# First Physics Results from the FASER Experiment

B. Petersen on behalf of the FASER collaboration  Department of Experimental
Physics, CERN,
Espl. des Particules 1, 1211 Meyrin, Switzerland

###### Abstract

FASER is a new LHC experiment designed to search for light, weakly-interacting
particles that are produced in proton-proton collisions at the ATLAS
interaction point and travel in the far-forward direction. The first physics
results from the initial year of data-taking are presented. A search for dark
photons decaying to an electron-positron pair found no events, yielding new
constraints on dark photons with couplings $\epsilon\sim 10^{-5}-10^{-4}$ and
masses $\sim 10\leavevmode\nobreak\ \text{MeV}-100\leavevmode\nobreak\
\text{MeV}$. A search for muon-neutrino charged-current interactions in a
tungsten target at the front of the FASER experiment found $153^{+12}_{-13}$
neutrino candidates with a negligible background. The reconstructed charge and
momentum distributions imply the observation of both neutrinos and anti-
neutrinos with an incident neutrino energy above 200 GeV.

##  1 Introduction

FASER [1, 2] is a new LHC experiment designed to search for dark photons and
other long-lived beyond-standard model particles as well as to detect high
energy neutrinos produced in proton-proton collisions. The FASER detector is
located approximately $480\text{\,}\mathrm{m}$ from the ATLAS interaction
point (IP1) along the beam collision axis line-of-sight (LOS). Because the
particles are extremely weakly interacting, dark photons and neutrinos
produced at IP1 may travel along the LOS, pass through
$100\text{\,}\mathrm{m}$ of rock and concrete without interacting, and then
decay or interact in FASER. At the same time, most other Standard Model (SM)
particles produced at the ATLAS IP will either be bent away by the LHC magnets
or stopped in the $100\text{\,}\mathrm{m}$ of shielding. In these proceedings,
the first results from the search for dark photons [3] decaying to
$e^{+}e^{-}$ and the first direct observation of neutrinos [4] produced at a
particle collider are presented.

##  2 The FASER Detector and Datasets

A detailed description of the FASER detector can be found elsewhere. [2] The
experiment is located in the TI12 tunnel, which connects the SPS and LHC
tunnels, approximately $480\text{\,}\mathrm{m}$ downstream of the ATLAS
interaction point. The FASER detector is partially immersed in a magnetic
field and consists of a passive tungsten/emulsion neutrino detector
(FASER$\nu$), two scintillator-based veto systems, additional scintillators
for triggering, a tracking spectrometer, a pre-shower, and an electromagnetic
calorimeter.

FASER$\nu$ consists of 730 layers of $1.1\text{\,}\mathrm{mm}$-thick
$25\text{\,}\mathrm{cm}$ by $30\text{\,}\mathrm{cm}$ tungsten plates
interleaved with emulsion films with a total mass of 1.1 metric tons. The
emulsion films are not used in the presented analyses and instead the
FASER$\nu$ detector is used as a target for neutrino interactions and as an
absorber of neutral hadrons produced in muon interactions upstream of FASER.
FASER$\nu$ is situated between two veto scintillator stations. These are used
to veto incoming charged particles, mainly muons, as well as particles
produced in FASER$\nu$ itself in the case of the dark photon search. The front
veto station has two layers of
$$30\text{\,}\mathrm{cm}$\times$35\text{\,}\mathrm{cm}$$,
$2\text{\,}\mathrm{cm}$-thick plastic scintillators, while the second veto
station has three layers of
$$30\text{\,}\mathrm{cm}$\times$30\text{\,}\mathrm{cm}$$,
$2\text{\,}\mathrm{cm}$-thick plastic scintillators. All scintillators are
read out with photomultiplier tubes (PMTs).

The tracking system consists of the interface tracking station (IFT) and the
three tracking spectrometer stations. [5] Each tracking station is composed of
three planes with eight ATLAS semiconductor tracker (SCT) barrel modules [6]
per plane, arranged as two columns of four modules covering
$24\text{\,}\mathrm{cm}$ by $24\text{\,}\mathrm{cm}$. In the presented
analyses, only the spectrometer is used to reconstruct charged particles,
while the IFT is only used to measure remnants of the neutrino interactions
through cluster counting.

