Physics at HERA. Summer Student Lectures August Katja Krüger Kirchhoff Institut für Physik H1 Collaboration

Transcription

1 Physics at HERA Summer Student Lectures August 28 Kirchhoff Institut für Physik H1 Collaboration katja.krueger@desy.de

2 Overview Part 2 Exotics Jet Physics Cross Sections Strong Coupling Heavy Quarks Model Dependent Searches Model Independent Searches Charm Beauty personal selection! many more analyses are done! Diffraction 2

3 Exotics or Beyond the Standard Modell 3

4 New Particles many theories predict more particles than the SM: SUSY: every Standard Model particle has a supersymmetric partner fermion partners are bosons, boson partners fermions leptoquarks particle with lepton and quark properties can be produced resonantly in ep collisions... exited fermions, contact interactions, large extradimensions... but experimentally search also model independent! 4

5 Leptoquarks e e k LQ xp q looks the same as NC process MLQ2 = (xp + k)2 = xs q derive limits on coupling Events / 2 GeV Events / 2 GeV compare measured cross section with SM expectation NC, P= 27% H1 data (prelim.) SM SM uncertainty MLQ / GeV 5 CC, P= 27% H1 data (prelim.) SM SM uncertainty MLQ / GeV 5

6 λ Limits on Leptoquarks 1 H1 preliminary S,L (e u, νd) ~.L. S,R (e u) 95 % C S,R (e d) S 1,L (e d, e u, νd) Ex c lu de d at MLQ/ GeV 6

7 SUSY R parity violation: single SUSY particle can be produced limits depend on many parameters (masses, couplings) example: stop 7

8 General Searches idea: new particles have typically large mass final state should contain particles with large transverse momentum from the decay jets electrons muons photons neutrinos (missing transverse momentum) 8

9 General Searches every channel in reasonable agreement with the standard model 9

10 Multi Leptons H1+ZEUS (common phase space) Data (prelim.) 3 SM SM Signal M12 [GeV] Tri electrons, HERA I+II (e±p,.94 fb 1) 3 3e H1+ZEUS (common phase space) Data (prelim.) SM 2 SM Signal HERA Exotics Working Group 2e Events Di electrons, HERA I+II (e±p,.94 fb 1) HERA Exotics Working Group Events in HERA1 a small excess of di and tri electron events at high transverse momenta observed by H M12 [GeV] HERA2 data show no significant excess

11 Isolated Leptons and Missing PT X X e e spectacular events excess in HERA1 data at large transverse momenta of the hadronic system (PTX) seen by H1 11

12 Isolated Leptons and Missing PT no excess in e data what about e+? 12

13 Isolated Leptons and Missing PT H1+ZEUS combined: 1.8 excess H1 alone: 2.9 excess? 13

14 Jet Physics & the Strong Coupling S 14

15 What are Jets? jets are narrow bundles of hadrons originating from quarks or gluons can be used to study QCD and the strong coupling 15

16 How Are Jets Produced? do analysis in a frame where photon and proton collide head on (e.g. Breit frame) LO DIS cannot produce transverse momentum jets with transverse momentum can originate from boson gluon fusion (BGF) or QCD Compton (QCDC) processes BGF QCDC s 16

17 Jet Cross Sections theory curve: NLO QCD calculation PDFs s hadronisation very good agreement of theory and data uncertainty on PDF and theory input leads to uncertainty on s 17

18 Jet Cross Sections Normalised Inclusive Jet Cross Section NLO hadr Z ET / GeV H ET / GeV H R 1. ET / GeV ET / GeV 5 < Q < 15 GeV σjet / σ NC.8 ratio of jet cross section to inclusive cross section has reduced uncertainties systematic PDFs 1. 7 < Q < 5 GeV < Q < 7 GeV σ jet/ σnc 2 2 H1 2 R σ jet/ σnc R ET / GeV 2 σjet / σ NC H1 2 < Q 2< 27 GeV 2 σ jet/ σ NC 1 27 < Q < 4 GeV R 2 H1 Data R R σ jet/ σ NC 15 < Q 2< 2 GeV 2 H ET / GeV 18

19 S from Jets running of S visible in one measurement theory uncertainties larger than experimental uncertainties 19

20 Running of S combine S measurements with smallest errors in fit 2

21 Running of S comparison with other HERA measurements 21

22 s M Z HERA measurements often dominated by systematic and theoretical uncertainties use only selected datasets to extract s with minimal uncertainty HERA value very competitive 22

