L’expérience LHCb (Large Hadron Collider beauty) explore les légères différences qui existent entre matière et antimatière grâce à l’étude d’un type de particule appelé « quark beauté » ou « quark b ».
Au lieu d’utiliser un détecteur fermé au niveau du point de collision, tel que ceux d’ATLAS et de CMS, l’expérience LHCb a recours à plusieurs sous-détecteurs conçus pour observer principalement les particules émises « à petits angles », vers l’avant, dans le sens du faisceau. Le premier sous-détecteur est placé à proximité du point de collision ; les autres se suivent sur une longueur de 20 mètres.
Le LHC produit une grande variété de quarks, qui se désintègrent rapidement en d’autres particules. Pour intercepter les quarks b, la collaboration LHCb a mis au point des trajectographes mobiles perfectionnés et les a installés au plus près de la trajectoire des faisceaux dans le LHC.
Le détecteur de LHCb, qui pèse 5 600 tonnes, est constitué d’un spectromètre à petits angles et de détecteurs planaires. Long de 21 mètres, haut de 10 mètres et large de 13 mètres, il est installé à 100 mètres sous terre, à proximité de la commune de Ferney-Voltaire (France). La collaboration LHCb compte plus de 1.565 scientifiques, ingénieurs et techniciens originaires de 20 pays (mars 2022)
https://home.cern/fr/science/experiments/lhcb
L’article ci-dessous donne un petit aperçu de son travail
Le premier tiers en a été traduit et résumé par nos soins ( JP.Baquiast, europe solidaire ) La suite est non traduite et non résumée
———————————————————
Le 20 janvier 2021 un petit groupe de physiciens attendait le résultat d’une mesure réalisée dans le cadre de l’expérience de physique des particules dite LHCb ou collision de protons produites au collisionneur Large Hadron Collider (LHC) du CERN.
La mesure concernait des particules subatomiques connues sous le nom de “beauty” ou “bottom” quarks. Si ces quarks se comportaient comme ils semblaient le faire, on aurait pu y voir l’influence d’une force encore inconnue, voire peut-être le besoin d’une nouvelle théorie des particules et des forces.
Malgré toutes ses qualités dans la description des forces de l’univers, le modèle standard des particules élémentaires avait des insuffisances. Ainsi il ne pouvait décrire la matière noire ou dark matter, dont le poids permet aux galaxies en rotation de conserver leur cohésion, ou l’énergie noire, dark energy, qui semble responsable de l’.expansion.u delà de cela, le modèle standard comporte un grand nombre de traits arbitaires qui nécessite des explications.
La saga des beauty quarks avait commencé à la mi-2000, quand Gudrun Hiller, une physicienne théoricienne travaillant à l’université de Munich, étudiait les flots de données résultants des expériences Belle au Japon et BaBar en Californie. Ces expériences produisaient des quarks Beauty en réalisant des collisions entre des électrons et leurs antiparticules, des positrons. Les quarks Beauty vivaient un court instant – environ un trillionième de seconde, avant de retomber parmi les autres particules.
Hiller était particulièrement intéressé par un phénomène extrêmement rare permettant à un beauty quark de se transformer en strange quark, le troisième des plus lourds types de quark. Ce faisant le quark émettait deu muons de charge opposé, le plus lourd des électrons. Le résultat de l’expérience serait comparé avec les prédictions du modèle standard. En cas de désaccord, l’on aurait la preuve de l’existence d’une nouvelle force.
Or l’expérience n’aboutit pas pour des raisons ne pouvant être décrite ici.
10 ans après, le Grand Collisionneur du CERN commença à produire des torrents de quark beauté . Alors une mesure suggéra que la production de quarks étranges et de muons survenait plus rarement que ne le suggérait le modèle standard.
Puis en 2013, la LHCb produisit un nouveau résultat montrant des vices dans l’analyse des mouvements respectifs des particules. En 2014, la LHCb montra de nouveaux désaccords entre les expériences et le modèle standard. Les Beauty quarks se dégradaient moins souvent en muons qu’en électrons. L’on estima que le phénomène ne correspondait pas à une découverte en physique des particules, mais plutot à une imprécisions dans les processus expérimentaux.
