Monte Carlo event generators contain a large number of parameters that must be determined by comparing the output of the generator with experimental data. Generating enough events with a fixed set of ...parameter values to enable making such a comparison is extremely CPU intensive, which prohibits performing a simple brute-force grid-based tuning of the parameters. Bayesian optimization is a powerful method designed for such black-box tuning applications. In this article, we show that Monte Carlo event generator parameters can be accurately obtained using Bayesian optimization and minimal expert-level physics knowledge. A tune of the PYTHIA 8 event generator using e+e− events, where 20 parameters are optimized, can be run on a modern laptop in just two days. Combining the Bayesian optimization approach with expert knowledge should enable producing better tunes in the future, by making it faster and easier to study discrepancies between Monte Carlo and experimental data.
In this paper, we describe the potential of the LHCb experiment to detect stealth physics. This refers to dynamics beyond the standard model that would elude searches that focus on energetic objects ...or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.
The LHCb simulation application. Gauss, consists or two independent phases, the generation of the primary event and the tracking of particles produced in the experimental setup. For the LHCh ...experimental program it is particularly important to model B meson decays: the EvtGcn code developed in CLEO and BaBah has been chosen and customized for non-coherent B production as occurring in pp collisions at the LHC, The initial proton-proton collision is provided by a different generator engine, currently PYTHIA 6 for massive production of signal and generic pp collisions events. Beam gas events, background events originating from proton halo, cosmics and calibration events for different detectors can be generated in addition to pp collisions. Different generator packages as available in the physics community or specifically developed in LHCb are used for the different purposes. Running conditions affecting the events generated such as the size of the luminous region, the number of collisions occuring in a bunch crossing and the number of spill-over events from neighbouring bunches are modeled via dedicated algorithms appropriately configured. The design of the generator phase of Gauss will be described: a modular structure with well defined interfaces specific to the various tasks, e.g. pp collisions, particles' decays, selections, etc. has been chosen. Different implementations are available for the various tasks allowing selecting and combining them as most appropriate at run time as in the case of Pythia 6 im pp collisions or HIJING for beam gas. The advantages of such structure, allowing for example to adopt transparently new generators packages will be discussed.
Precise knowledge of the location of the material in the LHCb vertex locator (VELO) is essential to reducing background in searches for long-lived exotic particles, and in identifying jets that ...originate from beauty and charm quarks. Secondary interactions of hadrons produced in beam-gas collisions are used to map the location of material in the VELO. Using this material map, along with properties of a reconstructed secondary vertex and its constituent tracks, a p-value can be assigned to the hypothesis that the secondary vertex originates from a material interaction. A validation of this procedure is presented using photon conversions to dimuons.
The data sample of Λ0b→J/ψpK− decays acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3 fb−1, is inspected for the presence of J/ψp or ...J/ψK− contributions with minimal assumptions about K−p contributions. It is demonstrated at more than nine standard deviations that Λ0b→J/ψpK− decays cannot be described with K−p contributions alone, and that J/ψp contributions play a dominant role in this incompatibility. These model-independent results support the previously obtained model-dependent evidence for P+c→J/ψp charmonium-pentaquark states in the same data sample.
A full amplitude analysis of Λ0b→J/ψpπ− decays is performed with a data sample acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3 fb−1. A ...significantly better description of the data is achieved when, in addition to the previously observed nucleon excitations N→pπ−, either the Pc(4380)+ and Pc(4450)+→J/ψp states, previously observed in Λ0b→J/ψpK− decays, or the Zc(4200)−→J/ψπ− state, previously reported in B0→J/ψK+π− decays, or all three, are included in the amplitude models. The data support a model containing all three exotic states, with a significance of more than three standard deviations. Within uncertainties, the data are consistent with the Pc(4380)+ and Pc(4450)+ production rates expected from their previous observation taking account of Cabibbo suppression.
In the Standard Model of particle physics, the strength of the couplings of the b quark to the u and c quarks, |Vub| and |Vcb|, are governed by the coupling of the quarks to the Higgs boson. Using ...data from the LHCb experiment at the Large Hadron Collider, the probability for the Λb0 baryon to decay into the p final state relative to the final state is measured. Combined with theoretical calculations of the strong interaction and a previously measured value of |Vcb|, the first |Vub| measurement to use a baryonic decay is performed. This measurement is consistent with previous determinations of |Vub| using B meson decays to specific final states and confirms the existing incompatibility with those using an inclusive sample of final states.
A search for CP violation in D-0 -> K-K+ and D-0 -> pi(-)pi(+) decays is performed using pp collision data, corresponding to an integrated luminosity of 3 fb(-1), collected using the LHCb detector at ...center-of-mass energies of 7 and 8 TeV. The flavor of the charm meson is inferred from the charge of the pion in D*(+) -> D-0 pi(+) and D*(-) -> (D) over bar (0)pi(-) decays. The difference between the CP asymmetries in D-0 -> K-K+ and D-0 -> pi(-)pi(+) decays, Delta A(CP) A(CP)(K-K+) - A(CP)(pi(-)pi(+)), is measured to be -0.10 +/- 0.08(stat) +/- 0.03(syst)%. This is the most precise measurement of a time-integrated CP asymmetry in the charm sector from a single experiment.
The LHCb collaboration has redesigned its trigger to enable the full offline detector reconstruction to be performed in real time. Together with the real-time alignment and calibration of the ...detector, and a software infrastructure to make persistent the high-level physics objects produced during real-time processing, this redesign enabled the widespread deployment of real-time analysis during Run 2. We describe the design of the Run 2 trigger and real-time reconstruction, and present data-driven performance measurements for a representative sample of LHCb's physics programme.
The production of the
η
c
(
1
S
)
state in proton-proton collisions is probed via its decay to the
p
p
¯
final state with the LHCb detector, in the rapidity range
2.0
<
y
<
4.5
and in the meson ...transverse-momentum range
p
T
>
6.5
GeV
/
c
. The cross-section for prompt production of
η
c
(
1
S
)
mesons relative to the prompt
J
/
ψ
cross-section is measured, for the first time, to be
σ
η
c
(
1
S
)
/
σ
J
/
ψ
=
1.74
±
0.29
±
0.28
±
0
.
18
B
at a centre-of-mass energy
s
=
7
TeV
using data corresponding to an integrated luminosity of 0.7 fb
-
1
, and
σ
η
c
(
1
S
)
/
σ
J
/
ψ
=
1.60
±
0.29
±
0.25
±
0
.
17
B
at
s
=
8
TeV
using 2.0 fb
-
1
. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the
η
c
(
1
S
)
and
J
/
ψ
decays to the
p
p
¯
final state. In addition, the inclusive branching fraction of
b
-hadron decays into
η
c
(
1
S
)
mesons is measured, for the first time, to be
B
(
b
→
η
c
X
)
=
(
4.88
±
0.64
±
0.29
±
0
.
67
B
)
×
10
-
3
, where the third uncertainty includes also the uncertainty on the
J
/
ψ
inclusive branching fraction from
b
-hadron decays. The difference between the
J
/
ψ
and
η
c
(
1
S
)
meson masses is determined to be
114.7
±
1.5
±
0.1
MeV
/
c
2
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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