Limits on the effective quark radius and the contact-interaction mass scales from inclusive ep scattering at HERA. Aleksander Filip Żarnecki

Transcription

1 38th International Conference on High Energy Physics August 5, 2016 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24 Limits on the effective uark radius and the contact-interaction mass scales from inclusive ep scattering at HERA. Aleksander Filip Żarnecki Faculty of Physics, University of Warsaw on behalf of the ZEUS collaboration

2 Quark radius and Contact Interactions at HERA Outline 1 Introduction 2 HERA data 3 Quark radius limits 4 Contact Interaction limits 5 Conclusions A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

3 Introduction HERA electron(positron)-proton collider at DESY e + /e 27.5 GeV p 820 GeV 920 GeV HERA I about 0pb 1 collected per experiment mainly e + p data Integrated Luminosity (pb -1 ) HERA luminosity e e e Days of running A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

4 Introduction HERA electron(positron)-proton collider at DESY e + /e 27.5 GeV p 820 GeV 920 GeV HERA I about 0pb 1 collected per experiment mainly e + p data, unpolarised H1 Integrated Luminosity / pb -1 Status: 1-July electrons positrons low E HERA-2 HERA-1 HERA II about 400pb 1 per experiment similar amount of e p and e + p data with longitudinal polarization of e ± beams (30-40%) and small samples collected at reduced proton beam energy Days of running A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

5 Introduction Two omni-purpose detectors ZEUS Both euipped with silicon tracking, drift chambers, hermetic calorimetry and muon detector system A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

6 Introduction Two omni-purpose detectors ZEUS Both euipped with silicon tracking, drift chambers, hermetic calorimetry and muon detector system Combining H1 and ZEUS measurements Different detectors complementary event reconstruction methods reduction of systematic uncertainties A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

7 XY View ZR View Introduction Deep Inelastic e ± p Scattering Main process studied at HERA NC DIS Q 2 = GeV 2 ; y = 0:56; M = 211 GeV CC DIS E t/gev jet e + Kinematic variables: e(k) e; (k 0 ) ; Z; W (xp ), 0 p(p ) p remnant Q 2 = (k k 0 ) 2 Q 2 x = 2P (k k 0 ) y = P (k k0 ) P k virtuality of the exchanged boson spatial resolution λ 1/Q sensitivity to mass scales Λ Q A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

8 XY View ZR View Introduction Deep Inelastic e ± p Scattering Main process studied at HERA NC DIS Q 2 = GeV 2 ; y = 0:56; M = 211 GeV CC DIS E t/gev jet e + Kinematic variables: e(k) e; (k 0 ) Q 2 = (k k 0 ) 2 virtuality of the exchanged boson p(p ) (xp ) ; Z; W, 0 p remnant Q 2 x = 2P (k k 0 ) y = P (k k0 ) P k fraction of proton momenta carried by stuck uark fraction of lepton energy transfered in the proton rest frame A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

9 HERA data Combination of HERA measurements All DIS data from H1 and ZEUS combined into one set of cross section measurements data points 1307 Good consistency between experiments and data sets χ 2 /N df = 1685/1620 Eur. Phys. J. C 75 (2015) 580, arxiv: A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

10 HERA data QCD analysis DIS cross sections can be described in terms of the parton distributions in the proton. We parametrise Parton Density Functions (PDFs) at a starting scale of Q 2 = 1.9 GeV 2. We then use QCD evolution euation to evolve them to arbitrary Q 2 scale. Fit to combined H1+ZEUS data HERAPDF2.0 xf xg ( 0.05) xs ( 0.05) 3 H1 and ZEUS HERAPDF2.0 NLO uncertainties: experimental model parameterisation HERAPDF2.0AG NLO 2 2 µ 2 = GeV f 1 1 x Eur. Phys. J. C 75 (2015) 580, arxiv: xu v xd v A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

