The LHCb experiment is one of the four large CERN LHC detectors. Its goal is to search for evidence of new physics phenomena through precise measurements of the decay properties of particles containing beauty and charm quarks. The requirements of particle physics experiments such as LHCb have many commonalities with information retrieval. Both domains deal with large datasets, rely heavily on computational power, and require sophisticated data analysis algorithms, which are based on similar principles. For this reason many potential benefits can be discerned in conducting interdisciplinary research in the two areas. Guy Wilkinson, LHCb spokesperson, will give a brief overview of LHCb and its goals, suitable for a non-specialist audience. He will then focus on the challenges that the large datasets present, and outline the technologies and approaches that are currently being deployed to manage and analyse these datasets. Effective solutions to these problems are critical for aiding the exploration of the frontiers of fundamental science at LHCb, and similar experiments.

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YSDA seminar
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Searching for New Physics at LHCb & the challenge of big data
Guy Wilkinson
University of Oxford and CERN
27/11/14
Introduction
- the Standard Model
- flavour physics
- open questions
- LHCb
Data processing
- the data challenge
- trigger
- offline
- the end-user: selected physics results

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Introduction
- the Standard Model - flavour physics - open questions - LHCb

3. ATLAS
CMS
ALICE
LHCb

4. LHCb – a flavour physics experiment at the LHC
A collaboration of ~1100 members from 68 institutes in 17 countries
An experiment to search for physics beyond the Standard Model, through
flavour studies of particles containing beauty (b) quarks & CP-violation.

5. LHCb – a flavour physics experiment at the LHC
A collaboration of ~1100 members from 68 institutes in 17 countries
An experiment to search for physics beyond the Standard Model, through
flavour studies of particles containing beauty (b) quarks & CP-violation.
Russian institutes have always had a big role in LHCb:
YANDEX & YSDA have also been making very important
contributions in the past few years – many thanks, and
let us hope this collaboration can grow even stronger !

6. The Standard Model of particle physics is a quantum-
field theory that describes the fundamental particles,
plus the electromagnetic, weak & strong interactions
All tests made of the Standard Model in particle colliders have been successful !
The most spectacular recent example was the discovery of the Higgs boson.
One very important part of the theory is the ‘flavour sector’ & its associated physics.
The Standard Model
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CERN, July 2012

7. What is flavour physics?
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The concept of ‘flavour’ in particle physics relates to the existence of
different families of quarks*, and how they couple to each other
i.e. 6 known flavours of quark, grouped into 3 generations
Open questions:
These mysteries make the ‘flavour sector’ of the Standard Model of great interest.
• why 3 generations ?
• why do the quarks exhibit this striking hierarchy in mass ?
No answer yet !
These values (i.e. ‘3’ &
the masses) are free
parameters of the SM
Not to linear scale !
mass in MeV/c2
* the concept of flavour extends
to the lepton sector too

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Flavour and the CKM matrix
In the Standard Model quarks can only change flavour through emission of a
W boson (i.e. weak force). For example a t quark can decay into a b, s or d quark:
But these decays are not equally likely. At the amplitude level they are weighted
by factors that are elements of the Cabibbo-Kobyashi-Maskawa (CKM) matrix, and
these factors vary dramatically – here is another hierarchy we don’t understand !
These elements of the CKM matrix are also fundamental parameters of the
Standard Model. Why they have these values is another great mystery.
The CKM matrix is also linked to another big puzzle of flavour physics…
=

9. 9
50 years ago – events of 1964
Nelson Mandela sentenced
to life imprisonment
Cassius Clay becomes heavyweight champion of the world
Martin Luther King Jnr.
wins Nobel Peace Prize
Change of face
in the Kremlin
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YSDA seminar

10. 10
50 years ago – events of 1964
Nelson Mandela sentenced to life imprisonment
Cassius Clay becomes heavyweight champion of the world
Martin Luther King Jnr.
wins Nobel Peace Prize
Change of face
in the Kremlin
Discovery of CP violation (in kaon decays)
Nobel Prize for physics in 1980
27/11/14
YSDA seminar

