EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)

 LHCb-DP-2016-003 January 12, 2018

The HeRSCheL detector: high-rapidity shower counters for LHCb

K. Carvalho Akiba1, F. Alessio2, N. Bondar2,3, W. Byczynski2, V. Coco2, P.
Collins2, R. Dumps2, R. Dzhelyadin4, P. Gandini5, B.R. Gruberg Cazon5, R.
Jacobsson2, D. Johnson2, J. Manthey2, J. Mauricio6, R. McNulty7, S. Monteil8,
B. Rachwal9, M. Ravonel Salzgeber2, L. Roy2, H. Schindler2, S. Stevenson5, G.
Wilkinson5

1 Universidade Federal do Rio de Janiero (UFRJ), Rio de Janeiro, Brazil
2 European Organization for Nuclear Research (CERN), Geneva, Switzerland
3 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia
4 Institute for High Energy Physics (IHEP), Protvino, Russia
5 Department of Physics, University of Oxford, Oxford, United Kingdom
6 FQA, ICC, Universitat de Barcelona, Avinguda Diagonal 647, Barcelona, Spain
7 School of Physics, University College Dublin, Dublin, Ireland
8 Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-
Ferrand, France
9 AGH - University of Science and Technology, Faculty of Physics and Applied
Computer Science, Kraków, Poland

The HeRSCheL detector consists of a set of scintillating counters, designed to
increase the coverage of the LHCb experiment in the high-rapidity regions on
either side of the main spectrometer. The new detector improves the
capabilities of LHCb for studies of diffractive interactions, most notably
Central Exclusive Production. In this paper the construction, installation,
commissioning, and performance of HeRSCheL are presented.

Submitted to JINST

© CERN on behalf of the LHCb collaboration, licence CC-BY-4.0.

##  1 Introduction

HeRSCheL (High Rapidity Shower Counters for LHCb) is a system of Forward
Shower Counters (FSCs) located in the LHC tunnel on both sides of the LHCb
interaction point. It was installed for Run 2 of the LHC, which began in 2015,
with the aim of enhancing LHCb’s capabilities in diffractive physics, in
particular Central Exclusive Production (CEP) analyses [1].

Each FSC comprises a quadrant of scintillator planes equipped with PMTs, which
are read out synchronously with the sub-detectors of the LHCb spectrometer.
The planes are situated close to the beampipe and detect showers induced by
very forward particles interacting in the beampipe or other machine elements
[2, 3]. In this manner HeRSCheL provides sensitivity to activity at higher
rapidities than is available from the other sub-detectors of the experiment.
This increased acceptance will be valuable in the classification of different
production processes in LHC collisions, for example the isolation of CEP
candidates.

This paper is organised as follows. The physics motivation for HeRSCheL is
elaborated in Sect. 2. The apparatus is described in Sect. 3, and the
calibration procedure and detector sensitivity are discussed in Sect. 4. The
improved performance that HeRSCheL brings to CEP analyses is assessed in Sect.
5. The role of HeRSCheL in the LHCb trigger is explained in Sect. 6.
Conclusions are drawn in Sect. 7.

##  2 Physics motivation: CEP studies at LHCb

LHCb is a forward spectrometer with tracking, particle identification and
calorimetry extending to a pseudorapidity of $\eta\sim 5$ [4]. The upstream
stations of the silicon-strip vertex detector (VELO) provide sensitivity to
charged particles down to $\eta\sim-3.5$ in the backward hemisphere. Although
the design of LHCb was optimised for studies of heavy-flavour hadron decays,
its operating conditions and sensitivity to low transverse-momentum
($p_{\mathrm{T}}$) particles make it ideally suited to measurements of CEP
processes.

In CEP interactions at the LHC a central system of one or more particles is
formed as the result of pomeron or photon exchange. The beam protons survive
the collision intact, remaining undetected inside the beampipe. As an example,
the Feynman diagram for the CEP formation of a $J/\psi$ meson is shown in Fig.
1. The low multiplicity of the final state and the absence of activity, or
large ‘rapidity gap’, either side of the central system provides a distinctive
signature of the CEP process. In practice, this signature can only be
exploited by the current LHCb trigger in beam crossings where there are no
additional interactions. The fraction of single interaction events within the
acceptance of the LHCb spectrometer is relatively large during normal
operation, being approximately 37% in 2015. Contamination to the signal
selection arises, however, from inelastic interactions in which one or both
protons disassociate, or gluon radiation accompanies the pomeron or photon
exchange, processes that are also displayed in Fig. 1. In these cases
additional hadrons are produced at high rapidities, which generally means that
they fall outside the acceptance of the sub-detectors of the spectrometer.

