Jet Physics Kenichi Hatakeyama Baylor University CTEQ - MCnet - PowerPoint PPT Presentation

Jet Physics Kenichi Hatakeyama 畠山 賢一 Baylor University CTEQ - MCnet Summer School Lauterbad (Black Forest), Germany 26 July - 4 August 2010

Contents  Introduction  Jet production  What are jets?  Inclusive jets and multijets  QCD  New physics search with jets  History of Jets  Jet fragmentation Jet physics motivation Underlying event   e + e - Boson+jets   ep Diffraction and exclusive   production Hadron collider  Jet commissioning and  Jet algorithms  preparation at the LHC Jet reconstruction and calibration  Jet plus track and particle  Detector response for jets  flow jet reconstruction Jet energy correction  Boosted jets for Higgs and  new physics searches Final remarks  July 26 - August 4, 2010 CTEQ Summer School 2010 2

Disclaimers I am an experimentalist, so I have a little more emphasis on  experimental aspects and findings A lot of new “results” were released from LHC experiments at  ICHEP 2010 in Paris about one week ago; however, since there are separate talks on early LHC results next week by Klaus Rabbertz and Jan Fiete Grosse-Oetringhaus, I will not talk about them extensively Although very interesting, I will not discuss jet physics in heavy ion  collisions due to time constraints July 26 - August 4, 2010 CTEQ Summer School 2010 3

What Are Jets?    jet jet anything p p A collimated spray of particles originating from hard scattered partons July 26 - August 4, 2010 CTEQ Summer School 2010 4

See lecture QCD by Dr. Olness The non-abelian SU(3) gauge theory of the strong interaction   Similar to QED, but there are important differences. QED Lagrangian  1               F A A L QED q ( i m ) q e q A q F F ,         4 photon field) ( A :  QCD Lagrangian  1           A L q ( i m ) q g ( q T q ) G F F ,   QCD a b a A b  4      gluon field) A A A B C A G G G g f G G ( G :         ABC   (quark color charges) (gluon color charges) [ a , b 1 , 2 , 3 , A , B , C 1 ,...., 8 ] This non-abelian term distinguishes QCD from QED (introduces triplet and quartic gluon self-interactions)      2 3 2 4 ( L QCD " q q " " G " g " q qG " g " G " g " G " ) Gluon self interactions July 26 - August 4, 2010 CTEQ Summer School 2010 5

QCD quark colors quark anticolors There are three color charges (c.f. one  electric charge in QED) Quarks carry one color charge  Gluons carry one color charge and  one anti-color charge (c.f. photons do not carry electric charge) Gluons have self-interactions (c.f. photons do not) Color charge is conserved at all vertices Gluon self-interaction leads to “anti -  screening” of color charge (c.f. electric charge screening) A quark can emit gluons, and gluons can make  a quark loop or gluon loop  Spread out original quark color (color cloud)  confinement and asymptotic freedom Both features important to describe jets  July 26 - August 4, 2010 CTEQ Summer School 2010 6

Basic Aspects of QCD Asymptotic freedom  A test charge inside the color “cloud” will  experience smaller force than at large distance  At small distances, quarks can interact through confine- color fields of reduced strength and ment asymptotically behaves as free particles Asymptotic The coupling constant  s decreases at  freedom small distances  12   2 ( ) Applicability of perturbation theory Q    s 2 2 ( 33 2 ) ln / n Q f Distance Confinement   The energy injected into a hadron does not separate the quarks but goes into creating qqbar pairs, and hence hadrons  answer the non-observation of free quarks Origin of jets: partons from hard scatter evolve  via radiation and hadronization processes to form a “spray” of collinear hadrons (limited k T relative to “jet” axis) July 26 - August 4, 2010 CTEQ Summer School 2010 7

Observation of Quark Jets First evidence of jets arising from quarks in e+e-  qq events was  obtained at the SPEAR e + e - collider in 1975.   Jet like: S=0  Use “ sphericity ”:  2 2 S 3 ( p ) /( 2 p )  , i min i Isotropic: S~1 i i  QCD predicts that, as the cms energy increases, events should become more jet-like; sphelicity should peak toward lower S values G. Hanson et al. (MARK-I Collaboration), PRL 35 (1975) 1609 July 26 - August 4, 2010 CTEQ Summer School 2010 8

