The CMS experiment at the LHC relies on 7 Tier-1 centres of the WLCG to perform the majority of its bulk processing activity, and to archive its data. During the first run of the LHC, these two ...functions were tightly coupled as each Tier-1 was constrained to process only the data archived on its hierarchical storage. This lack of flexibility in the assignment of processing workflows occasionally resulted in uneven resource utilisation and in an increased latency in the delivery of the results to the physics community. The long shutdown of the LHC in 2013-2014 was an opportunity to revisit this mode of operations, disentangling the processing and archive functionalities of the Tier-1 centres. The storage services at the Tier-1s were redeployed breaking the traditional hierarchical model: each site now provides a large disk storage to host input and output data for processing, and an independent tape storage used exclusively for archiving. Movement of data between the tape and disk endpoints is not automated, but triggered externally through the WLCG transfer management systems. With this new setup, CMS operations actively controls at any time which data is available on disk for processing and which data should be sent to archive. Thanks to the high-bandwidth connectivity guaranteed by the LHCOPN, input data can be freely transferred between disk endpoints as needed to take advantage of free CPU, turning the Tier-1s into a large pool of shared resources. The output data can be validated before archiving them permanently, and temporary data formats can be produced without wasting valuable tape resources. Finally, the data hosted on disk at Tier-1s can now be made available also for user analysis since there is no risk any longer of triggering chaotic staging from tape. In this contribution, we describe the technical solutions adopted for the new disk and tape endpoints at the sites, and we report on the commissioning and scale testing of the service. We detail the procedures implemented by CMS computing operations to actively manage data on disk at Tier-1 sites, and we give examples of the benefits brought to CMS workflows by the additional flexibility of the new system.
The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20-170 GeV. The experimental arrangement ...allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.
An overwhelming number of theoretical predictions for hadron colliders require parton distribution functions (PDFs), which are an important ingredient of theory infrastructure for the next generation ...of high-energy experiments. Here this whitepaper summarizes the status and future prospects for determination of high-precision PDFs applicable in a wide range of energies and experiments, in particular in precision tests of the Standard Model and in new physics searches at the high-luminosity Large Hadron Collider and Electron–Ion Collider. We discuss the envisioned advancements in experimental measurements, QCD theory, global analysis methodology, and computing that are necessary to bring unpolarized PDFs in the nucleon to the N2LO and N3LO accuracy in the QCD coupling strength. Special attention is given to the new tasks that emerge in the era of the precision PDF analysis, such as those focusing on the robust control of systematic factors both in experimental measurements and theoretical computations. Various synergies between experimental and theoretical studies of the hadron structure are explored, including opportunities for studying PDFs for nuclear and meson targets, PDFs with electroweak contributions or dependence on the transverse momentum, for incisive comparisons between phenomenological models for the PDFs and computations on discrete lattice, and for cross-fertilization with machine learning/AI approaches.
The CERN-NA-59 experiment examined a wide range of electromagnetic processes for multi-GeV electrons and photons interacting with oriented single crystals. The various types of crystals and their ...orientations were used for producing photon beams and for converting and measuring their polarisation.
The radiation emitted by 178GeV unpolarised electrons incident on a 1.5cm thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised photon beams.
A new crystal polarimetry technique was established for measuring the linear polarisation of the photon beam. The polarimeter is based on the dependence of the coherent pair production (CPP) cross section in oriented single crystals on the direction of the photon polarisation with respect to the crystal plane. Both a 1mm thick single crystal of Germanium and a 4mm thick multi-tile set of synthetic Diamond crystals were used as analyzers of the linear polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the photon beam into circular polarisation, was observed. This was achieved by letting the linearly polarised photon beam pass through a 10cm thick Silicon single crystal that acted as a “quarter wave plate” (QWP) as suggested by Cabibbo et al.
Experimental results for pair production by 20–150 GeV photons in a Ge crystal cooled to 100 K are presented. The results include total and differential enhancements for incidence of the photons ...along the (1
1
0) plane with angles 0–4.5 mrad to the
〈1
1
0〉
axis. For the first time, a comparison can be made with theoretical results which for incidence along the plane predict pronounced structural changes of a resonance type in the pair production yield as a function of angle to the axis. These changes originate from the coherent interaction with the strong field of the atomic strings in the crystal. Furthermore, results for pair production by 5–55 GeV photons in W and Ir crystals at temperatures between 100 and 300 K are presented. These results are for total enhancements with photons incident along the strongest axes in these materials, i.e. the
〈1
1
1〉
axis in W and the
〈1
1
0〉
axis in Ir. A clear disagreement with theoretical curves calls for improved calculations.
A method for photon linear polarization determination based on the measurement of the asymmetry of pairs produced by polarized photons in single crystals within the optimal intervals of pair particle ...energies is proposed. In difference to the well-known methods the asymmetry in this case is essentially larger. The optimal orientation of crystal is found, which provides the maximal values for analyzing power and figure of merit as well as minimal measurement time.
The distribution of the energy released in a silicon detector placed on the downstream side of thick germanium single crystals bombarded by 20–180 GeV gamma rays along directions close to the
〈1
1
0〉
...axis or along a random direction has been investigated. A large enhancement of the shower for axial incidence of the gamma rays has been found. The response of the system composed of a germanium crystal and a silicon detector to single gamma rays as a function of their energy has been deduced and compared with existing Monte Carlo simulations.
To carry out experimental investigation of radiation of positrons in crystalline undulators at the Yerevan synchrotron it is necessary to produce 0.2–2 GeV low intensity positron beams with moderate ...energy (1–5%) and small angular (∼10
−4 rad) spread on the basis of existing extracted 4.5 GeV/c electron beam line. As the results of Monte Carlo calculations show, the use of coherent Bremsstrahlung in crystalline targets instead of amorphous targets allows us to enhance the positron yield a few times. The dependence of the efficiency of positron production on the target material, thickness, orientation as well as parameters of the positron beam at various energies are presented. Such a method of direct high-energy positron beam production is simple since it does not require further acceleration of low energy positrons.
PDF
