arXiv subjects
Publications and source records attributed to Spencer R. Klein.
Ultra-peripheral collisions (UPCs) involving heavy ions and protons are the energy frontier for photon-mediated interactions. UPC photons can be used for many purposes, including probing low-$x$ gluons via photoproduction of dijets and vector mesons, probes of beyond-standard-model processes, such as those enabled by light-by-light scattering, and studies of two-photon production of the Higgs.
Silicon tracking detectors provide excellent spatial resolution, and so can provide excellent momentum resolution for energetic charged particles, even in compact detectors. However, at lower momenta, multiple scattering in the silicon degrades the momentum resolution. We present an alternate method to measure momentum and alleviate this degradation, using silicon detectors that also incorporate timing measurements. By using timing information between two silicon layers, it is possible to solve for the the radius of curvature, and hence the particle momentum, independent of multiple scattering within the silicon. We consider three examples: an all-silicon central tracker for an electron-ion collider, a simplified version of the CMS detector, and a forward detector for an electron-ion collider. For a 75 cm diameter tracker in a 1.5 T magnetic field, timing can improve the momentum determination for particles with momentum below 500 MeV/c. In the 3.8 T CMS magnetic field and 1.2 m radius tracker, timing can improve tracking up to momenta of 1.3 GeV/c. The resolution is best at mid-rapidity. We also discuss a simpler system, consisting of a single timing detector outside an all-silicon tracker.
Neutrino telescopes like IceCube, KM3NeT and Baikal-GVD offer physicists the opportunity to study neutrinos with energies far beyond the reach of terrestrial accelerators. These neutrinos are used to study high-energy neutrino interactions and to probe the Earth through absorption tomography. Current studies of TeV neutrinos use cross sections which are calculated for free nucleons with targets which are assumed to contain equal numbers of protons and neutrons. Here we consider modifications of high-energy neutrino interactions due to two nuclear effects: modifications of the parton densities in the nucleus, referred to here as shadowing, and the effect of non-isoscalar targets, with unequal numbers of neutrons and protons. Both these effects depend on the interaction medium. Because shadowing is larger for heavier nuclei, such as iron, found in the Earth's core, it introduces a zenith-angle dependent change in the absorption cross section. These modifications increase the cross sections by 1-2\% at energies below 100 TeV (antishadowing), and reduce it by 3-4\% at higher energies (shadowing). Nuclear effects also alter the inelasticity distribution of neutrino interactions in water/ice by increasing the number of low inelasticity interactions, with a larger effect for $ν$ than $\barν$. These effects are particularly large in the energy range below a few TeV. These effects could alter the cross sections inferred from events with tracks originating within the active detector volume as well as the ratio $ν/\barν$ inferred from inelasticity measurements. The uncertainties in these nuclear effects are larger than the uncertainties on the free-proton cross sections and will thus limit the systematic precision of future high-precision measurements at neutrino telescopes.
In the 1930's, nuclear physicists developed the first realistic atomic models, showing that nuclei were made up of protons and neutrons. In the 1960's, Deep Inelastic Scattering experiments showed that protons and neutrons had internal structure: quarks and gluons (collectively, partons), and later experiments showed that the parton momentum distributions are different in heavy nuclei, compared to those in free nucleons. This difference is not surprising; partons are sensitive to their environment, and two gluons from different nucleons may fuse together, for example. Understanding how quarks and gluons behave in the nuclear environment is a significant focus of modern nuclear physics. Recent measurements have provided us with an improved understanding of how quark and gluon densities are altered in heavy nuclei. We have also begun to make multi-dimensional pictures of the nucleus, exploring how these alterations are distributed within heavy nuclei. We naturally expect these modifications to be largest in the core of a nucleus, and smaller near its periphery; this can change the effective shape of the nucleus. We have also started to explore the transverse momentum distribution of the partons in the nuclei, and, using incoherent photoproduction as a probe, study event-by-event fluctuations in nucleon and nuclei parton densities. This article will explore recent progress in measurements of nuclear structure at high energy, with some emphasis on these multi-dimensional pictures. We will also discuss how a future electron-ion collider (EIC) with high luminosity and center-of-mass energy will make exquisitely detailed images of partons in a nucleus.