Observation of Gluon Jets TASSO [PETRA] PLB(1979)243; MARK-J [PEP] PRL43(1979)830; PLUTO [PETRA] PLB86(1979)418; JADE [PETRA] PLB91(1980)142 e + e - at E cm = 13 – 32 GeV 1 st three-jet event from TASSO July 26 - August 4, 2010 CTEQ Summer School 2010 9

Jets in e + e - Annihilations  e   jet   e     jet jet jet jet e e e + e - events are clean  No initial state QCD radiation  No beam remnant  No multiple interaction  Played a critical role in establishing QCD  July 26 - August 4, 2010 CTEQ Summer School 2010 10

Why Study Jets in e + e - ? Determine quark spin Measure a s , Study non-abelian Determine spin of gluon structure of QCD QCD Studies  Spin of quarks and gluons  SU(3) gauge structure of QCD, color  Z* factors, triple-gluon vertex  Measurements of a s Quark & gluon jet  properties/differences Fragmentation functions  Search for Higgs Search for the Higgs and new physics  July 26 - August 4, 2010 CTEQ Summer School 2010 11

Jets in e + e - : Spin of the Quark The quark spin can be inferred from the angular distributions of the  “thrust axis” (~direction of jets) Thrust is another event shape variable used in e + e - analyses      Thrust axis: maximize S |p i, parallel | S   p n    i T max T    S  | |  p i  d     2 1 cos  th cos d th  th TASSO (PETRA) 1984: Sphericity axis July 26 - August 4, 2010 CTEQ Summer School 2010 12

Jets in e + e - : Spin of the Gluon Study 3-jet events:   Order jets in decreasing E i Third jet more likely to be the  radiated gluon Angle  EK between axis of (2,3)  relative to 1 in the frame where 2 & 3 are back-to-back (Ellis-Karliner angle) sensitive to gluon spin July 26 - August 4, 2010 CTEQ Summer School 2010 13

Jets in e + e - : Three Gluon Vertex Study 4-jet events:   Order jets in decreasing E i Jets 3 & 4 more likely to be  “radiated” jets Angle  BZ between planes spanned by  (1,2) & (3,4) (Bengtsson-Zerwas angle) sensitive to the three-gluon vertex Full analysis of angular distributions  allows determination of contributions from different diagrams Confirm SU(3) gauge group  structure of QCD July 26 - August 4, 2010 CTEQ Summer School 2010 14

References You can find a lot more interesting  jet physics studies from e + e - in: July 26 - August 4, 2010 CTEQ Summer School 2010 15

Jet Production in ep Collisions     jet jet anything  e  jet  anything p ep (Photoproduction) (NC DIS) July 26 - August 4, 2010 CTEQ Summer School 2010 16

Why Study Jets in ep Collisions? NC DIS QCD Compton Born Process Boson-Gluon Fusion Photoproduction QCD Studies  Proton and photon PDFs  Measurements of  s  Fragmentation functions  Quark-gluon jet properties  Inclusive- and multi-jet production   Rapidity Gaps/Diffraction Search for new physics  July 26 - August 4, 2010 CTEQ Summer School 2010 17

Jets at Hadron Colliders    jet jet anything p p July 26 - August 4, 2010 CTEQ Summer School 2010 18

Jets at Hadron Colliders    jet jet anything pp July 26 - August 4, 2010 CTEQ Summer School 2010 19

Jets at Hadron Colliders Proton (Anti)Proton July 26 - August 4, 2010 CTEQ Summer School 2010 20

Jets at Hadron Colliders See lecture Partons inside proton: by S. Forte Parton Distribution Functions (PDF’s) Proton (Anti)Proton July 26 - August 4, 2010 CTEQ Summer School 2010 21

Jets at Hadron Colliders Hard scattered parton creates Hadronization Jet a “jet” of observable particles Parton showering Dominant hard process: Outgoing parton QCD 2 → 2 scattering of partons q , g Proton Anti(Proton) q , q , g g x 1 p x 2 p x 1 p 1 2 1 q , g Jet July 26 - August 4, 2010 CTEQ Summer School 2010 22

Jets at Hadron Colliders Hadronization Jet Parton showering Initial State Radiation Outgoing parton Beam Remnants Proton Anti(Proton) x 1 p 1 Multiple parton scattering In reality, a little more complicated. Jet Often need to use phenomenological models to account for non-perturbative effects July 26 - August 4, 2010 CTEQ Summer School 2010 23