New articles on High Energy Physics - Experiment


[1] 2208.08209

$\mathrm{t\bar{t}H/tH}$ production at CMS

After the Higgs boson discovery, a main focus at the CERN LHC has been the measurement of its properties. Observing the Higgs boson associated production with a top quark-antiquark pair ($\mathrm{t\bar{t}H}$) is particularly interesting because it provides tree-level access to measuring the Higgs boson-top quark Yukawa coupling. However, the production of a single top quark in association with the Higgs boson ($\mathrm{tH}$) is also sensitive to the sign of this coupling. This coupling is essential for the stability of the Higgs potential at high energy scales and can also be a probe for physics beyond the standard model (SM). The latest measurements of the $\mathrm{t\bar{t}H}$ and $\mathrm{tH}$ associated production rates performed by the CMS Collaboration with proton-proton collision events at $\sqrt{s} =13$ TeV in the diphoton and multilepton channels are presented here. Events are categorized according to the lepton and jet multiplicities, and multivariate classifiers are used to distinguish signal from background processes. The results are consistent with the SM predictions and feature the first observation of $\mathrm{t\bar{t}H}$ production in a single Higgs boson decay channel along with the first tests of its CP structure.


[2] 2208.08214

Precision luminosity measurement with proton-proton collisions at the CMS experiment in Run 2

A precision luminosity measurement is essential for LHC cross-section measurements to determine fundamental parameters of the standard model and constrain or discover beyond-the-standard-model phenomena. The luminosity of the CMS detector has been measured at the LHC Interaction Point 5 using proton-proton collisions at $\sqrt{s}=13$ TeV during the Run 2 data-taking period (2015-2018). The absolute luminosity scale is obtained using beam-separation scans and the Van der Meer (VdM) method, and several systematic uncertainty sources are investigated, from the knowledge of the scale of beam separation provided by LHC magnets to the nonfactorizability of the spatial components of proton bunch density distributions in the transverse direction. When the VdM calibration is applied to the entire data-taking period, the detector linearity and stability measurements contribute significantly to the total uncertainty in the integrated luminosity. In 2016-2018, the reported integrated luminosity was among the most precise measurements at bunched-beam hadron colliders.


[3] 2208.08250

QCD at the Future Circular $\rm{e^{+}e^{-}}$ Collider

The Future Circular Collider in the $\mathrm{e^{+}e^{-}}$ configuration offers the opportunity to significantly improve SM measurements with dedicated runs at the Z-pole, WW threshold, ZH (240 GeV), and $\mathrm{t}\bar{\mathrm{t}}$ threshold. With a factor of approximately $10^{5}$ more statistics at the Z-pole and $10^{4}$ at the WW threshold than at LEP, the FCC-ee will enable the extraction of the strong coupling $\alpha_{s}$ at the per-mille level. Parton showering studies exploiting the pure gluon sample from $\mathrm{ZH}({\rightarrow} \mathrm{gg})$ will greatly improve our understanding of gluon radiation and fragmentation, directly impacting quark vs gluon discrimination studies. Further possibilities include precision hadronization studies and colour reconnection studies at the WW threshold. Some elements of the rich precision QCD program of the FCC-ee are outlined below.


[4] 2208.07910

When, Where, and How to Open Data: A Personal Perspective

This is a personal perspective on data sharing in the context of public data releases suitable for generic analysis. These open data can be a powerful tool for expanding the science of high energy physics, but care must be taken in when, where, and how they are utilized. I argue that data preservation even within collaborations needs additional support in order to maximize our science potential. Additionally, it should also be easier for non-collaboration members to engage with collaborations. Finally, I advocate that we recognize a new type of high energy physicist: the 'data physicist', who would be optimally suited to analyze open data as well as develop and deploy new advanced data science tools so that we can use our precious data to their fullest potential. This document has been coordinated with a white paper on open data commissioned by the American Physical Society's (APS) Division of Particles and Field (DPS) Community Planning Exercise ('Snowmass') Theory Frontier [1] and relevant also for the Computational Frontier.


[5] 2208.07945

Nuclear-level Effective Theory of Muon-to-Electron Conversion

New mu-to-e conversion searches aim to advance limits on charged lepton flavor violation (CLFV) by four orders of magnitude. By considering P and CP selection rules and the structure of possible charge and current densities, we show that rates are governed by six nuclear responses. To generate a microscopic formulation of these responses, we construct in non-relativistic effective theory (NRET) the CLFV nucleon-level interaction, then embed it in a nucleus. We discuss previous work, noting the lack of a systematic treatment of the various small parameters. Because the momentum transfer is comparable to the inverse nuclear size, a full multipole expansion of the response functions is necessary, a daunting task with Coulomb-distorted electron partial waves. We perform such an expansion to high precision by introducing a simplifying local electron momentum, treating the full set of 16 NRET operators. Previous work has been limited to the simplest charge/spin operators, ignored Coulomb distortion (or alternatively truncated the partial wave expansion) and the nucleon velocity operator, which is responsible for three of the response functions. This generates inconsistencies in the treatment of small parameters. We obtain a "master formula" for mu-to-e conversion that properly treats all such effects and those of the muon velocity. We compute muon-to-electron conversion rates for a series of experimental targets, deriving bounds on the coefficients of the CLFV operators. We discuss the nuclear physics: two types of coherence enhance certain CLFV operators and selection rules blind elastic mu-to-e conversion to others. We discuss the matching of the NRET onto higher level EFTs, and the relation to mu-to-e conversion to other CLFV tests. Finally we describe a publicly available script that can be used to compute mu-to-e conversion rates in nuclear targets.


