Searcharxiv⌕ Search

arXiv subjects

J. Schacher

Publications and source records attributed to J. Schacher.

At least 19 recordsLinked to original sources
arXiv2024

The π^+π^- Coulomb interaction study and its use in the data processing

In this work the Coulomb effects (Coulomb correlations) in $π^+π^-$ pairs produced in p + Ni collisions at 24 GeV/$c$, are studied using experimental $π^+π^-$ pair distributions in $Q$, the relative momentum in the pair center of mass system (c.m.s), and its projections $Q_L$ (longitudinal component) and $Q_t$ (transverse component) relative to the pair direction in the laboratory system (l.s.). The $Q$, $Q_L$, and $Q_t$ distributions of the {\sl Coulomb pairs} in the c.m.s. have been simulated assuming they are described by the phase space modified by the known point-like Coulomb correlation function $A_C(Q)$, corrected for small effects due to the nonpoint-like pair production and the strong two-pion interaction. The same distributions of {\sl non-Coulomb pairs} have been simulated according to the phase space, but without $A_C(Q)$. It is shown that the number of {\sl Coulomb pairs} in all $Q_t$ intervals, including the small $Q_t$ (small opening angles $θ$ in the l.s.) is calculated with the theoretical precision better than 2\%. The comparison of the simulated and experimental numbers of {\sl Coulomb pairs} at small $Q_t$ allows us to check and correct the detection efficiency for the pairs with small $θ$ (0.06 mrad and smaller). It is shown that {\sl Coulomb pairs} can be used as a new physical tool to check and correct the quality of the simulated events. The special property of the {\sl Coulomb pairs} is the possibility of checking and correcting the detection efficiency, especially for the pairs with small opening angles.

hep-ph↗
arXiv2022

Investigation of $K^+K^-$ pairs in the effective mass region near $2m_K$

The DIRAC experiment at CERN investigated in the reaction $\rm{p}(24~\rm{GeV}/c) + Ni$ the particle pairs $K^+K^-, π^+ π^-$ and $p \bar{p}$ with relative momentum $Q$ in the pair system less than 100 MeV/c. Because of background influence studies, DIRAC explored three subsamples of $K^+K^-$ pairs, obtained by subtracting -- using time-of-flight (TOF) technique -- background from initial $Q$ distributions with $K^+K^-$ sample fractions more than 70\%, 50\% and 30\%. The corresponding pair distributions in $Q$ and in its longitudinal projection $Q_L$ were analyzed first in a Coulomb model, which takes into account only Coulomb final state interaction (FSI) and assuming point-like pair production. This Coulomb model analysis leads to a $K^+K^-$ yield increase of about four at $Q_L=0.5$ MeV/c compared to 100 MeV/c. In order to study contributions from strong interaction, a second more sophisticated model was applied, considering besides Coulomb FSI also strong FSI via the resonances $f_0(980)$ and $a_0(980)$ and a variable distance $r^*$ between the produced $K$ mesons. This analysis was based on three different parameter sets for the pair production. For the 70\% subsample and with best parameters, $3680\pm 370$ $K^+K^-$ pairs was found to be compared to $3900\pm 410$ $K^+K^-$ extracted by means of the Coulomb model. Knowing the efficiency of the TOF cut for background suppression, the total number of detected $K^+K^-$ pairs was evaluated to be around $40000\pm 10\%$, which agrees with the result from the 30\% subsample. The $K^+K^-$ pair number in the 50\% subsample differs from the two other values by about three standard deviations, confirming -- as discussed in the paper -- that experimental data in this subsample is less reliable.

hep-ex↗
arXiv2019

Summary Report of Physics Beyond Colliders at CERN

Physics Beyond Colliders is an exploratory study aimed at exploiting the full scientific potential of CERN's accelerator complex and its scientific infrastructure in the next two decades through projects complementary to the LHC, HL-LHC and other possible future colliders. These projects should target fundamental physics questions that are similar in spirit to those addressed by high-energy colliders, but that require different types of beams and experiments. A kick-off workshop held in September 2016 identified a number of areas of interest and working groups have been set-up to study and develop these directions. All projects currently under consideration are presented including physics motivation, a brief outline of the experimental set-up and the status of the corresponding beam and detector technological studies. The proposals are also put in context of the worldwide landscape and their implementation issues are discussed.

