# Dielectron production in proton–proton collisions at $\sqrt{s}=7$ TeV with
ALICE

H. Sebastian Scheid
for the ALICE Collaboration
<EMAIL_ADDRESS>

###### Abstract

The ALICE Collaboration measured dielectron production as a function of the
invariant mass ($m_{\rm ee}$), the pair transverse momentum ($p_{\rm T,ee}$)
and the pair distance of closest approach ($\rm DCA_{ee}$) in pp collisions at
$\sqrt{s}=7$ TeV. Prompt and non-prompt dielectron sources can be separated
with the $\rm DCA_{ee}$, which will give the opportunity in heavy-ion
collisions to identify thermal radiation from the medium in the intermediate-
mass range dominated by contributions from open-charm and beauty hadron
decays. The charm and beauty total cross sections are extracted from the data
by fitting the spectra with two different MC generators, i.e. PYTHIA a leading
order event generator and POWHEG a next-to-leading order event generator.
Significant model dependences are observed, reflecting the sensitivity of this
measurement to the heavy-flavour production mechanisms.111Presented at Hot
Quarks 2018 - Workshop for young scientists on the physics of
ultrarelativistic nucleus-nucleus collisions, Texel, The Netherlands,
September 7-14 2018

_K_ eywords ALICE $\cdot$ Dielectron $\cdot$ Heavy flavour

##  1 Introduction

The ALICE experiment at the LHC is dedicated to the study of strongly
interacting matter under extreme conditions, i.e. high temperature, which can
be reached in heavy-ion collisions. In such collisions, the formation of a
Quark-Gluon Plasma (QGP) is expected. Dielectrons are produced at all stages
of the collision and therefore carry information about the whole evolution of
the system. Since they do not interact strongly with the medium, they are a
prime probe to study the properties of the QGP. Dielectrons stem from decays
of pseudoscalar and vector mesons, from semi-leptonic decays of correlated
open-charm and open-beauty hadrons and from internal conversions of direct
photons. In heavy-ion collisions, additional sources are expected, i.e.
thermal radiation from the QGP and hadron gas. The medium introduces
modifications of the vector meson properties, in particular the short-lived
$\rho$, related to chiral symmetry restoration. In addition, the initial
conditions of the collisions are expected to change compared to elementary
collisions due to modifications of the parton distribution functions in
nuclei. The latter can be studied in proton-lead (p–Pb) collisions, whereas pp
collisions provide an important vacuum baseline. It is crucial to first
understand the dielectron production in vacuum to single out the signal
characteristics of the QGP. Moreover, proton-proton (pp) collisions can also
be used to study the heavy-flavour (HF) and direct photon production.

In the following, the steps of the data analysis are explained and the first
measurements of the dielectron production in pp collisions at $\sqrt{s}=7$ TeV
are presented and discussed [ALICE(2018)].

##  2 Data analysis and results

The analysis is performed with pp data taken during the first data-taking
period of the LHC in 2010 with the ALICE detector. The integrated luminosity
of the data sample is $L_{\rm int}=6.0\pm 0.2$ nb-1. After identifying
electrons in the ALICE detector it is not a priori clear which electrons
belong to the same pair. We follow a statistical approach to obtain the final
spectrum. The electrons and positrons are combined to an opposite-sign
spectrum (OS), which includes not only the signal but also background, that
can be purely combinatorial or have some residual correlation from jets or
cross pairs from double Dalitz decays. This background is estimated by the
same-sign spectrum (SS). Residual acceptance differences for OS and SS pairs
are estimated with mixed events and taken into account during the subtraction
of the background. Finally, the background-subtracted spectrum is corrected
for tracking and particle identification inefficiencies within the ALICE
acceptance ($p_{\rm T,e}>0.2$ GeV/$c$, $\eta_{\rm e}<0.8$).

In Fig. 1 the measured dielectron cross section as a function of $m_{\rm ee}$
is compared to a so-called hadronic cocktail, which includes all known sources
of dielectron production from hadron decays and uses parameterisations of
measured spectra as input when available. Where no measurements are available
$m_{\rm T}$-scaling [WA80(1995)] is applied. The HF contributions are
simulated using the Perugia2011 tune of PYTHIA 6 [Sjostrand, T. and Mrenna, S.
and Skands, P.(2006), Skands, P.(2010)]. The resulting dielectron pairs from
the hadron decays are then filtered through the ALICE acceptance.

Figure 1: Dielectron cross section as a function of $m_{\rm ee}$ compared to a
cocktail of known hadronic sources.

A good agreement is observed between the cocktail and the data. The charm
contribution already dominates the spectrum for $m_{\rm ee}\geq 0.5$
GeV/$c^{2}$. The very large heavy-flavour contribution makes the measurement
of thermal radiation from the medium in heavy-ion collisions very challenging
at LHC energies. To separate the heavy-flavour background from thermal
radiation from the QGP in a future heavy-ion run in the intermediate-mass
range (IMR, $\phi<m_{\rm ee}<J/\psi$), an additional variable, the pair-
distance-of-closest-approach ($\rm DCA_{ee}$), is added to the traditional
analysis as a function of $m_{\rm ee}$ and $p_{\rm T,ee}$. $\rm DCA_{ee}$ is
defined as:

${\rm DCA_{ee}}=\sqrt{\frac{({\rm DCA_{{\it
xy},1}}/\sigma_{xy{\rm,1}})^{2}+({\rm DCA_{{\it
xy},2}}/\sigma_{xy,2})^{2}}{2}}$ (1)

Here ${\rm DCA}_{xy,i}$ is the closest distance between the reconstructed
electron track and the primary collision vertex in the transverse plane. Its
resolution is estimated from the covariance matrix of the track reconstruction
and denoted as $\sigma_{xy,i}$. In the case of weak decays, the decay electron
candidates do not point to the vertex which leads to a broader DCA
distribution than for tracks from prompt decays. This can be seen in Fig. 2
and Fig. 3, where the $\rm DCA_{ee}$ spectra are shown for two invariant mass
regions. Fig. 2 shows the mass region between the $\pi^{0}$ and the $\phi$
mass. The light flavour template is taken from the $\pi^{0}$ shape, normalised
to the expected contribution from all light flavour sources. Fig. 3 shows the
mass region around the $J/\psi$ mass peak. In both mass regions we can see a
clear peak which can be described by the expected prompt contributions,
whereas the tail of the spectrum is described by the broader contributions
from charm and beauty.

Figure 2: Dielectron spectrum as a function of $\rm DCA_{ee}$ for $0.14<m_{\rm
ee}<1.1$ GeV/$c^{2}$ [ALICE(2018)].

Figure 3: Dielectron spectrum as a function of $\rm DCA_{ee}$ for $2.7<m_{\rm
ee}<3.3$ GeV/$c^{2}$ [ALICE(2018)].

In Fig. 3 the $J/\psi$ from $B$-mesons can be seen in addition to the open HF
contributions. In the so-called intermediate mass region, located between the
$\phi$ and $J/\psi$ in the mass spectrum, the dominant contribution is from
open HF. The dielectron cross section as function of $p_{\rm T,ee}$ and $\rm
DCA_{ee}$ is compared to a hadronic cocktail using PYTHIA 6 Perugia0 [Skands,
P.(2010)] to estimate the $\rm c\bar{c}$ and $\rm b\bar{b}$ contributions in
the left and right panels of Fig. 4, respectively.

Figure 4: Dielectron cross section as a function of $p_{\rm T,ee}$ (left) and
$\rm DCA_{ee}$ (right) in the IMR compared to a cocktail calculated with
PYTHIA 6 [ALICE(2018)].

The data are well described by the hadronic cocktail within the statistical
and systematic uncertainties. The contribution from $\rm c\bar{c}$ dominates
the dielectron yield at low $p_{\rm T,ee}$ and relatively small $\rm
DCA_{ee}$, whereas the $\rm b\bar{b}$ becomes relevant at high $p_{\rm T,ee}$
and large $\rm DCA_{ee}$. To investigate the processes of heavy-quark
production we changed the generator from PYTHIA to POWHEG, switching from
leading order in the HF quark generation to next-to-leading order. To quantify
the differences the total $\rm c\bar{c}$ and $\rm b\bar{b}$ cross sections are
extracted from the data by fitting the results two-dimensionally as a function
of $p_{\rm T,ee}$ and $m_{\rm ee}$ and one-dimensionally as a function of $\rm
DCA_{ee}$ in the IMR allowing the contributions of the two HF components to
vary. The results are shown in the left and right panels of Fig. 5 for PYTHIA
and POWHEG[Nason, P.(2004)], respectively. Both fits give consistent results
for a given MC event generator. The uncertainties are fully correlated between
the cross sections extracted with PYTHIA and POWHEG. Significant model
dependences are observed which reflect the different rapidity distribution of
charm quarks and different $p_{\rm T,ee}$ spectra of the $\rm c\bar{c}$ and
$\rm b\bar{b}$ contributions predicted by the two models.

Figure 5: Total $\rm c\bar{c}$ and $\rm b\bar{b}$ cross sections with their
systematic and statistical uncertainties, extracted from a fit of the measured
dielectron yield from heavy-flavour hadron decays in ($m_{\rm ee}$, $p_{\rm
T,ee}$) and in $\rm DCA_{ee}$with PYTHIA (left) and POWHEG (right) are
compared to published cross sections (lines) [ALICE(2018)].

The results are compared to independent measurements of $\sigma_{\rm
c\bar{c}}$[ALICE(2017)] and $\sigma_{\rm b\bar{b}}$[LHCb(2011)] from single
heavy-flavour particle spectra and found to be consistent within the large
uncertainties. Once these uncertainties are reduced, the dielectron
measurements can give further constraints on the MC event generators aiming to
reproduce the heavy-flavour production mechanisms.

##  3 Conclusion

To summarise, ALICE measured the dielectron cross sections as a function of
$m_{\rm ee}$, $p_{\rm T,ee}$, and $\rm DCA_{ee}$ in pp collisions at
$\sqrt{s}=7$ TeV. The hadronic cocktail is in good agreement with the measured
dielectron cross sections in the three discussed observables, which suggests a
good understanding of the dielectron cross section in the ALICE acceptance. We
show that $\rm DCA_{ee}$ makes it possible to separate prompt from non-prompt
dielectron pairs, and thus will be a key tool to determine the average
temperature of the QGP formed in heavy-ion collisions in the future. In the
heavy flavour sector we can report a significant dependence of the total cross
sections of charm and beauty when using PYTHIA and POWHEG, which reflects the
sensitivity of the dielectron measurement to the underlying heavy-flavour
production mechanisms implemented in the models.