Figure 1: Feynman diagrams of diffractive-production mechanisms of $J/\psi$
mesons at the LHC, where the double gluon system being emitted from the beam
proton constitutes the pomeron. (a) is the pure CEP process, (b) has
additional gluon radiation, and (c) and (d) involve proton dissociation.
(Taken from Ref. [5]).

LHCb has published studies of exclusive $J/\psi$, $\psi(2S)$ [6, 7, 5] and
$\Upsilon$ [8] production based on the Run-1 data set. The background from
inelastic events in which high rapidity particles were undetected in the
spectrometer was estimated by fits to the $p^{2}_{\rm T}$ spectra of the
candidates. The expected distribution of CEP signal peaks at
$\mbox{$p^{2}_{\rm T}$}\sim 0$, although the exact distributions of signal and
non-exclusive background depend on model-based assumptions. Using this
approach, the typical purity of the selection was determined to be $50-60\%$.

The sensitivity that HeRSCheL provides for high-rapidity particles will enable
the contamination from inelastic events in the CEP selection to be suppressed.
Information from HeRSCheL will typically be deployed in a veto mode. The
absence of any significant activity in the FSCs will be used to confirm the
existence of a rapidity gap extending beyond the spectrometer acceptance, and
to add confidence to the central-exclusive hypothesis of CEP candidates. In
this manner the purity of the CEP selection will be improved, and the
systematic uncertainties associated with the modelling of the residual
background can be reduced.

The information provided by HeRSCheL will be beneficial for other analyses in
which rapidity gaps are an important signature, for example the study of
single-diffractive events. It can also be used as a counter of general
inelastic activity, which has applications in the luminosity determination.

##  3 Description of the detector

###  3.1 Layout

Figure 2: Layout of the active areas of the HeRSCheL stations around the LHCb
interaction point (IP8), where for illustration the HeRSCheL stations have
been magnified by a factor of 20 with respect to the rest of the LHCb
detector. $z$-axis not to scale.

In the LHCb coordinate system the $z$-axis points from the LHCb interaction
point (IP8) towards the muon chambers and is collinear with the nominal beam
line. As shown schematically in Fig. 2, the HeRSCheL system comprises three
stations at negative $z$, known as ‘backward’ or ‘B’ stations, and two
stations at positive $z$, known as ‘forward’ or ‘F’ stations.

The station closest to the interaction point, named ‘B0’, is located at
$z\sim-7.5$ m, next to the MBXWH compensator dipole. The next closest stations
are ‘B1’ and ‘F1’, located at $z\sim-19.5$ m and $z\sim 20$ m, respectively,
in the proximity of the MBXWS corrector dipoles. The most distant stations,
‘B2’ and ‘F2’, are located at $z\sim\pm 114$ m, close to the point at which
the vacuum chamber splits into two separate chambers, one for each beam.

The active element of each station is a plastic scintillator plane with outer
dimensions of $600$ mm $\times$ 600 mm, centred around the beam line. The
shape and dimensions of the inner opening depend on the local vacuum chamber
layout. Stations B0, B1, and F1 have circular holes with radii of 47 mm (B0,
B1) and 61 mm (F1), respectively. For stations B2 and F2, the inner opening
has a half-width of 115 mm in the horizontal direction (to encompass the two
vacuum chambers), and a half-width of 54 mm in the vertical direction.

###  3.2 Acceptance

Stations B0, B1, and F1 are intended to register the showers produced by high-
rapidity collision products crossing the beam pipe inside or close to the
MBXWH and MBXWS magnets (which constitute local aperture restrictions).
Stations B2 and F2 are intended to detect showers from high-rapidity neutral
particles interacting with the copper absorber block of the LHC collision rate
monitors (BRAN [9]), which are located close to and in front of these HeRSCheL
stations.

In order to validate the detector concept and to determine the acceptance, the
expected activity in the HeRSCheL stations was simulated using the Gauss
simulation framework [10], generating minimum bias events using Pythia 8 [11]
and transporting the emerging final-state particles through the detector and
tunnel geometry using Geant4 [12, 13].

The simulations confirm that the signal in the HeRSCheL stations is dominated
by energy deposits from low-energy electrons and positrons produced in the
showers induced by high-rapidity particles interacting with machine elements
close to the scintillators. Fig. 3 illustrates the angular coverage, showing
the pseudorapidity of the particles produced in $pp$ collisions where the
contribution from each particle is weighted by the corresponding energy
deposit in the scintillators. The asymmetry between the distributions for B1
and F1 is a consequence of the asymmetric beampipe layout on the two sides of
the LHCb interaction point, leading also to the different hole radii for the
B1 and F1 counters shown in Fig. 2. The HeRSCheL stations are seen to add
detector acceptance from five up to nearly ten pseudorapidity units.

