills Theory

We study {2Q+1}-strings connecting two static charges Q in (2+1)-d SU(2) Yang-Mills theory. While the fundamental {2}-string between two charges Q = 1/2 is unbreakable, the adjoint {3}-string connecting two charges Q = 1 can break. When a {4}-string is stretched beyond a critical length, it decays into a {2}-string by gluon pair creation. When a {5}-string is stretched, it first decays into a {3}-string, which eventually breaks completely. The energy of the screened charges at the ends of a string is well described by a phenomenological constituent gluon model.

hep-lat↗