Searching for confirmation of charm CP violation
in K
S
0
final states at LHCb
Summary
Giulia Tuci
March 31, 2021
The Standard Model of particle physics is the theory describing electroweak and strong in-teractions. Although it has demonstrated huge success providing a very large number of precise predictions, it leaves some phenomena unexplained and falls short of being a complete theory of fundamental interactions. For example it does not fully explain the baryon asymmetry, it does not account for gravitation and it does not explain the nature of dark matter. It is reasonable to believe that the Standard Model is only a low-energy approximation of a more general theory.
Typical ways of putting the Standard Model under test are the production of particles at higher energies in the search of unexpected phenomena, or high-precision measurements of known quantities to compare experimental results with predictions. In the second approach, processes suppressed within the Standard Model are particularly interesting: enhancements of such processes are evidence of contributions from New Physics. The search for CP violation, i.e. the non-invariance of fundamental interactions under the combined symmetry transformations of charge conjugation (C) and parity inversion (P), in the charm sector plays a key role in this kind of research, and has therefore a relevant place in the LHCb physics program.
In the Standard Model, CP violation is described by the presence of a single complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Until now experimental results support the CKM phase, but to explain the cosmological observations on the abundance of matter and anti-matter in the Universe additional terms of CP violation are needed. D0 and ¯D0 mesons are very interesting particles to study because they are the only mesons with an up-type quark as the heavy quark that can oscillate. In fact top quarks decay before hadronizing and π0 and η
mesons, composed of u and ¯u, are their own antiparticle and therefore cannot oscillate. Since, in principle, there might exists some New Physics coupling only to up-type quarks, charm transitions provide a unique opportunity to eventually observe some effects that cannot be explained within the Standard Model.
The experimental study of CP violation in the charm sector is extremely challenging; CP viola-tion is in fact expected to be equal or less than 10−3and therefore huge samples of charm-hadrons decays are needed. For this reason CP violation in charm decays was unobserved until 2019, when LHCb made the first observation using about 100 million of D0→ K+K−and D0→ π+π−decays.
The interpretation of the observed effect is however nontrivial, because of the large theoretical un-certainties on the predicted value. The theoretical description is not straightforward, since the masses of charmed hadrons, O(2 GeV), are in a range where non-perturbative hadronic physics is operative. Current theoretical approaches to charm dynamics are based on establishing relations among decay rates in different modes through amplitudes and phases, allowing the extraction of the theoretical parameters by fitting the measured branching-fractions in several decay channels.
Obviously now it is the moment to start a systematic exploration of all the charm hadrons decay channels, to improve the existing measurement and eventually get for confirmation of CP violation. Because of the size of the expected effect, the D0→ K0
SK 0
S decay channel is very interesting. A
theoretical argument based on the Standard Model predicts in fact an upper limit on CP violation in this channel of 1.1%, but this value could result enhanced because of the interference of New
Physics. In addition, D0→ K0
SKS0 decays provide independent information with respect to the
D0→ π+π− and D0→ K+K−ones, since they are sensitive to a different set of decay amplitudes.
However, selecting K0
S at trigger level in an experiment like LHCb is challenging. KS0particles have
a relatively high lifetime τ ∼ 0.9×10−10s and are produced with a high boost in the beam direction (aligned with the detector). Therefore they often decay outside the vertex detector acceptance, whose information is used to select tracks at the first level of the software trigger. For this reason the amount of D0 → K0
SK 0
S decays collected by LHCb is limited (about 10 thousand using data
collected from 2015 to 2018) with respect to other D0decay channels, and the previously published LHCb measurements were not competitive with the world’s best measurement performed by Belle.
In this thesis I present a new measurement of the CP asymmetry in the D0 → K0 SK
0 S decay
channel, using data collected by LHCb from 2015 to 2018. To take full advantage of the available statistics, offline selections have been carefully studied and some enhancements of the methodology - with the respect to what is usually done in similar analysis at LHCb - have been introduced. Thanks to these improvements, I was able to reduce the uncertainty on the 2015 and 2016 data, already used by the LHCb collaboration in the last publication regarding this measurement, by ∼ 30%, and to perform the world’s best measurement using the full 2015-2018 dataset.
Looking ahead, in 2022 LHCb will start to acquire data with an almost completely new detector and trigger system. The instantaneous luminosity will increase by a factor of 5 and the amount of data collected by LHCb will jump from ∼ 9 fb−1 (collected up to now) to ∼ 50 fb−1in 2030. Since the measurement of CP asymmetry in D0→ K0
SKS0 decays is limited by the available statistics,
there is a large room for improvement. In addition to the increase of luminosity, there is an ongoing effort to increase the trigger efficiency for this channel, both introducing specific trigger lines at the first level of the software trigger, optimised to select KS0 decaying inside the vertex detector, and developing a specialised tracking device to reconstruct and trigger at the earliest possible stage on KS0 decaying outside the vertex detector acceptance.
Find tracks outside the vertex detector at the earliest trigger level is in fact not part of the baseline trigger scheme, on account of the significant CPU time required to execute the search. Not having access to this information limits efficiency for decay modes that cannot easily be triggered through another signature, as the D0 → K0
SKS0 one. The same is true for decays involving Λ
baryons and long-lived exotic particles. A system of specialised processors may be used to find rapidly the downstream tracks through look-up tables, and present these tracks to the software trigger in parallel with all the raw detector information in the event. Such a system is a large and ambitious project, which targets future upgrades of LHCb. However, the feasibility of the system can be demonstrated realising a smaller and cost-effective demonstrator, which performs the tracking of the vertex detector. In this thesis I therefore also describe the work that I have done in developing a software emulator for such a system, fine-tuning the algorithm and studying its performance on simulated data. The system has been approved to run parasitically during the next data-acquisition run of LHCb.
