This course introduces you to subatomic physics, i.e. the physics of nuclei and particles. More specifically, the following questions are addressed: - What are the concepts of particle physics and how are they implemented? - What are the properties of atomic nuclei and how can one use them? - How does one accelerate and detect particles and measure their properties? - What does one learn from particle reactions at high energies and particle decays? - How do electromagnetic interactions work and how can one use them? - How do strong interactions work and why are they difficult to understand? - How do weak interactions work and why are they so special? - What is the mass of objects at the subatomic level and how does the Higgs boson intervene? - How does one search for new phenomena beyond the known ones? - What can one learn from particle physics concerning astrophysics and the Universe as a whole? The course is structured in eight modules. Following the first one which introduces our subject, the modules 2 (nuclear physics) and 3 (accelerators and detectors) are rather self contained and can be studied separately. The modules 4 to 6 go into more depth about matter and forces as described by the standard model of particle physics. Module 7 deals with our ways to search for new phenomena. And the last module introduces you to two mysterious components of the Universe, namely Dark Matter and Dark Energy.
7.5 Hunting new physics with LHCb (optional)
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來自 University of Geneva 的課程
Particle Physics: an Introduction
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從本節課中
Discovering new phenomena
In this 7th module Anna discusses searches for new phenomena, beyond the known ones described by the standard model and covered in previous modules. We will remind you why we believe that the standard model is incomplete and new physics must be added. We will explain how hadron collider data are rendered usable for searches. And we will discuss examples, split into the two categories, based on how new phenomena might manifest themselves.
與講師見面
Martin PohlProfesseur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes PanicciaCollaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
Anna SfyrlaAssistant Professor
Nuclear and Particle Physics
[MUSIC]
So thank you very much, Nico, for being here with us for this interview.
Could you tell us a couple of words about yourself?
>> Okay. Thanks, Anna.
Thanks for inviting me.
So, I'm Nico Serra.
I'm professor at University of Zurich and I work in the LHCb experiment.
>> The LHCb experiment.
Could you tell us how the detector of the LHCb experiment differs with
respect to the bigger, general purpose detectors at the LHC?
>> So, LHCb has been designed and optimized in order to measure
properties of c and c hadrons, Hadrons containing the b and c quarks.
This explains the peculiar geometry,
which is similar to the geometry of fixed target experiments
rather than onion-like geometry that you can see at detectors like ATLAS and CMS.
And the reason is that the partons that compose the protons,
the quarks and gluons, have a much higher probability to carry a low
momentum fraction of the proton with respect to carrying a large fraction of its momentum.
Therefore, to produce relatively light objects like for
instance b hadrons, the probability is much higher to have asymmetric collisions.
This implies that b hadrons are produced predominantly in the beam direction,
forward and backward, and with a very large Lorentz boost.
So this geometry, therefore,
allows to collect a really large statistics of b hadrons even
if covering really small solid angles and operating at a lower luminosity.
In addition, it has the advantage, since the b hadrons have a large
Lorentz boost, decays are really displaced with respect to the proton-proton interaction.
And these are very big handles against background.
And in this geometry, we can easily accommodate RICH detectors for
particle identification. Particle identificatiob,
momentum resolution and vertex resolution are the key feature in order
to select the signal over the background.
>> Good, so, you said that the LHCb detector operates at lower luminosities
with respect to other experiments like ATLAS,
in whose cavern we are right now, by the way.
So that means that there are smaller event rates.
Does it mean that we do not need a trigger anymore?
>> No, we do need a trigger.
And starting from the 40 MHz bunch crossing rate,
we have a hardware level-0 trigger that reduces the rate to about 1 MHz.
After that, we run a software high-level trigger
that runs a mixture of inclusive and exclusive selections to further reduce the rate.
And recently we had a major shift in the paradigm of triggering in LHCb.
And this is done in preparation for the LHCb upgrade,
where we will run directly a fully software trigger.
So what we do is to buffer events in the disk, and to perform online
calibration and alignment of the detector such that we will run in
the upgrade the same event reconstruction as offline with the same quality.
>> That's very interesting, indeed.
Can we can now focus on the searches?
What type of searches do you at the LHCb?
>> So, at LHCb we do searches for new physics.
But rather than doing direct search of bumps and tails of new particles,
we hunt for these particles as virtual particles.
So, let me explain this with an example.
For instance, let's consider the decay of the B_s mesons in a di-muon pair.
This decay is a very rare decay in the standard model because it is
suppressed by the GIM mechanism and also helicity suppressed.
And its branching ratio is about 3 10^-9.
