KEK-PH Lectures and Workshops Hadron Collider Physics Zhen Liu University of Maryland 08/05/2020 Part I: Basics Part II: Advanced Topics Focus Collider Physics is a vast topic, one of the most systematically explored areas in particle physics, concerning many observational aspects in the microscopic world • Focus on important hadron collider concepts and representative examples • Details can be studied later when encounter References: Focus on basic pictures Barger & Philips, Collider Physics Pros: help build intuition Tao Han, TASI lecture, hep-ph/0508097 Tilman Plehn, TASI lecture, 0910.4182 Pros: easy to understand Maxim Perelstein, TASI lecture, 1002.0274 Cons: devils in the details Particle Data Group (PDG) and lots of good lectures (with details) from CTEQ summer schools Zhen Liu Hadron Collider Physics (lecture) KEK 2020 2 Part I: Basics The Large Hadron Collider Lyndon R Evans DOI:10.1098/rsta.2011.0453 Path to discovery 1995 1969 1974 1969 1979 1969 1800-1900 1977 2012 2000 1975 1983 1983 1937 1962 Electric field to accelerate 1897 1956 charged particles Synchrotron radiation 4 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 4 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 5 Why study (hadron) colliders (now)? • Leading tool in probing microscopic structure of nature • history of discovery • Currently running LHC • Great path forward • Precision QFT including strong dynamics and weakly coupled theories • Application to other physics probes • Set-up the basic knowledge to build other subfield of elementary particle physics Zhen Liu Hadron Collider Physics (lecture) KEK 2020 6 Basics: Experiment & Theory Zhen Liu Hadron Collider Physics (lecture) KEK 2020 7 Basics: How to make measurements? Zhen Liu Hadron Collider Physics (lecture) KEK 2020 8 Part I: Basics Basic Parameters Basics: Smashing Protons & Quick Estimates Proton Size ( ) Proton-Proton cross section ( ) Particle Physicists use the unit “Barn”2 1 = 100 The American idiom "couldn't hit the broad side of a barn" refers to someone whose aim is very bad. Enrico Fermi said the neutrons2 hits nuclear reactor nucleus as easy as hitting the broad side of a barn. Proton-Proton cross section is then 10 , actually ~20 mb for hard scattering and 100 mb for total scattering. In natural units ( ~)200 1 −1 Zhen Liu Hadron Collider Physics (lecture) Λ KEK 2020 ≃ 10 Basics: Smashing Protons & Quick Estimates Particle Physicists use the unit “Barn” Beam of colliding 1 = 100 = 10 particles are the Proton-Proton cross section2 is ~100−28 mb2 for microscope for us. total scattering. LHC is 3 Beam power is called (# / /sec) ILC 250 GeV− 12 Instantaneous Luminosity, typical value2 is FCC-ee/CEPC 5 −1 10 = 10 FCC-hh 30 −1 A typical year34 is− 210 sec−1 , for 10 years−1 , −1 −1 accumulating Integrated7 Luminosity, = 10 = 1 9 −1 −1 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 11 Basics: typical cross sections Amongst the collisions, most are not of too much interests to us (either soft scattering, corresponding to known low energy physics, or too massy to interpret), only a small fraction of the collisions are our primary targets. At 14 TeV LHC: How did we calculate the rate Inclusive: 100 and develop a strategy to Dijets (pT> 50 GeV):discover 20 Higgs at the LHC? W: 200 Z: 50 Top quark pair: 800 WW: 100 H: 50 (× =150 ) : 10 −1 3 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 12 Part I: Basics Basic Parameters Basic Cross Sections & Representation At hadron colliders, we are smashing protons. Cross Sections Protons are composite. We are colliding the constituents. ( ) = × × → → Center of 1 Mass 4 Energy × Σ − Σ 4 .. 2 0 � 3 2 =1 2 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 14 Partonic Cross Sections (Hard Scattering) “Parton”—proton constituents� ( ) = × × �→ � → Partonic 1 4 Center of × Σ − Σ 4 Mass .. 2 0 2 � 3 Energy =1 2 < ̂ Zhen Liu � Hadron Collider Physics (lecture) KEK 2020 15 Hadron Collider Representation lab frame center of mass frame but can be translated via Lorentz Boost in the Z-direction = ( , 0≠,0, ), = ( , 0,0, ) Collider (Beam) center of mass energy = , e.g., LHC is 14 TeV with 7 TeV on each beam, ILC 250 is 250 GeV S with 125 GeV on each1 beam. 2 2 Hadron colliders can only use part of the total2 energy as it effectively colliding− partons = ( , 0,0, ), = ( , 0,0, ) Viewing parton 1 and 2 as a system, its Invariant mass (c.o.m.1energy): 1 1 2 2 2 = + = = − Velocity ( ) & Rapidity ( ) in lab frame: 2 1 2 1 2 1 2 =̂ ( )/( + ), Y2= log( / ) 1 1 − 2 1 2 2 1 2 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 16 Rapidity Rapidity: 1 + ln 2 General 4-momenta: ≡ = cosh , sin , − cos , sinh with + , + 2 2 2 2 Rapidity ≡ is