Figure 3: Energy deposit in the scintillators as a function of the
pseudorapidity of the parent particle that caused the shower. The grey areas
indicate the nominal pseudorapidity coverage of LHCb.

###  3.3 Scintillators and photodetectors

Each station is segmented into four quadrants, i.e. four scintillator plates
read out by separate PMTs. The scintillator plates are 20 mm thick and are
manufactured from EJ-200111Eljen Technology, Sweetwater, Texas, United States
(http://www.eljentechnology.com). plastic scintillator material, with a rise
time of 0.9 ns and a decay time of 2.1 ns. Plexiglass light guides of fishtail
type are glued to one side of the scintillator plates, providing a transition
between the rectangular cross-section of the scintillators and the circular
cross-section of the PMT (Fig. 4, left). For safety reasons222Polyvinyltoluene
emits dense black smoke when burning without sufficient oxygen. and to prevent
external light leaking in, the scintillators and light guides are covered by
thin aluminium sheets.

Figure 4: Schematic design of the scintillator and light guide of a single
quadrant of B0/B1-type (47 mm inner radius), accompanied by a photograph of a
B2/F2-type quadrant with the PMT and ‘clipping cable’ attached.

Each light guide is coupled to a Hamamatsu333Hamamatsu Photonics K. K.,
Hamamatsu City, Japan (www.hamamatsu.com). R1828-01 $2^{{}^{\prime\prime}}$
PMT. This PMT is a suitable choice because of its relatively fast rise time of
1.3 ns and its large range of gain adjustment.

A general requirement for the stable operation of a PMT is that the resistive
divider current exceed the average anode current by a factor of at least 10 –
20. The R1828-01 PMT is rated for a maximum average anode current of
$200\,{\rm\upmu A}$, and the voltage divider should thus drain about $2-4$ mA
at the relatively low gain ($\sim 10^{3}-10^{4}$) used in operation. This is
achieved by a custom-made divider that provides an extra bias current to the
last set of dynodes, allowing the initial current through the divider to be
restricted, while ensuring that there is enough current provided in the
vicinity of the anode during nominal operation.

The use of Zener diodes for some of the stages allows a stable voltage to be
maintained for the last dynodes at a high current drain. The two voltages for
each PMT (bias and high voltage) are supplied from CAEN444CAEN S. p. A.,
Viareggio, Italy (http://www.caen.it/). A1535 and A1538D modules located on
the accessible side of the LHCb cavern.

A short coaxial cable terminated with a resistor-capacitor chain is connected
in parallel to the signal cable at the output of the PMT to ‘clip’ the tail of
the signal and thus ensure that the pulse width is less than 25 ns. A complete
assembly of scintillator, light guide and PMT is shown in Fig. 4 (right).

The PMT signals are transmitted to the readout electronics located in the LHCb
cavern using low-loss coaxial cables (C-50-11-1). The length of these cables
ranges between 137 m (for station B2), corresponding to a signal attenuation
of 15%, and 15.4 m (for station B0), corresponding to an attenuation of 5%.

###  3.4 Mechanics

Each photodetector is mounted in a steel housing, including a shielding tube
that provides protection against magnetic fields up to approximately 0.1 T.
The detectors are mounted on mechanical supports manufactured from standard
aluminium Bosch profiles. The stations are equipped with a remote-controlled,
pneumatic motion system, illustrated in Fig. 5, which allows the scintillators
to be retracted from the high-fluence region close to the vacuum chamber
whenever they are not required for data-taking for an extended period of time.
Figures 6 and 7 show photographs of the shower counter stations as installed
in the LHC tunnel.

Figure 5: Visualisation of a HeRSCheL station in its nominal data-taking
position and in its parking position, when the detectors are retracted and
rotated.

Figure 6: Photographs of the backward HeRSCheL stations B0, B1, and B2,
respectively.

Figure 7: Photographs of the forward HeRSCheL stations F1 and F2,
respectively.

###  3.5 Readout and data acquisition

Figure 8: Block diagram of the readout, powering, and control of the FSC
detectors. The diagram shows only one quadrant and one of the two front-end
crates.

In order to minimise the development cost and to ease the integration with the
LHCb data acquisition and trigger system, HeRSCheL makes use of the
electronics of the LHCb preshower system [14], comprised of a very-front end
(VFE) board, a front-end board (FEB), and a calorimeter readout card (CROC).
The FEB and CROC are housed in a front-end crate (VME 9U card cage with a
W-IE-NE-R555W-IE-NE-R, Plein & Baus, Burscheid, Germany (www.wiener-d.com)
MARATON power supply and a backplane specific to the LHCb calorimeter system).
Two front-end crates are installed in the LHCb cavern, one for each side. Fig.
8 gives an overview of the readout chain components and connections.

