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27/11/14 YSDA seminar 1

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


Introduction

- the Standard Model

- flavour physics

- open questions

- LHCb

ATLAS

CMS

ALICE

LHCb

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.

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 !

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

27/11/14 6

CERN, July 2012

What is flavour physics?


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


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

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

27/11/14 YSDA seminar

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

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

[LH

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Cosmological connections ?


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.

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


• 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)

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

Indirect measurements –

an established tradition in science


Eratosthenes was able to determine

the circumference of the earth

using indirect means…

…around 2.2 thousand years

prior to the direct observation.

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’

The power of indirect measurements –

the top quark mass


direct

indirect

Top m

ass

[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 !

LHCb – a forward spectrometer

for flavour physics



for flavour physics

27/11/14 19

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


for flavour physics

20

Array of RICH photodetectors

Assembling RICH 2;

note the mirrors Momentum [GeV/c]

Ch

ere

nko

v a

ng

le [ra

d]

Two ‘RICH’ detectors detect Cherenkov radiation.

the angle at which this is emitted tells us the particle

species – it provides ‘hadron identification’.


for flavour physics

YSDA seminar 21

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


for flavour physics

27/11/14 22

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

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


delivered by LHC

collected by LHCb


Data processing

- the data challenge

- trigger

- offline

- the end-user: selected physics results

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

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

Not all collisions are equally interesting


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

Triggering on beauty


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.

.





μ+

μ-

K+

p p

~ 1 cm

Interaction point




finite

impact

parameter

B+





2. Look for tracks with significant

‘impact parameter’ with respect

to primary vertex.





μ+

μ-

K+

p p

B+

~ 1 cm

Interaction point




b-hadron decay, or

‘secondary vertex’





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.

Trigger: L0


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]

Trigger: HLT


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.

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

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

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

Offline processing - event reconstruction


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

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

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

LHCb computing model

RRCKI + recent addition

Monte Carlo

production

(and some

user analysis)

HLT farm

Tier-2’s

YANDEX +

Snapshot of recent CPU provision


~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

Profile of recent CPU usage


Th

ou

san

ds o

f jo

bs

Simulation jobs

User analysis jobs

(Recall that currently no real data being processed, nor are any being reprocessed)


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.

Data storage


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

The end-user: selected physics results


The end-user: selected physics results


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

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)4.66( 2.1

3.3

Relations exist between the elements

of the matrix which can be displayed

geometrically – as triangles.

predicted

value


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

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


..and none is seen, since the decay rates are essentially the same.

[LH

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]

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-


Note the suppressed mode is very rare, and so a multivariate selection is essential.

[LH

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662

]


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.

[LH

Cb

, a

rXiv

:14

08

.27

48

] 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

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 !


Measuring γ – the current status

current LHCb measurement

(diagram approximate)

[LHCb-CONF-2014-004]

52

Bs →μμ – a twenty-five year old quest

We have been searching for Bs →μμ for a long time...

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 !


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

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


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

[LH

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7.5

042

]


[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

[LHCb, arXiv:1112.1600]


[LHCb, arXiv:1203.4493]






2010

+2011

+ 2011

Maybe a hint of a bump, but nothing can be claimed


[LHCb, arXiv:1112.1600]


[LHCb, arXiv:1211.2674]

[LHCb, arXiv:1203.4493]






2010

+2011

+ early 2012

+ early 2012

Evidence that

there is something there !


[LHCb, arXiv:1112.1600]


[LHCb, arXiv:1211.2674]

[LHCb, arXiv:1203.4493]

Bs→μμ - progress through run 1 Plot of invariant mass distribution in region





2010

+2011

+ early 2012

+ all 2012

[LHCb, arXiv:1307.5042]

+ all 2012

The evidence grows…

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.


Bs→μμ – the wait is over

Bd→μμ:

an even

rarer mode

Bs→μμ

[LH

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rXiv

:14

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13

]

The LHCb Upgrade


The Upgrade is an ambitions, approved

project, due to be installed in the second

long LHC shutdown of 2018-19

The LHCb Upgrade


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

Conclusions


• 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 !


Backup

Precise measurement of Bs oscillations


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.

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