B. Marsh

Axial channeling radiation from MeV electrons in Si resulting from spontaneous transitions between transverse energy states is studied theoretically and experimentally. Transitions between tightly bound states are well described within the single-string approximation with a thermally averaged Hartree-Fock continuum potential. This applies both for line energies and for absolute intensities as functions of incidence direction for the electron beam. For weakly bound states the extension of the calculations to describe itinerant states in the tight-binding approximation reproduces the band widths obtained from more complete many-beam calculations. For a thick crystal the discrete lines are Lorentzians with widths determined by incoherent scattering, whereas the finite crystal thickness modifies the line shape and the line width in thinner specimens. For tightly bound states the incoherent scattering is due to thermal displacements of atoms, and the formula presented for the scattering probabilities gives satisfactory agreement with observed line widths when correlations of displacements are included.

The orientation dependence of the radiative emission of 150 GeV electrons and positrons incident at small angles with respect to the 〈110〉 axial direction of a thin (0.185 mm) Ge crystal has been observed. The processes are well understood, except for channeled electrons, which radiate unexpected high energy photons.

We report on a search for elementary particles with charges much smaller than the electron charge using a data sample of proton-proton collisions provided by the CERN Large Hadron Collider in 2018, corresponding to an integrated luminosity of 37.5 fb$^{-1}$ at a center-of-mass energy of 13 TeV. A prototype scintillator-based detector is deployed to conduct the first search at a hadron collider sensitive to particles with charges ${\leq}0.1e$. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges between $0.006e$ and $0.3e$, depending on their mass. New sensitivity is achieved for masses larger than $700$ MeV.

We report on the expected sensitivity of dedicated scintillator-based detectors at the LHC for elementary particles with charges much smaller than the electron charge. The dataset provided by a prototype scintillator-based detector is used to characterize the performance of the detector and provide an accurate background projection. Detector designs, including a novel slab detector configuration, are considered for the data taking period of the LHC to start in 2022 (Run 3) and for the high luminosity LHC. With the Run 3 dataset, the existence of new particles with masses between 10 MeV and 45 GeV could be excluded at 95% confidence level for charges between 0.003 e and 0.3 e, depending on their mass. With the high luminosity LHC dataset, the expected limits would reach between 10 MeV and 80 GeV for charges between 0.0018 e and 0.3 e, depending on their mass.Received 14 April 2021Accepted 12 July 2021DOI:https://doi.org/10.1103/PhysRevD.104.032002Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHypothetical particle physics modelsParticle dark matterTechniquesScintillatorsGeneral PhysicsParticles & Fields

Channeling radiation emitted from 2–55 GeV/c electrons and positrons traversing Si and Ge single crystals has been measured for planar directions. The intensity is found to be 30 (Si) to 10 (Ge) times the intensity of normal bremsstrahlung. For channeled electrons, no structure is found in the spectrum, whereas strong and sharp peaks are found for positrons. This peak structure is extremely sharp around a certain “magic” beam momentum between 1 and 10 GeV/c. The spectra are compared to calculated ones based on classical electrodynamics. The agreement is good but for positrons the calculated photon energies are around 5% higher than the measured ones. This discrepancy also exists for practically all previous positron experiments but could be explained in the present investigation by errors in beam momentum calibration. The possibility of subharmonic peaks in the photon spectra is discussed and compared to Monte Carlo calculations. The radiation from {110}, {111} and {100} planes was measured for different momenta between 1 and 10 GeV/c and for different temperatures. Also the thickness dependence of channeling radiation is presented together with a comparison of photon spectra from Si and Ge crystals. The possibility of making a monoenergetic γ-source in the energy region of 1–100 MeV is discussed.

Radiation spectra for GeV electrons and positrons have been measured in a broad range of incidence angles in Si and Ge crystals. An enhancement up to a factor of 50 is found for axially channeled particles. In nearly all cases, the very low-energy part of the photon spectra is strongly reduced either due to equalization in the doughnut or due to the strong multiple scattering from the atomic rows. For large incident angles to the axis, the reduction is due to the so-called Landau-Pomeranchuk effect. For proper channeled positrons, stable oscillations give rise to a peak in the photon spectra. In the transition region from axial to planar channeling, the photon spectra change from being structureless to showing pronounced peaks from the first harmonics. The experimental results are compared to both classical and quantal calculations.