11 HERA data Resulting SM predictions (pb/gev 2 2 ) H1 and ZEUS 1 HERA NC e p 0.4 fb + 1 HERA NC e p 0.5 fb HERAPDF2.0 NC e p Good description of the data also at the highest Q 2 values dσ/dq 1 + HERAPDF2.0 NC e p Q 2 M 2 Z, M2 W 3 y < 0.9 s = 318 GeV NC and CC DIS cross sections comparable HERA CC e p 0.4 fb + 1 HERA CC e p 0.5 fb HERAPDF2.0 CC e p + HERAPDF2.0 CC e p 3 4 Q 2 / GeV 2 Sizable contribution also from γ Z interference Combined QCD+EW analysis shows good agreement with SM predictions see presentation by Amanda Cooper-Sarkar (ID: 314) Phys. Rev. D 93 (2016) , arxiv: A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

12 HERA data Resulting SM predictions (pb/gev 2 2 ) H1 and ZEUS 1 HERA NC e p 0.4 fb + 1 HERA NC e p 0.5 fb HERAPDF2.0 NC e p Good description of the data also at the highest Q 2 values dσ/dq 1 + HERAPDF2.0 NC e p Q 2 M 2 Z, M2 W 3 y < 0.9 s = 318 GeV NC and CC DIS cross sections comparable HERA CC e p 0.4 fb + 1 HERA CC e p 0.5 fb HERAPDF2.0 CC e p + HERAPDF2.0 CC e p 3 4 Q 2 / GeV 2 Sizable contribution also from γ Z interference High precision data could be used to look for possible BSM effects... However, PDF fit may have been biased by BSM contributions! new approach needed A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

13 Quark radius limits Quark form factor classical method to look for possible fermion (sub)structure. If a uark has finite size, the standard model cross-section is expected to decrease at high momentum transfer: e R Z / γ [ ] 2 [ ] 2 dσ dq 2 = dσsm dq 2 1 R2 6 Q2 1 R2 e 6 Q2 where R is the root mean-suare radius of the electroweak charge distribution in the uark. We do not consider the possibility of finite electron size... same dependence expected for e + p and e p! A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

14 Quark radius limits QCD+BSM fit Approach used for HERAPDF2.0 determination extended to take into account the possible BSM contribution χ 2 (p, s, η) = i [ m i + ] 2 j γi j mi s j µ i 0 ( ) + δi,stat 2 + δ2 i,uncor (µ i 0 )2 j p and s are vectors of PDF parameters p k and systematic shifts s j, η is the parameter describing BSM contribution (eg. η = R 2 ) we fit them simultaneously to the combined HERA data R 2 Data = cm 2 s 2 j µ i 0 and mi (p, η) are measured and predicted (SM+BSM) cross sections, γ i j, δ i,stat and δ i,uncor are the relative correlated systematic, relative statistical and relative uncorrelated systematic uncertainties of the input data point i A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, 2016 / 24

15 Quark radius limits Limit setting Limits derived using the techniue of MC replicas (freuentist approach). Replicas are generated sets of cross-section values that are calculated for given R 2 True and varied randomly according to the statistical and systematic uncertainties (including correlations) of the input data. Each replica is then used as an input to QCD+BSM fit R 2 Fit Number of replicas for each considered R 2 True value distribution of R 2 Fit R 2 Data R 2 True R 2 True is tested by comparing R 2 Fit distribution with the value of R 2 Data A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

16 Quark radius limits Limit setting The probability of obtaining a R 2 Fit value smaller than that obtained for the actual data Prob(R 2 Fit < R 2 Data ) ) (%) 2 Data < R 2 Fit Prob(R ZEUS PDF + R replicas PDF + R fit R only replicas R only fit 95% C.L. limit is studied as a function of R 2 True values corresponding to the 1 probability smaller than 5% are excluded at the 95% C.L R 2 True 2 True R R < cm limits obtained for fixed PDF parameters are too strong by about % A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24 (( 16 2 cm) )

17 Quark radius limits Results Phys. Lett. B757 (2016) 468, arxiv: ZEUS σ/σ SM a) HERA NC e p 0.5 fb 1 HERA NC e p 0.4 fb ZRPDF total unc Q (GeV ) σ/σ SM b) Quark Radius 95% CL Limits 16 2 R 2 = (0.43 cm) 16 2 R 2 = (0.47 cm) Q (GeV ) ( cm) 2 < R 2 < ( cm) 2 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