The multi-anode photomultipliers of the preshower system are connected
directly to the VFE board, whereas in HeRSCheL the signals are transmitted
over long coaxial cables. A dedicated adapter board was therefore developed
for HeRSCheL which receives the signal from the PMTs and adapts it to the
input requirements of the VFE board. Each channel of the VFE board has two
multiplexed differential integrators running at 20 MHz allowing the VFE
channel reading even-numbered bunch crossings an empty, relaxation period
while the other channel reads out, and vice-versa. The integrated analogue
signal is sent to the FEB which digitises it with a 10-bit ADC and distributes
it to the data acquisition and trigger paths. The digitised data are stored in
looping RAMs with a depth of 256 cycles, in groups of four channels. The fine
delay of the integration and sampling clock can be adjusted in steps of 0.78
ns in order to maximise the signal and minimise the contamination from signal
in $pp$ crossings preceding and following the one of interest.

On a positive decision from the global LHCb hardware trigger (L0), the data
from each station are sent from the corresponding RAM depth. This allows for
the data from the different stations, which acquire up to 750 ns relative
delays due to the different station positions, to be synchronised with the
global LHCb bunch counter. The digitised signals are sent to the CROC via the
crate backplane. The CROC plays a dual role. Firstly, it is responsible for
the control and monitoring of the front-end crate. It receives the timing and
fast control (TFC) information and distributes it on the backplane to the FEB.
The interface to the Experiment Control System (ECS) is based on the SPECS
protocol [15]. The ECS control and monitoring of the FEBs is communicated
through the backplane. Secondly, the CROC gathers the front-end data and sends
them to the acquisition board (TELL1 [16]) via optical fibres.

##  4 Detector sensitivity

In the case of a single CEP interaction, the produced central system may be
reconstructed inside the standard LHCb acceptance and will be accompanied by
no other activity within the main detector. For this signal no activity is
expected in the HeRSCheL system. In the case of non-exclusive processes
additional particles are produced together with the CEP candidate. These
particles lead to activity being registered in HeRSCheL. In this Section the
sensitivity of HeRSCheL to charged-particle activity is considered. Steps
taken to minimise noise in order to maximise sensitivity are described.
Finally the characteristic response of the detector in the absence of
additional particle activity is presented. This ‘empty detector’ signal is the
expected response in the case of a single CEP interaction in a $pp$ bunch
crossing.

###  4.1 Response to incident charged particles

The detector response to charged particles has been calibrated in the
laboratory, prior to installation, using cosmic muons. To achieve sensitivity
to single incident particles, the voltage applied to the PMT was set to 1.1
kV. Later, the PMT gain as a function of the applied high voltage has been
measured using LED light pulses. Using these measurements the signal per
charged particle is determined as a function of the applied high voltage. This
is shown in Fig. 9(a) for one quadrant of station B0.

Figure 9(b) shows the response (analogue pulse shape) of one of the quadrants
of station B0, recorded using a digital oscilloscope during LHC injection
tests in November 2014. In these tests, proton beams were extracted from the
Super Proton Synchrotron (SPS) and dumped on an absorber (named TED) located
approximately 340 m away from the LHCb interaction point, generating a shower
of secondary particles at small angles with respect to the beam line.

At the beginning of the 2015 data-taking period, the high-voltage settings
were configured such that an energy deposit in the scintillator corresponding
to one minimum ionising particle (MIP) resulted in a digitised signal between
two and five ADC counts. These settings were kept throughout 2015.

Irradiation leads to a reduction in attenuation length of scintillators and
light guides and consequently to a loss of signal. In order to quantify the
average signal, the tail of the ADC spectrum above the pedestal (see, for
example, Fig. 13) is fitted with an exponential distribution, and the mean of
the fitted function is used as a metric of the average signal. The ageing
process is illustrated in Fig. 10 which shows the average signal per $pp$
collision in the quadrants of station B0 as a function of the integrated
luminosity in LHCb. The PMTs of this station were operated at a high voltage
of $830-850$ V, depending on the quadrant. The average signal corresponded to
the equivalent of $\sim 72-95$ MIPs at the beginning of the data-taking period
and degraded to the equivalent of $\sim 35-40$ MIPs at the end of 2015.

In 2016 and 2017, the high-voltage settings were updated regularly to
compensate for the increasing loss of photoelectrons due to radiation damage.
For example, the high voltages of station B0 were set to $\sim 830-860$ V at
the beginning of the 2016 data-taking period and were raised gradually up to
$\sim 880-920$ V at the end of 2016. The increase in high voltage did not lead
to a significant change of the dark count rate.

