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Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
Mitesh Patel "Searching for new physics with the LHCb experiment"
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Mitesh Patel "Searching for new physics with the LHCb experiment"

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Семинар «Использование современных информационных технологий для решения современных задач физики частиц» в московском офисе Яндекса, 3 июля 2012 …

Семинар «Использование современных информационных технологий для решения современных задач физики частиц» в московском офисе Яндекса, 3 июля 2012

Mitesh Patel, Imperial College

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  • 1. Searching for new physicswith the LHCb experiment Mitesh Patel (Imperial College London) Yandex, Moscow 3rd July 2012
  • 2. What is Particle Physics?Particle physics is the study of the basic constituentsof matter and the forces that act between them 2  
  • 3. Subatomic structure•  The protons and neutrons that make-up ordinary matter are not fundamental – they are made of quarks 3  
  • 4. Subatomic structure•  The protons and neutrons that make-up ordinary matter are not fundamental – they are made of quarks •  proton –  up up down •  neutron –  up down down 4  
  • 5. The Standard Model Gauge Bosons 5  
  • 6. The Standard Model•  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… 6  
  • 7. 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, f   H H f   → Higgs mass blows-up to ∞ (aside which we will ignore: ‘or incredible fine-tuning’)  •  This can’t be the case … 7  
  • 8. Problems with the SM (cont’d) Two colliding clusters of galaxies•  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 ! 8  
  • 9. 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 –  … 9  
  • 10. 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 f   ~   +   f   H H H H f   ~   f   10  
  • 11. 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 f   ~   +   f   H H H H f   ~   f  •  In order to make this cancellation the superpartners cannot be too heavy•  Lightest supersymmetric partner good candidate for dark matter 11  
  • 12. The Large Hadron Collider•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the SM –  worlds 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 12  
  • 13. Searching for new particles•  Two ways of searching for new particles, X –  Try and produce X directly from pp interactions (and Direct production detect its subsequent decay into known particles) p   p   Designed to X study pp interactions –  Look for the effect of X as an intermediary in decay of well known particles •  So called ‘loop’ decays of B particles particularly Loop decay interesting Designed to B   study B decays X Have to integrate over •  Uncertainty principle means that, provided it exists all possible momenta of only for a very short time, X can be much heavier than intermediate partlcles allowed by energy conservation 13  
  • 14. 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  
  • 15. 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(Bs0→µ+µ-) = (3.5±0.2)×10-9 –  → 1 Bs0→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  
  • 16. 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 16  
  • 17. Latest Experimental Results2011 data (5fb-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   Part of 2011 data (2.4fb-1)   3.5 Events/60 MeV 2011 data (1fb-1)   3 ATLAS | |max< 1 s = 7 TeV -1 Data 2.5 Ldt = 2.4 fb Bs µ+µ- MC (10×) 2 1.5 1 0.5 0 4800 5000 5200 5400 5600 5800 mµ µ [MeV] 17   3.5 0 MeV ATLAS 3 | |max< 1.5
  • 18. The Future of Bs0→µ+µ- 12B(Bs → µ + µ -) [10 -9] 11 LHCb 10 Projection from 1 fb-1 9 8 7 0 6 5 4 time integrated SM 3 (arXiv:1204.1737) 2 1 1.5 2 2.5 3 3.5 4 4.5 5 Luminosity [fb-1] 18  
  • 19. Other Analyses relying on MVA •  Whole host of other analyses rely on MVA… 500Events / (0.5 a. u.) Bd → K*0 µµ Background 500 2011 Bd → J/ ψ K* 0 Signal 0 450 2010 Bd → J/ ψ K* Signal 400 2011 Bd → K* 0 µ+µ- Background 400 0 2010 Bd → K* µ+µ- Background 350 300 300 250 200 200 150 100 100 50 0 0 -1 -0.5 0 0.5 1 0 100 200 300 BDT Response (a. u.) Bd → K*0 µµ Signal `   19  
  • 20. 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 L0 “high pT” signals in calorimeter Hardware and muon systems HLT1 Partial reconstruction, selection Software based on one or two (dimuon) displaced tracks, muon ID HLT2 Global reconstruction (very close Software to offline) dominantly inclusive signatures – use BDT 20  
  • 21. 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 21  
  • 22. 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 Bs0→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 22  
  • 23. Backup 23  
  • 24. 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  
  • 25. Extra Dimensions•  Electromagnetism is confined to our usual three dimensions of space•  Maybe gravity is special: – maybe gravity sees other dimensions of space … ? Gravity•  As the force is spread out, it is weakened•  How can there be extra dimensions of space?! 25  
  • 26. 26  
  • 27. 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
  • 28. Cosmic rays are continuously bombarding Earths atmosphere with farmore energy than protons will have at the LHC, so cosmic rays wouldproduce everything LHC can produce  They have done so throughout the 4.5 billion years of the Earthsexistence, and the Earth is still here!The LHC just lets us see these processes in the lab (though at amuch, much lower energies than some cosmic rays)So, there is no danger at all!
  • 29. Pair Production and Annihilation•  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 p.29/41  
  • 30. The History of the Universe •  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 ???? p.30/41  
  • 31. Where did the antimatter go?•  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 p.31/41  
  • 32. 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  
  • 33. CP Violation ParityP   Inversion Spatial mirror p.33/41  
  • 34. CP Violation Charge InversionC   P   Particle-antiparticle C   CP   mirror Parity P   Inversion Spatial mirror p.34/41  
  • 35. CP Violation Charge InversionC   P   Particle-antiparticle C   CP   mirror Parity P   Inversion Spatial mirror p.35/41  
  • 36. 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  
  • 37. 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  
  • 38. The Large Hadron Collider•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the Standard Model –  worlds 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 38  
  • 39. The Large Hadron Collider•  CERN’s Large Hadron Collider (LHC) will explore the physics beyond the Standard Model –  worlds 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  
  • 40. The Higgs Boson H?•  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  
  • 41. 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 A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,   41   UK  Science  Minister  1993  (David  J.  Miller,  UCL)  
  • 42. 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 A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,   42   UK  Science  Minister  1993  (David  J.  Miller,  UCL)  
  • 43. 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 The Higgs field pervades all to their next neighbours who want to know about it too space, the Higgs boson is•  A wave of clustering passes through the like the clustering in that room. It may spread out to all the field. It is the interactions of corners, or it may form a compact bunch particles with the Higgs which carries the news along a line of boson that give particles workers from the door to some dignitary mass. 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 A  quasi-­‐poli?cal  Explana?on  of  the  Higgs  Boson;    for  Mr  Waldegrave,   43   UK  Science  Minister  1993  (David  J.  Miller,  UCL)  
  • 44. The Higgs Boson H? •  Existing measurements tell us that the Higgs Boson, or some other phenomena, must appear at energies accessible at CERN’s LHC e+e-→W+W- Only if we put the Higgs in with the couplings predicted in the SM do we get a theoryrelated to interaction prediction (the turquoise line) that agrees with theprobability: must be measurements (green points)less than ~17 May not be the Higgs boson but something is doing the job! Energy of e e collision •  The simplest theories predict only one boson, but others say there + - might be several 44  

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