Searching for new physics with the LHCb experiment

Mitesh Patel (Imperial College London) Yandex, Moscow 3rd July 2012

What is Particle Physics?

Particle physics is the study of the basic constituents of matter and the forces that act between them

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Subatomic structure

•  The protons and neutrons that make-up ordinary matter are not fundamental – they are made of quarks

Subatomic structure

•  proton –  up up down

•  neutron –  up down down

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•  The protons and neutrons that make-up ordinary matter are not fundamental – they are made of quarks

The Standard Model

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Gauge B

osons

The Standard Model

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•  Mathematical description (not a classification!) of particle interactions –  Quantitative and predictive theory –  Agrees with the results of virtually

all experiments …

•  Incredibly successful theory – describes virtually all known phenomena with amazing accuracy

•  Incomplete…

–  The Higgs Boson supposed to give mass to other particles

–  Theory doesn’t describe gravity –  Number of other open questions…

Problems with the SM Higgs •  Even if the Higgs boson is found at CERN’s Large Hadron Collider,

our problems aren’t over…

•  If we compute the Higgs mass find contributions from processes like,

→ Higgs mass blows-up to ∞ •  This can’t be the case …

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H H

f  

f  (aside which we will ignore: ‘or incredible fine-tuning’)  

Problems with the SM (cont’d)

•  Observations of the stars → much more mass that visible –  “Dark Matter” – 23% of the mass/energy

of the Universe is missing! –  SM has no Dark Matter candidate!

•  Observations also indicate that the Universe is expanding at an accelerating rate –  “Dark energy” – 73% of mass/energy in

Universe is missing! –  Try to compute this from the SM - find

something 1054 times too big !

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Two colliding clusters of galaxies

Problems with the SM (cont’d) •  Whole host of other open questions:

–  Why are there so many types of matter particles? •  Mixing of different flavours of quarks and leptons •  Observed matter-antimatter difference

–  Are fundamental forces unified? •  Do all the forces unify at some higher energy scale?

–  What is quantum theory of gravity? •  String theory?

–  …

•  → Expect to find new phenomena (“new physics”) at experiments at CERN’s latest accelerator, the Large Hadron Collider !

•  Solving the problems of the Standard Model: –  (Super-)partners to all existing particles –  Extra spatial dimensions –  …

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Supersymmetry… ? •  Supersymmetric theory (SUSY) postulates that every particle we

observe has a partner with spin different by 1/2 –  denoted by adding tildes (~) to the symbols for the SM particles → squarks, sleptons, gauginos

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H H

f  

f  +   H H

f  

f  ~  

~  

Supersymmetry… ? •  The symmetry must be “broken” – the partners must have higher

masses than the SM particles or we would have seen them!

•  Superpartners stablise the Higgs mass

•  In order to make this cancellation the superpartners cannot be too heavy

•  Lightest supersymmetric partner good candidate for dark matter

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H H

f  

f  +   H H

f  

f  ~  

~  

The Large Hadron Collider

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•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the SM

–  world's largest and highest-energy particle accelerator

–  contained in a circular tunnel, 27km around, at a depth ~100m underground

–  two adjacent parallel beam pipes that intersect at points where expts are placed

–  1600 superconducting magnets bend two proton beams into circular trajectory

–  ~96 tonnes of liquid helium used to keep the magnets at their operating temperature of 1.9K (−271.25 °C)

–  beams accelerated to 0.99999999 of the speed of light

–  Beam energy •  Channel tunnel train at 150km/h •  “Admiral Kuznetsov” cruiser @ 8 knots •  77kg of TNT, your car at ~1000 mph

… within the width of a human hair

Searching for new particles

•  Two ways of searching for new particles, X –  Try and produce X directly from pp interactions (and

detect its subsequent decay into known particles)

–  Look for the effect of X as an intermediary in decay of well known particles

•  So called ‘loop’ decays of B particles particularly interesting

•  Uncertainty principle means that, provided it exists only for a very short time, X can be much heavier than allowed by energy conservation

