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Improved tests for CP violation in charm decays

Amarjit Soni; HET, BNLEPS-2013, Stockholm, Sweden

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Outline• Intro: D-system is unique: interesting sensitivity

to top “compositeness”; BSM connection• SM confronts observed charm CP • Possible repercussions of Delta I=1/2 Rule

understanding from the lattice • FS with possibly enhanced CP• CPT constraints • Possibly critical test for SM or NP• Summary & Outlook

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Charm system is unique

• Sensitive to top physics, example: warp space theory of flavor

Delta F=2 mixings are an extremely valuable treasure in providing stringent constraints ………..

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Charm CP in the context of a geometric theory of

flavor

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Simultaneous resolution to hierarchy and flavor puzzles

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Fermion “geography” (localization) naturally explains:

• Why they are light (or heavy)• FCNC for light quarks are severely suppressed automatically• RS-GIM MECHANISM (Agashe, Perez,AS’04) flavor changing

transitions though at the tree level (resulting from rotation from interaction to mass basis)are suppressed roughly to the same level as the loop in SM=> CKM hierarchy

• O(1) CP ubiquitous;…..in fact for neutron a (mild) CP problem• Most flavor violations are driven by the top-> ENHANCED t-> cZ, (alsoD0 mixing w & w/o CP)….A VERY

IMPORTANT “GENERIC” PREDICTION..Agashe, Perez, AS’06

Grossman&Neubert; Gherghetta&Pomarol; Davoudiasl, Hewett & Rizzo

EXTENSIVE RECENT STUDIES by BURAS et al and by NEUBERT et al

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D-CP Experimental info: 2011--2012

• Measurements of CP violation in D0P+P- show results apparently larger than expected:

• Note in particular the >3-sigma signal in the combined result..

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New LHCb Results (Moriond 2013)

• Using soft pi D* tag:

• Using semileptonic tag for D’s originating from B-hadron decay (note “wrong” sign)

• Naïve average of all results is

• Again almost 3-sigma signal of CP violation [but opp. sign means experimental numbers are not yet robust]

• Taken at face value the new average is bit closer to the quark level estimate

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Peek @ PDG: old results

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EXACT U-SPIN LIMIT

Taking U-spin breaking into a/c

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Summary (so far) on Recent D-CP results

• SM explanation cannot be ruled out and is quite plausible; however, a compelling case for SM explanation can also not be made.

• Unless true result is , for sure, 1% or more , not a compelling sign of new physics

• theory estimates plagued by large hadronic (non-perturbative) uncertainties; NO RIGOUROUS METHOD IN SIGHT; LONG-TERM WORRY => Ghost of ‘/. However, unlike K-> , lattice methods appear exceedingly difficult

• More exptal input (many other modes) crucial & could change interpretation…

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I=1/2 Rule K=>• A[Ks => + - ]/A[K- => - 0 ] ~ 22!

• MUCH, MUCH speculations, role of penguins,[SVZ and the like]

• Folklore[see e.g. DGH book: section VIII-4]:“the factorization hypothesis works reasonably well in reproducing the experimental value of A2”

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Dissecting 3/2 Amp on the lattice

Simplest basic step isSignificantly different

from phenomenological

expectations

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Must try ascertain if D-CP is receiving contribution from

BSMSuggestions:

Atwood & AS, arXiv:1211.1026

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• Implications of CPT

• Final States with enhanced CP

• SM or not : A critical test

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Candidates for enhanced CP asymmetry[because of CPT]

• Since asymmetry arises from T and P interference and as a rule P<<T, need final states where T is suppressed => color suppressed modes: compare D0 => + + - versus 0 0

• 0 0 also gives 4 charged pi’s and possibility of triple correlation asymmetry

• Other examples: For KEKB D=> 0 0 (, ’)also imp but may not be CS

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Propose a new “critical” test for new physics

• Key idea: Hadronic matrix elements enhancement (factor O(5) needed for SM explanation) only operational for EXCLUSIVE (in particular 2 body pseudoscalar) MODES

• Inclusive (multibody) modes should exhibit quark level asymmetry[quark-hadron duality] ~6X10-4 if SM is the source, if these also show O(3X10-3 ) asymmetry then BSM-CP is the origin

• Look forward to imlementation at LHCb, BF & especially KEKB Atwood + AS, arXiv 1211.1026

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How to look for inclusive final states? Simple suggestion• Look for D => K K X • Operationally KKX is any final state

containing a K K with total energy in the 2 kaons less than the energy of the parent D

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Multibody FS can also be very useful

• Main point is esp avoid 2-body pseudos as they tend to have significant non-perturbative effects.

• [Of course tests need be done so that systematics respect CP]

• examples: KK + n(pi) with n = 2,3…

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Expected Progress

• S(LHCb)

• Super-KEKB/BELLEII

• Lattice [success of Flavor Physics/Intensity Frontier is intricably tied to the developments on the lattice]though D-CP is highly probematic for now.

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Summary & Outlook• With 1st observation of dir- CP in charm decays, LHCb has opened a new

avenue for probing CP but experimental numbers need confirmation.• Difficulties in hadronic matrix elements make precise predictions

difficult; observed asymmetry ( ACP in KK, ) is perhaps somewhat bigger than simple quark level expectations but may well be just SM (or part of it may be BSM).

• Charm system is quite unique and is sensitive to different sources of BSM-CP [e.g. warped theory]; so extremely important to clarify.

• Measurements in more modes may help, choose those with charged final mesons : D0=>0 0 , K(*)0 K(*)0 , Ds => 0 K(*)+ ; VV states allow T-odd CP tests w/o need for tagging; and rates are (tree) suppressed so CPT implies larger asymmetries • Suggested search in inclusive D=> K K X events could shed decisive light

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EXTRA

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Mass depends of ReA2, A0

Due to the cancellation, 3/2 amplitude decreases significantly as the

pion mass is lowered towards its physical value

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