hep-ex↗
arXiv2018

First measurement of a long-lived $π^+ π^-$ atom lifetime

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like $π^+π^-$ atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96~mm and got broken up in the 2.1~\textmu{}m thick platinum foil. Analysing the observed number of atomic pairs, $n_A^L= \left.436^{+157}_{-61}\right|_\mathrm{tot}$, the lifetime of the 2$p$ state is found to be ${τ_{2p}=(\left.0.45^{+1.08}_{-0.30}\right|_\mathrm{tot}) \cdot10^{-11}}$s, not contradicting the corresponding QED $2p$ state lifetime ${τ_{2p}^\mathrm{QED}=1.17 \cdot 10^{-11}}$s. This lifetime value is three orders of magnitude larger than our previously measured value of the $π^+π^-$ atom ground state lifetime $τ=(\left.3.15^{+0.28}_{-0.26}\right|_\mathrm{tot})\cdot 10^{-15}$s. Further studies of long-lived $π^+π^-$ atoms will allow to measure energy differences between $p$ and $s$ atomic states and so to determine $ππ$ scattering lengths with the aim to check QCD predictions.

hep-ex↗
arXiv2017

Measurement of the $πK$ atom lifetime and the $πK$ scattering length

After having announced the statistically significant observation (5.6~$σ$) of the new exotic $πK$ atom, the DIRAC experiment at the CERN proton synchrotron presents the measurement of the corresponding atom lifetime, based on the full $πK$ data sample: $τ= (5.5^{+5.0}_{-2.8}) \cdot 10^{-15}s$. By means of a precise relation ($<1\%$) between atom lifetime and scattering length, the following value for the S-wave isospin-odd $πK$ scattering length $a_0^{-}~=~\frac{1}{3}(a_{1/2}-a_{3/2})$ has been derived: $\left|a_0^-\right| = (0.072^{+0.031}_{-0.020}) M_π^{-1}$.

hep-ex↗
arXiv2014

First $πK$ atom lifetime and $πK$ scattering length measurements

The results of a search for hydrogen-like atoms consisting of $π^{\mp}K^{\pm}$ mesons are presented. Evidence for $πK$ atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic $πK$ pairs from their breakup in the same target ($178 \pm 49$) and from Coulomb final state interaction ($653 \pm 42$). Using these results the analysis yields a first value for the $πK$ atom lifetime of $τ=(2.5_{-1.8}^{+3.0})$ fs and a first model-independent measurement of the S-wave isospin-odd $πK$ scattering length $\left|a_0^-\right|=\frac{1}{3}\left|a_{1/2}-a_{3/2}\right|= \left(0.11_{-0.04}^{+0.09} \right)M_π^{-1}$ ($a_I$ for isospin $I$).

nucl-ex↗
arXiv2011

Determination of $ππ$ scattering lengths from measurement of $π^+π^-$ atom lifetime

The DIRAC experiment at CERN has achieved a sizeable production of $π^+π^-$ atoms and has significantly improved the precision on its lifetime determination. From a sample of 21227 atomic pairs, a 4% measurement of the S-wave $ππ$ scattering length difference $|a_0-a_2| = (.0.2533^{+0.0080}_{-0.0078}|_\mathrm{stat}.{}^{+0.0078}_{-0.0073}|_\mathrm{syst})M_{π^+}^{-1}$ has been attained, providing an important test of Chiral Perturbation Theory.

hep-ex↗
arXiv2009

Evidence for $πK$-atoms with DIRAC

We present evidence for the first observation of electromagnetically bound $π^\pm K^\mp$-pairs ($πK$-atoms) with the DIRAC experiment at the CERN-PS. The $πK$-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the $π^\pm$ and $K^\mp$-mesons from the atom dissociation are analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173 $\pm$ 54 $πK$-atoms. The mean life of $πK$-atoms is related to the s-wave $πK$-scattering lengths, the measurement of which is the goal of the experiment. From these first data we derive a lower limit for the mean life of 0.8 fs at 90% confidence level.