## References

  * [ALICE(2018)] ALICE, JHEP 2018, 09, 64, doi:10.1007/JHEP09(2018)064.
  * [WA80(1995)] WA80, Phys. Lett. 1995, B361, 14, doi:10.1016/0370-2693(95)01166-N.
  * [Sjostrand, T. and Mrenna, S. and Skands, P.(2006)] Sjostrand, T. and Mrenna, S. and Skands, P., JHEP 2006, 06, 26, doi:10.1088/1126-6708/2006/05/026.
  * [Skands, P.(2010)] Skands, P., Phys. Rev. 2010, D82, 26, doi:10.1103/PhysRevD.82.074018.
  * [Nason, P.(2004)] Nason, P., JHEP 2004, 11, 40, doi:10.1088/1126-6708/2004/11/040.
  * [ALICE(2017)] ALICE, Eur. Phys. J. 2017, C77, 550, doi:10.1140/epjc/s10052-017-5090-4.
  * [LHCb(2011)] LHCb, Eur. Phys. J. 2011, C71, 1645, doi:10.1140/epjc/s10052-011-1645-y.


11institutetext:  Instituto de Física Fundamental, CSIC, Calle Serrano 123,
E-28006 Madrid, Spain
11email<EMAIL_ADDRESS><EMAIL_ADDRESS>22institutetext:
Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, E-28014 Madrid,
Spain 33institutetext: Observatorio de Yebes, IGN, Cerro de la Palera s/n,
E-19141 Yebes, Guadalajara, Spain

# O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1:
detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3††thanks: Based on
observations carried out with the Yebes 40m telescope (projects 19A003,
20A014, and 20D023). The 40m radiotelescope at Yebes Observatory is operated
by the Spanish Geographic Institute (IGN, Ministerio de Transportes, Movilidad
y Agenda Urbana).

M. Agúndez 11 N. Marcelino 11 B. Tercero 2233 C. Cabezas 11 P. de Vicente 33
J. Cernicharo 11

(Received; accepted)

We report the detection of the oxygen-bearing complex organic molecules
propenal (C2H3CHO), vinyl alcohol (C2H3OH), methyl formate (HCOOCH3), and
dimethyl ether (CH3OCH3) toward the cyanopolyyne peak of the starless core
TMC-1. These molecules are detected through several emission lines in a deep
Q-band line survey of TMC-1 carried out with the Yebes 40m telescope. These
observations reveal that the cyanopolyyne peak of TMC-1, which is the
prototype of cold dark cloud rich in carbon chains, contains also O-bearing
complex organic molecules like HCOOCH3 and CH3OCH3, which have been previously
seen in a handful of cold interstellar clouds. In addition, this is the first
secure detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH
are detected in a cold environment, adding new pieces in the puzzle of complex
organic molecules in cold sources. We derive column densities of (2.2 $\pm$
0.3) $\times$ 1011 cm-2, (2.5 $\pm$ 0.5) $\times$ 1012 cm-2, (1.1 $\pm$ 0.2)
$\times$ 1012 cm-2, and (2.5 $\pm$ 0.7) $\times$ 1012 cm-2 for C2H3CHO,
C2H3OH, HCOOCH3, and CH3OCH3, respectively. Interestingly, C2H3OH has an
abundance similar to that of its well known isomer acetaldehyde (CH3CHO), with
C2H3OH/CH3CHO $\sim$ 1 at the cyanopolyyne peak. We discuss potential
formation routes to these molecules and recognize that further experimental,
theoretical, and astronomical studies are needed to elucidate the true
mechanism of formation of these O-bearing complex organic molecules in cold
interstellar sources.

###### Key Words.:

astrochemistry – line: identification – ISM: individual objects (TMC-1) – ISM:
molecules – radio lines: ISM

##  1 Introduction

Complex organic molecules (COMs) like methyl formate (HCOOCH3) and dimethyl
ether (CH3OCH3) have been traditionally observed in the warm gas around
protostars, the so-called hot cores and corinos, where they are thought to
form upon thermal desorption of ice mantles on grains (Herbst & van Dishoeck,
2009). In the last decade, these molecules have been observed as well in a few
cold sources, like the dense cores B1-b (Öberg et al., 2010; Cernicharo et
al., 2012) and L483 (Agúndez et al., 2019), the dark cloud Barnard 5 (Taquet
et al., 2017), the pre-stellar cores L1689B (Bacmann et al., 2012) and L1544
(Jiménez-Serra et al., 2016), and the starless core TMC-1 (Soma et al., 2018).
The low temperatures in these environments inhibit thermal desorption, and it
is still an active subject of debate how are these molecules formed, whether
in the gas phase or on grain surfaces followed by some non-thermal desorption
process (Vasyunin & Herbst, 2013; Ruaud et al., 2015; Balucani et al., 2015;
Chang & Herbst, 2016; Vasyunin et al., 2017; Shingledecker et al., 2018; Jin &
Garrod, 2020).

