LHC Physics. Christopher S. Hill University of Bristol. Warwick Week 12 th - 16 th April, 2010

Transcription

1 LHC Physics Christopher S. Hill University of Bristol Warwick Week 12 th - 16 th April, 2010

2 What these lectures will hopefully be A review of the reasons why will built the LHC, maybe a bit on how it works and how it is currently performing A review of the detectors (and how they work, and are performing) and experimental techniques that will be used at the LHC An introduction/review of the physics that we expect to see at the LHC (and what we are already seeing ) An introduction/review of some of the physics we might see at the LHC Roughly 1 lecture on each of the above Convey the excitement of this unprecedented time for particle physics

3 What these lectures will not be I will not go into detail with respect to the maths involved in some of the physics that we might encounter at the LHC I will not cover flavour physics at the LHC even though there is a dedicated flavour physics experiment at the LHC (LHCb) I presume this will be covered by Tim I will not cover relativistic heavy-ion physics at the LHC even though there is a dedicated relativistic heavy-ion physics experiment at the LHC (ALICE) Me standing up here deriving equations, and you busy scribbling it all down. I hope that we can foster discussion on each of the topics that I cover Generous time will be allotted for such discussion

4 Outline of Lectures Lecture 1 - The LHC Lecture 2 - The CMS/ATLAS detectors Lecture 3 - Higgs Physics Lecture 4 - Beyond the Standard Model (BSM) Physics Schedule is flexible - can adjust to your (and my) tastes :)

5 Lecture 1 - Discovery, The LHC, and Collider Physics

6 How are scientific discoveries made? I think there are really only three ways By complete accident (though these stories are often apocryphal) By very careful painstaking work, looking at lots of mundane things and noting discrepancies, etc. By looking somewhere we have never looked before, usually with some new apparatus that allows us to see further

7 The LHC which started operation last year at lower energies, is now routinely colliding at 7 TeV - significantly beyond that which has been done before The LHC is such an apparatus

8 Energy is the key to unlocking small distance scales From De Broglie: λ = h p Therefore by using particles with higher and higher momenta Can resolve smaller and smaller structures This mechanism, responsible for many of the experimental discoveries of 20 th century particle physics

9 In Bohr s Words

10 Cosmic rays are a good source of high energy particles Nature provides plenty of such high energy probes in the form of cosmic rays Successful strategy for discovery in particle physics Muon, Pion, etc. But limited Can t control the energy Can t control rate

11 Controlled Acceleration to High Energies Man-made particle accelerators Application of alternating electric fields gives charged particles a kick Use dipole magnets to bend particles in a circle Repeatedly accelerate Achieve ultra-relativistic velocities Use bunched beams of many particles to obtain high collision rates Fixed target or Colliding Beams? fixed target ECM 2E 1 m 2 E collider CM 4E 1 E 2

12 What to Collide? Matter on anti-matter is best (annihilation) E = γmc 2 e + e -? E turn rad 1 r ( E m )4 Difficult to accelerate e+e- to highest energies due to losses from synchrotron radiation Okay so ppbar? m p 2000 m e Difficult to make (and store) antiprotons in sufficient quantities So the LHC has was chosen to be a pp collider

13 Hadronic Collisions u u u d q Hadrons are composite Really collide constituent partons Valence (u,d) quarks Sea (virtual) quarks Gluons Momentum fraction carried by parton given by empirically determined parton distribution functions (PDFs) q q u d x f(x) q p x f(x) p x x

14 Summary of Advantages of Hadron Machines Higher Energies Multiple production mechanisms Quark-quark, gluon-gluon, quark-gluon Broad band of energies available With enough collisions can probe regimes significantly higher (and lower) than average constituent CM energy These facts make hadron colliders ideal for exploring the unknown Excellent discovery machines

15 Hadron Colliders as Discovery Machines Hadron colliders have historically been discovery machines The LHC should be no different Especially since we expect to see the SM break down at the Terascale (why? we ll get to that soon) W,Z t

16 The Challenge of Hadron Machines Probability for soft QCD interactions many orders of magnitude greater than hard scattering processes of interest Analysis of collision data has added complications»10 12 No beam energy constraint Possibility of more than one interaction per beam crossing Incoming partons may radiate gluons

17 The Large Hadron Collider

18 A better view (obviously taken while skiing)

19 CERN Accelerator Complex LHC is just the last in a series of accelerators needed to get pp collisions at 14 TeV Protons start being accelerated in a LINAC, then transferred to ever larger synchrotrons Eventually injected into LHC where final acceleration to 7 TeV occurs

20 Some LHC facts To bend 0.1 electron/ positron beams LEP magnets were only 0.1 T To bend 7 TeV proton beams, LHC needs 8.3 T This means superconducting magnets