Figure 9:  (a) shows the signal per particle as a function of PMT high voltage
for one of the quadrants of station B0. The absolute scale is determined using
measurements with cosmic muons, and the evolution with high voltage using a
pulsed LED setup. The analogue signal of one quadrant of station B0, recorded
during injection tests (‘TED shots’) in November 2014, is shown in (b). Figure
10: Average signal in $pp$ collisions as a function of integrated luminosity
in 2015, for the four quadrants of station B0. The high-voltage settings were
kept the same throughout the entire 2015 data-taking period.

###  4.2 Noise

For data collected during 2015 a significant common-noise component was found
between the detector channels within each station, arising either from
interference induced in the cables passing along the tunnel, or in the front-
end electronics. This common noise has been studied by considering the
correlation between the signal from each input channel and one spare cable for
each station, placed next to the signal cables in the tunnel and connected to
the readout electronics but left unconnected at the detector end.

The correlation between the signal and spare cables has been studied using
data collected during special calibration runs at the end of each fill, once
the beams have been dumped. An example of the observed correlation is shown
for one channel in Fig. 11. The common noise is then subtracted offline
according to the correlations observed in each fill, and the ADC distribution
is shifted to be centred at zero.

The adaptor board was replaced at the end of 2015 operation to employ a less
noisy input-matching mechanism. Although data taken during 2016 have not been
studied in depth yet, the uncorrected pedestal is seen to be much narrower
than that observed in 2015, and no offline noise subtraction is required. The
RMS for each channel in end-of-fill data is shown in Fig. 11, comparing the
raw 2015 signal with that after the calibration has been applied, as well as
demonstrating the much less noisy raw signal from 2016.

Figure 11: Subtraction of common noise. (a) illustrates the correlated noise
for the two integrator channels reading out one detector quadrant on station
B0 in 2015 end-of-fill data, with respect to the signal in the spare cable (B0
reference), not connected at the detector end. (b) compares the raw signal in
end-of-fill data taken during 2015 (red/darker filled histograms) with that
after common-noise subtraction (blue filled histogram) and the raw signal RMS
with the new adapter board in 2016 (dashed histogram). Each station houses
four PMTs, read out via the dual-channel VFE board, and there are therefore
eight channels to consider for each station. The relatively large contribution
of uncorrelated noise to the signal in the B2 station was the result of
imperfect grounding; this was resolved at the end of 2015.

###  4.3 Correlation with other LHCb sub-detectors

An important confirmation of the detector’s successful operation is the
observation of correspondence between activity registered in other LHCb sub-
detectors and that in the HeRSCheL scintillators. In Fig. 12 the sum of the
ADC counts from all the stations on each side is shown in the two cases that
there are small or large numbers of tracks reconstructed on the corresponding
side of the interaction point. For comparison with the B-side HeRSCheL
response, only ‘backward’ tracks, leaving deposits in the VELO modules behind
the interaction point and therefore reconstructed using only that sub-
detector, are considered. To compare with the F-side HeRSCheL response, only
‘long tracks’ are employed, where these tracks are reconstructed using
deposits in the VELO modules around the interaction point and the tracking
stations further downstream. As expected, more activity is seen in the
HeRSCheL counters when more tracks are reconstructed.

Figure 12: Correlation between sum of raw HeRSCheL ADC counts on (a) the B
side and (b) the F side of the interaction point, with reconstructed tracks on
the same side.

###  4.4 Empty-detector signal characterisation

In this section the expected response of the detector is presented in the case
that a single CEP interaction occurs during a $pp$ crossing. For illustration
Fig. 13 shows the signal recorded in one counter at each of the five HeRSCheL
detector stations, for an arbitrary sample of $pp$ collision events selected
by the LHCb trigger. The two principal features of the distribution are the
large portion of events around zero ADC counts in each counter, corresponding
to an absence of activity, and a long tail to higher numbers of ADC counts,
corresponding to significant activity in the counters.

Protons circulating in the LHC are distributed in bunches, separated from one
another by 25 ${\mathrm{\,ns}}$. Bunches are collected into ‘trains’ by virtue
of the injection procedure, separated by gaps. A 25 ${\mathrm{\,ns}}$ window
within which proton bunches cross in LHCb is referred to as a ‘bunch
crossing’. Whilst the dominant contribution to the HeRSCheL empty-detector
signal is electronic noise, secondary contributions arise as the result of
activity in nearby crossings which spill into the 25 ${\mathrm{\,ns}}$ time
interval of the triggered crossing. The largest of these secondary
contributions is the residual impact on detector electronics of successive
large signals in the detector, during a train of proton-proton crossings in
the LHC. It is found that the signal recorded in the counters in the window
immediately after such a train, where no particle activity can be present,
provides a good description of the empty-detector region of the ADC response,
as in the case of a CEP interaction. This ADC response for each of the example
counters is also shown in Fig. 13.