The cyanopolyyne peak of TMC-1, TMC-1(CP), is characterized by a carbon-rich
chemistry, with high abundances of carbon chains and a poor content of
O-bearing COMs (e.g., Agúndez & Wakelam 2013). Here we report the detection of
four O-bearing COMs toward TMC-1(CP). Propenal (C2H3CHO) has been reported
previously toward massive star-forming regions in the Galactic center (Hollis
et al., 2004; Requena-Torres et al., 2008) and in the hot corino IRAS
16293-2422 B (Manigand et al., 2021). Vinyl alcohol (C2H3OH) has been only
seen toward Sagittarius B2(N), where the high spectral density complicates the
identification (Turner & Apponi, 2001). Therefore, this is the first clear
detection of C2H3OH in space and the first time that C2H3CHO and C2H3OH are
detected in a cold environment. We also report the detection of HCOOCH3 and
CH3OCH3, recently detected (the latter tentatively) toward the methanol peak
of TMC-1 (Soma et al., 2018) but not toward TMC-1(CP).

##  2 Astronomical observations

The data presented here belong to a Q-band line survey of TMC-1(CP),
$\alpha_{J2000}$ = $4^{\rm h}$ $41^{\rm m}$ $41.9^{\rm s}$ and
$\delta_{J2000}$ = $+25^{\circ}$ $41^{\prime}$ $27.0^{\prime\prime}$,
performed with the Yebes 40m telescope. The cryogenic receiver for the Q band,
built within the Nanocosmos project 111https://nanocosmos.iff.csic.es and
which covers the 31.0-50.4 GHz frequency range with horizontal and vertical
polarizations, was used connected to FFTS spectrometers, which cover a
bandwidth of $8\times 2.5$ GHz in each polarization with a spectral resolution
of 38.15 kHz. The system is described in Tercero et al. (2021). The half power
beam width (HPBW) of the Yebes 40m telescope ranges from 36.4 ′′ to 54.4 ′′ in
the Q band. The intensity scale is antenna temperature, $T_{A}^{*}$, for which
we estimate an uncertainty of 10 %, which can be converted to main beam
brightness temperature, $T_{\rm mb}$, by dividing by $B_{\rm eff}$/$F_{\rm
eff}$ (see Table 1). The line survey was carried out during several observing
runs and various results have been already published. Data taken during
November 2019 and February 2020 allowed to detect the negative ions C3N- and
C5N- (Cernicharo et al., 2020a), and to discover HC4NC (Cernicharo et al.,
2020b), HC3O+ (Cernicharo et al., 2020c), and HC5NH+ (Marcelino et al., 2020).
A further observing run, carried out in October 2020, resulted in the
detection of HDCCN (Cabezas et al., 2021), HC3S+ (Cernicharo et al., 2021a),
CH3CO+ (Cernicharo et al., 2021b), and various C4H3N isomers (Marcelino et
al., 2021). Additional observations were taken in December 2020 and January
2021, which led to the discovery of vinyl acetylene (CH2CHCCH; Cernicharo et
al. 2021c), allenyl acetylene (CH2CCHCCH; Cernicharo et al. 2021d), and
propargyl (CH2CCH; Agúndez et al. 2021), and a final run was carried out in
March 2021. All observations were carried out using the frequency switching
technique, with a frequency throw of 10 MHz during the two first observing
runs and 8 MHz in the later ones. All data were reduced with the program CLASS
of GILDAS (Pety, 2005) 222http://www.iram.fr/IRAMFR/GILDAS.

Table 1: Observed line parameters of the target O-bearing COMs of this study
in TMC-1.