Between the three tracking spectrometer stations are two
$1\text{\,}\mathrm{m}$-long dipole magnets with magnetic field of 0.57 T. A
similar $1.5\text{\,}\mathrm{m}$-long magnet is placed between the last veto
station and the spectrometer and serves as the decay volume for dark photon
decays. All magnets have a circular aperture of 200 mm diameter, which defines
the active transverse area of the detector, and bend charged particles in the
vertical plane. In addition, the so-called timing scintillator station is
located between the decay volume and the front of the first tracking station
of the spectrometer. It is used primarily for triggering.

The electromagnetic energy of particles in the detector volume is measured by
the electromagnetic calorimeter, located as the most downstream component of
the detector. The calorimeter is made up of four spare modules from the LHCb
experiments outer ECAL calorimeter. [7] Each module is
$$12\text{\,}\mathrm{cm}$\times$12\text{\,}\mathrm{cm}$$ in the transverse
plane and the modules are arranged in a two-by-two formation. The calorimeter
modules are readout using PMTs. In order to maintain good PMT linearity and
avoid saturation for deposits above a few hundred GeV, optical filters were
installed in front of the PMTs for the second part of the data-taking period
in 2022. Between the calorimeter and the tracking spectrometer is a pre-shower
system built from two scintillator layers of the same type as used for the
second veto station, each preceded by a $3\text{\,}\mathrm{mm}$-thick tungsten
absorber plate.

The trigger and data acquisition system of FASER was designed to achieve high
efficiency and reliability. [8] Neutrino and dark photon candidate events are
triggered by scintillator signals that exceed a preset threshold below that of
a single minimum-ionizing particle (MIP), resulting in a typical trigger rate
of 0.5-$1.3\text{\,}\mathrm{kHz}$. Data was recorded throughout 2022 with an
integrated data taking efficiency of 96.1%.

The presented analyses use data from runs with stable beam conditions
collected between July (September) and November 2022 for the neutrino (dark
photon) analysis. The latter analysis requires the calorimeter to have the
optical filters installed. This corresponds to total luminosities as measured
by ATLAS of $35.4\text{\,}{\mathrm{fb}}^{-1}$ and
$27.0\text{\,}{\mathrm{fb}}^{-1}$, respectively after data quality selection.
Event reconstruction is carried out in FASER’s Calypso offline software
system, based on the open-source Athena framework from the ATLAS experiment.

Monte Carlo (MC) simulation samples are used to evaluate the signal efficiency
and in the estimation of some of the backgrounds, and systematic
uncertainties. All samples are simulated using GEANT4 [9] with a detailed
description of the detector geometry, including passive material. The samples
include a realistic level of detector noise, and are reconstructed in the same
way as the data.

##  3 Search for Dark Photon Decays

Dark photons are a common feature of hidden sector models motivated by dark
matter. If the dark sector contains a U(1) electromagnetic force, the dark
gauge boson can mix with the SM gauge boson, [10] leading to a new spin-1
gauge boson, the dark photon $A^{\prime}$, which is among the leading targets
for dark sector searches. The dark photon is characterized by its mass,
$m_{A^{\prime}}$, and coupling parameter, $\epsilon$. FASER is most sensitive
to dark photons with $m_{A^{\prime}}\sim 10-100\leavevmode\nobreak\
\text{MeV}$ and $\epsilon\sim 10^{-5}$–$10^{-4}$. In this region of parameter
space, the dark photon is primarily produced at the LHC through neutral pion
decays to $A^{\prime}\gamma$, but $\eta\rightarrow A^{\prime}\gamma$ decays
and dark bremsstrahlung $pp\rightarrow ppA^{\prime}$ also contribute. In the
forward direction, the produced dark photons are highly energetic which,
depending on the coupling and mass, can result in a long decay length. For
$E_{A^{\prime}}\gg m_{A^{\prime}}\gg m_{e}$, the dark photon decay length can
be expressed as [1]

$L=c\beta\tau\gamma\approx(80\leavevmode\nobreak\
\text{m})\left[\frac{10^{-5}}{\epsilon}\right]^{2}\left[\frac{E_{A^{\prime}}}{\text{TeV}}\right]\left[\frac{100\leavevmode\nobreak\
\text{MeV}}{m_{A^{\prime}}}\right]^{2}\ .$ (1)

This means a substantial number of these dark photons can decay at the FASER
location. For
$$1\text{\,}\text{MeV}$<m_{A^{\prime}}<$211\text{\,}\text{MeV}$$, the dark
photon will decay 100% to an $e^{+}e^{-}$ pair. The analysis therefore
searches for the appearance of a track pair in the decay volume before the
spectrometer with a large energy deposition in the electromagnetic
calorimeter.