11. CP violation (CPV) → difference in behaviour between matter and anti-matter. First discovered in the kaon system in 1964, opportunities of study were limited until colliders arrived that could make lots & lots of b-quark hadrons, e.g. the LHC A recent example from LHCb - look at B meson decaying into a pion & two kaons… …the decay probabilities are manifestly different for B- & B+ ! In the Standard Model CPV is accommodated, but not explained, by an imaginary phase in the CKM matrix
CP violation
B-
B+
signal
decays
background
[LHCb, arXiv:1408.5373]

12. Cosmological connections ?
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As first pointed out by Andrei Sakharov, CP-violation is one
requirement for explaining baryogenesis – the process that took
us from the equal amounts of matter and anti-matter produced
in the Big Bang, to the matter dominated universe of today
The problem is that the CP-violation that
appears in the Standard Model, is woefully
inadequate to explain the matter-antimatter
asymmetry we have today.
This is a big problem with the Standard Model !
More & better measurements
may point a way forward.

13. The Standard Model cannot be a final theory
We have already encountered the following shortcomings:
And there are plenty of others, for example:
More ambitious theories (e.g. supersymmetry or SUSY) can solve at least some of these problems. They generally predict new particles or effects outside the Standard Model. The goal of the LHC is to search for evidence of these effects !
Problems with the Standard Model
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• No explanation for baryogenesis
• No explanation for the quark or CKM hierarchy
• No real explanation for CP violation, and why it is only found in the weak interaction.
• No explanation for dark matter or dark energy
• No explanation for neutrino masses
• Gravity not included
• No explanation for why the Higgs boson has the mass it does (left to itself the theory would make it much, much heavier)

14. Attacking the fortress of the Standard Model
14
The LHC is searching for ‘New Physics’ - to find this we need to get behind the walls of the Standard Model fortress. There are two strategies used in this search Both methods are powerful. LHCb specialises (mostly) in the ‘indirect’ approach
Direct
Make precise measurements of
processes in which New Physics
particles enter through ‘virtual loops’
Use the high energy of the LHC
to produce the New Physics
particles, which we then detect
Indirect

15. Indirect measurements – an established tradition in science
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Eratosthenes was able to determine
the circumference of the earth
using indirect means…
…around 2.2 thousand years
prior to the direct observation.

16. For some processes, especially suppressed decays, more complicated Feynman diagrams are important. These contain ‘loops’ in which virtual particles participate Decays, & other processes, involving b-quarks are a good place to study role of these loops. In the loops the contribution of heavy particles, e.g. top, is important, even though mt >> mb. Hence these decays tells us about the particles in the loops. As drawn above, the loop contains Standard Model particles, but New Physics particles could also contribute, affecting decay rates, CP violation etc ! Indirect search Precise measurements of low energy phenomena principle tells us about unknown physics at higher energies
Loop diagrams & ‘indirect searches’

17. The power of indirect measurements – the top quark mass
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direct
indirect
Top mass [GeV]
LEP, the accelerator operation at CERN
in the 1990s, did not have sufficient energy
to produce ‘real’ top quarks.
The top quark was instead discovered
at the Tevatron, near Chicago in 1995
Nonetheless, the precise measurements
of Z boson properties made at LEP carry
information on the top through loop diagrams.
LEP was able to measure the top mass indirectly, even before Tevatron discovery !

18. LHCb – a forward spectrometer for flavour physics
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19. LHCb – a forward spectrometer for flavour physics
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The VELO is a silicon
detector around the
interaction point.
It approaches within 8 mm of the beamline and reconstructs the b-hadron decay vertex precisely.
One-half of the VELO under construction
A reconstructed b-hadron decay vertex
~1.5 cm

20. LHCb – a forward spectrometer for flavour physics
20
Array of RICH photodetectors
Assembling RICH 2; note the mirrors
Momentum [GeV/c]
Cherenkov angle [rad]
Two ‘RICH’ detectors detect Cherenkov radiation.
the angle at which this is emitted tells us the particle
species – it provides ‘hadron identification’.

