Recent results and trends in charm physics

Recent results and trends in charm physics Brian Meadows U. Cincinnati Leverhulme visiting professor Oxford University, UK . • “ An important goal in charm physics is not just • to observe CP Violation in D decays • but also to understand its origin” • -- IkarosBigi TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAAAAAAAAAAAAAAAAAAAA

Rudiments of the Standard Model (SM) Three Fermion families of quarks and of leptons with “flavours” Gauge Bosons for EM, Weak and strong interactions g, W§, Z0, g + Scalar field (Higgs Boson) to allow the above to have mass. + Weak interactions can allow decay of up-typedown-type quarks: with transitions among the 3 quark families BUT – NO FCNC !! 3x3 CKM coupling matrix Complex, allowing CPV

What is Interesting about Charm • Charm was “invented” to account for small FCNC interactions in nature (GIM mechanism). • Major step in the birth of the standard model (SM). • In this SM scenario, for the charm sector, • The mixing phenomenon in theD0-D0 system is greatly suppressed; • Many charm particle decays are also highly suppressed; • CP violation (CPV) is also expected to be small, mostly because weak phases are very small. • With “SM backgrounds” so small, charm is a good place to look for new physics (NP)– particles outside the SM set. • Charm, uniquely, allows study of the role of up-type quarks. In addition, vast samples of charm particles are available.

BUT SM is Difficulty to compute: • Charm mass is neither “light” (cPT) nor heavy (HQET) • Long range effects can mask those of new physics. For today: • Focus on prospects for improving our understanding ofCPV and mixingin the charm sector. • There are also many other interests in charm: • Spectroscopy – differences between D and Ds or baryons • Great tool for lighter quark states to which they decay (mesons, baryons, glueballs) • Probe for new generation of exotic states (with charmonium)?, …

Tools available: Hadron: LHCb, CDF, (CMS/ATLAS?) e+e- y(3770): BES3 (~10 fb-1), INFN (5 ab-1?) e+e- Y(4S): Belle2 (~50 ab-1) We are off to a good start: BaBar and Belle, evidence for D0-D0mixing confirmed by CDF. LHCb, observation of D0-D0mixing at 9.1 level confirmed by CDF. However,CPV: LHCb, evidence for direct CPV in D0 h+h-GONE

Lecture I Outline CP Violation what it is ... The CKMmodel for CPV Unitarity triangles Mixing and its role in CPV Prospects for oberving CPV from mixing. Direct CPV Bfactory measurements on D mesons

before that …? the ONION “America’s finest News source”

CPViolation (CPV) Age Change Particles Era Johannes Gutenberg-Universitaet Mainz, June 11, 2013 Johannes Gutenberg-Universitaet Mainz, June 11, 2013 Brian Meadows, U. Cincinnati

CPV and Baryogenesis • The excess of baryons over anti-baryons is only a small fraction of the observed number of photons. • Sakharov (1967) held that the excess of baryons requires: • Baryon number violating processes (in decay or production) • CPV so that, for any baryon-violating process, • A period in universe’s history when it was not in thermal equilibrium. In thermal equilibrium (Tinvariance),CPTinvariance is equivalent to CP invariance! [NOTE that CPT violation alone could also generate baryon excess • BUT would also violate locality, causality and Lorentz invariance.]

CPV – what it is • Cconverts particle  anti-particle; P inverts spatial coords. • Individually conserved in EM & strong, but not weakinteractions • CP Violation (CPV) is often observed as a particle - antiparticle asymmetry in the amplitudes for transitions: from initial state i to final state f • CP asymmetry (preponderance or lack of matter) can then arise from the difference in rates between matter and antimatter. [Bars indicate antiparticle (charge-conjugate) states]

CPV is not “T-violation” • CPV would be the same thing as “T violation” only if CPT were an unbreakable symmetry. This is not established. • An amplitude <f|H|i> describes a transition from if. Under CP this becomes<f|H|i>, not<i|H|f>. • “T-violation” (time-reversal symmetry breaking), has been found experimentally by the BaBar collaboration [BaBar used entangled states to define time lines along one of the B decay paths and compare it with another with the two events interchanged].