We report on absolute measurements of the orientation dependence of the ${\mathrm{e}}^{+}$-${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ pair production by 20--150-GeV photons aligned or nearly aligned with the 〈110〉 axial direction of a thin Ge crystal. The observed effects are very large, and are well described by theoretical predictions based on the existence of strong macroscopic fields in aligned crystals.

An increase of the ${e}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$ pair-creation rate above the Bethe-Heitler rate has been observed for photons incident along the $〈110〉$ axis of a Ge crystal at 100 K. The enhancement increases with increasing energy and the rate is more than doubled at 100 GeV. The observed features appear to be consistent with the theory of crystal-assisted pair creation, but the magnitude is less than predicted. Pair-creation enhancement is also observed off axis and this is attributed to coherent pair creation.

The electron-positron pair production for a beam of very energetic photons (or ultra-relativistic charged particles) is predicted to be significantly enhanced when the beam is aligned along crystallographic rows of atoms. Channeling in crystals lowers the total energy of the created electron thus enabling the photon to decay in a one-step process. The new mechanism yields a rapidly increasing flux of electrons and positrons of comparable energies above a threshold near 30 GeV.

We have measured the radiation emitted by 150-GeV ${e}^{\ensuremath{-}}$ incident along the $〈110〉$ axis of Ge crystals. The on-axis total radiated energy is 25 times larger than for nonaligned directions for 0.4-mm-thick Ge. The distribution of the radiated energy versus the angle of the electron beam yields a half-width much larger than the channeling critical angle. The on-axis results confirm the predictions of the crystal-assisted radiation theory. The Born approximation to the coherent bremsstrahlung fits the data at large angles.

The emission of radiation from 5 to 55 GeVc planar channeled positrons and electrons passing through a 135 μ thick silicon-crystal has been investigated. The intensity of the channeling-radiation is found to be 10 to 30 times the intensity of normal bremsstrahlung. For channeled electrons no structure is found in the spectrum, whereas strong and sharp peaks are found for positrons. This peak structure is extremely sharp at GeVc and for momenta above 20 GeVc the structure disappears. For a classical description of channeling, but using an anharmonic potential, certain energies are found for which the maximum energy of the channeling radiation is practically independent of transverse energy. The possibility of making a monoenergetic γ-source in the energy range of 10–100 MeV is mentioned.

Photon lines emitted in the forward direction from 2.0–4.5 MeV electrons channeled along a 〈110〉 axis in a 3400 Å Si crystal have been measured and compared with calculations and with previous results from a 12 000 Å Si 〈111〉 crystal. Nearly all features agree with predictions based on an isolated string.

The new technique of Charge Collection Ion Microscopy (CCIM) for investigating defects in semiconductors is described. It utilizes the SUNY/Albany ion microbeam and is based on the collection efficiency of electron-hole (e-h) pairs produced by individual ions penetrating the sample. Compared to the technique of electron beam induced conductivity (EBIC), CCIM produces more damage per projectile but has the compensating advantage that more e-h pairs are created. Analysis of individual pulses rather than an average current therefore is necessary and practicable. Illustrative results are presented for Schottky diodes which have been locally damaged by ion bombardment.

A matrix of twenty 2 × 12 mm2 surface barrier counters has been constructed for use in the image plane of a magnetic spectrometer. The outputs of individual crystals are displayed in predetermined channels of a 400-channel pulse height analyzer. This is done by connecting separate crystals to successive sections of a delay line and converting the time difference between signals arriving at the ends of the line into pulse heights. Simple procedures for producing reliable GeAu surface barriers and contacts and certain properties of such counters are discussed. Samples of data decoded by this unit are presented.

In this LOI we propose a dedicated experiment that would detect milli-charged particles produced by pp collisions at LHC Point 5. The experiment would be installed during LS2 in the vestigial drainage gallery above UXC and would not interfere with CMS operations. With 300 fb$^{-1}$ of integrated luminosity, sensitivity to a particle with charge $\mathcal{O}(10^{-3})~e$ can be achieved for masses of $\mathcal{O}(1)$ GeV, and charge $\mathcal{O}(10^{-2})~e$ for masses of $\mathcal{O}(10)$ GeV, greatly extending the parameter space explored for particles with small charge and masses above 100 MeV.