21 Challenges of the LHC - Magnets Superconducting (Nb-Ti) magnets are expensive Must be cooled to 1.9 K (liquid He) To accelerate proton beams in opposite directions requires two sets of magnets (2x the price) LHC uses a novel two-inone magnet design to avoid this cost

22 Luminosity Once the LHC is completed and commissioned, all the typical experimental particle physicist will care about is the beam s instantaneous luminosity Linst = cm -2 s -1 (LHC design goal) This is equivalent to 10 nb -1 /s Particle production is dependent on the cross-section of the physics process and the integrated luminosity (which has units of inverse barns) N = σ Ldt To observe rare processes (e.g. higgs with fb cross-sections) one needs a lot of integrated luminosity One experimental year = 10 7 s -> 100 fb -1 /year at 10 34

23 Beam Parameters that Affect Luminosity n = number of particles in bunch 1,2 f = collision frequency ε = transverse emittance (~size) L = fn 1 n 2 4 x β x y β y σ = gaussian beam profiles in x,y = πσ 2 /β β* = amplitude function (beam optics quantity) For high luminosity want high populations of bunches at low emittance to collide at high frequency with magnets focusing to as low beta as possible

24 Plans for LHC Operation ( ) Energy will be 7 TeV, no higher In 2010 Linst will be (eventually) Expect ~100 pb -1 this year and ~1 fb -1 by end of first run In 2011 Linst will get to 10 33

25 What happened to 14 TeV? Linst =10 34? Due to the accident of September 19, 2008 (which caused a delay of 1 year in the LHC startup), going to energies above 7 TeV has been deemed unsafe without significant work on the LHC (requires warm up of sectors which costs time) 2012 will be spent doing these repairs Will also make modifications necessary for higher lumi From ???, LHC will (hopefully) finally run at sqrt(s) = 14 TeV, with Linst = 10 34

26 Hadron Collider - Basic Kinematics Cylindrical geometry of experiments -> cylindrical coordinates (r,phi,theta) Phi is azimuthal angle Physics is symmetric in phi Theta is the polar angle (0 along the beamline) The fact that hadronic collisions are not collisions amongst point particles complicates the kinematics Longitudinal (z) momentum is not conserved in the partonic collision Momentum is only conserved in the transverse plane Transverse quantities are often used in analysis p T p sin θ E T E sin θ m 2 + p 2 x + p 2 y m T

27 Hadron Collider - Kinematics (Cont.) We usually do not use theta as the polar angle, but rather the pseudorapidity, eta η ln(tan θ 2 ) Eta is a good approximation to the rapidity, y y 1 2 ln(e + p z E p z ) y η, p m, θ 1/γ Charged particle production is constant per unit of rapidity Rapidity is invariant under lorentz boosts in z dn dy 7

28 Hadron Collider - Kinematics (Cont.) Partons carry only a fraction of the protons momenta x p parton p proton Effective center-of-mass energy usually much less than sqrt(s) ŝ = x1 x 2 s To produce a mass of 100 GeV at LHC requires x ~ To produce a mass of 5 TeV at LHC requires x ~ 0.36

29 Minimum ( Min ) Bias Events Vast majority of collisions have a minimum of triggerable activity and are correspondingly called min bias events Small momentum transfer Final state particles have large longitudinal, but small transverse momentum <pt> 500 MeV

30 Why do we expect New Physics at the LHC? Standard Model obviously not a complete theory No description of gravity, 3 generations, mass hierarchy, etc. Requires existence of unobserved Higgs boson to break electroweak symmetry (and give mass to the particles) The Higgs field is a scalar field which causes what is known as the Hierachy Problem N.B. These (and many other questions about SM) may/may not be addressed by the LHC but they were not why it was built

31 Hierarchy Problem Can t the SM be valid up to the scale where gravity is important? MPlanck GeV m 2 H (200 GeV ) 2 = m 2 tree H + δm 2 top H + δm 2 gauge H + δm 2 self H Not easily, even for Much lower energy scales (Λcutoff ~ 10 TeV) This must balance 4,000,000 3,000,000 2,000,000 Incredible fine-tuning required in loop corrections to Higgs mass 1,000, ,000,000 Mass 2 [GeV 2 ] δm 2 H Λ 2 cutoff These -2,000,000-3,000,000 top gauge self tree actual -4,000,000

32 Summary The LHC will offer us a new view into a region of nature that we have every reason to suspect holds significant surprises which we hope to discover It is a challenging machine, one of the biggest scientific endeavours ever embarked on Nevertheless, it has been working very well in 2009, and About 100 µb -1 so far -- more every day (10 6 more by end of 2010, 10 9 more, 1 fb -1, by end of 2011) Next lecture, we ll talk about the detectors with which we ll be observing these 7 TeV collisions...