23 Improved Parton Densities F2 is only indirectly sensitive to the gluon global fits (MRST, CTEQ) use Tevatron jet data alternative: use HERA (di )jet data gluon fractional error ZEUS Q = 1 GeV2 Q = 2.5 GeV without jet data with jet data Q = 2 GeV2 2 2 Q = 2 GeV2 Q = 7 GeV Q = 2 GeV x 1 improvement at medium to large x 23

24 Heavy Quarks 24

25 Production of Heavy Quarks predominantly via boson gluon fusion e b,c b,c p heavy quark contribution to structure function g large quark mass allows pqcd calculations directly sensitive to gluon density in the proton d 2 b b dx dq 2 = 2 2 xq 4 Y [ y2 b b 2 F x, Q F L x,q Y bb ]

26 Reconstruction of charm Quarks K D c Entries /.5 MeV D fragmentation c D meson (25,5%) golden decay (2,6%) D D s K s only charged decay particles small mass difference M =m D m D H1 Preliminary HERA II + ± Κ π fit 6 π ±slow N(D*) = 283 ± small momentum of the slow s good experimental resolution (~1 MeV) 5 < Q2 < GeV2.2 < y <.7 η (D*) < 1.5 pt (D*) > 1.5 GeV M(Kππ) M(Kπ) [GeV] 26

27 More charm Signals ± Ds Combinations / MeV D ZEUS ep e + D± + X Gaussmod + p1 N(D± ): 2181 ± 83 ZEUS (prel.) 135 pb 1 2D SDL > 3 HERA II 5 D± M(K ππ) (GeV) 27

28 D* Cross Section good description by NLO pqcd calculation (HVQDIS) in full measured Q2 range (> 4 orders of magnitude) PDF: ZEUS PDF extracted from inclusive DIS 28

29 Tagging of beauty Quarks large transverse momenta due to large mass semileptonic decay long lifetime (beauty ~5 µm, charm ~ 3 µm) lifetime tagging ptrel 29

30 beauty Cross Section Results some data higher than NLO QCD theory, but good agreement for the most precise data 3

31 Inclusive Lifetime Tagging signed impact parameter α < π/2 δ=+ δ α > π/2 δ= δ 1. Vertex δ δ Jet α 1. Vertex α Track Track resolution & lifetime resolution Events Jet H1 Data Total MC uds c b (b) both experiments have silicon vertex detectors inclusive method: use all tracks study significance of the (signed) impact parameter: S= / allows separation of beauty, charm and light quarks S2 31

32 Contribution to the Cross Section large charm fraction (up to ~3%) small beauty fraction ( to few %) charm and beauty thresholds reasonable description by theory 32

33 Contribution to the Structure Function 33

34 Diffraction 34

35 What is Diffraction? in general: in DIS events the proton breaks up in diffraction: the proton stays intact (but nevertheless W>MP) empty surprise: ~% of all events at HERA are diffractive! 35

36 Diffraction idea: interaction between photon and proton by a Pomeron colourless already used to describe low energy hadron hadron scattering no particle! 36

37 Physics in Diffraction many things similar to inclusive DIS diffractive parton densities jets in diffraction heavy flavour in diffraction test of factorization are the parton densities the same for all diffractive processes? or: does the Pomeron know what happens at the photon vertex? 37

38 Diffractive Parton Densities Singlet = Quark xip σrd(3) H1 Data H1 26 DPDF Fit A (extrapol. fit).5 β=.1 β=.4 β=.1 β=.2 β=.4 β=.65 Q2 Μ GeV2Ν 3.5 β= xip 38

39 Diffractive Dijet Cross Sections shape of the QCD theory prediction agrees with the data normalization is wrong by a factor 2 factorization is broken! 39

40 Vector Mesons in Diffration vector mesons (VM=,,, J/,...) have the same quantum numbers as the photon photon can fluctuate into a VM, afterwards the VM can scatter off the proton hadron hadron scattering e e VM p p 4

41 Vector Mesons in Diffration light VMs (,, ) show the same dependence on the p center of mass energy W heavier VMs (J/, ) show increasing W dependence with mass simple Pomeron picture doesn't work! 41

42 Deeply Virtual Compton Scattering production of a real photon by scattering of a virtual photon on a proton needs PDFs depending on x and skewedness Generalized PDFs, extending standard PDFs to elastic form factors 42

43 Deeply Virtual Compton Scattering similar W dependence as heavy vector mesons 2 gluon exchange 43

44 Summary HERA offered unique possibilities to study the structure of the proton perturbative QCD is a big success to desribe HERA data no significant deviation from the Standard Model found always prepare for the unexpected! 44