The LHCb vertex locator upgrade — the detector calibration overview

The Vertex Locator (VELO) is a silicon tracking detector in the spectrometer of the Large Hadron Collider beauty (LHCb) experiment. LHCb explores and investigates CP violation phenomena in b- and c- hadron decays and is one of the experiments operating on the Large Hadron Collider (LHC) at CERN. After run 1 and run 2 of LHC data taking (2011–2018), the LHCb detectors are being modernized within the LHCb upgrade I program. The upgrade aims to adjust the spectrometer to readout at full LHC 40 MHz frequency, which requires radical changes to the technologies currently used in LHCb. The hardware trigger is removed, and some of the detectors replaced. The VELO changes its tracking technology and silicon strips are replaced by 55 μm pitch silicon pixels. The readout chip for the VELO upgrade is the VeloPix ASIC. The number of readout channels increases to over 40 million, and the hottest ASIC is expected to produce the output data rate of 15 Gbit/s. New conditions challenge the software and the hardware side of the readout system and put special attention on the detector monitoring. This paper presents the upgraded VELO design and outlines the software aspects of the detector calibration in the upgrade I. An overview of the challenges foreseen for the upgrade II is given.

Direct CP violation and the ΔI = 1/2 rule in K → ππ decay in the Standard Model

We discuss the RBC & UKQCD collaborations’ recent [1] lattice calculation of ϵ′, the measure of direct CP-violation in kaon decays. This result significantly improves on our previous 2015 calculation, with nearly 4× the statistics and more reliable systematic error estimates. We discuss how our results demonstrate the Standard Model origin of the ΔI = 1/2 rule, and present our plans for future calculations.

CP violation in charm

The existence of CP violation in the decays of strange and beauty mesons is very well established experimentally. On the contrary, CP violation in the decays of charmed particles has been elusive for a long time and has been observed for the first time in 2019 by the LHCb experiment. Since then several studies have been performed in the charm sector. During the LHC Run 1 and Run 2, the LHCb collaboration has collected large samples containing charm hadron decays, on a scale never seen before. Collected data enabled physicists to obtain several new results, most of which surpassed previous results and became new world’s best measurements. Presently the LHCb spectrometer is being upgraded to enhance readout system, improve subdetector components and increase integrated luminosity to 50 fb−1 by the end of Run 4.

On the effective lifetime of Bs → μμγ

We consider the $$ {B}_s^0 $$ B s 0 → μ+μ−γ effective lifetime, and the related CP-phase sensitive quantity $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ , as a way to obtain qualitatively new insights on the current B-decay discrepancies. Through a fit comparing pre- to post-Moriond-2021 data we identify a few theory benchmark scenarios addressing these discrepancies, and featuring large CP violation in addition. We then explore the possibility of telling apart these scenarios with $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ , once resonance-modeling and form-factor uncertainties are taken into account. We do so in both regions of low and high invariant di-lepton mass-squared q2. For low q2, we show how to shape the integration range in order to reduce the impact of the ϕ-resonance modelling on the $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ prediction. For high q2, we find that the corresponding pollution from broad-charmonium resonances has a surprisingly small effect on $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ . This is due to a number of cancellations, that can be traced back to the complete dominance of semi-leptonic operator contributions for high q2 — at variance with low q2 — and to $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ behaving like a ratio-of-amplitudes observable. Our study suggests that $$ {A}_{{\Delta \Gamma}_s}^{\mu \mu \gamma} $$ A ΔΓ s μμγ is — especially at high q2 — a potentially valuable probe of short-distance CP-violating effects in the very same Wilson coefficients that are associated to current b → s discrepancies. Its discriminating power, however, relies on progress in form-factor uncertainties. Interestingly, high q2 is the region where $$ {B}_s^0 $$ B s 0 → μ+μ−γ is already being accessed experimentally, and the region where form factors are more accessible through non-perturbative QCD methods.