Pour la suite, voire
Still, when you combined the measurements of the muon-to-electron ratio, the angles and how often the decays happened, a coherent picture did seem to be emerging. Since then, almost every time a measurement has been updated with yet more beauty quark data, the deviations from theory have become stronger.
Almost, because there was one notable exception. When the Hiller-Krüger ratio was updated with more data in 2019, the measured value moved towards the standard model value. “We really thought we had it,” says Patel, who led the work. “We ended up feeling gutted.” So, when Patel and his colleagues met on Zoom in January 2021 to unveil a new measurement, emotions were running high.
“These anomalies could be the real deal”
University of Cambridge experimental physicist Paula Alvarez Cartelle pushed the button to reveal the result. The measured value of the ratio had stayed almost exactly the same, but the error on it had shrunk, creating an unmistakable tension with the standard model prediction. There was now less than a 1 in 1000 chance the discrepancy was a statistical fluke. Everyone on the call erupted. “There was an awful lot of swearing,” says Patel. However, the team also felt the weight of responsibility; they knew the result would create huge excitement. As Alvarez Cartelle puts it: “You don’t want to think, ‘I just broke the standard model’, but at the same time you’re a bit, ‘Oh shit!’.”
Anomalies come and go in particle physics, and no measurement of the muon-electron ratio on its own has yet crossed the threshold of statistical certainty for it to be regarded as a definitive discovery. But there is a coherency to what have become known as the “B anomalies” that has led a growing number of physicists to regard this as the real deal. “I’ve turned into a believer,” says Ben Allanach, a theorist at the University of Cambridge. “There’s always healthy scepticism, but the fact that it’s coming from lots of different angles and saying the same thing is pretty convincing.”
In which case, what could be causing these anomalies? Allanach has spent the past few years trying to figure that out. For him, the most promising candidate is a force carried by a hypothetical particle known as a Z prime. This would be very heavy, electrically neutral and, crucially, would interact with electrons and muons with different strengths. This could explain why beauty quarks decay into muons less often than to electrons – the Z prime is stopping them.
This could also explain one of the most mysterious, seemingly arbitrary features of the standard model: the fact that matter particles come in three “generations”. The first comprises the familiar particles that make up most ordinary matter: the electron, the electron neutrino and the up and down quarks. The second contains heavier copies of these particles: the muon, muon neutrino, charm and strange quarks. And the third generation is heavier still: the tau, tau neutrino, top (or “truth”) and beauty quarks. The existence of these generations has long been a puzzle, as has the peculiar fact that the masses of the matter particles vary so wildly, with the top quark being around 350,000 times heavier than the electron.
The different generations could be explained if the beauty quark anomalies are revealing the presence of a new force that acts almost exclusively on the third generation of particles. “The model I’m working on contains a symmetry which means that if you squint a bit, only the third generation is allowed to have a mass,” says Allanach – which would explain why these particles are so heavy.
The implications of this new force wouldn’t end there. In the second half of the 20th century, physicists discovered that the three forces of nature described by the standard model – the strong and weak forces and electromagnetism – could each be described using a mathematical symmetry. In the 1970s, there was a big push to bring all three forces together under a single bigger symmetry, to create a so-called grand unified theory, which promised to unify these forces and the matter particles into one elegant structure.
The problem was that the various grand unified theories predicted that protons should decay, while every experiment performed failed to see any sign of that. What’s more, the energies required to probe these theories are over a trillion times higher than even the LHC can achieve, meaning that the new particles they predict are well out of experimental reach. As a result, the quest to unify the forces and the matter particles has been stalled for decades.
The B anomalies appear to be resurrecting aspects of the old grand unified theories, but at far lower energies than anyone had expected. “What we’re doing is putting in a tiny bit of symmetry – it’s an element of a grand unified theory, but it’s only a little one,” says Allanach. He believes that the hints of a new force we are seeing at the moment could be a low-energy remnant of a much grander symmetry that only becomes apparent at very high energies. In other words, we might be catching a glimpse of the edge of a grand unified theory.
Hiller pioneered an alternative explanation for the B anomalies that goes further still – a particle known as a leptoquark. Again, a leptoquark would be the carrier of a new force. This force would transform quarks directly into electrons, muons and taus, collectively known as leptons – hence the particle’s name. Unlike Z prime models, leptoquark models also aim to explain a second set of anomalies that have appeared in another type of beauty quark decay, this time to charm quarks, while pointing to a unified theory that’s much closer at hand in terms of energy scales.