21. LHCb – a forward spectrometer for flavour physics
YSDA seminar
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A 4 Tm dipole, and the tracking detectors reconstruct the trajectory of charged particles, and allows their momentum to be determined.
Dipole magnet
Reconstructed tracks
Part of outer tracker

22. LHCb – a forward spectrometer for flavour physics
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The calorimeter system (ECAL & HCAL) reconstructs the energy of photons, electrons and hadrons. The muon system (M1-M5) identifies muons.
Part of calorimeter system (preshower)
These detectors are particularly important
for the role they play in the LHCb trigger

23. LHC run 1 went from 2010 to 2012, during which LHCb collected 3 fb-1 of data
(this corresponds to ~3 x 1011 b anti-b pairs being produced within LHCb).
We are coming to end of two year shutdown, necessary to upgrade LHC energy.
Run 2 will go on to 2018 – we hope to increase our beauty sample by x3 or more.
LHCb – the story so far
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delivered by LHC
collected by LHCb

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Data processing
- the data challenge - trigger - offline - the end-user: selected physics results

25. The data challenge
LHC operates at 40 MHz and
does so for ~15% of year
LHCb raw event
size ~100 kBytes
~ 15000 PetaBytes /yr (raw data alone)
~ 15000 PetaBytes/year is less
than dealt with by search engines,
but still considerably more than
e.g. Facebook (~ 180 PB/year).
’
Data LHCb ~15000 PB.yr
rate Facebook ~180 PB / yr
LHCb
Facebook

26. The data challenge
LHC operates at 40 MHz and
does so for ~15% of year
LHCb raw event
size ~100 kBytes
~ 15000
PetaBytes /yr
(raw data alone)
~ 15000 PetaBytes/year is less
than dealt with by search engines,
but still considerably more than
e.g. Facebook (~ 180 PB/year).
Public science has less money to
spend on computing than Facebook.
Storage costs money. Better to
process as much as possible in ‘real time’.
Data LHCb ~15000 PB.yr rate Facebook ~180 PB / yr
Computing LHCb ~10M$ / yr
budget Facebook ~600 M$ /yr
LHCb
Facebook

27. Not all collisions are equally interesting
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Core business of LHCb is beauty physics, and here we can be selective (Situation is complicated by the fact we also want to study charm physics. Charm is much more abundant, and the decays of interest are more common). So we only save to disk the potentially interesting collisions – task of the trigger.
Collision rate 40 MHz
(currently a little less,
but this sets the ballpark)
b-hadrons produced
about once every
~150 pp collisions
And most b-hadrons
decays don’t interest us.
The ones that do, occur
every 10-3 -10-10 of time.
Bs→μμ
occurs every
4 x 10-9
Bs decays

28. Triggering on beauty
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There exist characteristics of increasing complexity than can be searched for to
determine if the collision is of interest and should be preserved for offline analysis.
μ+
μ-
K+
p
p
~ 1 cm
Interaction point
or ‘primary vertex’
(many other particles
produced, not shown)
high pT
B+
1.Look for high transverse energy (ET) or momentum (pT) in calorimeters or muon system from decay products. .

29. Triggering on beauty
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There exist characteristics of increasing complexity than can be searched for to determine if the collision is of interest and should be preserved for offline analysis.
μ+
μ-
K+
p
p
~ 1 cm
Interaction point
or ‘primary vertex’
(many other particles
produced, not shown)
finite
impact
parameter
B+
1.Look for high transverse energy (ET) or momentum (pT) in calorimeters or muon system from decay products. 2. Look for tracks with significant ‘impact parameter’ with respect to primary vertex.

30. Triggering on beauty
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There exist characteristics of increasing complexity than can be searched for to
determine if the collision is of interest and should be preserved for offline analysis.
μ+
μ-
K+
p
p
B+
~ 1 cm
Interaction point or ‘primary vertex’ (many other particles produced, not shown)
b-hadron decay, or ‘secondary vertex’
1.Look for high transverse energy (ET) or momentum (pT) in calorimeters or muon system from decay products. 2. Look for tracks with significant ‘impact parameter’ with respect to primary vertex. 3. Reconstruct secondary vertex and full b-hadron decay products.
Each successive step provides improved discrimination,
but requires more information and time to execute.

31. Trigger: L0
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Earliest trigger stage, ‘L0’, makes decisions in hardware based on simple high ET, high pT signatures. Decision made with partial detector information. No time to build full event. Trigger decision made within 4 μs synchronous with bunch crossing rate While decision is being made local detector information is retained in a pipeline within front-end electronics. Reduces data rate down to 1 MHz ( = rate at which full event is read out)
[LHCb trigger – see LHCb, arXiv:1211.3055]

32. Trigger: HLT
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The High Level Trigger (HLT) is a software trigger (C++) that runs on the Event Filter Farm (EFF) The EFF is a farm of multiprocessor PCs (~1500 nodes), located at LHCb ~30,000 copies of HLT run on the EFF L0-accepted event assembled on the EFF. Placed in buffer that is accessed by HLT programs. Two steps: - HLT1: impact parameter info etc used to reduce rate to ~40 kHz (~35 ms/event) - HLT2: full event information used to reduce rate to ~12 kHz (~350 ms/event) Then written offline.