Molecule | Transition | $E_{\rm up}$ | $\nu_{\rm calc}$ |  $T_{A}^{*}$ peak |  $\Delta v$ a | $V_{\rm LSR}$ | $\int T_{A}^{*}dv$ |  S/N b |  $B_{\rm eff}$/$F_{\rm eff}$ c
---|---|---|---|---|---|---|---|---|---
|  | (K) | (MHz) | (mK) |  (km s-1) |  (km s-1) |  (mK km s-1) |  ($\sigma$) |
$trans$-C2H3CHO |  41,4-31,3 | 6.2 | 34768.987 | $3.06\pm 0.31$ | $0.62\pm 0.08$ | $5.77\pm 0.03$ | $2.03\pm 0.21$ | 14.5 | 0.603
40,4-30,3 | 4.3 | 35578.136 | $3.77\pm 0.33$ | $0.81\pm 0.08$ | $5.79\pm 0.03$ | $3.27\pm 0.27$ | 19.4 | 0.597
41,3-31,2 | 6.4 | 36435.990 | $3.17\pm 0.29$ | $0.85\pm 0.09$ | $5.73\pm 0.04$ | $2.87\pm 0.25$ | 19.2 | 0.590
51,5-41,4 | 8.3 | 43455.469 | $1.81\pm 0.49$ | $0.72\pm 0.27$ | $5.78\pm 0.08$ | $1.38\pm 0.37$ | 6.5 | 0.530
50,5-40,4 | 6.4 | 44449.749 | $3.67\pm 0.54$ | $0.54\pm 0.08$ | $5.80\pm 0.04$ | $2.11\pm 0.31$ | 10.5 | 0.521
51,4-41,3 | 8.6 | 45538.994 | $4.72\pm 0.66$ | $0.69\pm 0.09$ | $5.80\pm 0.04$ | $3.45\pm 0.43$ | 12.6 | 0.511
$syn$-C2H3OH |  40,4-31,3 | 9.3 | 32449.221 | $1.17\pm 0.32$ | $1.07\pm 0.21$ | $5.61\pm 0.11$ | $1.34\pm 0.27$ | 6.8 | 0.622
21,2-11,1 | 5.1 |  37459.184 d | – | – | – | – | – | 0.581
20,2-10,1 | 2.8 | 39016.387 | $1.90\pm 0.39$ | $0.89\pm 0.16$ | $5.81\pm 0.07$ | $1.79\pm 0.29$ | 9.0 | 0.568
21,1-11,0 | 5.3 | 40650.606 | $1.71\pm 0.41$ | $0.65\pm 0.19$ | $5.88\pm 0.08$ | $1.18\pm 0.28$ | 6.7 | 0.554
HCOOCH3 |  31,3-21,2 $E$ | 4.0 | 34156.889 | $1.92\pm 0.29$ | $0.62\pm 0.12$ | $5.93\pm 0.06$ | $1.27\pm 0.23$ | 9.6 | 0.608
31,3-21,2 $A$ | 3.9 | 34158.061 | $1.86\pm 0.29$ | $0.80\pm 0.16$ | $5.84\pm 0.07$ | $1.59\pm 0.28$ | 10.6 | 0.608
30,3-20,2 $E$ | 3.5 |  36102.227 e | – | – | – | – | – | 0.593
30,3-20,2 $A$ | 3.5 | 36104.775 | $2.61\pm 0.29$ | $0.74\pm 0.09$ | $5.80\pm 0.04$ | $2.05\pm 0.22$ | 14.6 | 0.592
31,2-21,1 $E$ | 4.4 | 38976.085 | $1.81\pm 0.35$ | $0.66\pm 0.14$ | $5.93\pm 0.07$ | $1.28\pm 0.25$ | 8.3 | 0.568
31,2-21,1 $A$ | 4.4 | 38980.803 | $2.45\pm 0.35$ | $0.57\pm 0.09$ | $5.92\pm 0.04$ | $1.49\pm 0.21$ | 10.4 | 0.568
41,4-31,3 $E$ | 6.1 | 45395.802 | $2.82\pm 0.57$ | $0.63\pm 0.18$ | $6.00\pm 0.07$ | $1.89\pm 0.40$ | 8.3 | 0.512
41,4-31,3 $A$ | 6.1 | 45397.360 | $2.39\pm 0.57$ | $0.42\pm 0.19$ | $5.91\pm 0.10$ | $1.07\pm 0.32$ | 5.8 | 0.512
40,4-30,3 $E$ | 5.8 | 47534.116 | $3.19\pm 0.75$ | $0.59\pm 0.12$ | $5.92\pm 0.07$ | $2.01\pm 0.44$ | 7.1 | 0.493
40,4-30,3 $A$ | 5.8 | 47536.905 | $2.76\pm 0.75$ | $0.98\pm 0.23$ | $5.99\pm 0.10$ | $2.88\pm 0.60$ | 7.9 | 0.493
CH3OCH3 |  21,1-20,2 $AE+EA$ | 4.2 | 31105.223 | $1.35\pm 0.39$ | $0.92\pm 0.25$ | $5.72\pm 0.13$ | $1.31\pm 0.36$ | 5.8 | 0.632
21,1-20,2 $EE$ | 4.2 | 31106.150 | $1.52\pm 0.39$ | $0.54\pm 0.34$ | $5.94\pm 0.08$ | $0.87\pm 0.29$ | 5.0 | 0.632
31,2-30,3 $AE+EA$ | 7.0 | 32977.276 | $1.37\pm 0.25$ | $0.63\pm 0.13$ | $5.84\pm 0.07$ | $0.91\pm 0.19$ | 7.8 | 0.618
31,2-30,3 $EE$ | 7.0 | 32978.232 | $1.37\pm 0.25$ | $0.95\pm 0.23$ | $5.84\pm 0.09$ | $1.38\pm 0.27$ | 9.6 | 0.618
31,2-30,3 $AA$ | 7.0 | 32979.187 | $1.06\pm 0.25$ | $0.85\pm 0.32$ | $5.99\pm 0.12$ | $0.96\pm 0.29$ | 7.1 | 0.618
41,3-40,4 $EE$ | 10.8 | 35593.408 | $1.22\pm 0.27$ | $0.56\pm 0.13$ | $5.82\pm 0.07$ | $0.73\pm 0.16$ | 6.4 | 0.597
11,1-00,0 $EE$ | 2.3 | 47674.967 | $3.31\pm 0.62$ | $0.59\pm 0.12$ | $6.02\pm 0.06$ | $2.08\pm 0.38$ | 8.9 | 0.492

The line parameters $T_{A}^{*}$ peak, $\Delta v$, $V_{\rm LSR}$, and $\int
T_{A}^{*}dv$ and the associated errors are derived from a Gaussian fit to each
line profile. a $\Delta v$ is the full width at half maximum (FWHM). b Signal-
to-noise ratio is computed as S/N = $\int T_{A}^{*}dv$ / [rms $\times$
$\sqrt{\Delta v\times\delta\nu(c/\nu_{\rm calc})}$], where $c$ is the speed of
light, $\delta\nu$ is the spectral resolution (0.03815 MHz), the rms is given
in the uncertainty of $T_{A}^{*}$ peak, and the rest of parameters are given
in the table. c $B_{\rm eff}$ is given by the Ruze formula $B_{\rm eff}$ =
0.738 $\exp{[-(\nu/72.2)^{2}]}$, where $\nu$ is the frequency in GHz, and
$F_{\rm eff}$ = 0.97. d Line overlaps with a hyperfine component of CH2CCH
(see Agúndez et al. 2021). e Line detected but not fitted because it overlaps
with a negative frequency-switching artifact.