The colour violet
Isidori is a proponent of leptoquarks. He says the models represent a “change of paradigm” compared with the old grand unified theories. While the old ones looked for symmetries that unified all three forces, the modern leptoquark models instead unify leptons with quarks.
They do this by differing from the standard model in a crucial way. In the standard model, the equivalent of electric charge for the strong force, which acts on quarks, is known as “colour”. It comes in three varieties, red, green and blue. Leptons don’t carry colour, so they don’t feel the strong force. In leptoquark models, however, there is a fourth colour, sometimes labelled violet, which arises from an enlarged version of the symmetry that describes the strong force. This larger symmetry then breaks down into the usual three-colour strong force with red, green and blue quarks, while the leftover fourth colour is carried by the leptons. Leptons are really just differently coloured quarks.
This is heady stuff – but the challenge now is to prove that these anomalies are the real deal. Isidori, for one, is convinced. “For me, the evidence is already very solid,” he says. But not everyone agrees. Although a series of unfortunate statistical flukes now seems like a very unlikely explanation given the range of different anomalies, the looming spectre is the chance of a conspiracy of missed biases, either in the theoretical predictions or the experimental measurements, or perhaps both.
New measurements are already under way at LHCb to confirm the picture and test for hidden experimental effects. In October 2021, my University of Cambridge colleague John Smeaton and I performed a new measurement of the Hiller-Krüger ratio using an unexplored part of the LHCb data sample. It revealed very similar effects to those seen in March, strengthening the case for a new force.
Meanwhile, the growing excitement around the anomalies has awoken the two big beasts of the LHC, the ATLAS and CMS experiments. In 2012, they discovered the Higgs boson, the long-predicted standard-model particle that gives all other fundamental particles their mass, and are now beginning to think about ways they might spy the predicted Z primes or leptoquarks. In Japan, the Belle II experiment is gradually accumulating data that will allow it to independently check several of LHCb’s results. Later this year, an upgraded LHCb will begin collecting data at a far higher rate than before, allowing us to seek out even rarer decays where the anomalies could be even stronger.
If the emerging picture is confirmed, we are in for a revolution in our understanding of the constituents of nature that could reveal a deeper structure beneath the standard model, while perhaps even giving us a handle on the nature of dark matter or the strange properties of the Higgs boson. If that happens, it will be the greatest discovery in fundamental physics since the standard model was put together. The stakes are high and the game is on.
L’article ci-dessous donne un petit aperçu de son
travailLe premier tiers en a été traduit et résumé par
nos soins ( europe solidaire ) Pour la suite, non traduite et non
résumés,
Le 20 janvier 2021 un petit
groupe de physiciens attendait le résultat d’une mesure réalisée
dans le cadrede
l’expérience de physique des particules dite LHCb ou collision de
protons produites au collisionneur Large
Hadron Collider (LHC)
du CERN. La mesure concernait des
particules subatomiques connues sous le nom de “beauty” ou
“bottom” quarks. Si ces quarks se comportaient comme ils
semblaient le faire, on aurait pu y voir l’influence d’une force
encore inconnue, voire peut-être le besoin d’une nouvelle théorie
des particules et des forces.
Malgré toutes ses qualités
dans la description des forces de l’univers, le modèle standard des
particules élémentaires avait des insuffisances. Ainsi il ne
pouvait décrire la matière
noire ou dark matter, dont le poids permet aux galaxies en
rotation de conserver leur cohésion, ou l’énergie
noire, dark energy, qui semble responsable de l’. Au delà de
cela, le modèle standard comporte un grand nombre de traits
arbitaires qui nécessite des explications.La saga des beauty quarks
avait commencé à la mi-2000, quand Gudrun
Hiller, une physicienne théoricienne travaillant à l’université
de Munich, étudiait les flots de données résultants des
expériences Belle au Japon et BaBar en Californie. Ces
expériences produisaient dexpansion
accélérée de l’universes quarks Beauty en réalisant des
collisions entre des électrons et leurs antiparticules, des
positrons. Les quarks Beauty vivaient un court instant – environ un
trillioniè de secondes, avant de retomber parmi les autres
particules.Hiller était particulièrement intéressé par un
phénomène extrémement rare permettant à un beauty quark de se
transformer en strange quark, le troisième des plus lourds types de
quark. Ce faisant le quark émettait deu muons de charge opposé, le
plus lourd des électrons. Le résultat de l’expérience serait
comparé avec les prédictions du modèle standard. En cas de
désaccord, l’on aurait la preuve de l’existence d’une
nouvelle force. Or l’expérience n’aboutit pas pour des raisons
ne pouvant être décrite ici.10 ans après, le Grand Collisionneur du CERN
commença à produire des torrents de quark beauté . Alors une
mesure suggéra que la production de quarks étranges et de muons
survenait plus rarement que ne le suggérait le modèle standard.