Figure 13: Activity registered, after calibration, in one integrator attached
to one counter for each HeRSCheL detector station during beam-beam crossings
in the solid histogram, showing only the range up to 100 ADC counts. The
empty-detector signal recorded after a bunch train is represented by the
dotted histogram.

##  5 Impact on physics studies

In the case of a single CEP interaction in a $pp$ crossing, no activity is
expected in the HeRSCheL detector other than that described in Sect. 4.4. In
this section a metric is presented, employing information from all counters in
the HeRSCheL detector, that can be used to discriminate between the empty-
detector signal and the increased activity associated with background
processes. The efficiency of this metric in selecting CEP signal is measured
using CEP production of continuum-dimuon pairs, an abundant process that can
be rather easily identified without HeRSCheL information. The effectiveness of
the same metric in suppressing non-CEP background is determined. Finally the
impact of using HeRSCheL information in the measurement of exclusive
photoproduction of ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ mesons, that
can be considered a benchmark for other CEP processes, is considered.

###  5.1 Efficiency of HeRSCheL in selecting empty events

In order to construct a quantity that combines the responses of all twenty
counters comprising the HeRSCheL detector effectively, it is beneficial to
account for the characteristic distribution of the empty-detector signal in
each counter. Examples of this empty detector signal have been discussed in
Sec. 4.4. The most natural way to combine the activity in all the HeRSCheL
detectors is to construct a $\chi^{2}$ quantity, $\xi_{\rm HRC}$, such that
values of $\xi_{\rm HRC}$ close to zero correspond to events with little or no
activity in all the HeRSCheL counters, as expected in the case of a single CEP
interaction, and high values of $\xi_{\rm HRC}$ correspond to events where the
counter activity is elevated, as expected for non-CEP background. In order to
construct such a $\chi^{2}$ it is necessary to account for two effects.
Firstly the non-Gaussian distribution of the empty detector HeRSCheL signals,
as displayed in Fig. 13, must be accounted for. Secondly it is required to
consider the correlation in the activities recorded in the different quadrants
of the HeRSCheL stations, even between the different stations on each side,
given the overlap in acceptance shown in Fig. 3. Both these effects have been
studied in events recorded immediately after a series of $pp$ bunch crossings
in the LHC. A single $\chi^{2}$ quantity, $\xi_{\rm HRC}$, is constructed to
quantify the activity above the noise in all the counters, taking account of
correlations between the counters.

The distribution of the HeRSCheL metric, $\ln(\xi_{\rm HRC})$, is shown in
Fig. 14 for the sample of events immediately after a bunch train discussed in
the previous Section. Since these events represent the activity expected in
HeRSCheL for CEP processes, the efficiency for these events to survive an
upper limit on $\ln(\xi_{\rm HRC})$ is also indicated.

Figure 14: Distribution of HeRSCheL metric for events immediately following a
bunch train (black line), representative of the response of the detector to
CEP signal. The efficiency for an upper limit on the metric for retention of
these events is also shown (red line).

###  5.2 Study of the performance on continuum-dimuon production

In order to enhance the CEP candidates in an event sample, an upper limit on
$\xi_{\rm HRC}$ should be chosen. Continuum-dimuon production, an abundant CEP
process with a comparatively clear experimental signature, is used to assess
the efficiency of such a limit. This process takes place by two-photon
exchange between the protons, and results in a very soft spectrum in
$p^{2}_{\rm T}$ for the dimuon system. By analysing the $p^{2}_{\rm T}$
spectrum of the dimuon candidates the CEP component may be reliably isolated.

A sample of dimuon candidates is selected in data collected during 2015. The
fraction of single-interaction CEP candidates within this sample, originating
from an event where only a single $pp$ interaction is detected, is enhanced by
making standard trigger requirements and imposing limits on the activity in
the other LHCb sub-detectors. Specifically only events with exactly two
tracks, satisfying standard LHCb muon-identification requirements, are allowed
in this two-track sample. The invariant mass of the candidate, shown in Fig.
15(a), is chosen to exclude regions of resonant production, which proceeds by
a different CEP mechanism and does not have the same $p^{2}_{\rm T}$
distribution. The $p^{2}_{\rm T}$ distribution of the sample is shown in Fig.
15(b). On Fig. 15(b) a second distribution has been superimposed corresponding
to the $p^{2}_{\rm T}$ distribution for dimuon candidates selected in the same
way, but where one additional VELO track is required. This category of events
is unlikely to be CEP and thus in the case that an additional VELO track is
detected, this provides a representation of the background distribution. This
distribution is normalised such that the number of background candidates above
0.75${\mathrm{\,Ge\kern-1.00006ptV^{2}\\!/}c^{2}}$ matches the number for the
same range in the two-track sample. The very low $p^{2}_{\rm T}$ region, where
continuum-dimuon CEP is expected to be concentrated, is magnified in the inset
histogram.

