Search for New Physics at the Early LHC

Transcription

1 Search for New Physics at the Early LHC Chong Sheng Li Institute of Theoretical Physics, School of Physics, Peking University July 16 th, 2011, at SDU, Weihai

2 The LHC era is coming Nov 30, 2009 center mass energy 2.36TeV!! Mar 19, 2010 energy of both beams 3.5TeV!! Mar 30, TeV collision!! Apr 22, 2011 beam intensity cm s!! Tevatron: cm s

3 Rediscovering the SM at the early LHC LHC is a collider with both high energy and high luminosity! We can rediscover the SM at the LHC. QCD EW Gauge bosons Top An over view of the physics from 200 MeV to 200GeV!

4 Rediscovering the SM at the early LHC A dijets event recorded by ATLAS ATLAS-CONF

5 Rediscovering the SM at the early LHC ATL-PHYS-SLIDE Even W+Z events have been recorded!

6 Rediscovering the SM at the early LHC ATL-PHYS-CONF A top pair event

7 Searching New Physics at the early LHC The motivation of building the LHC is not testing the SM but searching NEW physics! Higgs? SUSY? Extra dimension? Little Higgs? Technicolor? Unparticle?? And? ATLAS-CONF , CMS collaboration Phys. Lett. B699(2011) Thanks the experimentalists! We already have many new results. CMS collaboration arxiv:

8 Searching New Physics at the early LHC The motivation of building the LHC is not testing the SM but searching NEW physics! Higgs? SUSY? Extra dimension? Little Higgs? Technicolor? Unparticle?? And? ATLAS-CONF , CMS collaboration Phys. Lett. B699(2011) Thanks the experimentalists! We already have many new results. ATLAS collaboration arxiv: ,

9 Searching New Physics at the early LHC The motivation of building the LHC is not testing the SM but searching NEW physics! Higgs? SUSY? Extra dimension? Little Higgs? Technicolor? Unparticle?? And? ATLAS-CONF , CMS collaboration Phys. Lett. B699(2011) Thanks the experimentalists! We already have many new results. ATLAS-CONF

10 Searching New Physics at the early LHC Using dijet mass and angular distribution to search for new physics Excited quark and axigluon, quantum black hole, RS graviton Constraint to the 4- quark effective operator. ATLAS collaboration arxiv: , CMS collaboration arxiv:

11 Model independent study for New Physics J. J. Zhang, C. S. Li, J.Gao, H. Zhang, C.-P. Yuan, T.C. Yuan, PRL102, , 2009 Thousands of new physics models and many new parameters, thus the experimentalists prefers the model-independent study. For example, in the unconstraint MSSM there are 105 new parameters which is a nightmare for the experimental analysis. Effective operator approach: Top FCNC Measurements at the LHC: top FCNC decay, direct top production, single top production. Limits presented by D0, CDF using QCD NLO predictions for single top production (JJ Liu and C.S.Li et al, PRD 2005) : D0 Collaboration, PLB 693, 81, 2010

12 Top quark decay via anomalous couplings at the NLO in QCD To be consistent, besides obtaining limits on the coupling constants, we should deduce limits on the BRs as well. Moreover, experimentalists are more interest in measuring BRs. without mixing: J.J.Zhang, C.S. Li, C.-P. Yuan, et al., Phys.Rev.Lett,102,072001, 2009 with mixing: J.J.Zhang, C.S. Li, C.- P. Yuan, et al., PRD, 2010 Constraints from Tevatron measurement Using the upper limits of FCNC couplings measured by D0 and CDF and our predictions at the NLO in QCD, the following constraints on the branching ratios can be obtained D0 CDF

13 When considering the mixing effects of FCNC operator, the decay branching ratio of t->qg doesn t change, while t->qz and t-> qү are modified. In particular, mixing effects changes the running of anomalous couplings. (J.J.Zhang, C.S. Li, et al., PRD, 2010) Sensitivities at the LHC: Low Luminosity 10 fb -1,without mixing High Luminosity 100 fb -1, without mixing Low Luminosity 10 fb -1,with mixing High Luminosity 100 fb -1, with mixing

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15 Model independent study for New Physics Quark compositeness and other NP will induce the following effective interactions four-quark contact interaction Measurements at hardron colliders: angular distribution of dijet production. The constraints from D0, > 2.9 TeV, ATLAS, > 3.4 TeV, CMS, >4.0 TeV (only color singlet operator considered so far)

16 Quark compositeness search at the LHC J. Gao, C. S. Li, J. Wang, H.X. Zhu, C.-P. Yuan, PRL106, , 2011 The quark compositeness models have been existed since 1960 s. If the quarks are composite indeed, there should be four-quark contact interaction induced by constituent exchange. Thus, we can use dijet production to probe the quark compositeness scale efficiently. SM dijet: q q q q, t- channel dominant, prefer the forward backward region. change the angular distribution significantly NP dijet: isotropic In all the experimental studies at the Tevatron and LHC, they use the NLO predictions for the SM dijet production. But for the NP contributions, no exact NLO predictions existed before. The problem is that the D0 and ATLAS use the SM dijet K-factors to scale the NP contributions while the CMS just use the LO results for the NP ones. Which one is better? we need the exact NLO QCD corrections to match the experimental precision.