33. Trigger: alignment and calibration
Note this step (new to 2015), between the initial and final HLT selections Here the events are written to local disks (~4 PB available) while calibration and alignment is performed. Only when this is satisfactory is second stage of HLT executed.
This step important, as better alignment provides better signal discrimination. Also
means we can trust information we would not use otherwise in HLT (i.e. from RICH).
Goal is to improve background rejection, & to do
online much of what was traditionally done offline.
MeV/c2
Initial alignment
Final
alignment
Υ→μμ
system

34. Trigger: multivariate selections
The strategy to allow selections to be deployed in second-stage of HLT that would normally be reserved for offline has allowed for ever better performance. Multivariate selections, discussed later, play an increasingly important role in trigger
data
beauty decays
charm decays
Example: ‘boosted Bonzai decision
Tree’ [Gligorov, Williams, arXiv:1210.861]
used to select generic B decays.
This strategy should become ever
more powerful with the new calibration
& alignment information now available,
& opportunity to use e.g. RICH information.
→ first step towards real time physics analysis !
high purity
selection
possible

35. Trigger output
These data will be sent for offline reconstruction on the GRID, after which physics analysis can be begin. This is the classical approach. These data won’t be reconstructed for some years, probably not until after run 2. This is merely dictated by computing costs. These data will be analysed directly without further offline reconstruction. The idea is that the online reconstruction will be sufficient for physics analysis If this works well then this scheme will be expanded. It could be the future !
Full stream Parked stream Turbo stream

36. Offline processing - event reconstruction
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Event reconstruction:
Processing takes ~2 s / event. Occurs in ~5k concurrent jobs run on GRID.
Output is DST (data storage tape) – 2012 DST data require 2 PB of disk storage.
Improved detector understanding → need to reprocess data (only do this 1-2 times).
• reconstruct particles trajectories, providing momentum information and precise knowledge of behaviour close to interaction point
RICH 2
LHCb data (preliminary)
kaon ring
• perform particle identification – e.g. finding Cherenkov rings in RICH detectors and providing probability of particle assignment for each track

37. 37
Offline processing – data reduction or ‘stripping’
Any given physics analysis selects 0.1-1.0% of events
•Inefficient to allow individual physicists to run selection job over FULL DST
Assemble loose (but not too loose!) pre-selections for each set of analysis.
Group all these pre-selections in a single pass, executed centrally
•Runs over FULL DST
•Executes ~800 independent stripping “lines”
- ~ 0.5 sec/event in total
- Writes out only events selected by one or more of these lines
•Output events are grouped into ~15 streams
•Each stream selects 1-10% of events
Overall data volume reduction: x50 – x500 depending on stream
•Few TB (<50) per stream, replicated at several places
•Accessible to physicists for data analysis

38. Offline processing - simulated data
Almost all physics analyses require large quantities of simulated data for training selection algorithms, measuring performance, and understanding backgrounds Elements of simulation, or ‘Monte Carlo’ software: Simulation is a very major driver of CPU and storage requirements
•Simulation of physics events;
•Detailed detector and material description (GEANT);
•Pattern recognition, trigger simulation and offline event selection;
•Implements detector inefficiencies, noise hits, effects of multiple collisions.
GEANT description
of VELO
Simulated LHCb event

39. LHCb computing model
RRCKI
+
recent addition
Monte Carlo
production
(and some
user analysis)
HLT farm
Tier-2’s
YANDEX
+

40. Snapshot of recent CPU provision
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~50% CERN plus Tier 1 centres ~ 12% HLT farm (we use it for offline tasks when LHC is not running) ~ 5% YANDEX ~33% Tier 2 centres

41. Profile of recent CPU usage
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Thousands of jobs
Simulation jobs
User analysis jobs
(Recall that currently no real data being processed, nor are any being reprocessed)

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Tools for data access
Book-keeping tools needed to identify files
corresponding to datasets for e.g. analysis jobs.
Book-keeping has no information about individual events. But often desirable to select events according to given characteristics and download source files. . This possible thanks to Event Index tool developed by YSDA.