Figure 15: Selected continuum-dimuon sample. In (a) is shown the invariant
mass distribution of dimuon candidates. The shaded regions of resonant
production are excluded. In (b) is shown the full $p^{2}_{\rm T}$ distribution
of the selected dimuon candidates. Since the signal is expected to be
concentrated chiefly below 0.1 ${\mathrm{\,Ge\kern-1.00006ptV^{2}\\!/}c^{2}}$,
the candidates below this threshold are shown in the inset histogram. For both
regions, an additional histogram is superimposed, corresponding to the
distribution of candidates in a background sample.

These samples are used to obtain the response of the HeRSCheL metric to
exclusive continuum-dimuon production. The distribution of the natural
logarithm of $\xi_{\rm HRC}$ in the background sample is represented by the
dashed line in Fig. 16(a). To visualise the $\ln(\xi_{\rm HRC})$ response for
CEP signal, the distribution is first obtained for dimuon candidates with
$p^{2}_{\rm T}$ below 0.1${\mathrm{\,Ge\kern-1.00006ptV^{2}\\!/}c^{2}}$ in the
signal sample. The background shape can be subtracted from this distribution
according to the number of events in the background sample, again normalised
in the high-$p^{2}_{\rm T}$ region. The signal response is shown as the solid
line in Fig. 16(a). It is interesting to note that the background sample has a
peak at low values of $\ln(\xi_{\rm HRC})$, a feature which can be explained
by the presence of background where all additional activity associated with,
for example, proton dissociation lies outside the HeRSCheL acceptance. The
significant tail in HeRSCheL activity seen even for ‘CEP signal’ appears
because, although the activity in the LHCb spectrometer is consistent with
only one visible $pp$ interaction there, a second pile-up interaction may
still occur producing activity only in the geometrical acceptance covered by
the HeRSCheL detector.

In order to obtain the signal efficiency of a given limit on activity in the
HeRSCheL detector, the continuum-dimuon sample can again be used. This time a
fit is performed to the $p^{2}_{\rm T}$ distribution of signal candidates. A
template is taken from simulated samples to represent the signal component.
Studies of the three-track background sample and other, associated, data sets
give confidence that the background component can be well-modelled using a
probability density function (PDF) consisting of two exponential shapes. To
obtain the efficiency of the HeRSCheL activity limit on the continuum-dimuon
CEP signal, the $p^{2}_{\rm T}$ distribution is fit with a PDF containing the
signal template and the background model, where the slope parameters of the
background exponentials are free to vary along with their relative fraction.
One such fit is shown, for illustration, in Fig. 16(b). The effect on the
exclusive-signal yield of changing the HeRSCheL veto is determined and shown
in Fig. 16(c), where the uncertainty is systematically dominated.

It is of interest to consider the effect of the HeRSCheL activity limit on the
background component, although it should be noted that the kinematic
properties and likely HeRSCheL response to the background are rather specific
to the production mode under consideration. The effect of the HeRSCheL
activity limit in suppressing the background in this specific sample is also
shown in Fig. 16(c), where the uncertainty receives comparable contributions
from statistical and systematic sources.

An illustrative veto on activity in HeRSCheL is chosen, consisting of an upper
limit on $\ln(\xi_{\rm HRC})$ at 4.9. The signal efficiency for continuum-
dimuon production at this working point is 84% according to Fig. 16(c) and, as
seen in the same study, 65% of the background is rejected. It is clear from
Fig. 14 that the intrinsic HeRSCheL efficiency for selection of empty events
at this working point is nearly 100%. The difference between the empty-event
selection efficiency and the measured efficiency for selection of CEP
continuum-dimuon candidates arises because a limit on activity in the HeRSCheL
detector also suppresses events where additional, pile-up $pp$ interactions
take place. Such additional pile-up $pp$ interactions, if the resulting
particle production escapes the standard spectrometer acceptance but enters
the HeRSCheL detector, would be likely to produce a background-like signature
in some fraction of events also containing a genuine CEP candidate.

Figure 16: In (a) the distribution of $\ln(\xi_{\rm HRC})$ is shown for CEP
(solid line) and non-CEP (dashed line) continuum-dimuon candidates. In (b) an
example fit to the continuum-dimuon $p^{2}_{\rm T}$ distribution is shown,
with the CEP signal template and double-exponential background indicated. In
(c) the CEP signal efficiency and, for this process, background rejection are
shown as a function of the limit chosen on $\ln(\xi_{\rm HRC})$.