Crystal-assisted processes around the 〈110〉 axis of a Ge crystal cooled to 100 K have been investigated at CERN with 150 GeV e− and e+ incident beams and 30–150 GeV incident photons. A large enhancement of the radiation emitted by incident e+ and e− is observed. Electrons are found to radiate more than positrons. When the crystal is tilted around 〈110〉 clear channeling effects are still observed. However the radiation enhancement persists at angles much larger than the channeling critical angle. The total pair creation rate by photons aligned along 〈110〉 has also been measured: in contrast with what is observed for a random orientation, the rate increases sharply with the photon energy. A scan through 〈110〉 shows that the pair production does not reach a maximum for axial incidence but at a tilt angle which decreases when the photon energy increases. These measurements indicate that crystal assisted pair production is not governed by the channeling of the created electrons.

Angular distributions have been measured for ${\mathrm{C}}^{13}(d, \ensuremath{\alpha}){\mathrm{B}}^{11}$ (ground state) at 3.35, 3.55, 3.85, and 4.20 MeV. Differential excitation functions for this reaction were measured in 100-keV intervals over the 3.1- to 4.2-MeV region at 54\ifmmode^\circ\else\textdegree\fi{}, 84\ifmmode^\circ\else\textdegree\fi{}, 119\ifmmode^\circ\else\textdegree\fi{}, and 154\ifmmode^\circ\else\textdegree\fi{}. Angular distributions for the ${\mathrm{B}}^{11}({\mathrm{He}}^{3}, p){\mathrm{C}}^{13}$ (ground state) reaction have been measured at 8.6, 9.6, and 10.3 MeV. Differential excitation functions were measured in 200-keV intervals over the 8.0- to 11.0-MeV region at 35\ifmmode^\circ\else\textdegree\fi{} and 140\ifmmode^\circ\else\textdegree\fi{}. The two reactions studied are related in that both involve the transfer of a deuteron between ${\mathrm{B}}^{11}$ and ${\mathrm{C}}^{13}$ in a common momentum transfer region. Notwithstanding, the two reactions appear to proceed predominantly by different mechanisms, viz. compound-nucleus formation and direct process, respectively. The ${\mathrm{C}}^{13}(d, \ensuremath{\alpha}){\mathrm{B}}^{11}$ data are fitted using Legendre polynomials in $cos\ensuremath{\theta}$. The forward portions of angular distributions of ${\mathrm{B}}^{11}({\mathrm{He}}^{3}, p){\mathrm{C}}^{13}$ are fitted using two versions of the plane-wave Born approximation theory.

The experimental results obtained in a high energy experiment (30–200 GeV), with γ, e+, e−, incident around Ge crystallographic directions are presented. The pair creation process is well reproduced by QED in strong field. The radiation process in very strong field is much less understood especially in channeling conditions.

Enhanced radiation of up to 0.4 of the incident beam energy, measured for 4-, 15-, and 17.5-GeV positrons directed along the $〈100〉$ axis of an 80-\ensuremath{\mu}m-thick diamond crystal, is reported for the first time. The results are in excellent agreement with the recently proposed theory of crystal-assisted radiation based on the synchrotron formalism.

In this LOI we propose a dedicated experiment that would detect "milli-charged" particles produced by pp collisions at LHC Point 5. The experiment would be installed during LS2 in the vestigial drainage gallery above UXC and would not interfere with CMS operations. With 300 fb$^{-1}$ of integrated luminosity, sensitivity to a particle with charge $\mathcal{O}(10^{-3})~e$ can be achieved for masses of $\mathcal{O}(1)$ GeV, and charge $\mathcal{O}(10^{-2})~e$ for masses of $\mathcal{O}(10)$ GeV, greatly extending the parameter space explored for particles with small charge and masses above 100 MeV.

Mean values m of photon multiplicity in the radiation of 150 GeV electrons directed at and near the 〈110〉 axis of a 0.185 mm thick Ge crystal cooled to 100 K have been deduced from the measurements of pair conversion probabilities. Depending on the distribution of multiplicity assumed, values of m ranging from 3.8 to 4.3 are obtained for the previously reported anomalous radiation peak.

Abstract Photon lines emitted in the forward direction from 2.0–4.5 MeV electrons channeled along a 〈110〉 axis in a 3400 A Si crystal have been measured and compared with calculations and with previous results from a 12 000 A Si 〈111〉 crystal. Nearly all features agree with predictions based on an isolated string.

The presented document if the Letter of Intent for the Gamma Factory Proof-of-Principle Experiment.