18 Contact Interaction limits Contact Interactions For many scenarios of new physics at much larger energy scale, BSM interactions can be approximated as ee Contact Interactions (CI) e e LQ Z e e e e LQ, ~ e e e η e ee contact interactions (CI) A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

19 Contact Interaction limits Contact Interactions For many scenarios of new physics at much larger energy scale, BSM interactions can be approximated as ee Contact Interactions (CI) Effective Lagrangian for vector ee contact interactions: L CI = η e αβ (ē αγ µ e α )( β γ µ β ) η e αβ α,β=l,r - 4 possible couplings per flavor related to the coupling strength η or the new physics mass scale Λ by: η αβ = ε αβ η = ε αβ 4π Λ 2 where ε αβ = ±1 e η ee contact interactions (CI) Different CI scenarios assume different helicity structure of new interactions, given by set of ε αβ A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24 e

20 Contact Interaction limits General models Also referred to as compositeness models (Λ - compositeness scale) Models violating parity: Family universality assumed: η eu αβ = ηed αβ = ηes αβ = ηec αβ = ηeb αβ Model ε LL ε LR ε RL ε RR LL +1 RR +1 Parity conservation reuire: η e LL + ηe LR ηe RL ηe RR = 0 Models conserving parity: VV AA VA X X X A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

21 Contact Interaction limits Simplified fit procedure Limit setting in the replica method is very time consuming. Full fit of HERA data: QCD evolution of PDFs repeated at each iteration. R analysis: Monte Carlo replicas for each value of R 2 True over fits to set final limits Processing time was a limiting factor for including more models Simplified fit method, based on the Taylor expansion of the cross section predictions in terms of PDF parameters reduce the limit calculation time by almost two orders of magnitude. For details see arxiv: #events Full fit Simp. fit log (t CPU A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24 / s)

22 Contact Interaction limits Results Same limit setting procedure applied to contact interaction models. Use replicas to calculate: Prob(η Fit < η Data ) for η > η Data Prob(η Fit > η Data ) for η < η Data for different η True Probability (%) 2 1 VV model ZEUS preliminary Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

23 Contact Interaction limits Results Same limit setting procedure applied to contact interaction models. Use replicas to calculate: Prob(η Fit < η Data ) for η > η Data Prob(η Fit > η Data ) for η < η Data for different η True Probability (%) 2 1 VV model ZEUS preliminary Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) Excluded on 95% C.L. are η True resulting in probability below 5% η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

24 Contact Interaction limits Results Same limit setting procedure applied to contact interaction models. Use replicas to calculate: ZEUS preliminary Prob(η Fit < η Data ) for η > η Data Prob(η Fit > η Data ) for η < η Data Probability (%) 2 VV model 95% CL allowed for different η True 1 Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) Excluded on 95% C.L. are η True resulting in probability below 5% Λ > 14.7 TeV Λ + > 9.5 TeV -2 η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

25 Contact Interaction limits Results For AA scenario, QCD+CI fit gives improved description of the data Fitted coupling η Data = 0.32 TeV 2 corresponding to Λ Data = 6.2 TeV Probability (%) 2 1 AA model ZEUS preliminary Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

26 Contact Interaction limits Results For AA scenario, QCD+CI fit gives improved description of the data Fitted coupling η Data = 0.32 TeV 2 corresponding to Λ Data = 6.2 TeV Probability (%) 2 AA model ZEUS preliminary The probability of obtaining larger best-fit coupling for η True = 0 p SM = 0.7% (2.5 σ) 1 1 Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

27 Contact Interaction limits Results For AA scenario, QCD+CI fit gives improved description of the data Fitted coupling ZEUS preliminary η Data = 0.32 TeV 2 corresponding to Probability (%) 2 AA model 95% CL allowed Λ Data = 6.2 TeV The probability of obtaining larger best-fit coupling for η True = 0 p SM = 0.7% (2.5 σ) 1 1 Prob(η Fit < η Data ) Prob(η Fit > η Data ) Prob(η Fit > η SM ) TeV < Λ + <.4 TeV -2 η True (GeV ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