43. Data storage
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Data storage capabilities is a major consideration in LHCb computing planning.
Efficient use of existing
resources is essential
for optimising physics
performance.
Study of data popularity now
underway, led by YSDA:
• will help to improve replication and cleanup strategies
• multivariate tools very useful to predict evolution of demand
old, unused data –
remove or move to tape

44. The end-user: selected physics results
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45. The end-user: selected physics results
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Over 225 journal publications. Here will focus on two very important analyses with particular demands on data handling:
• Testing the Standard Model description of CP violation – measuring the angle γ
• Hunting for the ‘needle in the haystack’: the search for Bs→μμ

46. CP violation, the difference in behaviour between matter and
antimatter, is accommodated in the Standard Model
by a single complex phase in the CKM matrix.
One of the angles of this triangle, γ, is related very directly to the CKM phase.
Our knowledge of the other elements of the matrix allow us to predict the
angle with high precision. Now LHCb must measure CP-violating processes
to obtain a direct measurement of γ to compare with this prediction.
Measuring the angle γ - testing
CP violation in the Standard Model
Any discrepancy would point to New Physics effects in flavour physics
observables (e.g. SUSY particles, 4th generation of quarks etc )
o (66.4 ) 1.2
3.3

 
Relations exist between the elements
of the matrix which can be displayed
geometrically – as triangles.
predicted
value

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How to measure γ ?
• It is a hidden phase, which exist between different b-quark decays
• The only way to access this phase is to look for a decay mode that is common to both D0 and anti-D0. Then we don’t know by which path the decay has taken, and we get quantum inferference, as in the famous double-slit experiment
• The phase is CP-violating, which means it should generate different effects for B+ & B-.
B- →D0 K- B- →anti-D0 K-
phase γ
between
amplitudes
Measuring γ – quantum interferometry

48. B- →D0 K-
B- →anti-D0 K-
First look at the case where the (anti-)D0 is reconstructed in the ‘favoured’ mode K-π+ for the B- decay (or K+π- for B+ ). This is (almost) completely dominated by a single decay path, and so no quantum interference is expected…
Favoured
B-
B+
signal
Measuring γ – quantum interferometry
..and none is seen, since the decay rates are essentially the same.
[LHCb, arXiv:1203.3662]

49. B- →D0 K-
B- →anti-D0 K-
Now look at the case where the (anti-)D0 is reconstructed in the ‘suppressed’ mode K+π- for the B- decay (or K-π+ for B+ ). Here we expect significant interference, and indeed a large CP-violating difference in rates is seen between B- and B+ !
Favoured
B-
B+
signal
Suppressed
B+
B-
Measuring γ – quantum interferometry
Note the suppressed mode is very rare, and so a multivariate selection is essential.
[LHCb, arXiv:1203.3662]

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Measuring γ – adding other decays
B- →D0 K-
B- →anti-D0 K-
Another possibility is to look for (anti-)D0 decaying into three particles, such as K0Sπ+π- These three particles can share the energy of the decay in different ways, and so here the CP-violation effects are expected to vary, depending on the sharing.
[LHCb, arXiv:1408.2748]
B+
B-
more energy goes to K0Sπ+ in B- case
more energy
goes to K0Sπ-
in B- case
The multivariate selection must ensure efficiency is ~uniform over this 2D space !
Axis meaning

51. Using these decays and others, LHCb has now measured γ to precision of 10o Measurement agrees with the indirect prediction, but is not yet as accurate. Significant improvements are expected over the coming run and with the LHCb Upgrade (see later) which will allow for a very precise comparison. High performance data selection tools will be essential in reaching this goal !
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Measuring γ – the current status
current LHCb measurement
(diagram approximate) [LHCb-CONF-2014-004]

52. 52
Bs →μμ – a twenty-five year old quest
We have been searching for Bs →μμ for a long time...

