# The LHCb Upgrade

Lars Eklund, on behalf of the LHCb Collaboration

###### Abstract

The LHCb Experiment is preparing a detector upgrade fully exploit the flavour
physics potential of the LHC. The whole detector will be read out at the full
collision rate and the online event selection will be performed by a software
trigger. This will increase the event yields by a facto $10$ for muonic and a
factor $20$ for hadronic final states. Research towards the upgrade has
started with the target to install the detector in 2018.

###### Keywords:

flavour physics, instrumentation, detector upgrade

##  1 Introduction

The LHCb Experiments Alves Jr. et al. (2008) is one of the four main
experiments at the LHC. LHCb is designed for precision measurements of CP
violation and rare decays of $b$ and $c$ flavoured hadrons, but has broadened
its programme to a wide range particle physics topics. Its forward
spectrometer geometry enables world leading measurements both within and
outwith the original core programme.

LHCb has successfully collected collision data since 2010 and has to date
collected more than $2~{}\rm{fb^{-1}}$ of integrated luminosity. More than 92
% of the delivered luminosity is recorded with the detector fully operational
and 99 % of the recorded data passes the quality criteria for use in offline
analyses.

The experiment covers the pseudo-rapidity region of $2<\rm{\eta}<5$ and is
fully instrumented in the whole acceptance range. LHCb profits from the very
large heavy flavour production cross sections at LHC. Moreover, the majority
of the $b$ and $c$ flavoured hadrons are produced in the very forward region.
Even though LHCb only covers 2 % of the stereo angle, 27 % of the $b\bar{b}$
pairs are produced within its acceptance. The production cross sections within
the detector acceptance are $\sigma_{b\bar{b}}=75.3~{}\rm{\mu b}$ Aaij et al.
(2010a) and $\sigma_{c\bar{c}}=1.23~{}\rm{mb}$ Aaij et al. (2010b) for proton
collisions at $7~{}\rm{TeV}$.

Hence LHCb has access to a sample of heavy flavour decays of unprecedented
size. As an illustration, approximately $8\cdot 10^{10}$ $b\bar{b}$ pairs were
produced in the LHCb acceptance in the 2011 run ($1~{}\rm{fb^{-1}}$ integrated
luminosity), which is more than 50 times larger than the total number of
$b\bar{b}$ pairs produced in the B-factories (Belle and BaBar) combined. This
comes at the price of a more challenging experimental environment. The total
cross section Antchev et al. (2011) is several orders of magnitude larger than
the $b\bar{b}$ cross section, hence the signal candidates have to be selected
among a large number of background events. Moreover, the track multiplicity
for events containing the signal candidates is larger than in the B-Factories.
Hence the event selection is crucial for the performance of the experiment, in
particular the online selection performed by the trigger system.

LHCb has shown that it performs very well in these challenging experimental
conditions and that high precision measurements are possible at at a hadron
collider. This has already resulted in a large number of publications, many of
them representing world best measurements or first observations.

LHCb is currently operating at a luminosity of $4\cdot
10^{32}~{}\rm{cm^{-2}s^{-1}}$, which is twice the design value but still
significantly below what LHC could provide. A scheme of luminosity levelling
has been devised where the beams are slightly displaced from head-on
collisions and gradually re-centred during the fill as the beam intensity is
reduced. This allows the experiment to collect data at a constant luminosity
throughout the fill. Hence LHCb is not limited by the luminosity that the LHC
can deliver, but rather by the luminosity that could be turned in to high
precision measurements.

The aim of the LHCb Upgrade Aaij et al. (2012) is to build an experiment that
can operate at a higher luminosity while maintaining or increasing the
efficiency of selecting signal candidates. The details of the upgraded
experiment are outlined in this paper.

##  2 The Current and Upgraded Trigger

The current online event selection (trigger) consists of two levels. A first
level (L0) is is selecting events with large energy depositions and high
momentum muon tracks using information from the calorimeters and muon system
only. It is implemented in hardware and is reducing the rate from the up to
$30~{}\rm{MHz}$ collision rate to approximately $900~{}\rm{kHz}$ event rate.
The total latency for the L0 trigger decision is $4~{}\rm{\mu s}$. The full
detector information is read out for the selected events and sent to the High
Level Trigger (HLT) system which is software based and implemented in a CPU
farm. The events are fully reconstructed and those containing candidates
matching inclusive and exclusive signal criteria are written to tape for
offline analysis. The output event rate of the HLT is approximately
$4.5\rm{kHz}$.