Puis en 2013, la LHCb produisit un nouveau résultat montrant des
vices dans l’analyse des mouvements respectifs des particules. En
2014, la LHCb montra de nouveaux désaccords entre les expériences
et le modèle standard. Les Beauty quarks se dégradaient moins
souvent en muons qu’en électrons. L’on jugeau que le phénomène ne
corespondait pas à une découverte en physique des particules, mais
plutt à une imprécisions dans les processus expériementaux.
Pour la suite, voire
Still, when you combined the measurements of the
muon-to-electron ratio, the angles and how often the decays happened,
a coherent picture did seem to be emerging. Since then, almost every
time a measurement has been updated with yet more beauty quark data,
the deviations from theory have become stronger.Almost, because there was one notable exception. When
the Hiller-Krüger ratio was updated with more data in 2019, the
measured value moved towards the standard model value. “We really
thought we had it,” says Patel, who led the work. “We ended up
feeling gutted.” So, when Patel and his colleagues met on Zoom in
January 2021 to unveil a new measurement, emotions were running high.“These anomalies could be the real deal”University of Cambridge experimental physicist Paula
Alvarez Cartelle pushed the button to reveal the result. The
measured value of the ratio had stayed almost exactly the same, but
the error on it had shrunk, creating an unmistakable tension with the
standard model prediction. There was now less than a 1 in 1000 chance
the discrepancy was a statistical fluke. Everyone on the call
erupted. “There was an awful lot of swearing,” says Patel.
However, the team also felt the weight of responsibility; they knew
the result would create huge excitement. As Alvarez Cartelle puts it:
“You don’t want to think, ‘I just broke the standard model’,
but at the same time you’re a bit, ‘Oh shit!’.”
Anomalies
come and go in particle physics, and no measurement of the
muon-electron ratio on its own has yet crossed the threshold of
statistical certainty for it to be regarded as a definitive
discovery. But there is a coherency to what have become known as the
“B anomalies” that has led a growing number of physicists to
regard this as the real deal. “I’ve turned into a believer,”
says Ben Allanach,
a theorist at the University of Cambridge. “There’s always
healthy scepticism, but the fact that it’s coming from lots of
different angles and saying the same thing is pretty convincing.”In which case, what could be causing these anomalies?