additive for boost≡ along z-direction (reference rapidity of ) = 0 ′ Very useful Rapidity difference invariant under boost 0in z-direction = − properties for hadron colliders, Hence allow trivial differential′ translation′ (Jacobian of 1) − − which needs boost = in the z-direction ′ Zhen Liu Hadron Collider Physics (lecture) KEK 2020 17 Rapidity & Pseudo Rapidity Rapidity: Pseudo-Rapidity: 1 + 1 + ln ln 2 2 ⃗ Rapidity is additive≡ for boost along z- For massless particles≡ direction (reference rapidity− of ) =⃗ − = 0 inherit all the properties. ′ Rapidity difference invariant0 under In the limit of boost in z-direction − = ≫ ′ ′ all the properties approximately hold Hence allow trivial differential ≈ translation (Jacobian− of− 1) = No above properties for massive particles ′ Zhen Liu Hadron Collider Physics (lecture) KEK 2020 18 Pseudo Rapidity representing Geometry Pseudo-Rapidity: 1 + 1 1 + cos ln = ln = ln cot 2 2 1 cos 2 ⃗ ≡ Recall that two-body phase ⃗ − − space = cos Compared 1to cos , is 2 8 preferred phase space measure, e.g., a spinless particle decay is “flat” in cos not Approximately linear (in both cos and ) in the central region (|cos | < 1) “zoom in” for forward region (|cos |~1) Zhen Liu Hadron Collider Physics (lecture) KEK 2020 19 Pseudo Rapidity representing Geometry Pseudo-Rapidity: 1 + 1 1 + cos ln = ln = ln cot 2 2 1 cos 2 ⃗ ≡ ⃗ − − Experimentally, the hadron collider geometry are expressed in pseudo rapidity Zhen Liu Hadron Collider Physics (lecture) KEK 2020 20 Pseudo Rapidity representing Geometry Pseudo-Rapidity: 1 + 1 1 + cos ln = ln = ln cot 2 2 1 cos 2 ⃗ ≡ ⃗ − − Experimentally, the hadron collider geometry are expressed in pseudo rapidity Typical high mass/energy events (central: Barrel & EndCap): | | < 3 Forward (lots of QCD background) > 3 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 21 Part I: Basics Basic Parameters Basic Cross Sections & Representation Parton Distribution Functions Factorization Short distance physics (large momentum transfer) should not depend on long distances physics. = , → ∫ 12Σ 1 2 �→ ̂ Used as working assumption = QCD factorization proven for certain processes QED factorization proven ̂ 12 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 23 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 24 Parton Distribution Functions (PDFs) Partons are weakly bound (when probed) constituents of hadrons. Feynman; Bjorken, Paschos-1969 Collinear PDFs ( , ) are the simplest functions describing the nonperturbative hadron structure. (In the simplest interpretation) ( , ) is the probability to find a parton a carrying momentum of the initial hadron with momentum , with a probing scale of . Zhen Liu Hadron Collider Physics (lecture) KEK 2020 25 Parton Distribution Functions (PDFs) PDFs ( , ) are universal, independent of the hard- scattering process. The structure of the hadron drastically as a function of the probing scale Q, obeying Dokshitzer-Gribov-Lipatov- Altarelli-Parisi (DGLAP) equations ( , ) = , ( , ) 1 / , , , , ,… � � � � / are the probabilities= for colinear splitting. Zhen Liu Hadron Collider Physics (lecture) → KEK 2020 26 Hard: large momentum transfer Soft: small momentum transfer Zhen Liu Hadron Collider Physics (lecture) KEK 2020 27 Reasonable (boundary conditions) for PDFs Momentum sum-rule (conservation) , = 1 1 , , , , ,… � �0 Valence sum rule, =e.g., � for � protons (uud) [ , , ] = 2 1 � � [ , − , ] = 1 01 � � [ , − , ] = 0 Sea quark 0 1 � − ̅ Zhen Liu Hadron Collider0 Physics (lecture) KEK 2020 28 Recall that Up and Down are valence quarks, their PDF have a peak at x~1/3, valence quark effect at low 2 PDF rise rapidly at low x due to perturbative evolution (splitting) Zhen Liu Hadron Collider Physics (lecture) KEK 2020 29 As Q increases, high x partons lose momentum through QCD radiation Up Quark PDF evolution (splitting) Zhen Liu Hadron Collider Physics (lecture) KEK 2020 30 As Q increases, high x partons lose momentum through QCD radiation (splitting), gluon PDF Gluon PDF evolution dominants and sea quark Up anti-quark PDF evolution PDF follows Zhen Liu Hadron Collider Physics (lecture) KEK 2020 31 Zhen Liu Hadron Collider Physics (lecture) KEK 2020 32 PDF (as a function of ) = ̂ , = → 1 2 , 1 2 ()→ ∫ 1 Σ � ̂ ∫ Useful̂ tô Σ quickly1 estimate1 2 high2 energy�→ ̂ physics processes PDF drops as a high power (5-6 power) as a function of the enter of mass energy ̂ Zhen Liu Hadron Collider Physics (lecture) KEK 2020 33 “Inverse Process” of fragmentation and hadronization Fig. Credit: Zhen Liu Hadron Collider Physics (lecture) KEK 2020 Eric M. Metodiev 34 Part I: Basics Basic Parameters Basic Cross Sections & Representation Parton Distribution Functions