hep-ex↗
arXiv2005

HadAtom05, Workshop on Hadronic Atoms

These are the proceedings of the workshop "HadAtom05", held at the University of Bern, Switzerland, February 15 and 16, 2005. The main topics of the workshop concerned the physics of hadronic atoms, and in this context recent results from experiments and theory were presented. These proceedings contain the list of participants, the scientific program and a short contribution from each speaker.

hep-ph↗
arXiv2004

HadAtom03

These are the proceedings of the workshop "HadAtom03," held at the European Centre for Theoretical Nuclear Physics and Related Studies (ECT*), Trento (Italy), October 13 - 17, 2003. The main topics discussed at the workshop were the physics of hadronic atoms and in this context recent results in experiment and theory. Included here are the list of participants, the scientific program and a short contribution from each speaker.

hep-ph↗
arXiv2003

HadAtom02

These are the proceedings of the workshop "HadAtom02", held at the CERN, October 14 - 15, 2002. The main topic of the workshop concerned the physics of hadronic atoms and in this contest recent results from experiments and theory were presented. These proceedings contain the list of participants, the scientific program and a short contribution from each speaker.

hep-ph↗
arXiv2001

HadAtom01

These are the proceedings of the workshop "HadAtom01", held at the Institut fuer Theoretische Physik, Universitaet Bern, October 11 - 12, 2001. The main topics discussed at the workshop were the physics of hadronic atoms and in this context recent results in experiment and theory. Included here are the list of participants, the scientific program and a short contribution from each speaker.

hep-ph↗
arXiv2000

Antimatter and Matter Production in Heavy Ion Collisions at CERN (The NEWMASS Experiment NA52)

Besides the dedicated search for strangelets NA52 measures light (anti)particle and (anti)nuclei production over a wide range of rapidity. Compared to previous runs the statistics has been increased in the 1998 run by more than one order of magnitude for negatively charged objects at different spectrometer rigidities. Together with previous data taking at a rigidity of -20 GeV/c we obtained 10^6 antiprotons 10^3 antideuterons and two antihelium3 without centrality requirements. We measured nuclei and antinuclei (p,d,antiprotons, antideuterons) near midrapidity covering an impact parameter range of b=2-12 fm. Our results strongly indicate that nuclei and antinuclei are mainly produced via the coalescence mechanism. However the centrality dependence of the antibaryon to baryon ratios show that antibaryons are diminished due to annihilation and breakup reactions in the hadron dense environment. The volume of the particle source extracted from coalescence models agrees with results from pion interferometry for an expanding source. The chemical and thermal freeze-out of nuclei and antinuclei appear to coincide with each other and with the thermal freeze-out of hadrons.

nucl-ex↗
arXiv2000

Centrality dependence of K+ produced in Pb+Pb collisions at 158 GeV per nucleon

The NA52 collaboration searches for a discontinuous behaviour of charged kaons produced in Pb+Pb collisions at 158 A GeV as a function of the impact parameter, which could reveal a hadron to quark-gluon plasma (QGP) phase transition. The K+ yield is found to grow proportional to the number of participating ('wounded') nucleons N, above N=100. Previous NA52 data agree with the above finding and show a discontinuous behaviour in the kaon centrality dependence near N=100, marking the onset of strangeness enhancement -over e.g. p+A data at the same \sqrt{s}- in a chemically equilibrated phase.

hep-ex↗
arXiv2000

Centrality dependence of K+ produced in Pb+Pb collisions at 158 GeV per nucleon

The NA52 collaboration searches for a discontinuous behaviour of charged kaons produced in Pb+Pb collisions at 158 A GeV as a function of the impact parameter, which could reveal a hadron to quark-gluon plasma (QGP) phase transition. The K+ yield is found to grow proportional to the number of participating ('wounded') nucleons N, above N=100. Previous NA52 data agree with the above finding and show a discontinuous behaviour in the kaon centrality dependence near N=100, marking the onset of strangeness enhancement -over e.g. p+A data at the same \sqrt{s}- in a chemically equilibrated phase.

hep-ex↗
arXiv1999

HadAtom99

These are the proceedings of the workshop "HadAtom99", held at the Institut fuer Theoretische Physik, Universitaet Bern, October 14-15, 1999. The main topics discussed at the workshop were the physics of hadronic atoms and in this context recent results from Chiral Perturbation Theory. Included here are the program, the list of participants and a short summary provided by the speakers.

hep-ph↗