Figure 1: The calorimeter energy distribution after various levels of
selections for data and expected signal for three representative signal
models. (top left) after a reconstructed track requirement; (top right) after
a reconstructed track requirement, and with no signal in the veto
scintillators; (bottom left) after a fiducial track requirement and no veto
scintillator signal; (bottom right) after a fiducial two track requirement and
no veto scintillator signal.

The search uses simple, robust selection criteria optimized for an early
discovery with selections fixed before looking at data in the signal region.
All five veto scintillator layers are required to have no signal, vetoing
events with any incoming charged particles or interactions in the FASER$\nu$
detector. The timing and pre-shower scintillators layer are required to have
signals consistent with two or more MIPs. Exactly two good quality
reconstructed tracks with momentum above 20 GeV inside a fiducial radius of
$95\text{\,}\mathrm{mm}$ are required. The extrapolations of both tracks to
the first scintillator station are required to have a radius within
$95\text{\,}\mathrm{mm}$ in order to suppress particles coming at a large
angle. Finally at least $500\text{\,}\text{GeV}$ of deposited energy in the
calorimeter is required. This selection results in a typical signal efficiency
of 40-50% for dark photons decaying inside the decay volume.

The main backgrounds remaining after all selections are estimated to be from
neutrino interactions in the detector and neutral hadrons entering the
detector and decaying in the decay volume. The former is estimated from large
simulation samples to be $0.0018\pm 0.0024$ events where most of these
originate from neutrino interactions in the timing scintillator. Neutral
hadrons originate from muon interactions in the rock upstream of FASER and are
mostly suppressed by the detection of the muon and absorption of the hadrons
in the FASER$\nu$ tungsten. The background is estimated to be $(2.2\pm
3.1)\times 10^{-4}$ events based on low energy events with two and three
reconstructed tracks with different veto scintillator conditions. Background
events from undetected muons are negligible as each of the five scintillator
layers have a MIP-detection efficiency measured in data to be more than
$99.9998\%$. Similarly, backgrounds from cosmic ray events and non-collision
beam backgrounds have been studied and found to be negligible.

Figure 1 shows the calorimeter energy distribution after different stages in
the event selection. No data events remain after just requiring no signal in
the veto layers and at least one track in the fiducial region. Given that no
events are observed, the results are used to set exclusion limits on dark
photons at 90% confidence level. Figure 2 shows the exclusion limit in the
signal parameter space. The analysis excludes signal models in the range
$\epsilon\sim 1\times 10^{-5}-2\times 10^{-4}$ and masses $\sim
10\leavevmode\nobreak\ \text{MeV}-80\leavevmode\nobreak\ \text{MeV}$, and
provides exclusion for previously viable models in the range $\epsilon\sim
2\times 10^{-5}-1\times 10^{-4}$ and masses $\sim 17\leavevmode\nobreak\
\text{MeV}-70\leavevmode\nobreak\ \text{MeV}$. Figure 2 also includes an
example thermal relic contour [11], obtained for the scenario where the dark
photons couple to a light complex scalar dark matter field $\chi$,
demonstrating that FASER is sensitive to dark photons in a cosmologically
interesting region of parameter space.

Figure 2: The 90% confidence level exclusion contour in the dark photon model
parameter space. Regions excluded by previous experiments are shown in
grey[12]. The red line shows the region of parameter space which yields the
correct dark matter relic density, with the assumptions discussed in the main
text.

##  4 Observation of Neutrino Interactions

Until now no neutrino produced at a particle collider has ever been directly
detected. Colliders copiously produce both neutrinos and anti-neutrinos of all
flavors, and they do so in a range of very high energies where neutrino
interactions have not yet been observed. Nevertheless, collider neutrinos have
escaped detection, because they interact extremely weakly, and the highest
energy neutrinos, which have the largest probability of interacting, are
predominantly produced in the forward region, parallel to the beam line. In
2021, the FASER collaboration identified the first collider neutrino
candidates [13] using a $29\text{\,}\mathrm{kg}$ pilot detector, highlighting
the potential of discovering collider neutrinos in LHC collisions.