53. This decay mode can only proceed through suppressed loop diagrams. In the Standard Model it happens extremely rarely (~10-9), but the exact rate is very well predicted Many models of New Physics (e.g. SUSY) can enhance rate significantly ! A ‘needle-in-the haystack’ search !
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Bs →μμ – the physics interest
Prior to the LHC, the experiments at Fermilab
were pushing the search limits down towards 10-8
Standard Model
SUSY

54. There are lots of B-decays that look rather similar to Bs→μμ. And ‘rather similar’ is very dangerous when you are searching for such a rare decay. Traditional, ‘cut-based’ analyses are not adequate – must use a machine-learning multi-variate technique. We use a sequence of two boosted decision trees (BDTs). BDTs must not just search for a B-decay, as in trigger, but must look for one which is Bs→μμ Many signatures to use, and many improvements with time, but possible to do even better (e.g. muon identification itself is still handled in cut-based way)
Finding the needle
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e.g. compare momentum vector of decay with vertex separation vector
momentum vector of candidate
vector between interaction
point & secondary vertices
μ+
μ-
Bs
interaction point
[LHCb, arXiv:1307.5042]

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[LHCb, arXiv:1112.1600]
Plot of invariant mass distribution in region
of high BDT sensitivity – if there is a signal
we should see a peak here (but
the BDT uses much more information
than the invariant mass alone !)
2010
2010 Nothing
Bs→μμ - progress through run 1

56. [LHCb, arXiv:1112.1600]
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[LHCb, arXiv:1203.4493]
Plot of invariant mass distribution in region
of high BDT sensitivity – if there is a signal
we should see a peak here (but
the BDT uses much more information
than the invariant mass alone !)
2010
+2011
+ 2011
Maybe a hint of a bump, but nothing can be claimed
Bs→μμ - progress through run 1

57. [LHCb, arXiv:1112.1600]
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[LHCb, arXiv:1211.2674]
[LHCb, arXiv:1203.4493]
Plot of invariant mass distribution in region
of high BDT sensitivity – if there is a signal
we should see a peak here (but
the BDT uses much more information
than the invariant mass alone !)
2010
+2011
+ early 2012
+ early 2012
Evidence that
there is something there !
Bs→μμ - progress through run 1

58. [LHCb, arXiv:1112.1600]
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[LHCb, arXiv:1211.2674]
[LHCb, arXiv:1203.4493]
Bs→μμ - progress through run 1
Plot of invariant mass distribution in region of high BDT sensitivity – if there is a signal we should see a peak here (but the BDT uses much more information than the invariant mass alone !)
2010
+2011
+ early 2012
+ all 2012
[LHCb, arXiv:1307.5042]
+ all 2012
The evidence grows…

59. Signal becomes even more compelling, if we look at results of a joint analysis
performed on LHCb data and data from the CMS experiment….
…the branching ratio turns out to be consistent with Standard Model prediction.
This result is extremely important, as it rules out many New Physics models.
Improved analyses, with better multivariate selections, are needed to make
more precise measurement, & watch development of intriguing Bd→μμ signal.
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Bs→μμ – the wait is over
Bd→μμ: an even rarer mode
Bs→μμ
[LHCb, arXiv:1411.4413]

60. The LHCb Upgrade
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The Upgrade is an ambitions, approved
project, due to be installed in the second
long LHC shutdown of 2018-19

61. The LHCb Upgrade
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The Upgrade is an ambitions, approved project, due to be installed in the second long LHC shutdown of 2018-19 Its main feature is to remove hardware trigger, increase read-out rate to 40 MHz, and execute software HLT at this rate ! We will also run at higher luminosity… …and output to storage at 20-100 kHz
Very exciting physics potential, provided that the vastly increased data processing challenges can be overcome !
40 MHz
20-100
kHz
2020

62. Conclusions
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• The study of flavour, in particular beauty decays, is a powerful & necessary approach in the search for New Physics beyond the Standard Model.
• LHCb is leading this search. The experiment has produced many important results from LHC run 1, with great prospects for run 2 & the Upgrade.
• Success of experiment depends critically on efficient handling of very large data volume. So far we have done OK, but we are very open to new ideas !

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Backup

64. Precise measurement of Bs oscillations
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Loop diagrams allow Bs mesons to transform into
their own antiparticles, and back again. They do
this very, very rapidly, flipping back and forth many
times during the short time (~1 ps) the particle exists
The capabilities of LHCb have
allowed us to measure this
behaviour with great precision.
This capability is essential for
teasing out the effects from the
New Physics particles which
could contribute (e.g. in
CP-violation effects)
in the loop diagram.