Astrophysical and atmospheric neutrinos are important probes of the powerful accelerators that produce cosmic-rays with EeV energies. Understanding these accelerators is a key goal of neutrino observatories, along with searches for neutrinos from supernovae, from dark matter annihilation, and other astrophysics topics. Here, we discuss how neutrino observatories like IceCube and future facilities like KM3NeT and IceCube-Gen2 can study the properties of high-energy (above 1 TeV) neutrino interactions. This is far higher than is accessible at man-made accelerators, where the highest energy neutrino beam reached only 500 GeV. In contrast, neutrino observatories have observed events with energies above 5 PeV - 10,000 times higher in energy - and future large observatories may probe neutrinos with energies up to $10^{20}$ eV. These data have implications for both Standard Model measurements, such as of low Bjorken$-x$ parton distributions and gluon shadowing, and also for searches for beyond Standard Model physics. This chapter will review the existing techniques and results, and discuss future prospects.
The 1 km$^3$ IcCube neutrino observatory was built to find high-energy neutrinos that are associated with the sources of ultra-high energy cosmic rays. Its 5,160 optical sensors detect Cherenkov light from the charged particles produced when neutrinos interact in the ice. In this talk, I will describe the techniques that IceCube has used to search for astrophysical neutrinos. An emphasis will be given to diffuse neutrinos (not associated with a specific source), including analyses of contained events and energetic through-going neutrinos. At the end, I will discuss multi-messenger astronomy, and present an intriguing correlation between a high-energy IceCube neutrino and a blazar in outburst.
Tasks involving the analysis of geometric (graph- and manifold-structured) data have recently gained prominence in the machine learning community, giving birth to a rapidly developing field of geometric deep learning. In this work, we leverage graph neural networks to improve signal detection in the IceCube neutrino observatory. The IceCube detector array is modeled as a graph, where vertices are sensors and edges are a learned function of the sensors' spatial coordinates. As only a subset of IceCube's sensors is active during a given observation, we note the adaptive nature of our GNN, wherein computation is restricted to the input signal support. We demonstrate the effectiveness of our GNN architecture on a task classifying IceCube events, where it outperforms both a traditional physics-based method as well as classical 3D convolution neural networks.
Although they are best known for studying astrophysical neutrinos, neutrino telescopes like IceCube can study neutrino interactions, at energies far above those that are accessible at accelerators. In this writeup, I present two IceCube analyses of neutrino interactions at energies far above 1 TeV. The first measures neutrino absorption in the Earth, and, from that determines the neutrino-nucleon cross-section at energies between 6.3 and 980 TeV. We find that the cross-sections is 1.30 $^{+0.21}_{-0.19}$ (stat.) $^{+0.39}_{-0.43}$ (syst.) times the Standard Model cross-section. We also present a measurement of neutrino inelasticity, using $ν_μ$ charged-current interactions that occur within IceCube. We have measured the average inelasticity at energies from 1 TeV to above 100 TeV, and found that it is in agreement with the Standard Model expectations. We have also performed a series of fits to this track sample and a matching cascade sample, to probe aspects of the astrophysical neutrino flux, particularly the flavor ratio.
In "$η_c$ production in photon-induced interactions at the LHC," \cite{Goncalves:2018yxc} Goncalves and Moreira discuss inclusive and exclusive $η_c$ production at $pp$ and $pA$ collisions at LHC energies. The exclusive channels are via two-photon and photon-Odderon interactions. This comment points out that there is a large additional source of almost-exclusive $η_c$ in ultra-peripheral collisions: from the radiative decay of $J/ψ$ that are produced in photon-nucleon interactions. Although the $J/ψ\rightarrowγη_c$ branching ratio is small, the $J/ψ$ production cross-section is large enough that it dominates over the exclusive channels considered in \cite{Goncalves:2018yxc}, and is comparable to the non-exclusive production. In $J/ψ\rightarrowγη_c$, the photon is very soft and therefore easy to miss, and the $η_c$ will have very similar kinematics to the $J/ψ$.