##  3 Molecular spectroscopy

Figure 1: Lines of C2H3CHO observed in TMC-1 (see parameters in Table 1). In
red computed synthetic spectra for $N$ = 2.2 $\times$ 1011 cm-2, $T_{\rm rot}$
= 7.5 K, FWHM = 0.71 km s-1, and $\theta_{s}$ = 80 ′′. Figure 2: Lines of
C2H3OH observed in TMC-1 (see parameters and note on line 21,1-11,0 in Table
1). In red computed synthetic spectra for $N$ = 2.5 $\times$ 1012 cm-2,
$T_{\rm rot}$ = 10 K, FWHM = 0.87 km s-1, and $\theta_{s}$ = 80 ′′. Figure 3:
Lines of HCOOCH3 observed in TMC-1 (see parameters and note on line 30,3-20,2
$E$ in Table 1). In red computed synthetic spectra for a $N$ = 1.1 $\times$
1012 cm-2, $T_{\rm rot}$ = 5 K, FWHM = 0.67 km s-1, and $\theta_{s}$ = 80 ′′.
Figure 4: Lines of CH3OCH3 observed in TMC-1 (see parameters in Table 1). In
red computed synthetic spectra for $N$ = 2.5 $\times$ 1012 cm-2, $T_{\rm rot}$
= 3.6 K, FWHM = 0.72 km s-1, and $\theta_{s}$ = 80 ′′.

C2H3CHO has two conformers. The most stable, and the only one reported in
space (Hollis et al., 2004; Requena-Torres et al., 2008; Manigand et al.,
2021), is the $trans$ form, which is the one reported here as well. Level
energies and transition frequencies were obtained from the rotational
constants derived by Daly et al. (2015). The dipole moment along the $a$ axis
(all transitions observed here are of $a$-type) is 3.052 D (Blom et al.,
1984).

C2H3OH has also two conformers, named $syn$ and $anti$. Turner & Apponi (2001)
assigned various emission features to the two conformers in the crowded
spectra of Sgr B2(N). Here we report an unambiguous detection of the $syn$
form, which is the most stable one, in a colder source that is much less
affected by line confusion. Level energies and transition frequencies were
computed from the rotational constants given by Melosso et al. (2019). The
components of the dipole moment along the $a$ and $b$ axes are 0.616 D and
0.807 D, respectively (Saito, 1976). Both $a$\- and $b$-type transitions are
observed here.

HCOOCH3 is a well known interstellar molecule in which the internal rotation,
or torsion, of the methyl group makes each rotational level to split into $A$
and $E$ substates, with statistical weights A:E = 1:1. We used the
spectroscopy from a fit to the lines measured by Ogata et al. (2004)
implemented in MADEX (Cernicharo et al., 2012). Here all observed transitions
are of $a$-type and thus we adopted $\mu_{a}$ = 1.63 D (Curl, 1959).

CH3OCH3 is an asymmetric rotor in which the large amplitude internal motion of
the two equivalent methyl groups leads to level splitting into $AA$, $EE$,
$EA$, and $AE$ substates, with non-trivial statistical weights (Endres et al.,
2009). We adopted the spectroscopy from the CDMS catalog (Müller et al., 2005)
333https://cdms.astro.uni-koeln.de/. The geometry of the molecule makes it to
have non-zero dipole moment only along the $b$ axis, with a measured value of
1.302 D (Blukis et al., 1963).

##  4 Results

We detected various lines for each of the O-bearing COMs target of this study
(see Table 1), with signal-to-noise ratios (S/N) well above 3 $\sigma$. The
position of the lines is consistent with the calculated frequencies based on
laboratory data (see Sec. 3) and the systemic velocity of the source, $V_{\rm
LSR}$ = 5.83 km s-1 (Cernicharo et al., 2020b). Moreover, the relative
intensities of the lines are those expected for rotational temperatures in the
range 3-10 K, which are typical of TMC-1 (Gratier et al., 2016), and there are
no missing lines. We thus consider that the detection of the four O-bearing
COMs in TMC-1 is secure. Hereafter we discuss the particularities of each
molecule.

The detection of the $trans$ form of C2H3CHO is very solid since the six
observed lines are detected with S/N between 6.5 $\sigma$ and 19.4 $\sigma$
and the $V_{\rm LSR}$ of the lines are fully consistent with the systemic
velocity of TMC-1 (see Table 1 and Fig. 1). The rotational temperature
($T_{\rm rot}$) of C2H3CHO is not very precisely determined, 7.5 $\pm$ 3.5 K
from a rotation diagram, but synthetic spectra computed under thermodynamic
equilibrium indicate that values in the range 5-10 K are consistent with the
relative intensities observed. We thus adopted $T_{\rm rot}$ = 7.5 K to
compute the synthetic spectra and derive the column density. The arithmetic
mean of the observed C2H3CHO linewidths, 0.71 km s-1, is adopted when
computing the synthetic spectra (see Table 1). We also assumed a circular
emission distribution with a diameter $\theta_{s}$ = 80′′, as observed for
various hydrocarbons in TMC-1 (Fossé et al., 2001). For the other three
molecules we followed the same convention, adopting as linewidth the average
of the observed values and assuming the same emission distribution. The column
density derived for C2H3CHO is (2.2 $\pm$ 0.3) $\times$ 1011 cm-2.