CPV- very brief History • 1963: CPVwas discovered (Fitch and Cronin, 1963) in long-lived KLmeson decays to p+p- (CP=+1) states that were formerly found to decay only to (with CP=-1). Later observed as asymmetry in decays • 1999: A big issue was the origin of the CPV: in KL-Ks mixing or in the decay itself (direct CPV). • NA48andKTeVestablisheddirect CPVinK0decay from measurement of the ratio of ratios:

CPV - very brief History • 1973: Kobayashi and Moskawa (KM) pointed out that the 3-family nature of the SM led to the possibility for CPV through the weak phase in VCKM. • 2001: After almost 40 years during which CPV had only been observed in neutral Kdecays, the BaBar and Belleexperiments were able to observe a large CP asymmetry in decays of B0 mesons toJ/Ã Ks (CP=-1)final states through interference with mixing, much as predicted by KM.

Most recent BaBarbccs modes

2004: BaBar established direct CPV in decays; • May 2013:LHCbreports direct CPV in decays. Phys. Rev. Lett. 93, 131801 arXiv:1304.6173 B0 B0 Bs0 Bs0

So far, all seems to be in accord with the detailed expectations of the SM. • Our knowledge is yet incomplete, however: • The level of CPV from this source is orders of magnitude too small to account for the cosmological baryon excess • Virtually no evidence for CPV in mixing yet. • CPVhas yet to be observed in the charm sector. Perhaps this is simply what theSMpredicts, or perhaps up-typequarks are actually different from down-type ! • We discuss experimental prospects for the charm sector.

Arg{Vub} = - d s b u c t (, ) Vtb*Vtd Vcb*Vcd Vub*Vud Vcb*Vcd  Arg{Vtd} = -b   (1, 0) (0, 0) Unitarity of the CKM • Much studied by the B factories who measured a, b and g. • Two kinds of decay mechanisms were explored: Penguins and Trees (later) VCKM “The” unitarity triangle

Arg{Vub} = - d s b u c t Arg{Vtd} = -b Weak phases in Ddecays Phase “¯c” of order 4 Tree phases ¯c are tiny BUT penguin phase c= = 670 is large. VCKM • Not yet studied cutriangle – D decays Bevan, Inguglia, BM: Phys.Rev. D83 (2011) 051101

It is surely beyond experimental ability to measure bc~ 0.030 • But it is important to check that it is very small. • Also interesting to check other phases in the “cu” triangle. • An estimate of the level of “Penguin pollution” would also be interesting in light of recent focus on this.

D0 Mixing Flavour oscillations in the neutral D system arise from the propagation of two mass eigenstatesD1 and D2 that comprise the flavour states It is usual to define four mixing parameters: Decays to state “f’ and oscillations of neutral D mesons compete. Mixing Weak decay Strong decay Eigenvaluesare with means: CPV signalled by Time-dependence involves the quantity Define decay amplitudes:

Three types of CPV • In the mixing (“indirect CPV”) • In interference between mixing and decay (“indirect CPV” – a.k.a. “mixing-induced CPV”) • In the decay (“direct CPV”) In the last two, CP asymmetry can depend on decay mode (requires two amplitudes with different weak AND different strong phases).

D0 oscillations are hard to compute in SM Off-diagonal mass matrix – two leading terms: Hadronic intermediate states (long-range) C=2 (short-range) (contributes mostly to x ) C=1 C=1 • Difficult to compute (need to know all • the magnitudes and phases, …) • Most computations predict x and y • in the range 10-3–10-2and |x|<|y| • Recent predictions: • (consistent with current observation) • Down-type quarks in loop: • b : CKM-suppressed (|VubVcb|2) • d, s: GIM-suppressed • (almost 2 orders of magnitude less than current sensitivity)

New Physics and Mixing Several extensions to the SM have been considered that can increase the value of x include: Generally agreed that signals for new physics would be: EITHER |x|>>|y| ORAny evidence for CPV in mixing W c u c u c u Di Dk H 0 u u u c c W c ~ ~ ~ ~ g q q g Heavy, weak iso-singlet quarks Supersymmetry FCNC [ A survey:Phys. Rev. D76, 095009 (2007), arXiv:0705.3650 ]

Oscillations from heavy flavour Mixing Mixing measurements probe t-dependence of ratio of decays 0 1 2 t/t Some “fingerprint” is required to distinguish -4 0 +4 Dt/t Oscillation frequencies differ greatly. t/t NO mixing !