17 Quark compositeness search at the LHC The experimental observables and our strategy to compare with them ATLAS: angular distribution variables F: dividing the y 1 -y 2 region into 11 bins with equal width, F is defined as the number of events in the first 4 bins to the one in the total 11 bins. our strategy to compare with the experimental results: define the discriminator. CMS: Centrality ratio R: ratio of the number of events in the center region to the one in the outer region. Here we only consider the 5 invariant mass bins from 1.5 TeV to 3 TeV, since the NP effects are only significant here. Our strategy to compare with the experimental results: define the discriminator.

18 Quark compositeness search at the LHC The effects of our calculations to the experimental results ATLAS: CMS: NLO1 scaled NLO results used by ATLAS NLO2 our exact NLO results The scaled NLO prediction adopted by the ATLAS Collaboration has overestimated the new physics effect on some direct observables by more than 30% and renders a higher limit on the quark compositeness scale. From our results, the current exclusion limits of the quark compositeness scale set by the ATLAS and CMS Collaborations shall be lowered from 3.4 TeV to 3.1 TeV, and 4.0 TeV to 3.7 TeV, respectively.

19 Quark compositeness search at the LHC General parameter cases (both of singlet and octet exist): Here we use the method of ATLAS for the measurement of the angular distribution, but we raise the LHC energy to 14 TeV, and also the invariant mass cut to 3 TeV. (0 or 1 for singlet, 0.5 or 1.5 for octet) We can see that the color octet contributions are small and even can be cancelled with the singlet contributions. Thus if the Wilson coefficients localized in these regions, it will be hard to detect the quark compositeness through the angular distribution. Again, the NLO QCD effects are significant.

20 Search for anomalous top quark production J. Gao, C. S. Li, L.L. Yang, and H. Zhang, arxiv: The direct top quark production, where a single top quark is produced without any additional particle, is the most promising channel to search for the top FCNC couplings. Any observation of this characteristic process definitely indicates the existence of the tqg anomalous couplings, and the underlying new physics. A detailed study include the top quark subsequent decay and the complete NLO QCD effects needed for the early search at the LHC see Q. H. Cao, C. -P. Yuan, et. al., PRD71, for how to incorporate the QCD effects of top decay NLO QCD corrections: We find that after using the kinematic cuts and jet veto in the final states, the QCD corrections of the signal process become negative while the one for the inclusive rate is large and positive, which may have significant influence to the precision measurement of the anomalous couplings at the LHC.

21 Search for anomalous top quark production The discovery potentials of the LHC SM backgrounds 5σdiscovery limits Taking into account the current limits from the Tevatron, the LHC with c.m. energy 7 TeV may discover the anomalous coupling at 5 σ level for a very low integrated luminosity of 61 pb -1.

22 Search for anomalous top quark production Once the direct top quark production is observed, it is important to determine whether it comes from the up quark or charm quark initiated process for the understanding of the underlying new physics. Relative deviation of the lepton charge ratio The tcg coupling induces a positive deviation while it s negative for the tug coupling. The shadow region indicates the 3σfluctuation of the backgrounds.

23 Search for anomalous top quark production The ATLAS Collaboration has carried out the corresponding searches which will come out soon. We also performed a new NLO calculation based on their event selection cuts including jet veto. Once again, the NLO corrections are stable while the veto cuts are applied, within several percents. As can be seen, the NLO QCD corrections reduce the scale dependence significantly, which are about ±10% at the LO and ±3% at the NLO.

24 Dark Matter and photon associated production at the LHC Jiang Wang, C.S.Li, et.al, appear soon Motivation: Dark Mater (DM) exists! Methods to Detect DM: Indirect: PAMELA, ATIC,HESS and Fermi LAT Drawback: Many assumptions are applied and other astrophysical interpretation can not be excluded. Direct: DAMA, CDMS, CoGeNT and XENON Drawback: passive and much time was spent in waiting for the collision with the DM Colliders ( Model independent Methods ):

25 Dark Matter and photon associated production at the LHC DM Relic Abundance Dirac Fermion Singlet under SU(3)*SU(2)*U(1) Valid only if Signal at Colliders: DM associated production with a photon or a jet LO result (blue band),nlo result (red band), the NLO QCD K factor is nearly 1.4. NLO QCD correction increases the new physical scale by about 10%. The regions below the red band are all allowed.