The presented search targets $\nu_{\mu}$ and $\overline{\nu}_{\mu}$ charged-
current interactions in the tungsten/emulsion detector. Such interactions will
produce a high-momentum muon that can be reconstructed in the three stations
of the FASER tracking spectrometer as well as increased activity in the second
veto scintillator station and in the IFT tracking station from secondary
particles produced in the interaction. The event selection starts from events
triggered by any of the scintillators downstream of FASER$\nu$ that are
consistent with a colliding bunch crossing identifier and all downstream
scintillator layers are required to have a signal consistent with at least one
MIP. Events with signals in any of the two layers of the first veto station
are rejected as they are likely from incoming muons or other charged particle
backgrounds. Only events with a single, good quality track in the three
tracker stations with a reconstructed track momentum above
$100\text{\,}\mathrm{G}\mathrm{e}\mathrm{V}$ are accepted. The track is
required to be in the fiducial tracking volume ($r<$95\text{\,}\mathrm{mm}$$)
of the spectrometer and when extrapolated to the IFT. It should be within
$r<$120\text{\,}\mathrm{mm}$$ at the first veto scintillator. Based on
simulation of neutrino production from proton-proton collisions [14] and of
interactions [15] in FASER$\nu$, $151\pm 41$ neutrino events are expected to
be selected with the above selection. The uncertainty is given by the
difference between two different production models and no experimental
uncertainties are included in this estimate.

Figure 3:  Distribution of neutrino candidate tracks extrapolated to the first
veto station. (left) The extrapolated track radius $r_{\mathrm{veto}\,\nu}$
and reconstructed track momentum $p_{\mu}$ is depicted including outside the
final signal region. The region with lower momenta and larger radii is
dominated by background events consisting of charged particles that miss the
FASER$\nu$ scintillator station. (right) extrapolated transverse position of
the reconstructed tracks of neutrino-like events to the FASER$\nu$
scintillator station. The ATLAS LOS is indicated with a red marker.

The background from neutral hadrons is estimated from simulation of neutral
hadron production from muon interactions in the rock in front of the FASER
detector combined with simulation of neutral hadron interactions in FASER$\nu$
that produce charged particles with a momentum of more than
$100\text{\,}\text{GeV}$. A total of just $0.11\pm 0.06$ events are expected.
An additional background can come from large angle muons that miss the front
veto station and scatter into the spectrometer in FASER$\nu$. This is
estimated to be $0.08\pm 1.83$ events based on the rate of events in data
within a large-radius control region.

Figure 3 shows the selected events, as well as background-enriched regions
around the signal region. In total we observe 153 events passing all selection
steps. The number of neutrino events and the statistical significance are
estimated using a binned extended maximum likelihood fit taking into account
the background estimates above and fitting as well the number of events with
signals in one or both of the veto layers to account for the number of
incoming muons that fail to be detected by the front veto scintillator
station. The fitted number of neutrino events is $153^{+12}_{-13}$ events with
a statistical significance of 16 standard deviations over the background-only
hypothesis. The right side of Figure 3 shows that the events are distributed
around the LOS as expected. Additional properties of the neutrino events are
displayed in Figure 4. It confirms that the events have larger activity in the
IFT than incoming muon events and that the reconstructed muons and
consequently also the interacting neutrinos have energies that are
significantly larger than $200\text{\,}\mathrm{GeV}$. A clear charge
separation in $q/p_{\mu}$ for the reconstructed tracks is also observed with a
total of 40 events with a positively-charged track, demonstrating the presence
of anti-neutrinos in the analyzed data set.

Figure 4: The figures depict the number of reconstructed clusters in the IFT,
momentum, $p_{\mu}$ and $q/p_{\mu}$ for events in the signal region (black
markers) and compares them to the expectation from simulation GENIE (blue) and
muon-like events (grey markers). The muon-like events are from events where
both layers of the front scintillator station observed a signal, and show the
expected distributions for non-neutrino backgrounds. The blue bands correspond
to the statistical error of the simulated samples and are luminosity scaled
for $p_{\mu}$ and $q/p_{\mu}$ and normalized to unity for clusters.

##  5 Summary

The first physics results from the FASER experiment have been presented. No
signal events are observed in an almost background free search for dark photon
decays to $e^{+}e^{-}$ and new limits are set in a cosmological relevant
region of parameter space. FASER has performed the first direct detection of
neutrinos produced at a collider 111In addition to FASER, the SND@LHC
experiment reported evidence of neutrino interactions elsewhere in these
proceedings. with $153^{+12}_{-13}$ neutrino events observed from charged-
current interactions of $\nu_{\mu}$ and $\overline{\nu}_{\mu}$ in FASER$\nu$.
These results represent the beginning of the FASER physics programme with many
more results to come in the next years with up to an order of magnitude more
luminosity expected.

## References

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