Coherent photoproduction of vector mesons is sensitive to the shape of the target nucleus, as probed at four-momentum scale $Q^2 \propto (M_V/2)^2$. Previously STAR presented a high-statistics measurement of $dσ_c/dt$ for coherent $π^+π^-$ photoproduction in ultra-peripheral gold-gold collisions, and made a two-dimensional Fourier-Bessel (Hanckel) transformation to give the transverse distribution of interactions in the nucleus. Here, we study how $dσ_c/dt$ evolves with $Q^2$. We divide the $π^+π^-$ signal into three different mass ($Q^2$) bins to measure how $dσ_c/dt$ evolves with dipion mass. Furthermore, we find that the depth of the first diffractive minimum varies with pair mass. We perform a two-dimensional Fourier-Bessel transform to see how the effective transverse distribution of the interactions changes with decreasing pair mass. In the lowest mass bin, the nuclear profile is broader, which is consistent with expectations from saturation models.
The STAR collaboration has observed an excess production of $e^+e^-$ pairs in relativistic heavy ion collisions, over the expectations from hadronic production models. The excess pairs have transverse momenta $p_T < 150\ {\rm MeV}/c$ and are most prominent in peripheral gold-gold and uranium-uranium collisions. The pairs exhibit a peak at the $J/ψ$ mass, but include a wide continuum, with pair invariant masses from 400 MeV/c$^2$ up to 2.6 GeV/c$^2$. The ALICE Collaboration observes a similar excess in peripheral lead-lead collisions, but only at the $J/ψ$ mass, without a corresponding continuum. This paper presents a calculation of the cross-section and kinematic for two-photon production of $e^+e^-$ pairs, and find general agreement with the STAR data. The calculation is based on the STARlight simulation code, which is based on the Weizsäcker-Williams virtual photon approach. The STAR continuum observations are compatible with two-photon production of $e^+e^-$ pairs. The ALICE analysis required individual muon $p_T$ be greater than 1 GeV/c; this eliminated almost all of the pairs from two-photon interactions, while leaving most of the $J/ψ$ decays.
Ultra-peripheral collisions are the energy frontier for photon-mediated interactions, reaching, at the Large Hadron Collider (LHC), $γ-p$ center of mass energies five to ten times higher than at HERA and reaching $γγ$ energies higher than at LEP. Photoproduction of heavy quarkonium and dijets in $pp$ and $pA$ collisions probes the gluon distribution in protons at Bjorken-$x$ values down to $3\times10^{-6}$, far smaller than can be otherwise studied. In $AA$ collisions, these reactions probe the gluon distributions in heavy ions, down to $x$ values of a few $10^{-5}$. Although more theoretical work is needed to nail down all of the uncertainties, inclusion of these data in current parton distribution function fits would greatly improve the accuracy of the gluon distributions at low Bjorken-$x$ and low/moderate $Q^2$. High-statistics $ρ^0$ data probe the spatial distribution of the interaction sites; the site distribution is given by the Fourier transform of $dσ/dt$. After introducing UPCs, this review presents recent measurements of dilepton production and light-by-light scattering and recent data on proton and heavy nuclei structure, emphasizing results presented at Quark Matter 2017 (QM2017).
Ultra-peripheral collisions (UPCs) have been a significant source of study at RHIC and the LHC. In these collisions, the two colliding nuclei interact electromagnetically, via two-photon or photonuclear interactions, but not hadronically; they effectively miss each other. Photonuclear interactions produce vector meson states or more general photonuclear final states, while two-photon interactions can produce lepton or meson pairs, or single mesons. In these interactions, the collision geometry plays a major role. We present a program, STARlight, that calculates the cross-sections for a variety of UPC final states and also creates, via Monte Carlo simulation, events for use in determining detector efficiency.