26 Dark Matter and photon associated production at the LHC The p T distributions of the backgrounds decrease faster then that of the signal with increasing of the the transverse momentum of the photon, indicating good discovering potentional. The η distributions of the backgrounds are almost flat in the full region. In contrast, the signal lies mainly in the middle region of η. These characteristics may help to select the events in experiments.

27 Top AFB and the early LHC Top pair forward-backward asymmetry 3 sigma deviation from the SM prediction! CDF collaboration arxiv: Many new physics models are proposed to explain this anomaly Axigluon, effective operators, Z, FCNC Z, W, FCNC scalars, charged scalars, color sextet scalar Can we test them at the early LHC?

28 The FCNC Z model Top AFB and the early LHC E. L. Berger, Q.-H Cao, C.-R Chen, C. S. Li and H. Zhang, Phys.Rev.Lett. 106, (2011) ( ) '.. L = g uγ f P + f P tz + hc μ W L L R R S. Jung, et al, PRD ; J. Cao, et al, PRD ; Q.-H. Cao, et al, PRD ; B. Xiao, et al, PRD ; eta. μ Change the direction of the Fermion line Many same-sign top events at the early LHC?

29 Top AFB and the early LHC The SM background of the samesign top process is small (in fb) To explain the top AFB, large coupling constant is needed If the top AFB anomaly is caused by the FCNC Z, the same-sign top signature has to be detected -1 at the early LHC ( 1fb )! The handedness of the interaction can also be measured!

30 Top AFB and the early LHC Results from CMS (arxiv: ) CMS (arxiv: ) :

31 Top AFB from model indentpent 4 quark operator Ding Yu Shao, Chong Sheng Li, et.al, appear soon Dimensional 6 operators for top pair production can be induced from many new physics model. ~ ~ ~ ~

32 Combined fit to and at We set: Up to free parameters: best-fit point at LO : best-fit point at NLO : NLO corrections reduce the best-fit value about 7%

33 Combined fit to and at different CLs SM SM LHC has reported their first observation of top pair production Charge Asymmetry used by CMS at the LHC Measured value of Charge Asymmtery Uncertainty is large We find that the value of Chagre Asymmetry induced by t hese operators is much different from SM predictions, and LHC has p otential to discover these NP effects when the measurement precision increased.

34 NLO QCD corrections to top pair production through KK gluon H. X. Zhu, C. S. Li, L. Dai, J. Gao, J. Wang, C.-P. Yuan, arxiv: ,2011 KK Gluon in Randall-Sundrum model provides a promising explanation to top pair Forward-Backward asymmetry ( Djouadi et.al, ) Observation of resonance in top pair invariant mass may be the first hint of new physics signal (Agashe et.al, 2006) Previous LO study on top pair production suffers from large theoretical uncertainty. NLO corrections are needed for more accurate predictions for total cross section, FB asymmetry and various differential distribution. Difficulty of NLO corrections: KK gluon is a color-octet vector boson. Need to introduce ghost, Goldstone boson et.al to properly quantizing it.

35 NLO QCD corrections to top pair production through KK gluon Covariant gauge field in 5 dimension: KK decomposition of the 5D gauge field: Choose covariant gauge in which 4D gauge field doesn t mix with the fifth component of the gauge field Derive the ghost Lagrangian following the Faddev-Popov procedure Keep the zero mode and the first KK mode Keep the interactions whose form are fixed by gauge invariance

36 NLO QCD corrections to top pair production through KK gluon In covariant gauge, a set of interaction vertices between are derived from the 5D Lagrangian, most of them are not presented in the previous literature. They are essential for preserving 4D QCD gauge invariance. KK ghost gluon ghost

37 NLO QCD corrections to top pair production through KK gluon Some representative Feynman diagrams: Renormalization Extra terms in alpha_s renormalization KK-gluon mass renormalization Renormalization of the coupling between fermion and KK gluon

38 Summary The world largest collider LHC has been run for more than one year. The SM has been rediscovered at the LHC. The results of searching for new physics have added new constraints to new physics. Model independent study using effective operator approach is very promising for the new physics search at the LHC. It can be expected that the LHC may discover the anomalous couplings at the very early stage. And the QCD corrections will also improve the corresponding experimental measurements. If the top AFB anomaly is caused by the FCNC Z, the same-sign top signature has to be detected at the early LHC.

39 Thank you!