In the case of C2H3OH (see Table 1 and Fig. 2), three lines are clearly
detected, with S/N of 6.7 $\sigma$ and 9.0 $\sigma$. The frequency of the
$2_{1,2}$-11,1 transition, 37459.184 MHz, coincides with a hyperfine component
of CH2CCH, recently reported in TMC-1 (Agúndez et al., 2021). We predict
$T_{A}^{*}$ = 1.1 mK for the $2_{1,2}$-11,1 transition, which indicates that
the observed line (see Agúndez et al. 2021) has contributions from both CH2CCH
and C2H3OH. The high detection significance of three lines, the overlap of a
fourth line with CH2CCH, and the fact that there are no missing lines, makes
us to consider the detection of C2H3OH secure. The rotational temperature is
not well constrained for C2H3OH, although the observed relative intensities
indicate that it must be in the high range of the values typically observed in
TMC-1. We thus adopted $T_{\rm rot}$ = 10 K and a linewidth of 0.87 km s-1 to
compute the synthetic spectra. We derive a column density of (2.5 $\pm$ 0.5)
$\times$ 1012 cm-2 for C2H3OH.

We detected five $A$/$E$ doublets of HCOOCH3 with S/N in the range 5.8-14.6
$\sigma$ (see Table 1 and Fig. 3). The only line affected by a problem is the
30,3-20,2 transition of the $E$ substate, which accidentally lies close to a
negative frequency-switching artifact, making it to appear less intense and
slightly shifted from the correct position. We thus did not fit this line. The
rotational temperature derived is 5.1 $\pm$ 2.5 K, and we thus adopted $T_{\rm
rot}$ = 5 K, and a linewidth of 0.67 km s-1, to compute the synthetic spectra.
The total column density obtained for HCOOCH3, including both $A$ and $E$
substates, is (1.1 $\pm$ 0.2) $\times$ 1012 cm-2.

For CH3OCH3, we observed four triplets with the characteristic structure of an
intense component corresponding to the $EE$ substate lying between two equally
intense components corresponding to the $AE$+$EA$ and $AA$ substates. The $EE$
component of the four triplets is detected with good confidence levels,
between 5.0 $\sigma$ and 9.6 $\sigma$ (see Table 1 and Fig. 4). The weaker
components corresponding to the $AE$+$EA$ and $AA$ substates are sometimes
found to lie within the noise, although the computed synthetic spectra is
consistent with this fact. We consider that the detection of CH3OCH3 is
secure. From a rotation diagram we derive a low rotational temperature of 3.6
$\pm$ 0.6 K, which is well constrained by the availability of transition
covering upper level energies from 2.3 K to 10.8 K. We thus adopted $T_{\rm
rot}$ = 3.6 K and a linewidth of 0.72 km s-1 to compute the synthetic spectra,
which implies a total column density, including the four substates, of (2.5
$\pm$ 0.7) $\times$ 1012 cm-2 for CH3OCH3.

The variation in the column densities due to the uncertainty in $T_{\rm rot}$
is small, $\sim$ 15 %, for C2H3CHO and C2H3OH, and higher, a factor of two,
for HCOOCH3 and CH3OCH3.

##  5 Discussion

The abundances derived for C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3 are 2.2
$\times$ 10-11, 2.5 $\times$ 10-10, 1.1 $\times$ 10-10, and 2.5 $\times$
10-10, respectively, relative to H2, if we adopt a column density of H2 of
1022 cm-2 (Cernicharo & Guélin, 1987). Now, how are these four O-bearing COMs
formed in TMC-1?.

C2H3CHO and C2H3OH could be formed by gas-phase neutral-neutral reactions
between reactive radicals like OH, CH, or C2H and abundant closed-shell
molecules. However, among the potential sources of C2H3OH, the reactions OH +
C2H4 and OH + CH2CHCH3 seem to have activation barriers (Zhu et al., 2005;
Zádor et al., 2009) and the reaction CH + CH3OH seems to yield H2CO and CH3 as
products (Zhang et al., 2002). In the case of C2H3CHO, a potential formation
reaction is OH + allene, but the main products are H2CCO + CH3 (Daranlot et
al., 2012). Two more promising routes to C2H3CHO are the reactions CH +
CH3CHO, which has been found to produce C2H3CHO (Goulay et al., 2012), and C2H
+ CH3OH, which to our knowledge has not been studied experimentally or
theoretically.