X

X

Loop decay

Direct production

p  p  

B  

Designed to study pp interactions

Designed to study B decays

Have to integrate over all possible momenta of intermediate partlcles

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The LHCb Experiment •  LHCb is used to study a wide range of “golden decays” where we

have precise theory predictions

•  Perhaps, the highest profile measurement is the search for the

decay Bs0→µ+µ- 14  

The decay Bs0→µ+µ-

•  The decay Bs0→µ+µ- is very sensitive to contributions from new

particles e.g. Higgs boson A0

•  The decay is very suppressed in the SM but the rate expected from

SM processes can be computed precisely, –  B(Bs

0→µ+µ-) = (3.5±0.2)×10-9 –  → 1 Bs

0→µ+µ- decay in every 285 Million Bs0 decays…

–  … but only get 1 Bs0 in every 2000 pp interactions, some of which can

fake a Bs0→µ+µ- decay → few events in >> 285 Million decays

… and rate can be substantially modified in presence of e.g. Higgs boson, A0

•  Rely on combination of all event properties: Multivariate Analysis 15  

Multivariate Analysis •  This, and pretty much all other analyses at LHCb, use the package

Toolkit for MultiVariate Analysis (TMVA)

•  Boosted Decision Tree (BDT) seems to be best performing method

•  Not clear how optimal this is : –  Most people just use default boosting procedure (AdaBoost), choice of

depth, number of nodes etc. –  Notable feature of our problems: not enough training data

•  From analysis side application of MVA is the problem in extracting particle physics results : –  Acquiring the data is extremely time consuming and expensive –  Anything that allows you to get more “power” out of same data is

therefore vitally important → Something with a demonstrable advantage would be used everywhere, very quickly

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Latest Experimental Results

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2011 data (5fb-1)  

2011 data (1fb-1)  

[MeV]µµm4800 5000 5200 5400 5600 5800

Even

ts/6

0 M

eV

0

0.5

1

1.5

2

2.5

3

3.5ATLAS = 7 TeVs

-1 Ldt = 2.4 fb

< 1max||

Data)× MC (10-µ+µsB

[MeV]µµm4800 5000 5200 5400 5600 5800

Even

ts/6

0 M

eV

0

0.5

1

1.5

2

2.5

3

3.5ATLAS = 7 TeVs

-1 Ldt = 2.4 fb

< 1.5max||

Data)× MC (10-µ+µsB

[MeV]µµm4800 5000 5200 5400 5600 5800

Even

ts/6

0 M

eV

0

0.5

1

1.5

2

2.5

3

3.5ATLAS = 7 TeVs

-1 Ldt = 2.4 fb

< 2.5max||

Data)× MC (10-µ+µsB

Part of 2011 data (2.4fb-1)  

•  Profile is such that search made at all three LHC experiments

•  Intense rivalry to see the first signal events

•  Will then need to make a precise measurement of the decay rate  

The Future of Bs0→µ+µ-

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]-1Luminosity [fb1.5 2 2.5 3 3.5 4 4.5 5

]-9

) [10

-µ +

µ → 0 s

B(B

123456789

101112

time integrated SM(arXiv:1204.1737)

LHCb-1Projection from 1 fb

Other Analyses relying on MVA •  Whole host of other analyses rely on MVA…

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BDT Response (a. u.)-1 -0.5 0 0.5 1

Even

ts /

(0.5

a. u

.)

0

100

200

300

400

500 Signal0 K*ψ J/→ d2011 B

Signal0 K*ψ J/→ d2010 B

Background -µ+µ 0 K*→ d2011 B

Background -µ+µ 0 K*→ d2010 B

Signalµµ 0 K*→ dB0 100 200 300

Bac

kgro

und

µµ 0

K*

→ dB0

50100150200250300350400450500

`  

Triggering •  Accelerator collides particles 40Million times / second •  Cannot process or store all of events from collisions and look

afterwards for events we are interested in – have to chose which events to keep for further study

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L0 Hardware

“high pT” signals in calorimeter and muon systems

HLT1 Software

Partial reconstruction, selection based on one or two (dimuon) displaced tracks, muon ID

HLT2 Software

Global reconstruction (very close to offline) dominantly inclusive signatures – use BDT

HLT2 •  LHCb uses a BDT in the second level of the High Level Trigger,

HLT2 –  selects N-dimensional regions of parameter space to keep by learning

from training samples –  Have to ensure that selected regions are not so small relative to the

resolution and/or stability of the detector st they could cause the signal events to oscillate in and out of the kept regions (→ less efficiency, or a trigger that is impossible to understand the efficiency of)

–  Only allow decision tree to split at certain pre-defined points in the parameter space

•  e.g. know that the track quality of a particle discriminates between signal and background – requirement of χ2 < 4 or χ2 < 9 are sensible, effect of χ2

<1.000045 might vary between data-taking period –  Triggering is one of the major challenges for the experiment – any

advantage that could get from new methods would make a tremendous difference

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Conclusions •  Our knowledge of particle physics is embodied by a mathematical

description of particle interactions, ‘The Standard Model’

•  The model is tremendously successful but has some significant problems – latest experiments may find new phenomena!