###  5.3 Study of the performance on ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip
2.0mu}$ photoproduction

The effect of the limit placed on $\xi_{\rm HRC}$ can be considered for the
case of resonant dimuon production, near the
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ mass indicated by the first
shaded region in Fig. 15(a). In this case, the production mechanism is
predominantly photoproduction, neglecting a very small contamination from
continuum-dimuon production in this region, and the exclusive
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ candidate $p^{2}_{\rm T}$
distribution extends to higher values, even up to
$1{\mathrm{\,Ge\kern-1.00006ptV^{2}\\!/}c^{2}}$. The effect of placing a limit
on $\ln(\xi_{\rm HRC})$ of 4.9, chosen to have a signal efficiency of
approximately 84% according to the continuum-dimuon studies, is shown for the
distribution of ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ candidate
$p^{2}_{\rm T}$ in Fig. 17. The background yield, determined by the number of
candidates in the high $p^{2}_{\rm T}$ region above 1
${\mathrm{\,Ge\kern-1.00006ptV^{2}\\!/}c^{2}}$, where little
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ CEP signal is expected, is
reduced to less than a third of its original value. A similar suppression is
expected in the background component at lower $p^{2}_{\rm T}$, and a
corresponding improvement in the signal purity. The background rejection that
is achieved at a particular HeRSCheL working point depends on the production
process for that background, hence for this process the HeRSCheL cut is more
effective in suppressing background than in the continuum-dimuon study.

Figure 17: Effect on the $p^{2}_{\rm T}$ distribution of
${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ CEP candidates of applying a
limit of 4.9 on $\ln(\xi_{\rm HRC})$. This cut is expected to retain close to
100% of CEP signal which have no additional tracks due to pile-up or single
diffractive events in the combined LHCb and HeRSCheL geometrical acceptance.
This corresponds to about 84% of the events which would be accepted if only
the LHCb geometrical acceptance was used. At this working point, the yield of
non-CEP ${J\mskip-3.0mu/\mskip-2.0mu\psi\mskip 2.0mu}$ candidates falls to
nearly a third of the original value.

##  6 Employing HeRSCheL in the LHCb trigger

The LHCb trigger is divided into a low-level hardware trigger and a higher-
level software trigger. In 2016 HeRSCheL information was introduced into the
software trigger. Specifically, a loose limit is placed on the sum of ADC
counts for the counters in each station. The distributions of the sum of ADC
counts from each station are shown in Fig. 18 for events which pass the
hardware trigger relevant for hadronic CEP. The portion of the distribution
rejected is indicated. This conservative limit, far above the empty-detector
portion of the distribution, allows for the trigger rate to be reduced by
nearly a factor of two, allowing for other constraints to be relaxed in order
to select hadronic CEP signals more efficiently. Work is ongoing to include
HeRSCheL information in the hardware trigger.

Figure 18: HeRSCheL in the LHCb software trigger. Distributions of the sum of
ADC counts recorded by the four counters in each station. The portion of
events rejected by the limit in each station is indicated by the filled
region.

##  7 Conclusion and outlook

A new sub-detector has been constructed, installed and commissioned in the LHC
tunnel in order to extend the rapidity coverage of the LHCb experiment in the
high-rapidity regions either side of the interaction point during Run 2 of the
LHC. Five stations have been installed up to a maximum distance of
approximately 114 m from the interaction point. The HeRSCheL detector extends
the LHCb forward coverage up to a pseudorapidity of around 10.

The physics impact of the new detector has been described; in particular
HeRSCheL allows for significant improvement in the suppression of the most
problematic backgrounds in studies of Central Exclusive Production at LHCb,
whilst maintaining high signal efficiency. The new detector is already in use
in the LHCb software trigger for CEP processes, and will be integrated at the
hardware level in the L0 trigger chain in the near future.

## Acknowledgements

We thank all our LHCb collaborators who have contributed to the design,
construction, commissioning, and operation of HeRSCheL, in particular J.
Buytaert, P. Dziurdzia, C. Gaspar, A. Granik, Yu. Guz, C. Joram, F. Machefert,
P. Robbe, and B. Schmidt. Furthermore we are grateful to J. Bernhard from the
COMPASS collaboration who provided us with the PMT housings. We express our
gratitude to our colleagues in the CERN accelerator departments for the
excellent performance of the LHC. We thank the technical and administrative
staff at the authors’ institutes. We acknowledge support from CERN and from
the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); CNRS/IN2P3
(France); MNiSW and NCN (Poland); MinES and FASO (Russia); MinECo (Spain);
STFC (United Kingdom). We acknowledge the computing resources that are
provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy), SURF
(The Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC
(Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-GRID
(Poland) and OSC (USA). We are indebted to the communities behind the multiple
open-source software packages on which we depend.

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