In 2010, the STAR Collaboration collected 38 million low-multiplicity triggers for ultra-peripheral collision studies. We present new results involving photonuclear interactions in ultra-peripheral relativistic heavy ion collisions (UPCs), based on an analysis of 384,000 photoproduced pion pairs, from $ρ$, $ω$ and direct $π^+π^-$ pair production.The $ω$ component is clearly visible through its interference with the $ρ$ peak. The relative amplitudes of the three components have been measured, along with the phase angle between the $ρ$ and $ω$ components. Using a two-year (2010 and 2011) dataset, we explore higher mass final states, and observe a $π^+π^-$ state with a mass of $1653 \pm 10$ MeV and a width of $164\pm 15$ MeV (statistical errors only). The state is at least roughly compatible with the $ρ_3(1690)$. The $ρ^0$ squared momentum transfer ($t$) spectrum exhibits a spectrum with both coherent and incoherent prodution. The coherent component shows two characteristic diffraction minima, with positions that are sensitive to the hadronic radius of the gold nucleus.
Ultra-peripheral collisions (UPCs) of ions allow us to study photonuclear and two-photon interactions at energies above those available at fixed target accelerators. For heavy ions, the couplings are large enough so that multi-photon interactions are possible, and higher order corrections are expected to be significant. In this writeup, I present some recent UPC results from the Relativistic Heavy Ion Collider (RHIC), and discuss some future prospects. I also draw parallels between UPC data and that expected at an electron-ion collider (EIC), and show how UPCs are a useful lead-in to EIC physics. This writeup is based on a talk at "Initial State 2014," (IS2014), with a focus on the newest results. One important result is that comparison of the RHIC (and LHC) results on coherent $ρ^0$ photoproduction show evidence for nuclear suppression, compared to a calculating based on $γp$ cross-sections.
Atmospheric neutrinos are an important background to astrophysical neutrino searches, and are also of considerable interest in their own right. This paper points out that the contribution to conventional atmospheric $ν_e$ of the rare semileptonic decay of $K_S$ becomes significant at high energy. Although the $K_S\rightarrow πeν$ branching ratio is very small, the short $K_S$ lifetime leads to a high critical energy, so that, for vertical showers, the inclusion of $K_S$ semileptonic decay increases the conventional $ν_e$ flux by $\approx 30%$ at energies above 100 TeV. In this paper, we present calculations of the flux of $ν_e$ from $K_S$. At energies above their critical energies, the $ν_e$ fluxes from kaon decay may be simply related to the kaon semileptonic widths; this leads to a near-equality between the flux of $ν_e$ from $K^+$, $K_L$ and $K_S$.
There are currently several proposals to build a high-luminosity electron-ion collider, to study the spin structure of matter and measure parton densities in heavy nuclei, and to search for gluon saturation and new phenomena like the colored glass condensate. These measurements require operation with heavy-nuclei. We calculate the cross-sections for two important processes that will affect accelerator and detector operations: bound-free pair production, and Coulomb excitation of the nuclei. Both of these reactions have large cross-sections, 28-56 mb, which can lead to beam ion losses, produce beams of particles with altered charge:mass ratio, and produce a large flux of neutrons in zero degree calorimeters. The loss of beam particles limits the sustainable electron-ion luminosity to levels of several times $10^{32}/$cm$^2$/s.
We discuss the acceleration of secondary muons, pions, and kaons in gamma-ray bursts within the internal shock scenario, and their impact on the neutrino fluxes. We introduce a two-zone model consisting of an acceleration zone (the shocks) and a radiation zone (the plasma downstream the shocks). The acceleration in the shocks, which is an unavoidable consequence of the efficient proton acceleration, requires efficient transport from the radiation back to the acceleration zone. On the other hand, stochastic acceleration in the radiation zone can enhance the secondary spectra of muons and kaons significantly if there is a sufficiently large turbulent region. Overall, it is plausible that neutrino spectra can be enhanced by up to a factor of two at the peak by stochastic acceleration, that an additional spectral peaks appears from shock acceleration of the secondary muons and pions, and that the neutrino production from kaon decays is enhanced. Depending on the GRB parameters, the general conclusions concerning the limits to the internal shock scenario obtained by recent IceCube and ANTARES analyses may be affected by up to a factor of two by secondary acceleration. Most of the changes occur at energies above 10^7 GeV, so the effects for next-generation radio-detection experiments will be more pronounced. In the future, however, if GRBs are detected as high-energy neutrino sources, the detection of one or several pronounced peaks around 10^6 GeV or higher energies could help to derive the basic properties of the magnetic field strength in the GRB.