•  LHCb experiment searching for signatures of new phenomena by probing certain rare B particle decay modes such as Bs

0→µ+µ-

•  In this and in many other analyses, and in other aspects of the experiment, searching for small signal over large backgrounds – multivariate analysis a key requirement

•  Any improvement in MVA would be hugely beneficial and sought after by everyone working in this field, and in other fields

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Backup

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Extra Dimensions

•  Which is weaker: – Gravity or Electromagnetism?

•  Alternatively, which is more powerful: – The gravitational pull of the entire earth

or The boy with his magnet?

•  Gravity is extremely weak! Why? 24  

Extra Dimensions

•  Electromagnetism is confined to our usual three dimensions of space

•  Maybe gravity is special: – maybe gravity sees other dimensions of space … ?

•  As the force is spread out, it is weakened

•  How can there be extra dimensions of space?!

Gravity

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Black Holes •  Microscopic Black Holes! Not like astronomical Black Holes!

•  If matter is sufficiently compressed, its gravity becomes so strong that it carves out a region of space from which nothing can escape

•  Size you have to compress to depends on the mass -> smaller hole, greater amount of compression required

•  Gravity weak -> amount of compression required way beyond accelerators… but with extra-dimensions maybe gravity is strong on small enough scales… -> microscopic black holes at the LHC?

•  Hawking radiation -> black holes shrink

•  Quantum effects -> microscopic black holes “evaporate” -> produce lots of particles

Cosmic rays are continuously bombarding Earth's atmosphere with far more energy than protons will have at the LHC, so cosmic rays would produce everything LHC can produce  They have done so throughout the 4.5 billion years of the Earth's existence, and the Earth is still here! The LHC just lets us see these processes in the lab (though at a much, much lower energies than some cosmic rays) So, there is no danger at all!

Pair Production and Annihilation

p.29/41  

•  Picture shows pair-production: γ + γ -> e+ + e-

•  Observe that particle and antiparticle are always created in pairs

•  Annihilation also occurs in pairs: e+ + e- -> γ + γ

•  Hence,  Particles − Antiparticles = 0

The History of the Universe

p.30/41  

•  t = 13.7×109 yrs

•  All energy in Universe confined in a tiny region -> extremely hot and dense

•  ‘Soup’ of basic particles •  Only later, as Universe expanded and

cooled, temperature became low enough to form neutrons and protons, nuclei, atoms…

•  t=0 s ????

Where did the antimatter go?

p.31/41  

•  Shortly after the Big Bang (extremely dense/hot) -> equal amounts of matter and antimatter were created from the available energy

•  Where did the antimatter go?

•  Particle Physics – smallest of scales

Big Bang – largest of scales

A matter-antimatter asymmetry •  We have found a small difference between matter and antimatter

that could generate such an asymmetry

•  Some processes generate slightly more matter than antimatter

•  Such processes violate a symmetry known as “CP-symmetry” –  A process obeys CP-symmetry if its results are identical after changing

all particle positions to a mirror image and changing all particles to their antiparticles [… next slides…]

–  Processes that don’t obey CP-symmetry said to be “CP-violating” – can produce an excess of matter over antimatter as they treat particles and antiparticles differently

p.32/41  

CP Violation

Parity Inversion Spatial mirror

p.33/41  

P  

CP Violation

Parity Inversion Spatial mirror

CP  

Charge Inversion Particle-antiparticle mirror

p.34/41  

C   P  

P  

C  

CP Violation

Parity Inversion Spatial mirror

C   CP  

Charge Inversion Particle-antiparticle mirror

p.35/41  

P  C  

P  

CP Violation

CP  •  We have found that matter and antimatter behave differently after the C and P mirrors: “CP violation”

•  Allows for some reactions to proceed more easily that their CP-opposites

p.36/41  

A matter-antimatter asymmetry

•  While CP violation could generate a matter-antimatter asymmetry the effect we see is tiny – much too small to explain the matter-antimatter asymmetry in the Universe