D0 Mixing Measurements (D0f) Interference Accessible to D0 or D0 Point in Dalitz Plot (DP), etc (D0f) D0 “f”: Mix (D0D0) Decay through Mixing Direct decay D0-D0strong phase difference So weak phases change sign Exploit interference between direct decays D0fand decays through mixing: Time-dependence to 2nd order in x and y.

Mixing Measurements • Experimentally, tag D0flavourat t = 0 with sign of ¼§from D*§D0¼§decay or of ¹§from BD(*)m§n. • Record time t at decay. Mixing established from the non-exponential decay: • The term representing interference between mixing and direct decay, linear inx and y, affords best measurement of mixing parameters. • D0/D0rel.phase ±generally unkown, so we only measure: and • In some cases, independent information on dis available.

HFAG combination of all available observables Main parameters No CPV Arg{q/p} X y X No mixing Evidence for mixing. No evidence for CPV in mixing (or decay). |q/p | x HFAG presently: sx(y)~ 18(9)x10-4 We aim for s(|q/p|) ~ 1% we need sx(y)~1x10-4

Decays to two-hadron states • Measure time-dependence of ratio: • This is ratio is the deviation from exponential decay • Single point in phase space determines x’2and y’. • d unknown (BES3 measurement possible) • Measure average decay times t for decay to different CP final states: y Mixed CP. y K +K –or+- CP -even x x

Decays to multi-hadron states • Measure time-dependence of ratio: • Do so as function of position in 3-body phase-space – ”Dalitz Plot” (DP). Requires decay model. • Uses multiple points in phase space connected by model – so can determine x’ and y’. • BUTd0(D0-D0) offset in strong phase unknown. • As above, but use DP • CPself-conjugate state,sod0 = 0 orp • Can, therefore, determine x and y directly ! • Major problem is knowing DP decay model. • This introduces irreducible model uncertainty (IMU) y y x x

D0 Mixing at B Factories TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAA

Projection to 2022 (Belle2 ends?) Because these do not shrink Miss goal

Irreducible Model Uncertainty (IMU) • The problem – it is not easy to find a model for the Dalitz plot. • This introduces an uncertainty in mixing parameters BaBar: x = (0.16 ± 0.23 ± 0.12 ± 0.08)% y = (0.57 ± 0.20 ± 0.13 ± 0.07)% IMU (our goal ~ 0.01) D0 from D* D0ps Some see polesin Others see it as a set of complex numbers ! It is easy to describe event density, BUT it less obvious how to describe the phase :  We need data from charm threshold.

Charm threshold  Model independence • Decay of Ã(3770) prepares the D(1)D(2) system in a state • 3 TAGS: used • Ignore mixing - solve for & for each bin Flavour CP “Double Dalitz” ? D0 Signal e.g. KSpp (sig tag) D0 Semi-leptonic K-l+nl , etc. (CP=+1) KK, etc. TAG

Two improvements in mixing precision come from threshold data: Include strong phase measurements • Dalitz plot model uncertainty shrinks • Precision of overall strong • phase K() increases BES III SuperD Uncertainty inxDimproves more than that of yD

Prospects for observing CPV in mixing Asymmetries in measurements of x and y (or x’ and y’) values for D0 and D0 separatelycan provide information on CPV parameter |q/p|: Dependence on decay mode could indicate direct CPV. Weak mixing phase fM= Arg{q/p} can be directly measured in Ksh+h- time-dependent Dalitz plot analyses OR in measurement of thet-dependence of CP asymmetry