If you have new particles, like, for instance,
in supersymmetry, they can enter in mediating this decay.
And they can change the properties of this decay.
For instance, they can enhance this branching ratio by orders of magnitude.
Therefore, you compare what you measure with the predictions.
>> If you actually measure then this type of decay,
you can constraint supersymmetry as well.
>> Exactly.
For instance of all this particular decays, the fact that we measure a value
of the branching ratio that is in agreement with standard model predictions allows to
strongly contains supersymmetry.
Also consider that this is complimentary
with respect to direct searches in ATLAS and CMS.
Because since we hunt these particles as virtual particles,
we are not limited by the available energy in the center of mass.
So we can in principle constrain particle that have mass
higher than the collision energy at the LHC.
>> I see.
Could you explain to us how such a search is done?
>> So these are rare decays so
it's very important the selection.
You have to be efficient in rejecting the background and selecting the signal.
Then, we attribute the di-muon pair to the B_s,
because essentially, we see a mass bump in the di-muon invariant mass spectrum.
But the difference is that we do know what is the mass of the B_s,
so we do not have the so called « look elsewhere » effect.
But you do need to know
very well the efficiency of your detector and the background.
And you have to know how many B_s are produced originally.
What you do normally is to normalize with respect to another decay that you
know very well with a very well known branching ratio.
And you choose this decay, such that you can somehow
cancel some systematic uncertainties in the ratio.
>> Okay, now you talk about uncertainties.
How do these compromise this type of measurements?
>> So, first of all we have three types of uncertainties in these searches.
The statistical uncertainties like everywhere due to the limited statistics.
Then you have the systematic uncertainties.
In this case for instance, a source of systematic would be the limited
knowledge of the efficiency or the limited knowledge of the background.
And in addition, since we compare our measurement with the predictions,
we have uncertainties which are due to the accuracy of the theory predictions.
This is due to the fact that theories have to extrapolate from the Feynman
diagrams at the quark level, to what happens at the hadron level.
And this involves uncertainty due to QCD essentially.
>> Okay.
So, do you see any deviations with respect to the standard model in
this channel or elsewhere?
>> So we don't see deviations in B_s -> µ+ µ- but
we do see deviations with respect to the standard model elsewhere.
>> Could you tell us more about that, actually?
>> Yes, so one of the things we test at LHCb is
the lepton flavor universality in b decays.
So, as the students know, the electron, the muon and
the tau, they behave exactly the same way apart from their mass,
which comes from their different interactions with the HIggs boson.
But apart of that they behave exactly in the same way.
So if you have a branching ratio of a B meson decaying to charged leptons and
you factor out the effect of the mass in the phase space,
their branching ratios should be the same.
If you have a new particles, new virtual particles as I said before,
they might couple differently to electron, muon and tau and
they would spoil the lepton universality of the standard model.
So, what we see is exactly a departure from this lepton universality,
in some B decays and by now,
we need more data to be certain of this deviation but
it certainly looks like
an intriguing pattern we are seeing.
>> Okay, well it is certainly interesting the breadth of the research program of LHCb.
What else have you discovered?
Could you tell us a bit more about that?
>> Yes, so in addition to search for new physics,
what we do is also to do spectroscopy, to measure properties of
particles, of hadrons predicted by the quark model. For instance,
by doing bump searches, as you do in ATLAS, but at lower masses.
We discover new excited states of D mesons and B mesons.
And in addition, we also do different analysis which are called amplitude
analysis that allows us to measure the quantum numbers of these particles.
For instance, the resonance X(3872) that was long thought to be a D meson molecule.
We could discard this hyphothesis exactly by measuring
the quantum numbers of this resonance.
And also relatively recently, we discovered two particles,
which are consistent with being neither baryons not mesons but pentaquarks.
So particles composed by four quarks and one anti-quark.
And this was a breaking ground results.
>> That sounds really exciting indeed.
But now how about the prospects of LHCb for the future?
>> So, most of our measurements first of all are dominated
by the statistical errors.
So it means that, in order
to exploit fully the LHC potential, we need more statistics.
And therefore we are working to prepare an upgrade of the LHCb
detector to run at higher luminosity.
In addition as I said, in most measurements
we do see a striking agreement with respect to standard model and
prediction, but we also see some deviation.
So our hope is that in the future we will reveal a pattern of
this deviation that can lead us to the discovery of new physics.
>> The prospects for LHCb to be are really exciting from what you are telling us.
So, we really hope that you reveal this interesting patterns indeed.
Thank you very much
for this interview, Nico.
>> Thank you Anna, bye.