A matrix of twenty 2 x 12 mm/sup 2/ surface bsrrier counters was constructed for use in the image plane of a magnetic spectrometer. The outputs of individual crystals are displayed in predetermined channels of a 100-channel pulseheight analyzer. This is done by connecting separate crystals to successive sections of a delay line and converting the time difference between signals arriving at the ends of the line into pulse heights. Simple procedures for producing reliable Ge-Au surface barriers and contacts and certain properties of such counters are discussed. A sample of information decoded by this unit is presented. (auth)

Radiation spectra measured for 150 GeV positrons directed at or close to the (10) planar direction of a 0.185 mm thick Ge crystal show clear evidence for increasing lower cut-off frequency with decreasing pathlength in the planar electric field. The observed features are well reproduced by the theory of "synchrotronlike" radiation off a continuum planar potential.

Forward radiation spectra of electrons channeled along the 〈111〉 and 〈110〉 axes of a Ni single crystal (5000 Å thick) have been measured with a semiconductor detector for beam energies of 2.2 to 3.0 MeV and various crystal tilt angles θ. In the 〈111〉 case, the 2p-1s transition of channeling states can be identified. At E = 2.96 MeV, the line energy is 4.72 keV with a fwhm width of ~ 50%. The broad linewidth and the absence of a dip in the observed line intensity variation with tilt angle are reflections of large scattering effect. Such scattering effects will be larger for the more tightly bound states such as those in the 〈110〉 direction, and this explains the absence of resolved channeling radiation lines in the measured spectra for 〈110〉.

Angular distributions were measured for C/sup 13/(d alpha )B/sup 11/ ground state reaction at 3.35, 3.55, 3.85, and 4.20 Mev. Differential excitation functions for this reaction were measured in 100-kev intervals over the 3.1 to 4.2-Mev region at 54, 84, 119, and 154 c- . Angular distributions for the B/sup 11/(He/sup 3/,p)C/sup 13/ ground state reaction were measured at were measured in 200-kev intervals over the 8.0 to 11.0Mev region at 35 and 140 c- . The two reactions studied are related in that both involve the transfer of a deuteron between B/sup 11/ and C/sup 13/ in a common momentum transfer region. Notwithstanding, the two reactions appear to proceed predominantly by different mechanisms, viz. compoundnucleus formation and direct process, respectively. The n cos . The forward portions of angular distributions of e plane wave Born approximation theory. (auth)

We report on the measurement of the $^{7}$Be($n, p$)$^{7}$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n\_TOF facility at CERN. This reaction plays a key role in the lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a $^{7}$Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal $^{7}$Li($p, n$)$^{7}$Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+$^{7}$Li reaction is also discussed.

In recent years a great deal of theoretical work has been devoted to the description of the interaction of ultrarelativistic electrons and positrons and high energy photons moving along the major directions of crystals.1–9 It was predicted that quantum electrodynamic (QED) effects must be taken into account. In particular, the perfect alignment of photons with a crystal axis was predicted to increase the pair production rate above the Bethe-Heitler (BH) value.

Author(s): Marsh, Bennett | Advisor(s): Campagnari, Claudio | Abstract: A search for phenomena beyond the Standard Model (BSM) is performed using events with hadronic jets and significant transverse momentum imbalance. The results are based on a sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the Large Hadron Collider in 2016-2018 and corresponding to an integrated luminosity of 137 fb^-1. The search is based on signal regions defined by the hadronic energy in the event, the jet multiplicity, the number of b-tagged jets, and the value of the kinematic variable MT2 for events with at least two jets. For events with exactly one jet, the transverse momentum of the jet is used instead. No significant excess event yield is observed above the predicted Standard Model background. This is used to constrain a range of BSM models that predict the following: the pair production of gluinos and squarks in the context of supersymmetry models conserving R parity; the resonant production of a colored scalar state decaying to a massive Dirac fermion and a quark; and the pair production of scalar and vector leptoquarks each decaying to a neutrino and a top, bottom, or light-flavor quark. In most of the cases, the results obtained are the most stringent constraints to date. The analysis is published in the European Physical Journal C vol. 80, #3.Additionally, the first search at a hadron collider for elementary particles with charges much smaller than the electron charge is presented. These results are based on a sample of proton-proton collisions at a center-of-mass energy of 13 TeV provided by the LHC in 2018, corresponding to an integrated luminosity of 37.5 fb^-1. A prototype scintillator-based detector is deployed near the CMS interaction point to conduct a search sensitive to particles with charges l=0.3e. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges varying between 0.006e and 0.3e, depending on their mass. New sensitivity is achieved for masses larger than 700 MeV.