C2H3CHO and C2H3OH are not specifically considered in the chemical networks
UMIST RATE12 (McElroy et al., 2013) or KIDA uva.kida.2014 (Wakelam et al.,
2015), but acetaldehyde (CH3CHO), which is an isomer of C2H3OH, is included.
Since it often happens that astrochemical databases do not distinguish between
different isomers because information is not available, it is conceivable that
some of the reactions that are considered to produce CH3CHO could also form
C2H3OH. According to a standard pseudo-time-dependent gas-phase chemical model
of a cold dark cloud (e.g., Agúndez & Wakelam 2013), there are two main
reactions of formation of CH3CHO. The first is O + C2H5, for which formation
of C2H3OH does not seem to be an important channel, according to experiments
(Slagle et al., 1988) and theory (Jung et al., 2011; Vazart et al., 2020). The
second is the dissociative recombination of CH3CHOH+ with electrons, in which
case experiments show that the CCO backbone is preserved with a branching
ratio of 23 % (Hamberg et al., 2010), so that it is possible that both CH3CHO
and C2H3OH are formed. A different route to CH3CHO starting from abundant
ethanol (C2H5OH) has been proposed (Skouteris et al., 2018; Vazart et al.,
2020), but in L483, the only cold environment where C2H5OH has been detected,
its abundance is twice smaller than that of CH3CHO (Agúndez et al., 2019). The
column density of CH3CHO at TMC-1(CP) is (2.7-3.5) $\times$ 1012 cm-2 (Gratier
et al., 2016; Cernicharo et al., 2020c), which implies a C2H3OH/CH3CHO ratio
of $\sim$ 1\. Therefore, if the two isomers are formed by the same reaction,
then the branching ratios should be similar. The reaction CH3+ \+ H2CO
produces CH4 \+ HCO+ (Smith & Adams, 1978), and thus it is unlikely to form
C2H3OH, as suggested by Turner & Apponi (2001).

Grain-surface processes could also form C2H3CHO and C2H3OH in TMC-1.
Experiments show that C2H3OH is formed upon proton irradiation of H2O/C2H2
ices (Hudson & Moore, 2003), and electron irradiation of CO/CH4 and H2O/CH4
ices (Abplanalp et al., 2016; Bergantini et al., 2017), while C2H3CHO is
produced after electron irradiation of CO/C2H4 ices (Abplanalp et al., 2015).
Non-energetic processing of C2H2 ices, in which reactions with H atoms and OH
radicals occur on the surface, also produces C2H3OH (Chuang et al., 2020). It
remains however uncertain whether these experimental setups (e.g., in terms of
irradiation fluxes and ice composition) resemble those of cold dark clouds.
Abplanalp et al. (2016) made an effort in this sense by incorporating the
results of electron irradiation experiments in a chemical model of a cold dark
cloud, and found that cosmic rays could drive the formation of C2H3OH on grain
surfaces. Recently, Shingledecker et al. (2019) proposed that C2H3CHO can be
efficiently formed on grain surfaces by successive reactions of addition of an
H atom to HC3O. This process would also produce propynal (HCCCHO), which make
these authors to propose a chemical connection, and thus a potential
correlation, between HCCCHO and C2H3CHO. If this mechanism is correct, then
C2H3CHO would be more likely detected in those sources with intense HCCCHO
emission (Loison et al., 2016).

There is no yet consensus on how are HCOOCH3 and CH3OCH3 formed in cold
sources. Models where the synthesis relies on chemical desorption and gas-
phase radiative associations usually require a chemical desorption efficiency
as high as 10 % (Vasyunin & Herbst, 2013; Balucani et al., 2015; Chang &
Herbst, 2016), which can be relaxed if Eley-Rideal processes (Ruaud et al.,
2015), radiation chemistry (Shingledecker et al., 2018), or nondiffussive
grain-surface processes (Jin & Garrod, 2020) are considered. These models can
account for abundances relative to H2 around 10-10 for HCOOCH3 and/or CH3OCH3
under certain assumptions, although they rely on yet poorly constrained
chemical and physical processes. Astronomical observations pointed out that
HCOOCH3 and CH3OCH3 could have a chemical connection with CH3OH, based on the
slight abundance enhancement inferred for these O-bearing COMs at the CH3OH
peak with respect to the dust peak in the pre-stellar core L1544 (Jiménez-
Serra et al., 2016). The column densities derived here for HCOOCH3 and CH3OCH3
at TMC-1(CP) are similar, within a factor of two, to those reported by Soma et
al. (2018) at the CH3OH peak of TMC-1. A coherent study using the same
telescope and a detailed radiative transfer model is needed to see if there is
a significant abundance enhancement of HCOOCH3 and CH3OCH3 at the CH3OH peak
of TMC-1.

##  6 Conclusions

We reported the detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3 toward the
cyanopolyyne peak of TMC-1. This region, which is a prototypical cold dark
cloud with abundant carbon chains, has been revealed as a new cold source
where the O-bearing COMs HCOOCH3 and CH3OCH3 are present. In addition, we
provide the first evidence of two other O-bearing COMs, C2H3CHO and C2H3OH, in
a cold source, the latter being identified unambiguously for the first time in
space here. The abundances relative to H2 derived are a few 10-11 for C2H3CHO
and a few 10-10 for the three other molecules. Interestingly, C2H3OH has a
similar abundance to its isomer CH3CHO, with C2H3OH/CH3CHO $\sim$ 1\. We
discuss potential formation routes to these molecules and conclude that
further experimental, theoretical, and astronomical studies are needed to shed
light on the origin of these COMs in cold interstellar sources.

###### Acknowledgements.

We acknowledge funding support from Spanish MICIU through grants
AYA2016-75066-C2-1-P, PID2019-106110GB-I00, PID2019-106235GB-I00, and
PID2019-107115GB-C21 and from the European Research Council (ERC Grant 610256:
NANOCOSMOS). M.A. also acknowledges funding support from the Ramón y Cajal
programme of Spanish MICIU (grant RyC-2014-16277). We thank the anonymous
referee for a constructive report that helped to improve this manuscript.

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