Riassunto Tesi Laurea Magistrale
Measurement of time-integrated CP asymmetries
in D
0
→ K
S
0
K
S
0
decays
Candidato: Giulia Tuci
Relatori : Prof. Giovanni Punzi, Dott. Simone Stracka, Dott. John Walsh
The Standard Model (SM) of particle physics is the theory describing electroweak and strong interactions. Although the SM 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 SM is only a low-energy approximation of a more general theory.
A way to test the SM is to perform more precise measurement of known quantities and compare experimental results with predictions. Of particular interest are processes suppressed within the SM: enhancements of such processes are evidence of contribution of New Physics. The search for CP violation in charm sector plays a key role in this kind of research, and has therefore a relevant place in the high energy physics program.
In the SM CP violation can be explained 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. In particular in the charm sector CP violation has not yet been discovered, although time-integrated CP asymmetries in Singly-Cabibbo-Suppressed D0 → h+h− decays have reached a remarkable precision, O(0.1%).
This work is about the experimental measurement of a very promising channel for the discovery of CP violation: D0 → K0
SKS0. There are two amplitudes which contribute to this decay and
they are both suppressed. Therefore, because of the absence of a dominant amplitude, ACP could
be enhanced also within the SM. In fact a phenomenological SM prediction gives an upper limit for CP asymmetry of 1.1% (C.L. 95%), contrary to what happen in Singly-Cabibbo-Suppressed decays, where expectations are less than 10−3 and totally dominated by theoretical uncertainties. Therefore for the D0 → K0
SKS0 channel it’s very important to improve the experimental precision
of the measurement and go down the threshold of 10−2. Currently the world’s best measurement is the one from BELLE, who measured ACP = (−0.02 ± 1.53 ± 0.17)%. The BELLE detector
was a hermetic multilayer particle detector, located at the collision point of the asymmetric-energy electron-positron collider KEKB, and was built to study B-physics. The upgrade of the experiment, BELLE II, will start operation probably by the end of 2018. The second best existing measurement is from LHCb Run-1 and it is equal to ACP = (−2.9 ± 5.2 ± 2.2)%. The LHCb
detector is a single-arm forward spectrometer, located at the LHC accelerator at CERN, covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. LHCb is currently taking data in its Run-2 during which it will almost duplicate the integrated luminosity collected in the first Run.
In this thesis I have performed a time-integrated measurement of CP asymmetries in D0 → K0 SKS0
decays using 2015 and 2016 LHCb Run-2 data. The trigger selection is different with respect to
Run-1 and I have dealt with the new dataset to improve the previous results.
In such a measurement, because of the slow mixing rate of D0 meson, it is possible to
approx-imate
ACP ' Adir =
|Af|2− | ¯Af¯|2
|Af|2+ | ¯Af¯|2
,
where Af is the amplitude of the decay D0 → f and ¯Af¯ is the amplitude of the decay ¯D0 → ¯f .
To determine the flavour of the D0 decays are selected using D∗+ → D0π+decays (and the charge
conjugate process). The K0
S are reconstructed from the final state π+π
−. The quantity that is
really measured in LHCb is
Araw =
ND0 − ND¯0
ND0 + ND¯0
,
that is not equal to ACP, because of the production asymmetry Aprod of the D∗ meson and the
detection asymmetry Adet of the tag pion πtag. When Aprod and Adet are small, it’s possible to
approximate Araw ' ACP + Aprod+ Adet.
The strategy to remove these additional asymmetry terms is to measure ACP(D0 → KS0KS0)
with respect to the control channel D0 → K+K−, for which A
CP is measured very precisely
(uncertainties are O(10−3)). In this way the poorly known Aprod and Adet are cancelled. The
yield extraction is done by performing a fit to the m(D∗) − m(D0) distribution. This is a common
approach because the resolution on m(D∗) − m(D0) is ' 10 times smaller than the resolution on
m(D∗).
The reconstruction of neutral and long-lived particles in LHCb involves some challenges. In fact the trigger is not efficient because some requirements, that are usually helpful to separate signal from background, reject a lot of signal events in these particular decays. The D0 → K0
SK 0 S
decay falls within this kind of events. Therefore also if the number of charm meson produced in pp collision is high (σ(pp → D0X,√s = 13 TeV) ∼ 2 mb) the number of signal events in the final
sample is only ∼ 1000.
KS0 has an high mean life τ ∼ 0.9 × 10−10s, therefore it can decay inside the vertex detector, after this detector but before the magnet or after the magnet. In the first case we refer to K0 S
as “long KS0 ”, in the second case we to KS0 as “downstream KS0 ”and in the third case LHCb is not able to reconstruct the particles. Downstream KS0 are reconstructed only in the final stage of the trigger. The two kind of K0
S are separately analysed due to the difference in their mass
resolution.
Because of the small statistics, the main part of the analysis consists in identify and analyse all possible background sources in order to reduce them efficiently and to achieve a good sensitivity on the final measurement. The main background contribution in this channel is combinatorial background, which consists of random combinations of pions and/or KS0 that result in a fake D0 candidate within the selected m(D∗) − m(D0) mass window. However the most dangerous
back-ground sources are the ones that peak in m(D∗) − m(D0) distribution and which can therefore introduce a bias in the measured asymmetry. In particular in our sample we have contributions from D0 → K0
Sπ+π
−, where the two prompt pions are wrongly assigned to a fake K0
S, and from
secondary decays, i.e. decays where the D∗ comes from a b-hadron decay. The latter are charac-terised by the same CP asymmetry of signal events, but by a different production asymmetry due to the contribution of b-hadron production asymmetry. To suppress combinatorial background we rely on a k-NN (k-Nearest-Neighbour) multivariate classifier, based on the kinematics of the final state.
A preliminary estimate of systematic uncertainties has been done. Main systematic contribu-tions come from the remaining background events that could add a bias on the ACP value.
The final results, still blind waiting for collaboration approval, is
ACP = (xx ± 3.3 ± 1.0)%,
where the first uncertainty is statistic and the second is systematic.
With this measurement the statistical uncertainty of Run-1 result is improved by a factor of ∼ 1.6, while the systematic is improved by a factor of ∼ 2. This is yet enough to improve on the current world average.
In addition this study has allowed to make progress with the trigger performance considerably for 2017 and 2018 data taking, and a first look to 2017 data has shown an increase of the signal yield per fb−1 by a factor of 2. This is promising for a measurement with the full Run-2 sample to improve on the current world average and potentially be sensitive to CP -violating effects.
