Wγ Measurements at ATLAS using 7 TeV data

Transcription

1 Wγ Measurements at ATLAS using 7 TeV data BNL Jamboree February 10, 2010 Al Goshaw Duke University 1

2 Introduction Duke has been involved with several W/Z γ analyses at the Tevatron using CDF Run II data. A few of us have been thinking about what can be done using ATLAS data, especially now with the opportunity for an early measurement using 7 TeV data. Some of these studies are included in the CSC SM chapter for 14 TeV predictions. People involved: Andrea Bocci, Zhijun Liang, Jianrong Deng, Suen Hou, Miaoyuan Liu, Tom Phiilips, AG with others expressing interest in related measurements. A brief summary today: Cross section measurements and theory calculations Al ATLAS framework and photon fake rate studies Zhijun Data sets and organizational plans Andrea 2

3 An Approach to measuring the Wγ Cross section 3

4 Measuring W + V cross sections Step 1: Inclusive W data set Start from the selected inclusive W data set and use associated trigger and ID studies required for cross section measurement. p + p W + X e/µ + ν + X For the example here assume the cuts proposed in the CSC note for an inclusive W cross section measurement: E T > 25 GeV and η < 2.4 for e/µ MET > 25 GeV and M T (e/µ + ν) > 40 GeV 4

5 Measuring W + V cross sections Step 1: Inclusive W data set Calculate the W + and W - inclusive cross section for e and µ channels within the above kinematic cuts: σ Wcuts = N W /[ L (ε trig ε ID ) W ] where N W = N W (data) N W (background) 5

6 Measuring W + V cross sections Step 2: W + γ data set Use the inclusive W data set N W (data) from Step 1 and search for p + p W + V + X e/µ + ν + V + X where V = γ or W or Z Use some specific set of kinematic cuts for the V selection but W selection cuts identical to inclusive W selection. For V = γ use the following cuts: E T > 25 GeV and η < 2.4 and ΔR(l-γ) > 0.7 for good isolation (see plots latter on ) 6

7 Measuring W + γ cross sections Step 2: W + γ cross section In general the cross section for : σ Wγ cuts = N Wγ /[ L (ε trig ε ID ) W ε IDγ ] where N Wγ = N Wγ (data) N Wγ (background) Take from SM theory Compare σ Wγ cuts above to σ Wcuts on page 3 to get: σ Wγ cuts = [ N Wγ /N W ] [(ε trig ε ID ) W / (ε trig ε ID ) W ] [ 1/ε IDγ ] σ Wcuts Ratio of signal events Ratio ~ 1.0: evaluate from W inclusive analysis From photon selection cuts 7

8 Measuring W + γ cross sections Step 3: Final W γ corrected cross section In general the cross sections for: p + p W + γ + X l + ν + γ + X are quoted with kinematic acceptance corrections for: 1. full W decay phase space for the e/µ and ν 2. full phase space in η for the photon These are of course model dependent corrections. For example: σ SM (p + p -> e + + ν e + γ + X) = σ Wγ cuts [ σ SM eνγ /σsm eνγ(cuts) ] (E T > 25 GeV, ΔR(e-γ) > 0.7) from data from SM theory 8

9 Tevatron W + γ and Z + γ cross sections Using this general idea (not exactly same technique) some results from the Tevatron (CDF) are: data SM theory σ ( p + p -> l+ + ν + γ + X) pb pb σ ( p + p -> l + l - + ν + γ + X) pb pb M(l + l - ) > 40 GeV (E T > 7 GeV and ΔR(l-γ) > 0.7) photon E T distribution from Z γ events compared to SM predictions 9

10 Wγ and Zγ SM generators 10

11 Wγ and Z γ generators LO and NLO matrix elements are available from calculations by Baur, Berger, Han and Ohnemus. Uli Baur provides simple user packages to access these. Basics features of LO and NLO parton level generators: Input parameter file controls kinematic cuts and dynamic variables used in the matrix element calculation Ten-step VEGAS grid training to maximize efficiency Produces events with a weight that includes all initial state parton collisions for that kinematic configuration Anomalous Gauge Couplings (AGC) at WWγ vertex include all four (complex) parameters allowed by Lorentz and gauge invariance. 11