CPV Parameters |qD/pD|, M=Arg{q/p} 39 • D0- D0 parameter asymmetries: • az = (z+-z-)/(z++z-) ~ |q|2-|p|2 • where z is x, y, x’, y’, x”, y”, x’2 Decay (|q/p|) (M)0 mode x 100 Global 2 Fit to all modes: ± 18 ±9 (HFAG - direct CPV allowed) Current World Averages (HFAG): Time-dependent amplitude analysis of Golden channels Time-dependent CP asymmetry By 2022 sensitivity to CPV in mixing with |q/p|~1% and Arg{q/p}~10 is possible. Some distinction between decay modes also possible. Strategy:

Searches for direct CPV in D decays Requirestwo (or more) amplitudes,TandP for example, describing a decay process with differentweak anddifferentstrong phases. This leads to a direct CP asymmetry in decay rates weak phases strong phases Weak phases flip under CP ACP= 0 unless BOTH ACP is largest whenP=T.

Searches for direct CPVin D decays • Decays are classified by level of Cabibbo suppression - CF, DCS, SCS. • CF and DCS decays dominated by Tree diagrams but Penguins can contribute to SCS. We therefore do NOT expect CPV in CF or DCS decays, but we do inSCS. • In the SM, CPV is highly suppressed, but there could be NP in loops. F. Bucella et al., Phys. Rev. D51, 3478 (1995) S. Bianco et al., Riv. NuovoCim. 26N7, 1(2003) S. Bianco, F.L. Fabbri, D. Benson, and I. Bigi, Riv., NuovoCim. 26N7, 1 (2003). A.A. Petrov, Phys. Rev. D69, 111901 (2004) Y. Grossman, A.L. Kagan, and Y. Nir, Phys. Rev. D75,036008 (2007)

The simplest observable is time-integrated CP asymmetry: • Uncertainties in charge asymmetry of detector efficiency or D production can limit the precision of measurements. • Data-driven methods to control these asymmetries allow a precision in currently at a level of 1x10-3 (close to SM expectations). This should shrink as more data is available. • With its large samples, LHCbis most likely to make meaningful measurements but has, as yet, had to measure differences between pairs of modes in order to control systematics at this level.

Direct CPV in D0 decay Episode PRL 108, 111602 (2012)– 0.62 fb-1 • LHCbmeasured - a clever idea: • This cancels most of the production (and other) asymmetries: • U-spin conservation suggests that , doubling any asymmetry • Any asymmetry from time-dependent mixing effects cancels so measures ONLY direct CPV. production pion tag What we measure charge of h cancels What we want same for KK and pp Grossman, Kagan and Nir, PRD72, 036008 (2007) Confirmation with 3 fb-1 ? !!

Confirmation ? Seem to confirm the evidence from LHCb. “Wow – ACP ~ 1% Too !!”

BUT recent, higher precision results from LHCb: Seem to confirm the evidence from LHCb. “Whew !!” This was just an unlucky statistical glitch !!

D0h+h- (K+K-, p+p-or r+r-) K+K- :zero Tree (T): CKM phase p+p-:bc q h+ u q c Exchange (E): CKM phase same as T SoCombine T and E as “T” h+ q c u D0 h- D0 u h- u u u q u h+ c q Penguin (P): CKM phase gc b,s,d q D0 h- u u

Standard Model Penguins “cu” triangle condition • Small - could be larger with U-spin or QCD effects • Weak phase large (~ g) • Change iso-spin D I = ½ (c u) U-spin breaking a tunable parameter controls level of Ps, Pd effect on P. c u b, s, d SM: Brod, Grossman, Kagan, Zupan, JHEP 1210 (2012) 161 SM Penguins:

Time-Dependent CP asymmetry in D0 decays Bevan, Inguglia, BM, Phys.Rev. D84 (2011) 114009 • Mixing allows D0and D0 to interfere, exposing weak mixing and decay phases to measurement: • Since , then • Can measure from time-dependent CP asymmetry. D0 fCP D0 D0 Equal when f is CP eigestate Mixing phase Arg{q/p} = M

Recent results and trends in charm physics