28 Contact Interaction limits Results Improved description of the HERA data also for VA and X1 models (models with opposite coupling sign for left- and right-handed uarks) probability that the SM reproduces the data is p SM = 2.1% and 0.3% respectively The fits suggest a positive deviation in NC e p DIS at highest Q 2, while a negative deviation is preferred for e + p. A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

29 Contact Interaction limits Results Improved description of the HERA data also for VA and X1 models (models with opposite coupling sign for left- and right-handed uarks) probability that the SM reproduces the data is p SM = 2.1% and 0.3% respectively The fits suggest a positive deviation in NC e p DIS at highest Q 2, while a negative deviation is preferred for e + p. The discrepancy is unlikely to be an effect of experimental systematic uncertainties, which were largely reduced by combining H1 and ZEUS data. A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

30 Contact Interaction limits Results Improved description of the HERA data also for VA and X1 models (models with opposite coupling sign for left- and right-handed uarks) probability that the SM reproduces the data is p SM = 2.1% and 0.3% respectively The fits suggest a positive deviation in NC e p DIS at highest Q 2, while a negative deviation is preferred for e + p. The discrepancy is unlikely to be an effect of experimental systematic uncertainties, which were largely reduced by combining H1 and ZEUS data. Possible explanations, before we attribute it to new physics missing higher-order EW corrections limitations of the assumed PDF parametrisation/evolution scheme A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

31 Contact Interaction limits Results Cross section deviations corresponding to the allowed coupling range for VV and AA models ZEUS preliminary σ/σ SM + a) NC e p DIS Contact Interactions 95% CL Limits VV AA No PDF variation included! Q (GeV ) σ/σ SM - b) NC e p DIS Q (GeV ) A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

32 Contact Interaction limits Results LL RR VV AA VA X1 X2 X4 ZEUS preliminary ± HERA e p 95% C.L. Observed Expected ZEUS preliminary HERA e ± p data 95%C.L. limits (TeV) Observed Expected p SM Model Λ Λ + Λ Λ + (%) LL RR VV AA VA X X X ± 1/Λ (TeV ) A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

33 Contact Interaction limits Results LL RR VV AA VA X1 X2 X4 ZEUS preliminary ± HERA e p 95% C.L. Observed Expected ZEUS preliminary HERA e ± p data 95%C.L. limits (TeV) Observed Expected p SM Model Λ Λ + Λ Λ + (%) LL RR VV AA VA X X X ± 1/Λ (TeV ) ATLAS limits from 13 TeV data: arxiv: LL RR A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

34 Conclusions High-precision HERA inclusive data allow searches for new physics effects up to TeV scales. New method developed for BSM analysis of HERA data: simultaneous fit of PDF parameters and BSM contribution. A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

35 Conclusions High-precision HERA inclusive data allow searches for new physics effects up to TeV scales. New method developed for BSM analysis of HERA data: simultaneous fit of PDF parameters and BSM contribution. Limits on the uark radius in the form-factor approach ( cm) 2 < R 2 < ( cm) 2 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

36 Conclusions High-precision HERA inclusive data allow searches for new physics effects up to TeV scales. New method developed for BSM analysis of HERA data: simultaneous fit of PDF parameters and BSM contribution. Limits on the uark radius in the form-factor approach ( cm) 2 < R 2 < ( cm) 2 Limits from the CI analysis in the TeV range CI models with opposite coupling sign for left- and right-handed uarks give improved description of the data theoretical predictions need to be reexamined carefully before any conclusions can be drawn A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

37 Thank you! A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24

38 Backup Simplified fit procedure New procedure was validated by repeating R 2 limit setting procedure. Comparison of results for replicas generated with R 2 True χ 2 of the fit χ 2 Simp. fit Fitted value of R 2 ) 2 (GeV Simp. fit R = (R Limit ) Full fit χ Full fit R (GeV 2 ) 6 A.F.Żarnecki (University of Warsaw) Quark radius and CI at HERA August 5, / 24