•  Expect there are additional sources of CP violation -> hope to see evidence of these in the collisions at CERNs Large Hadron Collider (LHC)

p.37/41  

The Large Hadron Collider

38  

•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the Standard Model

–  world's largest and highest-energy particle accelerator

–  contained in a circular tunnel, 27km around, at a depth ~100m underground

–  two adjacent parallel beam pipes that intersect at four points where experiments are placed

–  1600 superconducting magnets bend protons into circular trajectory

–  ~96 tonnes of liquid helium used to keep the magnets at their operating temperature of 1.9K (−271.25 °C)

–  beams accelerated to 0.99999999 of the speed of light

–  Beam energy •  Channel tunnel train at 150km/h •  Aircraft carriers HMS invisible and HMS

Illustrious (combined) at 6.0 m/s •  77kg of TNT, your car at ~1000 mph

… within the width of a human hair

The Large Hadron Collider

•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the Standard Model

–  world's largest and highest-energy particle accelerator

–  contained in a circular tunnel, 27km around, at a depth ~100m underground

–  two adjacent parallel beam pipes that intersect at four points where experiments are placed

–  1600 superconducting magnets bend protons into circular trajectory

–  ~96 tonnes of liquid helium used to keep the magnets at their operating temperature of 1.9K (−271.25 °C)

–  beams accelerated to 0.99999999 of the speed of light

–  Beam energy •  Channel tunnel train at 150km/h •  Aircraft carriers HMS invisible and HMS

Illustrious (combined) at 6.0 m/s •  77kg of TNT, your car at ~1000 mph

… within the width of a human hair

39  

The Higgs Boson •  One of main problems of Standard Model – in its simplest form the

mathematical structure of theory does not allow the introduction of mass for the particles!

•  The Higgs Boson, through the Higgs mechanism, is the particle that

‘gives’ particles mass … –  How can a particle give mass to other particles?! –  Don’t particles just have mass?

40  

H?

The Higgs Mechanism

•  Imagine a cocktail party of political party workers who are uniformly distributed across the floor, all talking to their nearest neighbours

•  A certain ex-Prime-Minister enters and crosses the room. All of the workers in her neighbourhood are strongly attracted to her and cluster round her. As she moves she attracts the people she comes close to, while the ones she has left return to their even spacing

•  Because of the knot of people always clustered around her she acquires a greater mass than normal, that is, she has more momentum for the same speed of movement across the room. Once moving she is harder to stop, and once stopped she is harder to get moving again because the clustering process has to be restarted

41  A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,  UK  Science  Minister  1993  (David  J.  Miller,  UCL)  

The Higgs Boson

•  Now consider a rumour passing through our room full of uniformly spread political workers. Those near the door hear of it first and cluster together to get the details, then they turn and move closer to their next neighbours who want to know about it too

•  A wave of clustering passes through the room. It may spread out to all the corners, or it may form a compact bunch which carries the news along a line of workers from the door to some dignitary at the other side of the room

•  Since the information is carried by clusters of people, and since it was clustering which gave extra mass to the ex-Prime Minister, then the rumour-carrying clusters also have mass

•  The Higgs boson is predicted to be just such a clustering in the Higgs field

42  A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,  UK  Science  Minister  1993  (David  J.  Miller,  UCL)  

The Higgs Boson

•  Now consider a rumour passing through our room full of uniformly spread political workers. Those near the door hear of it first and cluster together to get the details, then they turn and move closer to their next neighbours who want to know about it too

•  A wave of clustering passes through the room. It may spread out to all the corners, or it may form a compact bunch which carries the news along a line of workers from the door to some dignitary at the other side of the room

•  Since the information is carried by clusters of people, and since it was clustering which gave extra mass to the ex-Prime Minister, then the rumour-carrying clusters also have mass

•  The Higgs boson is predicted to be just such a clustering in the Higgs field

43  A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,  UK  Science  Minister  1993  (David  J.  Miller,  UCL)  

The Higgs field pervades all space, the Higgs boson is like the clustering in that field. It is the interactions of particles with the Higgs boson that give particles mass.

The Higgs Boson •  Existing measurements tell us that the Higgs Boson, or some other

phenomena, must appear at energies accessible at CERN’s LHC

•  The simplest theories predict only one boson, but others say there might be several

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related to interaction probability: must be less than ~17

Energy of e+e- collision

e+e-→W+W- Only if we put the Higgs in with the couplings predicted in the SM do we get a theory prediction (the turquoise line) that agrees with the measurements (green points) May not be the Higgs boson but something is doing the job!

H?