12 General features of BHO LO Wγ and Zγ Advantages Can be coupled to shower/hadronization Monte Carlo s M 2 Baur + PYTHIA includes all LO diagrams including IS, FS and gauge boson photon couplings with options of applying anomalous gauge VVγ couplings. Has been field tested at CDF and D0 Good support from Uli Baur Disadvantages Only q q production mechanisms Magnitude of cross sections has significant higher-order corrections (factors of ~ 1.3 at the Tevatron) Kinematics for high-p T photon production can have large higher-order corrections 12

13 General features of BHO NLO Wγ and Zγ 1. Produces Monte Carlo events in a manner analogous to that of the LO program. That is, you can access 3 and 4-body final state 4-vectors with weights ( can be + or -). 2. All q q and q g initial states are included. No production of W/Zγ from gluon-gluon fusion, VBF or quartic couplings. 3. The order of the calculation is α s α weak α em. Therefore, e.g., the final states for W γ are l ν γ, l ν γ g, or l ν γ q 4. The narrow-width approximation is used for the W/Z and therefore there is no FS photon radiation 13

14 General features of BHO NLO Wγ and Zγ 5. The 3-body final state includes q/g fragmentation to a photon using a next-to-leading-log calculation. This requires a specification of a the hadronic isolation around the photon. 6. The splitting of a quark -> quark + W/Z has a logarithmic dependence ~ α weak ln[e T (q)/m w/z )] 2 which for high energy quarks produces very large k-factors at high photon E T. These can be suppressed by doing the NLO calculation with a cut on high E T jets, to say < 50 GeV. 14

15 Some Wγ SM predictions at 7 TeV 15

16 Baur LO Wγ 7 TeV predictions Apply W -> e/mu nu selection cuts as for inclusive W measurements: E T > 25 GeV and η < 2.4 for e/µ MET > 25 GeV and M T (e/µ + ν) > 40 GeV Then look at the associated photon production spectrum for: E Tγ > 10 GeV and ΔR(l-γ) > 0.7 photon E T photon η 16

17 Baur LO Wγ 7 TeV predictions With these W selection cuts about ½ the photons are from FSR and ½ from Wγ events (ISR plus WWγ coupling). FSR ISR + WWγ mass(e-ν ) 17

18 Baur LO Wγ 7 TeV predictions Next apply photon selection : E Tγ > 25 GeV η γ < 2.4 ΔR(l-γ) > 0.7 p + p -> e - + ν e + γ + X with CTEQ5L FSR suppressed ISR + WWγ M(e-ν ) M T (e-ν ) 18

19 Wγ 7 TeV predictions With the cuts listed above for W and photon selection E T > 25 GeV and η < 2.4 for e/µ/γ MET > 25 GeV and M T (e/µ + ν) > 40 GeV the LO cross section are: σ (W + γ) = 0.58 pb + σ(w - γ) = 0.43 pb The NLO corrections depend on exact cuts used to suppress jets: I did not have time to do this at 7 TeV. From some 14 TeV studies with E T (jet) < 50 GeV k ~ 1.4, but depends on photon E T. Therefore the cross section with the rather hard lepton and photon cuts above, before particle ID efficiency, is ~ 1.4 pb. 19

20 Conclusions The W γ channel is a good candidate for an early analysis using 7 TeV data. Use the W inclusive data set and all the trigger/efficiency studies that will be provided by these analyses. This measurement provides a nice SM calibration with leptons an photons, allows for a search for W boson structure via AGC and other more exotic sources of W γ events 20

21 Quirk Relaxation Products can be Soft Kang Luty 08 see also Chacko et al 07 Quirks W gamma + relaxation effects 400 soft photons soft charged pions? 300 GeV 500 GeV 800 GeV Correlation of hard W+gamma with oddly shaped active UE allows diagnosis 21 21

22 Backup slides 22

23 Example NLO prediction for Wγ Generate W + γ and W - γ events with BHO NLO generator, using MRST(c-g) PDF (3,89 in pdflib) Event selection as follows Trigger lepton : E T > 25 GeV and η < 2.5 MET 20 GeV Photon : E T > 10 GeV, η < 2.5 and ΔR(l-γ) > 0.7 Suppress uninteresting events with cuts: 1. M C (l ν γ) > 90 GeV/c 2 (suppress FSR) 2. E T (q/g) < 50 GeV (suppress ) 23

24 Example NLO prediction for Wγ cross section (pb) e + ν γ q/g e - ν γ q/g PDF σ Born CTEQ5L (4,46) σ NLO MRST(c-g) (3,89) σ NLO E T (g/q) < 50 GeV K-factor = σ NLO (E T < 50)/ σ Born MRST(c-g) (3,89)