Allanach has spent the past few years trying to figure that out. For
him, the most promising candidate is a force carried by a
hypothetical particle known as a Z prime. This would be very heavy,
electrically neutral and, crucially, would interact with electrons
and muons with different strengths. This could explain why beauty
quarks decay into muons less often than to electrons – the Z prime
is stopping them.This could also explain one of the most mysterious,
seemingly arbitrary features of the standard model: the fact that
matter particles come in three “generations”. The first comprises
the familiar particles that make up most ordinary matter: the
electron, the electron neutrino and the up and down quarks. The
second contains heavier copies of these particles: the muon, muon
neutrino, charm and strange quarks. And the third generation is
heavier still: the tau, tau neutrino, top (or “truth”) and beauty
quarks. The existence of these generations has long been a puzzle, as
has the peculiar fact that the masses of the matter particles vary so
wildly, with the top quark being around 350,000 times heavier than
the electron.The different generations could be explained if the
beauty quark anomalies are revealing the presence of a new force that
acts almost exclusively on the third generation of particles. “The
model I’m working on contains a symmetry which means that if you
squint a bit, only the third generation is allowed to have a mass,”
says Allanach – which would explain why these particles are so
heavy.The implications of this new force wouldn’t end
there. In the second half of the 20th century, physicists discovered
that the three forces of nature described by the standard model –
the strong
and weak
forces and electromagnetism
– could each be described using a mathematical symmetry. In the
1970s, there was a big push to bring all three forces together under
a single bigger symmetry, to create a so-called grand unified theory,
which promised to unify these forces and the matter particles into
one elegant structure.The problem was that the various grand unified
theories predicted that protons should decay, while every experiment
performed failed to see any sign of that. What’s more, the energies
required to probe these theories are over a trillion times higher
than even the LHC can achieve, meaning that the new particles they
predict are well out of experimental reach. As a result, the quest to
unify the forces and the matter particles has been stalled for
decades.The B anomalies appear to be resurrecting aspects of
the old grand unified theories, but at far lower energies than anyone
had expected. “What we’re doing is putting in a tiny bit of
symmetry – it’s an element of a grand unified theory, but it’s
only a little one,” says Allanach. He believes that the hints of a
new force we are seeing at the moment could be a low-energy remnant
of a much grander symmetry that only becomes apparent at very high
energies. In other words, we might be catching a glimpse of the edge
of a grand unified theory.Hiller pioneered an alternative explanation for the B
anomalies that goes further still – a particle known as a
leptoquark. Again, a leptoquark would be the carrier of a new force.
This force would transform quarks directly into electrons, muons and
taus, collectively known as leptons – hence the particle’s name.
Unlike Z prime models, leptoquark models also aim to explain a second
set of anomalies that have appeared in another type of beauty quark
decay, this time to charm quarks, while pointing to a unified theory
that’s much closer at hand in terms of energy scales.
The colour violet
Isidori is a proponent of leptoquarks. He says the
models represent a “change of paradigm” compared with the old
grand unified theories. While the old ones looked for symmetries that
unified all three forces, the modern leptoquark models instead unify
leptons with quarks.They do this by differing from the standard model in
a crucial way. In the standard model, the equivalent of electric
charge for the strong force, which acts on quarks, is known as
“colour”. It comes in three varieties, red, green and blue.
Leptons don’t carry colour, so they don’t feel the strong force.
In leptoquark models, however, there is a fourth colour, sometimes
labelled violet, which arises from an enlarged version of the
symmetry that describes the strong force. This larger symmetry then
breaks down into the usual three-colour strong force with red, green
and blue quarks, while the leftover fourth colour is carried by the
leptons. Leptons are really just differently coloured quarks.
This is heady stuff – but the challenge now is to
prove that these anomalies are the
real deal. Isidori, for one, is convinced. “For me, the evidence is
already very solid,” he says. But not everyone agrees. Although a
series of unfortunate statistical flukes now seems like a very
unlikely explanation given the range of different anomalies, the
looming spectre is the chance of a conspiracy of missed biases,
either in the theoretical predictions or the experimental
measurements, or perhaps both.New measurements are already
under way at LHCb to confirm the picture and test for hidden
experimental effects. In October 2021, my University of Cambridge
colleague John Smeaton and I performed a
new measurement of the Hiller-Krüger ratio using an unexplored part
of the LHCb data sample. It revealed very similar effects to
those seen in March, strengthening the case for a new force.Meanwhile, the growing
excitement around the anomalies has awoken the two big beasts of the
LHC, the ATLAS and CMS experiments. In 2012, they discovered
the Higgs boson, the long-predicted standard-model particle that
gives all other fundamental particles their mass, and are now
beginning to think about ways they might spy the predicted Z primes
or leptoquarks. In Japan, the Belle II experiment is gradually
accumulating data that will allow it to independently check several
of LHCb’s results. Later this year, an upgraded LHCb will begin
collecting data at a far higher rate than before, allowing us to seek
out even rarer decays where the anomalies could be even stronger.If the emerging picture is
confirmed, we are in for a revolution in our understanding of the
constituents of nature that could reveal a deeper structure beneath
the standard model, while perhaps even giving us a handle on the
nature of dark matter or the strange
properties of the Higgs boson. If that happens, it will be the
greatest discovery in fundamental physics since the standard model
was put together. The stakes are high and the game is on.
Europe Solidaire 