The goal for the LHCb upgrade is to operate the detector at a luminosity of
$1-2\cdot 10^{33}~{}\rm{cm^{-2}s^{-1}}$ to collect up to $50~{}\rm{fb^{-1}}$
during 5 years of data taking. Unfortunately the signal yields would saturate
if the luminosity were increased to these levels with the current experiment.
This is due to the limited information and short time available for the L0
trigger decision. The effect is particularly prominent for signal with
hadronic final states, as the muonic triggers scale better with increased
luminosity.

Hence the strategy of the LHCb upgrade is to remove the L0 trigger and move to
a fully software based trigger with the full event information available for
all events. The aim is to reduce the $30~{}\rm{MHz}$ collision rate to
$20~{}\rm{kHz}$ event rate written to tape. The option of a Low Level Trigger
(LLT) is currently being investigated, running a fast event selection using
non-CPU based processing based on for instance digital signal processors or
graphical processing units. This would allow a gradual increase of the input
rate to the HLT as the available computing resources increase, finally
reaching the full $30~{}\rm{MHz}$ rate. The goal is to increase the yield of
muonic channels with a factor $10$ and for hadronic channels with a factor
$20$ compared to the current experiment.

##  3 Detector Upgrade

The change of trigger strategy will require an upgrade of most of the on- and
off-detector electronics to provide read-out at the full collision rate for
all systems. In addition, the increased luminosity will lead to larger track
densities and higher radiation dose on the detector components. Hence a large
fraction of the detector hardware needs to be upgraded.

One of the principal challenges is to read out and process the copious amount
of data produced. Significant data reduction in the front-end electronics is
necessary to reduce the bandwidth required in the data links off detector.
This will be done with custom algorithms tailored to each detector system.
Nevertheless a massive network of optical links will be necessary and the off
detector electronics will have to receive, re-package and re-distribute tenths
of $\rm{Tb/s}$ of data to the trigger system.

Research is ongoing to study the detector layout and to develop components for
the upgrade. Several different design options are being evaluated, with many
key choices to be maid in the coming years. A few details on the upgrades of
the different systems are detailed below.

The Vertex Locator is investigating both a fine-pitched silicon strip detector
and a hybrid pixel detector, with a reduced inner radius to minimise the
extrapolation distance. Common challenges are to implement the on-chip zero-
suppression in the front-end ASIC and to handle the large radiation dose, up
to $5\cdot 10^{15}~{}1~{}\rm{MeV}$ neutron equivalent fluence.

The tracking system will use a combination of silicon strip sensors and
scintillating fibres while keeping the current straw tube tracking for the
large radii. Common developments for the front-end electronics between the
tracking systems and the Vertex Locator are being investigated.

The photon detectors for the RICH detectors have to be replaced to provide
read-out at the full collision rate. A possible extension to the baseline
design is a combined time-of-flight and RICH detector using photon detectors
with very high timing resolution. This would replace one of the RICH detectors
for low momentum particle identification.

The impact of the upgrade is smallest on the calorimeter and muon systems as
they are already part of the L0 trigger. However, the front-end electronics
need to be replaced to cope with the higher data rates and the first muon
station will be removed as it will be saturated by the high track
multiplicity.

The target is to install and commission the upgraded detector in the second
long LHC shut down, scheduled for 2018-2019.

## References

  * Alves Jr. et al. (2008) A. A. Alves Jr., et al., _JINST_ 3, S08005 (2008).
  * Aaij et al. (2010a) R. Aaij, et al., _Phys. Lett._ B694, 209–216 (2010a), 1009.2731.
  * Aaij et al. (2010b) R. Aaij, et al., _CERN_ LHCb-CONF-2010-013 (2010b).
  * Antchev et al. (2011) G. Antchev, et al., _EPL_ 96, 21002. 11 p (2011).
  * Aaij et al. (2012) R. Aaij, et al., _CERN_ CERN-LHCC-2012-007 (2012).