[6] 2208.07953

Opportunities for theory studies with public collider data: Snowmass 2021

Over the last 20+ years, experimentalists have presented tantalizing hints of physics beyond the standard model, but nothing definitive. With the wealth of data from experiments, in particular the collider experiments, it is imperative that the community leave no reasonable model untested and no search unsought. Open datasets from particle physics experiments provide a relatively new and exciting opportunity to extend the reach of these searches by bringing in additional personpower in the form of the theory community. Analysis of these datasets also provides the opportunity for an increased information flow between theorists and experimentalists, an activity which can only benefit the entire field. This paper discusses the potential of this effort, informed by the successes of the last 5 years in the form of results produced by theorists making use of open collider data, primarily the datasets released by the CMS collaboration. Concerns about the potential negative impact on the field are also discussed. For a more detailed accounting of these concerns, see Ref. [1] of the bibliography.


[7] 2208.08050

Neutrinos in Stellar Astrophysics

The physics of the mysterious and stealthy neutrino is at the heart of many phenomena in the cosmos. These particles interact with matter and with each other through the aptly named weak interaction. At typical astrophysical energies the weak interaction is some twenty orders of magnitude weaker than the electromagnetic interaction. However, in the early universe and in collapsing stars neutrinos can more than make up for their feeble interaction strength with huge numbers. Neutrinos can dominate the dynamics in these sites and set the conditions that govern the synthesis of the elements. Here we journey through the history of the discovery of these particles and describe their role in stellar evolution and collapse, the big bang, and multi-messenger astrophysics. Neutrino physics is at the frontier of elementary particle physics, nuclear physics, astrophysics and cosmology. All of these fields overlap in the neutrino story.


[8] 2208.08294

Silicon sensors with resistive read-out: ML techniques for ultimate spatial resolutionSilicon sensors with resistive read-out: ML techniques for ultimate spatial resolution

Resistive Silicon Detectors (RSDs) are based on the LGAD technology, characterized by a continuous gain layer, and by the innovative introduction of resistive read-out. Thanks to a novel electrode design aimed at maximizing signal sharing, the second FBK production of RSD sensors, RSD2, achieves a position resolution on the whole pixel surface of about 8 $\mu m$ for 200-$\mu m$ pitch. RSD2 arrays have been tested in the Laboratory for Innovative Silicon Sensors in Torino using a Transient Current Technique setup equipped with a 16-channel digitizer, and results on spatial resolution have been obtained with machine learning algorithms.


[9] 2208.08323

Unraveling Gluon Jet Quenching through $J/ψ$ Production in Heavy-Ion Collisions

Jet quenching has long been regarded as one of the key signatures for the formation of quark-gluon plasma in heavy-ion collisions. Despite significant efforts, the separate identification of quark and gluon jet quenching has remained as a challenge. Here we show that $J/\psi$ in high transverse momentum ($p_T$) region provides a uniquely sensitive probe of in-medium gluon energy loss since its production at high $p_T$ is particularly dominated by gluon fragmentation. Such gluon-dominance is first demonstrated for the baseline of proton-proton collisions within the framework of leading power NRQCD factorization formalism. We then use the linear Boltzmann transport model combined with hydrodynamics for the simulation of jet-medium interaction in nucleus-nucleus collisions. The satisfactory description of experimental data on both nuclear modification factor $R_{AA}$ and elliptic flow $v_2$ reveals, for the first time, that the gluon jet quenching is the driving force for high $p_T$ $J/\psi$ suppression. This novel finding is further confirmed, in a robust and model-independent way, by the data-driven Bayesian analyses of relevant experimental measurements, from which we also obtain the first quantitative extraction of the gluon energy loss distribution in the quark-gluon plasma.


[10] 2208.08372

Evidence for intrinsic charm quarks in the proton

The theory of the strong force, quantum chromodynamics, describes the proton in terms of quarks and gluons. The proton is a state of two up quarks and one down quark bound by gluons, but quantum theory predicts that in addition there is an infinite number of quark-antiquark pairs. Both light and heavy quarks, whose mass is respectively smaller or bigger than the mass of the proton, are revealed inside the proton in high-energy collisions. However, it is unclear whether heavy quarks also exist as a part of the proton wavefunction, which is determined by non-perturbative dynamics and accordingly unknown: so-called intrinsic heavy quarks. It has been argued for a long time that the proton could have a sizable intrinsic component of the lightest heavy quark, the charm quark. Innumerable efforts to establish intrinsic charm in the proton have remained inconclusive. Here we provide evidence for intrinsic charm by exploiting a high-precision determination of the quark-gluon content of the nucleon based on machine learning and a large experimental dataset. We disentangle the intrinsic charm component from charm-anticharm pairs arising from high-energy radiation. We establish the existence of intrinsic charm at the 3-standard-deviation level, with a momentum distribution in remarkable agreement with model predictions. We confirm these findings by comparing to very recent data on Z-boson production with charm jets from the LHCb experiment.